Patent GB-743553-A (GB)
Title: Improvements in or relating to processes for the production of alumina from aluminous ores
Inventor: PORTER JOHN L
IPC: C01F7/062
Filed: 1953-06-24
Abstract: <PICT:0743553/III/1> In the wet alkali aluminate process for the production of alumina from aluminous ore or ores containing both alumina trihydrate and monohydrate, a monohydrate alumina-containing ore is first digested in spent caustic aluminate liquor under conditions suitable to extract the monohydrate to obtain an intermediate liquor having an alumina/caustic ratio greater than that of the spent liquor but less than that normally obtainable in extraction of a trihydrate ore, then digesting a predominantly trihydrate ore in the liquor resulting from the monohydrate digestion to increase the caustic ratio to that normally obtainable in extraction of a trihydrate ore.  The alumina is then removed in conventional manner.  The alumina/caustic (A/C) ratio of the monohydrate stage preferably does not exceed 0.58 whilst the final A/C ratio from the trihydrate stage should approach 0.63. The ore employed may be a West Indian laterite containing both mono- and trihydrate, this ore being employed in both stages, with or without additional trihydrate ore in the second stage. Alternatively, the ore used in the first stage is predominantly monohydrate (boehmite) and that in the second stage a trihydrate ore (gibbsite) or an ore containing a mixture of mono- and trihydrate such as the above laterites.  In a typical process applied to Jamaican bauxite employed in both stages, a spent liquor of A/C ratio of 0.33 is fed to the monohydrate digestion stage which is conducted at 390 DEG F. and a caustic concentration of 190 grams per litre.  The A/C ratio of the effluent is 0.543 and this liquor is fed to the second trihydrate digestion, conducted at 300 DEG F. to obtain a final A/C ratio of 0.622.  Fig. 2 illustrates a treatment of Jamaican bauxite having 40 per cent trihydrate and 6 per cent monohydrate alumina content in the first, monohydrate digestion and a gibbsite ore containing 51 per cent trihydrate alumina in the second digester.  The Jamaican bauxite is fed to the monohydrate digester together with spent liquor at 320 DEG F. and an A/C ratio of 0.323 from line 26.  The partially pregnant liquor leaves digester 1 through line 2 to a flash chamber 3 where the temperature of the liquor is reduced from 385 to 318 DEG F. with corresponding increase in caustic concentration from 190 to 206 grams/litre.  The steam is employed to heat incoming spent liquor in heat-exchanger 4.  The concentrated liquor passes from chamber 3 to the trihydrate digester 6 to which gibbsite is fed thereby increasing the A/C ratio from 4.504 to 0.626 and reducing the caustic concentration from 206 to 186 grams/litre.  The liquor passes through flash chamber 8 similar to 3 and thence through line 9 to a mud settler 11.  The underflow from 11 is fed by line 12 to a washer 13 from which the wash water rejoins line 9 through line 14.  The clarified pregnant liquor from settler 11 passes to a filter 16 and then to a vacuum water 18 and a precipitator 20.  Here the alumina is precipitated and withdrawn, the temperature of the liquor decreases to 140 DEG F. and the A/C ratio decreases from 0.626 to 0.33.  The spent liquor is fed by pipe 21 to a caustic make up tank 22, an evaporator 24 and a heat-exchanger 4 thereby raising the temperature to 320 DEG F. and the caustic concentration to 226 grams/litre, after which the spent liquor passes to the monohydrate digester 1 for a fresh cycle.  In the treatment of a mixture of boehmite and gibbsite the monohydrate extraction may provide 30 per cent of the total alumina extracted.  On the other hand in the treatment of a high trihydrate-low monohydrate Jamaican ore in conjunction with a trihydrate ore as described above the trihydrate may constitute 90 per cent of the total alumina extracted.