Zinc-cadmium sulfide, atmospheric

OPERATION LARGE AREA COVERAGE (LAC). A U.S. project to estimate the path, spread, and duration of biological weapons (BW) when released in the atmosphere. Operation LAC, carried out in 1957-1958, involved the release of fluorescent zinc cadmium sulfide particles as a simulant for BW. The tests were carried out over the central part of the United States, and results showed that the particles traveled distances in excess of 1,000 miles, effectively covering large segments of the United States. This demonstrated the vulnerability of the United States, or any nation, to an atmospheric release of BW. 

Hazards of Production. In most zinc mines, zinc is present as the sulfide and coexists with other minerals, especiaHy lead, copper, and cadmium. Therefore, the escape of zinc from mines and mills is accompanied by these other often more toxic materials. Mining and concentrating, usuaHy by flotations, does not present any unusual hazards to personnel. Atmospheric poHution is of Httle consequence at mine sites, but considerable effort is required to flocculate and settle fine ore particles, which would find their way into receiving waters. 

Zinc (76ppm of the earth s crust) is about as abundant as rubidium (78 ppm) and slightly more abundant than copper (68 ppm). Cadmium (0.16 ppm) is similar to antimony (0.2 ppm) it is twice as abundant as mercury (0.08 ppm), which is itself as abundant as silver (0.08 ppm) and close to selenium (0.05 ppm). These elements are chalcophiles (p. 648) and so, in the reducing atmosphere prevailing when the earth s crust solidified, they separated out in the sulfide phase, and their most important ores are therefore sulfides. Subsequently, as rocks were weathered, zinc was leached out to be precipitated as carbonate, silicate or phosphate. 

The Sulfide Ores (e.g., sulfides of iron, nickel, zinc, copper, lead, cobalt, cadmium, mercury, silver).— These must be studied both in their dry melts (to obtain their fundamental characteristics) and in relation to water solutions under atmospheric pressure (problems of oxidation and... 

Cadmium pigments have been manufactured by both a direct calcination process and a precipitation-calcination process. In the first instance, a mixture of cadmium carbonate and sulfur (and zinc oxide and selenium if the hue to be produced requires their addition) is calcined at 520-600°C for 1-2 h. This direct calcination process is complicated by the volatility of cadmium oxide and selenium, both of which are toxic and require special handling. In the precipitation process, an alkali sulfide solution is added to a solution of cadmium and (in the case of green-shade yellows) zinc salts or to a solution of cadmium and (in the case of deep oranges, reds, and maroon) selenium metal to precipitate the appropriate compound. The precipitate is washed, dried, and calcined at 600-700°C in an inert or reducing atmosphere to convert the precipitated cubic structure to a more stable wurtzite crystal. The calcination conditions control particle size, which ranges from 0.2 to 1.0pm. 

Estimates of the abundance of zinc in the sun, in meteorites, in the Earth s core and crust, and in the oceans are very difficult to make, but its abundance in the Earth s crustal rocks and soils is of the order of 100 ppm, about 1000 times as abundant as its congeners cadmium and mercury. All three elements are Chalcophiles so that, in the reducing atmosphere that prevailed when the earth s crust solidified, they were deposited in the sulfide phase giving rise to the sulfide ores, their most important source. Eater, as weathering took place, zinc became soluble only to be precipitated as the carbonate, silicate, or phosphate. 

Iron and manganese occur in a number of soil minerals. Sodium and chlorine (as chloride) occur naturally in soil and are transported as atmospheric particulate matter from marine sprays (see Chapter 10). Some of the other micronutrients and trace elements are found in primary (unweathered) minerals that occur in soil. Boron is substituted isomorphically for Si in some micas and is present in tourmaline, a mineral with the formula NaMg3AlgB3Sig027(0H,F)4. Copper is isomorphically substituted for other elements in feldspars, amphiboles, olivines, p5Toxenes, and micas it also occurs as trace levels of copper sulfides in silicate minerals. Molybdenum occurs as molybdenite (M0S2). Vanadium is isomorphically substituted for Fe or A1 in oxides, pyroxenes, amphiboles, and micas. Zinc is present as the result of isomorphic substitution for Mg, Fe, and Mn in oxides, amphiboles, olivines, and pyroxenes and as trace zinc sulfide in silicates. Other trace elements that occur as specific minerals, sulfide inclusions, or by isomorphic substitution for other elements in minerals are chromium, cobalt, arsenic, selenium, nickel, lead, and cadmium. 

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