FIELD OF INVENTION:
This invention relates to a process for sintering chromite ore.
The sintered chromite ore is then used for the production of ferro-chrome/charge chrome and alloy compound.
BACKGROUND OF THE INVENTION:
In order to produce ferro-chrome and charge-chrome, chrome ore is an essential raw material. The demand for ferro-chrome and charge-chrome is continuing to increase because of increase in the manufacturing of alloy steel components. India has about 180 million tones of chromite ore reserves, which is mostly confined to Sukinda-Nausahi belt and most of it is of ferruginous type. From chromium extraction point of view, the associated gangue minerals in chrome ore can be classified into two types, namely, ferruginous and siliceous. During selective mining of lumpy chrome ore from the siliceous type deposits, about 50,000 tpa of low grade siliceous ores is also produced. This material requires value addition for effective marketing of this product.
Chrome ore concentrate has been a strategic material for Tata Steel's business activities. Currently, the ore is ground to below 75 micron for beneficiation and subsequent pelletisation. Green pellets are hardened through carbo-thermic reaction at 1300-1400°C and then charged along with lumpy ore and briquettes into the submerged arc furnace for ferro-alloy production. Physical properties of chrome ore pellet, even after hardening, are found to be inconsistent, similar is the case with briquettes. This leads to generation of more fines during melting and

causes increased electricity consumption while producing ferro-chrome / charge-chrome. Alternative route of using chrome ore concentrates / chips from lumps is through agglomeration and then feeding to arc furnace for ferro-chrome production are being practiced elsewhere. An attempt was made to produce chrome ore sinter at low temperature using pot grate sinter machine, and subsequently electrical resistivity was determined. All chromite ores are smelted in electric arc furnaces which consumes electrical energy to the extent of 3800-4000 kWh/tonne of ore. The chrome ore extraction process for Kazakhstan, Turkey and Pakistan ore may not be similar to Indian ore owing to their compositional difference (Table 1), while South African chromite ore being akin to Indian ore similar extraction process can be adopted.


Another object of this invention is to propose a process for sintering chromium ore to produce ferro-chrome alloy or charge chrome;
Still another object of this invention is to propose a process for sintering chromium ore which consumes less power;
Further, object of this invention is to propose a process which is very simple and economical.
BRIEF DESCRIPTION OF THE INVENTION:
According to this invention there is provided a process for sintering chromite ore comprising:
introducing chrome ore into the sinter pot;
adding fluxing agents to the pot containing the ore;
subjecting the mixture to the step of heating to produce the sintered chrome.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
Figure 1: Experimental set up for measurements of electrical resistance of charge materials.

Figure 2: Shifting of slag composition point of Test 2 to Test 5 by addition of sand to Test 2 mix, resulting in considerable reduction of melting temperature of slag as shown in pseudo ternary diagram of Al2O3-MgO-SiO2.
Figure 3: Sand addition on the mix of Test 2 showed decrease in maximum sintering temperature of Test 5.
Figure 4: Sand addition on the mix of Test 2 showed adverse effect on Test 5 sintering time.
Figure 5: Sand addition on the mix of Test 2 showed reduction in granulometry (+10mm) of Test 5 chrome ore sinter.
Figure 6: Sand addition on the mix of Test 2 showed increase in extended tumbler index of Test 5 chrome ore sinter.
Figure 7: Addition of lime fines/LD Slang on Test 5 sinter mix has inconsequential effect on maximum sintering temperature of Test 6.
Figure 8: Addition of lime fines / LD Slag on Test 5 sinter mix have reduced sintering time of Test 4 and Test 6 owing to better granulation of fines.
Figure 9: Addition of lime fines / LD Slag on Test 5 sinter mix have resulted in increase in granulometry (+10 mm) of Test 4 and Test 6 chrome ore sinters.

Figure 10: Addition of lime fines / LD Slag on Test 5 sinter mix have resulted in increase in extended tumbler index of Test 4 and Test 6 chrome ore sinters.
Figure 11: X-ray mapping of elements in chrome ore sinter of Test 6; suggests that bonding between the particles was owing to presence of silicates.
Figure 12: XRD analysis of chrome ore sinter showing presence of magnetite and maghemite phases.
DETAILED DESCRIPTION OF THE INVENTION:
All sintering tests were carried out in the experimental sinter pot under conditions as stated in Table 2. Usually, for iron ore sintering with similar bed depth (400 mm) suction is maintained at 1100 mmWC, but owing to the presence of relatively more fines (63% less than 0.5 mm) in the chrome ore, suction is maintained at 1200 mmWC as given in Table 3. Chemical analyses of different materials used for sintering are given in Table 4. Attempts were made to reduce sintering temperatures by using various fluxing agents like lime, LD slag, bentonite and sand. Thermocouples for temperature measurements were inserted 80-100 mm deep into the sinter bed. There were three such thermocouples inserted at different heights of the sinter bed. Maximum temperature acquired by these thermocouples is reported as maximum sintering temperature.
The analysis of Cr bearing materials, lump coke and other raw materials used is given in Table-4. Chrome ore sinter was evaluated for electrical

conductivity along with other metallic charge used for ferro-chrome production in sub-merged arc furnace in an apparatus shown in the Figure 1.
Different combinations of raw materials used for sintering of chrome ore is given in Table 5. Strengths of all the chrome ore sinters were evaluated based on extended tumbler tests (ETI).
Melting of chromite sinter was done in 10 kg scale arc furnace where smelting of chromium bearing materials (e.g. chromite ore, chromite pellets, sinter and briquettes) were carried out with lump coke in presence of spar as a flux.
Chromite ore was in the form of lump from Sukinda in the size range of 10-25 mm size. Pellets were made from friable Sukinda ore after grinding which were fired at around 1300-1400°C in a furnace to impart strength. All the Cr bearing raw materials melted were broken down in the size range of 10-25 mm for melting in the furnace except pellets. Table 2: Conditions maintained in the sinter pot

In order to sinter chromite ore fines, the following measures were taken to facilitate chrome ore sintering.
1. Since chrome ore is relatively fine ore, so proper granulation with some binder and flux was a must to facilitate flame front to travel to the bottom of the sinter bed.
2. In order to bring down the melting temperature of the mix, additives in the form of sand was added to the sinter mix.
To facilitate sintering of chrome ore, on the basis of above mentioned investigations, effects of following additives on chrome ore sinter mix on its sintering characteristics were studied:

• Effect of sand addition
• Effect of lime powder and LD slag addition
(a) Effect of sand addition
As discussed previously, the chrome ore concentrate consists of high melting oxide and amount of silica is only 4.5%. It is difficult to melt chrome oxide as such and therefore attempt was made to melt chrome ore by fusing slag part of the ore. From Table 4, the total slag forming oxides in chrome ore is (Al2O3+MgO+SiO2) = 27%. As it can be seen from the ternary diagram[1] of Al2O3, MgO and SiO2 shown in Figure 2, the region with nearly equal proportion of Al2O3, MgO and SiO2 has low melting temperature zone. In the compositional analyses of chrome ore concentrate, Al2O3 and MgO are present to the extent of 10-12.5%. The silica proportion is only 4.5%. So to raise SiO2 from 4.5 to 10-12%, 7-8% sand needs to be added so that the gangue minerals in the sinter mix would be in equal proportions and the overall gangue composition point lies in the low melting region of the ternary diagram as shown in the Figure 2.
Table 5 shows mode of addition of sand in the sinter mix. Apart from sand, there would be contributions of alumina and silica from bentonite and coke breeze as well. The percentage point contributions of silica and alumina from these compounds were 1.8% and 0.7% for Test 2 and 1.7% and 0.7% for Test 5 respectively.
Owing to increase of silica, resultant slag chemistry will also be changed. The calculated slag chemistries of Test 2 and Test 5 are given in Table 7. From Table 7, it is clear that silica content in the slag has increased from

21.8% in Test 2 to 39.4% in Test 5. It can be seen from Figure 2 that, slag composition of Test 2 falls between the liquidus isotherms of 1900°C and 2000°C, but as sand was added to the sinter mix, SiO2 content increased from 5.9% to 12.8% in Test 5. Owing to sand addition, slag composition point of Test 5 in Figure 2 has shifted close to 1700°C liquidus isotherm. As a result, melt temperature would be decreased by about 200-250°C. In contrast to the estimated 250°C drop in liquidus, the actual measured decrease in liquidus was 138°C which is happen to be the maximum sintering temperature (1608°C) as shown in Figure 3.

Figure 4 shows that sintering time could not be reduced and almost same owing to increase fines content in mix due to sand addition and improper granulation of the mix so that flame front could not travel to the bottom rapidly. As a result, sintering time of Test 5 has remained similar to Test 2.

With addition of the sand, slag volume increased and thus binding between the particles improved, but large size lump formation of proper strength was reduced, this might be due to SiO2 bonding was not good enough to hold large lumps. This has resulted in decrease of granulometry (cum. + 10mm) from 48.6 to 39.5 (Figure 5) and the relatively small sized lump formation have increased with higher strength resulted increased extended tumbler Index from 20 to 38 of product sinter as depicted in Figure 6.
(b) Effect of lime powder and LD slag addition
It is well established that addition of lime powder facilitates improvement in the granulation of the sinter mix. Since chrome ore contains large amount of fines so granulation should be proper so that the flame front reaches to the bottom of the sinter bed. It was observed that with addition of lime fines or LD slag (as in Test 6 and Test 4), sintering time reduced and sinter strength improved compared with the sinter mix of Test 5, wherein no lime or LD slag was added (Table 5). From Figure 7, it could be seen that there was insignificant change in the maximum sintering temperature by sintering the mix containing lime / LD slag with respect to the mix containing no lime.
Encouragingly, with addition of lime fines or LD slag, the sintering time significantly reduced from 29.8 minutes to 19.4 minutes for lime fines and to 23.6 minutes for LD slag, as shown in Figure 8. Reduction of sintering time might be attributed to improvement in green mix granulation by addition of lime fines / LD slag. The sinter strength has also increased as can be seen from Figure 9 and Figure 10. Granulometry and ETI of chrome ore sinters have increased owing to better bonding between the particles with the availability of CaO and

sand. ETI for sinters made from LDS and lime found to be similar. Therefore, it can be said that the addition of lime fines to the extent of 1.7% to sinter mix has enhanced chrome ore sintering process beneficially and lime powder was more effective than L.D.slag. Since lime fines or LD slag addition was made in conjunction with sand so all these improvements could be the results of synergistic effect with sand providing necessary slag fluidity while lime provided required granulation effect of the fine mix to facilitate faster sintering of the mix.
Chemical and metallography analyses of chrome ore sinter
Chemical analyses of chrome ore sinters, produced in the pot grate sinter machine are given Table 6. Amongst all the sinters produced sinter formed in Test 6 was found to be best. Chemical analysis of Test 6 sinter was comparable to that of the chrome ore with respect to Cr/Fe ratio. Cr/Fe ratio of the chrome ore was 2.02 and that of Test 6 was 1.94. This was due to reduction of metallic oxide and increase in flux content.


With the addition of sand to sinter mix in Test 6, siliceous type slag was predominant. Energy dispersion of X-ray (EDX) mapping of sinter from Test 6 is shown in Figure 11. Owing to flow of carbon monoxide gas through the bed, chrome oxide was getting reduced to metallic carbide. The metallic phases are identified as chromium carbide formed by reaction given below:
7Cr2O3 (s) + 33 CO (g) = 2Cr7C3 (s) + 27 CO2 (g) (1)
Cr2O3 is directly transforming into the carbides of type Cr7C3, on reduction by carbon monoxide gas.
Physical properties of chrome ore sinter was compared with iron ore sinter in order to have correct judgment, is presented in Table 7. Strength of chrome ore sinter is found to be lower than iron ore sinter, but strength requirements for chrome ore sinter may not be similar to iron ore sinter. It is clear that there exists a potential to produce sinter agglomerate of chrome ore fines similar to iron ore fines. However, maximum sintering temperature as well as sintering time can be decisive factors for chrome ore sintering Bed height for chrome ore sintering process should be kept low due to possibility of excessive heat generation during sintering for deep bed sintering process which may eventually damage the sinter machine.
The product chrome ore sinter was tested for its electrical resistivity for its suitability as a charge material in sub-merged arc furnace for production of ferro-chrome. The metallic charge for ferro-chrome production usually consists of hard lumpy chrome ore, hardened pellets and briquettes of chromite fines. Inconsistent quality of pellets as well as

briquettes in the charge increases the electricity consumption of the furnace. In order to reduce the power consumption, an alternative feed material is developed in the form of chrome ore sinter to replace fired pellets as well as briquettes. The chrome ore sinter thus produced using mix of Test 6 was evaluated for electrical conductivity by using electrical conductivity measurement furnace as shown in Figure 1. The sample was charged into the furnace and across the two steel plates, one at top and other at bottom of the furnace, a D.C. voltage was applied through a meger. Resistance across the bed of sample was measured. This was done at different temperature up to 800°C by increasing the electrical power to the furnace from time to time. It was interesting to find that chrome ore sinter possesses lowest electrical resistance compared with pellets, hard lumpy ore and briquettes. This is owing to reduction during sintering of metallic oxide phases to metallic or lower oxide phases, which are relatively better conductor of electricity. The XRD analysis of this sinter shown in Figure 12 confirms the presence of phases like magnetite and maghemite. These phases are good conductor of electricity. XRD analysis on another sample of chrome ore sinter is presented in Table 8. By employing Rietvelt powder structure refinement method crystal lattice structure and size were determined for the phases of crystobalite, chromite and Cr2O3.FeO. Electrically, magnetite is a semi conductor at low temperature but changes into a metallic conductor above 110-120°K. This could be the reason for better conductivity of chrome ore sinter material. This characteristic can have a favorable effect for lowering electricity consumption for subsequent ferro-alloy production in arc furnace.


Average heating data of several smelting experiments are shown in Table 9. It is observed that specific power consumption in case of sinter and pellets are lower than that in case of briquettes and lump ore. The metallic yield in case of briquettes and lump ore is very poor, whereas, it is very good for sinter and pellets. Sinter shows best metallic yield, because of its porous structure enhances its reducibility and also it is partially reduced material i.e in sinter, there are some lower oxides of Cr like CrO and other non stoichiometric oxides of Cr. Therefore, the sinter is reduced faster in EAF than lump ore, pellets and briquettes. In case of sinter, it has been observed that smelting time was higher (73 min) than pellets briquettes and lump ore (66-70 min). This is because, sinter has lower heat conductivity due to its spongy structure and hence heat transfer from direct arc to the materials is lower at the initial stage of arcing. More over, during smelting it has been physically observed that in case of pellets, sinter and ore the molten pool is generated very quickly (5-7 minutes) in front of electrodes and submerged arcing was possible.

However, in case of sinter, the molten pool was generated after a prolonged time (15-18 min).
The experimental result in Table 9 is showing very high power consumption, however, in actual Fe-Cr production through submerged arc furnace route, the same has been found to be 3.5-4 kWh/kg. That is because of severe energy loss in conduction and radiation owing to the small heat size and indirect arcing at the first stage and heating of cold furnace lining. In industrial furnace arc is submerged from the initial stage and the furnace remain hot for its continuous operation. Therefore, the result does not reflect the actual power consumption for Fe-Cr production, however, this exercise is done only for a comparative performance among sinter, pellets, briquettes and lump ore.
Cr recovery in case of sinter and pellets are better than briquettes and lump ore as shown in Table 10. Similarly iron recovery for sinter it is very high comparable to briquettes and lump ore. This is because the sinter is a pre-reduced material, hence both iron and Cr produced very quickly at high temperature (1600-1800°C) in EAF.



Summary
To produce sinter lumps of chrome ore fines, it is necessary to granulate chrome fines with lime fines and bentonite for giving strength to green balls for handling and provide necessary permeability to sinter bed during sintering. Besides lime and bentonite, sand addition is made to lower the liquidus temperature of sinter mix, while fuel requirements remained equivalent to iron ore sintering. Spar can not be used in the mix for sintering of chromite ore.
This study indicates that the sinter and pellets both are suitable for Fe-Cr production from energy point of view, because, it shows lower power consumption than ore and briquettes. The sinter provides better metallic yield than pellets and other Cr bearing materials. Hardened ellets making is energy intensive owing to grinding and firing process for strength improvement and sizing of fines etc. As sinter making is easier than pellet making and its performance in EAF is better than other Cr bearing materials, sinter can be used as raw material feed for Fe-Cr production. However, if the availability of chromite is very fine, it may loose the permeability of sinter bed, hence production loss, in this case one can select the sintering route for Fr-Cr production adopting suitable granulometry of chromite ore fines.

Based on the preliminary findings of the study conducted on sinter pot it is concluded that there exists a potential to sinter chrome ore in a manner similar to iron ore sintering process. As of now, strength criteria of chrome ore sinter for charging into sub-merged arc furnace is not known. Since chrome ore contains high amount of fine materials and excessive heat is generated due to exothermic reactions, so therefore, it is suggested to go for shallow bed rather than deep bed sintering option for chrome ore sintering process.

WE CLAIM:
1. A process for sintering chromite ore comprising:
introducing chrome ore into the sinter pot;
adding fluxing agents to the pot containing the ore;
subjecting the mixture to the step of heating to produce the sintered chrome.
2. The method as claimed in claim 1, wherein the said sinter pot
maintains the following conditions:
Cross-sectional Area, mm2 400x400
Bed Height, mm 400
Ignition time, min 2
Ignition Temperature, °C 1075
Bed suction, mm WC 1200
Green mix air velocity, m/min 0.5-0.6
Coke breeze in mix, % 5.0
3. The method as claimed in claim 1, wherein the fluxing agent comprises of lime, LD slag, bentonite and sand.
4. The method as claimed in claim 1, wherein the heating is done by the thermocouples.

5. The method as claimed in claim 1, wherein the pellets size of the chromite ore is in the range of 10-25 mm.
6. Sintered chrome ore as provided by the method as claimed in the preceding claims in used for production alloys like ferro-chrome.

A process for sintering chromite ore comprising: introducing chrome ore into the sinter pot; adding fluxing agents to the pot containing the ore; subjecting the mixture to the step of heating to produce the sintered chrome.