Ferroalloys Processing

Ferro-alloys are master alloys containing iron and one or more non-ferrous metals as alloying elements. The ferro-alloys are usually classified in two groups: bulk ferro-alloys (produced in large quantities in electric arc furnaces), and special ferroalloys (produced in smaller quantities, but with growing importance). Bulk ferro-alloys alloys (ferro-chrome, ferro-silicon, ferro-manganese, silico-manganese and ferro-nickel) are used in steel making and steel or iron foundries exclusively, while the use of special ferroalloys is far more varied. In total, about 90 % of the ferroalloys produced are used in the steel industry.

A variety of furnace types, including submerged electric arc furnaces, exothermic (metallothermic) reaction furnaces, and electrolytic cells can be used to produce ferroalloys.

In most cases, the submerged electric arc furnace produces the desired product directly. The submerged arc process is a reduction smelting operation. The reactants consist of metallic ores (ferrous oxides, silicon oxides, manganese oxides, chrome oxides, etc.) and a carbon-source reducing agent, usually in the form of coke, charcoal, high- and low-volatility coal, or wood chips. Limestone may also be added as a flux material. Raw materials are crushed, sized, and, in some cases, dried, and then conveyed to a mix house for weighing and blending. Conveyors, buckets, skip hoists, or cars transport the processed material to hoppers above the furnace. The mix is then gravity-fed through a feed chute either continuously or intermittently, as needed. At high temperatures in the reaction zone, the carbon source reacts with metal oxides to form carbon monoxide and to reduce the ores to base metal. A typical reaction producing ferrosilicon is shown below:

Fe 2O3 2SiO2 7C → 2FeSi 7 CO
Smelting in an electric arc furnace is accomplished by conversion of electrical energy to heat. An alternating current applied to the electrodes causes current to flow through the charge between the electrode tips. This provides a reaction zone at temperatures up to 2000°C (3632°F). Normally, 3 carbon electrodes arranged in a triangular formation extend through the cover and into the furnace shell opening.

The molten alloy and slag that accumulate on the furnace hearth are removed at 1 to 5-hour intervals through the tap hole. During tapping, and/or in the reaction ladle, slag is skimmed from the surface of the molten metal. It can be disposed of in landfills, sold as road ballast, or used as a raw material in a furnace or reaction ladle to produce a chemically related ferroalloy product. After cooling and solidifying, the large ferroalloy castings may be broken with drop weights or hammers. The broken ferroalloy pieces are then crushed, screened (sized), and stored in bins until shipment. In some instances, the alloys are stored in lump form in inventories prior to sizing for shipping.

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