Treatment of Gold Ores. The treatment of gold ores depends greatly upon their rich ness and associations with other minerals, their gangues and rock embediments and the eco nomic conditions in the district in which the work is performed.
An outline of the ordinary reduction and recovery can best be considered under the fol lowing headings: Elimination of waste; con veyance to mill or reduction plant; rock break ing; crushing; amalgamation; concentration; chemical treatment, chlorination, cyanide; treatment of amalgam, zinc precipitates, bullion, refining. The site of a reduction plant is in fluenced by the topography of the country, the relationship of mine, mill and water supply, also transportation facilities. The ideal situation is when the mine outlet and mill are close together upon a hillside, with slope allowing gravity movement of the ore pulp and tailings. This is rarely obtainable, but excellent results can be obtained on flat ground with suitable machinery.
Elimination of Waste. The elimination of waste before treatment was in early practice largely effected by hand-picking the ere on dressing floors; in more modern practice the coarser ore, after screening, is dumped upon revolving annular tables, or belts, where it is washed and the hand-sorted ore scraped or dumped into bins or rock breakers. In this way often a large per cent of the rock hoisted is not sent to the mill and the grade of the ore treated is correspondingly raised. This policy of sorting is of doubtful benefit when the cost of reduction is small, as there must always be some gold in the waste selected, as also added cost rrom the sorting. In some !nines sorting and packing of waste is done underground.
Conveyance to Mill or Reduction Plant. The mills are often some distance from the mines and made to serve several shafts, outlet adits and tunnels, or a mill may be consolidated with the hoisting appliances of one shaft or tunnel. The mines, on the other hand, may be far from the mill and various methods of trans portation, as pack animals, carts, wagons, tram cars, gravity inclines, aerial conveyors, loco motives and electrical haulage, may be resorted to. At some mines a head frame of a shaft, or the dump from a tunnel, and a mill have been joined together so that the mine hoist dumps the ore at such a height that it can be sorted, the rock and ore broken in rock breakers and the ore run by gravity direct into the mill bins.
Rock Breaking. There are two classes of rock-breaking machines in general use jaw breakers with reciprocating motion and the gyratory crusher. The breakers are often placed over the mill bins, sometimes in the head frames of the shafts, now more common in plants to themselves, and the broken rock conveyed to the mill bins by cars or conveyors.
In the larger size of crushers the rock as dumped from skip, or car, is led to them in chutes. In some installations there are two sizes of crushers so that the ore can be finer crushed before reaching the stamps or other fine crushers. In some mines the primary rock breakers are placed underground.
Crushing. The earliest crushing machine in use in America was the Arrastra, being in troduced at the same time as the Patio process, about 1557. It is an apparatus for grinding and mixing ores by means of heavy stones dragged around upon a circular bed paved with stones, and charged with mercury. It was cheaply constructed and worked by animal power; it ground very finely and often gave very high extractions. Except for prospecting, it is almost obsolete, as the output from it is so limited and modern crushing machinery so improved. The stamp battery is the most com mon form of fine crusher. A stamp is a heavy iron or steel pestle, raised by a cam keyed on a horizontal revolving shaft and let fall by its own weight. They are ranged in line in groups of five, having a mortar box in common. The box is made of heavy cast iron, especially strong and solid at the bottom. The pestle is shod with a removable iron or steel shoe, and the bottom of the mortar clad with iron or steel blocks known as dies. Water and ore are fed into the mortars continuously, the water by pipes and the ore by automatic feeders. The blows of the stamps splash the water and pulp against screens set in the side of the mortar box and the ore after being crushed to the re quired size thus ejected. The height of drop of the stamp is usually five to seven inches, the number of drops per minute about 90 and the size of screening or perforated plates varying from 40 mesh to the inch to perforations as large as one-quarter inch. The stamps vary in weight from 100 to 2,000 pounds, only the very early stamps were as light as 100, and only South African as heavy as 2,000. The most common weight is between 1,000 and 1,500 pounds, and with crushing capacity between three and five tons to the stamp in 24 hours. The heavy stamps with one-quarter inch screen apertures and supplemented by fine grinding tube mills have crushed 20 tons to the stamp. Crushing rolls, Huntington mills and ball mills have been used to replace the stamp mill and under certain favorable conditions have given excellent results, but as a whole the stamp mill may still be considered the most economical and satisfactory device.