Alumina extraction

Claim 2

IN A PROCESS FOR PREPARATION OF SODIUM ALUMINATE LIQUOR BY FORMATION OF A CAUSTIC SLURRY OF BAUXITE AT ATMOSPHERIC PRESSURE WITH SUBSEQUENT HIGH-PRESSURE DIGESTION THEREOF, THE IMPROVEMENT WHICH COMPRISES, AFTER SAID FORMATION OF SAID CAUSTIC SLURRY, AND PRIOR TO PRESSURE DIGESTION FORMING A DESILICATION PRODUCT IN A HOLDING ZONE CONTAINING SAID SLURRY BY COMBINATION OF DISSOLVED SILICA WITH ALUMINA AND CAUSTIC TO FORM AN INSOLUBLE SODIUM ALUMINUM SILICATE, SEPARATING SAID SLURRY INTO A HIGH-SOLIDS COARSE PORTION COMPRISING MATERIAL MORE THAN 200 MESH IN SIZE AND A LOW-SOLIDS FINES PORTION COMPRISING MATERIAL LESS THAN 200 MESH IN SIZE, CONDUCTING SAID LOW-SOLIDS FINES PORTION THROUGH AT LEAST ONE INDIRECT HEAT EXCHANGER, WHEREBY SAID LOW-SOLIDS FINES PORTION IS HEATED, AND THEREAFTER REUNITING SAID HIGH-SOLIDS COARSE PORTION WITH

Claim 3

The improvement of claim 1 wherein the holding zone comprises a classification zone wherein said low-solids fines portion is overflowed and said high-solids coarse portion is settled.

Claim 4

The improvement of claim 1 wherein the separating of the slurry into the high-solids portion and the coarse solids portion takes place in a plurality of tanks arranged in series.

Claim 5

The improvement of claim 1 wherein said slurry to be separated into said low-solids portion and said high-solids portion contains from about 5% to about 10% by weight solids and the resulting high-solids portion contains at least about 30% by weight solids.

Claim 6

In a process for preparation of sodium aluminate liquor by atmospheric digestion of bauxite followed by high-pressure digestion, the improvement which comprises, after said atmospheric digestion, forming a slurry containing desilication product by combination of dissolved silica with alumina and caustic to form an insoluble sodium aluminum silicate, in a classification zone separating said slurry into a high-solids coarse (-80 mesh) portion and a low-solids fines (-250 mesh) portion, conducting the low-solids portion through a plurality of tubular heat exchangers whereby said portion is heated, and reuniting the high-solids portion with the low-solids portion for high-pressure digestion.

Claim 7

In a process for preparation of sodium aluminate liquor by atmospheric digestion of bauxite followed by high-pressure digestion, the improvement which comprises, after said atmospheric digestion, forming a slurry containing desilication product by combination of dissolved silica with alumina and caustic to form an insoluble sodium aluminum silicate, in a classification zone separating said slurry into a high-solids coarse (-80 mesh) portion and a low-solids fines (-250 mesh) portion, conducting the low-solids portion through a plurality of tubular heat exchangers whereby said portion is heated, and reuniting the high-solids portion with the low-solids portion for high-pressure digestion.

Claim 8

The improvement of claim 5 wherein the classification zone comprises a plurality of tanks in series.

Claim 9

In a two-stage process for recovery of alumina from bauxite wherein a coarse bauxite fraction is separated-in an atmospheric digest-from a fines fraction comprising alumina-rich liquor containing dissolved silica and bauxite generally finer than 80-100 mesh, the improvement which comprises: conducting said alumina-rich liquor containing said bauxite fines and dissolved silica to one or more holding and settling tanks to provide sufficient time to complete a major portion of a desilication reaction by combination of said dissolved silica with alumina and caustic in said liquor to form insoluble sodium aluminum silicate desilication product, adjusting the settling rates so as to permit an overflow fines liquor containing solids less than 250 mesh in size and including said desilication product to overflow said holding tanks, while accumulating coarser solids as a thick slurry, subsequently heating said overflow liquor containing said fine muds by indirect heat exchange with flashed vapor and live steam, then joining said thick slurry with said heated overflow liquid for subsequent high-pressure, high-temperature digestion.

Claim 10

In a two-stage process for recovery of alumina from bauxite wherein a coarse bauxite fraction is separated-in an atmospheric digest-from a fines fraction comprising alumina-rich liquor containing dissolved silica and bauxite generally finer than 80-100 mesh, the improvement which comprises: conducting said alumina-rich liquor containing said bauxite fines and dissolved silica to one or more holding and settling tanks to provide sufficient time to complete a major portion of a desilication reaction by combination of said dissolved silica with alumina and caustic in said liquor to form insoluble sodium aluminum silicate desilication product, adjusting the settling rates so as to permit an overflow fines liquor containing solids less than 250 mesh in size and including said desilication product to overflow said holding tanks, while accumulating coarser solids as a thick slurry, subsequently heating said overflow liquor containing said fine muds by indirect heat exchange with flashed vapor and live steam, then joining said thick slurry with said heated overflow liquid for subsequent high-pressure, high-temperature digestion.

Claim 11

The improvement of claim 7 wherein the thick slurry is also heated by direct steam contacting.