Aluminum may be present in acid wastes as the trivalent aluminum ion or in alkaline wastes as an aluminate ion. Aluminum is precipitated as the hydroxide or hydrolysis species of polymeric aluminum. The precipitation and conditioning of precipitated solids has an important effect on the separation rate and on the settling and dewatering characteristics of the precipitate. Precipitation in the presence of previously formed solids produces denser and more rapidly settling floc particles. The addition of a polyelec-trolyte also improves settling characteristics. Low velocity gradients, with agitator peripheral speeds as low as 5 ft/sec, are desirable to avoid shearing the floc into small particles that settle slowly.

Depending on the method of precipitation and solids separation, aluminum hydroxide can be concentrated to 1.0 to 2.0% by weight. Adding suitable polyelectrolytes, it can be further dewatered by centrifugation or vacuum filtration. However, without preconditioning of the aluminum hydroxide sludge, either precoat vacuum filtration or filter presses are required for sludge dewatering.

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