Abstract: A system for cooling and recuperative heating of a slurry in a metallurgical process which includes heat exchangers, pumps and autoclaves is described herein. The heat exchangers use a non-scaling common liquid heat transfer medium. Preferably, the heat exchangers are tube-in-tube heat exchangers with 3 to 7 slurry tubes in each heat exchanger. An advantage of this system is that it does not use flash tanks. To minimize abrasive wear on impinged surfaces, the velocity of the slurry is not more than 5 meters per second. The slurry comprises a solids concentration of 25% to 50%. Preferably, the pumps in the system are float-type pumps in which the driven liquid from the discharge pumps is also used as the drive liquid for the feed pumps.

Abstract: Sodium oxalate impurity is removed from oxalate-enriched Bayer process caustic liquor by gradual increase of the caustic concentration of the liquor, thus providing a staged precipitation of the sodium oxalate in a well-crystallized and readily filterable form.

Abstract: A pretreatment system is provided for transforming the goethite constituent [FeO(OH)] of goethitic bauxites to hematite and/or magnetite at high temperatures and pressures. The system employs a number of float-type, liquid displacement pressure pumps, both for charging caustic-containing bauxite slurries of high solids concentration to the system and for discharging the treated slurries. The system operates at temperatures in excess of 250.degree. C and up to temperatures required for the total conversion of the goethite content to hematite or magnetite which, in turn, significantly increases the yield of recoverable alumina. Since the slurry to be pretreated is of high solids concentration, e.g.

Abstract: A process is provided which allows control of the impurity level of Bayer process streams with simultaneous generation of an environmentally acceptable effluent and recovery of valuable dissolved aluminum and sodium values. The process involves carbonation of a blow-down stream at 40.degree.-100.degree. C to produce sodium dawsonite [NaAl(OH).sub.2 CO.sub.2 ] and an effluent essentially free of dissolved alumina and significantly depleted in dissolved sodium. The sodium dawsonite can be used as such or returned to the Bayer process after thermal decomposition to sodium aluminate. The effluent can be directly discharged into natural receiving bodies without further treatment.