Production of High Purity Silver from Silver Chloride

摘要

Silver chloride is frequently generated during wet chlorination of a variety of precious metals-bearing materials, including copper and nickel electrolytic refinery slimes. Other sources of silver chloride include salts from the Miller refining and silver chloride precipitated from nitric and sulphuric acid silver-bearing process liquors. Typical practice is to process the silver chloride bearing material with a reductant, such as iron powder to convert metal values to the elemental state prior to smelting the material and fire refining the bullion so obtained to produce a feed to electrolytic silver refining. An alternative process is the selective extraction of the silver chloride from the chlorination residue by ammoniacal leaching. A pure silver chloride is recovered from the ammoniacal silver chloride - bearing solution by distilling off of the ammonia to produce a clean crystalline silver chloride product. The silver chloride is reduced to pure elemental silver chemically. This paper will discuss the sodium carbonate preleach, the aqueous ammonia leach, the explosive aspects of silver-ammonia chemistry, the energy considerations associated with ammonia recovery, purification of the ammoniacal leach liquor prior to silver chloride crystallisation and purification of the crystallised silver chloride and other silver chloride intermediates prior to reduction. At present this process is used at a major precious metals refinery for the recovery of pure silver from copper refinery slimes wet chlorination residue.