Title of Invention	A PROCESS FOR THE RECOVERY OF CHROMIUM FROM CHROMITE ORE AT LOW TEMPERATURE
Abstract	The present invention relates to a process for the recovery of chromium from chromite ore at low temperatures. The process of reduction can be affected using a furnace. This invention will be useful for reducing chromite ore at temperature significantly lower than that conventionally used for reducing the ore.

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The present invention relates to a process for the recovery of chromium from chromite ore at low temperatures. The process of reduction can be affected using a furnace. This invention will be useful for reducing chromite ore at temperature significantly lower than that conventionally used for reducing the ore, Chromite ore is conventionally reduced in the submerged-arc furnace where the temperature varies from a low of about 500 °C at the top of the charge bed to about 2500 °C at the tip of the electrodes. In this process, a mixture of about 72-78 wt% chromite ore; 15-22 wt% coke or coal and 4-7 wt% of flux such as quartzite is charged to the furnace. The chromite ore is reduced in the furnace and liquid ferro-chromium is tapped at the bottom at regular intervals. In US patent No.4981510, reduction of the ore is carried out in rotary kilns at temperatures above 1400 °C. In this case, a mixture of ore and coal in the ratio of 1:0.4 to 1:2 is charged into a rotary furnace and heated to 1480-1580 °C for 20-240 minutes. This partially reduced (ore+metal) mixture, also called as 'pre-reduced ore' is charged to the submerged-arc furnace to produce ferro-chromium. In S.A Patent No.8.410.101, highly reduced chromium pellets has been produced by a process using large excess of reducing agent. Reference may be made to N. S. Sundar Murti and V. Seshadri: Transactions of the Indian Institute of Metals, Vol.38, No.5, October 1985, pp.423-425, wherein the reduction of a chromite ore containing 17.85% Fe, 38.15% Cr, 3.50% Si and 7.64% Mg with graphite was carried out. Reduction was conducted in the temperature range 1240- 1310°C. 15% and 25%, by weight, of carbon were used in the different runs. The authors have reported the cumulative percentage of reduction. It is known from various studies (for example: W.J. Rankin: Trans. Inst. Min. Metall. C, June 1979, Vol.88, C107-C113) that up to about 1200° C, only iron oxide is reduced in the chromite ore. Above this temperature, chromium oxide gets reduced. Iron oxide is almost completely reduced before the oxide of chromium gets reduced as shown by B.M.C. Tsomondo, D.J.Simbi and E.Navara, Ironmaking and Stelmaking,1997, 24(5), pp.386- 391; by P. Weber and R.H. Eric: Metallurgical TransactionsB, Vol.24B, December 1993, pp.987-995; by A. Lekatou and R. D. Walker, Iron and Steelmaking, 1995, Vol.22, No.5, pp.393-404; by K. P. D. Perry, C. W. P. Finn and R. P. King: Met. Trans. B, Vol.l9B, August 1988, pp.677-684; by O. Soykan, R. H. Eric and R. P. King, Met. Trans. B, Vol. 22B, December 1991, pp.801-810; by A. Lekatou and R. D. Walker, Iron and Steelmaking, 1995, Vol.22, No.5, pp.378-392 and by W.J.Rankin, Arch. Eisenhuttenwesen, 50(9), 1979, pp. 373-378. Therefore, wherever the investigators have not given the extent of recovery of iron and chromium separately, it is assumed that all the iron gets reduced before chromium is reduced. From this, the extent of recovery of chromium is inferred. In the study of N. S. Sundar Murti and V. Seshadri: Transactions of the Indian Institute of Metals, Vol.38, No.5, October 1985, pp.423-425, it is found that a maximum chromium recovery of 5.22% was obtained at 1240 °C after 35 minutes of reaction. Chromium recovery, (Cr)R is defined as the ratio of the amount of chromium reduced to the total present in the ore expressed as a percentage: (Cr)R = (metallic chromium in the partly reduced sample / total chromium in the ore)* i 00 The level of recovery remained unchanged after this period. N. A. Barcza, P. R. Jochens, and D. D. Howat: Electric furnace proceedings, 1971, pp.88-93 found that no reduction of chromite occurred at temperatures less than 1000° C. They studied the reduction of chromite in the temperature 1100-1500° C. A maximum rate of reduction about 70% was obtained at 1300° C. The rate of reduction remained constant with time after an initial rise up to about 20 minutes. There was a decrease in the rate of reduction after the maximum rate had been reached. This study also showed that C^Os was not reduced under CO gas but was reduced when carbon was present along with CO. These authors also have reported cumulative reduction. Using the method employed in the previous case, it is deducted that in this study the recovery of chromium was 63% at 1300 °C after 30 minutes. This level remained unchanged after this. Downing, Electric furnace proceedings, 1971, pp.88-93 noted that unless the CO gas was extremely pure, it would not reduce the chromite. W.J. Rankin: Trans. Inst. Min. Metall. C, June 1979, Vol.88, C107-C113,studied the Kroondal mine ore of Transvaal in South Africa. The ore was mixed with graphite in the mass ratio 7:3. The ore and the reductant were crushed to 150-170 mesh before mixing. The ore was treated to remove gangue. The composition of the ore and that of the chromite are given in table 1. Reduction studies were also carried out in a stream of CO in contact with graphite. The studies were carried out in the temperature range of 1100°C-1500°C. The reduction sequence involved formation of Fe and Cr203 up to 1200°C. Above this temperature, iron A series of ores from Greek and Cypriot were investigated by H. G. Vazarlis and A. Lekatou, Ironmaking and Steelmakmg, 1993, Vol.20, No.l, pp.42-53. The chemical compositions of the various ores studied are given in Table 2. Metallurgical coke and lignite from different sources were used as the reducing agent. In some cases, carbon monoxide and hydrogen were used as reducing agent. The ground ore and reductant were thoroughly mixed in a vibratory device and introduced into a furnace in an alumina boat. Argon was used for maintaining an inert atmosphere. The experiments were carried out in the temperature range 1200-1500°C. Since pure hydrogen and carbon monoxide failed to reduce the Greek and Cypriot chromites, it was argued that reduction was carried out by solid carbon. Note : S.A.- South African Chromite. Reductants with higher fixed carbon content were superior reducing agents. Increase in temperature increased the rate of reduction. The rate of reduction increased with progress of time and then it dropped. A. Lekatou and R. D. Walker, Iron and Steelmaking, 1995, Vol.22, No.5, pp.393-404, studied the solid state reduction of chromite ore from the Skoumtsa area of Greece, in the temperature range 1100°C-1470°C. The composition of the ore is given in table 3. Natural graphite, -45fj,m size, was used as reductant. Briquettes of 1cm diam. were prepared from these. Graphite was taken in stoichiometric quantity required for the reduction of Cr2O3 to Cr7C3 and iron oxides to Fe3C. Experiments were carried out in a tubular furnace. It was demonstrated that CO was the only gaseous reaction product. The maximum metallisation was 91% and 67% for iron and chromium, respectively for a reduction of 75%. Analysis of the samples reduced at different temperatures and time periods showed that at low percentage reduction (up to 25%). Fe-C alloy formed. Occasionally Fe3C was formed. This was followed by the formation of (Cr,Fe)?C3 which was the main metallization product up to 72% reduction. At 46% reduction, Cr23Ce was detected which remained at advanced levels of reduction. The percentage reduction increased with time, temperature, and carbon content. At any given temperature, there was an optimum time and carbon addition after which no further increase in reduction occurred. Ralph H. Nafziger, Jack E. Tress and Jack I.Paige: Metallurgical Transactions B, Vol. 10B, March 1979, pp.5-14, studied the reduction of two types of chromite ores using • different reducing agents such as coal char, metallurgical coke, petroleum coke etc. The experiments were carried out in a horizontal tubular furnace in the temperature range 1100° C-15000 C. The size of the chromite ore was 100/200 mesh and the reductant was - 100 mesh size. The quantity of reductant taken was 109% of the stoichioimetric amount required to reduce iron oxide to metallic iron and chromium oxide to chromium carbide. Argon was used for maintaining an inert atmosphere. Nafziger et.al R. H. Nafziger, P. E. Sanker, J. E. Tress and R. A. McCune: Ironmaking and Steelmaking, 1982, Vol.9. No.6, pp.267-277, studied the reduction of four chromite ores from mines in Montana and California. They used a rotary kiln to carry out the studies. Samples were withdrawn at different temperatures as the charge moved down the kiln and the extent of metallization was estimated. They pelletized the ores with coal char and/or coke breeze. They used 125% and 150% of the carbon requirement for the formation of Cr?C3 and Fe by reduction of the ore. A temperature of 1150 °C was required for iron reduction. The ores ground to -65 mesh were blended with -150 mesh coal char. 5% bentonite binder was used. The pellets were about 0.6 cm diameter. These authors observed a maximum chromium recovery of 50-60% which was achieved at 1300 °C after 200 minutes of traverse through the kiln. 0. Soykan, R. H.Eric and R. P. King, Met. Trans. B, Vol. 22B, February 1991, pp.53-63, studied the reduction of chromite from Bushveld, South Africa at 1416°C. Finely divided ore and graphite were used in this study. The amount of graphite used was 30% in excess of the amount required to produce the carbide C^Cs from the ore. XRD analysis of the reduced sample showed that Fe?C3 was a reaction product from about 27.5% reduction. Table 6 summarises the degree of chromium metallisation obtained by various investigators as a function of temperature and time. This table shows that the extent of chromium recovery at any temperature and time of holding depends on the source of the ore and the nature of the reducing agent. It is seen that significant levels of recovery are obtained at 1300 °C and above. These studies have investigated the effect of time and temperature on the extent of chromium recovery> the influence of factors such as the flow arte of inert gas has not been studied. The conventional processes for the reduction of chromite ore suffer from various draw-backs. In the submerged-arc furnace, heat required for the reduction reaction is supplied through electric power which is converted to heat by the resistance of the charged material. Since the cost of electric power is very high, the cost of reduction is, consequently, very high in this process. In the conventional rotary kiln process for the pre-reduction of the ore, it is necessary to heat the charge material to at least 1400 °C , in order to effect reduction. This leads to large energy cost and also affects the equipment through high refractory wear at high temperatures. Further, the temperatures cause a softening of the pellets making it difficult to charge these in the submerged-arc furnace. It is very difficult to maintain a balance between reduction and softening. The main objective of the present invention is to provide a process for the reduction of chromite ore at low temperatures. Another objective of the present invention is to provide a process for the reduction of chromite ore at low temperatures with a significant recovery of chromium. In the figures accompanying this application, fig.l, represents the experimental apparatus used for carrying out the reduction studies. Fig.2 represents the quartz capsules used as the container for the ore which was reduced in the furnace. Reduction of chromite ore in submerged-arc furnace is expensive since the cost of electric power is high. Partial reduction of the ore outside the submerged-arc fumace(SAF) can reduce the cost in the SAF. However, the conventional processes of pre-reduction of the ore outside the SAF is carried out at 1400 °C or above under normal atmospheric conditions and is still expensive and inefficient due to pellet softening because of the high temperatures involved. Accordingly the present invention provides a process for the recovery of chromium from chromite ore at low temperatures which comprises: (a) preparing a mixture of chromite ore and a reducing agent, (b) placing the above said mixture in a furnace, (c) passing a stream of an inert gas through the above said furnace containing the above said mixture at a rate of 0.016-7 lit/minute, (d) heating the above said mixture at a temperature ranging between 1100-1250 °C for a period of 30-180 minutes followed by discharging from the furnace and (e) recovering the desired chromium from the mixture by known method. In an embodiment of the present invention the chromite ore used is selected from the following composition: Cr 20-40%; Fe 10-30%; SiO2 15-30%; MgO 2-4%; A1203 2- 4%. In yet another embodiment the reducing agent used is selected from coke and coal. In still another embodiment the inert gas used is selected from argon and helium. In the present invention, passing a stream of inert gas like argon reduces the partial pressure of carbon monoxide. This facilitates faster reduction of the ore. The extent of reduction achieved at a given temperature and time interval was equivalent to that achieved at a higher temperature when argon was not passed.At a temperature of 1250 °C, up to 38% of the chromium in the ore could be recovered as metal. The novelty of the investigations is that using a flowing stream of inert gas at appropriate flow rate has enhanced the recovery of chromium. This has helped in bringing down the temperature of reduction for obtaining good recovery of metallic chromium, compared to the conventional processes. The following examples of the process are given by way of illustration and should not be construed to limit the scope of the present invention. The following examples are results of experiments carried out in the laboratory. In these experiments a tubular furnace 90 mm in diameter and 500 mm length was used. This furnace is called the main furnace. The furnace was resistance-heated. A recrystallised alumina tube was kept inside the furnace. One end of this tube was connected to a similar tube kept inside another furnace (called the auxiliary furnace), through ground glass joints. The auxiliary furnace contained coke maintained at 800 °C. The other end of the tube in the auxiliary furnace was connected to a cylinder of argon through a rotameter to measure the flow rate of the gas. The gas from the rotameter was passed through a train of CaCh, before entering the alumina tube of the auxiliary furnace. The other end of the main furnace tube was connected to the atmosphere through a train of CaCb and KOH solution to dry the gas and reduce the amount of CO in the exit gas. The experimental apparatus is illustrated in fig.l. In a typical experiment 10 gms. of the ore was mixed with 4 gms. of coke, both ground to a size of-150 mesh ( 0.1125 mm). The mixture was taken in a quartz capsule closed at both ends. The capsule measured 82 mm in length and 20 mm in diameter. Two holes, each approximately 3 mm diameter were made on the curved surface of the capsule, one at each end of the capsule. Both the holes were on the same line parallel to the axis. The mixture was introduced into the capsule through these holes. )o Fig.2 illustrates the capsule used in the experiments. The mass of the empty capsule and that after addition of the charge was noted. The capsule was positioned at the center of the alumina tube in the main furnace. This tube was closed airtight using ground glass joints. The auxiliary furnace containing coke was also closed similarly. Argon was passed through the apparatus at the desired flow rate. Power supply to the auxiliary furnace was then switched on. When this furnace reached a temperature of 800 °C, power to the main furnace was switched on. This furnace was then heated to the desired temperature and maintained at the temperature for a pre-determined time interval. At the end of this period, power to both the furnaces was switched off simultaneously. The sample in the furnace was allowed to cool down to room temperature in the stream of argon flowing through the apparatus. After cooling to room temperature, the quartz capsule containing the (ore+coke) mixture was withdrawn from the furnace and weighed. The sample was analyzed for metallic chromium and metallic iron. A few experimental samples were studied using XRD. Temperatures were measured using chromel-alumel (type K) thermocouples. Ore/coke ratio, temperature, time and argon flow rate were varied during the course of this study. The influence of these parameters on the reducibility of the ore was studied. In preliminary runs, 10 gms of the coke was taken in the quartz capsule without any ore and maintained at 1200 C in the furnace in the presence of argon. The coke sample was maintained this for 120 minutes. At the end of this period, the sample was cooled in the furnace to room temperature, under argon. The sample was later withdrawn and the mass of the (capsule +coke) was measured. There was a loss of mass of the coke by about 50%. The empty quartz capsule did not register any change in mass. This is recorded in table A, below, for a few typical cases. M2- mass of coke after experiment F.R. - flow rate of argon The ore used in the study analysed 29.9%Cr, 16.09%Fe, 23.01% SiO2, 2.47%MgO, 2.38%Al2O3. The following schedules of experiments illustrate the recovery of metallic chromium for different conditions. The recovery is defined by: (Cr)R = (metallic chromium in the partly reduced sample / total chromium in the ore) 100 Metallic iron was analyzed in some of the samples. This analyzed to be about 50% of the total iron in the ore. A good part of the reduced iron is present as the carbide, FesC in the partly reduced ore. This was confirmed by XRD analysis. This is consistent with information available in literature. Metallic iron and metallic chromium were analysed in the samples using wet chemical methods. In the following examples: T - Temperature Mo - mass of ore taken for reduction Me -mass of coke taken for reduction F.R. - flow rate of argon (1/min.) t - time of holding at temperature, hrs. Wl=weight of ore+coke+boat(before reduction), gms W2= weight of ore+coke+boat( after reduction), gms W3=weight of ore+coke(before reduction), gms W4=weight of ore+coke(after reduction), gms %Cr - percentage metallic chromium in the partly reduced sample Example 1 10 gms of the ore was mixed with 4 gms. of coke and heated at 1100 °C for a period of one hour. Argon was passed at a rate of 0.016 lit/min. The change in the mass of the sample during the experiment and the recovery of chromium are given in the table below. 10 gms of the ore was mixed with 4 gms. of coke and heated at 1150 °C for a period of one hour. Argon was passed at a rate of 0.016 iit/min. The change in the mass of the sample during the experiment and the recovery of chromium are given in the table below. Example 3 10 gms of the ore was mixed with 1 gm. of coke and heated at 1200 °C for a period of 1-2 hours. Argon was passed at a rate of 0.25 lit/min. The change in the mass of the sample during the experiment and the recovery of chromium are given in the table below. Example 4 S.No I 2 F.R 0.25 0.25 Time 2 1 Wl 32.2277 27.6547 W2 30.2955 25.7507 W3 10.9787 10.9442 W4 9.0511 9.023 %Cr 0.45 0.72 (Cr)R 1.44 2.30 10 gms of the ore was mixed with 2 gms. of coke and heated at 1200 °C for a period of 1-4 hours. Argon was passed at a rate of 1-7 lit/min. The change in the mass of the sample during the experiment and the recovery of chromium are given in the table below. S.No Example 5 10 gms of the ore was mixed with 3 gms. of coke and heated at 1200 °C for a period of 1-3 hours. Argon was passed at a rate of 0.016-0.25 lit/min. The change in the mass of the sample during the experiment and the recovery of chromium are given in the table below. It is see from the table that at a flow rate of 0.1 lit/min. of argon, maximum recovery (32.92%) of chromium could be obtained after a holding time of 1 hour. Example 6 10 gms of the ore was mixed with 4 gms. of coke and heated at 1200 °C for a period of 1-4 hours. Argon was passed at a rate of 0.016-7 lit/min. The change in the mass of the sample during the experiment and the recovery of chromium are given in the table below. Maximum recovery of chromium obtained was at a flow rate of 51it/min. of argon. The holding tome was 5 hrs. Example 7 10 gms of the ore was mixed with 12 gms. of coke and heated at 1200 °C for a period of 1 hour. Argon was passed at a rate of 0.125-3 lit/min. The change in the mass of the sample during the experiment and the recovery of chromium are given in the table below. Maximum recovery of chromium was obtained at a flow rate of argon of 31it/min. Example 8 10 gms of the ore was mixed with 4 gms. of coke and heated at 1250 C for a period of 1 -3 hours. Argon was passed at a rate of 1 lit/min. The change in the mass of the sample during the experiment and the recovery of chromium are given in the table below. Maximum recovery was obtained at a holding time of 3 hours. Example 9 10 gms of the ore was mixed with 8 gms. of coke and heated at 1250 C for a period of 1-3 hours. Argon was passed at a rate of 1-2 lit/min. The change in the mass of the sample during the experiment and the recovery of chromium are given in the table below. Maximum recovery of chromium(38.47%) was obtained at a flow rate of argon of 2 lit/min. after a holding time of 3 hours. Example 10 10 gms of the ore was mixed with 12 gms. of coke and heated at 1250 C for a period of 1 hour. Argon was passed at a rate of 0.125-3 lit/min. The change in the mass of the sample during the experiment and the recovery of chromium are given in the table below. Maximu recovery of chromium was obtained at a flow rate of argon of 2 lit/min. Holding time is the period for which the sample was maintained at the experimental temperature. It is seen from the tables above that the recovery increases with the holding time. The recovery increases with decrease in the (ore/coke) ratio. The recovery of chromium increases with increase in flow rate of argon. However, the recovery decreases after a certain optimum flow rate. This optimum flow rate depends on factors such as the temperature of reduction, the holding time and the ore/coke ratio. Recovery does not increase monotonously with increasing flow rate of inert gas since the actual reduction reaction is made of two steps: the oxidation of carbon by CO2 and the reduction of the ore by CO. The examples reported here have established the optimum flow rate for different conditions of experimentation, the other parameters being varied being the temperature, the ore/coke ratio and the holding time. The chromium recovery reported is based on the chemical analysis of the reduced sample for metallic chromium. However, a part of the chromium is converted into the carbide during reduction. This is not included in the reported recovery. Though the examples given here report on one type of ore, the present invention is not limited to this ore. These data are given here as examples to illustrate the influence of various parameters on the recovery of chromium and should not be construed to limit the scope of the present invention. It is seen from the examples given above that about 38% chromium can be recovered at 1250 °C at a flow rate of argon of 2 1/min. after keeping the sample at this temperature for 180 minutes. The advantages of the process are: (1) It can reduce the chromite ore and recover significant proportion of chromium at low temperatures, compared to the conventional processes for the pre-reduction of these ores. This saves energy required for reduction. (2) It enhances the life of the refractory used in the furnaces used for pre-reduction. (3) Pre-reduction facility can be added to exiting manufacturing units where preheating is already being carried out, with minimum capital expenditure. (4) The process gives a chromium recovery rate of 38%. We claim: (1) A process for the recovery of chromium from chromite ore at low temperatures which comprises: (a) preparing a mixture of chromite ore and a reducing agent, (b) placing the above said mixture in a furnace, (c) passing a stream of an inert gas through the above said furnace containing the above said mixture at a rate of 0.016-7 lit/minute, (d) heating the above said mixture at a temperature ranging between 1100- 1250 °C for a period of 30-180 minutes followed by discharging from the furnace and (e) recovering the desired chromium from the mixture by known method. (2) A process as claimed in claim 1, wherein the chromite ore used is selected from the following composition: Cr 20-40%; Fe 10-30%; SiO2 15-30%; MgO 2-4%; Al2O32-4%. (3) A process as claimed in claims 1&2, wherein the reducing agent used is selected from coke and coal. (4) A process as claimed in claims 1-3, wherein the inert gas used is selected from argon and helium. (5) A process for reduction of chromium from chromite ore at low temperature, substantially as herein described with reference to examples and drawing accompanying this specification

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