Joyal-Three-ring Micro Powder Mill,Three-ring . The JOYAL Three-ring Micro Powder Mill (ultrafine mill) is for super-fine grinding materials with hardness less than 6 in Moh's scale and humidity less than 6 percent such as kaolin, limestone, calcite, marble,talcum, barite, gypsum, dolomite, bentonite mud,mica, pyrophyllite, sepiolite, diatomite, graphite, alunite, fluorite, potassium feldspar ...