Chlor-alkali cells mercury

Summary of Raw Waste Loadings Found in Verification Sampling of Unit Product of Chlor-Alkali (Mercury Cell and Diaphragm Cell Processes)... 

FIGURE 10.2.10. Process flowsheet for chlor-alkali mercury cell. (The numbers inside the process equipment refer to the enthalpy change in kWhrton" of chlorine.)... 

TABLE 10.2.2. Mass balance for Chlor-Alkali Mercury Cell Process (All mass units are kg/kg CI2)... 

The older chlor-alkali mercury cell is a cell for the electrolysis of aqueous sodium chloride in which mercury metal is used as the cathode (Figure 20.23). At the mercury cathode, sodium ion is reduced in preference to water. Sodium ion is reduced to sodium to form a liquid sodium—mercury alloy called sodium amalgam. (An amalgam is an alloy of mercury with any of various other metals.)... 

Chlor-alkali mercury cell a cell for the electrolysis of aqueous sodium chloride in which mercury metal is used as the cathode. (20.10)... 

To strip mercuric ion from effluent resulting from the chlor-alkali mercury cell process, the concentration being reduced from 5 to 0.06 mg dm". ... 

The principle of the chlor-alkali membrane cell has been known for a considerable time, and patents for this approach to chlor-alkali manufacture were granted in the early 1960s. Membrane cells combine the purity of mercury cell caustic with the power efficiency of diaphragm cells, while... 

Fig. 12.5. Chlor-alkali electrolysis cells (a) diaphragm cell (b) membrane cell (c) mercury cell. (Courtesy the Dow Chemical Company.y ...

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. 

Removal of brine contaminants accounts for a significant portion of overall chlor—alkali production cost, especially for the membrane process. Moreover, part or all of the depleted brine from mercury and membrane cells must first be dechlorinated to recover the dissolved chlorine and to prevent corrosion during further processing. In a typical membrane plant, HCl is added to Hberate chlorine, then a vacuum is appHed to recover it. A reducing agent such as sodium sulfite is added to remove the final traces because chlorine would adversely react with the ion-exchange resins used later in the process. Dechlorinated brine is then resaturated with soHd salt for further use. 

Electrolytic Preparation of Chlorine and Caustic Soda. The preparation of chlorine [7782-50-5] and caustic soda [1310-73-2] is an important use for mercury metal. Since 1989, chlor—alkali production has been responsible for the largest use for mercury in the United States. In this process, mercury is used as a flowing cathode in an electrolytic cell into which a sodium chloride [7647-14-5] solution (brine) is introduced. This brine is then subjected to an electric current, and the aqueous solution of sodium chloride flows between the anode and the mercury, releasing chlorine gas at the anode. The sodium ions form an amalgam with the mercury cathode. Water is added to the amalgam to remove the sodium [7440-23-5] forming hydrogen [1333-74-0] and sodium hydroxide and relatively pure mercury metal, which is recycled into the cell (see Alkali and chlorine products). 

H. Y. Castner (US/UK) and C. Kellner (Vienna) independently developed commercial mercury-cathode cell for chlor-alkali production... 

The wastewater generated in the membrane cell and other process wastewaters in the cell are generally treated by neutralization.28 Other pollutants similar to those in mercury and diaphragm cells are treated in the same process stated above. Ion exchange and xanthate precipitation methods can be applied in this process to remove the metal pollutants, while incineration can be applied to eliminate some of the hydrocarbons. The use of modified diaphragms that resist corrosion and degradation will help in reducing the amount of lead, asbestos, and chlorinated hydrocarbon in the wastewater stream from the chlor-alkali industry.28... 

FIGU RE 22.5 General wastewater treatment process flow diagram at a mercury cell plant for the production of chlor-alkali. 

The Hg-electrolysis technology is one of the major point sources of Hg contamination, and its impact on the environment has been studied worldwide [23-26]. Although mercury cell chlor-alkali industry is obsolete in most of the European Union countries [27], in Spain it will be allowed until the end of 2010. 

On the other hand, the industrial activity of the other two mercury cell chlor-alkali plants have caused important Hg pollution in Sabinanigo (capacity of 25 x 103 Mg chlorine/year) and Monzon (31 x 103 Mg chlorine/year) (see Fig. 1), two small and industrial cities located in the middle course of the Gallego and Cinca Rivers, respectively, two tributaries of the Ebro River [31]. 

Landis MS, Keeler GJ, Al-Wali KI, Stevens RK (2004) Divalent inorganic reactive gaseous mercury emissions from a mercury cell chlor-alkali plant and its impact on near-field atmospheric dry deposition. Atmos Environ 38 613-622... 

Raldua D, Diez S, Bayona JM, Barcelo D (2007) Mercury levels and liver pathology in feral fish living in the vicinity of amercury cell chlor-alkali factory. Chemosphere 66 1217-1225... 

A substance of main environmental and health concern is mercury. Tons of mercury are released annually form Flix and Monzon mercury cell chlor-alkali plants, and there is evidence that it reaches sediments and animals downstream these plants. In fact, a substantial fraction of fish from the low Ebro River (arriving as far downstream as Xerta) contains too much Hg in their bodies to be considered apt for human consumption [8, 9]. 

Dr M Harris ICI Halo chemicals, PO Box 13, The Heath, Runcorn, Cheshire, WA7 4QP, UK. Phase-out Issues for Mercury Cell Technology in the Chlor-Alkali Industry. 

The development of the membrane cell cut the energy consumption in chlor-alkali production. A good cell will produce a ton of caustic for around 2400 kWh. Membrane caustic can only be produced up to around 35%. Several cell designers have tried to develop a cell and membrane combination that would allow 50% caustic to be made, but this has proved to be commercially elusive so far. Membrane cells have probably reached the theoretical limit on energy consumption for a commercial plant. In Japan, power consumption has been cut by 30% over the last 20 years as the conversion from mercury cell progressed. 

Phase-out Issues for Mercury Cell Technology in the Chlor-Alkali Industry... 

For over a hundred years the chlor-alkali industry has used the mercury cell as one of the three main technologies for the production of chlorine and caustic soda. For historical reasons, this process came to dominate the European industry - while in the United States the asbestos diaphragm cell took the premier position. Over the last two decades developments in membrane cells have brought these to the forefront, and membrane cells of one kind or another now represent the technology of choice worldwide. 

Contracting Parties] RECOMMEND that existing mercury cell chlor-alkali plants be phased out as soon as is practicable. The objective is that they should be phased out completely by 2010. ... 

Lindley, A.A. (1997) An Economic and Environmental Analysis of the Chlor-Alkali Production Process Mercury Cells and Alternative Technologies. Prepared for the European Commission (DG III C-4, Chemicals, Plastics, Rubber). See also OSPAR Document WOCAI 99/5/8 (Madrid, 1999). 

Mercury Cell-based Chlor-alkali Capacities 1998—2020 and Beyond (SRIC Project No. 7659, Final Report). Prepared for Euro Chlor, Brussels. See also OSPAR Document WOCAI 99/5/4 (Madrid,... 

Euro Chlor Member Companies (1999) Voluntary Commitments by each Western European Chlor-Alkali Producer (Mercury Cells). Presented to OSPAR as OSPAR Document WOCAI 99/5/18 (Madrid, 1999). 

This chapter gives an overview of the chlor-alkali industry in Australia and examines the background to the decision to replace the mercury cell plants. It then describes the new plants, their technical and safety features and the process used to arrive at the selection of the technology supplier. 

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