Silicon is widely used in various branches of industry. So, due to its ability to create valuable alloys with unique properties Si is used for making rust-resistant pipes when obtaining silicon steel for electrical industry, in transformer, instrumental, corrosion-resistant, heat-proof, spring, constructive and other steels. Silumins (silicon and aluminum alloys) applied in space and aviation, automobile, instrument-building and other industries; corrosion-resistant silicon bronzes, silicon and magnesium alloys, abrasive materials based on silicon carbide are widely known. Silicon is used for the production of a wide range of organic silicon compounds. The ultrapure Si - is the main semi-conducting material for transistors, current rectifiers, radio waves enhancers, controllers, electronic chips for computing devices. Silicon serves as the basic material for making photoelectric converters (PEC) as well .
The technical (metallurgical) silicon (Sitech) is obtained by the carbo-thermal method out of silica-containing raw material in electric arc furnaces on the general reaction: SiO2+2C=Si+2CO [2,3]. The Sitech obtained at the melting process dissatisfy the consumer requirements on the ultimate product chemical purity. That is why refining is practiced nowadays.
A complex operation of refining should reduce the content of Al, Ca, Fe, Ti and other admixtures in the silica and also fully remove small and big slag pockets. The silicon refining methods (those not introduced into production as well) at the national and overseas plants are based on the following physical and physico-chemical phenomena:
- the slag separation by settling the liquid for coagulation and separation of small inclusions into a single phase;
- the slag separation by the silicon remelting and settling an additive agent of the degassing flux;
- the separation of metallic impurities by their transferring into fugitive chlorides and fluorides by blowing with gases or solid additions gases;
- the same with an additive oxidative agent - O2 and transfer of the admixtures (Ca, Al and others, for example) into oxides passing into slag;
- the purification of powdery silicon in solid by halogens, chlorhydric or sulphuric acids, etc.; the ultimate product being the refined silicon powder;
- the silicon treatment in plasma.
The main method of Sitech commercial refining at the CJSC "Kremny" (Shelekhov, Irkutsk Region) - is the oxidizing-flux one performed in scoops by air blowing (with adding siliceous sand as the flux) . We carried out the pilot plant tests on the operating equipment of the enterprise by two refining methods. 1. The refining of Sitech by the oxidating method with the following crystallization was performed at the following parameters: the compressed air flow = 29-34 m3/h; the melt temperature = 1550-15700С; the gas-and air mixture supply increase up to »0,6 МPа; the blowing time = 16 hours; the silicon crystallization period = 48 hours.
2. The refining of Sitech by the oxidating method by blowing the melt with oxygen-rich air was performed (without adding fluxes) at the following process variables: the pressure in the oxygen and compressed air supply lines »0,5 МPа; the melt temperature = 1472-14810С; the refining time (average) = 1,5 hours; the consumption indices of oxygen, compressed air, m3/h, accordingly: 4; 19,5. After the carried out tests the degree of Sitech refining from principal impurities made, %, accordingly: on the first method - Fe - 97,27; Al - 95,5; Ca - 99,64; on the second method - Fe - 6,7-8,2; Al - 70; Са - 94,95.
Thus, the silicon refining procedures suggested allow obtaining refined silicon of high chemical purity without significant changes of the process flow sheet existing at the plant.
The work was supported by the program of the Ministry of Education and Science of the Russian Federation «Development of Science potential in Higher Education (2006-2008)», project no. RNP 126.96.36.1992.
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The work was submitted to international scientific conference «Manufacturing Technology», Italy (Rome, Florence), September, 9-16, 2008, came to the editorial office on 23.07.2008