Title of Invention

"AN IMPROVED METHOD FOR THE PRODUCTION OF FERRO-CHROMIUM"

Abstract

An improved method for the production of ferro-chromium, characterised by preparing a mixture of 72 to 85% wt.% of ore, 15 to 22 wt.% of coke/coal and 4 to 7 wt.% of flux; (ii) preparing a second mixture of 72 to 85 wt.% of ore, 15 to 22 wt% of ore, 15 to 22 wt% of coke/coal and 0 to p wt% flux, where V is less than the wt% of flux taken in the first mixture and the ratio of wt% ore to coke in the second mixture is the same as in the first mixture; (Hi) charging the second mixture into a furnace through a temperature zone in the range of 250 - 2500°C and simultaneously charging the first mixture into the said furnace through a temperature zone in the range of 250 - 2500°C; (Iv) tapping the resultant ferro-chromium and slag by known methods.

Full Text

The present invention relates to an unproved process for the production of ferro-chromium. The production of ferro-chromium by the improved process of the present invention may be effected using a furnace. This invention will be useful to reduce the amount of silicon produced during the production of ferro-chromium and also will result in a reduction in the consumption of coke as well as power requirement in the process. The conventional process for the production of ferro-chromium involves the following steps: (1) mix 72 to 78 wt % chromite ore and 15 to 22 wt % coke or coal, and 4 to 7 wt % flux such as quartzite ; (2) charge the mixture , pre-heated or otherwise, into a furnace, such as a submerged -arc furnace having a plurality of electrodes, usually three, in such a manner that the charge is uniformly distributed throughout the cross-section of the furnace wherein the temperature varies from a low about of 500 °C at the top to a high of about 2500 °C near the tip of the electrodes, reducing to about 1700 °C at the bottom of the furnace; (3) tap the metal and slag at the bottom of the furnace. The known method described above is usually implemented using a submerged-arc furnace wherein charging is done through chutes which are located almost symmetrically about each electrode. As the charge descends through the furnace, it gets heated from about 500 °C at the top, passing through a maximum of 2500 °C near the tip of the electrodes and to about 1700 °C near the bottom. A part of the charge faces higher temperature (close to a maximum of about 2500 °C near the tip of the electrodes). The charge is symmetrically distributed around each electrode which maximises the amount of charge passing through the highest temperature regions of the furnace. No discrimination is made about where the individual components of the charge go and what temperature is experienced by them. The conventional process suffers from the drawback that the amount of silicon produced is very high. The reduction of silica to silicon consumes energy. Since silicon in the product is not desired, the energy used in the reduction of silica is a wasteful utilization of energy. The conventional solution to this problem is to reduce the amount of quartzite charged into the furnace. But this leads to a reduced level of silica in the slag which becomes more viscous and difficult to tap. Another method adopted is to reduce the proportion of coal/coke charged. This reduces the level of silicon in the hot metal but leads to a reduced recovery of chromium also. These methods cannot lead to a reduction in specific energy consumption in the production of metal since the proportion of silica reduced out of the total charged is not altered. The main object of the present invention is to provide an improved process for the production of ferro-chromium which obviates the drawbacks noted above. Another object of the present invention is to reduce the coke consumption in the production of ferro-chromium. Still another object of the present invention is to reduce the actual silicon level in the hot metal when the other operating parameters such as the type and quantities of the various charge materials, power supply etc. are not altered. Yet another objective of the present invention is to reduce the specific power consumption (i.e. the power required to produce one tonne of the hot metal) when the other operating parameters are not altered and results from the reduced amount of power utilised in the production of total silicon in the furnace. Production of silicon along with chromium and iron during the production of ferro-chromium is not desirable. It consumes power and coke. If the amount of silicon produced in the furnace is decreased, the available power and coke can be more effectively utilised. This will bring down the specific power consumption in the production of the alloy and can lead to an improved recovery of the principal elements also. Silica is reduced to silicon essentially in the high temperature zone of the furnace. This zone is localised and is close to the electrode-region. If the amount of silica passing through this zone is reduced, this will lead to a decrease in the recovery of silicon and make more energy available for the reduction of the oxides of chromium and iron. In the process of the present invention, silica gets reduced at lower temperatures also, under appropriate conditions. However, under the conditions prevailing in the process, this reduction occurs only at temperatures equal to or greater than 2000°C, which is our finding. Such temperatures prevail in a furnace only in the zones around the electrode, say, in the case of the submerged-arc furnace. From an analysis of the temperature profile in a furnace, it is possible to predict the size of the high temperature zones where the temperature is 2000°C and above. From an analysis of the location of the charging chutes with respect to this zone, it is possible to predict how much of the flux, such as quartzite will pass through this zone. The flux added may be distributed among the charging chutes such that less of this material goes through this high temperature zone. Say 'x' amount of the (ore+coke) and 'y' amount of quartzite is conventionally charged into the furnace uniformly through all the chutes. The amount of quartzite is split into two parts 'a' and 'b' such that y=(a+b), where va' can vary from 0 to (y*r) where Y is the ratio of the quantity of material charged through the chutes which put the material in the high temperature zone of the furnace to the total material charged, conventionally. The amount of (ore+coke) charged through these chutes will be xz' which is equal to (x*r). The amount of (ore+coke) charged through the other chutes will be (x-z). The charging through the chutes feeding the material to the high temperature zone may be made such athat Y amount of (ore+coke) and 'a' amount of quartzite is added to these chutes where (z/a) > (x/y). Lower is the amount of quartzite added through these chutes (i.e. lower is the magnitude of 'a'), better is the performance of the improved process . The rest of the material is added to the other chutes. Accordingly, the present invention provides an improved method for the production of ferro-chromium, characterized by; (i)preparing a mixture of 72 to 85% wt.% of ore, 15 to 22 wt.% of coke/coal and 4 to 7 wt.% of flux; (ii) preparing a second mixture of 72 to 85 wt.% of ore, 15 to 22 wt% of ore, 15 to 22 wt% of coke/coal and 0 to p wt% flux, where (p' is less than the wt% of flux taken in the first mixture and the ratio of wt% ore to coke in the second mixture is the same as in the first mixture; (iii) charging the second mixture into a furnace through a temperature zone in the range of 250 -2500°C and simultaneously charging the first mixture into the said furnace through a temperature zone in the range of 250 - 2500°C; (Iv) tapping the resultant ferro-chromium and slag by known methods. In an embodiment of the present invention, the ore used is such as chromite, oxide of chromium and iron. In another embodiment of the present invention, the flux used is such as quartzite, oxide of silicon. In yet another embodiment of the present invention, the first mixture is charged through a temperature zone in the range of 500 - 1800°C. During the production of ferro-chromium in the furnace, oxides of iron and chromium get reduced as the charge descends the furnace. Experiments in the laboratory have provided information on the sequence of reduction of the oxides which usually proceeds through the formation of carbides as intermediate phases. The product formed and the extent of reduction at different temperatures have been determined through experiments. In the upper regions of the fiirnace where the temperatures are low, the kinetics of reduction plays a predominant role. The reduction of chromium oxide would be complete by the time the charge reaches a temperature of 1400 °C. The charge starts fusing around this temperature. Therefore, it is reasonable to expect thermodynamic equilibrium to prevail between the molten metal, slag and the gas at high temperatures. It was assumed in the present analysis that the furnace consisted of a number of thermal zones each characterized by a typical temperature. Local thermo-chemical equilibrium was taken to prevail between the metal, slag and gas in each temperature zone. Therefore, each zone will have a different chemistry of the metal and that of slag determined by the temperature of the zone. The ultimate chemical compositions of the tapped metal and slag will be weighted averages of the compositions of those produced in these zones. A significant change in the chemistry of the alloy occurs as the temperature reaches 2000 °C. Below this temperature, silicon is not present in the alloy. At temperature T >= 2000 °C, silicon is present in the alloy. This helps in recognising the presence of two thermo-chemical zones in the high temperature regions of the furnace. One is characterized by temperatures equal to or above 2000 °C; the other by T % Si = X + Y*m where, %Si is the percentage of silicon in the hot metal and'm', the relative size of the high temperature zone. X and Y are constants. The parameter 'm' in the expression above represents the ratio of the volume of the region where the temperature is 2000 °C or above to the volume of the furnace lying between the same horizontal levels as this high temperature zone. This zone lies in the central region of the furnace around the electrodes as in the case of the submerged-arc furnace. In an embodiment of the process of the present invention using a conventional submerged-arc furnace, quartzite which is used as a flux in the production of ferro-chromium is mixed with ore and coke and fed into the furnace through six chutes. Out of these, three are arranged along the perimeter of an inner circle and the other three are along the perimeter of an outer circle. Each of three outer chutes subscribes an angle of 105° and each of the three inner chutes subscribes an angle of 218° towards the higher temperature zone of the furnace where the temperature is 2000 °C and above. Therefore, the inner chutes feed proportionately more material to the high temperature zone compared to the outer chutes. The inner chutes feed 0.61 part of the quartzite passing through these chutes to the high temperature zone. The rest 0.39 part of this quartzite passing through the inner chutes is fed to the outer region of the furnace where the temperature never reaches 2000 °C or beyond. In the case of the outer chutes, 0.29 part of the quartzite passing this chute is fed to the high temperature zone of the furnace where the temperature is 2000 °C and above. The rest part, i.e 0.71 part, of the quartzite charged through the outer chutes passes through the regions of the furnace where the temperature never reaches 2000 °C or beyond. This manner of differential distribution of material by the inner and outer chutes is taken advantage of. If all the quartzite in the charge material is allowed to pass only through the outer chutes, then the fraction of quartzite passing through the high temperature zone of the furnace is much less than that in the conventional process. In the process of the present invention, if there is a reduction in the level of silicon by 0.5% in the hot metal (say, from 4% to 3.5%), the power consumed will decrease by 35.7 KWH / tonne of hot metal or 1963 MWH /annum, in the case of an industry producing 50 000 tonnes of ferro-chromium per annum and a saving the carbon consumption by about 23 tonnes per annum. The following examples are given by way of illustration of the present invention and should not be construed to limit the scope of the present invention. The following examples are the results of trials conducted at a ferro-chrome plant, to reduce the percentage of carbon charged that is utilized in the production of silicon during ferro-chrome production. The compositions and the quantities of the various raw materials used and the composition of the hot metal tapped are given for each case. The chemical analysis of the raw materials and that of the metal produced are given in wt%. The results achieved and the weekly average used as a reference figure for comparison are provided. In these examples, the abbreviations refer to : HL - hard lumpy chromite ores F.C - fixed carbon FL -friableores VOL -volatile SP - sinter pellets THM - tonnes of hot metal CK -coke RM - raw material charged QTY - quantity charged QZ - quartzite (tonnes/day) HM - hot metal The % C consumed in silicon production listed in the tables below represent the percentage of carbon out of the total charged during the trial that is used in the production of silicon from the silica charged. The % Si recovered is the percentage of the total silicon charged in the form of SiO2 that is present in the hot metal tapped. EXAMPLE 1 77.56 wt% ore, 15.95 wt.% coke and 6.5 wt% quartz were taken in the first mixture. 79.96 wt% ore, 16.44 wt% coke and 3.61 wt% quartz were taken in the second mixture.. The quantities of ore and coke in the first mixture were the same as in the second mixture. The nrst mixture was passed through the three inner chutes of a submerged-arc furnace. The second mixture was passed through the three outer chutes of the furnace. As described above, the inner chutes proportionately charged more quantity of the charge material through the high temperature zone of the furnace where the temperature is 2000 C and above, compared to the outer chutes which charged a smaller proportion of the charge through the high temperature zone of the furnace where the temperature was 2000° C and above. The results of the trial are given below. RAW MATERIALS USED RM Cr2O3 SiO2 Fe A12O3 MgO CaO QTY HL 41.08 14.18 9.00 8.16 17.00 2.91 86.838 FL 44.46 5.82 15.96 13.60 8.22 0.45 0.0 SP 49.92 5.90 12.43 15.04 10.16 0.56 166.255 CK : F.C. - 85.06; VOL.- 3.12; ASH -11.82; QTY - 52.072 QZ : SiO2 - 96.6; A12O3 -1.10; Fe2O3 - 2.22; QTY - 16.310 HM : Cr - 64.82; Si - 2.81; C - 7.76; QTY -104 RESULTS OF TRIALS S.No. Parameter Level during Reference trial 1. % Si recovered 20.83 15.35 2. % C consumed in Si production 5.66 5 EXAMPLE 2 75.75 wt% ore, 16.68 wt.% coke and 7.58 wt% quartz were taken in the first mixture. 80.36 wt% ore, 17.69 wt% coke and 1.95 wt% quartz were taken in the second mixture. The quantities of ore and coke in the first mixture were the same as in the second mixture. The quantity of coke in the first mixture was the same as in the second mixture. The first mixture was passed through the three inner chutes of a submerged-arc furnace. The second mixture was passed through the three outer chutes of the furnace. The inner chutes proportionately charged more quantity of the charge material through the high temperature zone of the furnace where the temperature is 2000 C and above, compared to the outer chutes which charged a smaller proportion of the charge through the high temperature zone of the furnace where the temperature was 2000 C and above. The results of the trial are given below. RAW MATERIALS USED RM Cr2O3 SiO2 Fe A12O3 MgO CaO QTY HL 40.06 14.34 9.00 8.00 17.35 2.80 131.300 FL 46.60 4.47 12.36 15.60 10.16 0.67 70.672 SP 47.96 6.75 12.24 15.30 11.12 0.78 150.105 CK: F.C. - 85.81; VOL.- 3.26; ASH -10.93; QTY - 77.557 QZ : SiO2 - 96.4; A12O3 -1.19; Fe2O3 - 2.33; QTY- 23.567 HM: Cr - 63.75; Si - 4.02; C - 7.90; QTY -146 RESULTS OF TRIALS S.No. Parameter Level during Reference trial 1. % Si recovered 25.83 22.62 2. % C consumed in Si production 7.45 7.87 EXAMPLE 3 76.14 wt% ore, 15.84 wt.% coke and 8.01 wt% quartz were taken in the first mixture. 81.93 wt% ore, 17.04 wt% coke and 1.03 wt% quartz were taken in the second mixture. The quantities of ore and coke in the first mixture were the same as in the second mixture. The quantity of coke in the first mixture was the same as in the second mixture. The first mixture was passed through the three inner chutes of a submerged-arc furnace. The second mixture was passed through the three outer chutes of the furnace. The inner chutes proportionately charged more quantity of the charge material through the high temperature zone of the furnace where the temperature is 2000 C and above, compared to the outer chutes which charged a smaller proportion of the charge through the high temperature zone of the furnace where the temperature was 2000 C and above. The results of the trial are given below. RAW MATERIALS USED RM Cr2O3 SiO2 Fe A12O3 MgO CaO QTY HL 43.05 11.64 9.24 8.32 17.16 2.69 159.875 FL 46.87 5.40 12.00 10.32 15.00 0.56 111.139 SP 47.69 6.75 12.84 14.70 10.80 0.90 92.935 CK: F.C. - 86.31; VOL.- 3.03; ASH -10.66; QTY - 80.632 QZ : SiO2 - 96.4; A12O3 -1.19; Fe2O3 - 2.33; QTY- 24.972 HM : Cr - 63.50; Si - 3.34; C - 7.89; QTY -147 RESULTS OF TRIALS S.No. Parameter Level during Reference trial 1. % Si recovered 17.55 22.62 2. % C consumed in Si production 6.04 7.87 EXAMPLE 4 75.12 wt% ore, 16.7 wt.% coke and 8.2 wt% quartz were taken in the first mixture. 80.86 wt% ore, 17.97 wt% coke and 1.17 wt% quartz were taken in the second mixture.. The quantities of ore and coke in the first mixture were the same as in the second mixture. The quantity of coke in the first mixture was the same as in the second mixture. The first mixture was passed through the three inner chutes of a submerged-arc furnace. The second mixture was passed through the three outer chutes of the furnace. The inner chutes proportionately charged more quantity of the charge material through the high temperature zone of the furnace where the temperature is 2000 C and above, compared to the outer chutes which charged a smaller proportion of the charge through the high temperature zone of the furnace where the temperature was 2000 C and above. The results of the trial are given below. RAW MATERIALS USED RM Cr2O3 SiO2 Fe A12O3 MgO CaO QTY HL 42.51 12.49 9.60 8.10 17.08 2.58 82.460 FL 46.87 5.57 11.88 15.00 10.40 0.56 75.395 SP 48.23 6.41 13.08 14.70 10.48 0.67 64.193 CK: F.C. - 85.54; VOL.- 2.86; ASH -11.60; QTY - 47.563 QZ : Si02 - 96.4; A12O3 - 1.19; Fe2O3 - 2.33; QTY-14.241 HM : Cr - 64.70; Si - 3.78; C - 7.83; QTY - 83 RESULTS OF TRIALS S.No. Parameter Level during Reference trial 1. % Si recovered 17.99 22.62 2. % C consumed in Si production 5.98 7.87 EXAMPLES 74.84 wt% ore, 16.53 wt.% coke and 8.63 wt% quartz were taken in the first mixture. 81.42 wt% ore, 17.98 wt% coke and 0.61 lwt% quartz were taken in the second mixture. The quantities of ore and coke in the first mixture were the same as in the second mixture. The quantity of coke in the first mixture was the same as in the second mixture. The first mixture was passed through the three inner chutes of a submerged-arc furnace. The second mixture was passed through the three outer chutes of the furnace. The inner chutes proportionately charged more quantity of the charge material through the high temperature zone of the furnace where the temperature is 2000 C and above, compared to the outer chutes which charged a smaller proportion of the charge through the high temperature zone of the furnace where the temperature was 2000 C and above. The results of the trial are given below. RAW MATERIALS USED RM Cr2O3 SiO2 Fe A12O3 MgO CaO QTY HL 43.60 10.63 9.24 8.52 17.56 2.80 139.305 FL 48.23 5.57 11.76 14.00 10.16 0.56 129.163 SP 47.69 6.92 12.00 15.42 11.08 0.90 100.215 CK : F.C. - 86.23; VOL.- 2.70; ASH -11.07; QTY - 82.108 QZ : SiO2 - 96.4; A12O3 - 1.19; Fe2O3 - 2.33; QTY- 23.341 HM : Cr - 64.70; Si - 3.78; C - 7.83; QTY - 83 RESULTS OF TRIALS S.No. Parameter Level during Reference trial 1. % Si recovered 22.61 22.62 2. % C consumed in Si production 7.17 7.87 EXAMPLE 6 76.92 wt% ore, 16.98 wt.% coke and 6.11 wt% quartz were taken in the first mixture. 79.51 wt% ore, 17.56 wt% coke and 2.94 wt% quartz were taken in the second mixture. The quantities of ore and coke in the first mixture were the same as in the second mixture. The quantity of coke in the first mixture was the same as in the second mixture. The first mixture was passed through the three inner chutes of a submerged-arc furnace. The second mixture was passed through the three outer chutes of the furnace. The inner chutes proportionately charged more quantity of the charge material through the high temperature zone of the furnace where the temperature is 2000 C and above, compared to the outer chutes which charged a smaller proportion of the charge through the high temperature zone of the furnace where the temperature was 2000 C and above. The results of the trial are given below. RAW MATERIALS USED RM Cr2O3 SiO2 Fe A12O3 MgO CaO QTY HL 43.33 11.14 9.24 8.42 17.48 2.69 89.634 FL 47.42 3.80 14.16 13.80 9.67 0.56 91.763 SP 50.14 6.75 12.96 13.2 10.08 0.56 71.38 CK : F.C. - 85.38; VOL.- 2.82; ASH -11.80; QTY - 55.82 QZ : SiO2 - 96.4; A12O3 -1.19; Fe2O3 - 2.33; QTY-16.02 HM : Cr - 63.77; Si - 3.37; C - 7.96; QTY - 87 RESULTS OF TRIALS S.No. Parameter Level during Reference trial 1. % Si recovered 18.71 22.62 2. % C consumed in Si production 5.97 7.87 The main advantages of the present invention are: 1. It can reduce the amount silicon produced per tonne of ferro- chromium manufactured. 2. It can reduce the amount of power consumed in the production of silicon accompanying the manufacture of every tonne of ferro-chromium. 3. It can reduce the amount of coke consumed in the production of silicon accompanying t the manufacture of every tonne of ferro-chromium. 4. If other conditions are unaltered, the invention can lead to a reduction in the specific power consumption (power consumed to produce one tonne of ferro-chromium) in the manufacture of ferro-chromium. 5. Under suitable circumstances, this can lead to an increase in the level of chromium in the ferro-chromium manufactured. Claim: 1. An improved method for the production of ferro-chromium, characterised by (i) preparing a mixture of 72 to 85% wt.% of ore, 15 to 22 wt.% of coke/coal and 4 to 7 wt.% of flux; (ii) preparing a second mixture of 72 to 85 wt.% of ore, 15 to 22 wt% of ore, 15 to 22 wt% of coke/coal and 0 to p wt% flux, where 'p' is less than the wt% of flux taken in the first mixture and the ratio of wt% ore to coke in the second mixture is the same as in the first mixture; (iii) charging the second mixture into a furnace through a temperature zone in the range of 250 - 2500°C and simultaneously charging the first mixture into the said furnace through a temperature zone in the range of 250 - 2500°C; (Iv) tapping the resultant ferro-chromium and slag by known methods. 2. An improved process as claimed in claim 1 wherein the ore used is sleeted from chromite, oxide of chromium and iron. 3. An improved process as claimed in claims 1 and 2 wherein the flux used is sleeted from quartzite, oxide of silicon. 4. An improved process as claimed in claims 1 to 3 wherein the first mixture is charged through a temperature zone in the range of 500 - 1800°C. 5. An improved process for the production of ferro - chromium substantially as herein described with reference to the examples.

Documents:

469-del-2000-abstract.pdf

469-del-2000-claims.pdf

469-del-2000-correspondence-others.pdf

469-del-2000-correspondence-po.pdf

469-del-2000-description (complete).pdf

469-del-2000-form-1.pdf

469-del-2000-form-19.pdf

469-del-2000-form-2.pdf

469-del-2000-form-3.pdf