Brine electrolysis mercury cells

Potassium hydroxide is produced commerically by electrolysis of a saturated solution of potassium chloride in brine using mercury cells consisting of a titanium anode and mercury cathode. Potassium reacts with mercury forming the amalgam which, on treatment with water, forms potassium hydroxide and hydrogen. 

Sodium hydroxide was formerly made by the treatment of sodium carbonate with lime but its main source today is from the electrolysis of brine using mercury cells or any of a variety of diaphragm cells. The principal product demanded from these cells is chlorine (for use in plastics) and sodium hydroxide is almost reduced to the status of a by-product. It is strongly alkaline and finds many applications in the chemical industry, particularly in the production of soaps and paper. It is also used to adsorb acidic gases, such as carbon dioxide and sulphur dioxide, and is used in the treatment of effluent for the removal of heavy metals (as hydroxides) and of acidity. Sodium hydroxide solutions are extremely corrosive to body tissue and are particularly hazardous to the eyes. 

R.B. MacMulUn, Electrolysis of Brines in Mercury Cells. In J.S. Sconce (ed.). Chlorine Its Manufacture, Properties and Uses, R.E. Kreiger Publishing Company, Huntington, New York (1972), p. 127. 

MacMullin, R.B., Electrolysis of brines in mercury cells, in Chlorine Its Manufacture, Properties and Uses. Sconce, J.S., Ed., Robert E Krieger Publishing Co., Huntington, NY, 1972, p. 127. 

Two types of cells are used for the electrolysis of brine the mercury cell and the diaphragm cell. In the mercury cell the cathode is mercury, the anode is graphite and the cell reaction is... 

Manufacture is either by reaction of molten sodium with methyl alcohol or by the reaction of methyl alcohol with sodium amalgam obtained from the electrolysis of brine in a Castner mercury cell (78). Both these methods produce a solution of sodium methylate in methanol and the product is offered in two forms a 30% solution in methanol, and a soHd, which is a dry, free-flowing white powder obtained by evaporating the methanol. The direct production of dry sodium methylate has been carried out by the introduction of methanol vapors to molten sodium in a heavy duty agitating reactor. The sohd is supphed in polyethylene bags contained in airtight dmms filled in a nitrogen atmosphere. 

Recently it has been shown that the oxides of the platinum metals can have a higher corrosion resistance than the metals themselves , and have sufficient conductivity to be used as coatings for anodes, e.g. with titanium cores. Anodes with a coating of ruthenium dioxide are being developed for use in mercury cells for the electrolysis of brine to produce chlorine , since they are resistant to attack if in contact with the sodium-mercury amalgam. 

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.

In the mercury cell process chlorine is liberated from a brine solution at Ihe anodes which are. today, typically melal anodes (Dimensionally Stable Anodes or DSAl. Collection and processing of the chlorine is similar lo Ihe techniques employed when diaphragm cells are used. However. Ihe cathode is a flowing bed or mercury. When sodium is released by electrolysis it is immediately amalgamated with the mercury The inereury amalgam is then decomposed in a separate cell 10 form sodium hydroxide and Ihe mercury is returned for reuse. 

There are two well-established methods for electrolysing brine, the diaphragm cell and the mercury cell. However, recent developments in electrolysis technology, by chemical engineers, have produced the membrane cell (Figure 5.13). This method is now preferred to the other two because it produces a purer product, it causes less pollution and it is cheaper to run. 

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. 

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. 

Briefly, how do the brine pretreatment steps required for diaphragm cell electrolysis and mercury cell electrolysis compare, and what are the reasons for these differences ... 

Mercury compounds continue to have numerous commercial uses. Besides its use as a preservative, mercury is used in the manufacture of many technical and medical instruments including blood pressure measurement devices, manometers, thermometers, and barometers. Mercury is also used in production of certain types of fluorescent lamps and in the chloralkali industry, where chlorine and caustic soda are produced using brine electrolysis in mercury cells. Metallic mercury is used in the production of precious metals such as gold and silver. As part of the production process, metallic mercury can be used to concentrate gold from... 

About 75% of total chlorine capacity in the world is produced electrolytically using diaphragm and membrane cells, while 20% is made using mercury cells. In all these technologies, chlorine and caustic are produced simultaneously by electrolysis of sodium chloride (commonly termed brine ) following the overall reaction... 

Nearly all sodium compounds are soluble in water. Sodium hydroxide is produced commercially by the electrolysis of brine using diaphragm cells or mercury-cathode cells chlorine is a coproduct. 

Production of CI2 and NaOH by electrolysis of NaCl is a huge industry with annual production capacity in excess of 50 million tons of NaOH per year. Membrane cells are the state-of-the-art technology, but mercury and diaphragm cells are still used because the capital cost for their replacement is substantial. The mercury cell technology is more than a century old and stiU accounts for nearly half of the world s production capacity. Chlorine evolves from a DSA (dimensionally stable anode) situated above a pool of mercury with NaCl brine in between. Mercury reacts with sodium to form sodium amalgam, which is removed and hydrolyzed in a separate reactor. 

Table 3.2 lists the reversible potentials for the four electrode reactions which must be considered in brine electrolysis. The data are given for the pH s which prevail at the anodes and cathodes in the various cells. Qiloride ion is always oxidized in slightly acid solution to prevent hydrolysis of chlorine to hypochlorite while the pH at the cathode will, with the exception of the mercury cell, be at least 14. Two factors emerge immediately. First, in a mercury cell where the electrode reactions are A and D, the term - E/ ) of equation (3.5) is —3.25 V, a value... 

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