The effect of ultrasonication on the precipitation of zinc hydroxide, as well as the separation of zinc hydroxide and gypsum precipitates by use of dissolved air flotation are considered in this paper. Both ultrasonic treatment and the addition of carboxymethyl cellulose (CMC) resulted in flocculation of the zinc hydroxide formed during precipitation, The best flotation results could be obtained by, using the CMC during precipitation of the hydroxides, followed by ultrasonic conditioning prior to flotation. Apart from flocculating the zinc hydroxide particles, expediting precipitation and enhancing reagent adsorption, ultrasonic treatment also promoted the mechanical removal of the zinc hydroxide from the surfaces of the gypsum particles. (C) 2002 Published by Elsevier Science Ltd.