One of the first methods employed to refine gold and silver made use of sulphuric acid and was developed by C. D’Arcet. The method was introduced in France and the refining process was adopted by several refiners in Europe. The bullion must be pretreated by and oxidation procedure with air or niter in order to remove some impurities present in the bullion. Also, the alloy to be refined must be in granulated form in order to favor the dissolution of the metals. Once this operation is complete and the content of gold and silver has been determined, it is necessary to add three or five part of silver per one part of gold. The silver must not be of high purity, but the level of impurities should be low. It is a good idea to use unrefined silver or in some circumstances silver nitrate.
It is recommended to control and regulate the presence of base metals in the bullion because they react with sulphuric acid and their sulphates are very soluble in concentrated sulphuric acid and they may affect the reactions of the process. For example the presence of high levels of lead, more than 4%, is one of the main problems of the procedure. The sulphuric acid procedure requires the addition of copper in order to promote the dissolution of silver. Basically, the addition of copper is 8-12% of the total. Although, high content of lead are problematic, small quantities help to the dissolution process.
The addition of sulphuric acid is 2 to 3 times the weight of the final alloy to refine. Sulphuric acid is added in two parts, at the beginning is added approximately 50% of the total addition and the other part when most metals are dissolved. Since the process is performed at temperature, it is important to avoid the excessive turbulence of the bath and assure the homogeneity of the charge into the vessel employed for the dissolution. Once the reactions are complete, it is possible to distinguish undissolved granulates of alloy and insoluble sulphates. As a rule of thumb, one part of silver sulphate is soluble in 0.25 parts of hot sulphuric acid and the solubility is influenced by the temperature. For this reason, when the temperature decreases, it is added more sulphuric acid. The other important sulphate is copper sulphate, which is slightly soluble in hot concentrated sulphuric acid and precipitates completely when the bath is at ambient temperature. The reaction of zinc and tin are similar to copper, and the presence of excessive quantities of lead is manifested by a white color. Once the bath is totally cool, the solution and the solid are separated. The solution is basically silver sulphate and the solid contents almost all the gold.