Caustic liquor processing dilution

The precoat usually is salt itself. Some of the slurry that ordinarily would go to the secondary centrifuges bypasses that step and goes directly to the filter that requires precoating. It is also possible to use filter feed as the precoat, but the finer salt does not work as well. When the precoat becomes blinded or the chambers filled with salt, backwashing is necessary. Fresh water, cell liquor, or unsaturated brine can be used here, and the salt, precoat as well as process load, dissolves and returns to the process. Water is the most efficient backwash fluid, but the use of brine or cell liquor prevents dilution of the process. The caustic filtrate becomes the final product and is ready for storage or shipping. 

Water, after the preliminary treatment methods of Section 12.4.1, can be called purified. Here, we use the term to refer to the higher levels of purification in Table 12.1 or to those processes which remove dissolved contaminants. In the chlor-alkali process, the major uses of purified water are dilution of catholyte, processing of membrane-cell caustic liquor, preparation of ion-exchange system regenerants, manufacture of hydrochloric acid, acidification of brine, and, sometimes, dissolving of salt. It also serves as utility and seal water in the membrane preparation area and in certain parts of the process. 

Separation of the solids, primarily CaCOs, from the caustic solution. Primary separation is in a thickener that produces NaOH solution in the overflow and a suspension of solids in the caustic liquor in the underflow. The solids go to a second set of thickeners for washing with hot water and recycle filtrate (from step 3). The overflow, a dilute caustic solution, is used to prepare the process feed solution. 

B. Process Parameters. Figure 7.46 shows a typical chemical treatment process. Solutions of caustic and carbonate are stored in feed tanks. While direct use of Na2C03 slurry is possible, this diagram is based on solution feed. There may also be a preparation tank in which carbonate solutions are made off line. The caustic solution is received from the process, preferably before evaporation in a membrane-cell plant. A separate supply of a diluted solution (20% or less) is often used, and ion-exchange regenerant solutions are another possible source of treat liquor (Section 7.5.5.2B). 

The oil is mixed with its own volume of water, and the mixture left on a water-bath with red oxide of mercury, slaked lime, and ferrous chloride, out of contact with the air. After all the prussic acid has been decomposed, the oU is rectified and contains no trace of prussic acid. This process was suggested by Redwood. Liebig used oxide of mercury only. Mackay prefers agitating at intervals for forty-eight hours with a mixture of lime and liquor potassse. To detect any traces of prussic acid left in the oil, a little of it is dissolved in alcohol, and a few drops of solutions of ferrous sulphate and ferric chloride are added. Sl ht excess of caustic soda solution is then added, and the precipitate is dissolved hy the addition of dilute hydrochloric acid, when a blue colour or precipitate (due to the formation of Prussian blue) appears if any hydrocyanic acid is present. Benzaldehyde is very readily oxidised by the oxygen of the atmosphere to benzoic acid, so that the oil almost invariably contains... 

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