Ore dressing is one of the principal processes in the work of mining. It consists of separating economically valuable minerals from those with little or no value, or of separating valuable minerals from each other, so that they may be further processed, for example by smelting.
PurposeWhen minerals are brought up from the mine to the surface, they commonly contain a variety of materials. The contained metal may be either in the native uncombined state or a chemical compound (commonly sulfides or oxides of metals), but in either case the valuable mineral ore is always associated with minerals of little or no value (gangue). The province of the ore-dresser is to separate the valuables from the waste, for example, quartz, feldspar, or calcite, by mechanical means, obtaining thereby concentrates and tailings. It is then the province of the metallurgist is to extract the pure metal from the concentrates by chemical means, with or without the aid of heat.
There are also a number of non-metallic minerals which do not have any value, or at best do not reach their highest value until they have been subjected to some form of mechanical preparation; among them are diamonds, graphite, corundum, garnet, asbestos, and coal.
MethodsOre commonly has to be crushed, so that crystals of different minerals adhering to each other are broken apart. These then have to be separated. The crushing process is known as "spalling".
CrushingTraditionally, the raw mineral was broken up using hammers, wielded by hand. Later mechanical means were found to achieve this. An early example of this was the Cornish stamp, consisting of a series of iron hammers mounted in a vertical frame, raised by cams on the shaft of a waterwheel and falling on to the ore under gravity. These have been replaced by other machinery.
SeparationThe simplest method of separating ore from gangue consists of the picking out the individual crystals of each. However this is a very tedious process, particularly when the individual particles are small.
Another comparatively simple method relies on the various minerals having different densities, causing them to collect in different places: metallic minerals (being heavier) will drop out of suspension more quickly than lighter ones, which will be carried further by a stream of water (or other liquid). One variety of apparatus for this is the 'buddle'.
Alternatively, if a fluid of a suitable density can be provided, lighter minerals will float and heavier ones sink. This is known as floatation.
Glossary of ore dressing termsThese definitions are from Chambers Twentieth Century Dictionary:
- (noun), an inclined hutch for washing ore
- (verb), to wash with a buddle
- Hutch (noun), a trough used with some ore-dressing machines
- Keeve or kieve (noun), a large tub
- Spale (noun), a splinter or chip (Scottish)
- (noun), a chip or splinter, especially of stone
- (verb), to split, splinter or chip
- Various articles in J. Day & R. F. Tylecote, Metals in the Industrial Revolution (Institute of Metals, London 1991).