Discovered by Stromeyer in 1817 from an impurity of zinc carbonate (ZnCO3). Cadmium is a soft ductile metal with a bluish-silvery white crystalline physical appearance. It is easily cut with a knife. Small additions of zinc make it very brittle. It resembles tin and is similar in respects to zinc.
Cadmium (Cd) is often associated in small quantities (0.1–1%) with zinc ores such as sphalerite (ZnS), however the only commercially viable ore of the metal is greenockite (CdS), which theoretically contains 77.7% cadmium. Greenockite occurs in a yellow powdery form in the zinc ores of Missouri. Almost all cadmium is obtained as a by-product in the treatment of zinc, copper and lead ores.
Electrolytic cadmium is 99.95% pure and is obtained primarily as a by-product of the zinc industry by treating the flue dust and fumes from the roasting of the ores. As an example flue dust on average can contain approximately 600kg of cadmium per metric ton of dust. Half the world’s production of cadmium is carried out in the United States.
Cadmium is a known carcinogen. Cadmium and solutions of its compounds are extremely toxic, caution is required not to create dust or fumes.
Cadmium has many industrial applications. Some of these are described below:
- Cadmium is implemented as an alloying element in soft solders and in fusible alloys.
- Adding small quantities of cadmium to copper gives higher strength, improved hardness and wear resistance, however decreases the electrical conductivity. Copper containing 0.5 to 1.2 % cadmium is commonly referred to as cadmium copper or cadmium bronze. Hitenso is a commercially available cadmium bronze and exhibits 35% greater strength than hard-drawn copper and 85% the conductivity of copper. Conductivity bronze another commercially available copper used extensively for electric wires, contains 0.8% cadmium and 0.6% tin and exhibits a conductivity 50% that of copper.
- It is employed in bearing alloys with low coefficients of friction
- Cadmium is used as a white corrosion-resistant plating metal (as a anode).
- It is used as a shield against neutrons in atomic equipment. However, gamma rays are emitted when the neutrons are absorbed, and these rays require an additional shielding of lead.
- Electronic applications requiring high corrosion resistance.
- Electroplating for the corrosion resistance coating for iron or steel, where a 0.008mm (0.0003in) is equivalent in effect of a zinc coating 0.025mm (0.001in). Cadmium electroplated in this application is denser than zinc and harder than tin. However electroplated coatings are subject to hydrogen embrittlement. For this reason aircraft parts are generally coated by the vacuum process. N.B.: copper and brass are not generally electroplated due to its relative electronegativity.
Cadmium compounds also find a number of commercial uses. These include:
- Cadmium nitrate Cd(NO3)2 (a white powder) is used for making cadmium yellow and fluorescent pigments. It can also be used as a catalyst.
- Cadmium compounds are used in back and white television phosphors and in the blue and green phosphors for colour television tubes.
- Cadmium sulphide (CdS) is used for yellow pigment and when mixed with cadmium selenide (CdSe) (a red powder) gives a bright-orange pigment.
- In the semiconductor industry, sulphate in conjunction with cadmium is used to grow cadmium sulphate crystals in plate and rod form.