Hydraulic Gold-mining.
Vein-mining for gold differs but little from working any other kind of metalliferous lode. When the vein-stuff has been raised it is reduced to a pulverulent condition, to liberate the gold from the gangue. In some cases roasting is first resorted to. This causes friability, and facilitates the subsequent comminution. When the gold is in a very fine state, too, it helps it to agglomerate. But if any pyrites are present the effect is most detrimental, the gold becoming coated with a film of sulphur or a glazing of iron oxide. The powdering of the vein-stuff is usually performed in stamp batteries, which consist of a number of falling hammers. While simple in principle, the apparatus is complicated in its working parts, and is probably destined to give way to the improved forms of crushing-rolls and centrifugal roller mills, which are less costly, simpler, more efficient, and do not flatten the gold particles so much. One of the most effective is that by Jordan. When the vein-stuff has been reduced to powder, it is akin to alluvial wash-dirt, and demands the same or similar contrivances for arresting the liberated gold and releasing the tailings—that is, mercury troughs, amalgamated plates, blanket strakes, &c.; but, in addition, provision is made for catching the other metalliferous constituents, such as pyrites, which almost always carry a valuable percentage of gold. These pyrites or 'sulphurets' are cleansed by concentration in various kinds of apparatus, all depending on the greater specific gravity of the portion sought to be saved.
Of the metals and minerals with which gold is found intimately associated in nature are the following: Antimony, arsenic, bismuth, cobalt, copper, iridium, iron, lead, manganese, nickel, osmium, palladium, platinum, selenium, silver, tellurium, tungsten, vanadium, and zinc, often as an alloy in the case of palladium, platinum, selenium, silver (always), and tellurium. The methods of separation vary with the nature of the ore and the conditions of the locality.
TREATMENT OF ORE AND GOLD IN THE TRANSVAAL.
The method of treatment of ore and gold in the Transvaal, the most perfect and effective known at the present time, has thus been described by Arthur Stenhouse:
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.