Characteristics of fluoride emission from five clay minerals as affected by temperature, heating time and addition of calcium compounds

Characteristics of fluoride emission from five clay minerals (montmorillonite, kaolinite, vermiculite, geothite, and allophane) as affected by temperature, heating time and addition of calcium compounds were studied. Marked increase of the fluoride emission rate was noticed with increase of temperature. The fluoride release, began at 500°C–600°C, and the main bulk of the fluoride emission occurred at the temperature of about 800°C. The loss of crystalline water was primarily responsible for the increase of fluoride emission. When minerals were heated at 800°C, The fluoride emission rate from the clay minerals reached the highest after heating for 1 hour. The samples treated by CaO, CaCO₃, Ca(OH)₂, Ca₃(PO₄)₂, and CaSO₄ had 55.45%, 59.58%, 46.45%, 54.31%, 31.25% reduction in the fluoride emission from montmorillonite at the temperature of 800°C, respectively. CaCO₃ had the highest fluoride fixing capacity compared to other calcium compounds. Project supported by the Special Funds for Major State Basic Research Project 973 (G1999011809) and Natural Science Foundation of Zhejiang Province.