Abstract: An improvement in the process for extracting alumina from gibbsitic bauxites, wherein the basic, known process comprises mixing the bauxites with a sodium hydroxide-sodium aluminate digestion liquor, digesting the bauxites at a temperature in the range of 100.degree.-180.degree. C., diluting the resulting slurry, separating the red mud therefrom, precipitating alumina hydrate by cooling and agitation of the sodium aluminate liquor, and calcining the alumina hydrate precipitate to obtain alumina, wherein the improvement is characterized by so controlling the process that, in the liquid phase of the slurry leaving the digestion reactor, the reactive hydroxide ion concentration is maintained below 6 g/l, and preferably between 0 and 2 g/l, where the reactive hydroxide ion concentration is defined by the following formula: ##EQU1## where: C.sub.Al.sbsb.2 O.sub.3,gi.sup.eq is the equilibrium solubility of Al.sub.2 O.sub.3 for gibbsite, in g/l.C.sub.Al.sbsb.2 O.sub.3 is the actual concentration of Al.sub.2 O.sub.

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.

Abstract: The invention relates to a new method of reducing the organic substance content of alum earth production cycle performed according to the Bayer technology. By the method of the invention the majority of the organic substances present in the circulating lye can be removed easily and with only slight losses in sodium content.According to the invention one proceeds so that soda present in the hydrate wash and/or in the soda salt solution is caustified with lime, the solution is evaporated to reach a total sodium oxide concentration of 200 to 400 g/l, the precipitated solid is separated and removed from the system, and the liquid is recycled.

Abstract: An improved process for accelerating the digestion process in the Bayer alumina process is provided which promotes both the goethite-hematite conversion and a more perfect and quick dissolution of the diaspore. The above is achieved by carrying out the digestion in the process of 5-20% by weight calculated on the dry weight of the bauxite of a novel hydrogarnet catalyst.

Abstract: The invention relates to an improved process for the production of alumina according to the Bayer technology, of bauxites containing goethite and especially of boehmitic or diasporic bauxites, for the acceleration of the digestion process, for the transformation of goethite into hematite, for the increase of alumina yield and for the reduction of the caustic soda losses, the essence of which consists in carrying out the digestion in the presence of 2-6% CaO and 0.2-2.0% Mg.sup.++, Fe.sup.++, Mn.sup.++ Co.sup.++ ions based on the weight of the dry bauxite or of compounds or ores containing these ions in the indicated quantities.

Abstract: The invention concerns a process for solubilizing alumina in minerals, rocks or industrial waste products, in which the starting material in finely divided form is mixed with lime in an amount 1 to 3 moles or more of lime (CaO or Ca(OH).sub.2) for each mole of alumina and optionally 1 mole for each mole of any silica present in the starting material, and at the same time or thereafter with water, the resulting mix is maintained under hydrothermal conditions until the alumina has become solubilized by reaction with the lime, and the product is if necessary comminuted to bring it to a form suitable for leaching out of the solubilized alumina.