a process and apparatus for purifying water - insoluble particles is disclosed . the process comprises the steps of continuously recirculating a fluid aqueous suspension of the water - insoluble particles between a filtration system and a suspension reservoir ; continuously conducting ultrafiltration of the recirculating suspension at the filtration system to separate an aqueous permeate containing the water - soluble impurities from the recirculating fluid aqueous suspension ; continuously discharging the permeate generated by ultrafiltration ; adding water to the recirculating fluid aqueous suspension undergoing ultrafiltration at approximately the same volumetric rate per hour as permeate is being discharged ; monitoring the purity of the recirculating fluid aqueous suspension ; and recovering the purified fluid aqueous suspension after the recirculating fluid aqueous suspension has reached a predetermined purity condition . an apparatus is also disclosed for carrying out the above process . preferred water - insoluble particles may include lakes and particles used to make lakes such as finely divided aluminum hydrate . advantages of this invention include the ability to consistently produce high purity water - insoluble particles at low cost and with savings in manual labor . purification according to this invention requires substantially less water than prior art methods , reducing purified water consumption and waste product disposal costs .