Study Of Chemical Precipitation-Coagulation Purification Process For High-Concentration Fluoride Water

The high fluoride drinking water in Ma'e Town, Sanyuan County, Xianyang City, Shaanxi Province, China, was purified by chemical precipitation-coagulation methods. Main conclusions were as follows:(1) Two kinds of calcium salt precipitants, calcium hydroxide and calcium chloride, were tested. It was found that both of them could remove fluorine ion from water. The pH value of water, the amount of calcium salt, and sedimentation time had important effect on the final settlement of fluorine ion.(2) Based on orthogonal test, the factors influencing fluoride removal, such as pH value, calcium salt quantity and sedimentation time, were investigated. When calcium hydroxide was used as a precipitant, the optimized precipitation condition was the mole ratio of calcium hydroxide to fluoride in water being 2.5 to 1, pH value between 5.5 and 8.5, standing time at least 75 min, under which condition fluoride content in drinking water containing 20 mg/L fluoride reduced to 9 mg/L. When calcium chloride was used as precipitating agent, the optimized precipitation condition was the mole ratio of calcium chloride to fluoride in water being 2.5 to 1, pH value between 7 and 8.5, standing time at least 50 min, under which condition fluoride content in drinking water containing 20 mg/L fluoride decreased to 7 mg/L. Compared to calcium hydroxide, calcium chloride had a better ability of removing fluorine ion in water. So, calcium chloride was chosen for the precipitation of high fluoride drinking water.(3) On the basis of calcium chloride as precipitant, the flocculent effects of aluminum sulfate and poly-aluminum chloride were studied. Aluminum chloride worked better in fluoride removal. When used in amount of 30 mg/L and standing time being 60min, the remaining fluorine ion in water sample treated with poly-aluminum chloride dropped to about 6.1 mg/L while in the water treated with aluminum sulfate fluoride concentration reduced to about 6.4 mg/L.(4) In conclusion, fluoride removal processes were as follows. The first step is to add calcium chloride whose mole number was 2.5 times as fluorine ion, adjust the pH value to be 7 to 8.5. Sediments were filtered after standing for at least 50min. The second step is to add poly-aluminum chloride in an amount of 30 mg/L. Floccules were filtered after standing for 60 min.