METHOD OF REDUCING THE SOLUBILITY OF CALCIUM SULFATE DIHYDRATE IN AN AQUEOUS SUSPENSION AND METHOD OF MAKING THE SAME

摘要

The suspension of needle-shaped (acicular) calcium sulfate dihydrate particles is prepared by mixing freshly calcined calcium sulfate hemihydrate (CaSO4 âH2O), having an average particle size between 1 and 100 micrometers, and water in a reactor at medium to high shear rate, with either no additives or together with acids or with calcium or sulfate containing additives. The average length and width of the resulting calcium sulfate dihydrate particles are then about 5 to 35 micrometers and 1 to 5 micrometers, respectively. The calcium sulfate dihydrate particles of reduced solubility are obtained using a calcium chelating agent, with or without the addition of a weak acid. The resulting product is a suspension of calcium sulfate dihydrate particles that can be used as a filler in papermaking. The particles have a high retention and provide improved optical properties with minimal filler losses due to solubility.