All copper bullion output from furnaces is sampled during tapping and analyzed and then fed into a Top Blown Rotary Converter (TBRC) or a standard cupel. In these furnaces the copper is oxidized and removed by decanting. The copper oxide contains 18-25% silver. The lockout of silver is not deemed to be a problem due to oxide copper is recycled to act as fresh collector metal (reduced back to copper during smelting) or it may be sent for copper removal. Copper will be removed if the quantity of collector metal is larger than required. This can be done by milling the oxide and leaching out the copper. The copper solution can be then electrolyzed to produce standard copper cathode. The residue which remains after leaching is basically precious metals and is returned to the smelter. The precious metal bullion from cupellation is over 98% total precious metal and it is put into safe storage to await further refining. Gold, silver, platinum and palladium from cupellation undergo hydrometallurgical processing. The alloy is electrochemically refined to produce silver crystal and a mixture of gold, platinum and palladium which are then recovered as pure metals. Pure metals can be the end product or fabricated into a desired form or converted into an almost limitless range of precious metal products or chemicals.
The treatment of precious metals containing iridium, rhodium and ruthenium is done in the iron circuit that employs iron as collector. The principal furnace is an arc electrode. In operation three graphite electrodes are submerged in the melt and oscillate vertically to find a current path of least resistance. The heating of the material is through resistance heating. Once molten, PGM readily migrate into the iron phase. When the iron has been enriched to the desired level of around 8-12% total PGM, the slag is decanted and the iron tapped, sampled and analyzed by XRF. The slag is once more from the stable CaO-Al2O3-SiO2 slag system. The iron is then granulated by pouring into water. In this way a material suitable for the next stage of processing is produced. It is screened at 1/8” to remove any oversize which could be troublesome during leaching. The iron alloy undergoes hydrometallurgical treatment but of a more complex and involved nature. Firstly the iron is leached out to leave a PGM residue, which is dissolved and passes through complex separation stages such that individual pure sponges are produced. The recovered precious metals can, if required, be converted into precious metals or chemicals.
Electric Arc Furnace