Institute of Gas Technology, Consolidation Coal Co., and M. W. Kellogg, all in cooperation with the Office of Coal Research. Coatings. A rapidly growing mass application of fluidized bed technology is the coating of objects with organic and inorganic films. When the fluidized particles themselves are to be coated, the coating materials are contained in the fluidizing medium. The mechanism is similar to adsorption. For large objects the coating material is the particulate phase itself. The object to be coated is heated, usually electrically, as it is immersed in the bed. Through a complicated mechanism of sintering, the coating material bonds to the surface of the object, and presumably flows on the surface to form an integral coating. A variation of this process is continuous coating of wire which passes through a bed and is heated in situ. Among more novel new applications of fluidized bed processing is the particulate electrode. The latest example is the fluidized electrode exhibited by Constructors John Brown, Ltd., at the ACHEMA exposition in Frankfurt. The fluidized electrode was developed in response to a longstanding need for increasing current density of electrochemical cells without greatly increasing electrode size. The fluidized electrode is the bed of metallic or metal-coated particles. Particulate surface area is much greater than the surface area of conventional electrodes in the same size cell. Thus far, the CJB studies have included reduction of nitroorganics to amines, reduction of sodium bisulfite, and electrorefining of metals. Heater. A related application of fluidized electrodes is being studied at Iowa State University. A research group is producing synthesis gas from coal char with steam as the fluidizing medium. The energy for the process is supplied by electrical heating of the fluidized bed. In effect, the bed functions as a resistance heater. Though results to date have been encouraging, the investigators believe that electrical properties of the fluid bed are still too little understood to venture into large pilot plants. Among other established processes involving fluidized beds, there is a range of applications represented by the major design and construction companies. Dorr-Oliver, for example, currently has more than 450 installations around the world processing everything from iron ore to municipal wastes. The growing number of new applications, when appended to the established roster of successes, leaves little doubt that the fluidized bed will contribute more to the chemical industry in the future than it has in the past. 48 C&EN DEC. 14, 1970
Process removes mercury in plant wastes The criterion for mercury concentration in chemical plant wastes is now "nil." Biological magnification of the toxic metal by aquatic life has brought this fact home to the operators of a host of chemical plants using mercury in their processes. Mercury has a relatively high vapor pressure for a metal and is soluble in water in the parts-per-million range. The challenge facing mercury-using plants is now to remove the last traces of the metal from effluents. A first-generation approach was to use sulfides to precipitate mercury. Although this approach has been effective, Ventron Corp. says it has developed a system based on sodium borohydride reduction that is faster and more efficient than present techniques (C&EN, Nov. 30, page 39). Ventron is willing to make its knowledge in this field available to all. Optimistic. Ventron will start u p a system at its Wood-Ridge, N.J., mercury chemicals plant at the end of next month that the firm hopes will reduce mercury content in its plant effluent to nondetectable levels—levels well below 1 part per billion. While the process has not been proved out in the rigors of plant conditions, the simplicity of the process and known ability of sodium borohydride to reduce mercurous and mercuric ions to elemental mercury make Ventron optimistic. Moreover, the company says, the technique might be readily applied to treating mercuiy-containing streams from mercury-cell chlorine-caustic plants, and plants using the mercurycatalyzed sulfonation route to anthraquinone production. Ventron's Wood-Ridge plant is the nation's largest producer of mercury
chemicals. The system being installed is designed to recover u p to 100 p.p.m. of mercury from about 15,000 gallons of plant effluent generated in producing such chemicals as mercuric nitrate, chloride, oxides, sulfate, and organics such as phenyl mercurials. Mercury and its compounds find use in electrical and electronic hardware, batteries, catalysts, and biocides. Despite the fact that sodium borohydride costs $13 a pound, a pound of the reagent reduces up to 21 pounds of mercury to its elemental state. At today's prices, mercury is worth about $5.00 a pound, and Ventron sources estimate that the cost of recovery in its system will run less than $1.00 per pound of recovered mercury. Sodium borohydride is also faster and more efficient than alternative routes, Ventron says. To effect complete reduction, organic mercury compounds must be converted to salts, such as chlorides. Sodium borohydride requires use of a basic medium. Organics converted. At WoodRidge, Ventron proposes to adjust the pH of its meroiry-oontaining overflow to 7 to 9, convert organics to their inorganic salts, and meter NaBH 4 and caustic into the overflow. Reduction to mercury occurs in a static in-line mixer. A cyclone removes 80 to 90% of the precipitated mercury (particle size about 10 microns). The cyclone's mercury sludge is vacuum-distilled, while its aqueous phase goes to a knockout pot, and from there to a 5micron filter—where remaining mercury is collected. Any hydrogen evolved may be saturated with mercury vapor and is sent to a knockout pot and nitric acid scrubber.
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