Treatment of Silver Ores
Considering the variability of silver minerals, the treatment process is not unique and presents some variations. For many years the treatment of silver ores was oriented to recovery of silver in union with lead, copper or mercury, or in solution from which it can be precipitated as metals or as sulphide or chloride, or else it was separated by electrolysis from its combinations. Although the metallurgy is not simple, the methods of extraction can be classified in three groups; dry processes, wet processes and electrolytic methods of separation. To a certain extent the wet and dry methods of extraction are combined, argentiferous products obtained by dry methods being worked up by wet processes or vice versa. Electrolytic processes are also usually preceded by a preparation of the ore by dry methods.
Basically, the extraction of silver from ores is based in the conversion of the silver into silver-lead alloy, whilst the combined wet and dry methods include the conversion of silver into a silver-lead alloy, the production of silver amalgam and the production of a silver compound which is soluble in aqueous solutions. The electrolytic process for the separation of silver needs its conversion into copper-silver, lead-silver, zinc-silver or gold-silver alloys, from which the silver is obtained by making it the anode in an electrolytic cell. The adoption of any particular method of extraction is determined by the characteristics of the ores. The mineralogy plays an important role.
It has been noted that ore very rich in silver are chiefly worked in the dry way. For ore of poor or medium silver content, either wet or dry methods are used, depending on the ingredients associated with the silver compounds and upon the price of power and other local conditions. Ores containing large percentages of lead are mainly treated by dry processes and ores with large amounts of copper are advantageously treated by wet processes or electrolytic methods. In the case of ores free from copper and lead or containing only small amounts of these metals, the method of treatment depends upon the price of silver and upon the possibility of obtaining lead ores or plumbiferous materials cheaply, upon the possibility of treating the ore directly with mercury without any pre-treatment such as roasting, or of converting the silver in it into chloride, which can then be dissolved by sodium chloride or thiosulphate solution. It also depends upon the behavior of the ore towards chloride solutions, upon the price of some reagents such as sodium chloride, copper sulphate, sodium thiosulphate, mercury, sodium sulphide, iron, copper and zinc, as well as upon the arrangement for the transport of the silver ores. The cyanidation process is other alternative.
Argentiferous lead and copper ores are treated by the usual ways for obtaining their respective metals; the silver is usually present in the flotation concentrate which is treated by smelting processes. The silver obtained is in most cases impure containing copper, lead, arsenic, antimony, bismuth or selenium, which must be removed by a refining process.