Smelting with lead and fluxes

This is a large scale procedure of the well known fire assay procedure which uses lead to collect and separate precious metals from a molten charge. The precious metals are the recovered by vaporization, or less commonly by acid dissolution of the lead. The process was used widely until about 1950, and is still used occasionally for treatment of low grade materials and material with high lead content.

Fluxes, lead oxide (litharge) and, if necessary carbon are mixed with the charge or material t be smelted. Smelting is performed at similar temperatures to those required for simple flux smelting. The lead oxide is reduced by metals, sulphides and carbon, if present to form lead metal. The lead drains through the molten charge as fine droplets and collect the contained gold and silver. The collection process is highly efficient provided that the ratio lead: precious metals is kept high (> 10:1) the lead precious metals alloy is tapped out of the crucible and cooled. This alloy is then treated by cupellation in a strong stream of air to oxidize the lead back to lead oxide, which is removed as a vapor and can be recovered as a solid on cooling to allow recycling to the fusion stage. The gold and silver remain and can be poured or remelted into bars.

The process is slow, primarily because it requires two stages, it is labour intensive and lead fumes are produced, which present a health hazard. Despite these disadvantages the process is effective for treatment of low grade refinery products.