Papers - Lead - Betts Process at Oroya, Peru, S. A.

Electrolytic refining of all lead bullion produced at the Oroya smelter of the Cerro de Pasco Copper Corporation has been adopted as standard practice. A pilot refinery with a capacity of 25 metric tons daily has been in operation since June, 1934, and a new refining unit having an initial capacity of 100 tons per day is under construction. This program has been made possible by certain modifications of the usual Betts refining process1, developed at Oroya, which have the effect of making the purity of the lead produced, and the acid loss incurred in operating the process, entirely independent of the amount of impurities in the original bullion. The modified process, as developed and at present used, treats impure bullion direct, without presoftening or other prerefining step, except that if the bullion carries tin it is desirable to detin before casting into anodes, since tin carries over to some extent to the cathodes. The refining process developed differs from usual Betts practice in two essential particulars: the anodes are cast thinner and replaced at more frequent intervals, and anode slimes are washed by a system of counter-current dipping, prior to their removal from the surface of the anode scrap. Limiting the period of anode corrosion insures a cathode of high purity, since solution of impurities at the anode can occur only if the thickness of the anode slime blanket, and hence the voltage drop across it, becomes greater than a certain critical value. The dip washing system operates to prevent acid losses, by recovering the highly concentrated, nearly neutral lead fluosilicate solution present in the pores of the slime blanket on the corroded anode, and doing this in such a way that there is no opportunity for chemical decomposition to occur. The best illustration of the efficacy of these two expedients may be secured from records of more than a year's operation of the 25-ton pilot plant, during which period the acid losses were less than 2 Ib. of 100 per cent hydrofluosilic acid (H2SiF6) per ton of lead produced, and typical assays of bullion and refined lead were as shown in Table 1.