Base metals and silver chlorides bailed off from the chlorination process that are contaminated with the borax, silica, sodium chloride flux employed are transferred to the De-Golding area. At this point gold is present in the mixed chlorides in two forms. First, gold particles, which have been mechanically carried over from the melt during the bailing operation. Second, traces of gold formed during final stages of the chlorination process, which have been absorbed by the chloride layer. Gold chloride is unstable and breaks down to the metal and chlorine gas. The gold from this source is present in a very finely divided state. In the de-golding process the mixed chlorides are reheated and sodium carbonate added.
2AgCl + Na2CO3 = Ag2CO3 + 2NaCl
Ag2CO3 = Ag2O + CO2
2Ag2O = 4Ag + O2
The myriad of silver droplets thus formed percolate downwards through the molten chlorides, alloying with the fine gold particles held in suspension. The gold-silver alloy settles to the bottom, leaving the chlorides relatively free of gold (less than 100 ppm). Side reactions cause some base metals oxides to be formed. These float to the surface and are absorbed by the borax-silica flux. The chlorides are transferred from the gold refining furnaces to the de-golding area in a crucible containing chlorides. The chlorides are tipped into the clay/graphite crucible. For example, it will be necessary a 270 gas fired tilting furnace and about 120 kg of chlorides make up a charge for the furnace which is heated to 1,100 oC. at this temperature the flux used as a cover in the gold refining furnace again floats to the surface, inhibiting the volatilization of silver chloride. A total amount of 4 to 7 kg of sodium carbonate is sprinkled over the furnace of the charge, approximately 1.5 kg at a time. The reaction being allowed to subside between additions. This precipitates about 20% of the silver from the silver chloride present and ensures an adequate rain of metal all the gold in the charge. After ten minutes the reaction is complete, but the furnace is allowed to stand for a further 10 minutes. Thereafter the viscous layer of borax-silica flux is removed and most of the fused chlorides in the furnace decanted into a preheated ladle containing a crucible. The final few kilograms of chlorides, together with the gold/silver alloy at the bottom of the furnace are poured into a separate crucible, which is set aside to cool to 800 oC. When this temperature is reached, the gold- silver bullion solidifies, whereupon the still fluid residual chloride is poured into a crucible. Thereafter the gold-silver button is separately tipped out of the crucible. The button assays about 20-25% gold, 68-72% silver and 1-4% base metals and is recycled to the gold refinery.
The ladle containing the molten chlorides is then transported over a rubber lined mild steel quenching tank fitted with a splash proof lid and containing some of water. The contents of the ladle are slowly poured into the tanks through a funnel and the chlorides quenched to finely divided granules. The quench tank is transferred to the silver refinery where the quenched chlorides are leached preparatory to the final refining of the silver by electrolysis. Granulation is preferred to crushing the solidified chlorides as it provides a rapid, dust free reduction, well suited for the next stage of refining of the silver and has an additional advantage of providing the immediate leaching of up to 40% of the base metal chlorides. Spent crucible, furnace linings, chlorine pipes, borax, slags, etc., from al furnace operations in the gold refining area are collected in the sweep room. In this place, they are crushed, milled with the addition of zinc dust to reduce chlorides and screened. The metallics are returned to the refining circuit, while the minus 28 Tyler mesh fines are sent to by-products recovery.