Talc is a clay mineral made of hydrated magnesium silicate. Talc is formed from metamorphic reaction of minerals in aqueous conditions.
Talc is one of the softest minerals on earth, with a Mohs hardness of 1.0 (defining mineral). However, some abrasive impurities (such as quartz or sand) may be present in the mined talc. It is typically in a foliated structure. Talc has natural dry lubrication properties. Talc also does not dissolve in water and has a high chemical stability.
Ground talc is used for paper and plastic fillers. It is used in paints and coatings for its high opacity, for its matt (low gloss) finish and rheology properties. It is used in the cosmetic and pharmaceutical industry. It is also used for dry lubrication for plastic sheets, gloves and many other applications.
The Hosokawa Alpine AFG Fluidized Opposed Jet Mill is one of the standard equipment used to mill ultra-fine talc. The particle-to-particle impact tends to maintain the foliated structure of talc in order to best retain talc’s properties. In the Hosokawa Alpine AFG Jet Mill, material is fed into the milling chamber to form a bed. Opposing nozzles create high speed air streams (up to 1000 MPH) which fluidize the powder and cause the powder to impact against each other in the center of the chamber. The flowing air eventually rises up the chamber and flows into the rotating classifier wheel, which allows the particles which meet the particle size specification to flow through, while forcing the larger particles to fall back into the fluidized bed and be re-ground. Various pressures, nozzle sizes and variable speed classifier wheel provide a wide range of particle sizes that can be achieved with the AFG Jet Mill. Hot air is typically used with talc to increase the milling rate and milling efficiency.
Another common method to mill talc is using 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 of less than 5 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.