Caustic liquor processing cooling

In the process to produce alumina (Fig. 1), bauxite is crushed and wet ground to 100-mesh, dissolved under pressure and heated in digesters with concentrated spent caustic soda solution from a previous cycle and sufficient lime and soda ash. Sodium aluminate is formed, and the dissolved silica is precipitated as sodium aluminum silicate. The undissolved residue (red mud) is separated from the alumina solution by filtration and washing and sent to recovery. Thickeners and Kelly or drum filters are used. The filtered solution of sodium aluminate is hydrolyzed to precipitate aluminum hydroxide by cooling. The precipitate is filtered from the liquor, washed, and heated to 980°C in a rotary kiln to calcine the aluminum hydroxide. 

The glycerine, thus purified by one or the other of these two processes, contains only a very small quantity of sea-salt in solution, and may be distilled. The inventor also adopts another method when he desires to obtain the carbonated or caustic salts of soda in the condition of carbonates, instead of transforming them into chlorides by means of hydrochloric acid. Bor this purpose he evaporates the ley and introduces into it carbonic acid, so as to convert the caustic soda into carbonate. When the liquor indicates about 25 , he allows the ley to cool, when he introduces an excess of carbonic acid, whereby bicarbonate of soda is formed, which is only slightly soluble, especiallj in a glycerine solution of salt. The greater... 

All diaphragm cells produce cell liquor that contains ca. 11 wt% caustic soda and 18wt% sodium chloride. To market the caustic soda, its concentration must be increased to 50 %. During the evaporation and cooling processes, the salt becomes less soluble in the stronger caustic, and at 50 % NaOH the NaCl concentration is ca. 1 %. 

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