FIELD OF THE INVENTION
This invention relates to an improved method of producing ferrochrome in submerged arc furnace by smelt reduction of new charge mixture. The invention also relates to method of producing oxidized chrome ore of size to less than 25 mm.
BACKGROUND OF THE INVENTION
Chromite ore is main raw material for production of ferrochrome using the submerged arc furnace (SAF) process. The smelting reduction of chromite ores is carried out in submerged arc furnace using coke as the reductant and quartzite as the flux. Chromite ores in the form of lump are smelted in SAF directly whereas chromite ore fines are agglomerated to pellets or briquettes. Chromite ore fines are conventionally agglomerated to pellets without any prior heat treatment of ore fines. These agglomerates are then subjected to sintering process in vertical shaft furnace or horizontal steel belt sintering furnace prior to smelting in submerged arc furnace.
The prior art teaches at least five primary processes for the production of ferrochrome. These include conventional process with open or semi-closed submerged arc furnaces, the conventional process with closed submerged arc furnace, the Outokumpu process, the DC Arc route and the Premus process. Each of these processes are discussed briefly below.

Conventional Process with Open or Semiclosed Submerged Arc Furnaces
In the conventional process of ferrochrome production, a mixture of chrome ore, reductants and flux is fed cold with minimum pre-processing directly into open/semiclosed type submerged arc furnaces. The furnace off-gases are cleaned in a bag plant before being vented into the atmosphere. The ferrochrome metal and slag are then tapped from the furnace for further processing. The primary advantage of this process is that it requires lowest capital investment and is very flexible in terms of raw materials that can be used in the process. The main disadvantage of this process is that it is increasingly being perceived as being less environmentally friendly than other available processes and it has the lowest efficiencies.
Conventional process with closed submerged arc furnace
In conventional process with closed submerged arc furnaces, a mixture of sintered pellets, lumpy chromite ores, coke as reductant and flux are fed to closed submerged arc furnaces. The furnace off-gas is cleaned using wet scrubbers in the gas cleaning plant. The ferrochrome metal and slag are then tapped for further processing. The primary advantage of closed submerged arc furnace is that it is energy efficient process as compared to open or semiclosed furnaces. The main disadvantages of the process are that, the energy consumptions are still higher up to 4500 kWh/t of metal, requires costly metallurgical coke and high losses of chromium metal to slag upto 10% by weight.

Outokumpu process
Fine chrome ore is wet milled and then pelletized using a binder such as bentonite. The pellets are then sintered and then air cooled and stockpiled. The pellets together with fluxes and reductants may be heated in a pre-heater located above the furnace. Reductants consisting of coke and char, are added to the (preheated) raw materials and fed into closed submerged arc furnaces. The furnace off-gas is cleaned in wet scrubbers and used as an energy source in the sintering and preheating processes. The main advantage of this process is that the sintered pellets and preheating of the charge to the submerged arc furnaces results in reduced specific energy consumptions and improved chromium recoveries.
DC arc furnace
The furnace uses a single or double solid or hollow carbon electrode and produces a DC arc to an anode in the bottom of the furnace. The arc is normally an open or semi-submerged one. Raw materials can be charged either directly into the furnace, or by using a hollow electrode. The primary advantage of this process is that the process utilizes any of the available raw materials including 100% chromite fines with minimum or no pre-processing, thus eliminating the need for an expensive agglomeration plant. Inferior grades of reductants like coal and anthracite can be used in this process, which is also considered an additional advantage of the process.

Premus Technology
Fine chrome ore, bentonite and a reductant such as anthracite fines are dry milled, pelletized and preheated before being fed into rotary kilns where partial pre-reduction of the chromium oxidie and iron oxides take place. The metallized pellets are then hot charged into closed submerged arc furnaces to produce ferrochrome metal and slag. The furnace off-gas is cleaned in ventury scrubber and used throughout the plant as an energy source. Initial capital costs for this process are high and the level of operational control required to ensure smooth operation of the process is also very high.
The prior art processes mainly included pre-reduction or preheating of agglomerates in rotary kilns or fluidized bed followed by smelt reduction of the preheated or pre-reduced charge in submerged arc furnace or converter type furnace. For example, an improved process for reduction of iron-containing chrome ores is disclosed in patent application number US4772316, wherein mixture of chrome ore, coal and slag former (quartz sand) is reduced in a rotary furnace in CO containing atmosphere for 8 hours and the reduced product obtained from the rotary furnace is subsequently melted to produce Ferrochrome. A reduction level of about 95% was achieved for iron and chromium in rotary furnace by heating the mixture in different zones of rotary furnace. The process was claimed to have low coal consumption and eliminated the variations in reduction level in rotary furnace. Coals containing high volatile matter (VM >20.0%) were introduced into the rotary furnace only through the outer nozzle in the burner. The coal introduced through the burner ensures continuous layer of reducing or inert gas on the furnace charge thereby prevents the reoxidation of the reduced product. In patent application number US4981510, a process for producing ferrochrome with carbon content in the range of 0.02 to 10 % is disclosed wherein the iron containing chrome ore is mixed with coal in 1:0.4 to 1:2 proportion and slag former containing CaO, MgO

or Al203 and Si02 such as limestone and dolomite. The reduction process is carried out in rotary furnace for 20 to 240 minutes in CO containing atmosphere at temperature in the range of 1480 °C to 1580 °C. The reduction product is then mixed with additive in a water cooled rotary drum in order to make use of the heat loss from reduced product to the additive material and cool the entire reduced product. Since the mixing and cooling process takes place in rotary drum where continuous mixing action is present, the formation of large agglomerates of reduced product is prevented. The mixture of reduced product and additive (dolomite or limestone) is cooled to 600 °C to 1000 °C and hot charged in melting furnace, wherein 1600 to 1700 °C temperature and basicity (CaO+MgO/Si02) is maintained greater than 1.1 in order to obtain the ferrochrome product.
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JP58022356 discloses a method of reduction of chrome ore in rotary kiln wherein mixture of chrome ore, slag forming agent and carbonaceous agent are reduced in rotary kiln which is provided with lining of magnesia type basic refractory in order to avoid the wear of refractories. The coal is burnt by blowing oxygen to melt and reduce the ore.,The basicity (CaO/Si02) of the slag is maintained between 1.3 to 1.7 and MgO-AI203-CaO-Si02 is 90% or more. The MgO in slag is maintained between 15 to 35% and Al203 between 15 to 35%. A pre-reduction and melt reduction method is disclosed in patent application number JP59113158, wherein chrome ore is pre-reduced in rotary kiln and 90% of iron oxide in the ore is reduced. The pre-reduced material is charged into top and bottom blown converter type melting and reducing furnace and the method is claimed to be advantageous over conventional electric arc furnace. In patent application number JP59085840, a titled apparatus essentially composed of rotary furnace is disclosed wherein the preheated chrome ore, reducing agent and slag forming agents are smelt-reduced to produce ferrochrome. The exhaust gases from the tilted apparatus are used to preheat the chrome ore in rotary kiln. A method for producing chromium containing steel is disclosed in patent

application number KR9301131, wherein the powder is prepared by mixing chrome ore, carbonaceous reducing agent and reducing the ore in rotary furnace at 1200 °C to 1500 °C in reducing atmosphere. The reduced powder contains 3 to 10% by weight carbon, 22 to 48% chromium and 11 to 24% iron. The powder material is charged to converter to produce chromium containing steel. In patent application number DE3713883, a process for producing ferrochrome by thermal reduction of pellets in rotary kiln and subsequent smelting of the reduced pellets in melting furnace is disclosed wherein the iron containing chrome ore with coal and pelletizing agent starch, Si02 and a base especially NaOH are prepared and reduced in rotary kiln for two to five hours at 1200 °C to 1400 °C to carry out the pre-reduction of chrome ore. The pre-reduced product is then hot charged into a melting furnace at 1600 °C to 1700 °C to produce ferrochrome. A process to produce ferrochromium from chrome ore is disclosed in patent application number US4629506, wherein mixture of chrome ore, coal and slag forming constituents are reduced in rotary kiln at 1480 °C to 1580 °C in CO containing atmosphere for 20 to 240 minutes. The SiO2 is added to rotary kiln only when the mixture reaches a temperature of 1200 °C. The pre-reduced ore obtained is cooled to less than 700 °C, crushed to less than 25 mm size and separated by density or magnetic separation in to coal containing fraction to recycle to rotary kiln and other fraction containing slag and metal crushed to less than 0.5mm size and melted in a melting furnace at 1700 °C by blowing oxygen and coal to produce ferrochromium. The process minimizes the gangue content to be smelted in melting furnace. A method for preheating and pre-reduction of chrome ore is disclosed in patent application number JP63216910, wherein pre¬heated chrome ore is pre-reduced in fluidized bed furnace by using coal. The char obtained after reduction is separated from pre-reduced ore, crushed and used to preheat the chrome ore by burning the char outside fluidized bed furnace. The ash obtained during preheating of chrome ore is separated from pre-heated ore and pre-heated ore is fed to fluidized bed furnace for pre-reduction. In patent application JP63216934, a process for reduction of

chromium ore is disclosed wherein powdered chrome ore and high temperature char are charged to preheating fluidized bed furnace where oxygen or air and high temperature reducing gas obtained from another reducing fluidized furnace are fed. The preheated chrome ore obtained is charged to reducing fluidized furnace with additional coal for carrying out reduction of ore. Preheater is used for reduction furnace to heat the reduction gas and the char obtained from the reduction furnace is separated in a separator. The reduced ore with small quantity of char is melted in a melting reduction furnace and the separated char is used as fuel in the preheating fluidized bed furnace. The process is claimed to be useful in recovering the heat in the process and also to eliminate the need for pelletization of chrome ore. JP57032351 discloses a method for enhanced pre-reduction of chromium and iron oxide in chrome ore to carbides in solid phase by heating ore with liquid or solid reducing agent in fluidized bed reactor. The internal temperature in fluidized bed is maintained in the range of 1000 °C to 1300 °C to convert chromium oxide in ore to Cr4C and FeO in ore to Fe3C. The reducing gas CO and H2 alongwith inert N2 is blown in fluidized bed furnace from bottom and heavy oil or solid carbon reducing agent such as slack coal is fed from the top. The patent claims to have produced product in the form of carbides of iron and chromium wherein the preliminary reduction of 50 to 60% is achieved. In patent application number JP59153863 a method of producing stainless steel directly from chrome ore wherein chrome ore is pre-reduced in fluidized bed furnace by gas reduction. The semi-reduced chrome ore obtained from fluidized bed is carried by nitrogen gas to a converter vessel wherein coal and oxygen are blown in to vessel through bottom tuyers. Pre-reduced chrome ore powder helps in reducing the heat and carbon required for reduction in converter as a result of pre-reduction. In patent application number JP59113131, a method for refining of ferrochromium and effective use of slag formed is disclosed wherein the chrome ore powder is reduced in solid state in rotary kiln using coal or coke reductant and subsequently melt-reduced by coal in converter type melting furnace. The slag obtained after melt-reduction is separated from

alloy, disintegrated, pulverized and magnetically screened so that very low chromium (0.6%) containing slag is used as soil-reforming agent and or additive to cement. In patent application number JP59089751, a method of producing ferrochromium is disclosed wherein chrome ore, reducing agent and slag making agent are smelt reduced in rotary furnace having horizontal or slightly inclined axis centre by blowing oxygen. The basicity of slag is maintained in the range of 1.3 to 2.0 in order to alleviate the fusion damage of furnace refractories and produce high quality ferrochromium with high yield. A process for producing high carbon chromium iron is disclosed in patent application number CN1673403 wherein chrome ore powder, iron ore powder, coke or coal powder, lime & silica are mixed, pressed into blocks or pellets and then reduced at 1350 °C to 1500 °C to produced reduced product which is pulverized naturally during the cooling process. The product is further separated by sieving to obtain granulized ferrochrome product. In patent application number US4441921, a process for production of ferrochromium is disclosed wherein ferrochrome fines may form part of feed materials to produce ferrochrome. Feed materials consisting of unreduced or partly reduced oxides of chromium and iron, carbonaceous reductant and one or more slagging agents are fed to molten bath of liquid slag and metal in transferred arc furnace in absence of oxygen or air to produce ferrochrome. The feed materials to bath are controlled such that the liquidus temperature of slag is maintained giving high ferrochrome yields. A method of reducing metal oxide in rotary hearth furnace is disclosed in patent application number US5567224, wherein a layer of mixture made from pulverized metal oxide and pulverized reductant is placed on the hearth surface and heated up to 1200 °C by radiation heat from an oxidizing flame generated from stationary burner by burning a fuel. A second layer of reductant is placed on the preheated prereduced layer of mixture to prevent the reoxidation of the metalized product and heated further to desired reduction temperature of 1350 °C for further reduction of iron and chromium oxides. The total time required for reduction is about one hour and the partially reduced mixture containing Chromium and iron

metal units can be used in manufacturing of alloyed iron, alloyed steel or alloyed stainless steel. A method for producing ferrochrome is disclosed in patent application number JP58016053, wherein mixture of chrome ore, reducing agent and fluxes in the form of pellets or briquettes is preheated and prereduced in a rotary furnace such as rotary kiln or shaft furnace or fluidized bed at temperature of 1800 °C. The preheated, prereduced agglomerates are then melted in horizontal axis rotating furnace by addition of coke and oxygen enriched gas is blown at the bottom of the furnace to produce ferrochrome. The reducing gases generated in melt furnace are used as heat source to carry out preheating and prereduction in rotary kiln. The process claims to have 25% less energy consumption compared to the electric smelting operation.
CN101144110 discloses a method for producing low phosphorus and low sulphur containing ferrochrome directly from chrome ore powder and coal wherein chrome ore powder, coal and flux are mixed to maintain an alkaline slag which increases the speed of reduction and improves the metal percent in product. Excess coal is used and reaction temperature is maintained at softening and melting temperature of agglomerates or pellets. Due to alkaline slag, the phosphorus and sulphur are restricted in slag. Excess coal and thermal stress causes natural pulverization of reduced product during cooling of the product and ferrochrome is separated from slag by sieving operation. A starting material composition is disclosed in patent application number JP62211344 wherein coke/coal to chrome ore to flux ration is maintained as 0.1-1:1:0.1-5 by weight to enable high speed smelting and reduction of chrome ore. The invention uses grain size of ore and reductant as <= 300 mesh and flux size <=50 mesh.
CN101608261 discloses a method for producing high carbon ferro-chrome by using single chromite powder ore, comprising the steps of: mixing single chromite powder ore with a bonding agent and a slag-forming agent; performing

cold pressing and pelletizing with the palletizing time of 10-15 minutes; screening the fabricated pellets, and warehousing eligible pellets as the finished product for production use; simultaneously recovering the screened powder and returning and mixing the recovered powder; air drying the fabricated pellets to obtain the finished pellets; directly adding the finished pellets together with a solvent and a reducing agent into a submerged arc furnace for production to obtain high carbon ferro-chrome. In the invention, 100 percent single powder ore is adopted for slagging in advance and then directly enters into the submerged arc furnace to produce the high carbon ferro-chrome without adding chrome lump ore or other powder ore.
CN1924056 discloses a C-Cr Fe fusing technology through ore heating furnace, which is characterized by the following: adding 8-20% silica, 4-10% coke and 1.5% dolomite in the ore heating furnace; setting the adding quantity of silica and coke at 2: 1; finishing the fusing procedure within 2000kwh.
CN101962714 discloses a production method of low-silicon low-titanium and high-carbon ferrochrome smelted by a submerged arc furnace, comprising: proportioning chrome ore, coke and flux silica at the weight ratio of 70:13:1, wherein the chrome ore contains 35-42 wt% of Cr203, the granularity of the chrome ore is 5-60 mm, the coke contains 82-86 wt% of fixed carbon, the granularity of the coke is 8-30 mm, the flux silica contains more than or equal to 97 wt% of SiO2, and the granularity of the flux silica is 10-30 mm; after mixing, add into a 12500KVA submerged arc furnace by a bin, and continuously smelt at 1550-1750 DEG C, wherein the basicity of slag is 1.1-1.5; producing the low-silicon low-titanium and high-carbon ferrochrome containing less than 0.3% of silicon and less than 0.03% of titanium by a pouring mode after separating slag.
WO2010103343 teaches a method for improved process for production of High Carbon Ferrochrome (HCFeCr) and Charge Chrome comprising: blending dried

chrome concentrate and chromite fines in all possible proportions to form raw feed ore; subjecting the raw feed ore to the step of mixing hydrated lime, molasses and bentonite as the binders to the said mixture to form briquetting mixture feed; forming briquettes from the said mixture by compaction.
In prior art technologies, the advantage can be seen in reducing the cost of electrical energy in ferrochrome production. Chromium recoveries were improved and cost of smelting was reduced by the preheating or pre-reduction methods. Optimal smelting furnace operation was achieved by the right charge mix, depending on the raw materials. The preheating and pre-reduction methods have improved the performance of SAF process; however none of it could replace the SAF process completely. It is worthwhile to mention that, most of these methods for ferrochrome production included the pre-treatment such as preheating or pre-reduction of the agglomerates obtained either by agglomeration of chromite ores such as for sintering or composite agglomerates obtained from agglomeration of chromite ores, reductant, flux and binder. However, none of the above processes (pre-heating, pre-reduction) used the pre-reduction process based on oxidized chromite ores which are obtained typically by pretreatment of chromite ore such as oxidation of chromite ore fines prior to its agglomeration. The oxidation process of chromite ore fines ensures a uniform degree of oxidation across the oxidized chromite ore particles which help in enhancing the reduction reactions during subsequent reduction process of oxidized chromite ore fines composite agglomerates. The smelt reduction methods based on the smelting of chrome ore in converter type furnaces need oxygen during the smelting processes which are reasonably costly methods. The prior art technologies were also designed for processing of FeO-rich South African chromite ores for producing the charge chrome with relatively lower chromium content, varying between 50 to 55 percent. On the other hand, the

MgO-rich Indian chromite ores are highly refractory in nature and therefore needs a different approach.
OBJECTS OF THE INVENTION
It is therefore an object of the present invention to propose an improved process for production of ferrochrome by smelt reduction of a mixture of sponge chrome or pre-reduced chromite agglomerates, chromite ore in any form such as lump or pellets, reductant coke and slag former such as quartzite in submerged arc furnace.
- Another object of the invention is to use powdered oxidized chromite ore with less than 150 μm size which were obtained by heat treatment of run of mine chromite ore of size 0 to 25mm in rotary kiln, cooled and ground to powder of less than 150 μm size for using in production of sponge chrome or pre-reduced chromite agglomerates. The heat treatment of 0 to 25 mm size chromite ore is carried out in rotary kiln in presence of air and typically at temperature upto 1100 °C preferably at 900 °C for 30 to 300 minutes duration in order to break the chrome spinel in chromite ore particles or lumps due to oxidation and conversion of FeO present in chrome spinel to Fe-sequioxide lamellae on the surface of chromite ore particles.
- A still further object of the invention is to make use of special slag composition in the new feed charge mixture for smelting obtained by mixing the sponge chrome or pre-reduced chromite agglomerates, chromite ore in any form such as lump or pellets, reductant coke and slag former such as quartzite such that the basicity [CaO+MgO]/[Si02+AI203] of charge mixture is in the range of 0.2 to 1.5 preferably 0.5 with MgO/AI203 in the range of 0.7 to 1.5 preferably 1.1 and MgO in the range of 7.0 to 14.0%, Al203 in the range of 5.0 to 15.0%, Si02 in the range of 8.0 to 22.0% and CaO in the range of 0.2 to 8.0% by weight in order to

produce high chromium ferrochrome having carbon in the range of 0.01 to 10% by weight in submerged arc furnace with high yield and less energy consumption.
SUMMARY OF THE INVENTION
According to a first aspect of the invention, there is provided an improved method of producing ferrochrome in submerged arc furnace by smelt reduction of new charge mixture. The charge mixture consisting of sponge chrome or pre-reduced chromite agglomerates obtained by pre-reduction of oxidized chrome ore composite agglomerates in rotary hearth furnace or tunnel furnace, chrome ore in lump or any other form such as briquettes or pellets, reductant coal or coke and slag forming agents.
According to the invention, an optimum charge composition in the charge mixture of sponge chrome, chrome ore in lump or any other form, carbonaceous reductant and slag formers is obtained by using slag composition in new charge mixture such that the basicity [CaO+MgO]/[Si02+AI203] in the charge mixture is in the range of 0.2 to 1.5 preferably 0.5 with MgO/AI203 in the range of 0.7 to 1.5 preferably 1.1 and MgO in the range of 7.0 to 14.0%, AI2O3 in the range of 5.0 to 15.0%, Si02 in the range of 8.0 to 22.0% and CaO in the range of 0.2 to 8.0% by weight in order to produce high chromium, ferrochrome product having carbon in the range of 0.01 to 10% by weight in submerged arc furnace with high yield and less energy consumption.
According to the invention, process for production of high chromium ferrochrome containing carbon in the range of 0.01 to 10% by weight is developed by smelt reduction of mixture of sponge chrome or pre-reduced chromite agglomerates, chromite ore in lump or any other form, coke and quartzite in submerged arc furnace. The slag composition in the mixture is maintained such that the basicity

[CaO+MgO]/[Si02+AI203] of charge mixture is in the range of 0.2 to 1.5 preferably 0.5 with MgO/AI203 in the range of 0.7 to 1.5 preferably 1.1 and MgO in the range of 7.0 to 14.0%, Al203 in the range of 5.0 to 15.0%, Si02 in the range of 8.0 to 22.0% and CaO in the range of 0.2 to 8.0% by weight in order to produce high chromium, ferrochrome having carbon in the range of 0.01 to 10% by weight in submerged arc furnace.
The sponge chrome or pre-reduced chromite agglomerates were obtained by direct reduction of oxidized chromite ore composite agglomerates in moving hearth furnace such as rotary hearth furnace or tunnel kiln. The oxidized chromite ore composite agglomerates were prepared from mixture of powdered oxidized chromite ore, powder reductant coal and powdered slag formers such as quartz, lime and bentonite binder. The powdered oxidized chromite ore was obtained by heat treatment of run of mine chromite ores in the size range of 0 to 25mm in rotary kiln, cooling the product followed by grinding of oxidized chrome ore to powdered form having size less than 150 μm. The heat treatment of 0 to 25mm size run of mine chromite ore was carried out in rotary kiln at temperatures of less than 1100 °C, preferably at 900 °C for residence time in the range of 30 to 300 minutes preferably 30 minutes in presence of air in order to oxidize the FeO present in chrome spinel to Fe-sequioxide (Fe203). The Fe-sequioxide phase formed during oxidation process helps in enhancing the direct reduction process of oxidized chromite ore fines composite agglomerates in moving hearth furnace. The powdered oxidized chromite ore was mixed with carbonaceous reductant such as coal, quartz or quartzite and lime as slag formers and bentonite as binder to obtain the oxidized chromite composite agglomerates. The reductant, slag formers and binder were added in predetermined ratio to form composite agglomerates for achieving enhanced reduction of oxidized chromite ore fines and formation of low temperature slag during direct reduction of oxidized chromite ore composite agglomerates in moving hearth furnace to obtain sponge chrome or pre-reduced chromite

agglomerates. The sponge chrome thus obtained is mixed with chromite ore in lump or any other form, carbonaceous reductant coke and quartzite as slag former in such a way that the desired basicity and composition of slag forming agents in mixture is maintained to achieve enhanced smelt-reduction in submerged arc furnace with high yield and less energy consumption to produce high chromium ferrochrome having carbon in the range of 0.01 to 10% by weight.
BRIEF DESCRIPTION F THE ACCOMPANYING DRAWINGS
The invention is explained in greater details with the accompanying drawing
Figure 1 presents a comparison of optical micrographs of chromite ore particles before and after oxidation in rotary kiln, the microstructure of chromite ore particles after oxidation exhibiting an exolved lamellae of Fe-sequioxide on the surface of chromite grains developed due to oxidation process.
Figure 2 Microstructures of oxidized chromite particles of different size showing exolved Fe203 lamellae on the surface of chromite grains.
Figure 3 shows micrographs of sponge chrome or pre-reduced chromite agglomerates obtained by pre-reduction of oxidized chromite composite agglomerates in moving hearth furnace, the micrograph exhibiting three phases namely reduced Fe-Cr metal, slag and partially reduced chromite particles.
Figure 4 Image of a charge mixture for smelting in submerged arc furnace (a) typical conventional charge mixture consisting of sintered pellets, hard chromite lumps, friable chromite lumps, coke and quartzite (b) New charge mixture according to the invention, consisting of sponge chrome or pre-reduced chromites, hard chromite lumps, friable chromite lumps and coke.

DETAILED DESCRIPTION OF THE INVENTION
Chromite ore of size 0 to 25 mm are obtained by screening of the run of mine chromite ore. The 0 to 25 mm size chromite ore is subjected to heat treatment prior to grinding and agglomeration in order to obtain the oxidized chromite ore. The heat treatment of 0 to 25 mm size ore is carried out in presence of air in rotary kiln furnace at temperatures of less than 1100 °C preferably at 900 °C for residence time in the range of 30 to 300 minutes preferably less than 30 minutes in order to oxidize the FeO present in chrome spinel to Fe-sequioxide (Fe203). The optical micrograph of chromite ore particles before and after oxidation process is shown in Fig. 1. The optical micrograph of chromite ore particles after oxidation process (Fig. lb) shows the Widmanstätten structure of bright lines on chromite mineral. It can be seen that, the Fe-sequioxide phase has exolved on the surface of chromite particles. During oxidation reaction, the FeO present in chrome spinel gets oxidized to Fe203 as exolved Fe-sequioxide phase and in turn cation vacancies are generated in the spinel lattice. In an oxidizing atmosphere, in addition to the intrinsic driving force for phase transformation, the imposed oxygen chemical potential promotes the diffusion of Fe2+ ions from the core of chromite grain towards the solid-gas interface on the surface of the grain. This outward diffusion of Fe2+ cations and oxidation to Fe3+ cations takes place via following reactions:


O2- Vn
Where ° represents oxygen anions on the cubic closed packed lattice, cat is
cation vacancy, h. is hole, and e is electron. Oxidation of chromite ore also results in opening up of the spinel structure. It can be seen from Fig. lb that, the Fe-sequioxide phase has exolved on the surface on the chromite particles. The EDX analysis of oxidized chromite particles has indicated that the bright lines are iron rich phases and the matrix is magnesium rich phase. It can be concluded from above that iron is precipitating from the matrix on {111} crystallographic planes. The oxidation of chromite resulting in Widmanstatten structure is in accordance with established theory of oxidation and precipitation of sequioxide phase which states that preferential crystallographic orientation of Widmanstätten lamellae was along the {111} plane of spinel matrix phase. The {111} planes of spinel and the {001} planes of sequioxide have a similar close packing arrangement of oxygen ions, which account for the common orientation of sequioxide lamellae along the {111} plane of the spinel matrix. Tapered terminals develop at the intersection of two or more lamellae, which are indicative of a diffusion-controlled process. This newly formed Fe-sequioxide phase on chromite particles along with the vacancies generated during oxidation improves the reactivity of chromite ore during reduction. The optical micrographs of oxidized chromite particles of coarser size (upto 10 mm) are shown in Fig. 2. The Fe-sequioxide phase formed during oxidation process helps in enhancing the subsequent reduction process of oxidized chromite ore composite agglomerates in moving hearth furnace such as rotary hearth furnace.
The oxidized chromite ore in the size range of 0 to 25 mm obtained from the rotary kiln process are cooled and grinded further to less than 150 μm size. The selection of 0 to 25 mm size ore for oxidation process in rotary kiln helps in reducing the dust load during the operation of rotary kiln. The grinded powdered oxidized chromite ore is then mixed with powdered carbonaceous reductant such as coal, powdered quartz or quartzite and powdered lime such as calcined lime

or hydrated lime as slag formers and bentonite as binder. The typical chemical composition of different raw materials such as run of mine chromite ore (0-25mm), powdered oxidized chromite, coal (reductant), quartz, calined lime or hydrated lime (slag formers) and bentonite (binder) used for pre-reduction process in moving hearth furnace are shown in Table 1. The reductant, slag formers and binder are added in predetermined ratio to form oxidized chromite composite agglomerates for achieving enhanced reduction and formation of low temperature slag during reduction in moving hearth furnace. The typical chemical composition of oxidized chromite composite agglomerates used in the moving hearth furnace obtained after mixing and agglomerating predetermined ratio of raw materials is given in Table 1. The formation of low temperature slag during reduction in moving hearth furnace is achieved with the help of lowering the slag liquidus due to use of lime as slag former and also due to reduction of Fe-sequioxide (Fe203) phase in oxidized chromite ore formed during oxidation of FeO at low temperatures. Since the iron is present as separate exolved Fe-sequioxide (Fe2O3) phase in powdered oxidized chromite ore, it reduces quickly to FeO at low temperatures during reduction. This newly formed FeO acts as slag forming agent and enhances the diffiusion of iron and chromium ions in slag. One of the important feature of the newly formed FeO is that it further reduces to Fe3C (iron carbides) at low temperatures and the carbon present in iron carbides helps in reduction of chromium oxides thus further enhancing the reduction process. The composite agglomerates obtained by mixing and agglomerating the powdered oxidized chromite ore, reductant coal, slag formers and binder can be of spherical shape pellets or briquettes or lump or any other form of agglomerates/These composite agglomerates are then subjected to drying process for 30 to 120 minutes in the temperature range of 100 °C to 200 °C. The drying process is carried out in continuously moving steel grate belt dryer which typically uses hot air from hot air generator as drying medium. The dried oxidized chromite composite agglomerates are then fed to reduction process in moving hearth furnace. During the reduction process in moving hearth

furnace, the oxidized chromite composite agglomerates are heated up to temperature of less than 1450 °C for 15 to 860 minutes duration preferably for 30 minutes to obtain the sponge chrome or pre-reduced chromite agglomerates.
The typical chemical composition of sponge chrome or pre-reduced chromite agglomerates is given in Table 2 and Fig. 3 shows the typical micrographs of sponge chrome or pre-reduced chromite pellets and pre-reduced chromite briquettes obtained by reduction of oxidized chromite composite pellets or briquettes in moving hearth furnace. The micrographs shows three phases namely reduced Fe-Cr metal, slag and partially reduced chromite particles in the reduced agglomerates. The sponge chrome or pre-reduced chromite agglomerates are used in ferrochrome production in combination with chromite ore of any other form results in low energy consumption in ferrochrome production and also improves the productivity of the submerged arc furnaces due to presence of highly metalized iron and chromium in it. In the present invention, an improved process for production of ferrochrome is developed by smelt reduction of mixture of sponge chrome or pre-reduced chromite agglomerates obtained above along with chromite ore such as hard chromite lumps and or friable chromite lumps, reductant coke and slag former such as quartzite. The typical chemical composition of various raw materials used for smelting is given in Table 2. Conventionally ferrochrome is produced by smelting the mixture of sintered chromite pellets (with no prior treatment of chromite ore before peptization), chromite ores in hard or friable lump form, reductant coke and slag former such as quartzite. In the present invention, the sponge chrome or pre-reduced chromite agglomerates obtained by direct reduction of oxidized chromite ore is one of the essential constituent of raw material mixture for smelting in combination with chromite ore in any form such as hard chromite lump or friable chromite lump. The mixture of sponge chrome or pre-reduced chromite agglomerates alongwith hard chromite lump, friable chromite lump, reductant coke and quartzite is prepared for smelting in submerged arc furnace

in such as way that the basicity [CaO+MgO]/[Si02+AI203] of charge mixture is maintained in the range of 0.2 to 1.5 preferably 0.5 with MgO/AI203 in the range of 0.7 to 1.5 preferably 1.1 and MgO in the range of 7.0 to 14%, Al203 in the range of 5 to 15%, Si02 in the range of 8.0 to 22.0% and CaO in the range of 0.2 to 8.0% by weight in order to produce high chromium, ferrochrome having carbon in the range of 0.01 to 10% by weight with high yield and less energy consumption.
As an example in order to practice the method described in the present invention, test results of smelting conventional charge mixture and new charge mixture in submerged arc furnace with desired slag composition in the improved process are described below. Figure 4a shows the image of typical conventional charge mixture consisting of sintered pellets, hard chromite lumps, Friable chromite lumps, coke and quartzite and Figure 4b shows the image of new charge mixture consisting of sponge chrome or pre-reduced chromites, hard chromite lumps, friable chromite lumps and coke. In the new charge mixture the sintered pellets were replaced with sponge chrome. It should be noted that, in the example case the sintered pellets were completed replaced with sponge chrome or pre-reduced agglomerates, however partial replacement of sintered pellets can also be practiced as long as the desired slag composition in the new charge mixture is maintained and sponge chrome or pre-reduced chromite agglomerate is used as one of the constituent in the charge mixture for submerged arc furnace process. The smelting tests for the example case were carried out in a 15 kg batch submerged arc furnace and the chemical composition of various raw materials for smelting are given in Table 3. The basicity [CaO+MgO/Si02+AI2O3] in the new charge mixture using sponge chrome was maintained at 0.5 with MgO - 10.1%, CaO - 2.8%, Si02-17.5% and Al203 -9.4% by weight. The desired basicity in slag is obtained by adjusting the quantity of hard chromite lumps and sponge chrome in the new charge mixture. The smelting process in submerged arc furnace was carried out at temperature in the

range of 1500 °C to 1900 °C. The test conditions, composition of ferrochrome metal and slag obtained and smelting parameters results for conventional charge mixture and new charge mixture are presented in Table 4. It can be seen from Table 4, that the power required for smelting and the smelting duration for the new charge mixture was significantly lower due to enhanced smelting process by use of sponge chrome or pre-reduced chromite agglomerates which were obtained by pre-reduction of oxidized chromite composite agglomerates in moving hearth furnace. The chromium yield obtained by smelting new charge mixture has improved from 87.5% to 90.65% by weight. The energy requirement for smelting new charge mixture was about 22% less compared to the conventional charge mixture. The percentage chromium in the ferrochrome product obtained was higher up to 63% in case of new charge mixture compared to 61.5% in conventional charge mixture. The increase in chromium content and chromium yield in smelting new charge mixture is due to use of sponge chrome or pre-reduced chromite agglomerates and desired slag composition which helps in enhancing the smelt-reduction process and improves the slag metal separation in submerged arc furnace process. Thus, the use of sponge chrome or pre-reduced chromite agglomerates obtained by direct reduction of oxidized composite chromite agglomerates in moving hearth furnace along with the proper slag composition in the new charge mixture results in improving the ferrochrome production process in order to produce high chromium ferrochrome product at increased chromium yield with less energy consumption in submerged arc furnace process.

WE CLAIM
1. An improved method for production of ferrochrome containing carbon in the range of 0.01 to 10% by weight by smelt reduction of a charge mixture consisting of sponge chrome or pre-reduced chromite agglomerates, chromite ore in any form such as lump or agglomerate, reductant coke and slag former such as quartzite in submerged arc furnace, wherein the charge mixture has slag composition in such that the basicity [CaO+MgO]/[Si02+AI203] of charge mixture is in the range of 0.2 to 1.5 with MgO/AI203 in the range of 0.7 to 1.5 and MgO in the range of 7.0 to 14.0%, Al203 in the range of 5.0 to 15.0%, Si02 in the range of 8.0 to 22.0% and CaO in the range of 0.2 to 8.0% by weight.
2. The method as claimed in claim 1, wherein the sponge chrome or pre-reduced chromite agglomerates are produced by direct reduction of dried oxidized chromite composite agglomerates in a moving hearth furnace.
3. The method as claimed in claim 2, wherein the dried oxidized chromite composite agglomerates are obtained by mixture of powdered oxidized chromite ore, powdered coal, powdered quartz or quartzite, powdered lime or limestone and powdered bentonite as binder.
4. The method as claimed in claim 3, wherein the powdered oxidized chromite ore is obtained by heat treatment of 0 to 25 mm size run of mine chromite ore in rotary kiln, cooling the oxidized ore and grinding it further to less than 150 µm size in dry or wet mill operation.

5. The method as claimed in claim 4, wherein the 0 to 25 mm size run of mine chromite ore have Cr203 content in the range of 25 to 65% (by weight) and Fe(t) in the range of 10 to 25% (by weight).
6. The method as claimed in claim 4, wherein the heat treatment of 0 to 25 mm size run of mine chromite ore is carried out at a temperature of less than 1100 °C preferably at 900 °C for residence time in the range of 30 to 300 minutes in rotary kiln in presence of air atmosphere.
7. The method as claimed in claim 4, wherein the powdered oxidized chromite ore has Fe-sequioxide phase lamellae on atleast one of the chromite particles which is iron rich phase in the microstructure of oxidized chromite particle.
8. The method as claimed in claim 3, wherein the dried oxidized chromite composite agglomerates can be pellets or briquettes or any other form of agglomerates which are obtained by agglomerating the mixture of powdered oxidized chromite ore, reductant coal, slag formers quartz or quartzite, lime or limestone, bentonite binder and drying the composite agglomerates for 20 to 120 minutes at temperature in the range of 100 °C to 200 °C in steel grate belt dryer or any other suitable process.
9. The method as claimed in claim 8, wherein the dired oxidized chromite composite agglomerates are characterized by presence powdered oxidized chromite ore as one of the constituent. .'
10.The method as claimed in claim 3, wherein the reductant is carbonaceous agents such as powdered coal, powdered coke, anthracite or charcoal and slag formers are powdered quartz or quartzite and powdered lime can be calcined lime or hydrated lime.

11.The method as claimed in claim 10, wherein the the reductant carbonaceous agent has fixed carbon typically in the range of 60 to 87% by weight and volatile matter in the range of 5 to 45% by weight.
12.The method as claimed in claim 3, wherein the binder is of organic or inorganic base for example bentonite, dextrose, ligno-sulphonate, molasses or dextrin
13.The method as claimed in claim 2, wherein the direct reduction is solid state reduction of powdered oxidized chromite ore particles in dried oxidized chromite composite agglomerates using carbonaceous reductant such as coal, coke, anthracite or charcoal present in dried oxidized chromite composite agglomerates.
14.The method as claimed in claim 2, wherein the sponge chrome or pre-reduced chromite agglomerates are obtained by direct reduction of dried oxidized chromite composite agglomerates in moving hearth furnace such as rotary hearth furnace or tunnel kiln.
15.The method as claimed in claim 14, wherein the reduction process of dried oxidized chromite composite agglomerates in moving hearth furnace is carried out at temperature of less than and or equal to 1450 °C for residence time in the range of 15 minutes to 80 minutes in a CO containing atmosphere.
16.The method as claimed in claim 14, wherein the sponge chrome.or pre-reduced chromite agglomerates are characterized by presence Fe-Cr metal or metallic phase in atleast one of the sponge chrome agglomerate or pre-reduced chromite agglomerate.

17.The method as claimed in claim 14, wherein the sponge chrome or pre-reduced chromite agglomerate has typically three phases namely reduced metal (Fe-Cr metal) phase, slag phase and partially reduced chromite particles.
18.The method as claimed in claim 14, wherein the sponge chrome or pre-reduced chromite agglomerate is also characterized by percent chromium (Cr) metallization in the range of 10.0 to 75.0% by weight and percent iron (Fe) metallization in the range of 30.0 to 97.0% by weight in it.
19.The method as claimed in claim 1, wherein the new charge mixture is smelted in submerged arc furnace at temperature in the range of 1500 °C to 1900 °C
20.The method as claimed in claim 1, wherein the ferrochrome product can be high carbon ferrochrome or low carbon ferrochrome having carbon in the range of 0.01 to 10% by weight.
21.The method as claimed in claim 1, wherein the chromite ore can be sintered pellets and or hard chromite lumps and or friable chromite lumps.

ABSTRACT

The invention relates to an improved method for production of ferrochrome containing carbon in the range of 0.01 to 10% by weight by smelt reduction of a charge mixture consisting of sponge chrome or pre-reduced chromite agglomerates, chromite ore in any form such as lump or agglomerate, reductant coke and slag former such as quartzite in submerged arc furnace, wherein the charge mixture has slag composition in such that the basicity [CaO+MgO]/[Si02+AI203] of charge mixture is in the range of 0.2 to 1/5 with MgO/AI203 in the range of 0.7 to 1.5 and MgO in the range of 7.0 to 14.0%, Al203 in the range of 5.0 to 15.0%, Si02 in the range of 8.0 to 22.0% and CaO in the range of 0.2 to 8.0% by weight.