The precipitate or residue obtained after the sulphuric acid attack is basically gold and its recovery is performed by washing and melting the precipitate. In order to do this, the precipitate is treated with hot concentrate sulphuric acid. The operation may be repeated several times until make sure all sulphates have been dissolved. When the proportion of sulphates is not high, the process is simple and the precipitate is separated, washed with hot water and finally melted with borax, potassium nitrate and sodium carbonate. If the lead was not eliminated properly, the ingot will be a little brittle. The purity of this gold is 99.6% to 99.8%.
If the precipitate contents a high proportion of sulphates the process is more complicated. In this case, the precipitate is treated with hot dilute sulphuric acid. The basic idea is to help to dissolve the small quantity of silver present and the sulphates of copper, zinc, tin and lead. It is important to add the precipitate in small portions in order to promote the dissolution of sulphates. When the addition of the precipitate is extremely high, it will be formed a copper sulphate, which retards the dissolution process. At the end of the process, the residue is removed, washed and melted. The other option is to treat the bath with sodium bisulphate in order to help the dissolution of sulphates, mainly silver sulphate. Similarly to the previous case, the residue is washed, and melted with a flux prepared with borax, potassium nitrate and sodium carbonate. The success of the process depends on the dosage of sulphuric acid and the solubility of the sulphates. The process requires time and skill operators; otherwise the operating cost can be extremely high.