Three processes are industrially operated in which aqueous solutions of sodium chloride are electrolyzed for the manufacture of chlorine, sodium hydroxide and hydrogen: process; diaphragm process; membrane process.

Manufacture using the membrane process is gaining in importance, since new chlorine capacity exclusively utilizes this technology. In Japan sodium chloride electrolysis is exclusively carried out membrane plants. The percentage contributions of the three processes to chlorine production are given in Table 1.7-15 (Europe) and Table 1.7-16 (worldwide).

Mercury Process

The amalgam cells consist of slightly inclined steel troughs, over the bottoms of which flow a thin mercury layer, which absorbs the sodium and acts as the cathode. Horizontal anodes adjustable in height at which chlorine is produced are incorporated into the lid of the cells. The chlorine is drawn off upwards through gas extraction slits.

The amalgam emerging from the ends of the cells is converted on graphite into mercury, 50% sodium hydroxide solution and hydrogen in a strongly exothermic reaction (see Fig. 1.7-5, 1.7-6 and 1.7-7).

Description of mercury cells:
(1)cathode surface area: 10 to 30 m²
(2)mercury layer thickness: 3 mm
(3)sodium concentration in mercury: 0.2 to 0.4% (by weight)
(4)50 to 180 individual anodes per cell
(5)cathode-anode separation: 3 mm
(6)anode material: graphite or, preferably, titanium coated with a noble metal compound (so-called dimensionally stable anodes DSA;
(7)brine throughput per cell: 3 to 20 m³/h

A salt solution with a sodium chloride content of ca. 310 g/L is electrolyzed at ca. 80°C, during which the sodium chloride content falls to 260 to 280 g/L. This is then concentrated by adding solid salt and recycled. During electrolysis the following reactions take place:
(1)reaction at anode:
Cl^- → 0.5Cl₂ + e^-; deposition voltage ca. 1.24 V
(2)reaction at cathode:
xHg + Na⁺ + e^- → NaHg_x; deposition voltage ca. -1.66 V

Typical side-reactions are:
(1)at the anode:
Cl₂ + 2NaOH → NaOCl + NaCl + H₂O
(2)at the cathode:
Cl₂ + 2e^- → 2Cl^-
C1O^- + 2H⁺ + 2e^- → H₂O + Cl^-

The electrochemical yield is 94 to 97%, the energy consumption ca. 3300 kWh/t chlorine, the effective cell voltage 4.2 V and the current density 8 to 1.5 kA/m².

The amalgam formed at the cathode is decomposed with water:
NaHg_x + H₂O → 0.5H₂ + NaOH + xHg
The electrical energy stored in the amalgam is thereby converted into heat.

Capacities of industrial plants:
(1)50 to 300^.10³ t/a chlorine
(2)56 to 340^.10³ t/a sodium hydroxide

In modern units the height of the anodes is computer controlled. Chemical and physical processes are used to reduce the mercury concentration in the effluent, exit gases and products to the ppb level.

Diaphragm Process

Industrial diaphragm cells consist of a box in which the anode plates are mounted vertically parallel to one another. The cathodes are flat hollow steel mesh structures covered with asbestos fibers, optionally impregnated with fluoroorganic resins, and fit between the anodes (see Fig. 1.7-8).
(1)moar electrode arrangement: anode surfaces of up to 50 m² per cell (activated titanium). Cathodes and anodes are all electrically connected with one another
(2)bipolar electrode arrangement: electrode surface areas of up to ca. 35 m². Cathodes and anodes are connected back to back.

The salt solution fed into the anode chamber passes through the diaphragm into the cathode chamber. The chlorine produced at the anode is drawn off upwards and hydrogen and sodium hydroxide mixed with residual salt are produced at the cathode.

The asbestos diaphragm has a number of functions:
(1)it has to hinder the mixing of hydrogen and chlorine. The tangled fiber structure of the asbestos allows liquids to pass through, but not fine gas bubbles (the 4% of chlorine which dissolves in the brine does, however, pass into the cathode chamber where it is reduced thereby reducing the yield).
(2)it hinders to a large extent the back-diffusion of the cathodically-formed OH^- ions to the anode. The flow rate of the brine into the anode chamber is regulated to limit the back-diffusion and the hydrostatic pressure therein.

Upon electrolysis, the sodium chloride content of an initially saturated solution falls to ca. 170 g/L. The reactions at the anode are the same as in the mercury process. However, hydrogen is produced at the steel cathode:
H₂O+e^- → 0.5H₂+OH^-
The cell alkali leaving the cathode chamber contains ca. 12% NaOH and 15% NaCl (by weight).

Recovery of sodium hydroxide: The alkali solution is evaporated to 50% by weight of sodium hydroxide, whereupon the salt, except for a residual 1%, precipitates out. This salt is very pure and can be further utilized for concentrating depleted brine or, in the case of combined plants, in the mercury process.

Evaporation is carried out in multi (up to four)-stage forced circulation evaporators. 5 t of water have to be evaporated per t of 50% sodium hydroxide solution. A further purification of this salt-containing sodium hydroxide is possible, but very expensive.

Capacity of industrial plants:
(1)360^.10³ t/a of chlorine corresponding to
(2)ca. 410^.10³ t/a of sodium hydroxide
at a specific current density of 2.2 to 2.7 kA/m².

The electrical energy consumption is ca. 20% less than that in the mercury process.

Membrane process

In the membrane process the cathode and anode chambers are separated by a water-impermeable ion-conducting membrane (see Fig. 1.7-11).

The membrane has to be stable under electrolysis conditions i.e. high salt concentrations, high pH-jump between anode and cathode chambers and to the strong oxidizing agents chlorine and hypochlorite.

These demands are fulfilled by membranes with a perfluorinated polyethene main chain with side-chains with sulfonic acid and/or carboxylic acid groups as produced by DuPont and Asahi Glass.

Multilayer membranes are also used, which have, for example, thin sulfonamide layers on the cathode side.