The Miller Chlorination process is capable to treat bullions with gold content higher than 70%. When the bullion has lower gold contents the time required to refine the charge is long and the economy of the refining process is affected. The main impurity of the bullion is silver and when the silver content is very higher than 30%, the operation of the process tends to be complicated. The best solution is to mix bullions of different content of gold and silver until obtain an appropriate balance. Also, the elimination of other impurities is influenced by the presence of silver.
During the chlorination process, precious metals and base metals react with chlorine simultaneously and the presence of gold into the molten charge is growing constantly. This action produces more gold chloride every moment until the reaction is complete. Basically, chlorine gas reacts with metals following a selectivity order: zinc, iron, antimony, tin, arsenic, copper, lead bismuth, silver, tellurium, gold, and platinum group metals. Fumes transport chlorides and the behavior of lead is not stable due to it is removed discontinuously no matter if the flow rate is increased. Apparently, lead needs more time to react properly with chlorine and it is not removed, the last fractions of lead that remain produce a brittle product. In this case the product must be retreated again in order to reduce the content of lead. If much lead is present, it is good idea to increase the proportion of silver in order to extent the time of the refining process. In some occasions, the addition of air or oxygen helps to remove lead.
Although the elimination of lead is difficult, to remove tellurium is more difficult because it remains until the end and the problem is solved by the addition of potassium nitrate. The reaction of silver and copper is rapid and copper chloride is transported by the fumes. Platinum group metals are gold remain together. All the process takes 90 to 120 minutes and it is influenced by the level of impurities and silver in the bullion. For example, a charge reported 99.1% gold, 0.2% silver, 0.6% palladium, 0.01 % platinum and traces of copper and lead. The refining process lasted 120 minutes and the gold content was 99.4%.