Until a few years ago the non-diaphragm process was the only one used for water treatment and it will consequently be discussed first.
Non-diaphragm Process. The theoretical voltage required for the decomposition of sodium chloride is 2.3 but when the products recombine in the electrolyte, side reactions occur which increase the minimum voltage to 3.54. On this basis one kilowatt hour gives 272 ampere hours and as one ampere hour is theoretically capable of producing 1.33 grams of chlorine, 1.21 kilowatt hours are necessary for the production of 1 pound of chlorine by the decomposition of 1.65 pounds of salt.
Charles Watt (1851) discovered this process and was the first to recognize the necessary conditions which are (1) insoluble electrodes, (2) low temperature of electrolyte, and (3) rapid circulation of electrolyte from the cathode to the anode. The control of the temperature is very important, for as it increases, side reactions occur with the formation of chlorates, and the efficiency is decreased.
The non-diaphragm cells used in Europe (Haas and Oettel, Kellner, Hermite, Vogelsand, and Mather and Platt) have been described by Kershaw.[2] In the Haas and Oettel electrolyser the electrodes are composed of carbon but in the other types at least one electrode is made from platinum or a platinum alloy. The Dayton electrolyser, which is the cell most familiar in North America, is shown in [Fig. 9].
Fig. 9.—Dayton Electrolytic Cell.
The outer cell is made of soapstone and is approximately 21⁄2 feet long and 2 feet wide. The main electrodes consist of four pieces of Atcheson graphite connected together by screws and metal strips to which is attached a clamp for connecting electrical terminals. Circulation of the brine is produced by glass baffle plates and secondary electrodes placed one inch apart between the main electrodes. The cell is intended to be used at 110-volts pressure but by wiring two cells in series a 220-volt circuit may be employed. An inlet and outlet are provided at each end of the tank to permit the direction of the flow to be periodically reversed for the purpose of removing the lime deposit from the graphite plates.
The salt solution is prepared in wooden tanks from coarse clean salt (ground rock salt is unsuitable), containing as little iron as possible, in the proportion of 50 pounds to 100 gallons of water. After passing through a gravel or other suitable filter the brine solution is carried by brass pipes to the electrolyser. The rate of flow is adjusted to the temperature of the hypochlorite solution leaving the cell but under normal conditions it is stated that the cell described will pass 40 gallons per hour with a consumption of 70 amperes and produce 21⁄2 pounds of chlorine per hour. This is equal to 8 pounds of salt and 3.08 kilowatt hours per pound of chlorine. After the cells have been operated for several months the efficiency usually falls and 10-11 pounds of salt and 3.5-3.7 kilowatt hours are required for the production of one pound of chlorine. The concentration of the hypochlorite solution is usually about 6 grams per litre.
Rickard[3] stated that by cooling the Dayton cell with ice 1 pound of chlorine could be produced from 2.65 kilowatt hours and 6.9 pounds of sodium chloride; without cooling the figures were 3.62 kilowatt hours and 7.2 pounds of salt. Based on the figures that have been obtained from mature cells, the efficiency of the Dayton cell as compared with those described by Kershaw is as follows: