In the traditional production of polyaluminum chloride (PACl), calcium aluminate, caustic soda,lime or aluminum slag wasere used to adjust the basicity as alkali. In this work, a novel composite coagulantwas prepared by using red mud (RM), a high alkali waste from alumina industry, as alkali to adjust the basicityof the semi-product of polyaluminum chloride (ACl). The basicity increased from 34%to 77%with theincrease of RM/ACl ratio (w/v) from 0.1 to 0.5. Hydrolysis-polymerisation curves of the composite coagulantsshowed that short buffer stage was obtained as basicity increased. The Al (III) species of the compositecoagulants showed that the content of Aladecreased and Alcincreased with the increase of RM/ACl ratio,while the content of Albslightly changed. The composite coagulant was prepared at RM/ACl ratio of 0.25(w/v). Compared with commercial PACl, the composite coagulant exhibited better phosphate removalefficiency and faster settlement. Particle size distribution of flocs showed that smaller flocs formed by dosingthe composite coagulant. The coarse flocs surface is favorable for adsorption ability of the compositecoagulant.
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