IV. Chemical Identification, Production, and use of Cadmium

Cadmium (Chemical Services Registry Number 7740-43-9) is a soft, blue-white, malleable, lustrous metal or a grayish-white powder. It is insoluble in water and reacts readily with dilute nitric acid. It reacts slowly with hot hydrochloric acid and does not react with alkalies. It is slowly oxidized by moist air to form cadmium oxide. Cadmium occurs in nature in ores, and is obtained as a by-product from the extraction, separation and recovery of those metals in refinery plants.

According to the Mine Safety and Health Administration (MSHA), cadmium ore does not exist since cadmium is a by-product of ores mined for other metals. In fact, cadmium is considered a "contaminant" in mining because it simply is not profitable for industries to mine for it solely. Cadmium is always found with other elements such as zinc, lead, copper or arsenic. An ore must contain at least a trace of a mineral or an aggregate of minerals; however to be considered valuable, it must contain a certain percentage of a desired mineral(s), which is cost-effective and profitable for the mine to process out of the ore. Generally, an ore is named after the minerals it contains which are being recovered. It usually follows that the ore is named in the order of the percentage of the elements present. For example, if an ore is mainly zinc then selenium and finally cadmium, it would be called zinc-selenium-cadmium-ore [personal communication 4/27/90, Mine Safety and Health (MSHA)] Cadmium metal is produced by three basic processes: fractional precipitation and distillation of roasted zinc ores; direct distillation of cadmium-bearing zinc; and, electrolytic zinc processing. Presently, there are four sites where cadmium is processed: ASARCO Incorporated in Denver, Colorado and Corpus Christi, Texas, where cadmium is recovered from lead smelter baghouse dust; Big River Zinc in Sauget, Illinois; Jersey Miniere Zinc Company in Clarksville, Tennessee; and Zinc Corporation of America in Bartlesville, Oklahoma where cadmium is recovered as byproduct of smelting zinc concentrates. ("Mineral Industry Surveys...Cadmium in 1991", Bureau of Mines, Department of Interior, 4/9/92).

A primary use for cadmium metal is as an anticorrosive, electroplated onto steel. Cadmium may serve as an electrode component in alkaline batteries and may be used in alloys, silver solders, and welding. Cadmium exposures in general occur in refining and smelting operations. Relative to the metals with which it is found, cadmium volatilizes readily during these processes because of its low boiling point (765 deg. C) and high vapor pressure. The cadmium then condenses to form fine airborne particles that react almost immediately with oxygen to form respirable cadmium oxide. Other general industry groups where exposure to cadmium may occur include electroplating, battery manufacturing, and pigment and plastics manufacturing. In addition, cadmium exposure is associated with welding, brazing, and painting operations in many other industries.

Cadmium exists in +2 valence state, and does not form stable alkyl compounds or other organometallic compounds of known toxicologic significance. Although all cadmium and cadmium-containing compounds are covered under this standard, OSHA is focusing in this section on those compounds most commonly associated with industrial processes in the United States of America which pose potentially serious health effects: cadmium oxide, cadmium sulfate, cadmium chloride, and cadmium sulfide (Ex. 8-671).

Cadmium oxide (CdO; Chemical Services Registry No. 1306-19-0) occurs as dark brown infusible powder or cubic crystals. It is practically insoluble in water but is soluble in dilute acids. It is also slowly soluble in ammonium salts. It is used as an electroplating chemical; as a component of silver alloys, phosphors, and semiconductors; and, in glass and ceramic glazes. It is also used as a vermicide and is a starting material for polyvinyl chloride (PVC) heat stabilizers. Cadmium oxide is used as a second polarizer in silver-zinc storage batteries and in plastics such as Teflon to improve their high-temperature properties.

Cadmium oxide fumes are formed when cadmium compounds are heated and cadmium ions are driven off. As these ions cool, they condense out of the air as cadmium oxide. Cadmium oxide exposures can occur in a variety of industries where cadmium-containing compounds are heated, such as smelters, refiners, and copper-cadmium alloy manufacturing plants. Workers who perform welding on automobile parts in manufacturing plants, using cadmium containing solders, can be exposed to cadmium oxide fumes.

Cadmium chloride (CdCl(2); Chemical Services Registry No. 10108-64-2) is also widely used in industry. It is hygroscopic and occurs as rhombohedral crystals. It is freely soluble in water and acetone but slightly soluble in methanol and ethanol. It is used: in fungicides, in the manufacture of cadmium yellow pigments, in galvanoplasty, as an ice-nucleating agent, in laboratory analyses of sulfides to absorb H(2)S, as a test for pyridine bases, and in electroplating. It is also used as an addition to tinning solutions, as a mordant in the dyeing and printing of textiles, as a component of metal finishing baths and aerosols, as an agent in photocopying, as an agent in the manufacture of coatings for electronic vacuum tubes, as an agent in the manufacture of special mirrors, as a solid film lubricant, as a catalyst, as a fog inhibitor in photographic film emulsions, as a chemical intermediary for cadmium sulfide, as a colorant for pyrotechnics, and in pesticides. Workers can be exposed to cadmium chloride dusts and aerosols in: battery manufacturing, electroplating, alloy and solder production, ceramics and vapor lamps production, and in welding.

Cadmium sulfate (CdSO(4); Chemical Services Registry No. 10124-36-4) also has many applications in industry. It occurs as odorless monoclinic hydrate. It is freely soluble in water but is almost insoluble in alcohol and ethyl acetate. It is used: in fungicides; as an intermediary in recovery of cadmium from zinc ore; in the electrodeposition of cadmium, copper, and nickel; in analytical tests as a catalyst for determining for arsenic, hydrogen sulfide and fumaric acids; in the manufacture of cadmium salts of long-chain fatty acids; for stabilizing plastics, especially polyvinyl chloride; in the production of vacuum tubes, fluorescent screens, and phosphors; as an electrolyte in standard cells; in CdS, cadmium lithopone and cadmium sulfoselenide pigment production as a chemical intermediary; in cement formation, as a chemical accelerator; and in the manufacture of standard cadmium elements. In addition, cadmium sulfate has the ability to absorb hydrochloric acid from waste gases from chemical plants.

Cadmium sulfide (CdS; Chemical Services Registry No. 1306-23-6) also has many industrial uses. It occurs in nature as the mineral greenokite. It occurs as light-yellow or orange-colored cubic or hexagonal crystals. Its solubility in water is approximately 0.13 mg/100g at 18 deg. C. Some applications include: as a pure, inorganic photoconductor; as a pigment which is colorfast against light in glass; as a colorant for soaps, textiles, paper and rubber: in printing inks, ceramic glazes and fireworks; in x-ray fluorescent screens; in body temperature detectors; in rectifiers, transistors, photovoltaic cells, and in solar cells; in pigments which include phosphors; and, in lead-sealing glass-binders to provide smooth glass enamel surface that is durable and resistant to damage from development, cleaning and handling. Cadmium sulfide also provides stability against oxidation and UV radiation in specific industrial products.

A substantial amount of cadmium sulfide and cadmium sulfoselenide is used in pigments to yield colors ranging from yellow to deep red. These pigments have a high tolerance to heat and to light and are used primarily in coloring plastics, ceramics and paints. Cadmium stearate is used as a stabilizer in plastics because it inhibits the deterioration of the product. Cadmium compounds are also used in smaller amounts in electric batteries and electronic components. Of the many inorganic cadmium compounds, several are quite soluble in water.

[57 FR 42102, Sept. 14, 1992; 58 FR 21778, April 23, 1993]