Materials that contains significant concentrations of mercury (more than 0.1%) must be treated for its removal prior to smelting gold bullion. This practice may be necessary to minimize the release of toxic mercury vapor into the atmosphere, in the case of amalgamation may be used to recover mercury. Materials with lower mercury concentrate may be smelt directly as long as adequate ventilation and fume collection facilities are provided. The mercury content varies depending on the original mercury content of the ore and the recovery method used. For example pressed amalgams resulting from amalgamation processes typically contain 30-70% Hg.
The retort temperature is ramped up gradually to enable the material to dry completely before mercury is vaporized and to allow time for diffusion of mercury to the solid surface. The system is held at maximum temperature for 2-3 hours to ensure completely evaporation. Mercury removal efficiencies in excess of 99% are commonly achieved.
Retorts are operated under a slightly negative pressure and mercury vapor is usually exhausted into a water condensation system. The vapor is cooled rapidly to below the boiling point and the liquid mercury is collected under water to avoid re-evaporation. The mercury produce has small quantities of gold, silver and other metals but is usually of sufficient purity to be reused for amalgamation. Mercury losses vary between 0.2-0.4% for each distillation cycle. These losses are mainly result of uncondensed fumes leaving the condensation system and mercury vapor that permeates into the refinery during retort loading/unloading processes.
Retorting of mercury amalgams yields a mercury free product in the form of sponge gold which can be treated directly by smelting for the removal of residual base metal impurities.
Mercury is highly toxic and has cumulative physiological effect. This factor, coupled with its high vapor pressure can create problems in refineries treating materials with high mercury content. These effects can be counteracted effectively by good ventilation, routine mercury monitoring, good hygiene procedures, and efficient operation of retorting systems.