The valuable metal needs to be separated from the surrounding gangue (uneconomic) material. Much of this can be done by efficient mining methods so that there is not too much dilution of the ore.

Initial stages usually involve crushing (eg Jaw Breakers) and grinding (eg Ball Mills) of the ore to reduce the material to sand and silt sizes. Classifiers (essentially giant sieves) are used to check particle sizes, with the oversize material being recycled. This process makes handling the ore easier and raises the likelihood of being able to liberate all of the valuable metal elements (and maximising the surface area that will come into contact with subsequent chemical processes).
 
The next stage is normally a series of concentration processes; eg removing water and waste material. Where the ore has to be transported a considerable distance, this concentration will occur at the mine site rather than at the processing operation. (In these circumstances, the ore can be transported as slurry via a pipeline.)

Processing methods include:

Carbon in Leach - Recovery (of gold and silver) from finely-ground ore by simultaneous dissolution and adsorption of the precious metals onto fine carbon in an agitated slurry tank.

Flotation - This process has been used to separate minerals since the early 1920s, and involves treating the ground ore in a bubbling mixture of water and chemical constituents. The metallic minerals bond with the chemicals (ie they stick and rise to the surface) and can then be skimmed off and the chemicals washed or burnt off. The resultant material can then be subjected to refining and/or smelting processes to improve the product's purity.

Heap Leach - The dissolving-out from mined rock of the contained soluble metals by percolating a chemical solution through mounded material.

ISA Process - Patented by Xstrata this process is used in more than 35% of the world's copper refining operations. The technology is used to bypass complex processes by utilising permanent stainless-steel cathodes in electro-winning applications. Xstrata also developed the ISAsmelt high-intensity copper-smelting furnace.

Solvent-extraction Electro-winning (SX-EW) - Dissolving of copper from the rock by organic solvents, with the metal then being recovered from solution by electrolysis.

Examples of metals-recovery processes include:

Mined bauxite is ground and mixed with caustic soda to form slurry. This is treated, and the alumina trihydrate particles recovered and smelted to form alumina. Oxygen is driven off by electrolytic action (in a refinery) to produce aluminium.

Copper ore is concentrated by grinding and flotation, and smelted in a furnace to create copper matte. Iron and sulphur impurities are then removed in a converter (heated air is blown through the material) to create blister copper, which is fire-refined and then electrolytically-refined to produce copper cathodes.

Zinc concentrates are roasted and the resulting calcine is leached and purified. Electrolyte zinc is deposited on sheets, stripped from them and then melted in a furnace (with the molten metal being cast in slabs.

The waste material from these processes is usually transported to a tailings dam (although they are sometimes dumped, controversially, at sea). There are various types but all are expensive, and so generally the retaining walls are built with either the waste product itself or with material that is available locally.

Continue with Mining Knowledge by clicking other categories below:

Click here to return to Mining Info home page.