Abstract: In the production of alumina from aluminous ores containing alumina trihydrate and alumina monohydrate comprising a first-stage digestion by digesting alumina trihydrate from a slurry of the ores and a caustic solution and separating the resulting slurry into a caustic aluminate solution and an insoluble residue, and a second-stage digestion by digesting alumina monohydrate from a slurry of the insoluble residue separated in the first-stage digestion and a caustic solution, the present invention provides an improvement wherein scale formation onto a preheater for the slurry in the second-stage digestion is completely or substantially prevented and reduction in heat transfer coefficient of heat transfer surface is remarkably lowered. The improvement is attained by setting up a desilication reaction zone in the course of the first-stage digestion, whereby the slurry introduced into said zone is subjected to desilication reaction treatment.

Abstract: Coarse grains of aluminum hydroxide having high resistance to disintegration to powder in a calcination step are continuously produced from a supersaturated sodium aluminate solution of the Bayer process through(1) a first precipitation step of adding recycle seed aluminum hydroxide to a supersaturated sodium aluminate solution having a molar ratio of Na.sub.2 O as caustic soda to Al.sub.2 O.sub.3 in solution of less than 1.8 supplied to a precipitation tank of substantially complete mixing type in a ratio of 30-150 kg of the recycle seed to 1 m.sup.3 of the solution, thereby partially decomposing the seed-added solution until the molar ratio of the solution reaches 2.0-2.4 while keeping the solution at 65.degree.-80.degree. C.

Abstract: Coarse grains of aluminum hydroxide are economically produced in a high yield from sodium aluminate solution by dividing supersaturated sodium aluminate solution being prepared by the Bayer process and having a molar ratio of Na.sub.2 O as caustic soda to Al.sub.2 O.sub.3 in solution of less than 1.8 into two streams, adding a portion of recycle seed aluminum hydroxide and fine grains of aluminum hydroxide as seed to one of the streams of sodium aluminate solution, partially decomposing the sodium aluminate solution until the molar ratio of the sodium solution reaches 1.8-2.6, adding to the resulting partially decomposed sodium aluminate slurry the other stream of sodium aluminate solution, which has been cooled to a temperature low enough to lower the temperature of the partially decomposed sodium aluminate slurry by at least 3.degree. C.

Abstract: A process for the removal of sulfur oxides from exhaust gases by absorbing with a slurry of red mud containing calcium ion which comprises the steps of:(1) contacting an exhaust gas containing sulfur oxides of not higher than about 100.degree. C. with a slurry of red mud containing calcium ion led from the next step (2) to take up sulfur oxides in the gas until pH of the slurry becomes in the range of 4.3 to less than 6 while maintaining the molar fraction of SO.sub.4.sup.-- to total SO.sub.2 in the slurry within the said pH range at 0.05 to 0.7; and then,(2) contacting the treated gas with a fresh slurry of red mud containing calcium ion to further take up sulfur oxides in the gas while maintaining the concentration of NaHSO.sub.3 in the slurry at not more than about 8 g/l.

Abstract: In a process for extracting alumina from alumina-containing ores on a two-stream system by individually preheating a slurry of alumina-containing ores and an aqueous alkali solution each in preheaters of indirectly heating, mixing the preheated slurry with the preheated aqueous alkali solution, and extracting alumina from the ores in a digester of either the heating type involving the direct injection of live steam or the heat-insulated type, the alumina is extracted from the ores by preparing the slurry of alumina-containing ores with water or a diluted aqueous alkali solution, such as from the drain obtained by recovering steam from an evaporator for cooling the slurry resulting from extraction, and utilizing the recovered steam in preheating the slurry of the alumina-containing ores or decomposition solution or both, preheating the slurry of alumina-containing ores, and mixing the preheated slurry with an aqueous alkali solution preheated as another stream as the decomposition solution.