Adhesion of anode slime on anode surface in electrolytic refining of lead
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I H E demand for high p u r i t y metals is increasing recently. Much has been said about the production techniques of p u r e r g r a d e of Pb by Betts process. Adhesion of anode s l i m e on the anode surface p l a y s the most important role in maintaining the p u r i t y of Pb at 99.99 pct. In electrolysis, the impurities included in the anode must r e m a i n as an adherent s l i m e layer on the anode surface because dropping of the s l i m e into the solution causes an excessive contamination due to mechanical or electrochemical deposition of impurities on the cathode. The formation of a porous s l i m e l a y e r is also a practical necessity t o prevent an abn o r m a l rise in the cell voltage. The relation between the nature of adhesion and impurities in the anode has not been made c l e a r and the adhesion is controlled by experience at present. The p u r p o s e of the present study is t o r e v e a l the factors responsible for adhesion and to contribute t o the rational improvement of the practical operations. EXPERIMENTAL Electrolytic Pb, Bi, Sb, Ag and As of 99.99 pct p u r i t y were used. Anodes were prepared by melting these metals in a graphite crucible and casting in an iron mold. The surface layer of the casting was cut off by machining, and the dendritic structure was destroyed by cold-rolling. The anodes thus obtained were ann e a l e d in a s t r e a m of a r g o n at 250°C for 5 h and subsequently furnace-cooled or water quenched. The e l e c t r o l y t e contained 80 g/1 of Pb, 100 g/1 of free H2SiF~o and 3 g,/1 of glue. Electrolysis was c a r r i e d out at 30 C and 1 A/dm 2 for 26 h, with an electrode spacing of 30 mm. The dimensions of the anode and cathode are 40 × 80 × 5 mm and 40 × 80 × 0.5 mm, respectively. For a comparison of the form of the Bi s l i m e with the Bi deposited by the reduction of Bi ions by metallic Pb, an attempt was made to drop the Bi solution on the TOKIAKI TANAKA, Member AIME, is Professor, Department of Metallurgical Engineering, Faculty of Engineering, Hokkaido University, 060 Sapporo, Japan. Manuscript submitted December 13, 1976. METALLURGICAL TRANSACTIONS B
surface of a pure Pb anode d u r i n g electrolysis. In this c a s e , the Bi solution containing 5.2 g Bi/1, 100 g free H2SiF~/1 was supplied through a tip of L-shaped p o l y ethylene pipe i m m e r s e d in the electrolytic solution. The feed rate of the Bi solution was 1.3 c c / m i n . For quantitative analysis of Bi and Ag in cathode, the method for emission spectrochemical analysis b a s e d on Japanese Industrial Standards H 1123 was employed. For the determination of Sb and As, the method of JIS H 1121 was applied. This method is absorptiometric analysis by using rhodamine B and AgDDTC. The lower l i m i t of these quantitative determinations is 0.0003 wt pct Bi, 0.0002 wt pct Ag, 0.0004 wt pct Sb, and 0.0001 wt pct As, respectively. RESULTS AND DISCUSSION Impurities of the Solid Solution Type The solubility l i m i t of Bi in Pb is about 20 wt pct.1 Hence, the Pb anode with maximum addit
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