The most spectacular part of the process is the refining. Silica, borax and soda ash are added to the dried precipitate (or steel wool, in the case of carbon-in-pulp), which is heated in a furnace. This results in a miniature smelting operation. On the top of the melt is the slag containing the impurities, while the molten gold's greater density causes it to sink to the bottom.
When the contents in the furnace are melted completely, the furnace is tilted and the molten material is poured into a conical mold. Black slag that is considered worthless forms on the top and is broken from the underlying gold button once it cools.
The button or buttons (there may be a sufficient quantity of precipitate to need more than one melt) are placed in the furnace again; they are melted and poured into bar molds.
Finally the bars are weighed; to determine the purity of the bars (expressed as fineness in parts per thousand), small samples are removed and the bars, called doré bars, are packed for shipping. In due course, the mine receives a cheque for the gold.
Treating Base Metal Ores
To reach their ultimate commercial form, the base metal ores go through a more complex treatment process than gold. Due to the fact that most base metal ores contain metallic sulphide minerals, separating the sulphur from the metal and then ensuring the sulphur is contained in some manner so that it can't damage the surrounding environment is the greatest challenge.
Different from the gold, base metals are not usually produced in an almost pure form at the mine site. The reason being that most mines are not large enough nor are they well situated to give warranty to the construction at the mine site of all of the plants (mills, smelters, refineries, etc.) needed to convert the metal ore into pure metal.
Each base metal mine instead tries to remove as much of the waste rock as possible from its ore and ship the enriched product locally, or concentrate, to a smelter which is strategically situated.
Crushing and grinding in a concentrator are practised in the same form as in a cyanide mill, and for the same purpose — for example, to liberate the valuable minerals from the worthless rock that surrounds it.