Salt-brine electrolysis

Manufacture Co-produced with sodium hydroxide by the electrolysis of salt brine. electrical current 2NaCH-2H20 2NaOH -1- H2 -l- CI2 sodium water hydrogen chlorine chloride... 

Sodium chloride is found in salt beds, salt brines, and sea water throughout the world, and it is also mined is some locations. Consequently, sodium chloride is the source of numerous other sodium compounds. A large portion of the sodium chloride utilized is consumed in the production of sodium hydroxide (Eq. (11.23)). The production of sodium metal involves the electrolysis of the molten chloride, usually in the form of a eutectic mixture with calcium chloride. Sodium carbonate is an important material that is used in many ways such as making glass. It was formerly produced from NaCl by means of the Solvay process, in which the overall reaction is... 

In the brine electrolysis system, silica is also contained in raw salt. Silica will precipitate on to membranes in the presence of calcium, strontium, aluminium and iodine resulting in the loss of current efficiency [8-10]. Silica can also be removed in a column filled with ion-exchange resin containing zirconium hydroxide, just like the iodide ion. 

Gorden, G., Gauw, R., Emmert, G. Bubnis, B. (1998) The kinetics and mechanism of 003-formation following the electrolysis of salt brine what role do CIO2 and/or O3 play . Models in Chemistry, 135(5), 799-809. 

Lithium is contained in minute amounts in the mineral ores of spodumene, lepidolite, and amblygonite, which are found in the United States and several countries in Europe, Africa, and South America. High temperatures are required to extract lithium from its compounds and by electrolysis of lithium chloride. It is also concentrated by solar evaporation of salt brine in lakes. 

DuPont, presented at the beginning of this chapter, and Dow are the largest chemical companies in the United States. Dow Chemical Company was started by Herbert Henry Dow (1866-1930) in Canton, Ohio. Dow was a student at Case Institute in Cleveland who studied the characteristics of salt brines acquired from wells around the Great Lakes. Dow was determined to discover methods to extract chemicals from the salt brine. Rather than use the standard distillation method of his day to obtain chemicals, Dow employed electrolysis to separate... 

Figure 19.16. Basic designs of electrolytic cells, (a) Basic type of two-compartment cell used when mixing of anolyte and catholyte is to be minimized the partition may be a porous diaphragm or an ion exchange membrane that allows only selected ions to pass, (b) Mercury cell for brine electrolysis. The released Na dissolves in the Hg and is withdrawn to another zone where it forms salt-free NaOH with water, (c) Monopolar electrical connections each cell is connected separately to the power supply so they are in parallel at low voltage, (d) Bipolar electrical connections 50 or more cells may be series and may require supply at several hundred volts, (e) Bipolar-connected cells for the Monsanto adiponitrile process. Spacings between electrodes and membrane are 0.8-3.2 mm. (f) New type of cell for the Monsanto adiponitrile process, without partitions the stack consists of 50-200 steel plates with 0.0-0.2 ram coating of Cd. Electrolyte velocity of l-2 m/sec sweeps out generated Oz.

The largest scale synthesis based on electrolysis is the chlor-alkali process. Sodium ions in a salt brine migrate... 

The electrolysis of salt brine yields almost equal amounts of chlorine and caustic. It is rare that the growth or use-rates for the two products are equal. Furthermore, the chlor-alkali balance problem extends to related products such as soda ash, lime, salt cake, and... 

Any of the products of brine electrolysis, chlorine, sodium hydroxide, and hydrogen can be hazardous if released. When releases do occur, it is usually from process upsets or breakdowns, which may be minimized by the construction of fail-safe plants, proper maintenance, and by safe transport and storage practices. Probably of greater long-term concern is the mercury loss experienced through the process streams of a mercury cell chloralkali operation. These losses can also carry over to the products of the diaphragm cell, even though this does not use mercury, if a common brine well or common salt dissolver is used for both sets of cells. 

The molecular structure of a conventional polymer used for a PFSA membrane is shown in Fig. 1. Membranes registered as Nafion (DuPont), Flemion , (Asahi Glass), and Aciplex (Asahi Chemical) have been commercialized for brine electrolysis and they are used in the form of alkali metal salt. Figure 4 shows a schematic illustration of a membrane for chlor-alkali electrolysis. The PFSA layer is laminated with a thin perfluorocarboxylic acid layer, and both sides of the composite membrane are hydrophilized to avoid the sticking of evolved hydrogen and chlorine. The membrane is reinforced with PTFE cloth. The technology was applied to PEFC membranes with thickness of over 50 xm [14]. 

Nearly all applications of caustic soda require separation of the chloride from the hydroxide. In sodium-brine electrolysis, fortunately, the phase equilibrium allows a rather effective separation by evaporation of the liquor. If water is removed until the concentration of NaOH approaches 50%, nearly all of the NaCl falls out of solution. After cooling, the residual concentration is about 1.0-1.1%. This removal of salt causes the concentration of NaOH to increase. The solution produced by evaporation therefore can contain somewhat less than 50% NaOH. This is discussed in some detail in Section 9.S.3.3. Dissolved salt is not acceptable in some uses of NaOH, and so there has always been a split market. Part has been reserved to a purified version of the diaphragm-cell product and to mercury-cell, and now membrane-cell, NaOH. 

Electrodes for electrochemical production processes should be good conductors, mechanically strong, free from chemical attack and should be efficient electrocatalysts for the reaction in question, i.e. this reaction should have maximum value when taking place on that electrode. Most of the electrodes are solid metals or carbon (graphite) liquid electrodes of mercury, are used extensively as cathodes in brine electrolysis molten lead electrodes are sometimes also used in molten-salt cells. 

Membranes based on Nation , Flemion, and Aciplex have been commercialized for brine electrolysis and they are used in the form of alkali metal salt. The technology was applied to PEMFC membranes with a thickness of over 50 pm later [42]. For the synthesis of this type of polymer, a fluorosulfonyl monomer is frequently copolymerized with tetrafluoroethylene (TFE). The synthetic scheme of this monomer is shown in Fig. 2.2 [51]. The lEC is about 0.9-1.1 mequiv./g dry polymer. As the lEC increases, water absorption increases, and the crystallinity based on successive sequences of the TFE monomer unit becomes smaller, which lowers the mechanical strength. On the other hand, when the lEC decreases, water absorption decreases, which lowers the proton conductivity. 

Anode Applications. Graphite has been used as the primary material for electrolysis of brine (aqueous) and fused-salt electrolytes, both as anode and cathode. Technological advances, however, have resulted in a dimensionally stable anode (DSA) consisting of precious metal oxides deposited on a titanium substrate that has replaced graphite as the primary anode (38—41) (see Alkali and chlorine products). 

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. 

Brine Preparation. Rock salt and solar salt (see Chemicals frombrine) can be used for preparing sodium chloride solution for electrolysis. These salts contain Ca, Mg, and other impurities that must be removed prior to electrolysis. Otherwise these impurities are deposited on electrodes and increase the energy requirements. The raw brine can be treated by addition of sodium carbonate and hydroxide to reduce calcium and magnesium levels to below 10 ppm. If further reduction in hardness is required, an ion-exchange resin can be used. A typical brine specification for the Huron chlorate ceU design is given in Table 6. 

Fluorine comes from the minerals fluorspar, CaF, cryolite, Na3AlF6 and the fluorapatites, Ca,F(P04)3. The free element is prepared from HF and KF by electrolysis, but the HF and KF needed for the electrolysis are prepared in the laboratory. Chlorine primarily comes from the mineral rock salt, NaCl. The pure element is obtained by electrolysis of liquid NaCl. Bromine is found in seawater and brine wells as the Br ion it ts also found as a component of saline deposits the pure element is obtained by oxidation of Br (aq) by Cl,(g). Iodine is found in seawater, seaweed, and brine wells as the I" ion the pure element is obtained by oxidation of I (aq) by Cl,(g). 

The alkali metals (Group 1A) and the alkaline earth metals (Group 2A) are not found free in nature because they are so easily oxidized. Their primary sources are seawater, brines of their soluble salts and deposits of sea salt. The metals are obtained from the electrolysis of their molten salts. 

Sodium chloride, 22 797-822. See also Salt analytical methods for, 22 811-812 applications of, 22 814-820 from brine, 5 800-801 corrosive effect on iron, 7 806 deposits of, 22 798, 799, 805 described, 22 797 in detergent formulations, 3 418 economic aspects of, 22 810-811 electrolysis of, 22 760 electrolysis of fused, 22 769-772 electrolytic decomposition, 6 175-177 environmental impact of, 22 813-814, 817... 

In the brine system, sulphate ions are mixed with raw salt. These ions deposit on membranes in the electrolysis process and cause loss of current efficiency [3, 4]. 

Solution-mined brine is used for salt production, and is also the feed for the diaphragm electrolysis and soda-ash production. The cell-liquor derived from the electrolysis is also used for soda-ash production after pre-carbonation. 

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