S.V. Kim¹, Candidate of Engineering Sciences,

A.S. Golovach², master’s degree student,

A.A. Tursynova², master’s degree student,

¹Abishev Chemical-Metallurgical Institute, Karaganda, Kazakhstan

²Karaganda State Technical University, Kazakhstan

Аlternative carbon reductant for silicon metal smelting

Extensive research of the complex mechanism of Si recovery in carbothermal smelting of silicon metal confirms the role of gaseous component, silicon monoxide [1-3]. In the final stages of the recovery process, its concentration in the gas phase may reach 75% [4]. Further disproportionation of SiO gas causes excessive loss of silicon in the form of silica fumes by evaporation. The ability of carbon reductant to absorb gaseous SiO with further formation of silicon carbide becomes critically important for an efficient process. Sorption capacity of reductant in high-temperatures during silicon smelting is ensured primarily by high reactivity, developed porosity and large surface area.

Low-ash special coke was created for silicon smelting as a substitute for charcoal which, due to almost complete absence of forests, is very difficult to produce in Kazakhstan. The technology is basically a high-speed thermal-oxidative coking achieved by fast heating of coal by means of combustion of its volatile components.

Initial material for low-ash special coke is high-volatile non-coking coal of Shubarkol deposit (Kazakhstan). It can be used for silicon smelting as a substitute for charcoal which, due to almost complete absence of forests, is very difficult to produce in Kazakhstan. Shubarkol coal contains about 45% of volatile matter and only about 1,5-3% ash. Considering high silica level in coal ash (at least 55%) this material has good prospects in terms of chemical purity – very important parameter for silicon metal process.

Modeling of coal processing conditions simulating the real coking process was carried out in Tamman furnace which is a high-temperature resistance furnace with vertical graphite tube. Experiments were made in heat-resistant steel crucible with open access for air. Initial temperature varied from 600 to 1100°C with 100°C intervals. A batch of raw coal in a steel crucible was placed into the furnace pre-heated to certain temperature. Temperature measurement was continuous throughout the experiment.

Experimental modeling of thermal-oxidative coking showed that structural characteristics of carbon reductant from Shubarkol coal are closely linked to basic parameters of the process – coking temperature and heating speed. Processing of experimental data has allowed determining the area of optimal process parameters ensuring achievement of necessary physical characteristics of reductant. Results of described research can be summarized in the following conclusions:

thermal destruction of coal is an integral part of coking process responsible for formation of lump carbon material with required mechanical characteristics;

– intensity of destructive processes is directly proportional to the amount of input energy per unit of time, i.e. heating speed;

– both heating speed and coking temperature directly influence the structural characteristics of reductant and therefore can be used as control action instruments to achievef required quality of the final product;

– optimal temperature of the coking process lies above 800°C. Operation in the temperature range of 900- 1100°C eliminates formation of pyrolytic carbon inside the coke body thus ensuring high reactive capacity of reductant towards gaseous silicon monoxide;

– minimal heating speed necessary for development of fine-pore structure and formation of large internal surface of reductant is 40 °C/min;

– organization of coking process with strict control of above parameters allows producing high-quality carbon reductant efficient for carbothermal smelting of silicon metal in submerged arc furnaces. It is necessary to note that, aside from structural properties, the quality of carbon reductant is also determined by its mechanical strength, specific resistance and reactivity.

References:

[1] Katkov O., Kozlov S., “Scheme of silica recovery in arc furnace”, Proceedings of Universities, Non-ferrous metallurgy, 1991, No. 3, p. 59.

[2] Ryabchikov V., Schedrovitskiy Ya., “Role of gaseous phase in SiO2-carbon interaction”, Reports of Academy of Sciences of USSR, 1964, v. 158, Pp. 427-428.

[3] Tolstoguzov H., “Scheme of carbothermal reduction of silicon”, Proceedings of Universities, Non-ferrous metallurgy, 1992, No. 5-6, Pp. 71-81.

[4] Katkov O., Nuikin Yu., Karpov I., “Causes of silicon losses with gas in electric smelting”, Proceedings of Universities, Non-ferrous metallurgy, 1985, No. 6, p. 37.