A study on purification of metallurgical grade silicon by molten salt electrorefining
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to desired current density. At the end of electrolysis the cathodic deposit was withdrawn to the receiver chamber and rapidly cooled to room temperature. A new cathode was subsequently inserted in to the electrolyte chamber via the receiver compartment to continue next electrodeposition without exposing the melt to open atmosphere. The silicon electrodeposits were processed by boiling first in water and then dilute hydrochloric acid before finally washing in water and drying. The cathode current efficiency was computed by assuming a simplified electrode reaction for deposition of silicon as shown below: Si 4§ + 4e- ---+ Si The different experimental parameters studied were cathode current density (7.75 to 23.25 • l 0 2 amp/m2), temperature (923 to 1073 K), and KzSiF6 concentration in the electrolyte (6 to 18 mol pet). The influence of cathode current density on cathode current efficiency was studied during electrolysis in a bath consisting of 42.5 mol pet LiF, 42.5 mol pct KF, and 15 mol pet K2SiFr. The results are plotted in Figure 1. It can be seen that as high as 98 pet current efficiency could be achieved by using a current density of 13.5 x 102 amp/m 2. The deposit freshly removed from the cell appeared to be modular in growth and when leached with acid yielded shining crystals of silicon. A thin plating of silicon on the graphite electrode was also observed. Use of higher current density reduced the efficiency presumably because the deposit became more and more powdery in nature and consequently less adherent. Figure 2 presents the plots of temperature and K2SiF6 concentration in the equimolar KF-LiF electrolyte vs current efficiency. During studies on the effect of temperature, an electrolyte consisting of 15 mol pet K2SiF6, 42.5 mol pet LiF, and 42.5 mol pet KF was employed and the cathode curI00
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I. G. SHARMA, Scientific Officer-SE, and T. K. MUKHERJEE, Group Leader, are with Extractive Metallurgy Section, Metallurgy Dwtsion, Bhabha Atomic Research Centre, Trombay, Bombay-400 085, In&a. Manuscript submitted June 17, 1985. METALLURGICALTRANSACTIONS B
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Fig. 1- - Influence of cathode current density on current efficiency. Electrolyte composition: 42.5 mol pct LiF-42.5 mol pct KF-15 mol pct K2SiF6; temp.-1023 K. VOLUME 17B, JUNE 1986--395
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After determination of these optimum parameters in single deposit experiments, it was decided to prolong the electrolysis semicontinuously to determine the metal recovery and average cathode current efficiency values. Table I presents the results of such an experimental campaign. It can be seen from this table that 67.4 g silicon could be deposited out of 80 g anode feed representing a metal recovery of 84 pct at an average cathode current efficiency of 92 pct. The weight loss suffered by the anode feed was found to
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