GB857245A - Improvements in production of alumina - Google Patents

Abstract

Silica-free and iron-free alumina is produced from an iron-containing clay by calcining the clay at at least 1000 DEG F., and leaching with aqueous hydrochloric acid of about 20% strength to form a silica-free solution of iron and aluminium chlorides, which after separating from the insoluble residue is evaporated and the chlorides crystallised; these are then heated to form on decomposition a crude alumina containing iron oxide. The crude alumina is converted to sodium aluminate and <PICT:0857245/III/1> <PICT:0857245/III/2> the resulting solution of the latter after separation from the insoluble iron oxide, is treated to precipitate aluminium hydroxide which is separated and calcined. The crude alumina may be converted to sodium aluminate by digestion in caustic soda in which case the aluminium chloride from which the crude alumina is formed, is heated at 1000 to 1300 DEG F., or the crude alumina may be heated with soda in which case the aluminium chloride precursor is heated at 1500 to 1900 DEG F. Alumina is precipitated from the aluminate solution by cooling and seeding if a caustic soda digestion is employed, whilst if a soda sintering is employed, precipitation is normally effected with carbon dioxide. As shown in Fig. 1 clay is shredded and calcined at 1200 DEG C. and is thereafter ground and leached with 20% hydrochloric acid solution at at least 160 DEG F. with agitation. The solution of chlorides so formed is passed to a separator or filter, the solid residue discharged and the wash liquor passed to a subsequent stage in the process whilst the pregnant liquor containing aluminium and iron chlorides is evaporated to form a 25 to 30% solid slurry. The crystals are drained and calcined at 1000 to 1300 DEG F. The HCl evolved is absorbed in an aqueous medium which may be wash liquor from earlier in the process, preferably at its boiling temperature. The crude alumina is ground and digested with caustic soda of a strength from 100 to 400 grams per litre at 275 to 400 DEG F. and 50 to 225 lbs. per square inch pressure. The sodium aluminate solution after filtering is cooled to precipitate the aluminium hydroxide whilst the spent liquor is recycled, if desired after removing excess sodium chloride which gradually builds up. The precipitated alumina is filtered and washed and calcined at 1700 to 2100 DEG F. The sodium chloride may be removed from the spent liquor by evaporation and crystallisation and the sodium chloride thus recovered may be subjected to electrolysis to produce caustic soda which is recycled, and hydrogen and chlorine which are combined to form the necessary hydrogen chloride. Alternatively as shown in Fig. 2 the crude alumina obtained as in Fig. 1 (with the exception that the chloride decomposition kiln is operated at at least 1500 DEG F. and preferably at 1700 to 1900 DEG F.,) is treated after grinding with sodium carbonate. The latter may be mixed with the crude alumina as a concentrated aqueous solution recovered from later in the process and the mixture is sintered at 1700 to 1900 DEG F. thereby converting the alumina to sodium aluminate. The product is leached preferably by recycled wash water and filtered to obtain a sodium aluminate solution leaving an iron oxide residue. The aluminate solution is thereafter carbonated with carbon dioxide and passed through a thickener from which aluminium hydroxide is separated and passed through a calciner. The overflow from the thickener, consisting of sodium carbonate, is evaporated and recycled. The calcination of the washed aluminium hydroxide is preferably effected in a gas fired kiln and the kiln gas after scrubbing with water may be recycled to the carbonation step. If the sodium compound for the soda sinter step is supplied as sodium hydroxide it may be carbonated by the carbon dioxide gas recovered from the hydrate calcining kiln gas.