Gypsum is a mineral composed of calcium sulfate dihydrate (CaSO4-2xH20), which is formed when a lake or sea has evaporated.  The gypsum is then mined from the evaporated beds.  Another way gypsum is formed is during the flue gas desulfurization process in power stations.

Gypsum is used in construction, especially in sheet-rock and wall finishing.  When heated, it loses its hydration, forming plaster, which can be used as cement when adding water. 

Pure calcium carbonate is a relatively soft mineral/compound, with a Mohs hardness of 2 (defining mineral).  However, mined gypsum may have a minor portion of abrasive impurities such as quartz / sand. 

One of the standard methods of milling Gypsum is with the Hosokawa Mikro® ACM Air Classifying Mill .  The ACM has a rotating rotor disc with hammers which rotates with speeds up to 23,000 feet per minute.  The particle size is controlled by the internal air classifier, to provide sharp particle size distributions.  Various classifier designs are available, to produce from coarse (100 mesh) product down to a fineness of 400 mesh product or a D50 < 10 microns.  Abrasion Resistant options are available, including tungsten carbide tipping, Ni-Hard or Ceramic liners to assist with abrasive impurities and to assist with long life of mill components.  Warm air can also be used to dry the product while milling.

An alternate to the Hosokawa Mikro® ACM for coarser grade Gypsum is the Hosokawa Mikro Pulverizer® Hammer and Screen Mill .  The Mikro Pulverizer® uses swing-hammers with interchangeable screen liners to produce a variety of products from 50 mesh to 150 mesh products.   The Mikro Pulverizer® needs less air (less ancillary equipment) than the Mikro® ACM and is a lower cost options for the coarser grind Gypsum products.

Contact Hosokawa’s mineral applications department to learn more about Gypsum Milling.