Salt recovery

The sodium chlorate manufacturing process can be divided into six steps (/) brine treatment 2 electrolysis (J) crystallisation and salt recovery (4) chromium removal (5) hydrogen purification and collection and (6) electrical distribution. These steps are outlined in Figure 3. 

Molten Salt Extraction (MSE) - Salt Recovery. The salt residue from the americium extraction process is made up of NaCl, KC1, MgCl2, PuCl3, and AmCl3. A typical residue weighs approximately 2.0 kg and contains 200 g plutonium, 10-20 g americium, 50 g MgCl3 with the remaining equimolar NaCl-KCl. 

A widely used metal salt recovery technique is evaporation. With evaporation, plating chemicals are concentrated by evaporating water from the solution. Evaporators may use heat or natural evaporation to remove water.22 28 Additionally, evaporators may operate at atmospheric pressure or under vacuum. 

Effect of the salt molecular mass (MB) on the cation transport number (C) in the corresponding solution effective solute (tB) and water (%) transport numbers surface resistances (rc, ra) of, and counterion transport numbers (tc+, ta ) in cation- and anion-exchange membranes specific electric energy consumption (e) in the case of 90% salt recovery at 1 A, and maximum solute concentration theoretically achievable in the concentrating stream (CBCmax). Note NaCl, sodium chloride Na-A, acetate Na-P, propionate Na-L, lactate. 

Countercurrent extraction is favored over crosscurrent extraction at a fixed salt composition for the following reasons (1) less salt is required to obtain the same separation, (2) plutonium loss to the salt is lower, (3) less magnesium metal by-product is generated, and (4) salt feed to the salt recovery line is cut in half. 

Electrorefining (ER) Salt Recovery. Approximately 8% (500 g) of the plutonium in an electrorefining run is lost to the salt phase. Historically, this waste stream was recovered by aqueous processing. The salts are now recovered pyrochemically by salt stripping. This operation consists of contacting the molten salt residue with calcium metal, resulting in the reduction of PuCls to metal and the coalescence of metallic plutonium shot. The process results in the formation of a metallic plutonium button (96% yield), a large white salt phase, and a small black salt phase. 

Feed brines for solar salt recovery may be derived from seawater with salinity of about 3.5%, or from one of the enclosed seas or natural salt lakes of higher salinities (Table 6.3). The Caspian Sea is not used for solar salt production because of its much lower salinity (1.3%) and its temperate location. Also, its salts content is about 25% lower in chloride and about three times higher in sulfate than ordinary sea salt. 

These features are likely to encourage further development of vacuum or solar evaporation salt recovery operations to work these brines in proximity to the desalination plants. In this way, various salts may be more profitably recovered from these artificially enriched seawaters for reasons similar to the present incentives to use the rich natural brine sources for sodium chloride production (Table 6.3). Similar energy savings should be obtained. 

Solar evaporation, from primary and secondary ponds of 100 and 30 km in extent, the initial stage for potassium chloride recovery from the Dead Sea brines [26]. The smaller number of constituent ions present in these waters significantly simplifies salts recovery, and the fact that they contain nearly twice the relative potassium chloride concentration of seawater also improves profitability. Developed from a process, which was first operated in 1931, evaporation in the first pond reduces the volume of the brine to about one-half of the initial volume and brings down much of the sodium chloride together with a small amount of calcium sulfate (Fig. 6.5). The concentrated brines are then transferred to the secondary pond where evaporation of a further 20% of the water causes carnallite (KCl MgCli 6H2O) and some further sodium chloride to crystallize out. With care, a 95% potassium chloride product on a scale of some 910,000 tonne/year is obtained either by countercurrent extraction of the carnallite with brines, or by hot extraction of potassium chloride from the sylvinite matrix followed by fractional crystallization for its eventual recovery [16]. 

P. Sistat, G. Pourcelly, C. Gavach, N. Turcotte and M. Boucher, Electrodialysis of acid effluents containing metallic divalent salts recovery of acid with a cation exchange membrane modified in situ, J. Appl. Electrochem., 1997, 27, 65. 

Fig. 12.4 Salt-effect distillation. 1, Feed stream 2, ED column 3, salt recovery operation 4, bottoms product 5, salt recycle 6, dissolving chamber 7, overhead product.

Salts recovery from seawater desalination plants... 

Three principal methods are available for ammonia and/or ammonium salt recovery they are known as the direct, indirect, and semidirect processes, respectively. 

Wu RC, Xu YZ, Song YQ, Luo JA, Liu D. (2011). A novel strategy for salts recovery from 1,3-propanediol fermentation broth by bipolar membrane electrodialysis. Sep Purif Technol, 83, 9-14. 

G. Materials of Construction. Evaporator materials of construction and their costs play a large role in determining the optimum compression ratio. Monel is a standard material of construction for brine evaporator bodies. Stabilized titanium (e.g.. Grade 12) is a frequent choice for heater tubing and Type 316 stainless steel for the circulating pumps. The evaporator normally will be of the forced-circulation type described in Section 9.3.3.2. It will either contain a number of effects or operate with vapor recompression. Salt-recovery centrifuges may be Monel or stainless steel. 

Ferritic stainless steels in general are less resistant than the austenitics and are subject to caustic cracking. Cupronickel tubes are occasionally used for low- to medium-concentration caustic at mild temperatures and with restrictions on fluid velocity. These alloys are also resistant to hot 70% NaOH, but only in the complete absence of oxidants. Monel has a number of applications, but its use can be restricted by the slow development of color due to copper contamination. It is a frequent choice for processing equipment in the salt-recovery section of the diaphragm-cell process (Sections 9.3.2.S and 9.3.2.6). 

Example. Washed salt from the primary centrifuge (Stream 1) drops into a leaching tank. The composition of the salt depends on the processing conditions and the amount of sulfate in the cell liquor. Here, we assume that the centrifuge cake is about 96% sohds and the amount of Na2S04 precipitated is 2.8% of the total solids (dry basis). This tank will export clear liquor to the Glauber s salt crystallizer and slurry to a salt recovery system, for return to the brine plant. The leach tank also receives mother liquor from the crystallizer (Stream 3) and a small amount of evaporator condensate (Stream 2) to... 

Figure 11.16 Electrohydrolysis processes for salt recovery, (a) Two-compartment cell for electrohydrolysis. (b) Sodium citrate recovery in a three compartment cell.

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