FIELD OF THE INVENTION
The invention relates to a method for smelting of chrome ore or chrome ore concentrates in submerged arc furnace to produce carbon ferrochrome containing low silicon (<1.5% by weight) and high chromium (up to 70% Cr maximum by weight).
BACKGROUND OF THE INVENTION
Chromite ore and or chrome ore concentrates obtained by physical processing of mined chrome ore is major chromium bearing raw material for production of Ferrochrome which is used in production of alloy and stainless steel as prime raw material. Ferrochrome is mostly produced by carbothermic smelt reduction of chromite ore lumps or agglomerates. Chromite ore in the form of lump are directly charged to the submerged arc furnace whereas fine chromite and chrome ore concentrates are fed after their agglomeration in the form of sinter pellet or briquette or any other form of agglomerate or combination of lumps and agglomerates. The smelt reduction is generally carried out using coke as the reductant and quartzite and/or bauxite as the flux. Chromite ores in the form of sintered pellets or briquettes or lumps having Cr2O3 content less than 50% by weight are conventionally smelt reduced in submerged arc furnaces to produce Ferrochrome having greater 60% chromium content and silicon greater than 1.5% by weight. In conventional process, chromite ore batch is prepared comprising of chromite ores having Cr2O3 content less than 50% by weight in the form of sintered pellets, briquettes, lumps and their combinations. The required quantity of reductants consisting of coke or coal for reduction of iron and chromium oxides (FeO, Fe2O3, Cr2O3) are mixed with ore batch. Quartzite flux is then added to prepare composite batch comprising of ores in the form of sintered pellets and or briquettes and or lumps and reductant. Ferrochrome product

having chromium content less than 63% by weight and silicon typically greater than 1.5% by weight and carbon in the range of 6-8% by weight is then produced by smelt reduction of composite batch (chromite ores, reductant and fluxes) in submerged arc furnace. However, Ferrochrome product with low silicon <1.5% by weight, carbon in the range of 6-10% by weight and chromium greater than or equal to 63% chromium is desired for cost effective refining operations while production of alloy and stainless steel. Therefore in the present invention a process is disclosed for producing low silicon (<1.5% weight), carbon in the range of 6-10% by weight and chromium up to 70% (max) by weight in submerged arc furnace smelting process. The method comprise of smelting of initial starting material (batch) comprising of chromite ore or concentrates in the form of lump or agglomerates or their combinations wherein atleast 25% of input chromium in charge mix is comprised from chromium bearing materials or ores containing at-least 43% or more by weight chromium oxide (Cr2O3) and silica (SiO2%) less than 8% by weight preferably less than or equal to 6.0% by weight. The high chromium (less than 70% Cr), low silicon (<1.5%) and carbon (≤9.0%) is controlled by combination of methods as using sub-stoichiometric carbon containing starting input material in the form of charge mix or batch with low lime slag composition (CaO<13.0%) or more preferably in the range of 4.0 to 10% by weight and basicity ratio (CaO by weight +MgO by weight/SiO2 by weight) equal to or less than 1.2, Input MgO/Al2O3 weight ratio in composite batch= 0.9-1.8, slag MgO/Al2O3 weight ratio in the range of 0.8-1.1, slag CaO/SiO2 in the range of 0.08-0.5 and slag CaO/Al2O3 greater than 0.16 and preferably in the range of 0.16 to 0.35 by using a combination of lime or limestone and Bauxite and or addition of external compound comprising of metal and metal oxide wherein the metal or metal oxide is preferably Fe or FeO or mixture thereof such as in various mill scales and or by addition of fluxing agent such as ladle furnace slag from a steel plant comprising of predominantly oxides of calcium and aluminum.

There are five primary processes that are currently in use for the production of ferrochrome. These include conventional process with open or semiclosed submerged arc furnaces, the conventional process with closed submerged arc furnace, the Outokumpu process, the DC Arc route and the Premus process. Convention process with open or semiclosed submerged arc furnace, the primary advantage is lower capital cost and flexibility in terms of raw materials use and handling during smelting. The main disadvantage of this process is that it has the lowest efficiencies. Conventional process with closed submerged arc furnace is more energy efficient as compared to open or semiclosed furnaces but also has higher energy consumption and requires costly metallurgical coke for the smelt reduction process. Outokumpu process uses sintered pellets and preheating of the charge to the submerged arc furnaces which results in reduced specific energy consumptions and improved chromium recoveries. In DC arc furnace smelting process, 100% chromite fines with minimum or no pre-processing can be fed to SAF process, thus eliminating the need for an expensive agglomeration plant and inferior grades of reductants can be used in this process. Premus process uses pre-reduced composite pellets obtained from rotary kiln by partial pre-reduction of the chromium oxide and iron oxide. The pre-reduced composite pellets are then hot charged to submerged arc furnace for Ferrochrome production. Two types of Ferrochrome product namely charge chrome and Ferrochrome are produced from submerged arc furnace process. Charge chrome contains less than 60% chromium whereas Ferrochrome contains high chromium in the range of 60-70% and carbon 6-8% by weight. The silicon content in typical high carbon ferrochrome is in the range of 2.5 to 4.0% (max).
A method for production of low silicon (<0.3%) low titanium (<0.03%) and high carbon (6-8%) Ferrochrome by smelt reduction process in submerged arc furnace is disclosed in patent application number CN101962/14 wherein chrome ore with 35-42% Cr2O3 having size range of 5-60 mm is smelted with

coke and silica flux having more than 9% SiO2 to obtain ferrochrome alloy with Cr in the range of 55-65%, Ti < 0.03%, Si<0.3% and carbon in the range of 6-10% by weight. The basicity of slag of smelting furnace is maintained in the range of 1.1-1.50 and it is claimed that the silicon content in the high carbon Ferrochrome alloy does not exceed 0.3% by mass of silicon and titanium not greater than 0.03% by weight.
In patent application number JP2011094209, a process is disclosed to produce low carbon ferrochromium with silicon content less than 0.25% by weight using electric arc or ladle furnace. Chrome oxide and iron oxide are melted with silicochrome thus reducing the oxides using the silicon reductant in silicochrome followed by deoxidation using aluminum alloy resulting in low carbon ferrochrome alloy. The slag contains calcium oxide (0.5-1.5%), magnesium oxide (5-15%) and alumina. The CaO/Al2O3 ratio is in the range of 0.5-1.5 and the ferrochrome alloy contains oxygen and vanadium less than 0.1% by weight.
A method for recovering ferrochrome metal from slag is disclosed in patent application number WO2011045755, wherein slag is mixed with molten ferrochrome metal having silicon so that the silicon in metal reduces the Cr2O3 in slag thus resulting in increased chromium recovery from 70-84% to 90-95%. It is claimed that additional Cr2O3 may also be added to achieve silicon less than 0.5% by weight.
A method to produce low-silicon, low-sulfur and high-carbon ferrochrome is disclosed in patent application number JP07003379 wherein, the molten high carbon ferrochrome from electric arc furnace is received in ladle and chromium ore, scale and slag making agent such as CaO and/or MgO are added and the mixture is agitated, desiliconized and desulfurized. The scales

used may be hammer scale produced in forging and hammering or a mill scale.
In patent application number US4971622, a method for sulfur and silicon control is disclosed wherein the pre-reduced chromite with slag forming components and residual carbonaceous reductant resulting from the pre-reduction process is melted in melter by adjusting the CaO content of final slag. Oxygen reduction potential of the furnace is controlled by controlling the oxidation/ reduction potential of slag and by controlling the air ingress to the vessel. Silicon is also controlled by controlling the activity of silica in slag by adding CaO in slag.
In US3765871, a method for controlling the amount of silicon in high carbon ferrochrome is disclosed wherein crushed raw material from the group of magnesia clinker, dolomite, magnesite, dunite, serpentine, corundum, bauxite, hydrated aluminous ore and aluminous shale are agglomerated and smelted with coke to produce ferrochrome. The silicon concentration of high carbon ferrochrome is controlled by mixing chrome ore with material which is rich in Mg and Al oxide, at a weight ratio of MgO to Al2O3 varying from 0.6 to 2.0, which corresponds to a silicon concentration in the product from 0.5 to 9.0 weight percent.
In US3301669, a two-step process to produce high chromium containing ferrochrome alloy with low silicon is disclosed, wherein in step 1, primary ferrochrome silicon alloy is treated with an intermediate slag from step 2 to produce chromium enriched secondary ferrochrome silicon alloy and discard slag. In step 2, the chromium enriched secondary ferrochrome silicon alloy is treated with chrome ore and lime melt to produce intermediate slag (for step 1) and high chromium low silicon ferrochrome alloy.

In US3158464, a refining process is disclosed wherein the high silicon and low sulfur alloy produced from arc furnace is tapped in ladle, chrome ore is added and blown with oxygen to produce low silicon low sulfur ferrochrome alloy and a recyclable chromium rich slag.
A process for the manufacture of low silicon ferrochrome or ferromanganese from high silicon containing starting material (SiMn-15-35% Si and ferrochrome silicon: 30-60% Si) is disclosed in patent application number DE1608613, wherein basic slag former quicklime, MgO or calcined dolomite and optionally a flux CaF2 (5-40% by weight) with chrome ore is mixed with the initial high silicon ferrochrome and ferrochrome silicon and melted in a melting pot. The starting ferrochrome or ferromanganese with high Si content was ground to less than 1 mm and melted. The process claimed to result in alloy of ferrochrome or ferromanganese with silicon content of about 1.5% by weight.
In patent application number GB-996221, process for producing low sulfur ferrochromium (<0.015%) and silicon less than 2% by weight is disclosed wherein, mixture of chrome ore, carbon and slag forming material is smelted to give slag having molar ratio of CaO+MgO = 2.5-8.5 and CaO/SiO2 > 0.75, viscosity of 25 poise at temperature of at-least 1575 0C and 95% of ferrochrome produced is at-least in the form of (Fe,Cr)7C3. Slag used was containing 13-35% CaO, MgO-22-35%, Al2O3: 25-42%, SiO2: 9-28%.
Most of the above prior art technologies for producing low silicon ferrochrome product are either based on refining method of high carbon high silicon Ferrochrome or silico-chrome as starting material and use additives such as metal oxides (Cr2O3, FeO, etc) in the form of ferrochrome slag or other additives in ladle or atleast involves some refining operation outside submerged arc furnace or involves intermittent production of low and high

silicon product in alternate tapping operation. Few submerged arc furnace processes disclosed in prior art comprise of starting material having maximum input chromium contributed from chrome ores with less than 42% Cr2O3 and also use high lime slag (CaO>13% by weight) with CaO/SiO2 ratio > 0.75. The present invention discloses a method for producing low silicon (<1.5%) and high chromium (up to 70%) carbon (<10.0%) breakable Ferrochrome using slag composition in combination of low lime, low silica activity slag and also control the carbon content preferably to less than 9.0% in Ferrochrome product by use of sub-stoichiometric carbon containing batch, increase metal yield and also to limit the chromium oxide (Cr2O3%) loss to slag to less than 12 to 15% by weight.
OBJECTS OF THE INVENTION
It is therefore an object of the present invention to propose a submerged arc furnace method for smelt reduction of chromite ore or chrome ore concentrates in the form of lump or agglomerate with silica less than 6.0% by weight in combination with other raw materials such as coke, fluxes and or additive to produce a breakable low silicon (Si<1.5%) and high chromium (up to 70% max) Ferrochrome product with carbon in the range of 6 to 10% by weight preferably below 9.0% carbon.
Another object of the present invention is to propose a novel slag composition with low lime slag (<13%) preferably in the range of 4.0 to 10% by weight and basicity ratio (CaO by weight +MgO by weight/SiO2 by weight) equal to or less than 1.1, slag CaO/SiO2 in the range of 0.08-0.5, slag CaO/Al2O3 in the range of 0.1-1.0 preferably greater than 0.16 and more preferably by using fluxes and additive to minimize the silica activity below 0.022 in slag and control the carbon content in Ferrochrome product to less than 10% preferably less than or equal to 9.0% by weight.

A still further object of the present invention is to propose starting material charge mixture for smelting which comprise of sub-stoichiometric carbon with respect to the total carbon requirement for reduction of metal oxides and residual carbon in the Ferrochrome product.
A yet further object of the present invention is to produce a low silicon, high chromium and high carbon ferrochrome alloy which can be breakable to desired size of lumps.
A still further object of the present invention is to disclose a charge mixture to produce very low silicon (<0.5% Si) Ferrochrome product with high chromium (up to 70% by weight max) preferably greater than 63% and carbon in the range of 6 to 10% preferably below 9.0% by weight by smelting of beneficiated chrome ore without increase in typical total energy consumption in ferrochrome production.
SUMMARY OF THE INVENTION
Accordingly, there is provided A method of smelting chromite ore and or chrome ore concentrates in submerged arc furnace process to produce low silicon (Si<1.5%) and high carbon containing, breakable Ferrochrome alloy having carbon content less than 10.0% by weight, comprising ; smelting charge mix (batch) consisting of chromite ores in which at-least more than 25% of input chromium in charge mix (batch or starting material for smelting) is comprised of chromium bearing materials/concentrates or ores containing at-least 43% or more by weight chromium oxide (Cr2O3) and silica (SiO2 %) less than 6% by weight, wherein the smelting the charge mix further comprising sub-stoichiometric input carbon with respect to the total theoretical carbon required for reduction of chromium oxide, iron oxides, silica and the residual carbon in ferrochrome alloy such that the carbon

content in the low silicon and high chromium ferrochrome product is controlled within 10% by weight, wherein the chromite ore or chrome ore concentrates obtained by physical processing of mined chrome ore are smelted in submerged arc furnace either in lump or agglomerate form such as sinter pellet and or briquette or any other form of agglomerate and their combinations with reductant (coke or coal) and slag former, and wherein the desired slag composition is low lime slag with CaO less than 13% by weight preferably in the range of 4.0-10% by weight with silica activity below 0.022 and the slag basicity (B3) less than or equal to 1.2 with composition of slag as CaO<13.0%, CaO/SiO2 = 0.08-0.50, input MgO/Al2O3 in composite batch equals to 0.9-1.8 and slag CaO/Al2O3 greater than 0.16 and preferably in the range of 0.16 to 0.35, MgO= 20-32%, Al2O3=25-35% by weight.
DETAILED DESCRIPTION OF THE INVENTION
High carbon Ferrochrome with carbon in the range of 6-8% is mostly produced in submerged arc furnace process. The silicon content in typical Ferrochrome product is 2 to 4% by weight and chromium content is generally less than or equal to 63% by weight. The process is energy intensive and requires smelting energy in the range of 3400-3700 kWh per ton of metal produced depending on open, semi-closed or closed type of furnace smelting operation. Chromite ores in the form of lumps are fed directly to the furnace whereas fines are agglomerated in the form of sinter pellet or briquette. Chromite concentrates of different grades are basically obtained by physical processing or beneficiation of the mined chrome ores wherein the silica and clay gangue is separated to enrich the concentrates in Cr2O3 and Cr/Fe ratio. Mined chromite ores with Cr2O3% content greater than or equal to 45% are either fed as lumps or in agglomerate form (pellet/briquette) to submerged arc furnace. However, mined chrome ores having Cr2O3 % by weight in the range of 25-40% are beneficiated to obtain chrome concentrates with Cr2O3

in the range of 50-60% and silica (SiO2) < 5.0% by weight. Chrome concentrates are fine materials (size <1.0mm) therefore typically agglomerated with other low grade materials and fed in the form of agglomerate such as sinter pellet or briquettes. In the present invention, the chromium bearing agglomerates are prepared by using chrome ore or chrome concentrates obtained by beneficiation of mined chrome ores and smelted with other raw materials coke (reductant) and quartzite (flux). Chromite ores or chromite concentrates with Cr2O3%, greater than 45% by weight and silica less than 6% by weight can be used either in the form of lump or agglomerates (sinter pellet and or briquette) obtained from chromite ore or concentrates wherein the agglomerate contains atleast 25% or more input chromium comprised from chromite ore or chromite concentrates with Cr2O3 greater than 45% by weight and silica less than 6.0% by weight. The chemical composition of chrome ore and chrome ore concentrates used for preparation of sintered pellets and the corresponding chemical composition of sintered pellets is given in Table 1. The chrome ore and concentrate were ground to pellet size (i.e. 80% passing through 75 micron size) for preparation of sintered pellets. The ground chrome ore was then mixed with about 2.0% by weight coke and bentonite binder to produce green pellets. The sintered pellets were prepared by heating the dried pellets at temperature of about 1300 0C for one hour. The chemical composition of sintered pellets obtained by using chrome ore and chromite concentrates and other chromium bearing raw materials used for smelting in submerged arc furnace. The chemical composition of coke reductant and fluxes (lime and quartzite) are provided in Table 3. The input batch for smelt reduction in submerged arc furnace was comprised of pre-determined quantities of sintered pellets or briquettes made from chrome ore or chrome ore concentrates, hard lumpy chrome ore, coke and the fluxes like limestone and quartzite. The chemical composition of input batch is controlled in such manner that the slag is low lime slag with CaO in the range of 4.0 – 10.% by

weight, basicity ratio equal to or less than 1.1, slag CaO/SiO2 in the range of 0.08-0.5 and CaO/Al2O3 in the range of 0.1-1.0 preferably greater than 0.16 and or using the fluxes to minimize the silica activity below 0.022 in slag and control the carbon content in ferrochrome product to less than 10.0% by weight. The input batch is smelt reduced in conventional submerged arc furnace process to produce slag and metal. The chemical composition (by weight) of various input batches for smelt reduction in submerged arc furnace are shown in Table 4 and the corresponding chemical composition of slag and metal product containing low silicon are shown in Table 5.

WE CLAIM :
1. A method for producing ferrochrome, the method comprising steps of: sintering/ briquetting of low silica chromite ore, the silica chromite ore comprising (all in wt.%) Cr2O3: 49.7-57.9, FeO: 12.5-18.1, SiO2: 0.01-4.4, Al2O3: 9.8-11.7, MgO: 10.5-10.7, CaO: 0.04-1.3, rest: unavoidable impurities to generate sintered chrome ore; and
charging the sintered chromite ore in submerged arc furnace for smelt reduction of chromite ore with coke, quartzite and limestone in wt. % ratio 60-80, 10-20, 5-15 and 2-10 respectively, the submerged arc furnace being operated at 3000-3300 KWH per tonne.
2. The method as claimed in claim 1, wherein the composition of the chromite ore preferably being (all in wt%) Cr2O357.9, FeO14 14.3, SiO2 0.01, Al2O3 11.7, MgO 10.6, CaO 0.04, rest 5.45.
3. The method as claimed in claim 1, wherein activity of silica in slag forming oxide is < 0.022.
4. The method as claimed in claim 1, wherein the ferrochrome obtained comprises Si< 1.5 wt.%.
5. The method as claimed in claim 1, wherein the composition of slag obtained after smelting in the submerged arc furnace is
CaO (by wt.%) 4-10 CaO/SiO2 = 0.08-0.5 CaO/Al2O3= 0.10-1.00, preferably 0.16.

6. The method as claimed in claim 1, wherein coke comprises FeO: 0.95, SiO2: 5.61, Al2O3: 3.36, MgO: 0.14, Ash: 11.26 fixed carbon: 87.84.
7. The method as claimed in claim 1, wherein composition of quartzite being FeO: 1.76, SiO2: 94.76, Al2O3: 0.50, MgO: 0.70, CaO: 1.16 Rest 1.12.
8. The method as claimed in claim 1, wherein composition of limestone being FeO: 0.08, SiO2: 0.01, Al2O3: 0.01, MgO: 0.43, CaO: 71.76, Rest: 27.71.
9. A ferrochrome comprising (all in wt%): Cr: 61.30-64.40, Si: 0.07-1.5, C: 9.10-9.50, P: 0.018-0.030 and rest 26.43-28.43.