In the non-submerged diaphragm types the invasion of the anodic compartment by caustic is much reduced and the efficiency and life increased.
An electrolyser of the non-submerged diaphragm type is the Allen-Moore cell which has been adopted by the Montreal Water and Power Co. This has been described by Pitcher and Meadows.[8] The general lay-out of the installation is shown in Fig. 10, and the essential features are: a salt storage bin having a capacity of 40 tons; the brine saturating and purifying apparatus; duplicate 15 horse-power motor-generator sets; four chlorine cells; and the silver ejectors and distributing lines for carrying the chlorine solution to the point of application.
[Illustration: FIG. 10--Brine Saturating and Purifying Equipment.]
The brine solution, which is prepared by passing water through the saturators previously filled with salt, is delivered to the two concrete reaction tanks where an amount of soda ash and caustic liquor sufficient to combine with the calcium and magnesium salts is added, and the mixture filtered through sand and stored in the purified brine tanks. To prevent the formation of hypochlorites by the interaction of chlorine and alkali, the alkalinity of the liquor is determined and sufficient hydrochloric acid added to ensure an acidity of 0.01 per cent. The acid brine is delivered at one end of the four cells (Fig. 11) each of which is 7 feet long and 20-3/8 inches wide and consumes 600 amperes at 3.3 volts. The cell box is built of concrete and is provided with a perforated wrought iron cathode box and graphite anode plates which are separated by an unsubmerged asbestos paper diaphragm.
[Illustration: FIG. 11.--Sections of Allen-Moore Cell.]
Each cell has a capacity of 32 pounds of chlorine per day and the gas flow is determined by measuring the volume of caustic soda produced in a given period of time and calculating the weight from the volume and concentration as determined by titration with standard acid; each gram of NaOH is equal to 0.88 gram of chlorine. The efficiency of the cell is obtained by dividing the number of grams of chlorine produced per hour by the product of the current volume (in amperes) and the factor 1.33, the theoretical production of chlorine for one ampere hour. The average efficiency of the Montreal cells was found to be 93 per cent. The installation comprises four cells, one being held in reserve, and the annual cost of producing 90 pounds of chlorine per day is given as $2,500. The details are: