The coagulation process. II. Effect of pH on the precipitation of aluminium hydroxide

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Abstract
The precipitation of aluminium hydroxide following the addition of aluminium sulphate to solutions containing sodium bicarbonate, sodium hydroxide and sodium chloride was investigated. The degree of precipitation was determined both by measuring the amount of residual soluble aluminium and the change of turbidity.The optimum pH for the coagulation of dilute clay suspensions with aluminium sulphate corresponds to that at which aluminium hydroxide is most rapidly precipitated and coagulated in the absence of clay. It is concluded that mutual coagulation of aluminium hydroxide and clay must be insignificant. The clay particles are probably removed by enmeshment in the mass of flocculating aluminium hydroxide.