Assay Sampling Protocols for Industrial Metal Scrap Valuation
Assay sampling protocols establish true dry weight and payable metal content, directly governing settlement values, working capital allocations, and lender borrowing base limits.
Lead Fusion
Meaning
High-temperature fire assay smelting represents the chemical process used to extract precious metals from rock samples. The lead fusion process takes place in a crucible where the pulverized sample is mixed with a flux containing litharge. This mixture is heated to over one thousand degrees Celsius, causing the litharge to reduce to metallic lead.
Chemical Reaction
The fluxing agents dissolve the silicate minerals to form a liquid slag. This allows the metallic lead droplets to sink. As the lead falls, it collects the gold and silver by forming a dense alloy, which separates the metals from the waste.
Melt Separation
The molten mixture is poured into an iron mould to cool and solidify. A dense lead alloy collects at the bottom to form a button, while the lighter slag sits on top. This clear separation allows the chemist to break away the slag with a hammer.
The clean lead button is then ready for the next stage of analysis.
Extraction Efficiency
Proper flux composition is required to ensure that all precious metals are collected by the lead. If the flux is too acidic or too basic, some gold may remain trapped in the slag. This leads to an underestimation of the metal content of the ore.
The chemist must balance the flux chemistry for each specific ore type. If the ore contains high sulphur, additional oxidants must be added to the crucible to prevent the formation of a matte phase.