抄録
The coagulation treatment of Cr2(SO4)3-Ca(OH)2-H2O system was made, and the effects of CaSO4 produced in the coagulation and the structure of sediment were investigated to make clear the coagulation mechanism of that system. The optimum condition of additional amount of Ca(OH)2 to Cr(SO4)3 concentration was at the mol ratio of Ca2+/Cr23+=3.0, according to the following equation: Cr2(SO4)3+3Ca(OH)2→2Cr(OH)3+3CaSO4. The pH value was at 7-8, at which the highest rate of sedimentation was observed, and the concentration of residual Cr3+ in the supernatant was less than 10-6mol/l under the above condition. When Cr2(SO4)3 concentration was varied under the above optimum condition, there was a difference in the composition of sediment between higher and lower concentrations of Cr2(SO4)3. On the side of the concentration lower than 10.2×10-3mol/l, water content and chromium hydroxide concentration were high, but calcium sulfate concentration was low. Whereas, on the side of the concentration higher than 12.75×10-3mol/l, the inverse results were obtained. Consequently, it was recognized that the relation between water content and calcium sulfate concentration in the sediment was in inversely proportional. After calcination of the sediments, the structures of these hydroxides were investigated by DTA. X-ray diffraction, and infrared absorption spectroscopy. The results of investigations showed that the structure of sediment on the lower side of concentration was chromium hydroxide containing Ca2+ and SO42-, and that on the higher side of concentration was the mixture of chromium hydroxide and CaSO4·2H2O.
