Preparation of pure silicon by electrowinning in a bytownite-cryolite melt
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I.
INTRODUCTION
IN addition to quartz and bauxite, there are other kinds of natural minerals potentially suitable for production of silicon and aluminum. Aluminosilicate is one of them, and it can also be used to produce silumin. By using sodium fluoride as a flux, Lyakhovich et al.[1] obtained silicon from Na2SiO3 by electrodeposition. With the same method, silicon also was produced in the system Na3AlF6-SiO2 by Monnier.[2] To make aluminum or aluminum-silicon alloys, different types of aluminosilicates were employed as raw materials by several authors.[3–10] Because the decomposition voltages of SiO2 and Al2O3 at 1000 7C are 1.10 to 1.30 and 1.45 to 1.50 V, respectively,[11] depending on the electrolyte composition, there is a possibility that silicon and aluminum can be deposited separately when aluminosilicates are electrolyzed. The present experiment was designed to prepare high-purity silicon by electrowinning in a bytownite-cryolite molten mixture. II.
cell. In the electric power system, KANTHAL* wire was *KANTHAL is a trademark of Kanthal Corporation, Bethel, CT.
employed as a heating element. The temperature was measured by a Pt-PtRh10 thermocouple, with an accuracy of 54 7C. The direct current was controlled by a switchbox (SB 15-20, made by Powerbox AS, Oslo, Norway). Both the temperature and the current were recorded by a pen recorder. The electrolysis cell had graphite electrodes made from the Le Carbone gold quality (,0.3 pct ash). The anode was a graphite crucible, and its dimensions were o.d. 5 90 mm, i.d. 5 75 mm, and depth 5 175 mm. To assure that the cathode and anode had an even current distribution, a corundum tube (74 mm in o.d. and 60 mm in i.d.) was inserted in the crucible. The cathode had a diameter of 31 mm, and its position was adjusted by a mechanical device. The electrodes had stainless steel current connectors. The cathode connector was covered with an alumina tubelet to protect it from corrosion. The apparatus is shown schematically in Figure 1.
EXPERIMENTAL
A. Materials
C. Procedure
Mineralogically, bytownite refers to a solid solution of anorthite (CaAl2Si2O8) and albite (NaAlSi3O8), containing 90 to 70 mol pct anorthite. It is a member of the feldspar group and its chemical formula can be expressed as (Ca, Na)Al2-1Si2-3O8. The bytownite used ((Ca0.76Na0.24) Al1.76Si0.24O8) in the present experiments is natural and its chemical composition is shown in Table I. The cryolite (NA3AlF6) is a chemically pure synthetic reagent, used for production of metallurgical grade aluminum (99.85 pct Al).
The electrolyte, containing 12 mass pct bytownite with total mass of 650 g, was mixed evenly before it was placed in the crucible. After the electrosis cell was installed in the furnace, electric power was turned on and the temperature started to increase. At the same time, commercially pure nitrogen was flushed into the furnace to protect the cell from oxidizing. A constant direct current, which was 2 or 4 A, was passed through the cell while the temperature reached about 1000 7C. Then t
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