Asbestos diaphragms

Fig. 9. Dow diaphragm cell, section view a, perforated steel back plate b, cathode pocket c, asbestos diaphragm d, DSA anode e, copper back plate f,...

L. C. Curhn, T. F. Florkiewicz, and R. C. Matousek, Polyramix (PM), A Depositable Replacementfor Asbestos Diaphragms, Paper presented at London... 

In the diaphragm-cell process, a solid cathode (iron) is used where hydrogen is evolved [reaction (15.4)]. Porous asbestos diaphragms are used to prevent mixing of the catholyte and anolyte, but owing to the finite permeability of these diaphragms, the alkaline solution that is produced near the cathode stiU contains important levels of chloride ions as an impurity. 

Asbestos-cement products, 3 311, 313 alternatives to, 3 315 Asbestos-containing insulation, 74 211 Asbestos diaphragms, in caustic soda manufacture, 22 839 Asbestosis, 3 316... 

For over a hundred years the chlor-alkali industry has used the mercury cell as one of the three main technologies for the production of chlorine and caustic soda. For historical reasons, this process came to dominate the European industry - while in the United States the asbestos diaphragm cell took the premier position. Over the last two decades developments in membrane cells have brought these to the forefront, and membrane cells of one kind or another now represent the technology of choice worldwide. 

The chlor-alkali cell in this diagram electrolyzes an aqueous solution of sodium chloride to produce chlorine gas, hydrogen gas, and aqueous sodium hydroxide. The asbestos diaphragm stops the chlorine gas produced at the anode from mixing with the hydrogen gas produced at the cathode. Sodium hydroxide solution is removed from the cell periodically, and fresh brine is added to the cell. 

The anionic groups almost completely inhibit transport of hydroxide ions from the cathode, at the same time letting current flow in the form of sodium ions. The resulting caustic is purer and more concentrated while still avoiding the potential pollution of mercury cells. These cells have larger power requirements than asbestos diaphragm cells. 

In chlor-alkali diaphragm cells, a diaphragm is employed to separate chlorine hberated at the anode from the sodium hydroxide and hydrogen generated at the cathode. Without a diaphragm, the sodium hydroxide formed will combine with chlorine to form sodium hypochlorite and chlorate. In many cells, asbestos diaphragms are used for such separation. Many types of diaphragm cells are available. 

In the obsolescent asbestos diaphragm cell, the product on the cathode side is typically 11% in NaOH and 16% NaCl (i.e., about 2.7 mol of each per kilogram of solution). Evaporation to about 50% NaOH causes most of the NaCl to crystallize, leaving about 1% NaCl in solution this caustic soda is sufficiently pure for many industrial uses. The Si-0 links in the asbestos are attacked by the alkali (Section 3.5 and Chapter 7), and the diaphragm soon deteriorates. 

Colonel Benard of Paris1 (1890) prepared hydrogen, for balloon work, in a cylindrical iron cathode vessel in which was suspended a cylindrical iron anode surrounded by an asbestos diaphragm. The electrolyte was caustic soda, and 250 litres of hydrogen were obtained per hour. 

The membrane cell (Fig. 1) uses a cation exchange membrane in place of an asbestos diaphragm. It permits the passage of sodium ions into the catholyte but effectively excludes chloride ions. Thus the concept permits the production of high-purity, high-concentration sodium hydroxide directly. 

Diaphragm cell (Fig. 15.3b). In this cell there is a physical barrier between the anode (DSA) and the cathode (steel) which is an asbestos diaphragm supported by a steel net. Sometimes separator and cathode are joined. Caustic soda is generated directly at the cathode with the... 

A — Nickel plated, Iron electrodes, B — Gas collecting bells, O — Current supply, D — Current conductors to tho olectrodes, B — Insulation pipe, G, H — Current oonduotors, I — Gas collecting pipe from tho individual bells, K — Gas pipe, L — Asbestos diaphragm. 

In this case asbestos is the only suitable material for diaphragms being fully resistant against caustic while at least partially resistant against chlorine. Asbestos diaphragms are particularly suitable for electrolyzers with horizontal diaphragms as they are entirely surrounded by caustic solution (see Fig. 88). In such a medium the durability of asbestos is almost unlimited and the diaphragm requires replacement only after its permeability has been considerably impaired by precipitated hydroxides of alkaline-earth metals. 

The permeability of the asbestos diaphragm is considerable at the start of the operation but becomes normal after 5 to 6 days of forming . The voltage applied equals 3.7 V with new electrodes with graphite electrodes operating... 

The cathodes are enveloped on the outer side by an asbestos diaphragm which is formed by dipping the cathode assembly into a suspension of asbestos fibres in salt solution then a suction is applied at the hydrogen outlet or caustic solution drain. During this evacuation the cathodes become coated with a sufficiently thick layer of asbestos. After some minutes the cathode is removed from the suspension and air is drawn through until the asbestos becomes dry and compact. It takes about one hour to make a new diaphragm. 

---