Brine, electrolysis

Sodium hydroxide brine electrolysis lime-sodium carbonate reaction (not presently used)... 

P. S. S. Hayfteld and W. R. Jacob, "Platinum—Iridium-Coated Titanium Anodes ia Brine Electrolysis," paper presented 2i. A.dvances in Chlor-A.lkali Technology, London, 1979. 

At present about 77% of the industrial hydrogen produced is from petrochemicals, 18% from coal, 4% by electrolysis of aqueous solutions and at most 1% from other sources. Thus, hydrogen is produced as a byproduct of the brine electrolysis process for the manufacture of chlorine and sodium hydroxide (p. 798). The ratio of H2 Cl2 NaOH is, of course, fixed by stoichiometry and this is an economic determinant since bulk transport of the byproduct hydrogen is expensive. To illustrate the scde of the problem the total world chlorine production capacity is about 38 million tonnes per year which corresponds to 105000 toimes of hydrogen (1.3 x I0 m ). Plants designed specifically for the electrolytic manufacture of hydrogen as the main product, use steel cells and aqueous potassium hydroxide as electrolyte. The cells may be operated at atmospheric pressure (Knowles cells) or at 30 atm (Lonza cells). 

The largest uses of platinum group metals in electronics are ruthenium for resistors and palladium for multilayer capacitors, both applied by thick film techniques . Most anodes for brine electrolysis are coated with mixed ruthenium and titanium oxide by thermal decomposition . Chemical vapour deposition of ruthenium was patented for use on cutting tools . 

Metals are the most important electrode materials. Because of the readily renewable surface of mercury electrodes, they have been for several decades and, to a certain degree, still remain the most popular material for theoretical electrochemical research. The large-scale mercury electrode also plays a substantial role in technology (brine electrolysis) but the general tendency to replace it wherever possible is due to the environmental harmfulness of mercury. 

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. 

Hydrogen is also obtained as a by-product of brine electrolysis for the production of chlorine and sodium hydroxide. Small electrolytic plants (hydrogen generators) are often used for in situ applications when small quantities of hydrogen are required at remote locations. 

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.

Chlorine is produced almost entirely by the electrolysis of aqueous solutions of alkali metal chlorides (Fig. 1), or from fused chlorides. Brine electrolysis produces chlorine at the anode and hydrogen along with the alkali hydroxide at the cathode. At present, three types dominate the industry the diaphragm cell, the membrane cell, and the mercury cell, and there are many variations of each type. 

An acid brine electrolysis will give the best results if smooth platinum anodes are used. If chromate is added and the electrolyte temperature > maintained at 75 to 80 °C, a current efficiency of 95 per cent can be obtained. Nowadays, graphite or magnetite is substituted for platinum. 

Alkali chloride electrolysis — Chlorine with its coproduct sodium hydroxide (also called caustic soda or simply caustic) has been produced on an industrial scale for more than a century by the electrolysis of a saturated solution of sodium chloride (brine). The process, which is often referred to as brine electrolysis or chlorine-... 

In the last twenty-five years a new process has been developed in the chlor-alkali industry that uses a membrane to separate the anode and cathode compartments in brine electrolysis cells. The membrane is superior to the diaphragm used in diaphragm cells because the membrane is impermeable to anions. Only cations can flow through the membrane. Because neither Cl- nor OH- ions can... 

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. 

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