Claims:WE CLAIM:
1. A method for producing high strength chromite ore pellet for ferrochrome (FeCr) production, comprising mixing granulated blast furnace (BF) slag with chromite ore fines wherein the concentration of BF slag is about 0.1% to 3% with respect to the chromite ore fines, and the obtained pellet has a cold crushing strength (CCS) of about 135 kg/pellet.

2. The method of claim 1, wherein the mixed BF slag-chromite ore fines are subjected to grinding in presence of coke dust and water to obtain a slurry, filtering the slurry to obtain filter cake, pelletizing the filter cake to obtain pellet and sintering the pellet.

3. The method of any of the preceding claims, wherein the concentration of BF slag is about 2.5% to 3% with respect to the chromite ore fines, and the BF slag comprises Calcium oxide (CaO) at a wt% of about 30 to 40, Silicon dioxide (SiO2) at a wt% of about 30 to 40, Aluminium oxide (Al2O3) at a wt% of about 13 to 23 and Magnesium oxide (MgO) at a wt% of about 5 to 11.

4. A method for producing high chromium ferrochrome (FeCr), comprising charging the chromite ore pellets obtained by the method of claims 1-3 into a submerged arc furnace (SAF).

5. The method of claim 4, wherein the ferrochrome is produced by carbothermic reduction of the chromite ore pellets in the SAF between a temperature ranging from about 350 °C to 2800 °C.

6. The method of claim 4 or claim 5, wherein the presence of chromite ore pellet results in high basicity ranging from 0.10 to 0.15, low viscosity ranging from 3.71 poise to 4.18 poise and low liquidus temperature ranging from 1786 °C to 1659 °C of the SAF slag produced during the method.

7. The method of any of the claims 4-6, wherein the method increases the chromium content in the Ferrochrome by at least about 0.94%, reduces the coke rate by about 15 kg/ton of Ferrochrome, reduces power consumption by about 72 kwh/ton of Ferrochrome, and increases the ferrochrome production by about 4%.

8. The method of any of the claims 4-7, wherein the chromium content in the Ferrochrome ranges from about 59.5% to 63%.

9. A method of increasing chromium content of ferrochrome (FeCr) during the ferrochrome production process, comprising employing the chromite ore pellets obtained by the method of claims 1-3.

10. The method of claim 9, wherein the chromium content in the ferrochrome is increased by at least about 0.94%.
, Description:TECHNICAL FIELD
The present disclosure is in the field of metallurgy and relates to product quality development. In particular, the disclosure provides high chromium (Cr) ferrochrome (FeCr) production method using Submerged Arc furnace (SAF) by increasing basicity, reducing viscosity and liquidus temperature of the SAF slag produced during the method.

BACKGROUND OF THE DISCLOSURE
Ferrochrome (FeCr) is the major source of chromium for stainless steel production. With rapid growth of world’s stainless steel industry, ferrochrome demand is very high. Ferrochrome alloy is produced from Submerged Arc Furnace (SAF). SAFs are used to smelt chromite ores and chromite pellets into ferrochrome by using carbonaceous reductants (anthracite, char, coke) with the help of electrical energy. Electrical power is the main source of heat energy.

Improvements of ferrochrome quality is a major factor in the prevailing scenario. High chromium (Cr) ferrochrome pays more premiums and is the need of hour. It is a known methodology that high quality (high Cr/Fe) chromite lumps and chromite pellets charged into SAF produces high Cr ferrochrome. However, based on the current scenario, availability of high Cr/Fe chromite lumps is an issue, and rather chromite ores quality is getting deteriorated. Hence, it is a major challenge to produce premium grade/high Cr containing ferrochrome using inferior quality raw materials.

It is also known that high basicity (%CaO/%SiO2) slag makes the slag fluid that helps to produce high Cr ferrochrome. To increase slag basicity, the present methods employ raw limestone which is charged into SAF. But raw limestone (CaCO3) has endothermic calcination reaction at 900°C which demands more heat and increases SAF conductivity leading to process disturbance. Therefore, it is not suitable to increase slag basicity by charging raw lime stone into SAF. Further, it is also established that low liquidus temperature slag requires less heat to melt. Hence, more super heat in the metal would increase Cr in the ferrochrome. High Al2O3 in ferro-chrome slag helps to reduce said slag liquidus temperature. Accordingly, bauxite having high Al2O3 can be charged into SAF, but the same would incur more costs. Moreover, only reduction of liquidus temperature of slag will have little effect on increase in Cr content in the ferrochrome.

Accordingly, there is a need for sustained and cost-effective methods for increasing Cr content in ferrochrome thereby obtaining premium grade (high Cr) ferrochrome, especially using the current or even inferior raw materials and also addressing the aforesaid concerns of the presently practiced methods.

SUMMARY OF THE DISCLOSURE
The present disclosure relates to a method of producing high strength chromite ore pellet for ferrochrome (FeCr) production.
In an embodiment, the chromite ore pellet production comprises mixing of granulated blast furnace (BF) slag with chromite ore fines.
The present disclosure further relates to a method of producing high chromium ferrochrome (FeCr) comprising charging the chromite ore pellets described above into a submerged arc furnace (SAF).
In an embodiment, the use of chromite ore pellets described above increases basicity, reduces viscosity and reduces liquidus temperature of the SAF slag resulting in high chromium ferrochrome production along with other process advantages.
BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES
Figure 1 shows the developed chromium (Cr) prediction model based on the statistical relation of various parameters such as slag B2, MgO/CaO ratio, Al2O3, slag viscosity and liquidus temperature with Cr.

Figure 2 is a flow diagram showing the steps involved in production of chromite ore pellet and the SAF process for high Cr ferrochrome production.

Figure 3 shows BF slag mixed with blended chromite fines through pay loader.

DETAILED DESCRIPTION OF THE DISCLOSURE
The present disclosure relates to a method for producing high strength chromite ore pellet for ferrochrome (FeCr) production, comprising mixing granulated blast furnace (BF) slag with chromite ore fines wherein the concentration of BF slag is about 0.1% to 3% with respect to the chromite ore fines, and the obtained pellet has a cold crushing strength (CCS) of about 135 kg/pellet.

In an embodiment of the above method, the mixed BF slag-chromite ore fines are subjected to grinding in presence of coke dust and water to obtain a slurry, filtering the slurry to obtain filter cake, pelletizing the filter cake to obtain pellet and sintering the pellet.

In another embodiment of the above method, the concentration of BF slag is about 2.5% to 3% with respect to the chromite ore fines, and the BF slag comprises Calcium oxide (CaO) at a wt% of about 30 to 40, Silicon dioxide (SiO2) at a wt% of about 30 to 40, Aluminium oxide (Al2O3) at a wt% of about 13 to 23 and Magnesium oxide (MgO) at a wt% of about 5 to 11.

The present disclosure further provides a method for producing high chromium ferrochrome (FeCr), comprising charging the chromite ore pellets obtained by the method as described above into a submerged arc furnace (SAF).

In an embodiment of the above method for producing high chromium ferrochrome (FeCr), the ferrochrome is produced by carbothermic reduction of the chromite ore pellets in the SAF between a temperature ranging from about 350 °C to 2800 °C.

In an embodiment of the above method for producing high chromium ferrochrome (FeCr), the presence of chromite ore pellet results in high basicity ranging from 0.10 to 0.15, low viscosity ranging from 3.71 poise to 4.18 poise and low liquidus temperature ranging from 1786 °C to 1659 °C of the SAF slag produced during the method.

In an embodiment of the above method for producing high chromium ferrochrome (FeCr), the method increases the chromium content in the Ferrochrome by at least about 0.94%, reduces the coke rate by about 15 kg/ton of Ferrochrome, reduces power consumption by about 72 kwh/ton of Ferrochrome, and increases the ferrochrome production by about 4%.

In an embodiment of the above method for producing high chromium ferrochrome (FeCr), the chromium content in the Ferrochrome ranges from about 59.5% to 63%.

The present disclosure also relates to a method of increasing chromium content of ferrochrome (FeCr) during the ferrochrome production process, comprising employing the high strength chromite ore pellets obtained by the method described above.

In an embodiment of the above described method of increasing chromium content of ferrochrome (FeCr), the chromium content in the ferrochrome is increased by at least about 0.94%.

The present disclosure relates to a method of obtaining a high chromium ferrochrome in a submerged arc furnace (SAF) by charging chromite ore pellets into the SAF. In particular, the disclosure relates to mixing of granulated blast furnace (BF) slag from iron making process with blended chromite fines to produce chromite ore pellets of desired chemistry that helps in achieving high basicity, low viscosity and low liquidus temperature SAF slag during the ferrochrome production process resulting in increased Cr in the produced ferrochrome. Further, the pellets produced after said BF slag mixing possess high cold crushing strength (CCS) that also improves permeability in SAF leading to improved process stability in SAF.

The method of the present disclosure also decreases coke rate and power consumption during ferrochrome production through optimization of SAF slag chemistry by charging desired quality of pellet produced by mixing granulated BF slag with blended chromite fines through pelletization.

In an embodiment, the permeability and stability of SAF process for high chromium ferrochrome production is improved by using high CCS chromite ore pellets which is obtained by mixing granulated BF slag with blended chromite ore fines.

High Cr Ferrochrome is used in steel making process to produce stainless steel up to 18% Cr in it. Cr in stainless steel increases its corrosion resistance. High Cr ferro-chrome is a very important alloy addition in stainless steel making to produce high quality stainless steel with lower power consumption.

It is a prime objective of the present disclosure to produce high Cr Ferrochrome with same or inferior quality raw materials thereby improving the existing approach/methods of Ferrochrome production. Accordingly, long term data of ferrochrome plant was collected. Slag viscosity and liquidus temperature models were developed to understand the slag system better. Based on the plant data analysis and study of slag oxide system, important parameters such as slag B2 (CaO/SiO2), MgO/CaO ratio, Al2O3 concentration, slag viscosity and liquidus temperature that impacts Cr content in the ferrochrome were studied. Based on the statistical relation of all these parameters with the Cr content, a Cr prediction model was developed (Fig. 1). Based on said Cr prediction model, it was found that high slag B2, low slag MgO/CaO and high Al2O3 in slag helped in increasing Cr in ferrochrome.

In an embodiment, blast furnace (BF) slag has high B2, low MgO/CaO ratio and high Al2O3. The chemical composition of BF slag is shown in Table 1.

Table 1: Typical chemical composition of BF slag
Components Unit Range
SiO2 % 30 - 40
CaO % 30 - 40
Al2O3 % 13 - 23
MgO % 5 - 11

BF slag chemical analysis shows that it comprises all important chemical constituents such as CaO, Al2O3 and MgO which are important to produce desired slag in SAF during ferrochrome production. Hence, BF slag was mixed at appropriate percentages with blended chromite fines to produce chromite ore pellets to achieve desired quality of SAF slag, thereby increasing Cr in the ferrochrome. In an embodiment of the present disclosure, about 3% BF slag (with respect to chromite fines) was employed for pellet production wherein using said pellets helped in achieving desired quality of SAF slag and resulting in high increased Cr in the ferrochrome.

In an embodiment, the pellet cold crushing strength (CCS) is increased due to the presence of high CaO in the BF slag which also improves SAF process stability during ferrochrome production.

The present disclosure particularly describes an effective process to produce high Cr ferrochrome by producing desired quality of chromite ore pellets through the addition of granulated blast furnace (BF) slag. These chromite ore pellet was charged into SAF to achieve favorable slag in SAF to produce premium grade/ high Cr ferrochrome.

In an exemplary embodiment of the present disclosure, the method of producing high strength/desired quality chromite ore pellet comprises the following:

BF slag is weighed as per the requirement (preferably between 0.1% to 3% with respect to chromite ore fines) and is mixed with chromite blended fines at raw materials yard through pay loader. This mixed chromite blended fines (0-25 mm) is fed into Ball Mill through conveyor along with coke dust (0 – 10 mm size) and water. After grinding in Ball Mill, the slurry is fed to a drum filter where filter cake is produced. This filter cake (moisture <15%) is stored in filter cake bin. This cake is thereafter mixed with bentonite, an absorbent clay (1% by weight) and fed to disc pelletizer. The green pellet formed in the pelletizer is fed to shaft furnace for sintering at a temperature of about 1250 °C. The sintered pellets are screened through 6 mm screen. Oversize pellet is stored in the pellet bins and charged into Submerged arc furnace (SAF). The undersize pellet (less than 6mm) is reused as feed to the Ball Mill. The schematic depiction of this method is shown in Figure 2.

In an embodiment, the method for producing high strength chromite ore pellet comprises mixing granulated BF slag with chromite ore fines wherein the concentration of BF slag is about 0.1% to 3% with respect to the chromite ore fines.

In a preferred embodiment, the method for producing high strength chromite ore pellet comprises mixing granulated BF slag with chromite ore fines wherein the concentration of BF slag is about 1% to 3% with respect to the chromite ore fines.

In another preferred embodiment, the method for producing high strength chromite ore pellet comprises mixing granulated BF slag with chromite ore fines wherein the concentration of BF slag is about 1.5% to 3% with respect to the chromite ore fines.

In a most preferred embodiment, the method for producing high strength chromite ore pellet comprises mixing granulated BF slag with chromite ore fines wherein the concentration of BF slag is about 2.5% to 3% with respect to the chromite ore fines.

In another exemplary embodiment of the present disclosure, the method to producing high Cr ferrochrome by employing the high strength/desired quality chromite ore pellet obtained above comprises the following:

The pellet produced by BF slag mixing as described above is charged into SAF for carbothermic reduction of chromite ore pellets to ferrochrome. The presence of said pellet improves the slag physical properties in SAF. Due to the improved physical properties, Cr content in ferrochrome increases, coke rate decreases and specific power consumption per ton of ferrochrome is reduced.

In an exemplary embodiment, the carbothermic reduction of chromite ore pellets to ferrochrome in SAF proceeds according to the following stages/temperature zones:

Temperature: 350 -900° C
This is the pre-heating zone wherein gasification of carbon reacting with air and CO2 occurs and require a temperature >700 °C. Preheating of solid burdens happen here.

Temperature: 900-1100°C
This is the pre-heating zone and indirect reduction of FeO & partial metallization (Cr) takes place here.

Temperature:1100 -1400° C
The reaction proceeds wherein the presence of semi-viscous material in this region is found.

Temperature: 1600 -2000° C
This is the maximum temperature zone where most of the endothermic direct reduction of Cr2O3 takes place. Exemplary reactions taking place are as follows:
Cr2O3 +3Cr7C3 = Cr23C6+3CO
2Cr2O3 + Cr23C6 = 27Cr+6CO
Cr2O3 + 3C = 2Cr + 3CO
Temperature: 2000 -2800° C
This is again the maximum temperature zone where most of the endothermic direct reduction of Si takes place, preferably at the electrode tip, and require a temperature >2000°C
SiO2+C=Si O+CO

With respect to the use of substantially any plural and/or singular terms herein, those having skill in the art can translate from the plural to the singular and/or from the singular to the plural as is appropriate to the context and/or application. The various singular/plural permutations may be expressly set forth herein for sake of clarity. The use of the expression “at least” or “at least one” suggests the use of one or more elements or ingredients or quantities, as the use may be in the embodiment of the disclosure to achieve one or more of the desired objects or results. Throughout this specification, the word “comprise”, or variations such as “comprises” or “comprising” or “containing” wherever used, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

Based on the present scenario, availability of high quality chromite ores is a major challenge. Hence, alternative methodologies are needed to produce high Cr ferrochrome with the existing/inferior raw materials. Thus, the present disclosure is successful in providing a simple and economical method comprising an innovative idea of mixing granulated BF slag in blended chromite fines to produce pellet which can be charged into SAF for high Cr ferrochrome production. The produced high Cr ferrochrome has applications including it’s use as an alloy in the making of stainless steel and other related/similar applications.

In an embodiment, the foregoing descriptive matter is illustrative of the disclosure and not a limitation. While considerable emphasis has been placed herein on the particular features of this disclosure, it will be appreciated that various modifications can be made, and that many changes can be made in the preferred embodiments without departing from the principles of the disclosure. Those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the embodiments as described herein.

EXAMPLES

EXAMPLE 1: Production of Ferrochrome using SAF process
(a) The objective of this experiment was to increase CaO % in the sintered chromite ore pellet from 0.76% to 1.15% that will be charged into SAF. This will increase SAF slag basicity from 0.10 to 0.12. This action will also help in reducing SAF slag viscosity from 4.18 to 3.99 poise and SAF slag liquidus temperature from 1786 °C to 1712 °C which is favorable to increase Cr in the ferrochrome.

Accordingly, 4.05 ton (1.35%) granulated BF slag was mixed with 300 ton chromite fines in yard through pay loader. These mixed blended fines were fed into Ball Mill along with coke dust of 0-10 mm size and water. After grinding in Ball Mill, the slurry was fed to the drum filter where filter cake was produced. This filter cake (moisture <15%) was stored in filter cake bin. Then this cake was mixed with bentonite (1% by weight) and fed to disc pelletizer. The green pellet formed in the pelletizer was fed to shaft furnace for sintering at a temperature of ~1250°C. The sintered pellets were screened through 6 mm screen. Oversize pellet was stored in the pellet bins and charged into SAF for ferrochrome production.

(b) The objective of this experiment was to increase CaO% in the sintered pellet to 1.36% that will be charged into SAF and will increase SAF slag basicity to 0.13. This action will also help in reducing SAF slag viscosity to 3.71 poise and SAF slag liquidus temperature to 1705°C which is favorable to increase Cr in the ferrochrome.

Accordingly, 6.0 ton (2.0%) granulated BF slag was mixed with 300 ton chromite fines and the same procedure was followed as described in (a) above.

Result: There was a slight increase in Cr in the ferrochrome.

(c) The objective of this experiment was to increase CaO% in the sintered pellet to 1.52% that will be charged into SAF and will increase SAF slag basicity to 0.14. This action will also help in reducing SAF slag viscosity to 3.84 poise and SAF slag liquidus temperature to 1669°C which would increase Cr in the ferrochrome.

7.5 ton (2.5%) granulated BF slag was mixed with 300 ton chromite fines and the same procedure was followed as described in (a) above.

Result: (i) % Cr in the ferrochrome was increased by an average of 0.41%.
(ii) In one casting ferrochrome Cr was increased by 1.25%.
(iii) Pellet average strength increased by 16 kg/pellet.
(iv) Percentage of pellet with CCS less than 60 kg was dropped by 10%.
(v) Reduction in specific power consumption was not observed.

(d) The objective of this experiment was to increase CaO% in the sintered pellet to 1.78% that will be charged into SAF and will increase SAF slag basicity to 0.15. This action will also help in reducing SAF slag viscosity to 3.82 poise and will reduce SAF slag liquidus temperature to 1659°C which will increase Cr in the ferrochrome.

9.0 ton (3 %) granulated BF slag was mixed with 300 ton chromite fines and the same procedure was followed as described in (a) above.

Result: The results at 3.0% BF slag mixing is shown in Table 2 below.

Table 2

As can be observed -
(i) % Cr in the metal (ferrochrome) was increased by an average of 0.94%.
(ii) Day average ferrochrome production increased by 4%.
(iii) Coke rate in SAF was reduced by 15 kg/ton of ferrochrome.
(iv) Specific power consumption reduced by 72 kWh/ton of ferrochrome.
(v) Pellet average strength increased by 40 kg/pellet.
(vi) Day average pellet production increased by 9%

The above experiments/trials (a) to (d) were performed in a 30 MVA Submerged Arc Furnace. BF slag was mixed up to 3.0% with blended chromite fines to produce desired quality pellet that was charged into SAF. At lower % of BF slag mixing, there was an impact on metal Cr, but the same was less. Further, when BF slag mixing was increased to 2.5 and 3.0%, Cr in metal was increased, coke rate decreased, productivity increased and power consumption decreased due to better SAF slag quality that improved the overall SAF process. As pellet strength improved, ferrochrome production was also increased due to more process stability. More specifically, pellet which was produced by mixing 3% BF slag with blended chromite fines and was charged into SAF impacted the process conditions significantly resulting in increased Cr in ferrochrome by 0.94%, decreased coke rate by 15 kg per ton of ferrochrome, reduced specific power consumption by 72 kwh per ton of ferrochrome, increased pellet strength by 40 points, increased overall ferrochrome production by 4% and pellet production by 9%. It is significant to note that even a slight increase in Cr content of ferrochrome along with an improvement in other parameters discussed above is highly advantageous in the field of ferrochrome production.

Further, BF slag mixing with chromite fines was done maximum up to 3%, preferably between 0.1% to 3% of BF slag with respect to the amount of chromite ore fines. It was not increased beyond 3% as very high CaO% in SAF slag would increase conductivity of SAF slag which will deteriorate the stability of SAF process. Hence, BF slag addition was limited to 3.0%, preferably between 0.1% to 3%.