FIELD OF THE INVENTION
The invention relates to utilisation of ferrochrome to make value added products such as refractory material and ferro-silico-chrome alloy. The invention also relates to carbo-alumino thermic smelting of granulated ferrochrome slag to produce spinel refractory and recover metallic values.
BACKGROUND OF THE INVENTION
Stainless steel producers are the largest consumer of ferrochrome. Most of the world’s Ferro-Chrome is produced in South Africa, Kazakhstan and India. High carbon ferrochrome is produced by carbothermic reduction process. Most commonly submerged arc furnaces are employed for smelting of chromite ores by carbonaceous reductants. Carbothermic reduction is the dominating technology practiced worldwide for production of high carbon ferrochrome alloy. The chromite ore used for production of ferrochrome alloy is of spinel mineral type having general formula of (Fe2+, Mg+2)O.(AI3+, Cr3+, Fe3+)2O3. Smelting of chromite ores are carried out in submerged arc furnaces. During the smelting process slag is produced and molten metal is produced. During smelting process the heavier metallic ferrochrome coalesces into droplets and settled through the slag layer. There by the metallic and non-metallic part is separated by the density difference between two. The melting point and viscosity of ferrochrome slag plays a vital role during smelting process which depends on the chemistry of the slag. Ferrochrome slag composition is influenced by the gangue materials present in chrome ore and fluxing materials such as quartzite used for production. The main components present in ferrochrome slag are SiO2, Al2O3, MgO, Cr2O3 and CaO. The Chromium part in ferrochrome slag exists in form of unreduced chromite, partial altered chromite (PAC) and metallic entrapment. The optimum smelting point has been practically noted between 1650-1750 °C. The tapping of slag and metal are done through same tape hole and collected in a ladle. The top layer slag collected is separated by pouring. High pressure water jets are employed to granulate the molten slag during pouring which breaks the molten slag into small granules. The quantity of ferrochrome slag generation is approximately 1.2 to 1.5 times of the amount of ferrochrome alloy produced.

The prior art processes developed in the past mainly included production of refractory type of material by changing the chemistry of ferrochrome slag or direct utilisation of ferrochrome slag for low temperature refractory application without changing its chemical composition.
Patent application number US4818290 disclosed that mineral wool can be manufactured from ferrochrome slag by addition of aluminium oxide and silicon oxide in molten slag by changing the viscosity to adjust the defibaration temperature. 2.5 -5.0: 1 ratio of silicon oxide to aluminium oxide can be added to ferrochrome slag to get low temperature mineral fibers. Aluminium oxide in the range of 20-30% by weight can be added to the molten slag for making high temperature mineral fibers.
Patent application number US4946811 has also claimed that mineral fibers can be obtained by mixing molten iron silicate slags with ferroalloy slags while maintaining the molar alakalinity (FeO+CaO+MgO/SiO2+Al2O3) within 0.5 to 0.7. According to the patent US4751208, chromiferrous slag waste discharge from sodium chromate production can be used to produce spinel type ceramic sintered body by selecting R2O3/MgO to 0.9-2.0 and SiO2/MgO to 1-6 where R represents Al, Fe, and Cr.
Patent no RU2182140 disclosed that refractory material comprises of mineral phases forsterite 43%, aluminomagnesium spinel 14 to 22%, magnesia pedalferic spinellide 12 to 20%, periclase 4 to 11 % and motecellite 1 to 4 % can be produced from ferrochrome slag which can be used for making magnesia silica refractory for lining of heating and roasting furnaces.
Patent application no SU672184-A disclosed making of refractory materials from molten ferrochrome slag and calcined magnesite. 10-40% calcined magnesite can be added to the molten slag to produce better refractory material.
Patent application no SU775092-B disclosed that the the refractoriness of the ferrochrome slag (of compsn. (in wt. %): SiO2 28-35; MgO 35; 45; Al2O3 15-20; FeO 1-3; CaO to 100) 70-90) can be increased by enriching chrome-picotite phase with addition of chromium ore (of composition (in wt. %): Cr2O3 38-59; FeO 7-12; MgO 10-19; Al2O3 9-18; SiO2 1-8.

Patent no US3016447, is related to producing the magnesium aluminium spinel from ferrochrome slag. This patent claims a process for beneficiating high carbon ferrochrome slag by removing silica and recovering magnesium aluminium containing spinel material. According to this patent high carbon ferrochrome slag is mixed with iron and carbon and sufficient aluminium oxide and magnesium oxide and heating the mixture in between 18500C to 19000C results silica free refractory material and ferrosilicon alloy. This patent described the carbothermic reduction of silica present in the ferrchrome slag containing Cr2O3 less than 5%. In this prior art there were no attempts made to enrich the chromium content of the alloy during the process. This prior art technology limits the spinel phase enrichment by restricting the ratio of magnesia to alumina by weight from 0.67 to1.1. Considering all the above prior art to produce refractory materials from ferrochrome slag the present invention discloses a novel process approach based on carbo alumino thermic reduction of ferrochrome slag containing higher Cr2O3 content (>5%) to produce spinel enriched refractory material and chromium enriched ferro-silicon-chrome alloy.
OBJECTS OF THE INVENTION
It is therefore an object of the present invention to propose a process for production of Ferro-silico-chrome alloy and spinel refractory material.
Another object of the invention is to establish a method to enrich the spinel content by decreasing the silica composition from slag.
Still another object of the invention is to enrich the chromium content and silicon content of the alloy produced by addition of settling agents and Ferrochrome alloy.
SUMMARY OF THE INVENTION
In the process of development of refractory material, the composition of the ferrochrome slag is engineered to produce spinel enriched refractory material. In this process, the metallic values from the waste ferrochrome slag are also extracted through pyro metallurgical process. Production of spinel enriched refractory material and chrome enriched ferro-silico-chrome alloy in presence of settling agents such as iron and/or

ferrochrome, reductant containing carbonaceous materials such as coke or coal and booster aluminium containing reductant in form of aluminium shots or chips/powders as reduction enhancer, spinel phase enhancer such as calcined alumina or calcined bauxite.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig – 1 shows the Mgo-Al2O3 binary phase diagram Fig – 2 shows Mgo-Al2O3-SiO2 ternary phase diagram
DETAILED DESCRIPTION OF THE INVENTION
During slag metal tapping operation in ferrochrome production process, the ferrochrome slag and metal are collected in a ladle through same tap hole from submerged electric arc furnace during slag metal tapping operation. The molten ferrochrome slag being lighter in density compared to molten metal gets separated on the top of the metal layer. The top slag layer present above the molten metal is removed by careful pouring of it into a slag quenching area. In the slag quenching area the molten ferrochrome slag is granulated by applying stream of water jet. The size of water granulated slag varies from 150 micron to 5mm. The chemical composition of the water granulated slag is close to the refractory composition and is given in Table 1.

Ferrochrome slag contains mineral phases as spinel and silica bearing phases such as forsterite and enstatite and siliceous glass enriched with oxides of calcium, aluminium. Presence of high amount of silica decreases the melting temperature of the slag. Presence of entrapped metal is also deleterious to the refractory property of slag. Fig 1

explains the MgO-Al2O3 binary phase diagram shows that magnesium aluminate spinel can be formed at 72% Al2O3 and 28% MgO. The MgO-Al2O3-SiO2 ternary phase diagram in fig 2 is used to explain ferrochrome slag composition and target refractory material composition. So if theoretically all silica can be removed from the ferrochrome slag, the final composition will end up with the periclase and spinel region. It is evident from figure 2 that addition of alumina can shift the ferrochrome slag composition towards spinel enriched region in the MgO-Al2O3-SiO2 ternary phase diagram.
The present invention focuses on enriching the ferrochrome slag chemistry with magnesium aluminate spinel phase by removing the silica component and enriching alumina composition by carbo-aluminothermic reduction of silica and unreduced
chromite and addition of alumina as spinel phase enhancer. To facilitate the reduction of silica, iron scrap or cast iron chips and high carbon ferrchrome alloy fines or high carbon ferrchrome jigging chips of size less than 3mm are added during the process so that silicon can form alloy with iron and chromium and settles to the bottom. The metal produced in this process is of chromium enriched ferro-silco-chrome alloy and it settled to the bottom due to the density difference between the metallic and non-metallic component. Carboaluminothermic reduction of the ferrochrome slag can be done with reductants such as carbonaceous materials such as coke or coal and booster aluminium containing reductant in form of aluminium shots/chips/powder for reduction enhancer. Carbon is added sufficiently to reduce all the silica and Cr2O3 present in the slag. Aluminium is added as booster or enhancer in the smelting and fusion of ferrochrome slag. The benefits of aluminium addition are: (1) acts as a reductant and set up alumina thermic reduction reaction (2) help to in situ generation of alumina inside molten slag which has better dissolution than alumina added in form in form of calcined alumina or calcined bauxite (3) increases the temperature of molten slag by aluminothermic reaction (4) Provides A+3 ions for enrichment of magnesium aluminate spinel. Depending on the observation on exothermicity of the aluminothermic reaction during the process, the quantity of aluminium chip/shot/powder can be limited. The quantity can be added such that it will replace the alumina addition up to 30% of the stoichiometric requirement of alumina to form spinel with magnesium oxide. Spinel

phase can be more enhanced by addition of alumina containing source such as calcined alumina or calcined bauxite to the melt during the process. The furnace could be of submerged arc furnace with water cooled shell or electric arc furnace or tilting arc furnace with provision of charge feeding through chutes in furnace in non-choked manner feed.
In the present invention, the process comprises of mixing high carbon ferrochromium slag with settling agents, reductants and spinel phase enhancer and then slowly adding the mixture to the furnace through non choking type of feed system. The temperature of the bath is maintained at temperature above 21000C for smelting and fusion reaction to occur. The main principle of this technology consists of enriching the refractory components by reduction of silica and unreduced chromite present in the ferrchrome slag by carbometallothermic reduction process. The present invention relates to production of ferro-silico-chrome alloy and spinel refractory from high carbon ferrochrome slag containing Cr2O3>5%. The slag generated from the production of high carbon ferrochrome contains high amount of silica and unreduced chromite and entrapped metals.
The carbometallothermic reduction of the ferrchrome slag is carried out in the arc furnace in presence of reductant such as carbonaceous material like coke or coal and aluminium inform of aluminium chips or aluminium shots. Aluminum serves three purposes in this process: (1) Act as reductant (2) Provides in situ Al+3 ions for spinel formation (3) reduces the power requirement for raising the temperature to the desired level due to the exothermicity of aluminothermic reaction. The temperature of the bath can be maintained by heat input through arc and carbometallothermic reaction.
The settling agent is preferable a metallic phase which can form alloy with silicon and chromium that are produced from reduction of silica and unreduced chromite present in slag and helps to settle the metallic part to the bottom of furnace and separates the metallic part from nonmetallic part due to density difference. The settling agent used for this purpose is metallic iron scrap or cast iron chips and ferrochrome alloy fines or ferrchrome alloy jigging chips.

Alumina in form of calcined alumina or calcined bauxite is added as spinel phase enhancer. Alumina reacts with the MgO present in the slag to form magnesium aluminate phase.
Example-1
1kg of water granulated Ferrochrome slag (size range 0.5 to 2mm) of chemical composition mentioned in table 1, 100gm of FeCr alloy fines, 150gm of iron scrap, 400gm of calcined alumina and 160gm of coke is mixed and then charged to an electric arc furnace and heated to temperature of above 21000C. The arcing was continued till all the silica and Cr2O3 reduces to metallic silicon and chromium. After completion of the reduction the molten slag and metal are allowed to cool inside the furnace. The metallic and non-metallic parts are physically separable and can be recovered by breaking the cakes. The metallic part is of ferrosilicochrome alloy type and its composition: Si-18.6 wt%, Fe-44.5 wt%, Cr-36.9 wt%, C-1.8 wt%. The non-metallic composition is of spinel refractory type and its composition: MgO-26 wt%, Al2O3-65 wt%, SiO2-4.5 wt%, Cr2O3-0.85 wt%, FeO- 0.6 wt%, TiO2-2.1 wt%.

WE CLAIM:-

1. A submerged arc furnace based method of smelt reduction of water granulated ferrochrome slag for production of spinel refractory material and ferro-silico-chrome alloy in presence of settling agents such as iron and/or ferrochrome, reductant containing carbonaceous materials such as coke or coal and booster aluminium containing reductant in form of aluminium shots or chips/powder as reduction enhancer, and spinel phase enhancer such as calcined alumina or calcined bauxite, wherein the charge is fed through chutes in the furnace in non-choked manner feed, and wherein the temperature of the bath is maintained at a temperature above 2100ºC.
2. The method as claimed in claim 1, wherein the ferrochrome slag contains Cr2O3 more than 5%, and wherein the water granulated ferrochrome slag comprises, in weight%, MgO: 20 to 28, Al2O3:22 to 30, SiO2:25 to 35, Cr2O3:5 to 15, CaO: 2 to 3.5, FeO: 2 to 4.
3. The method as claimed in claim 1, wherein the alloy composition contains Si: 10-30 wt%, Cr: 10-35%, C: 1.5-3%, Fe: balance.
4. The method as claimed in claim 1, wherein the spinel refractory material contains MgO: 10-35%, Al2O3: 60-85%,CaO: 1-3%, SiO2: 1-10%, TiO2: 1-3%, Cr2O3 <1:
5. The method as claimed in claim 1, wherein the silicon settling agent for the reduced silicon is :
(i) metallic iron scrap, cast iron chips, or
(ii) High carbon ferrochrome alloy fines or jigging ferrochrome alloy chips (<3mm).
6. The method as claimed in claim 1, wherein the spinel phase enhancer is high
alumina containing source such as calcined alumina, calcined bauxite.

7. The method as claimed in claim 1, wherein the reductant is carbonaceous materials such as coke or coal and booster aluminium containing reductant in form of aluminium shots or powder for reduction enhancer.
8. The method as claimed in claim 1, wherein the furnace employed for the process can be of submerged arc furnace with water cooled shell or electric arc furnace or tilting arc furnace.