• Amalgamation.— The pulp or discharge from the mortar, or tube mill, passes over cop per plates. These plates are commonly 15 feet by 4 feet 9 inches and given a grade of one and one-quarter inches per foot, sometimes made shorter in two or three sections with slight drops between. The plates are treated with quicksilver and dressed so that the gold amalgam formed on the plates is made use of to catch more gold, the quicksilver is fed partially into the mortar which is sometimes supplied with inside plates, and also the outside plates are sprinkled with quicksilver, the amount depending upon the amount of gbld in the ore. The amalgam on the plates should be neither too hard nor soft. Cleanups of amalgam are made daily, and the amalgam squeezed in cloths or chamois skins so as to eliminate the excess quicksilver. The amalgam is then stored for periodical retorting.
The amount of recovery by amalgamation varies from 90 per cent for very rich free-mill ing ores to only 30 or 40 per cent for complex ores; the average for the Witwatersrand con glomerates was about 60 per cent. A higher percentage of amalgamation is obtainable by finer grinding and longer contact with mercury, as by the Arrastra and Patio processes employed in South America, and amalgamation pans.
Concentration.— The mill pulp after pass ing over amalgamated plates, or sometimes di rectly from the battery, is subject to classifica tion and concentration of the small percentage of the coarse and heavier materials which carry much of the remaining gold. The amount of gold so obtained is dependent upon the charac ter and treatment of the ore and varies greatly. In some cases concentration is omitted and the ore directly treated by the cyanide process. In early practice, after amalgamation, concentrates were obtained by the use of rough blankets placed on inclined planes and frequently washed and renewed. The practice of concentration was further developed by the use of convex and concave buddies and the use of large areas of canvas on inclined floors frequently hosed down and cleaned. More perfect concentration is now obtained by the use of Frue and other vanners, on which concentrates are caught and subjected to slow constant longitudinal and slight rapid side-shake movements. Percussion tables, such as the gilt-edge concentrator, with an end bump, were often used; as also Wilfley tables, with riffles and smooth surface, and lon gitudinal jerking action; and Hartz and other jigs, especially useful for coarse-crushed ore. Hydraulic classifiers are used for obtainment of concentrates as well as for the separation of sands and slimes for cyanide treatment. There
are several kinds in use — the Spitzkasten, an inverted pyramid box into which the pulp stream enters at one side and is discharged at the other, and the coarser material drawn off at the apex. The regulation of the settlement is effected by baffles, the angle of sides of the box and the discharge valve at bottom. The Spitzlutte is similar in principle to the Spitz kasten, but with the addition of an ascending current of water at the apex and modification in the dimensions of the pointed box. The cone classifier works on the same principle but with perfected details. The Dorr classifier is in the form of an inclined trough, open at one end, in which mechanically operated rakes are placed to remove the heavy material as fast as it settles, the liquid and slimes overflowing at the closed end.
Flotation.— The flotation process is the most modern, new and interesting method of concentration. The Elmore process, in 1897, demonstrated on a practical scale that by using residuum oil in the crushing of ore, sul phides and metallic particles could be floated on top of a pulp stream, skimmed and treated as concentrates, and the earthy and stone materials kept at the bottom and dis charged as tailings. Many improvements and innovations have since been made and the proc ess is being extensively used. The fundamen tal principles are film suspension, oil buoyancy and bubble levitation. The latter phase de pends upon the aid of bubbles of gas which by attaching themselves to particles of minerals buoy them to the surface. For a aetailed ex planation of the various devices of flotation, special literature should be consulted.
Chemical treatment, chlorination, cyanide.
—The chlorination process — one of the early attempts at chemical treatment of gold ores has been extensively used since 1850 for ex tracting gold from concentrates. It has prac tically given place to the cyanide process. The Plattner process, originated in Freiberg in 1848, is the best known. It depends on the fact that chlorine readily attacks gold and forms soluble gold chloride, which is precipitated by suitable reagents. It requires that the sulphide concen trates should be subjected to a dead roast in reverberatory furnaces before the chlorine treatment is given; it also requires that the vats have removable tops that may be closed and sealed. Besides the Plattner there are several barrel chlorination processes, the best known being the Thies. In this process chlorine gas is generated inside the barrel by means of bleaching powder and sulphuric acid, and a has pressure of a few pounds to the square inch maintained.