GB648248A - Method for making a water solution of raw aluminates of lime and its application to the manufacture of alumina - Google Patents

Abstract

<PICT:0648248/III/1> In making solutions high in alkali aluminate and low in calcium aluminate contents by dissolving water - soluble raw calcium aluminates in dilute alkali lyes, containing, e.g. less than 30 gms. of Na2CO3 per litre, and recovering pure alumina therefrom without recourse to the usual purifying salts, the quantity of lye used is reduced below that at which the calcium aluminate is completely decomposed under the reaction conditions obtaining. Alumina may be obtained from the filtered solutions by carbonation at 30 DEG C. to give an amorphous form, or by carbonation at 50 DEG C. to give a crystalline form, Al2O3.2H2O, a granular product being obtained in both cases by effecting precipitation in the presence of suitable seed. In continuous operation, crude aluminate is dissolved at a in return solution from filters and wash waters, drained sludge from a being further drained and washed at F1, from which recovered water is returned to a via tank e. Saturated solution from a is filtered at F2 and make-up Na2CO3 is added thereto in tank b, after which precipitated CaCO3 is filtered and washed at F3, the precipitate being discharged at 6 while draining and wash water are returned to tank e from which a byepass 8 allows requisite amounts of regenerated Na2CO3 solution into the make-up tank b. Lime-free sodium aluminate solution from b is then filtered at F4, carbonated at c and passed into a settling tank d from which it is continually returned to c as build-up seed until the alumina is sufficiently granular, when a portion is withdrawn through 14 and filtered at 15, while the remainder is recirculated at c, residual liquid from d and water from F5 being returned to tank e, where it may be necessary to add lime to decompose bicarbonates formed through over-carbonation.