Since gold and silver are the main elements in the bullion, there are some reactions between chlorine gas and both elements. It is important to know how gold is present into the slag. Typically, after 20 – 35 minutes of adding chlorine gas, gold starts its presence into the slag and the level of gold grows easily in the final stages of the refining process. In some refineries, the gold content into the slag is 15-20%, and it is present most the time in metallic form, as tiny particles, whose gold content is as high as 99.9%.
It is possible to detect small globules of gold, but its occurrence is unusual. Apparently, the addition of air promotes the formation gold globules. The presence of gold chloride is possible, but only in small quantities and they are formed at the end of the process. The transference of gold into the slag is favored by the excessive turbulence of the molten charge as a result of the addition of chlorine and the mechanical transference is not very important. Probably, there is a phenomenon of entrainment, which facilitates the transference of gold into the slag. In the case of gold chloride, it is volatilized and trapped by the slag. It has been noted that the gold content into the slag is lower when air is employed in conjunction with chlorine gas. The loss of gold into the slag is mall and most gold refineries report losses of 1.5% to 3.0%. There is more variability in the content of gold as chloride. The addition of chlorine has a direct relationship with the transference of gold into the slag phase.
The reaction between silver and chlorine gas is immediate and it is important to minimize the presence of silver chloride into the slag due to the recovery of silver is not easy. For example, if there is appreciable content of copper, the recovery of silver is very complicated because copper chloride is leached and forms some insoluble salts or if the slag is ground to recover silver, the chlorides produce excessive quantities of fine material. A good practice is to crush the slag in pieces of 20-25 mm and treat them with a hot solution of sodium chloride with small quantities of sulphuric acid. In this process, copper chloride is dissolved and the remaining solid is silver chloride is treated with iron metallic, and the product is impure silver, which is filtered and washed and melted. The purity of the silver is 99.6% to 99.9%. Obviously, part of the copper chloride dissolved content silver and it is treated separately in the same process employed to recover copper.