Caustic liquor processing evaporation

The caustic liquor produced in a membrane cell is weaker than that from a mercury cell. Typical concentrations are 30-35% NaOH and 28-32% KOH. The net production of caustic solution goes to evaporation and final processing. The caustic solution also recycles around the catholyte side of the cells. This allows control of both temperature and concentration. A cooler removes the waste heat generated in the cells, and water addition keeps the catholyte concentration at the desired level. 

Additional operations essential to commercial bauxite processing are steam and power generation, heat recovery to minimise energy consumption, process liquor evaporation to maintain a water balance, impurity removal from process liquor streams, classification and washing of ttihydrate, lime caustication of sodium carbonate [497-19-8] to sodium hydroxide [1310-73-2] repair and maintenance of equipment, rehabiUtation of mine and residue disposal sites, and quaUty and process control. Each operation in the process can be carried out in a variety of ways depending upon bauxite properties and optimum economic tradeoffs. 

When manufacturing solid sodium hydroxide on industrial scale a caustic solution containing 50 per cent NaOH is evaporated. This is either done directly into the final product or by an intermediate concentration to a 75 per cent NaOH solution. For intermediate concentration a standard single-effect vacuum evaporator is used. As no salt is separated in the course of this evaporation process, the design of the evaporator is very simple. All pumps as well as the piping must be steam jaeketed to prevent the solidification of the highly concentrated liquor. 

Historically, a classic example of an evaporation process is the production of table salt. Maple syrup has traditionally been produced by evaporation of sap. Concentration of black liquor from pulp and paper processing constitutes a large-volume present application. Evaporators are also employed in such disparate uses as desalination of seawater, nuclear fuel reprocessing, radioactive waste treatment,preparation of boiler feed waters, and production of sodium hydroxide. They are used to concentrate stillage waste in fermentation processes, waste brines, inorganic salts in fertilizer production, and rinse liquids used in metal finishing, as well as in the production of sugar, vitamin C, caustic soda, dyes, and juice concentrates, and for solvent recovery in pharmaceutical processes. 

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). 

Recirculating crystals return to the agitator through the annulus between the draft tube and the baffle. The latter as shown here is an extension of the shell of the upper chamber. The volume between the baffle and the outer wall allows the slurry to separate. The crystals fall toward the bottom, and the clarified mother liquor overflows from the top of this section. This mother liquor can go on to further processing or return to the bottom of the crystallizer. The rate of withdrawal of mother liquor determines the clarifying efficiency behind the baffle. It can be varied to control the amount or size of fine crystals that leave the crystallizer. These crystals can redissolve in a fines killer if the circulating mother liquor is heated or if an unsaturated feed stream is introduced. The latter is the normal practice in the recovery of Glauber s salt from caustic evaporators. This action keeps the fines out of the final crystal product and makes the particle size distribution narrower. 

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