Quality mercury cells

In 1988 diaphragm cells accounted for 76% of all U.S. chlorine production, mercury cells for 17%, membrane cells for 5%, and all other production methods for 2%. Corresponding statistics for Canadian production are diaphragm cells, 81% mercury cells, 15% and membrane cells, 4% (5). for a number of reasons, including concerns over mercury pollution, recent trends are away from mercury cell production toward the more environmentally acceptable membrane cells, which also produce higher quality product and have favorable economics. 

A wide range of operating conditions and design philosophies affect mercury cell efficiency. For example, the fundamental distinction between a resaturation and a waste brine process influences the temperature and brine strength profile along the length of the cell and hence the overall efficiency. Another important factor is the quality of the brine. Impurities in the brine can cause base-plate deposits, which tend to reduce the anode/cathode gap. This gradual reduction in gap requires either manual or automatic adjustment and, eventually, the cell must be taken off-line and the thick mercury removed. 

In 1997 Bayer AG increased chlorine production at its Uerdingen site by adding Krupp Uhde membrane electrolysers to the existing mercury cells. In order to achieve a high chlorine quality (minimum oxygen concentration) and maximum chlorine yield, the acidic process is run - namely the brine leaves the cells at a low pH of 2.5-3. 

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. 

Three forms of caustic soda are produced to meet customer needs purified diaphragm caustic (50% Rayon grade), 73% caustic, and anhydrous caustic. Regular 50% caustic from the diaphragm cell process is suitable for most applications and accounts for about 85% of the NaOH consumed in the United States. However, it cannot be used in operations such as the manufacture of rayon, the synthesis of alkyl aryl sulfonates, or the production of anhydrous caustic because of the presence of salt, sodium chlorate, and heavy metals. Membrane and mercury cell caustic, on the other hand, is of superior quality and... 

In mercury cells, impurities play a significant role during the formation of sodium amalgam and, hence, quality control of brine purity is an important unit operation. 

The process energy consumption in a membrane cell is small compared to diaphragm and mercury cell operations and the membrane cell caustic is of the same quality as mercury cell caustic. Hence, the membrane cell technology is recognized as the most economical and preferred method for producing chlorine and caustic (see Section 6 for additional details regarding membrane cells). 

The concentration and quality of the caustic produced in a mercury cell depend entirely on the flow rate and quality of the water fed to the decomposer. The use of demineralized water is standard. The caustic and hydrogen produced in the decomposer are quite hot. Some of the water thus evaporates and leaves with the hydrogen gas. 

Note a common chlor-alkali process uses the mercury cell which gives excellent production quality products. However, mercury loss to the environment has made the use of this very efficient electrochemical process difficult to justify. 

It was the development of ion-exchange membranes which opened a new era in brine electrolysis. These membranes have the extraordinary property of allowing the passage of electrolytic current and yet almost completely preventing the mixing of the hydroxide formed in one compartment of the electrolytic cell with brine introduced as raw material into the other compartment. Hence, a "membrane cell" could be developed for large scale industrial electrolysis, giving satisfactory quality of products without environmental hazard. This is likely to make the "mercury cell" vanish as the enormous investment into new installations pays off. 

Caustic soda is made by the electrolysis of brine using both mercury cells and diaphragm cells to give caustic soda of di erent qualities. 

Production of caustic soda solution. In 1998, the worldwide production capacity was about 54 million tons per year. Ca. 96-98% of this amount is produced by chloralkali electrolysis [313). The three processes are described in detail in chapter 5 (Mercury Cell Process), chapter 6 (Diaphragm Process) and chapter 7 (Membrane process), a comparison of the relative qualities is given in chapter 9. 

The textile industry uses NaOH in the manufacture of viscose and of cellophane. It was this application which promoted the development of the mercury cell because this process produces the chloride-free rayon quality caustic soda. Cotton and wool can be improved by mercerization, a treatment with NaOH. 

In the diaphragm process, a KCl-containing, 8 -10 % potassium hydroxide solution is initially formed, whose salt content can be reduced to ca. 1.0 -1.5 % KCI by evaporation to a 50% liquor. Further purification is complicated, and the quality of liquor from mercury cells cannot be achieved. 

Although die zinc-mercuric oxide battery has many excellent qualities, increasing environmental concerns have led to a deemphasis in the use of this system. The main environmental difficulty is in the disposal of the cell. Both the mercuric oxide in the fresh cell and the mercury rcducrion product in the used cell have long-term toxic effects. 

The potentials of the two electrodes of the lead—acid cell are measured vs. a reference electrode. Thus, the lead—acid cell turns into a three-electrode cell. During measuring the potential of the two electrodes of the LA cell, the reference electrode should not be polarized, i.e. its potential should remain constant. The most common reference electrodes are hydrogen, cadmium, mercury-mercurous sulfate and silver-silver sulfate electrodes. Cadmium sticks are widely used in industrial quality control laboratories to measure the electrode potentials of the manufactured batteries. Cadmium does not form poorly soluble cadmium sulfate, which is the reason why during the measurement the electrolyte in the cell absorbs a few Cd ion impurities that do not affect the performance of the battery, however. 

Because there is no change in electrolyte composition dnring operation—the overall cell reaction involves only solid substances—the mercnry battery provides a more constant voltage (1.35 V) than the Leclanche cell. It also has a considerably higher capacity and longer life. These qualities make the mercury battery ideal for use in pacemakers, hearing aids, electric watches, and light meters. 

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