Limestone, which is primarily calcium carbonate, is a sedimentary rock made from the minerals calcite or aragonite. A large portion of limestone is made from the skeletal remains of sea creatures and organisms. Ground Calcium Carbonate (GCC) differs from Precipitated Calcium Carbonate (PCC) in that the GCC is formed directly from grinding limestone rock into a powder, while PCC is chemically produced and precipitated out as powder.
Ground Calcium Carbonate is used in paper and plastic fillers, coatings and paints. It is also used in cement, is converted to quick-lime (calcium oxide) and slaked lime (calcium hydroxide). Ground calcium carbonate is slightly basic and can be used to neutralize combustion vapors or increase the pH in water or soils.
Pure calcium carbonate is a relatively soft mineral/compound, with a Mohs hardness of 2.5 – 3.0. However, mined limestone may have a minor portion of abrasive impurities such as quartz / sand.
One of the standard methods of milling Limestone/Calcium Carbonate is with the 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 635 mesh (20 µm) product or a D50 of less than 5 µm. 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. Hot air can also be used to dry the product while milling.
An alternate to the ACM for coarser grade Calcium Carbonate is the Mikro Pulverizer Hammer & Screen Mill. The Pulverizer uses swing-hammers with interchangeable screen liners to produce a variety of products from 50 mesh to 150 mesh products. The Mikro Pulverizer Hammer & Screen Mill needs less air (less ancillary equipment) than the ACM and is a lower cost options for the coarser grind Calcium Carbonate products.
Wet milling with Hosokawa Alpine Agitated Ball Mill ANR is used to produce ultra-fine calcium carbonate slurries from 60% to 99% < 2 µm, at relatively low specific energy.
For plastic fillers, stearic acid coating on ground calcium carbonate is often performed. Very uniform coatings are produced using Hosokawa Alpine’s Contraplex (CW) pin mill coating system or Mikro LGM Long Gap Mill for flash drying and coating at the same time.
Contact Hosokawa’s mineral applications department to learn more about Limestone / Calcium Carbonate Milling.