Caustic liquor processing

The mercury returns to the cell, and the caustic liquor is ready for processing. By controlling the amount of water fed to the decomposer, the operator can control the strength of a mercury-cell product and can produce commercial concentrations directly. 

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. 

For added security, vents from the tails tower and the product storage tank can be scrubbed with a caustic liquor. The acid solution itself will contain a small amount of dissolved chlorine. In most plant applications, as for example in the acidification of depleted brine, this is not a problem. Other uses, such as the regeneration of the brine softening resin in a membrane-cell plant, may require that this chlorine be removed. Adsorption on activated carbon is probably the simplest technique for this small-scale process, which is similar to that described in Section 7.5.9.3B. 

Recovery of solid salt from the caustic liquor is primarily by centrifugation, normally in two stages. Section 9.3.2.S has already covered this operation. Processing of the recovered salt depends on the measures taken for sulfate removal. Postponing discussion of the sulfate, we consider first the handling of the salt for reuse. 

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. 

The ore is washed, crushed and milled (Figure 7.1, step A) to reduce the particle size and make the minerals more available for extraction. It is then combined with the process caustic liquor and sent as slurry to a heated pressure digester for extraction (step B). 

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. 

Two cocrystallization processes employ dibasic crystals as intermediates. The PPG process (199—202) is discussed under commercial processes. The PPC process (203) forms dibasic crystals from lime and recovered filtrates. The dibasic crystals are separated from thek mother liquor by decantation, slurried in caustic solution and chlorinated to produce a cocrystalline slurry of Ca(OCl)2 and NaCl. The slurry is sent to a flotation cell where the larger salt crystals settle out and the smaller hypochlorite crystals float to the top with the aid of ak and flotation agent. The hypochlorite slurry is centrifuged the cake going to a dryer and the centrate to the flotation cell. The salt-rich bottoms from the flotation cell are centrifuged and washed with dibasic mother Hquor. The centrates are recycled to the precipitation step. 

Red and blue acid-resistant bricks are resistant to all inorganic and organic chemicals, except for hydrofluoric acid and hot concentrated caustic alkalis. Acid-resistant fireclay bricks are used for conditions involving elevating temperatures and corrosive condensates. Highly vitrified materials such as chemical stoneware, porcelain and basalts are used for extremely severe duties or where contamination of the process liquors is undesirable. 

A process lean stream and an external MSA are considered for removing H2S. The process lean stream, S1, is a caustic soda solution which can be used as a solvent for the reactive separation of H2S. An added bonus for using the process MSA is the conversion of a portion of the absorbed H2S into Na2S, which is needed for white-liquor makeup. In other words, H2S pollutant is converted into a valuable chemical which is needed in the process. The external MSA, S2, is a polym ic adsorbent. The data for the candidate MSAs are given in Table 8.2. The equilibrium... 

There is a so-called dry mercerisation process [275] in which the fabric is padded with caustic soda liquor at 20-25 °C and then dried in a stenter at about 130 °C. An immersion time in the pad trough of 7-10 seconds is sufficient but the goods need a total saturation time in the alkaline liquor of 30-40 seconds, i.e. from the nip to entry into the drying zone. 

Generally, although not exclusively, a scrubber with a recycle loop of the caustic scrubbing liquor is used cases of once-through scrubbing liquor operation do exist. The scrubber may be operated in batch, semi-batch or continuous mode with respect to the liquid. Process hazards exist in batch and continuous mode, the most significant of which is over-chlorination. Batch-wise operations leads to periodic high loads on the hypochlorite destruction unit. In order to even out these loads, and improve the process safety, a study of alternative treatment options has been undertaken. 

The process concept is shown in Fig. 26.7 where the recycle loop of the caustic scrubbing liquor passes through a fixed-bed reactor and then through the normal cooler. The blow-down of spent caustic and make-up with fresh caustic can be carried on in the same fashion as without the in-loop hypochlorite decomposition. Consideration of the optimum locations for removal and addition may, however, be slightly different. 

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