Abstract: A method for the separation of magnesium hydroxide from a mixture of particulate magnesium hydroxide and dissolved sodium sulphate in water and for the washing and thickening of the magnesium hydroxide, includes the steps of pumping the mixture from a holding tank 14 through an ultrafiltration module comprising two banks 16, 18 of tubular membranes to which is applied a back pressure so that the dissolved sodium sulphate passes through the membrane pores positioned substantially at right angles to the direction of flow as a permeate; adding water to the holding tank 14 at a rate substantially equal to the permeate flow rate until the sulphate concentration in the magnesium hydroxide is less than a desired level; and then discontinuing the water addition thus allowing the magnesium hydroxide to thicken until a desired viscosity is reached.