Title of Invention	METHOD FOR REGULATING AN ELECTROLYTIC CELL
Abstract	ABSTRACT ELECTROLYTIC CELL REGULATION METHOD The invention relates to a regulation method for an electrolytic cell for the production of aluminium by means of reduction of alumina dissolved in a molten cryolite bath, wherein a solidified bath ridge is 5 formed on the internal walls of the pot, a quantity B, referred to as the "ridge variation indicator", which is sensitive to the variation of said solidified bath ridge, is determined and at least one of the setting means of the pot (such as the anode-metal distance) 10 and/or at least one control operation (such as the addition of AIF3) is modified as a function of the value obtained for said indicator. The indicator may be determined from electrical measurements on the pot and/or from measurements of the liquid metal surface 15 area. The method according to the invention makes it possible to regulate an electrolytic cell effectively at currents of up to 500 kA with an electrolyte bath with an AIF3 content greater than 11% and reduce the number of AIF3 content measurements in the bath 20 considerably- Figure 6

Title of Invention

METHOD FOR REGULATING AN ELECTROLYTIC CELL

Abstract

ABSTRACT ELECTROLYTIC CELL REGULATION METHOD The invention relates to a regulation method for an electrolytic cell for the production of aluminium by means of reduction of alumina dissolved in a molten cryolite bath, wherein a solidified bath ridge is 5 formed on the internal walls of the pot, a quantity B, referred to as the "ridge variation indicator", which is sensitive to the variation of said solidified bath ridge, is determined and at least one of the setting means of the pot (such as the anode-metal distance) 10 and/or at least one control operation (such as the addition of AIF3) is modified as a function of the value obtained for said indicator. The indicator may be determined from electrical measurements on the pot and/or from measurements of the liquid metal surface 15 area. The method according to the invention makes it possible to regulate an electrolytic cell effectively at currents of up to 500 kA with an electrolyte bath with an AIF3 content greater than 11% and reduce the number of AIF3 content measurements in the bath 20 considerably- Figure 6

Full Text	Field of the invention The invention relates to a regulation method for an aluminium production cell by means of electrolysis of alumina dissolved in an electrolyte based on molten cryolite, particularly according to the Hall-Heroult method. State of the art Metal aluminium is produced industrially by igneous electrolysis, i.e. by means of electrolysis of alumina in solution in a molten cryolite bath, referred to as an electrolyte bath, particularly according to the well-known Hall-Heroult method. The electrolyte bath is contained in pots, referred to as "electrolytic pots", comprising a steel shell, which is lined internally with refractory and/or insulating n The electrolytic current, which flows in the electrolyte bath and the pad of liquid aluntiniuTL via the anodes and cathode components, brings about the aluminium reduction reactions and also makes it possible to maintain the electrolyte bath ai: a temperature of the order of 950 "C by means of the Joule effect. The electrolytic ceil is regularly supplied with alumina so as to compensate for the alumina consumption.produced by the electrolytic reactions. The productivity and current efficiency of an electrolytic cell are influenced by several factors, such as the intensity and distribution of the electrolytic current, the pot temperature, the dissolved alumina content and the acidity of the electrolyte bath, etc., which interact with each other. For example, the melting temperature of a cryolite bath decreases with the excess aluminium trifluoride (AIF3) with reference to the nominal composition (3 NaF.AlFs) . In modern plants, the operating parameters are adjusted to aim for current efficiencies of over 90%. However, the effective current efficiency of a cell is significantly influenced by variations in said cell's parameters. For example, an increase in the electrolyte temperature by around ten degrees Celsius may cause the current efficiency to fall by approximately 2% and a decrease in the electrolyte temperature by around ten degrees Celsius may reduce the already low solubility of alumina in the electrolyte and favour the "anode effect", i.e. anode polarisation, with a sudden rise in the voltage at the cell tennir.sls and the release of a large quantity of» fluorinated and fluoro-carbonated products, and/or insplsting deposits on the cathode surface. Therefore, the operation of an electrolytic cell requires precise control of its operating parameters, such as its temperature, alumina content, acidity, etc., so as to maintain them at determined set-point values. Several regulation methods have been developed to achieve this objective. These methods generally relate to the regulation of the alumina content of the electrolyte bath, the regulation of its temperature, or the regulation of its acidity, i.e. the excess AIF3. statement of the problem Aluminium producers, in the continuous aim to increase electrolytic plant production and productivity at the same time, try to push back these limits. In particular, in order to increase plant productivity, it is aimed to reach current efficiencies above 95% operating with AIF3 excesses of over 11%, and which may reach 13 to 1^%, which makes it possible to decrease the cell operating temperature (the liquidus temperature drops approximately 5'C/IAir3) and, as a result, reduce the energy consumption of said cells. However, in this chemical composition range, the solubility of alumina is considerably reduced, which increases the risks of anode effects and forming of insulstinc deposits on the cathode. In addition, in order to increase plant production, it is airbed to increase the unit capacity of cells and, in correlation, increase the intensity of the electrolytic current. The current trend is to develop electrolytic cells with a current greacer thsn or equal to 5GO k,-. The increase in the capacity cf electrolytic cells may be obtained, as a general rule, either by increasing the permissible intensity of ceils of known type cr existing cells, or by developing very large cells. In the first case, the increase in the permissible intensity results in a decrease in the electrolyte 'OBth mass, which exacerbates the instability effect. In the second case, the increase in the cell size increases their thermal and chemical inertia. Consequently, the increase in cell capacity not only increases the rate of alumina consumption but also amplifies instability generation and cell deviation phenomena, which increases difficulties in controlling electrolytic cells. Therefore, the applicant searched for a regulation method for an electrolytic cell, particularly of the electrolyte bath acidity [i.e. its AlFj content) and the overall thermics of the cell, which makes it possible to control, in a stable manner with a current efficiency greater than 93S, or even greater than 95%, without having to use frequent AIF3 content measurements, electrolytic cells wherein the excess AIF3 is greater than 11% and wherein the current may be greater than or equal to 500 kA, Descriptiori of the invention The invention relates to a regulation method for an electrolytic cell intended for the production of aluminium by means of igneous, electrolysis, i.e. by flowing current in an electrolyte bath based on molten cryolite and containing dissolved alumina, particularly according to the Hall-Heroult method. The regulation method according to the invention comprises the addition of alumina in the electrolyte bath of an electrolytic cell, and is characterised in that it comprises the determination of a quantity B, referred to as the "ridge variation indicator", which is sensitive to variations of the solidified bath ridge formed on the side walls of the pot, and the modification of at least one of the setting means of the pot and/or at least one control operation as a function of the value obtained for said indicator. The applicant noted that, surprisingly, accounting for the variation in the solidified bath mass in the [regulation of an electrolytic pot made it possible to reduce the amplitude and dispersion of the fluctuations of the pot operating parameters, such as its acidity. According to one embodiment of the invention, said indicator is determined from an electrical measurement on the electrolytic ceil which is capable of detecting variations in the current lines induced by the variation of the ridge. In a preferred embodiment of the invention, said indicator is determined from a quantity referred to as the "specific resistance variation" ARS which is determined from the resistance R of the electrolytic ceil. According to another embodiment of the invention, said indicator is determined from a determination of the Surface area of the liquid metal pad, which is capable of detecting variations in the surface area of the liquid metal induced by the variation of the ridge. According to another embodi.~ent of the invention, said indicator is determined from a combination of electrical meesureme.its and raeasurerfients of the metal surface area. The invention may be implemented advantageously in electrolyte bath acidity regulation. In particular, the regulation method according to the invention may :omprise the addition, in the electrolyte bath of an slectrolytic ceil, during pre-determined time intervals . referred to as "regulation periods", of a quantity !(p) of aluminium trifluoride (AIF3) determined by the um of at least one basic term Qo{p) corresponding to he net average AIF3 requirements of the cell, and of a orrective term Qi(p) including at least one term sol(p), referred to as the "ridge term", which is etermined from at least one ridge variation indicator. Therefore, the quantity Q{p) is determined using the formula: Q(p) = Qo(p) + Qi(p) = Qo(p! + QsoL(p) + ... The applicant noted that the ridge term Qsol(p) makes it possible to reduce the number of analyses of the AlFj content of the liquid electrolyte bath significantly; these measurements add to cell operating costs and are, in any case, usually affected by significant errors. Said modifications of at least one cell setting means and/or at least one control operation may advantageously be combined. Figures Figure 1 represents, in a transverse section, a typical electrolytic cell. Figure 2 illustrates the principle of the regulation sequences according to the invention. Figures 3 and 4 show typical functions used to determine the terms of 0(p). Figure 5 illustrates s method to determine the specific electric resistance variation of the 2lectrolytic cell. Figure 6 is a schematic illustration of the shape >f the current lines flowing in the electrolyte bath between an anode and the liquid metal pad. Figure 7 illustrates a method to determine the ;urface area of the liquid metal pad. Figure 8 shows the variations in total AIF3 equireraents of an electrolytic cell. As illustrated in figure 1, an electrolytic cell 1 or the production of aluminium by means of the Hall-6rouit electrolysis method typically comprises a pot 0, anodes 7 supported by attachment means 8, 9 to an anode frame 10 and alumina supply means 11. The pot 20 comprises a steel shell, internal lining components 3, A and a cathode assembly 5, 6. The internal lining components 3, 4 are generally blocks made of refractory materials, which may be heat insulators. The cathode assembly 5, 6 comprises connection bars 6 to which the electric conductors used to route the electrolytic current are attached. The lining components 3, A and the cathode assembly 5, 6 form, inside the pot 20, a crucible capable of containing the electrolyte bath 13 and a liguid metal pad 12 when the cell is in operation, during which the anodes 7 are partially immersed in the electrolyte bath 13. The electrolyte bath contains dissolved alumina and, as a general rule, an alumina cover 14 covers the electrolyte bath. The electrolytic current transits in the electrolyte bach 13 via the anode frame 10, the attachment means 8, 9, anodes 7 and cathode components S, 6. The purpose of the alumina supply to the cell is to compensate for ■ the approximately continuous consumption of the cell which is esseriticlly due to the reduction of alumina into metal Eiurainium, The alumina supply, which is made by adding alunina into the liquid bath 13 is generally regulated separately. The metal aluminium 12 which is produced during the electrolysis is accumulated at the bottom of the cell and a relatively clear interface between the liquid metal 12 and the molten cryolite bath 13 is established. The position of this bath-metal interface varies over time: it rises as the liquid metal accumulates at the bottom of the cell and it goes down when the liquid metal is remo-ved from the cell. Several electrolytic cells are generally arranged in a row, in buildings referred to as electrolysis rooms, and connected electrically in series using connection conductors. The cells are typically arranged so as to form two or more parallel lines. The electrolytic current thus flows in cascade from one cell to the next. Detailed description of the invention According to the invention, the regulation method for an electrolytic cell 1 for the production of aluminium by means of electrolytic reduction of alumina dissolved in an electrolyte bath 13 based on cryolite, said cell 1 comprising a pot 20, at least one anode 7,■ at least one cathode component 5, 6, said pot 20 comprising internal side walls 3 and being capable of containing a liquid electrolyte bath 13, said cell 1 comprising at least one setting means of said cell including a mobile anode frame 10 to which said at least one anode 7 is attached, said cell 1 being capable of circulating a so-called electrolytic current in said bath, said current having an intensity I, the aluminium produced by means of said reduction forming a pad referred to as a "liquid metal pad" 12 on said cathode component (s) 5, 6, said cell 1 comprising a solidified bath ridge 16 on said wails 3, comprises control operations of said cell including the addition of alumina and the addition of hlFj in said bath and is characterised in that it comprises: - the determination of the value of at least one indicator B referred to as the "ridge variation", capable of detecting the variation of said solidified bath ridge 15; - the adjustment of at least one setting means and/or at least one control operation according to the value obtained for the or each ridge variation indicator. Variations in the solidified bath ridge are generally conveyed by variations in the thickness and, to a lesser degree, the shape of said ridge. Said adjustment of at least one setting means of the cell typically comprises at least one modification of the position of said mobile anode frame 10, either upwards, or downwards, so as to modify the anode/metal distance (AMD). Said at least one control operation typically comprises the addition of a quantity Q of AIF3 into said electrolyte bath 13. Said adjustment may then con-.prise at least one modification of said quantity Q as a function of the value obtained for one or each ridge variation indicator. In a preferred embodiment of the invention, the regulation method' is characterised in that said ec least one ridge variation indicator includes an indicator, referred to as "BE", which is determined fron at least one electrical measurement on said cell 1 capable of detecting the variations of the current lines induced by the variation of said ridge. Preferentially, said indicator BE is determined from at least one determination of said intensity I and at least one determination of the drop in voltage U at the terminals of said cell 1. In an alternative version of this embodiment, said at least one ridge variation indicator BE is equal to a specific resistance variation ARS which may be determined using a measurement method comprising; - the determination of at least one first value II foe said intensity I and at least one first value Ul for the drop in voltage U at the terminals of said cell 1; - the calculation of a first resistance Rl from at least said values II and Ul; - the movement of the anode frame 10 by a determined distance AH, from a so-called initial position, either upwards (AH being positive in this case), or downwards (AH being negative in this case); - the determination of at least one second value 12 for said intensity 1 and at least one second value U2 for the drop in voltage U at the terminals of said cell 1; ■;- the calculation of a second resistance R2 from at least said values 12 and U2; - the calculation of a resistance variation AR using the formula AR = R2-R1;' - the calculation of said specific resistance ARS usinc -he formula ABS = AR/AH. Preferentially, the measurement method also cor.prises (at least after the determination of the values of II, 12, Ul and U2) , the movement of the anode frame 10 so as to return it to its initial position and restore the initial cell setting. Said first and second resistance Rl and R2 may be calcul&ted using the formula R = (U-Uo)/I, where Uo is a constant typically between 1.6 and 2.0 V. For Example, Rl and R2 may be given by Rl = (Ul-Uo)/il and R2 = {U2-Uo)/I2. According to an alternative embodiment Df the invention, Bl and R2 may be given by a mean value obtained from a determined number of values of the voltage U and intensity I. In practice, it was found to be simpler to give an order of movement of the anode frame 10 for a determined time and measure the resulting frame displacement AH. According to this embodiment of the invention, the regulation method advantageously comprises: the determination of a specific resistance variation ARS using the formula: ARS = A9./AH; - the adjustment of at least one control means and/or at least one control operation using a determined function of said specific resistance variation ARS. Said adjustment may be a determined function of the difference between said specific resistance variation ARS and a reference value Ai^So, i.e. ARS-ABSo. As shown in figure 5, said resistance is typically measured using means 18 to measure the intensity I of the current circulating in the cell (where I is equal to the sum of the cathode currents Ic or anode intensity la) and means 15,17 to measure the resulting drop in voltage U at the cell terminals (typically the resulting drop in voltage between the anode frame and the cathode components of the cell). Said resistance R is generally calculated using the equation: R = (U -Uo)/I, wheire Oo is a ctmstant. The resistance R depends not only on the resistivity p of the electrolyte bath 13, on the distance H between the anode (s) 7 and the liquid metal pad 12, and on the surface area Sa of the anode (s) 7, but also on the spreading r) of the lines of current Jc, Js which are established in said bath, particularly between the anode(s) 7 and the solidified bath ridge 15 [lines Jc in figure 6) , The applicant had the idea to make use of the fact that the specific electric resistance variation ARS is not only sensitive to the resistivity of the electrolyte bath, but integrates an electric current spreading factor, which is sensitive to the presence, size and, to a lesser degree, shape of the solidified bath ridge 15 on the walls of the pot. The applicant also observed that, unlike that which is normally admitted, the spreading T^ is in fact a preponderant factor in the establishment of electfic resistance. The applicant considers that the contribution of spreading Co the specific electric resistance variation is typically between 75 and 90%, which means that the contribution of the resistivity is very low, or typically between 10 end 25% {that is typically 15%). In its tests on 500 '■. pots, the applicant observed a mean ARS value of the order of 100 icfi/mm, which decreases by approximstely -3 rli/csr. when the bath temperature increases by 5'C and when the AIFB content decreases by 1%, and conversely. The contribution of the resistivity to this variation is estimated to be only of the order of -C.5 nQ/mm (that is only approximately 15% of the total value) , the contribution attributable to spreading, i.e. -2.5 nfi/mm being dominant. It is possible to take into account the spreading of the current in the resistance measured (for example by modelling the current lines), which improves the reliability of the specific resistance variation considerably as an indication of the variation of the ridge BE (itself an indicator of the thermal state of the cell). In another preferred embodiment of the invention, the regulation method is characterised in that said at least one ridge variation indicator includes an indicator, referred to as "BM", which is determined from a determination of the surface area S of said liquid metal pad 12. According to this embodiment of the invention, the regulation method advantageously comprises: - the determination of a surface area S for the liquid metal pad 12; - the adjustment of at least one control means and/or at least one control operation using a determined function of the surface area S. Said adjustment may be a determined function of the so-called "metal surface area" difference between the value obtained for said surface area S and a set-point value So (i.e. S-So). The surface area S, which corresponds approxirxat-sly to the irietal/bath interface, is aoproximately equal to the horizontal right section of the electrolytic pot. The presence of solidified electrolyte bath on the walls of the pot decreases this surface area by a quantity which varies as a function of time and pot operating conditions. In the preferred embodiment of this alternative embodiment of the invention, the surface area S is calculated from a measurement of the volume Vm of metal tapped and the corresponding fall AHm of the metal level Hm (see figure 1). More specifically, said metal surface area may be deterrnined using a measurement method comprising: - the removal of a quantity of liquid metal from the electrolytic cell; - the determinatiorv of the volume Vm of said quantity of liquid metal removed from the electrolytic cell; - the determination of the change AHm of the resulting level of said liquid metal pad in said pot; - the determination of a surface area S for said liquid metal pad 12 using the formula S = Vm/AHm. Said volume Vm may be determined by measuring the mass of said quantity of liquid metal removed from the electrolytic cell. In practice, the anodes 7 are normally lowered at the same time as the level of liquid metal so as to keep the anode/metal distance (AMD) constant. Said at least one control operation may also comprise at least one addition of solid or liquid electrolyte bath so as to increase the level of said liquid electrolyte bath 13 in said pot 20. Said adjustments of at least one setting means of the cell and/or at least one control operation may advantageously be combined. Implementation of the invention in bath acidity regulation According to an embodiment of the invention, the regulation method for an electrolytic cell 1 for the production of aluminium by means of electrolytic reduction of alumina dissolved in an electrolyte bath L3 based on cryolite, said cell 1 comprising a pot 20, it least one anode 7, at least one cathode component 5, 6, said pot 20 comprising internal side walls 3 and being capable of containing a liquid electrolyte batti 13, said cell 1 also comprising at least one setting means of said cell inciuding a mobile anode frame 10 to which said at least one anode 7 is attached, said cell 1 being capable of circulating a so-called electrolytic current in said bath, said current having an intensity r, the aluminium produced by said reduction forming a pad referred to as the "liquid metal pad" 12 on the cathode component (s) 5, 6, said cell 1 comprising a solidified bath ridge 15 on said walls 3, comprises control operations of said cell including the addition of alumina and the addition of AlPa into said bath and is characterised in that it comprises: - the set-up of a regulation sequence comprising a series of time intervals p of pre-determined length Lp hereafter referred to as "regulation periods" or simply "periods"; - the determination of the value of at least one indicator B referred to as the "ridge variation" capable of detecting the variation of said solidified bath ridge 15; - the determination of a quantity .Qo (p), referred to as the "basic term", corresponding to the net average AIF3 requirements of the cell; - the determination of a corrective term Qi (p) including at least one term Qsol(p), referred to as the "ridge term", which is determined froin at least one or each ridge variation indicator 15; - the determination of a quantity Q(p) of AIF3 to be added during the period p, referred to as the "determined quantity Q(p)", by adding the corrective term Qi (p) to the basic term Qo (p) , i.e. Q(p) = Qo(p) + Qi(p); ~ the addition into said electrolyte bath, during Che period p, of an effective quantity of aluminium trifluoride {RIF3) equal to said determined quantity Q(p} . The intervals (or "periods") p are preferentially approximately equal in length Lp, i.e. the length Lp of the periods is approximately the same for all the periods, enabling easier implementation of the invention. Said length Lp is generally between 1 and 100 hours. The term Qsol(p) is a function of variations in the mass of the solidified bath ridge 15 formed on said walls 3; said variations are generally conveyed by variations in the thickness (and, to a lesser degree, the shape) of said ridge. In an advantageous alternative version of said embodiment of the invention, the term Qsol(p) includes at least one terir. referred to as Qr{p) which may be determined from at least one electrical measurement on the cell 1 capable of detecting variations in the current lines induced by the variation of said ridge. The term Qr(p} is advantageously determined from at least one measurement of said intensity I and at least one measurement of the drop in voltage U at the terminals of said cell 1. In the preferred embodiment of this alternative version of the invention, the method comprises: - the determination of at least one first value II for said intensity I and at least one first value Ul for the drop in voltage U at the terminals of said cell 1; - the calculation of a first resistance Rl from at■ least said values II and Ul; the movement of the anode frame 10 by a determined distance AH, from a so-called initial position, either upwards (AH being positive in this case), or downwards (AH being negative in this case); - the determination of at least one second value 12 for said intensity I and at least one second value U2 for the drop in voltage (J at the terminals of said cell 1; - the calculation of a second resistance R2 from at least said values 12 and U2; - the calculation of a resistance variation AR using the formula AR = R2-R1; - the calculation of said specific resistance AES using the formula ARS = AS/AH; - the determination of a term Qr(p) using a determined function of said specific resistance variation ARS; - the determination of the corrective term Qi Cp) including at least the term Qr(p) in the ridge term QsoKp). Preferentially, the measurement method also comprises (at least after the determination of the values of II, 12, Ul and 02), the movement of the anode frame 10 so as to return it to its initial position and restore the initial cell setting. Said first and second resistance Rl and R2 may be calculated using the formula R = (U-Uo)/I, where Uo is a constant typically between 1.6 and 2.0 V. For example, Rl and R2 may be given by Rl = (Ul-Uo)/II and R2 = (U2-Uo)/l2. According to an alternative embodiment of the invention, Rl and R2 may be given by a mean value obtained from a determined number of values of the voltage U and intensity I. Said determined function, which is typically decreasing, is preferentially limited. It is advantageously a function of the difference between ARS and a reference value ARSo. Figure 3 shows a typical function used to determine the term Qr. In a simplified alternative embodiment of the invention, the tern Qr {p) may be given by a simple equation such as: Or (p) = Kr x (ARS - ARSo), where Kr is a constant which may be set empirically and whose value is typically between -0.01 and -10 kg/hour/nli/mm, and more typically between -0.05 and -0.3 kg/hour/nn/min (corresponding, in the latter case, to approximately -0.5 to -2 kg/period/nC3/mm for an 8-hour period) for 300 kA to 500 kA pots. The term Qr(p) is preferentially limited by a minimum value and by a maj;i.~um value. These minimum and maximum values may be negative, null or positive. In practice, it is possible to make Nr measurements of ARS (i.e. two or more measurements) during the period p- The ARS value used to calculate Qr(p) will in this case be the mean of the Nr measured ARS values, except, if applicable, values considered to be aberrant. It is also possible to use a sliding mean on two or more periods to smooth the thermal fluctuations related to the operating cycle. An operating cycle is determined by the frequency of interventions on the electrolytic cell, particularly anode' replacements and liquid metal- sampling. The length of an operating cycle is generally between 24 and 48 hours {for example 4 x 8-hour periods). tn another advantageous alternative embodiment of the method according to the invention, the term Qsolfp) includes at least one term referred to as Qs(p), which may be. determined from at least one determination of the surface area S(p) of said liquid metal pad 12. The term Qs(p) is advantageously determined from the so-called "metal surface area" difference between the value obtained for said surface area S(p} and a set-point value So. According to the preferred embodiment of this alternative version, the method comprises: ~ the removal of a quantity of liquid metal from the electrolytic cell; - the determination of the volume Vm of said quantity of liquid metal removed from the electrolytic cell; - the determination of the change AHm of the resulting level of said liquid metal pad in said pot; - the determination of a surface area S {p} for said liquid metal pad 12 using the formula S = Vm/AHm; - the determination of a term Qs(p) using a determined function of the surface area S(p} of said liquid metal pad 12; - the determination of the corrective term Qi (p) including at least the term Qs{p) in the ridge ter-m Qsol(p). Said volume Vm may be determined by measuring the mass of said quantity of liquid metal removed from the electrolytic cell. Said determined function, which., is typically increasing, is preferentially limited. It is advantageously a function of the difference between the surface area S(p) of the liquid metal pad 12 and a set-point value So. Figure 4 shows a typical function used to determine the term Qs. In a simplified alternative embodiment of the invention, the term Qs (p) may be given by a simple equation such as: Qs(p) = Ks x (S(p) - So), where Ks is a constant which may be set empirically and whose value is typically between 0.0001 and 0.1 kg/hour/dm=, and more typically between 0.001 and 0.01 kg/hour/dm (corresponding, in the latter case, to approximately 0.01 to 0.05 kg/period/dm' for an 8-hour period) for 300 kA to 500 kA pots. The term Qs(p) is preferentially limited by a minimum value and by a maximum value. These jninimuni and maximum values may be negative, null or positive. The applicant noted that the corrective terms QrCp) snd Qs(p) according to the present application are effective indicators of the overall thermal state of the electrolytic cell, which take into account both the liquid electrolyte bath and the solidified bath ridge on the walls of the pot. These terms, taken separately or in combination, particularly make it possible to reduce the number of analyses of the AIF3 content in the liquid electrolyte bath markedly. The applicant observed that the frequency of the analyses of the AIF3 content may be reduced typically to one analysis per cell approximately every 30 days. The' terms Qr{p) and Qs(p), which may be combined, make it possible to only perform AiFj content analyses in exceptional cases or in order to characterise a cell or a series of cells statistically. The terms Qr(p) and Qs(p) also enable long-term thermal regulation of the ridge thickness. In a preferred alternative embodiment of the invention, the basic term Qo(p) is determined using a so-called "integral" (or "self-adaptive") term Qint(p), which represents the total actual AIF3 requirements of the pot. The term Qint (p) is calculated from a mean Qm(p) of the actual AlFj supplies made during the last N periods. The term Qint(p) takes into account AIF3 losses in the bath occurring during normal ceil operation and which are essentially produced by absorption by the pot crucible and emissions in gaseous effluents. This term, the mean value of which is not null, is particularly used to monitor pot ageing, without having to model it, by means of a memory effect of pot behaviour over time. It also takes into account the specific ageing of each pot, that the applicant generally found to be markedly different to the average ageing of the population of pots of the same type. In this case, the method also comprises: - the determination of a mean Qm{p) of the total AIF3 additions per period during the last H periods; the determination of a quantity Qint(p), advantageously using the following "smoothing" formula: Qint(p) = (1/D) X Qm(p) + (1 ~ 1/D) y. Qint(p-ll, where D is a smoothing parameter setting the temporal smoothing horizon; - the determination of the basic term Qo (p) using the formula Qo(p) = Qint{p). The horizon term D, which makes it possible to do away with medium and long-term thermal and chemical fluctuations, is equal to Pc/Lp, where Pc is a period which is typically of the order of AOO to 8000 hours, and more typically from 600 to 4500 hours, and Lp is the length of a period. Therefore, the term D is typically equal to 50 to 1000 8-hour periods if this work organisation method is applied. The term Qo(p) may be corrected so as to take into account the impact of alumina additions on the effective composition of the electrolyte bath. For this purpose, the method according to the invention may also comprise: - the determination of a compensating term Qcl(p) corresponding to the so-called "equivalent" quantity of AIF3 contained in the alumina added to the cell during the period p; the modification of the term Qo(p) by subtracting the term Qcl(p) from said term Qo{p), i.e. using the formula Qo(p) = Qo(p) - Qcl(p). The term Qcl(p) corresponds to the so-called "equivalent" quantity of AlFs added to the cell by means of the alumina added to the electrolytic cell during the period p, where said quantity may be positive or negative. This term is determined by producing the chemical balance of the fluorine and sodium contained in said alirr.ina from one or more chemical analyses. The effect of the sodium contained in the alumina is to neutralise fluorine, thus being equivalent to a negative quantity of AIF3. The term Qlc(p) is positive if said alumina is "fluorinated" (which is the case when it has been used to filter electrolytic cell effluents) and negative if the alumina is "fresh", i.e. if it is produced directly from the Bayer process. In a preferred alternative embodiment of the invention, the term Qm(p) is calculated using the equation: Qmlp) = + , where = {Q(p-N) + Q(p-N+1) + Q(p-N+2) +...+ Q(p-1})/N, = (Qcl(p-N) + Qcl(p-N+1) + Qcl(p-N+2) +...+ Qcl(p~l))/N, where N is a constant. The term Qm(p) is then equal to Q(p-l) + Qcl(p-1) when N = I: lQip-2) + Qcl(p'2) + Q(p-1> + Ocl(p-l))/2 when N = 2; (Q{p~3) + Qcl {p-3) + Q(p-2) + Qcl(p-2) + O(p-I) + Qcllp-l})/3 when N = 3, ... The value of the parameter N is selected according to the cell reaction time and is normally between 1 and 100, and more typically between 1 and 20. The term Om(p) chen takes into account total equivalent AlFj supplies, i.e. "direct" supplies from additions of AlFj and "indirect" supplies fro-; additions of alumina. In another advantageous alternative embodi.T,ent of the invention, the determination of Qi(p} comprises an additional so-call "damping" corrective term Qc21'p), which takes into account the delay in the reaction of the cell with the AlFj additions. The term Qc3 is a prospective correction term which is used to take into account the effect of an addition of AIF3 in advance, which normally only appears after a few days. Indeed, the applicant noted the surprising degree of the difference between the time constant of the temperature variation, which is low (of the order of a few hours) and that of the AIF3 content, which is very high (of , the order of a few tens of hours} . In its tests, it found that it was very advantageous to anticipate the variation of the acidity of the bath of the cell when adding AIF3, which is made possible effectively by the term Qc2. This alternative embodiment may be implemented by including in the method according to the invention: - the determination of an additional corrective term Qc2(p) using a typically decreasing, preferentially limited, function of the difference between Qm(p) and Qintfp), i.e. Om(p) - Qlnt {p); - the addition of the corrective term Qc2(p) in the determination of Qi{p). In a simplified alternative embodiment of the invention, the term Dc2(p) may follow a simple equation, such as Qc2(p) = Kc2 x CQm(p) - Qint (p) ) , where Kc2 is a constant which is typically negative and which may be set empirically and whose value is typically between -0.1 and -1, and more typically between -0.5 and -1 for 300 kA to 500 kA pots. fc^-^ term Qc2(p) is preferentially limited by a minimum value and by a maximum value. These minimum and maximum values may be negative, null or positive. In order to converge the integral term Qint{p) rapidly to the quantity Q' corresponding to actual cell requirements, it is possible to start the method by simoly taking Qint(O) = Qtheo, where Qtheo corresponds to the total theoretical AlFj requirements of the cell when regulation is started. The AIF3 requirements of an electrolytic cell are essentially due to losses through absorption in the walls of the pot and emission of fluorinated products. Qtheo is a function of the age of the pot which can be determined statistically for each type of cell. This alternative embodiment may bs implemented by including in the method according to the invention: the determination of a quantity Qtheo corresponding to the total theoretical AIF3 requirements of the cell when regulation is started; - the start-up of the method by taking Qint{0) = Qtheo. Figure 8 illustrates, using typical values, the term Qtheo(p) and the operating principle of the integral term Qint(p). In another advantageous alternative embodiment of the invention, the determination of Qi{p) includes an additional corrective term Qt(p) which is a function of the bath temperature measured of the electrolyte bath. The term Qt(p) also makes it possible to avoid having to use regular bath AIF3 content measurements. This alternative embodiment may be implemented by including in the method according to the invention: - the determination of a mean temperature T (p] of the electrolyte bath; - the determination of an additional corrective term Qt(p) using a determined function, which is typically increasing and preferentially limited [i.e. it is limited by a maximum value and by a minimum value), of the difference between said temperature T(p) and a set-point temperature To; - the addition of the corrective term Qt (p) in the determination of Qi(p). In a simplified alternative embodiment of the invention, the term Qt(p) may follow a simple equation, such as Ot{p) = Kt X (T(p) ■- To), where Kt is a constant which is typically positive and which may be set empirically and whose value is typically between 0.01 and 1 kg/hour/°C, and more typically between 0.1 and 0.3 kg/hour/'C (corresponding, in the latter case, to approximately 1 to 2 kg/period/"C for an 8-hour period) for 300 kA to 500 kA pots. The term Qt(p) is preferentially limited by a minimum value and by a maximum value. These minimum and maximum values may be negative, null or positive. The mean temperature T(p) is normally determined from temperature measurements made on the period p and on the previous periods p-1, etc., so as to obtain a reliable and significant value of the average condition of the pot. The terms Qt(p) and Qc2(p) are regulation terms wherein the mean value over time normally tends towards zero fi.e. they are normally null on average). In another advantageous alternative embodiment of rr.e invention, the quantity Qi (p) comprises an additional corrective term Qe{p) which is a function of the difference between the excess AlFj measured E(p) and its target value Eo. This alternative embodiment nay be implemented by including in the method according to the invention: - the measurement of the excess AlFa E(p); - the determination of an additional corrective terTr" Qeip) using a determined function (typically decreasing and preferentially limited} of the difference between the excess AIF3 measured E(p) and its target value Eo, i.e. the difference E(p) - Eo; the addition of the term Qe(p} in the determination of Qi(p}. In a simplified alternative embodiment of the invention, the term Qe (p) may be given by a simple equation such as: Qe(p) = Ke x (E(p) - Eo), where Ke is a constant which may be set empirically and whose value is typically between -0.05 and -5 kg/hour/%AlF3, and more typically between -0.5 and -3 kg/hour/%AlF3 (corresponding, in the latter case, to approximately -20 to -5 kg/period/%AlF3 for an 8-hour period) for 300 kA to 500 kA pots. The term Qe(p) is preferentially limited by a minimum value and by a maximum value. These minimum and maximum values may be negative, null or positive. The applicant found it was satisfactory to only apply the term Qe(p) exceptionally, for a short length of time, when the thermal operation of the cell leaves the normal operating range, i.e. when the temperature values and values of the regulation terms (Qr, Q$, etc.) leave the so-called safety ranges. The applicant noted in its tests that the corrective term Qe enabled the indicators (temperature, Qr, Qs, etc.) to return rapidly to che normal operating range. According to another alternative embodiment of the invention, it is also possible to add corrective terms to take into account individual interfering events. In particular, the corrective term Qi(p) may comprise a so-called anode effect term Qea to take into account the impact of an anode effect on the thermics of an electrolytic cell. An anode effect particularly induces significant AIF3 losses by emission and, generally, heating of the electrolyte bath. The term Qea is applied for a limited time following the observation of an anode effect. The terra Qea is calculated using either a scale which is a function of the anode effect energy (AEE), or a fixed mean value. In the first case, the terra Qea is given by a typically increasing and preferentially limited function of the energy AEE. The term Qea(p) is preferentially limited by a minimum value and by a maximum value. These minimum and maximum values may be negative, null or positive. The term Q(p) corresponds to an addition of pure AIF3 and is typically expressed in kg of pure AIFB per period (kg/period). The expression "addition of an effective quantity of RlFj" corresponds to an addition of pure AiFa- In industrial practice, AIF3 additions are generally made using so-called industrial AlFs with a purity of less than 100% (typically 90%). In this case, a sufficient quantity of industrial AIF3 is added to obtain the effective quantity of AIF3 required. Typically, a quantity of industrial AIF3 equal to the effective quantity of AIF3 required divided by the purity of tihe .industrial AlFj used is adtied. The expression "total AlFa additions" refers to the sum of the effective additions of pure P.lTj and the "equivalent" AlFa additions from alumina. AIF3 may be added in different ways. It may be added manually or mechanically (preferentially using a a point feed, such as an crustbreaker-feeder which makes it possible to add determined doses of AIF3, in an automated fashion if required) . AIF3 laay be added with alumina or at the same time as alumina. Industrial bath and pure cryolite additions are sometimes performed on industrial cells. These additions have an impact on the composition of the electrolyte bath which must generally be taken into account in the regulation. For this purpose, the regulation method may also comprise a corrective term Qb to take into account the modification of the pure AIF3 content induced by these additions. The different terms of Q(p) are determined preferentially at each period p. If the cell is very stable, it may be sufficient to determine the quantity Q(p) and some of the terms forming it, in a more staggered manner over time, for example once every two or three periods. The applicant observed that it was sufficient to only apply some of the terms of Q (p) , such as Qe[p), exceptionally and for a limited length of time, which makes it possible to limit costs relating to their determination. In order to prevent excess AIF3 additions, it is preferable, as a precaution, to limit Q(p} to a maximum value Qmax. It is also preferable to limit the application of the regulation terras in time when they cannot be determined at each period. The quantity Q{p) is normally determined at each period. If one or more terms of Q(p) cannot be calculated during a given period, then it is possible to^maintaih the value of said term(s) used during the previous period, i.e. the value of said term{s) will be determined by making it equal to the value used during the previous period. If one or more terms cannot be calculated during several periods, then it is possible to retain the value of said term(s} used during the last period for which it could be calculated and maintain this value for a limited number Ns of periods (Ns being typically equal to 2 or 3) . In the latter case, if said term{s) still cannot be calculated after the Ns periods, then it is possible retain the pre¬determined fixed value, referred to as the "standby value". These different situations may occur, for example, when the mean temperature of the pot cannot bs determined or when the equivalent AIF3 quantity contained in the alumina could not be determined. The term Q(p) may be positive, null or negative. In the last case, it is assumed that Qlp) = 0, i.e. AIF3 is not added during the period p. When the term Q{p} is negative, it is also possible to correct the composition of the electrolyte bath 13 by adding soda, i.e. calcined soda or sodium carbonate, referred to as soda ash. As shown in figure 2, the additions of AIF3 may be made at any time during said regulation periods (or sequences), which may correspond to the work shifts which determine the frequency of the changes of the shifts in charge of cell control and maintenance. The quantity Q(p) of AlFj determined for a period p may be added in one or more times during said working period. Preferentially, the quantity Q(p) is added practically continuously using crustbreaker-feeders which make it possible to add predetermined doses of AlFj throughout the period p. Examples of embodiments of the invention The following examples illustrate the calculations inherent to the regulation method according to the invention. These calculations are typical of those made for the 500 kA cells tested by the applicant. The length of the periods is 8 hours. Example 1 Example illustrating the use of the additional terms Qr and Qs in combination with the basic terms Qint, Qcl, Qc2 and Qsol, The value of Qtheo at 28 months is +31 kg/period. The average requirements of the pot Q' determined by the integral term Qint are +39 kg/period. The alumina analysis gives a value of 1.36% fluorine and 5250 ppm of Na20 equivalent. The term Qcl is then equal to +22 kg/period in equivalent pure AIF3 supply. By taking N = 12, the total actual AIF3 supplies per period over the last N periods is 44 kg/period. The difference between the actual supplies (AA kg/period) and the mean requirements (39 kg/period) is then +5 kg/period. The term Qc2 is then equal to -3 kg/period. The temperature measured is 964°C and the set-point temperature 9b3'C, i.e. a difference of +10.e'C. The corrective term Qt is then equal to +18 kg/period. The ARS value measured is 101.8 nO/mrr. and the set-point value ARSo is 106.0 nfi/rom. The term Qr(p) is then equal to +5 kg/period. The S value measured is 6985 dm^ and the set-point value So is 6700 dm'. The term Qs (p) is then equal to +5 kg/period. The quantity of AlFs to be added during the period p is then equal to: Q(p) = Ointtp) - QcKpl + Qc2 (p) + Qt{p) + Qr{p) + Qs(p) =39-22-3 + 18 + 5 + 5 = +42 kg. The terms Qr and Qs make a significant correction to the quantity Q(p). Tests The method according to the invention was used to regulate electrolytic cells with intensities of up to 500 kA. The length of the periods was 8 hours. The tests related to different types of pots. Table 1 contains the characteristics of some of the electrolytic cells placed under test and the typical results obtained. In case A, the pots were regulated using the embodiment of the invention wherein Q(p) was determined using the terms Qint(p), Qcl(p), Qc2(p) and Qt (p) . In case B, the pots were regulated using the embodiment of the invention wherein Q(p) was determined using the terms Qint{p), Qcl(p), Qc2(p), Qt(p) and Qe (p) . In case C, the pots were regulated using the embodiment of the invention wherein Q(p) was determined using the terms Oint , Qcl(p), Qc2(p), Qt(p), Qr (p) and Qs(p)' Table 1 Case A Case B Case C Intensity {kA) 300 kA 330 kA 500 kA Anode density (A/cm*) 0.78 0.85 0.90 Liquid bath mass 25 22 17 (kg/kA) Excess AlFj {%) li.8 11.8 13.2 Total standard 1.5 1.3 1.3 deviation (CT %} Dispersion of excess 8.8 - 14.8 9.2 - 14.4 10.6-15.8 AIF3 at ± 2 0 % Bath temperature ("O 962 962 961 Total standard 6 6 3.5 deviation (0 %) Dispersion of 950-974 950-974 954-968 temperature at ± 2 CT % Current efficiency (%) 95.0 95.0 95.5 The results show that the regulation metho according to the invention makes it possible to regulate electrolytic cells effectively wherein the excess AIF3 of the bath is greater than 11% and wherein the bath temperature is in the vicinity of 960°C. Accounting for the terms Qr(p) and Qs(p) in the determination of QCpl makes it possible to regulate effectively, and with a surprising stability, electrolytic cells wherein the intensity and anode density are very high and wherein the liquid bath mass is low. The applicant observed during its tests that the regulation method according to the invention makes it possible to control, with high stability, the AIF3 content of electrolytic cells, over a period of several months, without having to take into account measured AIF3 contents, said measured contents are, in any case, easily affected by significant errors. WE CLAIM : 1. A regulation method for an electrolytic cell (1) for the production of aluminium by means of electrolytic reduction of alumina dissolved in an electrolyte bath (13) based on cryolite, said cell (1) comprising a pot (20), at least one anode (7), at least one cathode component (5,6), said pot (20) comprising internal side walls (3) and being capable of containing a liquid electrolyte bath (13), said cell (1) comprising at least one setting means of said cell comprising a mobile anode frame (10) to which said at least one anode (7) is attached, said cell being capable of circulating a so-called electrolytic current in said bath, said current having an intensity I, the aluminium produced by means of said reduction forming a pad referred to as a "liquid metal pad" (12) on said cathode component(s) 5, 6, said cell 1 comprising a solidified bath ridge (15) on said walls (3), said method comprising control operations of said cell comprising the addition of alumina and the addition of AIF3 in said bath and being characterized in that it comprises: the determination of the value of at least one indicator B referred to as the "ridge variation", capable of detecting the variation of said solidified bath ridge (15); the adjustment of at least one setting means and/or at least one control operation according to the value obtained for the or each ridge variation indicator. 2. The regulation method as claimed in claim 1, wherein said at least one ridge variation indicator comprises an indicator, referred to as "BE", which is determined from at least one electrical measurement on said cell (1) capable of detecting the variations of the current lines induced by the variation of said ridge. 3. The regulation method as claimed in claim 2, wherein said indicator "BE" Is determined from at least one determination of said intensity 1 and at least one determination of the drop in voltage U at the terminals of said cell (1). 4. The regulation method as claimed in claim 3, wherein said at least one ridge variation indicator BE is equal to a specific resistance variation ARS which is determined using a measurement method comprising: the determination of at least one first value II for said intensity I and at least one first value Ul for the drop in voltage U at the terminals of said cell (1); the calculation of a first resistance Rl fi-om at least said values II and Ul; the movement of the anode frame (10) by a determined distance AH, from an initial position; either upwards, AH being positive in this case, or downwards, AH being negative in this case; the determination of at least one second value 12 for said intensity I and at least one second value U2 for the drop in voltage U at the terminals of said cell (1); the calculation of a second resistance R2 from at least said values 12 and U2; the calculafion of a resistance variation AR using the formula AR=R2-R1; the calculation of said specific resistance AR using the formula ARS = AR/AH. 5. The regulation method as claimed in claim 4, wherein the measurement method also comprises, at least after the determination of the values of II, 12, Ul and U2, the movement of the anode frame (10) so as to return it to its initial position and restore the initial cell setting. 6. The regulation method as claimed in claim 5, wherein said first and second resistance are calculated using the formula R=(U-Uo)/I, where Uo is a constant. 7. The regulation method as claimed in claim 6, wherein the constant Uo is between 1.6 and 2.0 V. 8. The regulation method as claimed in any one of claims 4 to 7, wherein said adjustment is a determined fimction of the difference between said specific resistance variation ARS and a reference value ARSo. 9. The regulation method as claimed in any one of claims 1 to 8, wherein said at least one ridge variation indicator comprises an indicator, referred to as "BM", which is determined from a determination of the surface area S of said liquid metal pad (12). 10. The regulation method as claimed in claim 9, wherein said metal surface area is determined using a measurement method comprising: the removal of a quantity of liquid metal from the eJectroIytic cell; the determination of the volume Vm of said quantity of liquid metal removed from the electrolytic cell; the determination of the change AHm of the resulting level of said liquid metal pad in said pot; the determination of a surface area S for said liquid metal pad (12) using the formula S=Vm/AHm. 11. The regulation method as claimed in claim 10, wherein said volume Vm is determined by measuring the mass of said quantity of liquid metal removed from the electrolytic cell. 12. The regulation method as claimed in any of claims 9 to 11, wherein said adjustment is a determined function of the so-called "metal surface area" difference between the value obtained for said surface area S and a set-point value So. 13. The regulation method as claimed in any one of claims 1 to 12, wherein said adjustment comprises at least one modification of the position of said mobile anode frame (10), either upwards, or downwards, so as to modify the anode/metal distance (AMD). 14. The regulation method as claimed in any one of claims 1 to 13, wherein said adjustment comprises at least one addition of solid or liquid electrolyte bath so as to increase the level of said liquid elecfrolyte bath (13) in said pot (20). 15. The regulation method as claimed in any one of claims 1 to 14, wherein said adjustment comprises at least one modification of said AIF3 addition. 16. A regulation method for an electrolytic cell (1) for the production of aluminium by means of electrolytic reduction of alumina dissolved in an electrolyte bath (13) based on cryolite, said cell (1) comprising a pot (20), at least one anode (7), at least one cathode component (5,6), said pot (20) comprising internal side walls (3) and being capable of containing a liquid electrolyte bath (13), said cell (1) also comprising at least one setting means of said cell comprising a mobile anode frame 10 to which said at least one anode (7) is attached, said cell (1) being capable of circulating a so-called electrolytic current in said bath, said current having an intensity 1, the aluminium produced by said reduction forming a pad referred to as the "liquid metai pad" (12) on the cathode component(s) (5,6), said cell comprising a solidified bath ridge (15) on said walls (3), said method comprising control operations of said cell comprising the addition of alumina and the addition of AIF3 into said bath and being characterized in that it comprises: the set-up of a regulation sequence comprising a series of time intervals of pre¬determined length Lp referred to as "periods"; the determination of the value of at least one indicator B referred to as the "ridge variation" capable of detecting the variation of said solidified bath ridge (15); the determination of a quantity Qo(p), referred to as the "basic term", corresponding to the net average AIF3 requirements of the cell; the determination of a corrective term Qi(p) comprising at least one term Qsol(p), referred to as the "ridge term", which is determined from at least one or each ridge variation indicator; the determination of a quantity Q(p) of AIF3 to be added during the period p, referred to as the "determined quantity Q(p)", by adding the corrective term Qi(p) to the basic term Qo(p), i.e. Q(p)=Qo(p)+Qi(p); the addition into said electrolyte bath, during the period p, of an effective quantity of AIF3 equal to said determined quantity Q(p). 17. The regulation method as claimed in claim 16, wherein said length Lp of said periods is roughly the same for all the periods. 18. The regulation method as claimed in claim 16 or 17, wherein said length Lp of said periods is between 1 and 100 hours. 19. The regulation method as claimed in any one of claims 16 to 18, wherein the term Qsol(p) comprises at least one term referred to as Qr(p) which is detenmined from at least one electrical measurement on said ceil (1) capable of detecting variations in the current lines induced by the variation of said ridge. 20. The regulation method as claimed in claim 19, wherein the term Qr(p) is determined from at least one measurement of said intensity I and at least one measurement of the drop in voltage U at the terminals of said cell (1). 21. The regulation method as claimed in claim 20, wherein it comprises: the determination of at least one first value H for said intensity I and at least one first value Ul for the drop in voltage U at the terminals of said cell (1); the calculation of a first resistance R1 from at least said values 11 and U1; the movement of the anode frame (10) by a determined distance AH, form an initial position, either upwards, AH being positive in this case, or downwards, AH being negative in this case; the determination of at least one second value 12 for said intensity I and at least one second value U2 for the drop in voltage U at the terminals of said cell (1); the calculation of a second resistance R2 from at least said values 12 and U2; the calculation of a resistance variation AR using the formula AR=R2-RI; the calculation of a quantity referred to as the "specific resistance variation" AR using the formula ARS= AR/AH; the determination of the term Qr(p) using a determined function of said specific resistance variation ARS; the determination of the corrective term Qi(p) comprising at least the term Qr(p) in the ridge term Qsol(p). 22. The regulation method as claimed in claim 21, wherein it also comprises, at least after the determination of the values of II, 12, Ul and U2, the movement of the anode frame (10) so as to return it to its initial position and restore the initial cell setting. 23. The regulation method as claimed in claim 21 or 22, wherein said first and second resistance are calculated using the formula R=(U-Uo)/I, where Uo is a constant. 24. The method as claimed in claim 23, wherein the constant Uo is between 1.6 and 2.0 V. 25. The regulation method as claimed in any one of claims 21 to 24, wherein the term Qr(p) is given by the ftinction Qr(p)=Kr x (ARS-.ARSo), where Kr is a constant and ARSo is a reference value. 26. The regulation method as claimed in claim 25, wherein Kr is between -0.01 and -10 kg/hour/n.fi/mm. 27. The regulation method as claimed in any one of claims 21 to 26, wherein the term Qr(p) is limited by a minimum value and by a maximum value. 28. The regulation method as claimed in any one of claims 16 to 27, wherein the term Qsol(p) comprises at least one term Qs(p) which is determined from at least one determination of the surface area S(p) of said liquid metal pad (12). 29. The regulation method as claimed in claim 28, wherein it comprises: the removal of a quantity of liquid metal from the electrolytic cell; the determination of the volume Vm of said quantity of liquid metal removed from the electrolytic cell; the determination of the change AHm of the resulting level of said liquid metal pad in said pot; the determination of a surface area S for said liquid metal pad (12) using the formula S=Vm/AHm; the determination of a term Qs(p) using a determined llmction of the surface area S(p) of said liquid metal pad (12); the determination of the corrective term Qi(p) comprising at least the term Qs(p) in the ridge term Qsol(p). 30. The regulation method as claimed in claim 29, wherein said volume Vm is determined by measuring the mass of said quantity of liquid metal removed from the electrolytic cell. 31. The regulation method as claimed in claim 29 or 30, wherein the term Qs(p) is determined from the so-called "metal surface area" difference between the value obtained for said surface area S and a set-point value So. 32. The regulation method as claimed in any one of claims 29 to 31, wherein the term Qs(p) is given by the function Qs(p)=Ks x (S(p)-So), where Ks is a constant. 33. The regulation method as claimed in claim 32, wherein Ks is between 0.0001 and 0.1 kg/hour/dm. . 34. The regulation method as claimed in any one of claims 29 to 33, wherein the term Qs(p) is limited by a minimum value and by a maximum value. 35. The regulation method as claimed in any one of claims 16 to 34, wherein it comprises: the determination of a mean Qm(p) of the total AIF3 additions per period during the lastN periods; the determination of a quantity Qint(p), advantageously using the following "smoothing" formula: Qint(p)=(l/D) x Qm(p)+(1-1/D) x Qint(p-l), where D is a smoothing parameter setting the temporal smoothing horizon; the determination of the basic term Qo(p) using the formula Qo(p)=Qint(p). 36. The regulation method as claimed in claim 35, wherein it comprises: the determination of a compensating term Qcl(p) corresponding to the so-called "equivalent" quantity of AIF3 contained in the alumina added to the cell during the period p; the modification of the term Qo(p) by subtracting the term Qcl(p) from said term Qo(p), i.e. using the formula Qo(p)=Qo(p)-Qcl(p). 37. The regulation method as claimed in claim 36, wherein the term Qm(p) is given by the equation: Qm(p)=+, where =(Q(p-N)+Q{p-N+l)+Q(p-N+2)+ .. . +Q(p-1))/N, =(Qcl(p-N)+Qcl(p-N+l)+Qcl(p-N+2)+ . . . +Qcl(p-1))/N, where N is a constant. 38. The regulation method as claimed in claim 37, wherein N is between 1 and 100. 39. The regulation method as claimed in any one of claims 35 to 38, wherein the parameter D is equal to Pc/Lp, where Pc is between 400 and 8000 hours. 40. The method as claimed in any one of claims 35 to 39, wherein it comprises: the determination of a quantity Qtheo corresponding to the total theoretical AIF3 requirements of the ceil when regulation is started; the start-up of the method by taking Qint(0)=Qtheo. 41. The regulation method as claimed in any one of claims 35 to 40, wherein it comprises: the determination of an additional corrective term Qc2(p) using a ftinction of the difference between Qm(p) and Qint(p); the addition of the term Qc2(p) in the determination of Qi{p). 42. The regulation method as claimed in claim 41, wherein the term Qc2(p) is given by the formula Qc2{p)=Kc2 x (Qm(p)-Qint(p)), where Kc2 is a constant. 43. The regulation method as claimed in claim 42, whereinKc2 is between -0.1 and -1. 44. The regulation method as claimed in any of claims 41 to 43, wherein the term Qc2(p) is limited by a minimum value and by a maximum value. 45. The regulation method as claimed in any one of claims 16 to 44, wherein it comprises: the determination of a mean temperature T(p) of the electrolyte bath; the determination of an additional corrective term Qt(p) using a determined function of the difference between said temperature T{p) and a set-point temperature To; the addition of the corrective term Qt(p) in the determination of Qi(p). 46. The regulation method as claimed in claim 45, wherein the term Qt(p) is given by the formula Qt(p)=Kt. x (T(p)-To), where Kt is a constant. 47. The regulation method as claimed in claim 46, wherein Kt is between 0.01 and 1 kg/hourA'C. 48. The regulation method as claimed in any one of claims 45 to 47, wherein the term Qt(p) is limited by a minimum value and by a maximum value. 49. The regulation method as claimed in any one of claims 16 to 48, wherein it comprises: the measurement of the excess AIF3 E(p); the determination of an additional corrective term Qe(p) using a function of the difference between the excess AIF3 measured E(p) and its target value Eo; the addition of the corrective term Qe(p) in the determination of Qi(p). 50. The regulation method as claimed in claim 49, wherein the term Qe(p) is given by the formula Qe(p)=Ke x (E(p)-Eo), where Ke is a constant. 51. The regulation method as claimed in claim 50, wherein Ke is between -0.05 and -5 kg/hour/% AIF3. 52. The regulation method as claimed in any one of claims 49 to 51, wherein the term Qe(p) is limited by a minimum value and by a maximum value. 53. The regulation method as claimed in any one of claims 16 to 52, wherein the quantity Q(p) comprises an additional term Qea(p) which is given by a function of the anode effect energy AEE. 54. The regulation method as claimed in claim 53, wherein the term Qea(p) is limited by a minimum value and by a maximum value. 55. The regulation method as claimed in any one of claims 16 to 54, wherein the quantity Q(p) is limited to a maximum quantity Qmax. 56. The regulation method as claimed in any of claims 16 to 55, wherein, when the detennined value of the term Q(p) is negative, its value is taken as equal to zero, i.e. no AIF3 is added during the period p.

Full Text

Field of the invention
The invention relates to a regulation method for an aluminium production cell by means of electrolysis of alumina dissolved in an electrolyte based on molten cryolite, particularly according to the Hall-Heroult method.
State of the art
Metal aluminium is produced industrially by igneous electrolysis, i.e. by means of electrolysis of alumina in solution in a molten cryolite bath, referred to as an electrolyte bath, particularly according to the well-known Hall-Heroult method. The electrolyte bath is contained in pots, referred to as "electrolytic pots", comprising a steel shell, which is lined internally with refractory and/or insulating n The electrolytic current, which flows in the electrolyte bath and the pad of liquid aluntiniuTL via the anodes and cathode components, brings about the aluminium reduction reactions and also makes it possible to maintain the electrolyte bath ai: a temperature of the order of 950 "C by means of the Joule effect. The electrolytic ceil is regularly supplied with alumina so as to compensate for the alumina consumption.produced by the electrolytic reactions.

The productivity and current efficiency of an electrolytic cell are influenced by several factors, such as the intensity and distribution of the electrolytic current, the pot temperature, the dissolved alumina content and the acidity of the electrolyte bath, etc., which interact with each other. For example, the melting temperature of a cryolite bath decreases with the excess aluminium trifluoride (AIF3) with reference to the nominal composition (3 NaF.AlFs) . In modern plants, the operating parameters are adjusted to aim for current efficiencies of over 90%.
However, the effective current efficiency of a cell is significantly influenced by variations in said cell's parameters. For example, an increase in the electrolyte temperature by around ten degrees Celsius may cause the current efficiency to fall by approximately 2% and a decrease in the electrolyte temperature by around ten degrees Celsius may reduce the already low solubility of alumina in the electrolyte and favour the "anode effect", i.e. anode polarisation, with a sudden rise in the voltage at the cell tennir.sls and the release of a large quantity of» fluorinated and fluoro-carbonated products, and/or insplsting deposits on the cathode surface.
Therefore, the operation of an electrolytic cell requires precise control of its operating parameters, such as its temperature, alumina content, acidity, etc., so as to maintain them at determined set-point values. Several regulation methods have been developed to achieve this objective. These methods generally relate to the regulation of the alumina content of the electrolyte bath, the regulation of its temperature, or the regulation of its acidity, i.e. the excess AIF3.

statement of the problem
Aluminium producers, in the continuous aim to increase electrolytic plant production and productivity at the same time, try to push back these limits.
In particular, in order to increase plant productivity, it is aimed to reach current efficiencies above 95% operating with AIF3 excesses of over 11%, and which may reach 13 to 1^%, which makes it possible to decrease the cell operating temperature (the liquidus temperature drops approximately 5'C/IAir3) and, as a result, reduce the energy consumption of said cells. However, in this chemical composition range, the solubility of alumina is considerably reduced, which increases the risks of anode effects and forming of insulstinc deposits on the cathode.
In addition, in order to increase plant production, it is airbed to increase the unit capacity of cells and, in correlation, increase the intensity of the electrolytic current. The current trend is to develop electrolytic cells with a current greacer thsn or equal to 5GO k,*-. The increase in the capacity cf electrolytic cells may be obtained, as a general rule, either by increasing the permissible intensity of ceils of known type cr existing cells, or by developing very large cells. In the first case, the increase in the permissible intensity results in a decrease in the electrolyte 'OBth mass, which exacerbates the instability effect. In the second case, the increase in the cell size increases their thermal and chemical inertia. Consequently, the increase in cell capacity not only increases the rate of alumina consumption but also amplifies instability generation and cell

deviation phenomena, which increases difficulties in controlling electrolytic cells.
Therefore, the applicant searched for a regulation method for an electrolytic cell, particularly of the electrolyte bath acidity [i.e. its AlFj content) and the overall thermics of the cell, which makes it possible to control, in a stable manner with a current efficiency greater than 93S, or even greater than 95%, without having to use frequent AIF3 content measurements, electrolytic cells wherein the excess AIF3 is greater than 11% and wherein the current may be greater than or equal to 500 kA,
Descriptiori of the invention
The invention relates to a regulation method for an electrolytic cell intended for the production of aluminium by means of igneous, electrolysis, i.e. by flowing current in an electrolyte bath based on molten cryolite and containing dissolved alumina, particularly according to the Hall-Heroult method.
The regulation method according to the invention comprises the addition of alumina in the electrolyte bath of an electrolytic cell, and is characterised in that it comprises the determination of a quantity B, referred to as the "ridge variation indicator", which is sensitive to variations of the solidified bath ridge formed on the side walls of the pot, and the modification of at least one of the setting means of the pot and/or at least one control operation as a function of the value obtained for said indicator.
The applicant noted that, surprisingly, accounting for the variation in the solidified bath mass in the [regulation of an electrolytic pot made it possible to

reduce the amplitude and dispersion of the fluctuations of the pot operating parameters, such as its acidity.
According to one embodiment of the invention, said indicator is determined from an electrical measurement on the electrolytic ceil which is capable of detecting variations in the current lines induced by the variation of the ridge. In a preferred embodiment of the invention, said indicator is determined from a quantity referred to as the "specific resistance variation" ARS which is determined from the resistance R of the electrolytic ceil.
According to another embodiment of the invention, said indicator is determined from a determination of the Surface area of the liquid metal pad, which is capable of detecting variations in the surface area of the liquid metal induced by the variation of the ridge.
According to another embodi.~ent of the invention, said indicator is determined from a combination of electrical meesureme.its and raeasurerfients of the metal surface area.
The invention may be implemented advantageously in
electrolyte bath acidity regulation. In particular, the
regulation method according to the invention may
:omprise the addition, in the electrolyte bath of an
slectrolytic ceil, during pre-determined time intervals
. referred to as "regulation periods", of a quantity
!(p) of aluminium trifluoride (AIF3) determined by the
um of at least one basic term Qo{p) corresponding to
he net average AIF3 requirements of the cell, and of a
orrective term Qi(p) including at least one term
sol(p), referred to as the "ridge term", which is
etermined from at least one ridge variation indicator.

Therefore, the quantity Q{p) is determined using the formula: Q(p) = Qo(p) + Qi(p) = Qo(p! + QsoL(p) + ...
The applicant noted that the ridge term Qsol(p) makes it possible to reduce the number of analyses of the AlFj content of the liquid electrolyte bath significantly; these measurements add to cell operating costs and are, in any case, usually affected by significant errors.
Said modifications of at least one cell setting means and/or at least one control operation may advantageously be combined.
Figures
Figure 1 represents, in a transverse section, a typical electrolytic cell.
Figure 2 illustrates the principle of the regulation sequences according to the invention.
Figures 3 and 4 show typical functions used to determine the terms of 0(p).
Figure 5 illustrates s method to determine the specific electric resistance variation of the 2lectrolytic cell.
Figure 6 is a schematic illustration of the shape >f the current lines flowing in the electrolyte bath between an anode and the liquid metal pad.
Figure 7 illustrates a method to determine the ;urface area of the liquid metal pad.
Figure 8 shows the variations in total AIF3 equireraents of an electrolytic cell.
As illustrated in figure 1, an electrolytic cell 1 or the production of aluminium by means of the Hall-6rouit electrolysis method typically comprises a pot 0, anodes 7 supported by attachment means 8, 9 to an

anode frame 10 and alumina supply means 11. The pot 20 comprises a steel shell, internal lining components 3, A and a cathode assembly 5, 6. The internal lining components 3, 4 are generally blocks made of refractory materials, which may be heat insulators. The cathode assembly 5, 6 comprises connection bars 6 to which the electric conductors used to route the electrolytic current are attached.
The lining components 3, A and the cathode assembly 5, 6 form, inside the pot 20, a crucible capable of containing the electrolyte bath 13 and a liguid metal pad 12 when the cell is in operation, during which the anodes 7 are partially immersed in the electrolyte bath 13. The electrolyte bath contains dissolved alumina and, as a general rule, an alumina cover 14 covers the electrolyte bath.
The electrolytic current transits in the electrolyte bach 13 via the anode frame 10, the attachment means 8, 9, anodes 7 and cathode components S, 6. The purpose of the alumina supply to the cell is to compensate for ■ the approximately continuous consumption of the cell which is esseriticlly due to the reduction of alumina into metal Eiurainium, The alumina supply, which is made by adding alunina into the liquid bath 13 is generally regulated separately.
The metal aluminium 12 which is produced during the electrolysis is accumulated at the bottom of the cell and a relatively clear interface between the liquid metal 12 and the molten cryolite bath 13 is established. The position of this bath-metal interface varies over time: it rises as the liquid metal accumulates at the bottom of the cell and it goes down when the liquid metal is remo-ved from the cell.

Several electrolytic cells are generally arranged in a row, in buildings referred to as electrolysis rooms, and connected electrically in series using connection conductors. The cells are typically arranged so as to form two or more parallel lines. The electrolytic current thus flows in cascade from one cell to the next.
Detailed description of the invention
According to the invention, the regulation method for an electrolytic cell 1 for the production of aluminium by means of electrolytic reduction of alumina dissolved in an electrolyte bath 13 based on cryolite, said cell 1 comprising a pot 20, at least one anode 7,■ at least one cathode component 5, 6, said pot 20 comprising internal side walls 3 and being capable of containing a liquid electrolyte bath 13, said cell 1 comprising at least one setting means of said cell including a mobile anode frame 10 to which said at least one anode 7 is attached, said cell 1 being capable of circulating a so-called electrolytic current in said bath, said current having an intensity I, the aluminium produced by means of said reduction forming a pad referred to as a "liquid metal pad" 12 on said cathode component (s) 5, 6, said cell 1 comprising a solidified bath ridge 16 on said wails 3, comprises control operations of said cell including the addition of alumina and the addition of hlFj in said bath and is characterised in that it comprises:
- the determination of the value of at least one indicator B referred to as the "ridge variation", capable of detecting the variation of said solidified bath ridge 15;

- the adjustment of at least one setting means and/or at least one control operation according to the value obtained for the or each ridge variation indicator.
Variations in the solidified bath ridge are generally conveyed by variations in the thickness and, to a lesser degree, the shape of said ridge.
Said adjustment of at least one setting means of the cell typically comprises at least one modification of the position of said mobile anode frame 10, either upwards, or downwards, so as to modify the anode/metal distance (AMD).
Said at least one control operation typically comprises the addition of a quantity Q of AIF3 into said electrolyte bath 13. Said adjustment may then con-.prise at least one modification of said quantity Q as a function of the value obtained for one or each ridge variation indicator.
In a preferred embodiment of the invention, the regulation method' is characterised in that said ec least one ridge variation indicator includes an indicator, referred to as "BE", which is determined fron at least one electrical measurement on said cell 1 capable of detecting the variations of the current lines induced by the variation of said ridge. Preferentially, said indicator BE is determined from at least one determination of said intensity I and at least one determination of the drop in voltage U at the terminals of said cell 1.
In an alternative version of this embodiment, said at least one ridge variation indicator BE is equal to a specific resistance variation ARS which may be determined using a measurement method comprising;

- the determination of at least one first value II foe said intensity I and at least one first value Ul for the drop in voltage U at the terminals of said cell 1;
- the calculation of a first resistance Rl from at least said values II and Ul;
- the movement of the anode frame 10 by a
determined distance AH, from a so-called initial
position, either upwards (AH being positive in this
case), or downwards (AH being negative in this case);
- the determination of at least one second value
12 for said intensity 1 and at least one second value
U2 for the drop in voltage U at the terminals of said
cell 1;
■;- the calculation of a second resistance R2 from at least said values 12 and U2;
- the calculation of a resistance variation AR
using the formula AR = R2-R1;'
- the calculation of said specific resistance ARS
usinc -he formula ABS = AR/AH.
Preferentially, the measurement method also cor.prises (at least after the determination of the values of II, 12, Ul and U2) , the movement of the anode frame 10 so as to return it to its initial position and restore the initial cell setting.
Said first and second resistance Rl and R2 may be calcul&ted using the formula R = (U-Uo)/I, where Uo is a constant typically between 1.6 and 2.0 V. For Example, Rl and R2 may be given by Rl = (Ul-Uo)/il and R2 = {U2-Uo)/I2. According to an alternative embodiment Df the invention, Bl and R2 may be given by a mean

value obtained from a determined number of values of the voltage U and intensity I.
In practice, it was found to be simpler to give an order of movement of the anode frame 10 for a determined time and measure the resulting frame displacement AH.
According to this embodiment of the invention, the regulation method advantageously comprises:
the determination of a specific resistance variation ARS using the formula: ARS = A9./AH;
- the adjustment of at least one control means and/or at least one control operation using a determined function of said specific resistance variation ARS.
Said adjustment may be a determined function of the difference between said specific resistance variation ARS and a reference value Ai^So, i.e. ARS-ABSo.
As shown in figure 5, said resistance is typically measured using means 18 to measure the intensity I of the current circulating in the cell (where I is equal to the sum of the cathode currents Ic or anode intensity la) and means 15,17 to measure the resulting drop in voltage U at the cell terminals (typically the resulting drop in voltage between the anode frame and the cathode components of the cell). Said resistance R is generally calculated using the equation: R = (U -Uo)/I, wheire Oo is a ctmstant.
The resistance R depends not only on the
resistivity p of the electrolyte bath 13, on the distance H between the anode (s) 7 and the liquid metal pad 12, and on the surface area Sa of the anode (s) 7,

but also on the spreading r) of the lines of current Jc, Js which are established in said bath, particularly between the anode(s) 7 and the solidified bath ridge 15 [lines Jc in figure 6) , The applicant had the idea to make use of the fact that the specific electric resistance variation ARS is not only sensitive to the resistivity of the electrolyte bath, but integrates an electric current spreading factor, which is sensitive to the presence, size and, to a lesser degree, shape of the solidified bath ridge 15 on the walls of the pot.
The applicant also observed that, unlike that which is normally admitted, the spreading T^ is in fact a preponderant factor in the establishment of electfic resistance. The applicant considers that the contribution of spreading Co the specific electric resistance variation is typically between 75 and 90%, which means that the contribution of the resistivity is very low, or typically between 10 end 25% {that is typically 15%). In its tests on 500 '■. pots, the applicant observed a mean ARS value of the order of 100 icfi/mm, which decreases by approximstely -3 rli/csr. when the bath temperature increases by 5'C and when the AIFB content decreases by 1%, and conversely. The contribution of the resistivity to this variation is estimated to be only of the order of -C.5 nQ/mm (that is only approximately 15% of the total value) , the contribution attributable to spreading, i.e. -2.5 nfi/mm being dominant.
It is possible to take into account the spreading of the current in the resistance measured (for example by modelling the current lines), which improves the reliability of the specific resistance variation

considerably as an indication of the variation of the ridge BE (itself an indicator of the thermal state of the cell).
In another preferred embodiment of the invention, the regulation method is characterised in that said at least one ridge variation indicator includes an indicator, referred to as "BM", which is determined from a determination of the surface area S of said liquid metal pad 12.
According to this embodiment of the invention, the regulation method advantageously comprises:
- the determination of a surface area S for the liquid metal pad 12;
- the adjustment of at least one control means and/or at least one control operation using a determined function of the surface area S.
Said adjustment may be a determined function of the so-called "metal surface area" difference between the value obtained for said surface area S and a set-point value So (i.e. S-So).
The surface area S, which corresponds approxirxat-sly to the irietal/bath interface, is aoproximately equal to the horizontal right section of the electrolytic pot. The presence of solidified electrolyte bath on the walls of the pot decreases this surface area by a quantity which varies as a function of time and pot operating conditions.
In the preferred embodiment of this alternative embodiment of the invention, the surface area S is calculated from a measurement of the volume Vm of metal tapped and the corresponding fall AHm of the metal level Hm (see figure 1). More specifically, said metal

surface area may be deterrnined using a measurement method comprising:
- the removal of a quantity of liquid metal from
the electrolytic cell;
- the determinatiorv of the volume Vm of said quantity of liquid metal removed from the electrolytic cell;
- the determination of the change AHm of the resulting level of said liquid metal pad in said pot;
- the determination of a surface area S for said
liquid metal pad 12 using the formula S = Vm/AHm.
Said volume Vm may be determined by measuring the mass of said quantity of liquid metal removed from the electrolytic cell.
In practice, the anodes 7 are normally lowered at the same time as the level of liquid metal so as to keep the anode/metal distance (AMD) constant.
Said at least one control operation may also comprise at least one addition of solid or liquid electrolyte bath so as to increase the level of said liquid electrolyte bath 13 in said pot 20.
Said adjustments of at least one setting means of the cell and/or at least one control operation may advantageously be combined.
Implementation of the invention in bath acidity regulation
According to an embodiment of the invention, the regulation method for an electrolytic cell 1 for the production of aluminium by means of electrolytic reduction of alumina dissolved in an electrolyte bath L3 based on cryolite, said cell 1 comprising a pot 20, it least one anode 7, at least one cathode component 5,

6, said pot 20 comprising internal side walls 3 and being capable of containing a liquid electrolyte batti 13, said cell 1 also comprising at least one setting means of said cell inciuding a mobile anode frame 10 to which said at least one anode 7 is attached, said cell 1 being capable of circulating a so-called electrolytic current in said bath, said current having an intensity r, the aluminium produced by said reduction forming a pad referred to as the "liquid metal pad" 12 on the cathode component (s) 5, 6, said cell 1 comprising a solidified bath ridge 15 on said walls 3, comprises control operations of said cell including the addition of alumina and the addition of AlPa into said bath and is characterised in that it comprises:
- the set-up of a regulation sequence comprising a series of time intervals p of pre-determined length Lp hereafter referred to as "regulation periods" or simply "periods";
- the determination of the value of at least one indicator B referred to as the "ridge variation" capable of detecting the variation of said solidified bath ridge 15;
- the determination of a quantity .Qo (p), referred to as the "basic term", corresponding to the net average AIF3 requirements of the cell;
- the determination of a corrective term Qi (p)
including at least one term Qsol(p), referred to as the
"ridge term", which is determined froin at least one or
each ridge variation indicator 15;
- the determination of a quantity Q(p) of AIF3 to
be added during the period p, referred to as the
"determined quantity Q(p)", by adding the corrective

term Qi (p) to the basic term Qo (p) , i.e. Q(p) = Qo(p) + Qi(p);
~ the addition into said electrolyte bath, during Che period p, of an effective quantity of aluminium trifluoride {RIF3) equal to said determined quantity Q(p} .
The intervals (or "periods") p are preferentially approximately equal in length Lp, i.e. the length Lp of the periods is approximately the same for all the periods, enabling easier implementation of the invention. Said length Lp is generally between 1 and 100 hours.
The term Qsol(p) is a function of variations in the mass of the solidified bath ridge 15 formed on said walls 3; said variations are generally conveyed by variations in the thickness (and, to a lesser degree, the shape) of said ridge.
In an advantageous alternative version of said embodiment of the invention, the term Qsol(p) includes at least one terir. referred to as Qr{p) which may be determined from at least one electrical measurement on the cell 1 capable of detecting variations in the current lines induced by the variation of said ridge. The term Qr(p} is advantageously determined from at least one measurement of said intensity I and at least one measurement of the drop in voltage U at the terminals of said cell 1.
In the preferred embodiment of this alternative version of the invention, the method comprises:
- the determination of at least one first value II for said intensity I and at least one first value Ul for the drop in voltage U at the terminals of said cell 1;

- the calculation of a first resistance Rl from at■
least said values II and Ul;
the movement of the anode frame 10 by a determined distance AH, from a so-called initial position, either upwards (AH being positive in this case), or downwards (AH being negative in this case);
- the determination of at least one second value 12 for said intensity I and at least one second value U2 for the drop in voltage (J at the terminals of said cell 1;
- the calculation of a second resistance R2 from at least said values 12 and U2;
- the calculation of a resistance variation AR using the formula AR = R2-R1;
- the calculation of said specific resistance AES using the formula ARS = AS/AH;
- the determination of a term Qr(p) using a
determined function of said specific resistance
variation ARS;
- the determination of the corrective term Qi Cp)
including at least the term Qr(p) in the ridge term
QsoKp).
Preferentially, the measurement method also comprises (at least after the determination of the values of II, 12, Ul and 02), the movement of the anode frame 10 so as to return it to its initial position and restore the initial cell setting.
Said first and second resistance Rl and R2 may be calculated using the formula R = (U-Uo)/I, where Uo is a constant typically between 1.6 and 2.0 V. For example, Rl and R2 may be given by Rl = (Ul-Uo)/II and R2 = (U2-Uo)/l2. According to an alternative embodiment

of the invention, Rl and R2 may be given by a mean value obtained from a determined number of values of the voltage U and intensity I.
Said determined function, which is typically decreasing, is preferentially limited. It is advantageously a function of the difference between ARS and a reference value ARSo. Figure 3 shows a typical function used to determine the term Qr.
In a simplified alternative embodiment of the invention, the tern Qr {p) may be given by a simple equation such as: Or (p) = Kr x (ARS - ARSo), where Kr is a constant which may be set empirically and whose value is typically between -0.01 and
-10 kg/hour/nli/mm, and more typically between -0.05 and -0.3 kg/hour/nn/min (corresponding, in the latter case, to approximately -0.5 to -2 kg/period/nC3/mm for an 8-hour period) for 300 kA to 500 kA pots.
The term Qr(p) is preferentially limited by a minimum value and by a maj;i.~um value. These minimum and maximum values may be negative, null or positive.
In practice, it is possible to make Nr measurements of ARS (i.e. two or more measurements)
during the period p- The ARS value used to calculate Qr(p) will in this case be the mean of the Nr measured ARS values, except, if applicable, values considered to be aberrant. It is also possible to use a sliding mean on two or more periods to smooth the thermal fluctuations related to the operating cycle. An operating cycle is determined by the frequency of interventions on the electrolytic cell, particularly anode' replacements and liquid metal- sampling. The

length of an operating cycle is generally between 24 and 48 hours {for example 4 x 8-hour periods).
tn another advantageous alternative embodiment of the method according to the invention, the term Qsolfp) includes at least one term referred to as Qs(p), which may be. determined from at least one determination of the surface area S(p) of said liquid metal pad 12. The term Qs(p) is advantageously determined from the so-called "metal surface area" difference between the value obtained for said surface area S(p} and a set-point value So.
According to the preferred embodiment of this alternative version, the method comprises:
~ the removal of a quantity of liquid metal from the electrolytic cell;
- the determination of the volume Vm of said quantity of liquid metal removed from the electrolytic cell;
- the determination of the change AHm of the resulting level of said liquid metal pad in said pot;
- the determination of a surface area S {p} for
said liquid metal pad 12 using the formula S = Vm/AHm;
- the determination of a term Qs(p) using a determined function of the surface area S(p} of said liquid metal pad 12;
- the determination of the corrective term Qi (p) including at least the term Qs{p) in the ridge ter-m Qsol(p).
Said volume Vm may be determined by measuring the mass of said quantity of liquid metal removed from the electrolytic cell.
Said determined function, which., is typically increasing, is preferentially limited. It is

advantageously a function of the difference between the surface area S(p) of the liquid metal pad 12 and a set-point value So. Figure 4 shows a typical function used to determine the term Qs.
In a simplified alternative embodiment of the invention, the term Qs (p) may be given by a simple equation such as: Qs(p) = Ks x (S(p) - So), where Ks is a constant which may be set empirically and whose value is typically between 0.0001 and 0.1 kg/hour/dm=, and more typically between 0.001 and 0.01 kg/hour/dm* (corresponding, in the latter case, to approximately 0.01 to 0.05 kg/period/dm' for an 8-hour period) for 300 kA to 500 kA pots.
The term Qs(p) is preferentially limited by a minimum value and by a maximum value. These jninimuni and maximum values may be negative, null or positive.
The applicant noted that the corrective terms QrCp) snd Qs(p) according to the present application are effective indicators of the overall thermal state of the electrolytic cell, which take into account both the liquid electrolyte bath and the solidified bath ridge on the walls of the pot. These terms, taken separately or in combination, particularly make it possible to reduce the number of analyses of the AIF3 content in the liquid electrolyte bath markedly. The applicant observed that the frequency of the analyses of the AIF3 content may be reduced typically to one analysis per cell approximately every 30 days. The' terms Qr{p) and Qs(p), which may be combined, make it possible to only perform AiFj content analyses in exceptional cases or in order to characterise a cell or a series of cells statistically. The terms Qr(p) and

Qs(p) also enable long-term thermal regulation of the ridge thickness.
In a preferred alternative embodiment of the invention, the basic term Qo(p) is determined using a so-called "integral" (or "self-adaptive") term Qint(p), which represents the total actual AIF3 requirements of the pot. The term Qint (p) is calculated from a mean Qm(p) of the actual AlFj supplies made during the last N periods. The term Qint(p) takes into account AIF3 losses in the bath occurring during normal ceil operation and which are essentially produced by absorption by the pot crucible and emissions in gaseous effluents. This term, the mean value of which is not null, is particularly used to monitor pot ageing, without having to model it, by means of a memory effect of pot behaviour over time. It also takes into account the specific ageing of each pot, that the applicant generally found to be markedly different to the average ageing of the population of pots of the same type. In this case, the method also comprises:
- the determination of a mean Qm{p) of the total
AIF3 additions per period during the last H periods;
the determination of a quantity Qint(p), advantageously using the following "smoothing" formula: Qint(p) = (1/D) X Qm(p) + (1 ~ 1/D) y. Qint(p-ll, where D is a smoothing parameter setting the temporal smoothing horizon;
- the determination of the basic term Qo (p) using
the formula Qo(p) = Qint{p).
The horizon term D, which makes it possible to do away with medium and long-term thermal and chemical fluctuations, is equal to Pc/Lp, where Pc is a period which is typically of the order of AOO to 8000 hours,

and more typically from 600 to 4500 hours, and Lp is the length of a period. Therefore, the term D is typically equal to 50 to 1000 8-hour periods if this work organisation method is applied.
The term Qo(p) may be corrected so as to take into account the impact of alumina additions on the effective composition of the electrolyte bath. For this purpose, the method according to the invention may also comprise:
- the determination of a compensating term Qcl(p) corresponding to the so-called "equivalent" quantity of AIF3 contained in the alumina added to the cell during the period p;
the modification of the term Qo(p) by subtracting the term Qcl(p) from said term Qo{p), i.e. using the formula Qo(p) = Qo(p) - Qcl(p).
The term Qcl(p) corresponds to the so-called "equivalent" quantity of AlFs added to the cell by means of the alumina added to the electrolytic cell during the period p, where said quantity may be positive or negative. This term is determined by producing the chemical balance of the fluorine and sodium contained in said alirr.ina from one or more chemical analyses. The effect of the sodium contained in the alumina is to neutralise fluorine, thus being equivalent to a negative quantity of AIF3. The term Qlc(p) is positive if said alumina is "fluorinated" (which is the case when it has been used to filter electrolytic cell effluents) and negative if the alumina is "fresh", i.e. if it is produced directly from the Bayer process.

In a preferred alternative embodiment of the invention, the term Qm(p) is calculated using the equation:
Qmlp) = + , where
= {Q(p-N) + Q(p-N+1) + Q(p-N+2) +...+ Q(p-1})/N,
= (Qcl(p-N) + Qcl(p-N+1) + Qcl(p-N+2) +...+ Qcl(p~l))/N, where N is a constant.
The term Qm(p) is then equal to Q(p-l) + Qcl(p-1) when N = I: lQip-2) + Qcl(p'2) + Q(p-1> + Ocl(p-l))/2 when N = 2; (Q{p~3) + Qcl {p-3) + Q(p-2) + Qcl(p-2) + O(p-I) + Qcllp-l})/3 when N = 3, ...
The value of the parameter N is selected according to the cell reaction time and is normally between 1 and 100, and more typically between 1 and 20.
The term Om(p) chen takes into account total equivalent AlFj supplies, i.e. "direct" supplies from additions of AlFj and "indirect" supplies fro-; additions of alumina.
In another advantageous alternative embodi.T,ent of the invention, the determination of Qi(p} comprises an additional so-call "damping" corrective term Qc21'p), which takes into account the delay in the reaction of the cell with the AlFj additions. The term Qc3 is a prospective correction term which is used to take into account the effect of an addition of AIF3 in advance, which normally only appears after a few days. Indeed, the applicant noted the surprising degree of the difference between the time constant of the temperature variation, which is low (of the order of a few hours) and that of the AIF3 content, which is very high (of , the order of a few tens of hours} . In its tests, it found that it was very advantageous to anticipate the

variation of the acidity of the bath of the cell when adding AIF3, which is made possible effectively by the term Qc2.
This alternative embodiment may be implemented by including in the method according to the invention:
- the determination of an additional corrective term Qc2(p) using a typically decreasing, preferentially limited, function of the difference between Qm(p) and Qintfp), i.e. Om(p) - Qlnt {p);
- the addition of the corrective term Qc2(p) in the determination of Qi{p).
In a simplified alternative embodiment of the invention, the term Dc2(p) may follow a simple equation, such as Qc2(p) = Kc2 x CQm(p) - Qint (p) ) , where Kc2 is a constant which is typically negative and which may be set empirically and whose value is typically between -0.1 and -1, and more typically between -0.5 and -1 for 300 kA to 500 kA pots.
fc^-^ term Qc2(p) is preferentially limited by a minimum value and by a maximum value. These minimum and maximum values may be negative, null or positive.
In order to converge the integral term Qint{p) rapidly to the quantity Q' corresponding to actual cell requirements, it is possible to start the method by simoly taking Qint(O) = Qtheo, where Qtheo corresponds to the total theoretical AlFj requirements of the cell when regulation is started. The AIF3 requirements of an electrolytic cell are essentially due to losses through absorption in the walls of the pot and emission of fluorinated products. Qtheo is a function of the age of the pot which can be determined statistically for each type of cell.

This alternative embodiment may bs implemented by including in the method according to the invention:
the determination of a quantity Qtheo corresponding to the total theoretical AIF3 requirements of the cell when regulation is started;
- the start-up of the method by taking Qint{0) = Qtheo.
Figure 8 illustrates, using typical values, the term Qtheo(p) and the operating principle of the integral term Qint(p).
In another advantageous alternative embodiment of the invention, the determination of Qi{p) includes an additional corrective term Qt(p) which is a function of the bath temperature measured of the electrolyte bath. The term Qt(p) also makes it possible to avoid having to use regular bath AIF3 content measurements.
This alternative embodiment may be implemented by including in the method according to the invention:
- the determination of a mean temperature T (p] of
the electrolyte bath;
- the determination of an additional corrective
term Qt(p) using a determined function, which is
typically increasing and preferentially limited [i.e.
it is limited by a maximum value and by a minimum
value), of the difference between said temperature T(p)
and a set-point temperature To;
- the addition of the corrective term Qt (p) in the
determination of Qi(p).
In a simplified alternative embodiment of the invention, the term Qt(p) may follow a simple equation, such as Ot{p) = Kt X (T(p) ■- To), where Kt is a constant which is typically positive and which may be set empirically and whose value is typically between

0.01 and 1 kg/hour/°C, and more typically between 0.1 and 0.3 kg/hour/'C (corresponding, in the latter case, to approximately 1 to 2 kg/period/"C for an 8-hour period) for 300 kA to 500 kA pots.
The term Qt(p) is preferentially limited by a minimum value and by a maximum value. These minimum and maximum values may be negative, null or positive.
The mean temperature T(p) is normally determined from temperature measurements made on the period p and on the previous periods p-1, etc., so as to obtain a reliable and significant value of the average condition of the pot.
The terms Qt(p) and Qc2(p) are regulation terms wherein the mean value over time normally tends towards zero fi.e. they are normally null on average).
In another advantageous alternative embodiment of rr.e invention, the quantity Qi (p) comprises an additional corrective term Qe{p) which is a function of the difference between the excess AlFj measured E(p) and its target value Eo.
This alternative embodiment nay be implemented by including in the method according to the invention:
- the measurement of the excess AlFa E(p);
- the determination of an additional corrective
terTr" Qeip) using a determined function (typically
decreasing and preferentially limited} of the
difference between the excess AIF3 measured E(p) and
its target value Eo, i.e. the difference E(p) - Eo;
the addition of the term Qe(p} in the determination of Qi(p}.
In a simplified alternative embodiment of the invention, the term Qe (p) may be given by a simple equation such as: Qe(p) = Ke x (E(p) - Eo), where Ke is

a constant which may be set empirically and whose value is typically between -0.05 and -5 kg/hour/%AlF3, and more typically between -0.5 and -3 kg/hour/%AlF3 (corresponding, in the latter case, to approximately -20 to -5 kg/period/%AlF3 for an 8-hour period) for 300 kA to 500 kA pots.
The term Qe(p) is preferentially limited by a minimum value and by a maximum value. These minimum and maximum values may be negative, null or positive.
The applicant found it was satisfactory to only apply the term Qe(p) exceptionally, for a short length of time, when the thermal operation of the cell leaves the normal operating range, i.e. when the temperature values and values of the regulation terms (Qr, Q$, etc.) leave the so-called safety ranges.
The applicant noted in its tests that the corrective term Qe enabled the indicators (temperature, Qr, Qs, etc.) to return rapidly to che normal operating range.
According to another alternative embodiment of the invention, it is also possible to add corrective terms to take into account individual interfering events.
In particular, the corrective term Qi(p) may comprise a so-called anode effect term Qea to take into account the impact of an anode effect on the thermics of an electrolytic cell. An anode effect particularly induces significant AIF3 losses by emission and, generally, heating of the electrolyte bath. The term Qea is applied for a limited time following the observation of an anode effect. The terra Qea is calculated using either a scale which is a function of the anode effect energy (AEE), or a fixed mean value. In the first case, the terra Qea is given by a typically

increasing and preferentially limited function of the energy AEE.
The term Qea(p) is preferentially limited by a minimum value and by a maximum value. These minimum and maximum values may be negative, null or positive.
The term Q(p) corresponds to an addition of pure AIF3 and is typically expressed in kg of pure AIFB per period (kg/period). The expression "addition of an effective quantity of RlFj" corresponds to an addition of pure AiFa- In industrial practice, AIF3 additions are generally made using so-called industrial AlFs with a purity of less than 100% (typically 90%). In this case, a sufficient quantity of industrial AIF3 is added to obtain the effective quantity of AIF3 required. Typically, a quantity of industrial AIF3 equal to the effective quantity of AIF3 required divided by the purity of tihe .industrial AlFj used is adtied.
The expression "total AlFa additions" refers to the sum of the effective additions of pure P.lTj and the "equivalent" AlFa additions from alumina.
AIF3 may be added in different ways. It may be added manually or mechanically (preferentially using a a point feed, such as an crustbreaker-feeder which makes it possible to add determined doses of AIF3, in an automated fashion if required) . AIF3 laay be added with alumina or at the same time as alumina.
Industrial bath and pure cryolite additions are sometimes performed on industrial cells. These additions have an impact on the composition of the electrolyte bath which must generally be taken into account in the regulation. For this purpose, the regulation method may also comprise a corrective term

Qb to take into account the modification of the pure AIF3 content induced by these additions.
The different terms of Q(p) are determined preferentially at each period p. If the cell is very stable, it may be sufficient to determine the quantity Q(p) and some of the terms forming it, in a more staggered manner over time, for example once every two or three periods. The applicant observed that it was sufficient to only apply some of the terms of Q (p) , such as Qe[p), exceptionally and for a limited length of time, which makes it possible to limit costs relating to their determination.
In order to prevent excess AIF3 additions, it is preferable, as a precaution, to limit Q(p} to a maximum value Qmax. It is also preferable to limit the application of the regulation terras in time when they cannot be determined at each period.
The quantity Q{p) is normally determined at each period. If one or more terms of Q(p) cannot be calculated during a given period, then it is possible to^maintaih the value of said term(s) used during the previous period, i.e. the value of said term{s) will be determined by making it equal to the value used during the previous period. If one or more terms cannot be calculated during several periods, then it is possible to retain the value of said term(s} used during the last period for which it could be calculated and maintain this value for a limited number Ns of periods (Ns being typically equal to 2 or 3) . In the latter case, if said term{s) still cannot be calculated after the Ns periods, then it is possible retain the pre¬determined fixed value, referred to as the "standby value". These different situations may occur, for

example, when the mean temperature of the pot cannot bs determined or when the equivalent AIF3 quantity contained in the alumina could not be determined.
The term Q(p) may be positive, null or negative. In the last case, it is assumed that Qlp) = 0, i.e. AIF3 is not added during the period p. When the term Q{p} is negative, it is also possible to correct the composition of the electrolyte bath 13 by adding soda, i.e. calcined soda or sodium carbonate, referred to as soda ash.
As shown in figure 2, the additions of AIF3 may be made at any time during said regulation periods (or sequences), which may correspond to the work shifts which determine the frequency of the changes of the shifts in charge of cell control and maintenance. The quantity Q(p) of AlFj determined for a period p may be added in one or more times during said working period. Preferentially, the quantity Q(p) is added practically continuously using crustbreaker-feeders which make it possible to add predetermined doses of AlFj throughout the period p.
Examples of embodiments of the invention
The following examples illustrate the calculations inherent to the regulation method according to the invention. These calculations are typical of those made for the 500 kA cells tested by the applicant. The length of the periods is 8 hours.
Example 1
Example illustrating the use of the additional terms Qr and Qs in combination with the basic terms Qint, Qcl, Qc2 and Qsol,

The value of Qtheo at 28 months is +31 kg/period. The average requirements of the pot Q' determined by the integral term Qint are +39 kg/period.
The alumina analysis gives a value of 1.36% fluorine and 5250 ppm of Na20 equivalent. The term Qcl is then equal to +22 kg/period in equivalent pure AIF3 supply.
By taking N = 12, the total actual AIF3 supplies per period over the last N periods is 44 kg/period. The difference between the actual supplies (AA kg/period) and the mean requirements (39 kg/period) is then +5 kg/period. The term Qc2 is then equal to -3 kg/period.
The temperature measured is 964°C and the set-point temperature 9b3'C, i.e. a difference of +10.e'C. The corrective term Qt is then equal to +18 kg/period.
The ARS value measured is 101.8 nO/mrr. and the set-point value ARSo is 106.0 nfi/rom. The term Qr(p) is then equal to +5 kg/period.
The S value measured is 6985 dm^ and the set-point value So is 6700 dm'. The term Qs (p) is then equal to +5 kg/period.
The quantity of AlFs to be added during the period p is then equal to: Q(p) = Ointtp) - QcKpl + Qc2 (p) + Qt{p) + Qr{p) + Qs(p) =39-22-3 + 18 + 5 + 5 = +42 kg. The terms Qr and Qs make a significant correction to the quantity Q(p).
Tests
The method according to the invention was used to regulate electrolytic cells with intensities of up to 500 kA. The length of the periods was 8 hours.

The tests related to different types of pots.
Table 1 contains the characteristics of some of the
electrolytic cells placed under test and the typical
results obtained. In case A, the pots were regulated
using the embodiment of the invention wherein Q(p) was
determined using the terms Qint(p), Qcl(p), Qc2(p) and
Qt (p) . In case B, the pots were regulated using the
embodiment of the invention wherein Q(p) was determined
using the terms Qint{p), Qcl(p), Qc2(p), Qt(p) and
Qe (p) . In case C, the pots were regulated using the
embodiment of the invention wherein Q(p) was determined
using the terms Oint

, Qcl(p), Qc2(p), Qt(p), Qr (p)
and Qs(p)'
Table 1

Case A Case B Case C
Intensity {kA) 300 kA 330 kA 500 kA
Anode density (A/cm*) 0.78 0.85 0.90
Liquid bath mass 25 22 17
(kg/kA)
Excess AlFj {%) li.8 11.8 13.2
Total standard 1.5 1.3 1.3
deviation (CT %}
Dispersion of excess 8.8 - 14.8 9.2 - 14.4 10.6-15.8
AIF3 at ± 2 0 %
Bath temperature ("O 962 962 961
Total standard 6 6 3.5
deviation (0 %)
Dispersion of 950-974 950-974 954-968
temperature at ± 2 CT %
Current efficiency (%) 95.0 95.0 95.5

The results show that the regulation metho according to the invention makes it possible to regulate electrolytic cells effectively wherein the excess AIF3 of the bath is greater than 11% and wherein the bath temperature is in the vicinity of 960°C. Accounting for the terms Qr(p) and Qs(p) in the determination of QCpl makes it possible to regulate effectively, and with a surprising stability, electrolytic cells wherein the intensity and anode density are very high and wherein the liquid bath mass is low.
The applicant observed during its tests that the regulation method according to the invention makes it possible to control, with high stability, the AIF3 content of electrolytic cells, over a period of several months, without having to take into account measured AIF3 contents, said measured contents are, in any case, easily affected by significant errors.

WE CLAIM :
1. A regulation method for an electrolytic cell (1) for the production of aluminium by
means of electrolytic reduction of alumina dissolved in an electrolyte bath (13) based on
cryolite, said cell (1) comprising a pot (20), at least one anode (7), at least one cathode
component (5,6), said pot (20) comprising internal side walls (3) and being capable of
containing a liquid electrolyte bath (13), said cell (1) comprising at least one setting means of
said cell comprising a mobile anode frame (10) to which said at least one anode (7) is attached,
said cell being capable of circulating a so-called electrolytic current in said bath, said current
having an intensity I, the aluminium produced by means of said reduction forming a pad
referred to as a "liquid metal pad" (12) on said cathode component(s) 5, 6, said cell 1
comprising a solidified bath ridge (15) on said walls (3), said method comprising control
operations of said cell comprising the addition of alumina and the addition of AIF3 in said bath
and being characterized in that it comprises:
the determination of the value of at least one indicator B referred to as the "ridge variation", capable of detecting the variation of said solidified bath ridge (15);
the adjustment of at least one setting means and/or at least one control operation according to the value obtained for the or each ridge variation indicator.
2. The regulation method as claimed in claim 1, wherein said at least one ridge variation indicator comprises an indicator, referred to as "BE", which is determined from at least one electrical measurement on said cell (1) capable of detecting the variations of the current lines induced by the variation of said ridge.
3. The regulation method as claimed in claim 2, wherein said indicator "BE" Is determined from at least one determination of said intensity 1 and at least one determination of the drop in voltage U at the terminals of said cell (1).
4. The regulation method as claimed in claim 3, wherein said at least one ridge variation

indicator BE is equal to a specific resistance variation ARS which is determined using a measurement method comprising:
the determination of at least one first value II for said intensity I and at least one first value Ul for the drop in voltage U at the terminals of said cell (1);
the calculation of a first resistance Rl fi-om at least said values II and Ul;
the movement of the anode frame (10) by a determined distance AH, from an initial position; either upwards, AH being positive in this case, or downwards, AH being negative in this case;
the determination of at least one second value 12 for said intensity I and at least one second value U2 for the drop in voltage U at the terminals of said cell (1);
the calculation of a second resistance R2 from at least said values 12 and U2;
the calculafion of a resistance variation AR using the formula AR=R2-R1;
the calculation of said specific resistance AR using the formula ARS = AR/AH.
5. The regulation method as claimed in claim 4, wherein the measurement method also comprises, at least after the determination of the values of II, 12, Ul and U2, the movement of the anode frame (10) so as to return it to its initial position and restore the initial cell setting.
6. The regulation method as claimed in claim 5, wherein said first and second resistance are calculated using the formula R=(U-Uo)/I, where Uo is a constant.
7. The regulation method as claimed in claim 6, wherein the constant Uo is between 1.6 and 2.0 V.
8. The regulation method as claimed in any one of claims 4 to 7, wherein said adjustment is a determined fimction of the difference between said specific resistance variation ARS and a reference value ARSo.
9. The regulation method as claimed in any one of claims 1 to 8, wherein said at least one

ridge variation indicator comprises an indicator, referred to as "BM", which is determined from a determination of the surface area S of said liquid metal pad (12).
10. The regulation method as claimed in claim 9, wherein said metal surface area is
determined using a measurement method comprising:
the removal of a quantity of liquid metal from the eJectroIytic cell;
the determination of the volume Vm of said quantity of liquid metal removed from the electrolytic cell;
the determination of the change AHm of the resulting level of said liquid metal
pad in said pot;
the determination of a surface area S for said liquid metal pad (12) using the formula S=Vm/AHm.
11. The regulation method as claimed in claim 10, wherein said volume Vm is determined by measuring the mass of said quantity of liquid metal removed from the electrolytic cell.
12. The regulation method as claimed in any of claims 9 to 11, wherein said adjustment is a determined function of the so-called "metal surface area" difference between the value obtained for said surface area S and a set-point value So.
13. The regulation method as claimed in any one of claims 1 to 12, wherein said adjustment comprises at least one modification of the position of said mobile anode frame (10), either upwards, or downwards, so as to modify the anode/metal distance (AMD).
14. The regulation method as claimed in any one of claims 1 to 13, wherein said adjustment comprises at least one addition of solid or liquid electrolyte bath so as to increase the level of said liquid elecfrolyte bath (13) in said pot (20).
15. The regulation method as claimed in any one of claims 1 to 14, wherein said adjustment comprises at least one modification of said AIF3 addition.

16. A regulation method for an electrolytic cell (1) for the production of aluminium by
means of electrolytic reduction of alumina dissolved in an electrolyte bath (13) based on
cryolite, said cell (1) comprising a pot (20), at least one anode (7), at least one cathode
component (5,6), said pot (20) comprising internal side walls (3) and being capable of
containing a liquid electrolyte bath (13), said cell (1) also comprising at least one setting means
of said cell comprising a mobile anode frame 10 to which said at least one anode (7) is attached,
said cell (1) being capable of circulating a so-called electrolytic current in said bath, said current
having an intensity 1, the aluminium produced by said reduction forming a pad referred to as the
"liquid metai pad" (12) on the cathode component(s) (5,6), said cell comprising a solidified bath
ridge (15) on said walls (3), said method comprising control operations of said cell comprising
the addition of alumina and the addition of AIF3 into said bath and being characterized in that it
comprises:
the set-up of a regulation sequence comprising a series of time intervals of pre¬determined length Lp referred to as "periods";
the determination of the value of at least one indicator B referred to as the "ridge variation" capable of detecting the variation of said solidified bath ridge (15);
the determination of a quantity Qo(p), referred to as the "basic term", corresponding to the net average AIF3 requirements of the cell;
the determination of a corrective term Qi(p) comprising at least one term Qsol(p), referred to as the "ridge term", which is determined from at least one or each ridge variation indicator;
the determination of a quantity Q(p) of AIF3 to be added during the period p, referred to as the "determined quantity Q(p)", by adding the corrective term Qi(p) to the basic term Qo(p), i.e. Q(p)=Qo(p)+Qi(p);
the addition into said electrolyte bath, during the period p, of an effective quantity of AIF3 equal to said determined quantity Q(p).
17. The regulation method as claimed in claim 16, wherein said length Lp of said periods is
roughly the same for all the periods.

18. The regulation method as claimed in claim 16 or 17, wherein said length Lp of said periods is between 1 and 100 hours.
19. The regulation method as claimed in any one of claims 16 to 18, wherein the term Qsol(p) comprises at least one term referred to as Qr(p) which is detenmined from at least one electrical measurement on said ceil (1) capable of detecting variations in the current lines induced by the variation of said ridge.
20. The regulation method as claimed in claim 19, wherein the term Qr(p) is determined from at least one measurement of said intensity I and at least one measurement of the drop in
voltage U at the terminals of said cell (1).
21. The regulation method as claimed in claim 20, wherein it comprises:
the determination of at least one first value H for said intensity I and at least one first value Ul for the drop in voltage U at the terminals of said cell (1);
the calculation of a first resistance R1 from at least said values 11 and U1;
the movement of the anode frame (10) by a determined distance AH, form an initial position, either upwards, AH being positive in this case, or downwards, AH being negative in this case;
the determination of at least one second value 12 for said intensity I and at least one second value U2 for the drop in voltage U at the terminals of said cell (1);
the calculation of a second resistance R2 from at least said values 12 and U2;
the calculation of a resistance variation AR using the formula AR=R2-RI;
the calculation of a quantity referred to as the "specific resistance variation" AR using the formula ARS= AR/AH;
the determination of the term Qr(p) using a determined function of said specific resistance variation ARS;
the determination of the corrective term Qi(p) comprising at least the term Qr(p) in the ridge term Qsol(p).

22. The regulation method as claimed in claim 21, wherein it also comprises, at least after the determination of the values of II, 12, Ul and U2, the movement of the anode frame (10) so as to return it to its initial position and restore the initial cell setting.
23. The regulation method as claimed in claim 21 or 22, wherein said first and second resistance are calculated using the formula R=(U-Uo)/I, where Uo is a constant.
24. The method as claimed in claim 23, wherein the constant Uo is between 1.6 and 2.0 V.
25. The regulation method as claimed in any one of claims 21 to 24, wherein the term Qr(p) is given by the ftinction Qr(p)=Kr x (ARS-.ARSo), where Kr is a constant and ARSo is a reference value.
26. The regulation method as claimed in claim 25, wherein Kr is between -0.01 and -10 kg/hour/n.fi/mm.
27. The regulation method as claimed in any one of claims 21 to 26, wherein the term Qr(p) is limited by a minimum value and by a maximum value.
28. The regulation method as claimed in any one of claims 16 to 27, wherein the term Qsol(p) comprises at least one term Qs(p) which is determined from at least one determination of the surface area S(p) of said liquid metal pad (12).
29. The regulation method as claimed in claim 28, wherein it comprises:
the removal of a quantity of liquid metal from the electrolytic cell; the determination of the volume Vm of said quantity of liquid metal removed from the electrolytic cell;
the determination of the change AHm of the resulting level of said liquid metal pad in said pot;

the determination of a surface area S for said liquid metal pad (12) using the formula S=Vm/AHm;
the determination of a term Qs(p) using a determined llmction of the surface area S(p) of said liquid metal pad (12);
the determination of the corrective term Qi(p) comprising at least the term Qs(p) in the ridge term Qsol(p).
30. The regulation method as claimed in claim 29, wherein said volume Vm is determined by measuring the mass of said quantity of liquid metal removed from the electrolytic cell.
31. The regulation method as claimed in claim 29 or 30, wherein the term Qs(p) is determined from the so-called "metal surface area" difference between the value obtained for said surface area S and a set-point value So.
32. The regulation method as claimed in any one of claims 29 to 31, wherein the term Qs(p) is given by the function Qs(p)=Ks x (S(p)-So), where Ks is a constant.
33. The regulation method as claimed in claim 32, wherein Ks is between 0.0001 and 0.1 kg/hour/dm. .
34. The regulation method as claimed in any one of claims 29 to 33, wherein the term Qs(p)
is limited by a minimum value and by a maximum value.
35. The regulation method as claimed in any one of claims 16 to 34, wherein it comprises:
the determination of a mean Qm(p) of the total AIF3 additions per period during
the lastN periods;
the determination of a quantity Qint(p), advantageously using the following "smoothing" formula: Qint(p)=(l/D) x Qm(p)+(1-1/D) x Qint(p-l), where D is a smoothing parameter setting the temporal smoothing horizon;

the determination of the basic term Qo(p) using the formula Qo(p)=Qint(p).
36. The regulation method as claimed in claim 35, wherein it comprises:
the determination of a compensating term Qcl(p) corresponding to the so-called "equivalent" quantity of AIF3 contained in the alumina added to the cell during the period p;
the modification of the term Qo(p) by subtracting the term Qcl(p) from said term Qo(p), i.e. using the formula Qo(p)=Qo(p)-Qcl(p).
37. The regulation method as claimed in claim 36, wherein the term Qm(p) is given by the
equation:
Qm(p)=+, where
=(Q(p-N)+Q{p-N+l)+Q(p-N+2)+ .. . +Q(p-1))/N,
=(Qcl(p-N)+Qcl(p-N+l)+Qcl(p-N+2)+ . . . +Qcl(p-1))/N, where N is a constant.
38. The regulation method as claimed in claim 37, wherein N is between 1 and 100.
39. The regulation method as claimed in any one of claims 35 to 38, wherein the parameter D is equal to Pc/Lp, where Pc is between 400 and 8000 hours.
40. The method as claimed in any one of claims 35 to 39, wherein it comprises:
the determination of a quantity Qtheo corresponding to the total theoretical AIF3 requirements of the ceil when regulation is started; the start-up of the method by taking Qint(0)=Qtheo.
41. The regulation method as claimed in any one of claims 35 to 40, wherein it comprises:
the determination of an additional corrective term Qc2(p) using a ftinction of the
difference between Qm(p) and Qint(p);
the addition of the term Qc2(p) in the determination of Qi{p).

42. The regulation method as claimed in claim 41, wherein the term Qc2(p) is given by the formula Qc2{p)=Kc2 x (Qm(p)-Qint(p)), where Kc2 is a constant.
43. The regulation method as claimed in claim 42, whereinKc2 is between -0.1 and -1.
44. The regulation method as claimed in any of claims 41 to 43, wherein the term Qc2(p) is limited by a minimum value and by a maximum value.
45. The regulation method as claimed in any one of claims 16 to 44, wherein it comprises:
the determination of a mean temperature T(p) of the electrolyte bath; the determination of an additional corrective term Qt(p) using a determined function of the difference between said temperature T{p) and a set-point temperature To; the addition of the corrective term Qt(p) in the determination of Qi(p).
46. The regulation method as claimed in claim 45, wherein the term Qt(p) is given by the formula Qt(p)=Kt. x (T(p)-To), where Kt is a constant.
47. The regulation method as claimed in claim 46, wherein Kt is between 0.01 and 1
kg/hourA'C.
48. The regulation method as claimed in any one of claims 45 to 47, wherein the term Qt(p) is limited by a minimum value and by a maximum value.
49. The regulation method as claimed in any one of claims 16 to 48, wherein it comprises:
the measurement of the excess AIF3 E(p);
the determination of an additional corrective term Qe(p) using a function of the difference between the excess AIF3 measured E(p) and its target value Eo;
the addition of the corrective term Qe(p) in the determination of Qi(p).

50. The regulation method as claimed in claim 49, wherein the term Qe(p) is given by the
formula Qe(p)=Ke x (E(p)-Eo), where Ke is a constant.
51. The regulation method as claimed in claim 50, wherein Ke is between -0.05 and -5 kg/hour/% AIF3.
52. The regulation method as claimed in any one of claims 49 to 51, wherein the term Qe(p) is limited by a minimum value and by a maximum value.
53. The regulation method as claimed in any one of claims 16 to 52, wherein the quantity
Q(p) comprises an additional term Qea(p) which is given by a function of the anode effect
energy AEE.
54. The regulation method as claimed in claim 53, wherein the term Qea(p) is limited by a minimum value and by a maximum value.
55. The regulation method as claimed in any one of claims 16 to 54, wherein the quantity Q(p) is limited to a maximum quantity Qmax.
56. The regulation method as claimed in any of claims 16 to 55, wherein, when the detennined value of the term Q(p) is negative, its value is taken as equal to zero, i.e. no AIF3 is added during the period p.

Documents:

1334-chenp-2003 abstract duplicate.pdf

1334-chenp-2003 abstract.jpg

1334-chenp-2003 abstract.pdf

1334-chenp-2003 claims duplicate.pdf

1334-chenp-2003 claims.pdf

1334-chenp-2003 correspondence-others.pdf

1334-chenp-2003 correspondence-po.pdf

1334-chenp-2003 description (complete) duplicate.pdf

1334-chenp-2003 description (complete).pdf

1334-chenp-2003 drawings duplicate.pdf

1334-chenp-2003 drawings.pdf

1334-chenp-2003 form-1.pdf

1334-chenp-2003 form-18.pdf

1334-chenp-2003 form-26.pdf

1334-chenp-2003 form-3.pdf

1334-chenp-2003 form-5.pdf

1334-chenp-2003 pct.pdf

1334-chenp-2003 petition.pdf

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Patent Number

224875

Indian Patent Application Number

1334/CHENP/2003

PG Journal Number

49/2008

Publication Date

05-Dec-2008

Grant Date

23-Oct-2008

Date of Filing

25-Aug-2003

Name of Patentee

ALUMINIUM PECHINEY

Applicant Address

7, PLACE DU CHANCELIER ADENAUER, F-75218 PARIS CEDEX 16,

Inventors:

#	Inventor's Name	Inventor's Address
1	VANVOREN, CLAUDE	L'ECHAILLON, F-73300 ST. JEAN DE MAURIENNE,
2	BONNARDEL, OLIVIER	LES GRANDES TERRES, F-73140 ST. MARTIN D'ARC,

PCT International Classification Number

C25C3/20

PCT International Application Number

PCT/FR02/00692

PCT International Filing date

2002-02-26

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	01/02723	2001-02-28	France