US3525608A - Method for increasing the chromium content of ferrochrome with low chromium content - Google Patents

Description

United States Patent Oflice 930/6 Int. Cl. C2Zc 35/00, 39/14 US. Cl. 75-1305 5 Claims ABSTRACT OF THE DISCLOSURE A method for increasing the chromium contact of loW chromium content ferrochrome derived from ores with a low ratio of chrome to iron. Such material is smelted in a conventional manner into ferrochrome, controlling the carbon and silicon content of the charge such that the carbon content of the produced smelt is within the range of 4-8% and silicon content is within the range of 0.2-6.0% and that the molten product is cooled in controlled crystallization conditions, or normally cooled ferrochrome is then heat treated similarly in controlled crystallization conditions, in such a manner that the produced chromium enriched chromiurn-iron-carbide crystals on one hand and crystals containing mainly iron on the other hand exist separate in the solid material in suitable sizes, and that the crystals of the phase containing mainly chromium, after crushing and grinding, are separated by mechanical separation methods, by magnetic or electrostatic means or by means based on differences in specific gravity.

BACKGROUND OF THE INVENTION Field of the invention The present invention relates to a method for increasing the chromium content of ferrochrome with low chromium content, produced from ores with a low ratio of chromium to iron.

Description of the prior art When assessing the adaptability of natural chrome ores or concentrates thereof to the production of ferrochrome, their ratio of chromium to iron is held as a standard of valuation. If this ratio in an ore is less than 2.8 by present production methods it yields ferrochrome of low chromium content, the use of which is partly limited e.g. by the need for addition of chromium steel scrap.

Furthermore steel producers are not willing to use ferrochrome with low chromium content in steels to be highly chromium alloyed because of the resulting difiiculties in connection with smelting. Ferrochrome with low chromium content, in order to reach the same chromium content in the steel, must be added in considerably greater quantities than in the case of ferrochrome with higher chromium content. This would result in excessive cooling of the smelt, which, with conventional alloy steel production methods would be uneconomical, because of among other things the necessary additional smelting time.

In order to improve the possibilities of utilizing ferrochrome produced from low ratio ores, methods for upgrading of ores have been investigated. Many of these methods are rather complicated, and the removal of excessive iron takes place directly from the ore by reducing the ore either in its solid state with subsequent dissolving or other treatment steps, or in its molten state,

3,525,608 Patented Aug. 25., 1970 the thus separated iron with some chromium content being cast into ingots or plates.

Attempts have been made to adapt these methods to commercial production, but because of their complexity and/or high fuel consumption they have proved to be uneconomical.

SUMMARY OF THE INVENTION One of the principal characteristics of the method according to the present invention is that an ore or concentrate with a low ratio of chromium to iron is smelted in a conventional manner into ferrochrome, controlling the carbon and silicon contents of the smelt, whereupon with utilization of controlled crystallization and afterwards crushing and grinding the product it can be separated into a chromium enriched fraction and another fraction containing a major portion of iron.

The resulting fraction containing chromium enriched chromium iron carbides can be used for the production, by suitable conventional methods, of high quality ferrochrome, or it can directly be utilized as alloying element for purposes where the carbon content is not an obstacle.

The other fraction containing a major portion of iron, can be utilized in steel works and foundries or for other suitable purposes.

The method comprises controlling the carbon and silicon contents of the low chromium ferrochrome smelt and controlled cooling of the smelt and/or heat treatment of the cooled product in such a manner that in the low chromium ferrochrome obtained by said method the portion containing metallic iron has a low chromium content and that the crystals of this portion comprise a phase separate from the chromium enriched chromiumiron-carbide crystals.

DESCRIPTION OF THE PREFERRED EMBODIMENTS During the smelting operation the carbon and silicon content of the smelt must be controlled in such a manner that the carbon content of the produced smelt is within the range of 4-8% and the silicon content is within the range of 0.2-6.0%.

The controlled crystallization is carried out either by controlled cooling of the product or if the cooling has not been controlled then by additional heat treatment of the cooled product or by both of these operations in such a manner that in the obtained ferrochrome with low chromium content the crystals containing iron have a low chromium content and comprise a phase by themselves, separate from the chromium enriched chromiumiron-carbide crystals.

These separately crystallized phases can be separated from each other, after crushing and grinding, by magnetic or electrostatic means, means based on difierencs in specific gravity or means based on other mechanical separation methods.

EXAMPLE 1000 kgs. of chrominum concentrate was smelted with coke and flux materials, whereby 57-8 kg. of ferrochrome and 635 kg. of slag were obtained. The composition of the ferrochrome was:

Percent Cr 50.6 C 6.0 Si 0.8

The slag contained 3.7% of Cr O The ferrochrome was cooled as follows:

From 1600 to 1400 C., 2 hours From 1400 to 700 C., 48 hours From 700 to 25 C., fast.

The cooled ferrochrome was crushed, ground to 100 mesh and separated by means of a low-magnetic separator.

The separation result was as follows:

The non-magnetic portion, 381 kg.: Percent Cr 68.8

Si 0.5 Yield 89.2

The magnetic portion, 197 kg: Percent Cr 15.1

What is claimed is:

1. A method for increasing the chromium content of ferrochrome with low chromium content, produced from low chromium-ferro ratio ore by smelting enriched ferrochrome together with coke and flux materials, comprising: controlling carbon and silicon content of the smelt during the smelting operation so that smelt produced has a carbon content Within the range of 4% to 8% and has a silicon content within the range of 0.2% to 6%; crystallizing the smelt produced under controlled conditions by rapidly cooling the smelt produced from a temperature near the smelting point of ferrochrome to a temperature of about 1400 C., then slowly cooling the smelt down to a temperature of about 700 C. then rapidly completing the cooling down to about room temperature for producing chromium enriched chromium carbide crystals and crystals containing mainly iron; crushing and grinding the product; and after such crushing and grinding, separating crystals with enriched chromium content from crystals with low chromium content.

2. A method according to claim 1 wherein the separation is eifected mechanically.

3. A method according to claim 1 wherein the separa* tion is effected magnetically.

4. A method according to claim 1 wherein the separa tion is effected electrostatically.

5. A method according to claim 1 wherein the separation is eifected by sorting out the crystals by difi'erences in their specific gravity.

References Cited UNITED STATES PATENTS 2,031,947 2/1936 Haglund 75l30.5 X 2,370,608 2/1945 Udy 75-.5 X 3,301,669 1/1967 Rathmann et al. 75-1305 CHARLES N. LOVELL, Primary Examiner US. Cl. X.R.