Native copper ores at Lake Superior are crushed in breakers to about 3 in., further reduced in steam stamps to about in. and then graded by hydraulic classifiers into a series of products, usually six. The coarser products are jigged and the finer prod ucts are tabled. Coarse middlings are reground in ball mills and retabled. Flotation may be used for slimes.
Iron is a relatively cheap metal so the concentration of iron ores must be on a large scale and at low cost. If the mineral is magnetite, the process is to crush the ore in breakers and rolls sufficiently to free the mineral and then use low power electro magnets, either wet or dry, to take out the magnetite. On hematite and limonite ores, the concentration of the crushed ore is usually by log washers or similar devices, but sometimes by jigs and tables. Slimes are usually thrown away without any attempt to recover the iron in them.
Manganese ores containing pyrolusite as the valuable mineral are handled in much the same way as hematite iron ores. For the special problem of the franklinite ores at Franklin, N.J., the ore, after crushing and sizing is run dry over high power Wetherill magnets which take out the franklinite and send the residue to wet jigs and tables which separate oxide and silicate zinc minerals from the waste limestone tailings. The treatment of gold ores is covered elsewhere under AMALGAMATION and CYANIDE PROCESS.
Many non-metallic minerals require concentration which may involve special processes. Serpentine rock containing asbestos is
first cobbed and hand picked for coarse masses of solid asbestos. The residue is disintegrated by a form of hammer mill which re duces the rock to sand and liberates the fibre as a fluffy mass which when sent to a screen remains on the screen as oversize, while the sand passes through as undersize. Diamondiferous rock in South Africa is crushed in breakers and rolls, agitated with water in large circular pans to remove light fine waste material and leave a concentrated residue to go to jigs. The diamonds are recovered from the jig concentrates by hand picking on the coarse, and greased tables on the fine.
Phosphate deposits in hard rock use crushers and log washers or in soft rock or pebble form only log washers. Graphite ores em ploy crushing and flotation with special electrostatic process for removing mica from the concentrates. Ores, of corundum, emery, ilmenite and such like, generally require a combination of gravity treatment on tables followed by further cleaning on magnets.
Coal preparation is a form of ore dressing which is based on the same fundamental principles as ore dressing, but which employs some additional processes and apparatus to meet the needs of the special problem of saving the light weight coal from the heavier slate impurity on a large scale at a low cost and with a minimum loss of coal. (C. E. L.; X.)