Chemical equilibria between silicon and slag melts
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I.
INTRODUCTION
M E T A L L U R G I C A L grade (MG) silicon is the starting material for the production of solar grade silicon and other high-purity grades. It is made in electric arc furnaces by reduction of quartz sand with coke. The purity is usually 98 to 99 pct silicon with the main other components being aluminum, calcium, and iron. The classical route for purification is the Siemens process. Unfortunately, this process is very expensive. Therefore, other methods of purification or prepurification have been explored in recent years. Among them is the so-called reactive oxygen blowing tl,2] in which elements with higher oxygen affinity than silicon are converted to their oxides forming a silicate slag. A related process is the treatment of the silicon melt with a silicate slag. tl,2,31 It has been found that the calcium and aluminum contents of the silicon can be decreased considerably and, to some extent, also the boron content. So, it is known that purification of MG silicon from certain elements is possible in principle by oxidizing reactions. The true potential of such processes, however, is still unclear, because systematic studies of the chemical equilibria of most of the relevant reactions have not been performed so far. We carried out equilibrium measurements on the distribution of calcium, aluminum, magnesium, titanium, sodium, and boron between liquid silicon and various silica-saturated slags.
H. GENERAL OUTLINE OF EXPERIMENTAL METHOD The equilibria of the following reactions were investigated: (CaO) + 1/2[Si] = [Ca] + 1/2(SIO2)
[1]
1/2(AlzO3) + 3/4[Si] = [All + 3/4(SiO:)
[2]
(MgO) + 1/2[Si] = [Mg] + 1/2(SIO2)
[3]
TORSTEN WEISS and KLAUS SCHWERDTFEGER, Professor, are with the Institut fiir Allgemeine Metallurgie, Technische Universitat Clausthal, D-38678 Clausthal-Zellerfeld, Germany. Manuscript received November 2, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS B
(TiOx) + x/2[Si] = [Ti] + x/2(SiO2)
[4]
1/2(B203) + 3/4[Si] = [B] + 3/4(SIO2)
[5]
1/2(Na20) + 1/4[Si] = [Na] + 1/4(SiOz)
[6]
The parentheses denote the slag phase and the bracket, the silicon phase. About 2 g of silicon and about 5 g of slag were melted in a silica crucible. Hence, the slag attains saturation in SIO2, and the SiOz activity is fixed at asio: = 1. Most of the experiments were carried out at 1500 ~ The slags chosen for the investigation of Reactions [1] through [5] were of the binary CaO-SiO2 system and the corresponding ternary systems CaO-SiO2-Y with Y being A1203, MgO, TiOx, and B203, respectively. Reaction [6] was studied using Na20-SiO2 and Na20-SiO2-CaO slags. The specimens were held at the reaction temperature for the specified time, then quenched in the water-cooled top of the furnace. The slag and silicon were separated from the crucible and from each other and chemically analyzed for the elements of interest. In most of the experiments, high-purity silicon (electronic grade, Wacker) was taken as the starting material so that the equilibria were attained from the left sides of Reactions [ 1] through
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