Gold refineries are used to 20 kg to 40 kg charges. The initial step is to preheat the crucibles at 900-1000 oC and then add the ingots, which must fit into the crucible. At this moment, borax and silica are added to cover the crucible and protect the slag. Normally, there are two or three additions of borax and silica, and the final addition depends on the slag viscosity. Approximately after 25-30 minutes, the material begins to melt and two pipes preheated are introduced partially and slowly into the molten charge with small additions of chlorine gas. In some refineries, there are two pipes, one for chlorine gas and the other for air. The formation of metallic chlorides helps to melt the charge uniformly.
As soon as the chlorine gas is bubbled into the molten charge, some air bubbles tend to appear at the top of the molten charge. Chlorides are formed and volatilized and carried by the fume rapidly and as they are produced, the addition of chlorine gas is increased gradually until the system is under full pressure. At this moment the formation of chlorides of silver and copper is complete and the slag is removed. Basically, the chlorides and slag formed by oxides are poured into a large mould. The operation is repeated and after a second or third elimination of slag, the addition of chlorine gas is stopped slowly.
The final point is detected by the appearance of a reddish coloration in the pipes. The color is produced by the presence of gold chloride out of the molten charge and presence of high levels of copper tends to make difficult to detect the final point of the refining process because the final coloration is darker. At this moment, the addition of chloride is totally stopped and the slag and chlorides are removed. Potassium nitrate (niter) is added, and the addition is stopped when a green layer appears. This layer is the final slag of the refining process and indicates the presence of tellurium. Once this slag is removed, the molten charge is poured in ingots.
Once the all the chlorides have been removed and cooled, they are homogenized and prepared in lots of 500-600 kg. The material is melted and the flux is prepared with borax and sodium chloride. It is common practice to add small dosages of sodium carbonate. Once the charge is melted and its homogeneity has been verified, the slag is removed and the molten charge (chlorides) is poured into moulds. The bottom of each mould contains a button of silver and gold that is refined again. The chlorides are crushed and treated with hot sulphuric acid, which dissolves copper chloride. The remaining material is silver chloride and reduced with iron. The final product is treated with oxygen and potassium nitrate, and final step is to filter and wash the cake. The purity of the silver is 99.5% to 99.9%. Chlorides contents 60-70% Ag, 12-18% Cu, 2-4% Pb, 0.05% Te, 0.01% Au and small quantities of sodium chloride.