Mercury cells operation

The mercury cell operates efficiently because of the higher overpotential of hydrogen on mercury to achieve the preferential formation of sodium amalgam. Certain trace elements, such as vanadium, can lower the hydrogen overpotential, however, resulting in the release of hydrogen in potentially dangerous amounts. 

Ru losses can occur during electrolysis, as well as those due to shorting in mercury cell operations, from erosion, loss of Ru-based intermediates involved during the course of chlorine and oxygen evolution reactions and during shut-downs. 

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). 

To avoid getting into diffusion-limited regimes arising from the solubility limitations, the amalgam should be stirred sufficiently to reduce the thickness of the diffusion layer. However, impurities such as Mg2+ in the brine can deposit on the amalgam cathode as Mg(OH)2 retarding the convection of the amalgam. Hence, brine purity becomes an important issue in mercury cell operations. 

A typical cell room (Fig. 3.5) consists of a large number of cells in electrical series so as to make use of the available 480 V, i.e. about 100 cells. Mercury cells operate at current density in the range 0.8— 1.4 A cm so that the total cell current will vary... 

FIGURE 3.8. Brine processing scheme for mercury-cell operations. 

Amalgam butter—a commonly used term in mercury cell operations—is a multiphase material, which has not been characterized thoroughly. The butter formed with Fe, Ni, and C consists of finely divided metal particles dispersed in mercury, whereas graphite butter is shiny and foamy. Ca butter is hard and compact, while Fe butter is blackish. 

TABLE 4.4.4 Effect of Metallic Impurities on Hydrogen Evolution at the Cathode of a Mercury Cell Operated at 4 kA and at 80°C... 

Hence, the thermodynamic decomposition voltage of a mercury cell operating at SO C with 0.2% Na amalgam is equal to 3.058 V as shown below. 

Diaphragm cell operating at 2.32 kA Membrane cell operating at 3.5 kA Mercury cell operating at lOkA m ... 

A general process description for mercury-cell operations is in Chapter 5 and hence, is not elaborated here. It should be noted that these energy flow diagrams do not take into consideration all the process details and can only provide a general description of the energy flows. 

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