Historically, grinding became a mechanised industrial process with the advent of water and wind powered mills to process wheat, barley, animal feed, etc. The mills used a flat stationary stone with a moving mill wheel revolving on-top. A derivative of this type of mill, an edge-runner mill, can still be found in use today, albeit electrically driven. In old mills classification was important to separate the flour from the husk; this was often achieved by sieving. Modern mills often combine classification and milling within the same device by having an up-draught to carry the finer particles away from the milling section. These are known as air swept devices.
Milling of minerals has been an important part of the recovery of metals and industrial minerals for many centuries. Often a mineral of interest is surrounded by rock of a different type, which may be worthless; i.e. a gangue mineral. The grain boundary between the desired mineral and the gangue will be the weakest mechanical point and the most likely to break. Thus, grinding to the liberation size will release the valuable mineral so that it may then be separated from the gangue. This is a process that is employed for metal ore mining as well as precious minerals recovery–see the box below Table 11.1.In the table the Moh’s scale of hardness is shown. It is based on the mineral lower in the table being able to scratch the mineral above. "Soft” minerals are 1 to 3,”medium” are 4 to 6 and "hard” are 7 to 10. However, the hardness value is entirely arbitrary; it is merely a ranking of the described minerals. The table also includes the Bond Work Index, which is discussed in the next section.
An initial size reduction from material several centimeters in diameter down to one centimeter, or so, is often termed primary crushing. This may be followed by further size reduction, secondary crushing, and then pulverizing, or fine grinding. The term micronising has become popular for the reduction of particle size to this dimension. Thus, coal being fed into a pulverized fuel burner for electricity generation will undergo the first three grinding operations.
Examples include Jaw and Cone Crusher. Secondary crushing can be by rotating surfaces such as swing hammer mills, for brittle materials, and roll crushers. Finer grinding usually takes place in rotating vessels, such as ball and rod mills. Very fine grinding, to sizes less than 10μm, requires high energy and attrition mills are often used.
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