Silicon metal

The location of a silicon metal plant is determined by balancing market costs against processing ones. Principal elements in the cost of silicon production, which ate site-dependent, are the deUvered cost of the raw materials, energy cost, and labor. Typical costs for production of silicon are given in Table 1. 

The only new sihcon capacity since the early 1990s is by the Gulf Ferroalloys Company in Saudi Arabia. This plant is expected to have four furnaces and produce silicon metal, ferrosihcon, sihcomanganese, and manganese alloys by late 1996. 

D. W. Ksinsik, Silicon Metal Production in an Open DCMrc Furnace, in Ref. 12, p. 25. 

P. deLinde, Silicon Metal Mn Era of Growth and Prosperity SiHcon for SiHcones Conference III, Sande ord, Norway, June 1996, p. 337. 

Fluidized bed reactors do not have to perform poorly, but special conditions must be maintained for good performance. A basic process for silicone manufacturing, which is not practiced much anymore, is the reaction of silicon metal with methyl chloride to form dimethyl dichlorosilane ... 

This reaction is carried out in tall fluidized beds of high L/dt ratio. Pressures up to 200 kPa are used at temperatures around 300°C. The copper catalyst is deposited onto the surface of the silicon metal particles. The product is a vapor-phase material and the particulate silicon is gradually consumed. As the particle diameter decreases the minimum fluidization velocity decreases also. While the linear velocity decreases, the mass velocity of the fluid increases with conversion. Therefore, the leftover small particles with the copper catalyst and some debris leave the reactor at the top exit. 

Metal Oxide - Since metals are less electrophilic than silicon, metal oxide adsorbents show even stronger selectivity for polar molecules than do siliceous materials. The most commonly used metal oxide adsorbent is activated alumina, used primarily for gas drying. Occasionally, metal oxides find applications in specific chemisorption systems. For example, several processes are under development utilizing lime or limestone for removal of sulfur oxides from flue gases. Activated aluminas have surface areas in the range of 200 to 1,000 ftVft Average pore diameters range from about 30 to 80 A. 

Investigations of silicon-metal systems are of fundamental interest, since stable coordination compounds with low valent silicon are still rare [64], and furthermore, silicon transition-metal complexes have a high potential for technical applications. For instance, coordination compounds of Ti, Zr, and Hf are effective catalysts for the polymerization of silanes to oligomeric chain-silanes. The mechanism of this polymerization reaction has not yet been fully elucidated, but silylene complexes as intermediates have been the subject of discussion. Polysilanes find wide use in important applications, e.g., as preceramics [65-67] or as photoresists [68-83],... 

The growing interest in volatile silyl-metal complexes for chemical vapor deposition reactions should also be mentioned. This technique is extremely useful for the preparation of silicide films in microelectronic devices. Further examples of applications of silicon-metal compounds are given in the appropriate sections. 

Also under investigation are MOCVD reactions from precursors having silicon-metal bonds such as SiH3Mo(CO)3.P i... 

Aylett, B. J., and Tannahill, A. A., Chemical Vapour Deposition of Metal Silicides from Organometallic Compounds with Silicon-Metal Bonds, SIRA Int. Seminar on Thin Film Preparation and Processing Technolgy, Brighton, UK (March 1985)... 

See Silicon Metal hexafluorides See other METAL HALIDES, OXIDANTS... 

The reactivity of hexacoordinated silicon species themselves has been considered [7, 8] when we have tried to find a way to obtain organosilanes directly from silica. There is a challenge for chemistry to find a way to obtain silanes avoiding the route through silicon metal, (Scheme 4). 

Other markets for char include iron, steel, and sili-con/ferro-silicon industries. Char can be used as a reducing agent in direct reduction of iron. Ferro-silicon and metallurgical-grade silicon metal are produced carbothermally in electric furnaces. Silica is mixed with coke, either iron ore or scrap steel (in the case of ferro-silicon), and sawdust or charcoal in order to form a charge. The charge is then processed by the furnace to create the desired product. Char can be substituted for the coke as a source of reducing carbon for this process. Some plants in Norway are known to have used coal-char in the production of silicon-based metal products as late as mid-1990.5 The use of char in this industry is not practiced due to lack of char supply. 

Substitution Reactions at Silicon without Cleavage of the Silicon-Metal... 

The demand for silicon in the 1990s has exceeded the installed Western world capacity. The difference has been supplemented by shipments from China, the Ukraine, and Russia. In 1993, Chinese exports reached 117,000 t, whereas exports from Ukraine and Russia were around 40,000 t (19). In 1995, the exports from China increased to 155,000 t, whereas the exports from the CIS (former USSR) countries declined. The silicon metal shipped from these countries has been high in iron and calcium and has been used primarily in the secondary aluminum market. 

The average annual import price of silicon metal based on Metals Week U.S. dealer import price is shown in Figure 2 (22). The price of silicon over... 

Fig. 2. Silicon product prices where ( ) represents silicon metal (°), 50% FeSi and ( ), 75% FeSi (22).

A 75% ferrosilicon furnace operating at 90% silicon recovery would give 0.24 kg of by-product silica fume per kg of silicon produced. A silicon metal furnace operating at 85% recovery (typical of the industry) would give 0.38 kg of fume silica per kg silicon produced. 

J. B. May, Silicon Metal Market, Canadian Industrial Minerals Conference, Vancouver, B.C., Canada, Oct. 1995. 

Fig. 2. Molecular orbital representation of silicon-metal rr-interaction.

Fig. 6. Typical silicon metal furnace. Courtesy of UCAR Carbon Technology Corp.

Due to their similarity to a-c furnaces, d-c furnaces can be substituted for neady any a-c furnace including the open-arc, submeiged-arc, and arc-resistance furnaces, provided that design criteria, particulady electrical parameters, are propedy chosen. Currendy, steel and ferrochrome is being made commercially in d-c furnaces and a silicon metal pilot plant is being built. 

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