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.
Fire Assay
Meaning
Quantitative thermal analysis is the industry standard laboratory method for determining the concentration of gold, silver, and platinum group elements in ores. Conducting a fire assay requires mixing a pulverized sample with chemical fluxes and a lead oxide collector before melting it in a furnace. This high-temperature extraction separates the precious metals from the waste rock.
Fusion Process
Technicians mix the rock powder with a dry flux containing borax, soda ash, silica, and litharge. This mixture is heated in a crucible to over one thousand degrees Celsius to melt the rock into a liquid slag. The lead oxide reduces to metallic lead, which settles to the bottom of the crucible while gathering the precious metals.
This produces a lead button after cooling.
Cupellation Extraction
The lead button is separated from the slag and placed in a porous cupel. Heating the cupel in a furnace oxidises the lead, which is absorbed into the cupel walls. This leaves a small, shiny metal bead containing only gold and silver.
The bead is then recovered for final measurement.
Gravimetric Finish
Chemists weigh the bead on a microbalance before separating the metals with acid. Dissolving the silver in hot nitric acid allows the remaining gold to be washed, dried, and weighed again. This difference in weight reveals the original silver content of the ore.
The final mass of gold gives the gold grade.