Abstract: The invention relates to an apparatus for the classification of solid materials, preferably that of hard and highly pure materials, comprising a housing provided with an inlet stub, fine fraction outlet stub and a coarse fraction outlet stub as well as vane-crowns. The fluid carrying the material to be separated or classified radially passes through two vane-crowns with the vanes oriented at an angle to the tangent of the vane-crowns. Upon passage through the first vane-crown the separating or classifying chamber is reached whereupon the coarse fraction is separated out and the fluid flows radially through the second vane-crown whereupon the flow becomes axial with the fluid exiting against gravity and the separated material being collected below in a closed storage tank associated with the coarse product outlet stub.

Abstract: An air-jet mill provided with pregrinding chambers and inner sizer suitable for fine grinding (below 10 .mu.), for cryogenic grinding and also for surface treatment during grinding of hard, elastic and/or thermoplastic materials.During operation there are no moving components to be exposed to heavy wear, and an inner sizer, forming an integral part of the grinding space, uses the energy left after grinding in the system to enable a sharp sizing or separation of fine fractions.

Abstract: The invention relates to a process for the reduction of sodium hydroxide losses of the Bayer-cycle by modified causticization of red mud which comprisessubjecting an optionally previously concentrated red mud slurry led off from the red mud washing line by ramification and containing a liquid phase which has a concentration of 5-40 g. caustic Na.sub.2 0/1. caustification by adjusting the lime content expressed as CaO through addition of lime or lime milk to 1-3.2 moles per mole Al.sub.2 O.sub.3, preferably to 2-2.5 moles per mole Al.sub.2 O.sub.3, related to the total Al.sub.2 O.sub.3 content of the slurry;stirring the slurry for 0.3-5 hours at a temperature of 70.degree.-110.degree. C., preferably at 80.degree.-100.degree. C.;adding to the slurry 3-4 moles, preferably 3.2-3.4 moles, of soda related to 1 mole of the originally dissolved Al.sub.2 O.sub.3, and continuing the stirring for 1-4 hours at 80.degree.-100.degree. C., preferably at a temperature of 90.degree.-95.degree. C.