Hafnium hydroxide filtration, expression, and washing

Hafnium hydroxide filter cake and ammonium sulfate are being recovered from slurry using an expression (membrane) filter press. The process cycle time is approximately 6 hours. Each cycle produces 150 kg of cake containing 75 kg of hafnium oxide. The hafnium hydroxide filter cake has a 4 × 105 Pa-specific cake resistance of 1013 m/kg and is highly compressible. Washing the cake requires two cake volumes water displacement. The four-hour wash cycle requires washing with 5.5 × 105 Pa pressure water while compressing the cake to 1.5 × 106 Pa. The expressed cake contains 55% moisture, has a dry texture, and crumbles to a powder during handling. The expression dewatering rate varies linearly with moisture content. The filter cake is nearly self-releasing from the multi-monofilament filter cloth.