Determination of Gold and Silver in Lead Ore Concentrate or Copper Ore Concentrate by Fire Assay with Desulfurization by Roasting

Sample of lead ore concentrate or copper ore concentrate (30.00 g) was smooth out on a layer of silicon dioxide (20 g) which was priliminarily placed on the bottom of a combustion boat and roasted for 60 min at 750℃ in an assay furnace for desulfurization. The desulfurized sample mixture was then transferred to a clay crucible, and a flux mixture containing 50 g Na₂CO₃, 200 g PbO and 20 g Na₂B₄O₇ together with 4.0 g of starch as reductant was added. After thorough mixing, and covering a layer of NaCl with its thickness ca. 5 mm, the mixture was fused at 1 100℃ in an assay furnace for the 1st fire assay, and a lead button of 30 to 40 g was obtained. The lead button was cupelled at 850℃ and a metal bead of gold and silver was obtained. The molten slag and cupel left from the 1st fire assay were crushed and placed into the crucible used in the 1st fire assay, and the 2nd fire assay and cupellation were carried out in the same way as the 1st assay and cupellation, except using less amounts of flux mixture (40 g Na₂CO₃, 20 g Na₂B₄O₇ and 25 g SiO₂), and a second metal bead of gold and silver was obtained. The two metal beads of gold and silver were put together and treated with hot HNO₃ (1+7) solution to separate gold from silver and other impurities. The gold was determined by gravimetry or by AAS, and the silver was determined by difference method (the impurities, i.e. Cu, Pb, Bi, As, Sb were determined by ICP-AES). Values of recovery found by test for recovery were in the range of 99.2% to 100% for gold and were 100% for silver, and values of RSDs (n=7) of determination values were ranged from 0.37% to 2.8% for gold and from 0.28% to 2.4% for silver.