In this section, bullion from gold mines and any extraneous deposits are received and their contents assessed. Standard shape of tapered bar has been adopted by most gold mines, the top being 241 mm by 140 mm, the depth 73 mm and the bottom 178 mm by 76 mm. with these dimensions the mass of a bar is 32 kg. When two bars are placed face to face the outer surfaces approximate to a cylinder, the diameter of which is about 10 mm less than the internal diameter of the melting crucible. Up to four bars from a single gold mine are combined to make a nominal deposit of 125 kg. Each deposit has its mass determined on a scale. Determination of the gold content is obtained after melting the whole deposit, since it is considered that the only reliable method of sampling heterogeneous alloys such as may be found in unrefined bars is by means of a dip sample from the molten bullion.
For melting, a deposit is transferred to a clay/graphite crucible which is then loaded onto the hydraulically operated platform of the induction furnace. After lowering the crucible into the furnace, heating by approximately 90-120 kw for 15-25 minutes is needed to melt the deposit at a temperature between 1050 -1100 oC. The field set up in the melt by the induction coil is such that turbulence is induced, guaranteeing homogeneity for accurate sampling. After a few minutes at peak temperature, allowing about 100oC of super heat for fluidity when casting, four dip samples are taken, by means of a carbon coated mild steel dipping spoon. The samples are dispatched to the assay section for the determination of gold and silver. The crucible is then raised out of the induction furnace, gripped by tongs and the bullion cast into tapered moulds set in a revolving casting wheel. This machine automatically tips the solidified ingots, with masses up to 12.5 kg, out of the graphite dressed moulds, conveys them through a cooling through and delivers them to a marking table.
Each ingot is stamped with the deposit number and the mass of the deposit determined to obtain the mass after melting. Strict control is practiced over any spilling and crucibles are scraped after each melt. The deposit is then stored in a strong room to await assay results. The refinery assay is compared with the depositor’s assay and the fine gold calculated. Up to this stage each deposit retain its original identity, but loses it once agreement of the gold content is reached, when it is transferred to the refining room.
Melting Casting Wheel