which are suited for finer mate rial (1 mm. maximum) are much less used since the development of the flotation process. The vanner feeds the ore and water on an endless rubber belt which is shaken crosswise while moving uphill. Wash water, together with the slope of the plane and the speed of the belt travel, determines the character of the grain that can pass over the upper roller and be washed off into the concen trates tank. All other grains of lighter waste mineral are washed down the plane and into the tailings. A revolving type of slime table consists of a flat cone. Ore and water are fed at the cen tre over one-half of the circle. Light minerals reach the circum ference of the table first and are discharged as tailings. Middlings are carried around by the revolution of the table and are washed off by a curved water jet while concentrates are finally removed by strong jets of water from a straight pipe. Many other forms of vanners and fixed and moving slime tables and buddles have been used for concentrating fine slimes.
are in use in most modern mills to remove metallic iron, such as bolts, nuts and broken drills, which has be come mixed with the ore during mining. Magnetic separation is also applied where one of the minerals to be separated possesses magnetic properties. Magnets are of many types to operate on different sizes of ore, wet or dry, or to give high or low strength of field.
may be classi fiers, jigs or tables. In every case the separation is by specific gravity, and air currents replace water currents as the separating medium. Flotation machines are described under FLOTATION. Other miscellaneous separators based on principles outside of specific gravity, magnetism and flotation, are not of sufficient im portance to warrant any description here.
In addition to the machinery for crushing, grading and concentrating, an ore dressing plant includes other apparatus which is absolutely essential even though perform ing no actual separation. Under such apparatus may be listed storage bins for ore and products; sampling devices; automatic feeders and distributors or dividers; chutes and troughs or laun ders for dry and wet material ; conveyors, of which the belt and pan types are most common; bucket elevators for dry ore or ore with water; centrifugal pumps for water mixed with sand or slime or for water alone ; unwatering devices for recovering water ; filters for fine concentrates; driers for removing moisture from ore or from concentrates; dust collecting systems; automatic weighing machines.
are of wide variety according to the nature of the material to be concentrated. Perhaps the simplest
process is the treatment of sands and gravels which contain valu able minerals such as gold, platinum and gems. A long trough or sluice is used on the bottom of which are small depressions called riffles in which the heavy values settle. The rough concentrates thus produced are removed or "cleaned up" periodically and are usually further enriched by some finishing treatment such as amal gamation, hand panning, tabling or magnetic separation.
Sulphide ores consisting of gangue mixed with sulphide of a single metal such as of iron as pyrite or of zinc as sphalerite or of lead as galena or of copper as chalcocite, bornite or chalco pyrite, may have the sulphides occurring as relatively large masses in the gangue, in which case the ore is crushed in a series of steps using breakers and rolls or Symons crushers until the sul phide grains are largely freed from the adhering gangue. This size may vary from i in. down to i mm. according to the ore. The crushed material is next graded by screens and classifiers into one or more coarse sizes for jigs, one or more sand sizes for tables and slimes for flotation. The jigs, tables and flotation each make clean concentrates to be saved and waste tailings to be discarded and usually also a middling product which, if coarse, must be recrushed and retreated or, if fine, needs only retreatment with out any crushing. If the sulphides are finely disseminated through the gangue, the ore is crushed down to mm. or finer, using ball mills or rod mills as the last step in crushing and all of it treated by flotation. Occasionally the metals occur, not as sulphides, but as oxides, carbonates or silicates but the process is similar to that for sulphides, except that special flotation reagents are required for these minerals.
Where ores are complex, that is, containing sulphides of two or more metals, the tendency of modern practice is to discard gravity concentration and separate the different metallic sulphides from one another and from the gangue by differential flotation.
Tin ore in Cornwall contains cassiterite associated with sul phides of other metals, with wolfram and with siliceous gangues. The ore is broken to about 3 in. and hand picked to remove any lumps of clean minerals and the residue is crushed in gravity stamps to about 3- mm. and concentrated by gravity by a rather complex system of jerking tables, vanners and slime tables of various kinds. Wolfram may be removed from the final concen trates by magnets. Sulphides may be separated from cassiterite by flotation as is the common practice on tin ores in Bolivia.