Mercury cell operating conditions

Alercury has a high vapor pressure at the normal cell operating conditions hence it is always found in the reaction products. Although the mercury is almost completely recovered and returned to the process, environmental problems associated with mercury, combined with the less efficient eneigy utilization compared to the modem membrane cell process, has effectively stopped the building of new mercury cell plants. Furthermore, in the 1990s, membrane cells will most likely replace most of the present mercury cells. For details related to mercury cells, see references 8 and 16 and general references. 

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. 

Mercury vaporization losses to the cell room air can amount to 1-5 g/tonne chlorine [30]. Therefore, ventilation is important to ensure safe working conditions, which require that the mercury vapor concentration be kept below 0.05 mg/m. This is achieved by tight cell construction, localized hoods and venting of critical cell areas, and cell room ventilation rates of six to eight air changes per hour (e.g., [31]). Mercury cell chloralkali plants in moderate climates are able to operate their cells outside, which avoids these ventilating problems, but does not control potential emissions. 

In seeking the conditions for operating the cell, both the absolute energy consumption and the slope of the lines, essentially the cell resistance, are important since there will be a trade-off between energy consumption and the rate of production (i.e. current density) when the total cost is taken into account the slopes indicate the additional energy which must be consumed to permit a faster production rate. It is the low resistance of the mercury cell which allows the use of high current densities without an unreasonable voltage penalty. 

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. 

With brine at or near its full process temperature, the operating pressure is reduced to about a third or a half of an atmosphere. Nearly all the chlorine in the depleted brine is recovered and can be returned to the process. The resulting brine is not suitable for return to a membrane-cell process. There is need for further dechlorination, which is the topic of Section 7.S.9.3. In a mercury-cell process, on the other hand, there actually are advantages to incomplete dechlorination. The presence of free chlorine in the brine returned to the salt dissolver, given suitable materials of construction, helps to keep mercury in solution and prevents its deposition on the brine sludge that will be removed from the process. Typical concentrations are 10-50 ppm CI2. Especially given the inherently lower solubility of chlorine, conditions used to dechlorinate mercury-cell brines therefore can and should be less rigorous. 

The flow of liquid water in the decomposer is adjusted to obtain a 50 wt% NaOH solution and mercury is pumped back to the electrolysis cell. Typical operating conditions and performances of a mercury cell are as follows ... 

Many cathode catalyst materials have been used. For noble metal catalysts, platinum was mainly used in fuel cells for space applications. For terrestrial use, one has to use less expensive materials, and non-noble metal catalysts are therefore mainly employed. Bacon used lithium-doped nickel oxide as a cathode catalyst for high-temperature AFCs. Lithium-doped nickel oxide has a sufficient electrical conductivity at temperatures above 150 °C. Currendy, mainly Raney silver and pure silver catalysts are favored. Developments of silver-supported materials containing PTFE are sometimes successful. Silver catalysts are usually prepared from silver oxide, Raney silver, and supported silver. Typically, the catalysts on the cathode are supported by PTFE because it is highly stable under basic and acidic conditions. In contrast, carbon is oxidized at the cathode in contact with oxygen, when carbon is used as an inexpensive support material. In the past, the silver catalysts frequentiy contained mercury as part of an amalgam to increase the stability and the lifetime of the cathode. Because mercury is partially dissolved during the activation procedure (see below) and during the fuel-cell operation, some electrolyte contamination can be observed. Because of the environmental hazard of mercury, this metal is currently not used in silver catalysts. 

Brine Purification. In mercury cells, traces of heavy metals in the brine give rise to dangerous operating conditions (see p. 32), as does the presence of magnesium and to a lesser extent calcium (521. In membrane cells, divalent ions such as Ca or Mg are harmful to the membrane. The circulating brine must be rigorously purified to avoid any buildup of these substances to undesirable levels [7]. Calcium is usually precipitated as the carbonate with sodium carbonate magnesium and iron, as hydroxides with sodium hydroxide and sulfate, as barium sulfate. 

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