Plants mercury

The results from the operation show that SRS is able to operate successfully in a mercury plant, which has brine containing greater impurities than expected from brine in a membrane cell plant. 

Of the chlorine production capacity installed in Germany, which totalled 4.4 million tonnes in 2003, 50% were from the membrane cell process, 27% from the mercury cell process and 23% from the diaphragm cell process. The mercury cell process has been the subject of environmental policy criticism for years because of its use of mercury cathodes and resulting pollutant emissions. Hence, no new mercury plants will be... 

Generally, the brine is free of mercury in new membrane-cell installations. However, if a mercury plant is converted to membrane technology, it is very likely that the brine will be contaminated with mercury. Laboratory tests have shown that the concentration of mercury in the catholyte will be about 1,000-fold lower than the concentration in the anolyte, and it is usually considered that lOppb Hg in the catholyte has no short-term effect on voltage. 

Two disadvantages of this approach are the constraint on membrane-cell capacity and the fact that all the caustic can be regarded as the less marketable mercury-plant product. A larger membrane capacity can of course be installed to eliminate the first disadvantage and alleviate the second, but only at the expense of installing and operating an evaporator. 

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. 

The choice of technology, the associated capital, and operating costs for a chlor—alkaU plant are strongly dependent on local factors. Especially important are local energy and transportation costs, as are environmental constraints. The primary difference ia operating costs between diaphragm, mercury, and membrane cell plants results from variations ia electricity requirements for the three processes (Table 25) so that local energy and steam costs are most important. 

Vinyl acetate (ethenyl acetate) is produced in the vapor-phase reaction at 180—200°C of acetylene and acetic acid over a cadmium, 2inc, or mercury acetate catalyst. However, the palladium-cataly2ed reaction of ethylene and acetic acid has displaced most of the commercial acetylene-based units (see Acetylene-DERIVED chemicals Vinyl polymers). Current production is dependent on the use of low cost by-product acetylene from ethylene plants or from low cost hydrocarbon feeds. 

The high temperatures in the MHD combustion system mean that no complex organic compounds should be present in the combustion products. Gas chromatograph/mass spectrometer analysis of radiant furnace slag and ESP/baghouse composite, down to the part per biUion level, confirms this behef (53). With respect to inorganic priority pollutants, except for mercury, concentrations in MHD-derived fly-ash are expected to be lower than from conventional coal-fired plants. More complete discussion of this topic can be found in References 53 and 63. 

This secondary reaction starts at about 180°C, but the mass must be heated to 350—400°C to bring the reaction to completion and produce a nitrate-free product. The off-gases are extremely corrosive and poisonous, and considerable attention and expense is required for equipment maintenance and caustic-wash absorption towers. Treatment of the alkaline wash Hquor for removal of mercury is required both for economic reasons and to comply with governmental regulations pertaining to mercury ia plant effluents. 

Mercury spills should be cleaned up immediately by use of a special vacuum cleaner. The area should then be washed with a dilute calcium sulfide solution. Small quantities of mercury can be picked up by mixing with copper metal granules or powder, or with zinc granules or powder. To avoid or minimize spills, some plants use steel trays as pallets so that a spih, whether of mercury or a mercury compound, is contained on the steel tray. 

Problems of removal of mercury from aqueous effluents are more comphcated in plants that manufacture a variety of inorganic and organic mercury compounds it is generally best to separate the effluent streams of inorganic and organic mercurials. When phenyhnercuric acetate is precipitated from its solution in acetic acid by addition of water, the filtrate is collected and reused for the next precipitation. This type of recycling is necessary not only for economic reasons but also to minimise recovery operations. 

The primary environmental concern for the coating plant is actually the residual material on the anode stmctures being returned for recoating. Therefore the anode user must enact effective cleaning procedures prior to shipment. For example, anodes in chlorine use must be cleaned of all traces of mercury and asbestos (qv). Anodes used in electrogalvanizing or in copper-foil production must similarly be cleaned to remove all traces of process materials. If cleaning at the user s plant is not done effectively, the anode may well be shipped back to the user for appropriate action before it is considered for recoating. 

Environmental awareness is a prime concern in all KOH plants. Safety precautions required in KOH and chlorine operations are well documented in operating manuals and sales brochures pubUshed by all commercial producers. Discharges of waste effluents containing mercury are strictly forbidden. 

Cadmium and mercury are usually recovered ia separate processes at the ziac plant. The others are shipped as enriched residues to plants that specialize ia their recovery. 

Caustic soda concentrations of 50% are produced directly from equation 11. This advantage is offset by higher operating cell voltages and some mercury contamination of the environment. This latter problem has been diminished or solved to an acceptable extent (31) however, it continues to influence the choice of cells for new plants. No new mercury cells have been installed in the United States since 1970 (32). 

Air pollutants that present a hazard to livestock, therefore, are those that are taken up by vegetation or deposited on the plants. Only a few pollutants have been observed to cause harm to animals. These include arsenic, fluorides, lead, mercury, and molybdenum. 

---