Cadmium volatilization

Selenium and cadmium. As furnace temperatures increase, selenium and cadmium volatilize and report to the fume (note, the boiling points of selenium and cadmium are 685 and 765°C, respectively). No appreciable quantities of these elements are found in the lead bullion, although cadmium levels can be found in the slag. 

Cadmium is usually found in zinc ores and is extracted from them along with zinc (p. 416) it may be separated from the zinc by distillation (cadmium is more volatile than zinc. Table 14.2) or by electrolytic deposition. 

Because intense heat is generated in these furnaces it is understandable that the arc volatilizes such metals as tin, zinc, lead, cadmium, and the like. In addition, both melting and smelting furnaces may generate large amounts of carbon monoxide. As a result all new furnace installations require pollution... 

Reduction to Gaseous Metal. Volatile metals can be reduced and easily and completely separated from the residue before being condensed to a hquid or a soHd product in a container physically separated from the reduction reactor. Reduction to gaseous metal is possible for 2inc, mercury, cadmium, and the alkah and aLkaline-earth metals, but industrial practice is significant only for 2inc, mercury, magnesium, and calcium. 

Volatilization. In this simplest separation process, the impurity or the base metal is removed as a gas. Lead containing small amounts of zinc is refined by batch vacuum distillation of the zinc. Most of the zinc produced by smelting processes contains lead and cadmium. Cmde zinc is refined by a two-step fractional distillation. In the first column, zinc and cadmium are volatilized from the lead residue, and in the second column cadmium is removed from the zinc (see Zinc and zinc alloys). 

Selective solution of the aluminum from the ahoy using a volatile metal, such as mercury, lead, bismuth, cadmium, magnesium, or zinc, has been investigated. After extracting the aluminum from the original ahoy into the volatile metal, the volatile metal is distilled, leaving pure aluminum. Neither electrolysis nor volatile metal extraction can extract aluminum from iron aluniinide [12004-62-3J, EeAl, titanium aluniinide [12004-78-3] TiAl, or Al C. ... 

Dimethylcadmium has found use as a volatile source of Cd for metal organic chemical vapor deposition (MOCVD) production of cadmium-containing semiconductor thin films (qv) such as CdS, Cdi 2 Hg -Te, or Cdi 2 Mn -Te, as multiple quantum weU species (32). Semiconductor-grade material seUs for... 

The principal constituents of the paniculate matter are lead/zinc and iron oxides, but oxides of metals such as arsenic, antimony, cadmium, copper, and mercury are also present, along with metallic sulfates. Dust from raw materials handling contains metals, mainly in sulfidic form, although chlorides, fluorides, and metals in other chemical forms may be present. Off-gases contain fine dust panicles and volatile impurities such as arsenic, fluorine, and mercury. 

The impurities that occur in the crude zinc produced by the carbothermic reduction of zinc oxide are 2-3% lead, 0.3-0.4% cadmium, and 0.05% iron. Zinc is more volatile than... 

In relation to the separation of cadmium from zinc by volatilization, it is worth noting that the ratio Pcd/P2n increases with decreasing temperature (from 5.09 at 850 °C to 7.30 at 650 °C and 12.69 at 450 °C). The liquid solution of zinc and cadmium exhibits a regular solution behavior and, therefore, the activity coefficient of cadmium dissolved in liquid zinc increases with decreasing temperature. Both these features suggest that the elimination of cadmium from zinc is more efficient at relatively lower temperatures. This is reflected in the choice of the temperatures in the second column. 

Hazardous waste burning incinerators, cement kilns, and LWAKs do not follow a tiered approach to regulate the release of toxic metals into the atmosphere. The MACT rule finalized numerical emission standards for three categories of metals mercury, low-volatile metals (arsenic, beryllium, and chromium), and semivolatile metals (lead and cadmium). Units must meet emission standards for the amount of metals emitted. For example, a new cement kiln must meet an emission limit of 120pg/m3 of mercury, 54pg/m3 of low-volatile metals, and 180 pg/m3 of semivolatile metals. 

The rule proposes emission standards for dioxins, furans, mercury, cadmium, lead, PM, hydrochloric acid, chlorine, carbon monoxide, hydrocarbons, and several low-volatile metals. It also proposes a new comparable fuels exclusion and makes significant changes to the existing combustion regulations. 

To increase precursor volatility, interest focused on using ligands of considerable bulk to reduce the degree of molecular association. This was of limited success, with mercury proving to be an exception. Bradley and Kunchur218 reported that Hg(SBu )2 has only weak intermolecular interactions and occurs as discrete Hg(SR)2 units even in the solid state. Other attempts to prepare complexes of zinc or cadmium with limited degrees of polymerization have been undertaken. 

For even better volatility the fluorinated aryl substituent C6H2(CF3)3 has been employed, as in the cadmium thiolate [Cd S(C6H2(CF3)3) 2] which sublimes at 160 °C under vacuum. When used in deposition the hexagonal phase of CdS was predominant with growth temperatures in the range 425 175 °C, with no fluorine incorporation in the CdS films being observed.232... 

International Agency for Research on Cancer (IARC). 1976. IARC monographs on the evaluation of the carcinogenic risk of chemicals to man cadmium, nickel, some epoxides, miscellaneous industrial chemicals and general considerations on volatile anaesthetics. Volume 11. World Health Organization, IARC, Lyon, France. 306 pp. 

Tetronics A process for treating dusts from electrical arc furnaces for making steel and nonferrous metals. Volatile metals (zinc, lead, cadmium) are recovered, and residual slag is nontoxic and suitable for landfill. The dusts, mixed with coal dust and a flux, are fed to a furnace heated by a plasma gun. The metal oxides present are selectively reduced and the vapors of zinc, lead, and cadmium are condensed in a modified Imperial Smelting furnace. Developed by Tetronics Research Development Company, United Kingdom, and first commercialized for steel dusts at Florida Steel, Jackson, TN, in 1989. Seven plants were operating in several countries in 1992. 

A number of workers have described methods for the determination of mercury in which the mercury is first reduced to the element or collected as the sulfide on a cadmium sulfide pad. It is then volatilized into a chamber for measurement. These techniques are extremely sensitive. Thillez108) recently described a procedure for urinary mercury in which the mercury is collected on platinum and then volatilized into an air stream. Rathje109) treated 2 ml of urine with 5 ml of nitric acid for 3 min, diluted to 50 ml, and added stannuous chloride to reduce the mercury to the element. A drop of Antifoam 60 was added and nitrogen was blown through the solution to carry the mercury vapor into a quartz end cell where it is measured. Six nanograms of mercury can be detected. Willis 93) employed more conventional methods to determine 0.04 ppm of mercury in urine by extracting it with APDC into methyl-n-amyl ketone. Berman n°) extracted mercury with APDC into MIBK to determine 0.01 ppm. 

Drying, usually by physical methods, is one of the most common unit operations in both laboratory or industrial scale process chemistry, and since heating is usually employed to remove volatiles, thermally unstable materials may decompose if overheated. As a light-hearted example, when a faulty oven thermostat led to overheating of mercuric thiocyanate, a monstrous Pharaoh s serpent resulted. Drying moist cadmium propionate in an electric oven led to explosive ignition of the diethyl ketone vapour produced as an unforeseen by-product. Drying 3,5-dinitro-2-toluamide had more serious consequences. 

On a worldwide basis, toxic concentrations of the heavy metals have thus far been limited to industrialized harbors. The only metals that appear to have accumulated to toxic levels on a regional scale are mercury, cadmium, and lead in the Arctic Ocean. This concentration of mercury and lead has been fecilitated by a natural process, called the grasshopper effect, which acts to transport volatile compoimds poleward. This transport plays a major role in redistributing the volatile organic pollutants, such as the PCBs, and, hence, is discussed at further length in Chapter 26.7. The process responsible for the cadmium enrichment in the Arctic appears to involve low-altitude transport of the fine particles that compose Arctic haze. 

---