Processing Nickel Ores

One Mine's Nickel Processing of Pyrrhoite,
Chalcopyrite and Pentalandite Ores

The ore is crushed to -5 inches in primary cone crushers, then reduced to -1/2" in short head cone crushers. The ore is then ground to -100 mesh in ball mills. Using wet magnetic separators the magnetic ore is separated (pyrrhotite) and further reduced to -200 mesh in a ball mill. Classification is accomplished with screens and cyclones. The pyrrhotite is then sent to froth flotation cells, and produces a 3% nickel concentrate.

The non magnetic ore is sent to a series of rougher, cleaner flotation cells, and produces a 31% copper concentrate. The tailings from these cells is sent to another flotation cell, to recover the nickel, and the concentrate is combined with the 3% nickel concentrate to produce a 12% nickel concentrate.

Most nickel ores have several recoverable metals in them and the process if commonly a multiple stream of metals recovered. The copper concentrate is sent to the copper smelter and 99.99% copper cathode is produced in electrowinning cells. The nickel ore is sent to the nickel refinery, where a complex set of reactions is carried out. These include leaching in autoclaves using ammonia, heated to around 200 deg F at elevated pressures of 100-150 PSI. The application of heat and pressure dramatically speeds up the chemical reaction and produces the nickel much faster than leaching at room temperature and atmospheric pressure. There is a primary and a secondary leach circuit, where the solids remaining in the first circuit are sent to a second autoclave to recover any metals the first leach process missed. A slurry of the liquid and solids are then pumped through a thickening and filtration system, separating the non-valuable solids from the liquid containing the nickel. The nickel and ammonia solution contains 2:2 ratio of nickel to ammonia (molar), which is roughly 50 grams nickel per liter of solution.

The nickel-ammonia solution is then reduced with hydrogen in autoclaves, adding a a small amount of ferrous ammonium sulphate. This solution is heated to 250 deg F at 350 PSI, in a hydrogen atmosphere. The nickel is reduced by hydrogen and precipitates as fine metallic nickel (Ni). The nickel settles and the solution is pumped out to a holding tank. Since the solution still contains some nickel , it is re-processed again to recover the remaining nickel. The nickel can then be removed through a cone bottomed tank or by a thickener/filter operation to yield the nickel powder. It is then dried and sent to a briquette process where it is made into pellets or briquettes. It can also be melted and poured into ingots for sale to the market.

Information provided by Charles Kubach, Mining and Mineral Processing Engineer

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