Brine mercury cell

Mercury cells are operated to maintain a 21—22 wt % NaCl concentration in the depleted brine and thus preserve good electrical conductivity. The depleted brine is dechlorinated and then resaturated with soHd salt prior to recycling back to the electroly2er. 

Manufacture is either by reaction of molten sodium with methyl alcohol or by the reaction of methyl alcohol with sodium amalgam obtained from the electrolysis of brine in a Castner mercury cell (78). Both these methods produce a solution of sodium methylate in methanol and the product is offered in two forms a 30% solution in methanol, and a soHd, which is a dry, free-flowing white powder obtained by evaporating the methanol. The direct production of dry sodium methylate has been carried out by the introduction of methanol vapors to molten sodium in a heavy duty agitating reactor. The sohd is supphed in polyethylene bags contained in airtight dmms filled in a nitrogen atmosphere. 

Chlorine—hydrogen ha2ards associated with mercury cells result from mercury pump failures heavy-metal impurities, particularly those with very low hydrogen overvoltage, ie. Mo, Cr, W, Ni excessively low pH of feed brine low NaCl concentrations in feed brine and poor decomposer operation, which leads to high sodium amalgam concentrations in the cell. 

Recently it has been shown that the oxides of the platinum metals can have a higher corrosion resistance than the metals themselves , and have sufficient conductivity to be used as coatings for anodes, e.g. with titanium cores. Anodes with a coating of ruthenium dioxide are being developed for use in mercury cells for the electrolysis of brine to produce chlorine , since they are resistant to attack if in contact with the sodium-mercury amalgam. 

In the decomposer, deionized water reacts with the amalgam, which becomes the anode to a short-circuited cathode. The caustic soda produced is stored or evaporated, if higher concentration is required. The hydrogen gas is cooled by refrigeration to remove water vapor and traces of mercury. Some of these techniques are employed in different facilities to maximize the production of chlorine, minimize the consumption of NaCl, and also to prevent the buildup of impurities such as sulfate in the brine.26 The production of pure chlorine gas and pure 50% sodium hydroxide with no need for further concentration of the dilute solution is the advantage that the mercury cell possesses over other cells. However, the cell consumes more energy and requires a very pure brine solution with least metal contaminants and above all requires more concern about mercury releases into the environment.4... 

Toxic pollutants found in the mercury cell wastewater stream include mercury and some heavy metals like chromium and others stated in Table 22.8, some of them are corrosion products of reactions between chlorine and the plant materials of construction. Virtually, most of these pollutants are generally removed by sulfide precipitation followed by settling or filtration. Prior to treatment, sodium hydrosulfide is used to precipitate mercury sulfide, which is removed through filtration process in the wastewater stream. The tail gas scrubber water is often recycled as brine make-up water. Reduction, adsorption on activated carbon, ion exchange, and some chemical treatments are some of the processes employed in the treatment of wastewater in this cell. Sodium salts such as sodium bisulfite, sodium hydrosulfite, sodium sulfide, and sodium borohydride are also employed in the treatment of the wastewater in this cell28 (Figure 22.5). 

There are different ways of producing chlorine from brine, for example, Dow cells, Hooker cells, and mercury cells. Which process is to be used must be known in order to make an accurate economic evaluation, since the capital costs and operating costs are different for each of these processes. The process engineer may have to investigate the different processes and economically evaluate each before deciding which process is best. 

De Nora An electrolytic process for making chlorine and sodium hydroxide solution from brine. The cell has a mercury cathode and graphite anodes. It was developed in the 1950s by the Italian company Oronzio De Nora, Impianti Elettrochimici, Milan, based on work by I. G. Farbenindustrie in Germany during World War II. In 1958 the Monsanto Chemical Company introduced it into the United States in its plant at Anniston, AL. See also Mercury cell. 

An assessment was made as to whether the mercury cells only could be replaced, while re-utilising the existing brine, electrical systems etc., but this was rejected due to cost and the duration of the required shutdown period. 

This chapter concentrates on the mercury technology with waste brine system, as used at Runcorn and in particular a mathematical model of a mercury cell which is being used to improve the operability and efficiency of chlorine production. In general, however, the use of the techniques described here is applicable to all cell technologies. 

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. 

Potassium hydroxide is produced commerically by electrolysis of a saturated solution of potassium chloride in brine using mercury cells consisting of a titanium anode and mercury cathode. Potassium reacts with mercury forming the amalgam which, on treatment with water, forms potassium hydroxide and hydrogen. 

The mercury cell proceeds in two stages that occur separately in two cells. The first is known as the brine cell or the primary electrolyzer in which sodium ion... 

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. 

Figure 19.16. Basic designs of electrolytic cells, (a) Basic type of two-compartment cell used when mixing of anolyte and catholyte is to be minimized the partition may be a porous diaphragm or an ion exchange membrane that allows only selected ions to pass, (b) Mercury cell for brine electrolysis. The released Na dissolves in the Hg and is withdrawn to another zone where it forms salt-free NaOH with water, (c) Monopolar electrical connections each cell is connected separately to the power supply so they are in parallel at low voltage, (d) Bipolar electrical connections 50 or more cells may be series and may require supply at several hundred volts, (e) Bipolar-connected cells for the Monsanto adiponitrile process. Spacings between electrodes and membrane are 0.8-3.2 mm. (f) New type of cell for the Monsanto adiponitrile process, without partitions the stack consists of 50-200 steel plates with 0.0-0.2 ram coating of Cd. Electrolyte velocity of l-2 m/sec sweeps out generated Oz.

In the mercury cell process chlorine is liberated from a brine solution at Ihe anodes which are. today, typically melal anodes (Dimensionally Stable Anodes or DSAl. Collection and processing of the chlorine is similar lo Ihe techniques employed when diaphragm cells are used. However. Ihe cathode is a flowing bed or mercury. When sodium is released by electrolysis it is immediately amalgamated with the mercury The inereury amalgam is then decomposed in a separate cell 10 form sodium hydroxide and Ihe mercury is returned for reuse. 

There are two well-established methods for electrolysing brine, the diaphragm cell and the mercury cell. However, recent developments in electrolysis technology, by chemical engineers, have produced the membrane cell (Figure 5.13). This method is now preferred to the other two because it produces a purer product, it causes less pollution and it is cheaper to run. 

Chlorine is produced almost entirely by the electrolysis of aqueous solutions of alkali metal chlorides (Fig. 1), or from fused chlorides. Brine electrolysis produces chlorine at the anode and hydrogen along with the alkali hydroxide at the cathode. At present, three types dominate the industry the diaphragm cell, the membrane cell, and the mercury cell, and there are many variations of each type. 

The technological process for purifying brine is almost identical with both diaphragm and mercury cell methods of the manufacture of either sodium or potassium hydroxide. Therefore, it is sufficient to describe only the preparation of brine in the manufacture of sodium caustic by the amalgam method. 

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