Title of Invention

"AN ECO-FRIENDLY ELECTROLYTIC PROCESS FOR THE PRODUCTION OF ALUMINUM"

Abstract

The present invention relates to an eco-friendly electrolytic process for the production of aluminum from fused salts such as alumina dissolved in cryolite melt. This eco-friendly electrolytic process for the production of aluminum involves single step electrolysis of alumina to give aluminum metal and pure oxygen gas, which can be concentrated and sold as a valuable co-product along with the primary product, that is aluminum metal. Presently aluminum is produced using consumable carbon anodes. Hitherto during the production of aluminum from alumina, green house gases such as carbon dioxide and carbon monoxide are evolved. The present invention mitigates this, by the use of cermet anode at which only pure oxygen is evolved. Thus the process is eco-friendly not only by the elimination of CO2 and CO during the electrolysis but also enhances the environmental hygiene by the addition of pure oxygen gas. The electrolysis can be carried out in closed electrolysers to give higher space time yield. The process of the invention enables the production of aluminum metal by a green process. The purity of the product is of the order of 99.4% at an average current efficiency of 85% and an energy consumption of 13.4 Kwh per Kg of aluminum.

Full Text

The present invention relates to an eco-friendly electrolytic process for the production of aluminum. The present invention particularly relates to an ecofriendly electrolytic process for the production of aluminum from fused salts such as alumina dissolved in cryolite melt. The present invention more particularly relates to an eco-friendly electrolytic process for the production of aluminum wherein the process liberates pure oxygen gas, which can be concentrated and sold as a valuable co-product along with the primary product, that is aluminum metal. Metals, which are less noble than hydrogen in the emf series, have necessarily to be electrowon from fused salts. Even other reduction processes may not be economically viable for production, as in the case of aluminum, titanium, Be, Ca, Mg, Na. Hence, electrolytic route becomes necessary for the extraction of such metals. The aluminum metal presently is manufactured by the Hall-Heroult process by the electrolysis of aluminia - cryolite melt between consumable carbon anode and carbon block cathode at temperatures ranging from 950 to 980° C. During the electrolysis aluminum metal is pooled to the cathode at the bottom and carbon dioxide, (CO2) is liberated at the carbon anode. The evolution of CC^, a green house gas is by the inherent consumption of anode carbon according to the following reaction: 2 AI203 + 3 C -> 4 Al + 3 CO2 Theoretically 334 Kg of carbon is required for one ton of aluminum but in practice about 500 kg is consumed, liberating 2 tons of CO2 to the atmosphere. The use of carbon as anode for aluminum electrolysis has the following disadvantages: 1. Liberation of carbon dioxide, a green house gas with harmful effects. 2. The carbon must be very high purity, free from silicon, sulphur, vanadium. 3. Frequent lowering and adjustment of the anode to maintain optimum anode - cathode distance. 4. Turbulence and unsteady voltage conditions resulting in higher voltage and higher energy consumption. 5. Process is labour intensive and environmentally hazardous. 6. Necessity of a separate anode shop. Reference may be made to U.S.Patent 3930967 (1976) in which electrodes made from Sn02, Sb2O3 and CuO were used for aluminum production. The main drawback is that very low current density ranging from 0.001 to 0.025 A/cm2 only has been applied. Reference may be made to another U.S Patent No.4057480 (1977), wherein mixed materials of SnC>2, Fe203, CoO, ZnO are used as inert anodes for aluminum electrolysis. The drawback being the contamination of the cathode product with excess tin. In a Japanese patent No. 1461155 (1977), oxygen ion conducting anodes are disclosed in which both spinel and perovskite structures containing ZrO2 as major constituent have been tested. The trial run faced many operational difficulties. Electrolysis with anodes fabricated from oxides of Fe, Co, Ni, Zn, Sn, Y and Cu powder up to 0 - 30% is given in U.S patent 6126799 (2001). The problem encountered was the contamination and high wear rate of the electrodes. Work on similar lines with cermet anodes incorporating Ag powders have been patented in Canada pat 2317596, (2001). The difficulty with the electrolysis being dissolution of the electrode and contamination of the aluminum metal produced. Reference may be made to a paper by T.R.AIcorn etal, "Light metals 1993" (Ed) Subodh.K.Das Minerals, Metals and Materials Society, Warrendale PA (1993) P. 433-443, in which operational results of a pilot cell test with inert anode has been published. The drawbacks are the instability of the electrode materials and tested up to a current density of 0.5 A/cm2 only. In a paper by J.S.Gregg etal., Light Metals, 1993 (Ed) Suboth.K.Das, Minerals, Metals and Materials Society, Warrendale P.A. (1993) P.455-464, have discussed the testing of copper - nickel ferrite / NiO with or without cerium oxide coating for electro-winning of aluminum. The high energy cost due to low conductivity of the anode was attributed as the main drawback. From the above prior art references it is clear that an eco-friendly electrolytic extraction process of metals such as aluminum is definitely needed. The main objective of the present invention is to provide an eco-friendly electrolytic process for the production of aluminum, which obviates the drawbacks of the hitherto known prior art processes, as referred above, specially the Hall Heroult process for aluminium production with carbon anodes. Another object of the present invention is to provide an eco-friendly electrolytic process for the production of aluminum from fused salts such as alumina dissolved in cryolite melt, which allows reduced constant inter-polar distance, reduced voltage and energy consumption, application of high current density up to SA.cm ~2, hygienic pot rooms and enhanced environmental condition and possibility of vertical drain type electrodes and improved designs with monopolar / bipolar electrodes. Yet another object of the present invention is to limit the generation of green house gases especially CO2 from metallurgical industries particularly from aluminum industry, as the elimination of C02 will reduce the environmental pollution as well as other harmful effects associated with C02. Still another object of the present invention is to provide an eco-friendly electrolytic process for the production of aluminum wherein the process liberates pure oxygen gas, which can be concentrated and sold as a valuable co-product along with the primary product, that is aluminum metal. Still yet another object of the present invention is to provide an eco-friendly electrolytic process for the production of aluminum wherein all the unit operations are easy to control. The present invention provides an eco-friendly electrolytic process for the production of aluminum from fused salts such as alumina dissolved in cryolite melt. This eco-friendly electrolytic process for the production of aluminum involves single step electrolysis of alumina to give aluminum metal and pure oxygen gas, which can be concentrated and sold as a valuable co-product along with the primary product, that is aluminum metal. Presently aluminum is produced using consumable carbon anodes. Hitherto during the production of aluminum from alumina green house gases such as carbon dioxide and carbon monoxide are evolved. The present invention mitigates this, by the use of cermet anode at which only pure oxygen is evolved. Thus the process is eco-friendly not only by the elimination of CO2 and CO during the electrolysis but also enhances the environmental hygiene by the addition of pure oxygen gas. The electrolysis can be carried out in closed electrolysers to give higher space time yield. The process of the invention enables the production of aluminum metal by a green process. The purity of the product is of the order of 99.4% at an average current efficiency of 85% and an energy consumption of 13.4 Kwh per Kg of aluminum. The eco-friendly electrolytic process for the production of aluminum from alumina of the present invention is illustrated in the drawings accompanying this specification. In figure 1 of the drawings is shown an electrolyser for electrolyzing molten alumina - cryolite, consisting of a carbon block cathode (1) and cermet anode (2). Figure 2 shows the top view of the same electrolyser of the process. Accordingly, the present invention provides an eco-friendly electrolytic process for the production of aluminum, which comprises electrolyzing molten alumina as herein described dissolved in fused cryolite in an electrolyser consisting of a carbon block cathode (1) and one or more cermet anode (2), at a temperature in the range of 950 to 990° C, and at a constant current density in the range of 0.50 A/cm2 to 1.25 A/cm2. In an embodiment of the present invention, the electrolyte is molten cryolite saturated with 5 to 9 wt % pure alumina. In another embodiment of the present invention, the carbon block cathode (1) and cermet anode (2) are connected to a DC rectifier to impress current of the order of 100A. In yet another embodiment of the present invention, the electrolysis is carried out continuosly for a duration of the order of 100 to 180 hours. In still another embodiment of the present invention, the feeding of alumina, make up salts and collection of aluminum metal is done at regular intervals of the order of 4 hours. In still yet another embodiment of the present invention, the cermet anode consists of 80 to 90 wt.% dry powders of oxides of one or more of the metals selected from Cu, Ni, Co, Fe, Zn, Al, Ti, Cr, Ce, La, Nb, and 10 to 20 wt. % metal powders selected from Au, Ag, Bi, Sb, Cu, Ni, Zn, either alone or in combination, as described and claimed in patent application no. NF - 359 / 02. In a further embodiment of the present invention, the process yields primary aluminum metal along with pure oxygen as anodic gas. In a still further embodiment of the present invention, the aluminum metal produced has a purity of 99.4 wt %. In another embodiment of the present invention, the average current efficiency is of the order of 85%. In yet another embodiment of the present invention, the average energy consumption is of the order of 13.4 kwh per kg. of aluminium at 120 mmpy wear rate of the anode. In our co-pending patent application no. 34DEL2003, we have described and claimed a process for the manufacture of a novel cermet anode useful for eco-friendly electrolytic extraction of metals from fused salts, which comprises mixing intimately 80 to 90 wt.% dry powders of oxides of one or more of the metals selected from Cu, Ni, Co, Fe, Zn, Al, Ti, Cr, Ce, La, Nb, and 10 to 20 wt. % metal powders selected from Au, Ag, Bi, Sb, Cu, Ni, Zn, either alone or in combination, to obtain a dry blend of powder composition, compacting the dry blend to obtain green compacts, sintering the green compacts at a temperature of up to 1400°C under normal atmospheres, allowing the sintered compact to cool and fixing current leads to obtain a novel cermet anode. The novel cermet anode is fabricated from mixed oxides of one or more of the metals such as Cu, Ni, Co, Fe, Zn, Al, Ti, Cr, Ce, La, Nb up to 90 wt % and 10-20 wt % metal powders selected from the Au, Ag, Bi, Sb, Cu, Ni, Zn, either alone or combined, by processing of dry powders with or without using binders adopting ceramic technology methods, starting with size reduction of metal powders, metallic oxides by different techniques and their size analysis using laser particle sizer, blending of the powders in required compositions. Compacting of the blend is done in an hydraulic press to get compacts of various size ranging from 5-12 cm diameter and 5 - 12 cm height. The green compacts are sintered at a temperature of up to 1400°C, in an electric furnace under normal atmospheres. holding at intermediate temperatures for several hours. The sintered compacts are characterized followed by fixing of current leads and assembling into one unit. The novelty of the present invention is a single stage eco-friendly process for the electrolysis of the alumina into aluminum metal and oxygen without the use of carbon and thereby not only mitigate the drawbacks of the carbon anode but also enhance the atmospheric hygiene by addition of oxygen to the atmosphere. The inventive step is the incorporation of one or more cermet anodes in the electrolyzer having carbon block as cathode, for the production of aluminum metal from molten alumina (AI2O3). The novel electrolytic process of the present invention for the production of aluminum involves electrolyzing molten cryolite saturated with pure 5 to 9 wt % alumina in an electrolyser, as depicted in figures 1 and 2 of the drawings, consisting of mild steel outer shell lined inside with high magnesia bricks and having carbon blocks as cathode. The inner area is partitioned into an electrolytic zone and a heating zone to maintain the thermal balance by additional ac input. A thick pool of molten aluminum metal over the carbon block acts as the cathode of the electrolyzer and an assembly of cermet electrodes acts as anode of the electrolyzer. The electrolyzer is completely covered and sealed to collect the anodic gas. Anode and cathode are connected to a DC rectifier to impress 100 A current. Arrangements are made to feed the alumina and make up salts as well as to collect the aluminum metal regularly. The following examples are given by way of illustration and therefore should not be construed to limit the scope of the present invention. Example: 1 An eco-friendly electrolytic process for the production of aluminum using cermet anode in which electrolysis was carried out in an electrolyser in fused cryolite with high purity aluminium pool over a carbon block cathode and alumina in cryolite containing excess AIF3, kept at 960° C to maintain a bath ratio of 1.2 and using an assembly of 4 cylinderical cermet anodes, 5 X 5 cm (figure 2) at a current density of 1.25 A/cm2 for 100 hrs with feeding of alumina and aluminum metal collection at an intervals of 4 hours. The current efficiency obtained was 85%. Example: 2 An eco-friendly electrolyser was operated with molten aluminium over carbon block cathodes containing cryolite with excess aluminium fluoride and 8 wt% of alumina as electrolyte at a constant current density of 0.50 A/cm2 and with a composition of cermet anode containing 85 wt% NiO, Fe2O3, NbaOs and 15 wt % metal powders of Cu, Bi combined. Continuous electrolysis was carried out at 950°C, for 180 hrs at an interelectrode distance of 3.5cm. Aluminum metal was produced at an energy consumption of 13.4 kwh/ kg of aluminum. Example: 3 An electrolytic cell for the production of aluminum metal was operated with cermet anode of composition 80 wt.% of NiO, Fe2O3, Ce2O3 and 20 wt % of Bi plus Cu powders in a cryolite electrolyte containing 6 wt % of AI2O3 maintained at a temperature of 980° C and an inter-electrode distance of 4 cm at a current density of 0.80 A/cm2. Pure aluminum was deposited to cathode at a current efficiency of 82%. The purity of the metal was 99.4 wt % with the impurities of Fe = 0.15 wt %, Ni = 0.04 wt % and Cu = 0.04 wt %. Example: 4 An eco-friendly electrolyser consisting of non-consumable cermet anodes containing 90 wt % of Fe2O3, NiO and La2Os and metal powders of Ni, Cu and Sb of 10 wt %, carbon block cathode, molten cryolite electrolyte, was operated at a current density of 1.0 A/cm2 at a temperature of 960° C for 140 hrs. to obtain primary aluminum at the cathode and pure oxygen at the anode at a current efficiency of 85%. The metal purity was 99.2% with the impurities Cu 0.05 wt %, Fe 0.20wt % and Ni 0.05 wt %. Example: 5 An eco-friendly electrolytic cell consisting of an anode made up of 4 numbers of cylindrical cermet electrodes of 5 X 5 cm, a cathode and cryolite electrolyte with a bath ratio 1.2, 8 wt % dissolved AI203, was operated at a temperature of 990° C and a current density of 0.7 A/ cm2 for 110 hrs duration. The metal tapping and alumina feeding was done for every four hours and was able to realize a current density of 85% and aluminium purity of 99.4 % From the above examples it is clearly confirmed that the eco-friendly electrolytic process of the present invention for the production of aluminum is novel and produces aluminum metal and oxygen gas from alumina dissolved in fused cryolite. The main advantages of the present invention are: 1. Production of primary aluminum from fused alumina - cryolite electrolyte, by an eco-friendly electrolytic process. 2. No hazardous carbon monoxide nor carbon dioxide are evolved, only oxygen an eco-friendly gas is produced during the electrolysis. 3. No emission of fluoride fluro-carbon compounds from the electrolyser. 4. Allows reduced and constant interpolar distance. 5. Reduced energy consumption. 6. Application of high current density. 7. The aluminum metal produced has a purity of 99.4 wt %. 8. The average current efficiency is of the order of 85%. 9. The average energy consumption is of the order of 13.4 kwh per kg. of aluminium at 120 mmpy wear rate of the anode. We claim: 1. An eco-friendly electrolytic process for the production of aluminum, which comprises electrolyzing molten alumina as herein described dissolved in fused cryolite in an electrolyser consisting of a carbon block cathode (1) and one or more cermet anode (2), at a temperature in the range of 950 to 990° C, and at a constant current density in the range of 0.50 A/cm2 to 1.25 A/cm2. 2. The eco-friendly electrolytic process as claimed in claim 1, wherein the electrolyte is molten cryolite saturated with 5 to 9 wt % pure alumina. 3. The eco-friendly electrolytic process as claimed in claim 1-2, the carbon block cathode (1) and cermet anode (2) are connected to a DC rectifier to impress current of the order of 100A. 4. The eco-friendly electrolytic process as claimed in claim 1-3, wherein the electrolysis is carried out continuosly for a duration of the order of 100 to 180 hours. 5. The eco-friendly electrolytic process as claimed in claim 1-4, wherein the feeding of alumina, make up salts and collection of aluminum metal is done at regular intervals of the order of 4 hours. 6. The eco-friendly electrolytic process as claimed in claim 1-5, wherein the cermet anode consists of 80 to 90 wt.% dry powders of oxides of one or more of the metals selected from Cu, Ni, Co, Fe, Zn, Al, Ti, Cr, Ce, La, Nb, and 10 to 20 wt. % metal powders selected from Au, Ag, Bi, Sb, Cu, Ni, Zn, either alone or in combination as herein described. 7. The eco-friendly electrolytic process for the production of aluminum, substantially as herein described with reference to the examples and drawings accompanying this specification.

Documents:

35-DEL-2003-Abstract-(04-02-2009).pdf

35-del-2003-abstract.pdf

35-DEL-2003-Claims-(04-02-2009).pdf

35-DEL-2003-Claims-(19-01-2009).pdf

35-del-2003-claims.pdf

35-del-2003-complete specification (granted).pdf

35-DEL-2003-Correspondence-Others-(04-02-2009).pdf

35-DEL-2003-Correspondence-Others-(19-01-2009).pdf

35-del-2003-correspondence-others.pdf

35-del-2003-correspondence-po.pdf

35-del-2003-description (complete)-04-02-2009.pdf

35-del-2003-description (complete).pdf

35-del-2003-drawings.pdf

35-del-2003-form-1.pdf

35-del-2003-form-18.pdf

35-del-2003-form-2.pdf

35-del-2003-form-3.pdf