Cadmium plants

Some of the melted ziac is fed to the ziac-dust unit where the molten ziac may be dropped from a cmcible through a small orifice (2.5 mm) to be atomized ia a blast of air. SoHdifted droplets are collected ia a chamber and screened to the proper size for purification and cadmium plant cementation. Frequently, coarse (+70 200 and fine (—70 fiva) fractions are required. 

Smilde, K.W., Luit, B. van and Driel, W. van (1992) The extraction by soil and absorption by plants of applied zinc and cadmium. Plant Soil, 143, 233-238. 

Wang, J., V. P. Evangelou, M. T. Nielsen, and G. J. Wagner. 1991. Computer-simulated evaluation of possible mechanisms for quenching heavy metal ion activity in plant vacuoles. I. Cadmium. Plant Phys. 97 1154—1160. 

Keltjens, W.G. and Van Beusichem, M.L. (1998) Phytochelatins as biomarkers for heavy metal stress in maize (Zea mays L) and wheat (Triticum aestivum L) combined effects of copper and cadmium. Plant and Soil, 203, 119-126. 

Tukendore a and Rauser WE (1990) Changes in f utathione and phytochelatins in roots of maize seedling exposed to cadmium. Plant Sd 70 155-166. 

Deknudt GH and Leonard A (1975) Cytogenetic investigations on leucocytes of workers from a cadmium plant. Environ Physiol Biochem 5 319-327. 

Impure SLP electrolyte is treated using cold-hot-polish zinc dust purification. The cold stage cement is treated in the cadmium plant to produce cadmium metal and a copper cake that is fed to the smelter. Hot stage cement is leached to minimize the loss of zinc and is then directed to the smelter. 

The zinc dust piuification circuit in the oxide leaching plant was originally conunissioned as a purification system for an old oxide cellhouse, decommissioned about 20 years ago when the oxide and calcine leaching plant electrolytes were combined. However, the oxide purification circuit continued to operate with about one gram per liter addition of dry zinc dust, principally to cement cadmium in the oxide electrolyte prior to entering the calcine leaching plant. A portion of the purification residue was then forwarded directly to the cadmium plant. Some arsenic was also removed fix>m solution. 

There were several problems with the oxide electrolyte purification circuit. The arsenic concentration in the zinc fiime fi-om the smelter had increased since the start-up of KIVCET, resulting in an increased tendency for arsenic breakthrough to the oxide electrolyte. This, in turn, had led to an increased potential for arsine generation during zinc dust purification as well as in the storage tank for purification residues in the cadmium plant. In addition, the oxide purification equipment was in very poor working condition. 

I. Almaguer, Cadmium Plant s Project Book. Met-Mex Penoles, Torreon, Mexico, 1995. 

Cadmium Plant Direct injection ETAAS 0.1-1.Opgr Online ultrasound-assisted digestion or acidified subcritical water (semiautomatic procedure)... 

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 New Jersey Zinc Company patented a fluidized-peUet roaster which was instaUed in several zinc plants. CaUed a fluid-column roaster, it resembles a shaft furnace and can handle 370 t of concentrate per day. This roaster can be operated at 1080—1100°C to eliminate 90% of the cadmium and 92% of the lead. The fluid-column roaster has the same advantages as the MHO roaster the pelletizing cost is a disadvantage for both systems. 

Aeration must be avoided since it can oxidize and resolubiUze the cemented (precipitated) impurities. Filter presses are used after each step and the cakes are leached to recover various values. For example, cadmium is dissolved, recemented with zinc, and recovered on site either electrolyticaHy or by distillation. A copper residue of 25—60% copper is sold for recovery elsewhere. The other impurities cannot be recovered economically with the exception of cobalt in some plants. 

The Palmerton, Pennsylvania, plant had 43 retorts with an output of ca 8 t/d per retort. Recovery was ca 94% when the plant was shut down in 1980. The zinc contained ca 0.3% lead, 0.10% cadmium, and 0.01% iron plus minor impurities. Lead and aluminum are added to produce galvanizer s zinc. 

Analysis of zinc solutions at the purification stage before electrolysis is critical and several metals present in low concentrations are monitored carefully. Methods vary from plant to plant but are highly specific and usually capable of detecting 0.1 ppm or less. Colorimetric process-control methods are used for cobalt, antimony, and germanium, turbidimetric methods for cadmium and copper. Alternatively, cadmium, cobalt, and copper are determined polarographicaHy, arsenic and antimony by a modified Gutzeit test, and nickel with a dimethylglyoxime spot test. 

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. 

Air pollution problems and labor costs have led to the closing of older pyrometaHurgical plants, and to increased electrolytic production. On a worldwide basis, 77% of total 2inc production in 1985 was by the electrolytic process (4). In electrolytic 2inc plants, the calcined material is dissolved in aqueous sulfuric acid, usually spent electrolyte from the electrolytic cells. Residual soHds are generally separated from the leach solution by decantation and the clarified solution is then treated with 2inc dust to remove cadmium and other impurities. 

Some electrochemicals are produced in very large quantity. Chlorine and sodium hydroxide production in 1991 were 10,727,000 t and 11,091,000 t, respectively (1). Aluminum was produced at the rate of 4,100,000 t/yr and had an annual market value of about 5.4 biUion. Other electrochemically produced products are required in smaller volume. The production of the metals cadmium, lithium, and nickel were at the rates of 1600 t, 2800 t, and 8400 t, respectively for 1991 (see Table 1). Electrochemical processing plants produce a variety of products in a wide range of capacities. 

Silver-brazed joints are used when temperature or the combination of temperature and pressure is beyond the range of soldered joints. They are also more reliable in the event of plant fires and are more resistant to vibration. If they are used for fluids that are flammable, toxic, or damaging to human tissue, appropriate safeguarding is required by the code. There are OSHA regulations governing the use of silver brazing alloys containing cadmium and other toxic materials. 

Solutions in contact with polyvinyl chloride can become contaminated with trace amounts of lead, titanium, tin, zinc, iron, magnesium or cadmium from additives used in the manufacture and moulding of PVC. V-Phenyl-2-naphthylamine is a contaminant of solvents and biological materials that have been in contact with black rubber or neoprene (in which it is used as an antioxidant). Although it was only an artefact of the separation procedure it has been isolated as an apparent component of vitamin K preparations, extracts of plant lipids, algae, livers, butter, eye tissue and kidney tissue [Brown Chem Br 3 524 1967]. 

Fluorides and dust are emitted to the air from the fertilizer plant. All aspects of phosphate rock processing and finished product handling generate dust, from grinders and pulverizers, pneumatic conveyors, and screens. The mixer/reactors and dens produce fumes that contain silicon tetrafluoride and hydrogen fluoride. A sulfuric acid plant has two principal air emissions sulfur dioxide and acid mist. If pyrite ore is roasted, there will also be particulates in air emissions that may contain heavy metals such as cadmium, mercury, and lead. 

Precipitation is often applied to the removal of most metals from wastewater including zinc, cadmium, chromium, copper, fluoride, lead, manganese, and mercury. Also, certain anionic species can be removed by precipitation, such as phosphate, sulfate, and fluoride. Note that in some cases, organic compounds may form organometallic complexes with metals, which could inhibit precipitation. Cyanide and other ions in the wastewater may also complex with metals, making treatment by precipitation less efficient. A cutaway view of a rapid sand filter that is most often used in a municipal treatment plant is illustrated in Figure 4. The design features of this filter have been relied upon for more than 60 years in municipal applications. 

In the profiles of the core from the industrial area, mercury displays the highest accumulation. Mercury in this area, close to the industrial district, has probably derived from a large chloralkali plant which has employed mercury cathodes since the fifties. Whereas, at present, very severe measures are taken to prevent mercury spills into the Lagoon, in the past, polluted waters and solid materials were discharged almost untreated. In the most superficial strata a marked decrease in the accumulations is, in fact, recorded. Lead and Cd accumulations are lower here by a factor of 5-10. The presence of cadmium in the sediments of the Lagoon has been referred to sphalerite (ZnS) processing on the basis of a strict concomitant... 

Nickel is required by plants when urea is the source of nitrogen (Price and Morel, 1991). Bicarbonate uptake by cells may be limited by Zn as HCOT transport involves the zinc metal-loenzyme carbonic anhydrase (Morel et al., 1994). Cadmium is not known to be required by organisms but because it can substitute for Zn in some metalloenzymes it can promote the growth of Zn-limited phytoplankton (Price and Morel, 1990). Cobalt can also substitute for Zn but less efficiently than Cd. 

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