Fire assay is a standard method for quantitative determination of precious metals contained at trace levels in ores. It is performed for different purposes (e.g. exploration, process control or accounting) by precious metal producers, companies producing copper or nickel with precious metals as a by-product, companies producing precious metals from recycled materials, and contract or service laboratories.
Fire Assay vs OES
In the traditional fire assay method, an ore sample containing precious metal traces is crushed, ground and weighed. It is then mixed in a crucible with a flux, which contains among other materials litharge (lead oxide) and a reducing agent (e.g. carbon), and heated in a furnace at high temperature. The litharge is reduced and forms lead droplets that quantitatively collect the precious metals contained in the sample. The sample is poured into a cone shaped mold to cool down. The lead is recovered in the form of a lead button that is separated from the slag and cleaned before being weighed. In the next operation, called cupellation, the button is placed in a cupel (a special MgO crucible), and heated and melted at high temperature in an open furnace. Lead is oxidized when it comes in contact with air and absorbed by the cupel, leaving a precious metal bead or prill. The bead is generally dissolved in aqua regia and the precious metals concentrations determined by several methods (AAS, ICP-OES, ICP-MS).
OES is The Clear Winner
Optical Emission Spectrometry (OES) offers an alternative to the fire assay method, which is time- and labor-intensive, expensive, and environmentally hazardous. OES instrumentation can determine the gold, silver, platinum, palladium and other precious metal concentrations directly in the fire assay lead button, thereby streamlining and speeding up the process. Eliminating cupellation reduces power and chemical consumption as well as equipment and cupel costs. Costs are further reduced because a single OES analyzer replaces several instruments and delivers results for all the precious metals in minutes, increasing throughput and freeing personnel for other tasks. OES is suitable for the most demanding industrial operations as well as a range of analytical requirements including geological samples in the gold and platinum mining industries, precious metal samples, and industrial samples such as catalytic converters.