Operation of mercury cells

In order to understand how anti-corrosive rubber linings are used in the caustic soda industry it is useful to have a broad understanding of the design, construction and operation of the process, mainly about the cell house where corrosion is severe. A brief description of design, construction and operation of mercury cells in the caustic soda industry is given next [11]. 

Dechlorination of depleted brine is an important step in the operation of mercury cells since the dissolved chlorine affected the... 

The aspects of the operation of mercury cells that typically differ from those of the other processes are the brine circulation system, the cell room, treatment of the products, measurement and control, and reduction of mercury emissions. 

The components of the diaphragm, membrane, and mercury cell voltages presented ia Table 8 show that, although the major component of the cell voltage is the term, ohmic drops also contribute to the irreversible energy losses duting the operation of the cells. 

Other remaining technical concerns with membrane cells relate to somewhat lower current efficiencies and to relatively short membrane lifetimes. At present, this is limited to 2-3 year of operation when coupled to much more careful brine pretreatment than is required for conventional asbestos diaphragm cells. A combination of mercury cell and membrane cell technologies has been recently tested for commercial feasibility [19]. The economics of the three primary chloralkali technologies have also been reviewed [20]. 

Sodium Amalgam. Sodium amalgam reduces nitrobenzene to azobenzene as well as to hydrazobenzene knd oximes to amines. The I. G. Farben-industrie, in its development of mercury cells for the production of chlorine and caustic, found that it could lise the intermediate sodium amalgam for the reduction of nitrobenzene to ajzobenzene on a very economical basis. The operation was carried out in batches in nickel reactors, and 50 per cent caustic soda was obtained simultaneously. It was not found feasible to produce hydrazobenzene directly by this method. Hallie has, however, obtained a patent on the reduction of nitrobenzei e to hydrazobenzene using sodium amalgam in aqueous alcohol. 

Solubilities and the physicochemical data related to these amalgams are readily available in the literature.62-65 Of these, sodium amalgam is of importance from the operational viewpoint of mercury cells. The phase diagram of sodium amalgam has been well established (see Fig. 10), and there are various compounds of... 

DeNora also developed a new anode design, called the Runner anode, with the THM (Titanium Hydrodynamic Means) configuration (Fig. 5.11) which utilized the gas lifting force to improve mixing of the brine, thereby eliminating the concentration gradient of brine from the bulk to the mercury interface. This allowed the operation of the cells at the... 

P.M. Mayo, Conversion of Mercury-Cell Plants to Membrane-Cell Technology, 36th Chlorine Institute Plant Operations Seminar, Washington, DC (1993). 

A more specialized application is the filtration of mercury-cell liquor to remove mercury from the product. We consider two types of filter here, the candle filter using carbon tubes and the pressure-leaf filter precoated with activated carbon. The operation of candle filters also has been described in Section 7.S.4.2. 

Mercury-Contaminated Solids. Many plants have recovery facilities that include a retort for vaporizing mercury in order to recover it from inert solids. This is useful for treating small items or small lots of material that are to be removed from the area. Examples include gaskets, decomposer packing, etc. Some wastes, such as the decomposer packing, can be pretreated to remove most of the mercury beforehand. Mercury butter, or thick mercury, is another source that yields mercury by vaporization. This is recovered from cells periodically, and stable operation of the cells with high-quality brine can minimize its formation [101]. 

Of the three processes, the mercury process uses the most electric energy however, no steam is required to concentrate the caustic solution. The use of large quantities of mercury demands measures to prevent environmental contamination. In addition, the hydrogen gas and sodium solution must be freed from mercury. Generally, the operation of the cells is not simple. 

Sodium hydroxide is manufactured by electrolysis of concentrated aqueous sodium chloride the other product of the electrolysis, chlorine, is equally important and hence separation of anode and cathode products is necessary. This is achieved either by a diaphragm (for example in the Hooker electrolytic cell) or by using a mercury cathode which takes up the sodium formed at the cathode as an amalgam (the Kellner-Solvay ceW). The amalgam, after removal from the electrolyte cell, is treated with water to give sodium hydroxide and mercury. The mercury cell is more costly to operate but gives a purer product. 

Overvoltages for various types of chlor—alkali cells are given in Table 8. A typical example of the overvoltage effect is in the operation of a mercury cell where Hg is used as the cathode material. The overpotential of the H2 evolution reaction on Hg is high hence it is possible to form sodium amalgam without H2 generation, thereby eliminating the need for a separator in the cell. 

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. 

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 appHcations and accounts for about 85% of the NaOH consumed in the United States. However, it caimot 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 quaUty and... 

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