Limestone is a natural rock, composed primarily of calcium carbonate. It may also contain magnesium carbonate in chemical combination, in which case it is called Dolomitic Limestone or, in the proper proportions, Dolomite. The hardness and physical nature of limestone varies widely – usually in the neighborhood of 3 or 4 on the Mohs scale. It can also take on the form of Marl or Chalk which is extremely soft.
Normally limestone or Dolomitic Limestone is a rather hard dense material. A typical specific gravity is in the neighborhood of 2.7.
The fine grinding of limestone in large capacity is an ideal application for the vertical roller mill. Tremendous quantities of relatively fine limestone are used for agricultural purposes, mineral fillers, mine dusting, chemical processing and many other uses.
In order to enable fine grinding, the limestone product must be thoroughly dry. Most vertical roller mill installations grinding limestone are equipped with heat to dry and grind in one operation. The initial moistures are usually low, 4 or 5%, for any hard dense limestone. The only way limestone can hold more moisture than this is to be porous or to contain excessive fines.
The range of fineness in this industry is tremendous. Agricultural limestone may be no more than 80% through 8 mesh and fillers as fine as 99.9% through 325 mesh. There is a wide variety of products between these extremes. Roller mill classifiers must be selected as appropriate to achieve the product size requirements.