This invention relates to a process for the production of ferrosilicon in a closed two-stage reduction furnace. In the present invention, carbon monoxide released as a result of the smelting process, is used to prereduce higher oxides of iron, for example Fe.sub.2 O.sub.3 and Fe.sub.3 O.sub.4, to iron monoxide (FeO). The use of a closed furnace and a pre-reduction process results in substantial savings both in energy utilization and the cost of certain feed materials.
In the manufacture of ferrosilicon today with greater than about 45% silicon content, an open electric furnace utilizing a submerged arc as an energy source is typically used. This process requires feed materials to be in lump form to prevent the positive pressure, which forms around the electrode, from venting the feed materials from the reaction zone. In this process silicon (S.sub.i) is typically prepared by the carbothermic reduction of silicon dioxide (SiO.sub.2) with carbonaceous reducing agents. The overall reduction reaction for silicon dioxide to silicon metal can be represented by the equation EQU SiO.sub.2 +2C=Si+2CO. (1)
Iron is typically added to the molten silicon of this process in the form of small steel scraps or filings to form ferrosilicon alloy. Alternatively, iron can be added to the process as oxides which are reduced to elemental iron as follows: EQU Fe.sub.2 O.sub.3 +3C=2Fe+3CO (2) EQU FeO+C=Fe+CO (3)
However, iron oxides are typically not used in this process even though inexpensive sources such as ore concentration tailings are available. A major reason is because of the high energy consumption required to reduce the iron oxides to elemental iron.
Present commercial furnaces used in the production of ferrosilicon alloy are estimated to consume approximately three times the theoretical amount of energy needed to effect the reduction of silicon dioxide to silicon. Approximately 50 percent or more of the energy input to this reduction process can be accounted for in the carbon content of the carbonaceous reducing agents. Much of this energy is presently lost as gaseous by-products, mainly carbon monoxide (CO).
Theoretically, if the carbon monoxide lost from the carbothermic reduction of silicon dioxide was used to prereduce the oxides of ore tailings, for example taconite, as great as 0.47 kWh of electricity per kilogram of reduced iron could be achieved. This energy savings along with inexpensive sources of iron oxides such as ore tailings could result in significant savings in the production of ferrosilicon. The tailings could also serve as an inexpensive source of silicon dioxide.
Udy, U.S. Pat. No. 2,637,648, issued May 5, 1953, discloses a process for producing ferrosilicon in an electric furnace. The process uses silicates of a base metal of the group consisting of magnesium, aluminum, potassium, sodium and lithium as a source for the silicon metal. Iron is provided in the form of metallic scrap or the form of iron oxide such as iron ore. The minerals are smelted in the presence of a carbonaceous reducing agent such as, for example, coke.
Eriksson et al., U.S. Pat. No. 4,526,612, issued July 2, 1985, discloses a process comprising introducing a starting material containing a powdered silica-containing material and a powdered iron-containing material, with a carrier gas, into a plasma gas generated by a plasma generator. The heated silica and iron-containing material along with the plasma gas are introduced into a reaction chamber surrounded substantially on all sides by a solid reducing agent in lump form, thereby bringing the silica to molten state and reducing it to silicon which combines with the iron to form ferrosilicon. The iron containing material may be iron oxide. No mention is made of a prereduction step for the iron oxide.
Wilson et al., U.S. Pat. No. 3,704,114, issued Nov. 28, 1972, describes a process for preparing ferrosilicon in an electric arc furnace. The process uses an agglomerated feed material consisting of a particulated silica comprising a fine fraction and a course fraction, a particulate carbonaceous reducing agent, and a particulate iron-bearing material.
Herold et al., U.S. Pat. No. 4,450,003, issued May 22, 1984, discloses a process for recovering combustible gases, in a single-stage, open electrometallurgy furnace, by means of a suction apparatus. Herold suggests the recovered gases can be used by any known process immediately or after a period of deferment, and in particular for preheating or prereducing the components of the furnace charge before they are introduced into the furnace. This patent does not teach a two-stage closed furnace process for prereducing of higher oxides of iron.
Johansson, U.S. Pat. No. 4,269,620, issued May 26, 1981, discloses a two-zone furnace used in a process for the production primarily of silicon metal. The zone of energy supply is divided into a first zone, essentially free from silicon and silicon carbide, and a second zone essentially containing silicon and silicon carbide. Silicon raw material together with reducing agent is charged to the first-mentioned zone and the product gases are conveyed into contact with the second zone wherein the SiO is further reduced to SiC. Mention is made that ferrosilicon can be made by adding iron or iron oxide to the process. No attempt is reported to prereduce the iron oxide with carbon monoxide.
Dosaj et al., co-pending U.S. Pat. No. 239,144, filed Aug. 31, 1988, discloses a cyclic two-step batch operation in a furnace to which a shaft containing a bed of carbon is affixed. In the disclosed process, SiO.sub.2 and SiC are reacted to form molten silicon, SiO, and CO, the SiO then being contacted with the bed of carbon to regenerate SiC. The furnace used in this process is similar to the two-stage furnace described in the process for the present invention.