A process for the recovery of Chromium and Iron oxides in High Cartoon Ferro Chrome Slag to obtain Chromium and Iron in the form of saleable metal.

This invention relates to a process for recovery of chromium from the slag of High Carbon Ferro Chrome, which is now being discarded as waste.

With the advent of stainless steel production in India for which chromium is the major additive for its luster and non-corrosive properties, the ferrochrome production has gained importance. High Carbon Ferro Chrome is produced in submerged electric arc smelting furnace. The raw material in the production of High Carbon Ferro Chrome consists of (a) Chromite, which is chrome and iron oxides containing mineral, (b) metallurgical coke to reduce the metal oxides present in the ore and (c) fluxing material like Quartz, Bauxite to get the right composition of the slag formed out of gangue materials in the ore and ash of coke. The smelted products obtained from the smelting furnace are High Carbon Ferro Chrome Alloy and Slag, which are tapped and generally collected in cast iron receptacles at regular intervals. The slag production is 1.10 to 1.2 MT for every ton of alloy produced depending on the chemical composition of the chromite ore taken as feed material. The temperature of the liquid slag during tapping is about 1700°C. The typical Ferro chrome slag composition is SiO2 - 26 to 30%, Al2O3 - 24 to 26%, MgO - 24 to 26%, CaO - 2 to 2.5%, Cr2O3 - 12 to 14% and FeO - 2 to 3%. Out of 12 to 14% Cr2O3 for total Cr reported in the slag about 2 to 2.5% Cr is in the form of metal drop lets and particles entrapped in slag and the remaining 10 to 11% is in oxide form.

Limitation in conventional method of High Carbon Ferro Chrome Production :-

The present process has got a limitation of recovery of only 84% to 86% of chromium even after following the best process control methods and loosing about 14% to 16% of chromium in slag (in metallic and oxide form) which is generally being used for landfill,

Thus we are loosing the non-replenishable mineral to a tune of about 15% for every ton of High Carbon Ferro Chrome produced. The present annual production capacity of High Carbon Ferro Chrome in India is about 9 to 10 Lac MT per year and a quantity of about 22,50,000 MT of chromite ore is being consumed. Because of the limitations in the process of producing High Carbon Ferro Chrome in sub-merged arc furnace, we are loosing about 5,65,000 MT of chromite in slag and thus depleting the non-replenishable mineral to such a great extent.

Till date efforts are made only to recover the entrapped metalics in the slag, which are around 2 to 3%, by mechanical means of crushing, grinding and gravity separation but no economic method is found to recover the chromium present in the slag in oxide form.

Object of the invention: The object of this invention is therefore to recover the total chromium lost in the slag, to an extent of 98% by smelting the liquid slag along with coke and required fluxes in an electric open arc furnace after adjusting the slag basicity, in the form of saleable metal. Thus conserving the non-replenishable

mineral source, the chromite ore besides achieving overall cost reduction in the production of High Carbon Ferro Chrome by way of getting additional about 10% quantity of metal.

The target is to achieve at least 98% recovery chromium and iron present in the Ferro chrome slag. The alloy produced from the slag can be directly used for the production of alloy and stainless steels.

Process proposed: According to this invention, the process for the reduction chromium oxide and iron oxide available in the slag is reduced by carbon to chrome iron carbide in the following steps.

1. The liquid High Carbon Ferro Chrome slag collected during the tapping is weighed and poured into an Electric Arc Furnace (EAF)

2. As per the stoechiometry requirement, the coke quantity is calculated, weighed and kept about one third in the furnace bottom and balance for addition later.

3. Calcined lime is calculated for maintaining a slag basicity (CaO + MgO : SiO2) of about 1.6 weighed and put about one third in the furnace with coke and balance for later addition after pouring slag.

4. After pouring the liquid slag in the EAF, the furnace is switched on and the arc is stabilized.Then the balance calculated quantities of coke and calcined lime are added in small increments to complete the smelting process.

5. After smelting, if required, little quantity of fluorspar is added to obtain the required slag fluidity. Spoon sample of final slag is collected to check for residual Chromium. Sample on cooling will become white instead of green in colour.

6. After attaining the temperature of about 1700°C and ensuring sufficient slag fluidity, metal and slag are unloaded in a refractory lined ladle. Slag is decanted and metal cast in a cast iron pan and crushed and sized after solidification.
Slag is cast in pans and discarded.

7. Samples of slag and metal are collected for analysis of the required components.
The slag being highly basic can be used as a component in Portland cement.

This invention will now be described in further detail with reference to the initial & final slag analysis and for the obtained metal analysis. Also illustrate by way of actual example and not by way of limitation (surmise).

In Table.1 High carbon Ferro Chrome Analysis

In Table.2 Smelted slag (obtained after smelting in EAF) Analysis

In Table.3 Metal Analysis.

EXAMPLE

1. Bench scale studies at National Metallurgical Laboratory, Tatanagar,
Jharkhand, India.

Bench scale test was performed in a 50 KVA Electric Arc Furnace having secondary voltage 80-85 V, Secondary current 0.75 KA with 50-55 KW load. Since in the molten form slag of H.C Ferro Chrome was not available at the laboratory, the solidified slag in the size 10-20 mm was taken for the experimental smelting.

7Kgs of High Carbon Ferro Chrome slag was initially melted in the furnace and after attaining the temperature above 1200 °C started adding the required quantities of coke and calcined lime in small increments to carry out the smelting. Smelting was completed in about 20 minutes and attaining a temperature of 1700 °C . About 50 grams of flour spar was added to increase the fluidity of the slag.

Looking at the fluidity and temperature of the slag, the furnace was switched off and tilted to collected the metal in a graphite crucible and slag in sand beds. Next day morning the solidified metal and slag were collected, weighed and analyzed,

Total weight of the metal collected was 911 grams.

2. Trials at FACOR Steel Division, at NAGPUR, Maharashtra with larger
quantities.

To establish the results on a larger scale, solidified slag in the size 10-20mm was initially melted and subsequently smelted in a 20 MT capacity Electric Open Arc furnace normally used for the production of alloy and stainless steel.

7 MT of slag was melted after adding in small batches and after complete melting the slag and attaining a temperature of 1479 °C started adding the required quantities ( as per stoichiometry calculation ) of coke and calcined lime in small increments by shovels. Load kept on the furnace was 7 MW with secondary voltage of 350 volts and secondary current of 20 KA.

After the slag attains a temperature of 1722 °C spoon sample was collected and checked for the reduction of Cr2O3 by observing the colour and weight of the slag. Flour spar was added to further thin the slag and after the melt attaining a temperature of 1780 °C the melt was unloaded in a refractory lined ladle. Slag was decant in slag pot and metal in cast iron pan.

After solidification the metal cake is handled and weighed. The quantity of metal obtained was 820 KGs.

As molten slag was not available for laboratory scale and large-scale studies, Solidified slag was melted in the respective furnace and carried out the reduction studies. Since, liquid slag from High Carbon Ferro Chrome production is available after the tapping in the Ferro Alloy plant premises, chromium oxide reduction from liquid High Carbon Ferro chromium slag can be directly carried out on tap to tap basis on commercial scale.

We claim a process for winning in form of saleable metal all the Chromium present in High Carbon Ferro Chrome slag issuing an Electric Open Arc furnace where by the chromium oxide and Iron oxide content in the said slag, is reduced to Chromium Iron carbide comprising of the steps of unloading the liquid slag obtained in the tapping of sub-merged arc furnace in to the electric open arc furnace; after stabilization of arc addition of the required quantities ( as per the Stoichiometry calculation ) coke and calcined lime in increments to promote reduction of Chromium Oxide in to metal and also win metallic present in slag with new metal formed.

After the melt attains a temperature of about 1750 °C, if required little quantity flour spar is to be added for thinning the slag. Spoon sample has to be taken to check the completion of the reduction of oxides to metallic state. After confirming the reduction by change in colour of slag sample from original green to white, the melt is unloaded in a refractory lined ladle, slag is decanted and metal cast in cast iron pans.

A process for reduction of Chromium Oxide present in High Carbon Ferro Chrome slag in electric arc furnace substantially as herein described with reference to and as illustrated by the input and output analysis enclosed.

Input and output Analysis

Table -1

Analysis of High Carbon Ferro Chrome slag (the input material):

Cr2O3% 14.06

FeO% 04.14

SiO2% 29.18

CaO% O3.36

MgO% 26.06

Al2O3% 23.00

Table - 2

Analysis of reduced slag (after completion of smelting):

Cr2O3% 0.76

FeO% 0.42

SiO2% 30.44

CaO% 27.72

MgO% 21.56

Al2O3% 17.84

Table - 3

Analysis of Metal obtained

Cr% 55.10

Si% 06.25

C% 07.21

S% 0.O29

With the advent of stainless steel production in India for which chromium is the major additive, the ferrochrome production gained importance. Conventionally High Carbon Ferro Chrome is produced in submerged electric arc smelting furnace. The raw material in the production of High Carbon Ferro Chrome consists of (a) Chromite, which is chrome and iron oxides containing mineral, (b) metallurgical coke to reduce the metal oxides present in the ore and (c) fluxing material like Quartz, Bauxite to get the right composition of the slag. The smelted products obtained from the smelting furnace are High Carbon Ferro Chrome Alloy and Slag, which are tapped in cast iron receptacles at regular intervals. The slag production is 1.15 to 1.2 MT for every ton of alloy produced depending on the composition of the feed material. The temperature of the slag in tapping is about 1700 C. The typical Ferro chrome slag composition is SiO2 - 28 to 30%, Al2O3 - 24 to 26%, MgO - 22 to 24%, CaO - 2 to 2.5%, Cr2O3 - 12 to 14% and FeO - 2 to 3%. Out of 12 to 14% Cr2O3 present in the slag about 2 to 2.5% is in the form of metal entrapped in slag and the remaining 10 to 11.5% is in oxide form.

Limitation in conventional method of High Carbon Ferro Chrome Production.

The present process has got a limitation of recovery of only 84% of chrome even after following the best process control methods and loosing about 16% of chrome in slag (in metallic and oxide form) which is being used for landfill, Thus we are loosing the non-replenishable mineral to a tune of 25% for every ton of High Carbon Ferro Chrome produced. The present production capacity of High Carbon Ferro Chrome in India is about 9 to 10 Lac MT per year and a quantity of 22,57,100 MT of chromite is consumed.

Because of the limitation in the conventional process of producing High Carbon Ferro Chrome in sub-merged arc furnace, in a year, we are loosing about 5,64,275 MT of chromite in the form of Cr2O3 in slag and thus depleting the non-replenishable mineral to such a great extent.

Till date efforts are paid only to recover the entrapped metal in the slag, which is around 2 to 2.5%, by mechanical means. No research work is made to recover the chromium Iron present in the slag in oxide form.

Object of the invention: An object of this invention is therefore to recover the chromium lost in the slag to an extent of 98% by smelting the slag along with coke and required fluxes in electric open arc furnace by adjusting the slag basicity. Apart from the object of this invention, another important object is to conserve the non-replenishable mineral source, the chromite besides achieving overall cost reduction in the production of High
Carbon Ferro Chrome.

The target is to achieve at least 98% reduction of chromium oxide and Iron oxide in the Ferro chrome slag. The alloy produced from the slag can be directly used for the production of alloy steel and stainless steel.

ANNEXURE- 3 Claims:

As molten slag was not available for laboratory scale and large-scale studies, Solidified slag was melted in the respective furnace and carried out the reduction studies. Since, liquid slag from High Carbon Ferro Chrome production is available after the tapping in the Ferro Alloy plant premises, chromium oxide reduction from liquid High Carbon Ferro chromium slag can be directly carried out on tap to tap basis on commercial scale.

We claim a process for winning in form of saleable metal all the Chromium present in High Carbon Ferro Chrome slag issuing an Electric Open Arc furnace where by the chromium oxide and Iron oxide content in the said slag, is reduced to Chromium Iron carbide comprising of the steps of unloading the liquid slag obtained in the tapping of sub-merged arc furnace in to the electric open arc furnace; after stabilization of arc addition of the required quantities ( as per the Stoichiometry calculation ) coke and calcined lime in increments to promote reduction of Chromium Oxide in to metal and also win metallic present in slag with new metal formed.

After the melt attains a temperature of about 1750 °C, if required little quantity flour spar is to be added for thinning the slag. Spoon sample has to be taken to check the completion of the reduction of oxides to metallic state. After confirming the reduction by change in colour of slag sample from original green to white, the melt is unloaded in a refractory lined ladle, slag is decanted and metal cast in cast iron pans.

A process for reduction of Chromium Oxide present in High Carbon Ferro Chrome slag in electric arc furnace substantially as herein described with reference to and as illustrated by the input and output analysis enclosed.