The solution produced by the reaction between sulphuric acid and the bullion is mainly silver sulphate. The recovery of silver starts by the addition of the hot solution into cold water in order to promote the precipitation of silver sulphate as small crystals. In order to keep in solution some impurities, the precipitated is boiled again. There are two alternatives to recover silver. The first is the direct treatment of small crystals of silver sulphate with ferrous sulphate. In order to do this, the solution and the silver sulphate are separated, the crystal are treated with hot solution of ferrous sulphate and as a result of this reaction, metallic silver is precipitated, filtered, washed, and melted.
The second alternative involves the addition of small particles of metallic copper into the hot bath and the precipitation of silver is influenced by the agitation of the bath, the concentration of sulphuric acid and the temperature of the bath. The final point of the precipitation process is determined with sodium chloride. In order to do this, a small sample of solution is taken and some drops of sodium chloride are added. The presence of silver is indicated by the presence of white stains in the solution. When the reaction is complete, the precipitate and the solution are separated. The precipitate is silver and before melting, it is filtered and washed. The purity of this gold is 99.6% to 99.8%. The final solution contents copper sulphate, which is treated by a crystallization process to get copper sulphate or by electrowinning if the content of copper and sulphuric acid and appropriate.
It is important to indicate that the election of the silver process is based on the level of impurities present into the solution, mainly lead, content of silver, sulphuric acid as free acid and the estimated weight of silver to recover. All the factors have an impact on the operating costs, efficiency and quality of the silver ingot obtained at the end of the process. A key factor is the level of knowledge and skill of the operators because one mistake may affect the next stage of the process.