Mercury chlor-alkali process

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

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). 

Castner, Hamilton Young — (Sep. 11, 1858, Brooklyn, New York, USA - Oct. 11,1899, Saranac Lake, New York, USA) Castner studied at the Brooklyn Polytechnic Institute and at the School of Mines of Columbia University. He started as an analytical chemist, however, later he devoted himself to the design and the improvement of industrial chemical processes. He worked on the production of charcoal, and it led him to investigate the Devilles aluminum process. He discovered an efficient way to produce sodium in 1886 which made also the production of aluminum much cheaper. He could make aluminum on a substantial industrial scale at the Oldbury plant of The Aluminium Company Limited founded in England. However, - Hall and - Heroult invented their electrochemical process which could manufacture aluminum at an even lower price, and the chemical process became obsolete. Castner also started to use electricity, which became available and cheap after the invention of the dynamo by - Siemens in 1866, and elaborated the - chlor-alkali electrolysis process by using a mercury cathode. Since Karl Kellner (1851-1905) also patented an almost identical procedure, the process became known as Castner-Kellner process. Cast-... 

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)... 

It is essential for all technical chlor-alkali electrolysis processes - as subsequently discussed in Section 6.19.2.2 - that the transport of hydroxide ions formed at the cathode into the anode compartment is excluded (membrane process) or at least largely suppressed (diaphragm process). In the mercury cell process, OH ions are not formed in the entire process. 

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". ... 

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). 

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. 

Finally, it should be mentioned that three out of the eight Spanish chlor-alkali plants operating with the mercury process are located in the Ebro River basin in the cities of Sabinanigo and Monzon - along the tributaries Gallego and Cinca Rivers, respectively - and Flix along the Ebro River (Fig. 1). Indeed, mercury emissions from the Hix and Monzon have already been reported [28]. Therefore, the mid-low Ebro River watershed might be considered as a hot spot of aquatic pollution of mercury in Spain. 

A second risk zone corresponds to Monzon (17,042 inhabitants), a highly industrialized city in the middle Cinca River. Its industrial activity has caused the historical release of organic and inorganic compounds to the river coming from chlor-alkali industry, from production and utilization of solvents and organochlo-rine pesticides, and from the use of brominated flame retardants in the production processes. Very high concentrations of mercury have been recorded for sediment and fish samples in the downstream of Monzon [2-4]. 

SAMEX A process for removing traces of mercury from the waste brine from the chlor-alkali process. 

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. 

It is for this reason that the Chief Executive Officers of every West European chlor-alkali producer have, through Euro Chlor, entered voluntarily into six binding commitments [15]. All but one of these are unconditional. In this sense they do not represent part of some kind of negotiation rather, they are an attempt to demonstrate by action that the industry, whatever its past, is now totally committed to addressing public concerns about its use of the mercury process. It matters not that these concerns are almost entirely without foundation they are there, and we have to address them. 

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. 

Chlorine (from the Greek chloros for yellow-green ) is the most abundant halogen (0.19 w% of the earth s crust) and plays a key role in chemical processes. The chlor-alkali industry has been in operation since the 1890s and improvements in the technology are still important and noticeable, for example, the transition from the mercury-based technology to membrane cells [60]. Most chlorine produced today is used for the manufacture of polyvinyl chloride, chloroprene, chlorinated hydrocarbons, propylene oxide, in the pulp and paper industry, in water treatment, and in disinfection processes [61]. A summary of typical redox states of chlorine, standard potentials for acidic aqueous media, and applications is given in Scheme 2. 

Approximately two-thirds of atmospheric mercury comes from human activities, including coal burning, waste incineration, and Cl2 production by the chlor-alkali process (Problem 17-7). 

Mercury is directly below cadmium in the periodic table, but has a considerably more varied and interesting chemistry than cadmium or zinc. Elemental mercury is the only metal that is a liquid at room temperature, and its relatively high vapor pressure contributes to its toxicological hazard. Mercury metal is used in electric discharge tubes (mercury lamps), gauges, pressure-sensing devices, vacuum pumps, valves, and seals. It was formerly widely used as a cathode in the chlor-alkali process for the manufacture of NaOH and Cl2, a process that has been largely discontinued, in part because of the mercury pollution that resulted from it. 

Another interesting application of the photocatalytic technology is the removal (more than 99%) of mercury from hazardous wastes of a chlor-alkali plant. The process begins by acid attack of the solid wastes and treatment of the acid solution under UV irradiation in the presence of Ti02 and citric acid. The selective precipitation of reduced mercury took place, while the other metal compounds remained in the solution. It was claimed that the final effluents reached a quality close to that of the standards imposed by international environmental agencies (Bussi et al., 2002). 

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