抄録
The precipitation reaction of crystalline hydroxyapatite (HAp) was investigated for the removal of orthophosphate in industrial wastewater. Experimental results in the Ca-PO4-OH system showed that the log [phosphorus residuals] were decreased with increasing pH during pH 7-10 region. Increasing calcium and phosphorus concentration in the initial solution increased phosphorus removal. The initial phosphorus concentration gave greater effect for the phosphate removal than calcium concentration. At pH 8.0, Ca/P=1.6, the phosphorus residuals were 0.48 mmol/l and 0.08mmol/l for 0.7 mmol/l and 7 mmol/l of the initial phosphorus concentration, respectively. Amount of calcium dosage was sufficient at ratio Ca/P=1.6. This phenomena indicates that the precipitated calcium phosphate acts as seed for the crystallization of HAp and it accelerates the crystallization velocity.
The HAp seeded phosphoric acid solution was continuously treated by reaction vessel. The pH was maintained by means of the pH-stat-controlled addition of milky lime. The log [phosphorus residuals] was decreased with increasing pH. From the experimental results, a new process with a two stage addition of lime was proposed. For the first stage, the addition of milky lime was done at pH 8.0-9.0. The second stage, theoretical amount of lime was added to phosphorus residuals under the pH 8.5-9.0. A crystalline HAp of small sludge volume was obtained.
