The rock when hoisted out of the mine is first assorted, the waste rock being thrown on one side and the gold-bearing ore broken into lumps by a stone-breaker. The lumps of ore now pass by gravitation and feeders through a battery (or stamp mill), each stamp of which weighs about 1150 pounds, every stamp being lifted and dropped separately by the cam shaft at a speed of about 95 drops a minute. A stream of water is introduced, the ore is crushed into fine sand, and is carried by the water over a series of inclined copper plates, which are coated with quicksilver. The free gold in the sand at once amalgamates with the quicksilver on the plates, and the sand-laden stream continues on its course.
The sand, having now passed over the plates, is carried by launders on to the concentrators, or frue vanners. These concentrators separate and retain the heavy sand (or concentrates), whilst the lighter sand is carried by gravitation through a trough (or launder) to the cyanide vats.
The stream of water carrying the lighter sand empties itself into the cyanide vats, and as each successive vat is filled up, the water is allowed to drain through the sand. A solution of cyanide of potassium is then pumped up and evenly distributed (by distributors) over the sand, and dissolves the gold in its progress, leaving pure sand alone in the vat. The gold-containing liquid (or solution) having left the vat, is led into a series of boxes filled with zinc shavings, the gold separates from the liquid, and settles on the zinc shavings in the shape of a small black powder. The cyanide solution now freed from the gold runs into the solution vats, and is restrengthened and ready for further use.
_Gold Recovery._--In the mill or battery the copper plates are scraped daily, and the amalgams (that is, quicksilver and gold) are weighed and placed in the safe in charge of the battery manager. This amalgam is generally retorted once a week, that is to say, the quicksilver is evaporated (but not lost) and the gold is left in the retort. This retorted gold is then smelted into bars.
The concentrates recovered by the frue vanners are generally treated by chlorination (roasted). This process is gone through so that the iron can be separated from the gold. Concentrates are sometimes treated by cyanide, but the process, if cheaper, is slow and less effective. Chlorinated gold is also smelted into bars.