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Metallurgist Fire Assay

The method of choice for gold analysis through time has been fire assay.

The basic procedure for fire assay involves mixing an aliquot of powdered sample (10g, 15g, 30g, or 50g are the common sizes used) with soda ash (sodium carbonate), borax (sodium borate), litharge (PbO), flour (baking flour used to add carbon as a reductant), silica, and possible nitre (potassium nitrate).  To this mixture, Ag as a collector can be added in solution or as a foil. The well mixed material is fired at temperatures ranging from 1100C to 1200C (depending on the laboratory). As the Pb and Ag in the melt settle to the bottom of the crucible, its scavanges the Au (+/- Pd, Pt) from the melt. The lead button is cupelled at 950 degrees C in a magnesia cupel. A tiny Ag bead which contains Au, Pt and Pd can be dissolved and analyzed by atomic absorption, ICP (Code 1C-OES) or ICP/MS (Code 1C expl, Code 1Cexpl2, Code 1C Res). Alternatively the Ag can be dissolved out leaving a tiny Au flake which can be weighed (gravimetric technique-Code 1A3) or the entire bead can be irradiated for direct INAA determination (Code 1A1). A variation of the technique using Au as a collector is used for Rh determination (Code 1C-Rh).

The platinum group elements (PGE) can use a nickel sulphide fire assay procedure using borax, soda ash, silica, high purity nickel powder and sulphur. The sample is fired at 1100 degrees C and a nickel sulphide droplets scavenge the PGE and form a button at the bottom of the crucible. The nickel sulphide button is digested in HCl and the insoluble residue is analyzed by INAA Code 1B) or further digested and analyzed by ICP/MS (Code 1B2).
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