Mercury rocking cell

The mercury cell process has developed extensively since its inception in 1892. A number of distinct phases can be identified, from the rocking cell to the 400 000 A modem mercury cell unit. The cell chemistry is identical in all cases... 

The first operating membrane cell was the rocking cell, largely developed by Baker, who was Castner s chief chemist, at Oldbury, and later at Runcorn. The history of the Castner Kellner plant at Runcorn gives the history of the development of the mercury cell, and indicates the way in which a technology has developed in the drive to increase production quantities and efficiencies while minimizing capital costs. 

Castner turned his interest to gold extraction, which required high-quality sodium hydroxide. Castner developed a three-chambered electrolytic cell. The two end chambers contained brine and graphite electrodes. The middle chamber held water. The cells were separated excepted for a small opening on the bottom, which contained a pool of mercury that served as the cell s cathode. When current flowed through the cell and the cell was rocked, sodium reduced from the brine came into contact with water in the middle cell to produce a sodium hydroxide solution. As Castner built his mercury cell, Kellner was working on a similar design. Rather than compete with each other, Castner and Kellner joined forces to establish the Castner-Kellner Alkali Company to produce sodium hydroxide, which competed with soda ash and potash as an industrial base, and chlorine, which was used primarily to make bleach. 

During the electrolysis the cell is rocked, causing the amalgam to flow back and forth. The amalgam produced in the end compartment is mixed with the mercury in the central compartment, and it then reacts with the water in this compartment ... 

FIGURE 8.3 Vertical section of a Castner-Kellner rocking mercury cell, which demonstrates the principle of operation. The cell had provision for brine entry and removal of depleted brine from the left compartment, and for purified water entry and removal of sodium hydroxide from the right compartment. The eccentric revolved slowly to move the mercury on a regular basis between the two compartments. A cell divider extended close enough to the bottom of the cell to maintain a mercury seal, which prevented any mixture of the water phases of the two compartments. 

McCurdy and Katz have described a simple visual cell for measuring vapour-liquid equilibrium. The equilibrium cell had a volume of 92 cm , was fitted with double glass windows and could be rocked to agitate the fluid phases and thus bring them into equilibrium. The sample was confined over mercury. To sample the gas phase, mercury was driven into the cell at the bottom at the same rate as gas sample was removed from the top. The liquid phase was sampled by completely removing the vapour sample and then the liquid sample by injection of mercury into the bottom of the cell. The gas and liquid samples were analysed by chemical gas absorption methods. 

Crude salt, whether it be rock or sea salt, contains a number of chemical constituents (see Table 5.1). The saturated NaCl solution prepared from these salts must be treated to remove impurities before going to a cell. Brine specifications depend on the type of cell used (diaphragm, mercury, or membrane) and the operating conditions. Water is transported from the anolyte to the catholyte through the membrane, as stated above, but more is required to maintain the water balance in the cathode compartment. 

Hildebrand based his double-cup mercury cathode on the Castner-Kellner process for the industrial production of caustic soda. This process, which has been called the most elegant electrolytic process ever invented," was patented in 1894 by Hamilton Castner, an American analytical chemist. By 1902, Castner had increased its efficiency to 90 percent. The heart of the process was Castner s "rocking" mercury cell. The cell had two chambers through which mercury moved back and forth, continuously removed metallic sodium from the decomposing chamber, thus preventing the recombination of the sodium and chlorine. (24)... 

Salt. The basic raw material for the mercury cell and membrane cell processes is usually solid salt. This may be obtained from three sources rock salt, solar salt, or vacuum-evaporated salt from purifying and evaporating solution-mined brine. 

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