What Is Cadmium?
Cadmium, a metallic element with element number 48, is widely found in the natural environment, including in minerals and soil.
Cadmium is distributed in the earth’s crust and is released into the environment through natural phenomena such as weathering of rocks, so naturally occurring cadmium is found in soil and water. Chemically similar to zinc, cadmium occurs not alone but with zinc minerals, especially sphalerite (ZnS) and rhyozincite (ZnCO3). Various foods and water contain trace amounts of cadmium, as it is taken up from the soil and water during plant and animal growth.
Consumption of foods with high levels of cadmium for many years can cause kidney dysfunction.
Uses of Cadmium
Cadmium has a silvery-white luster, is highly ductile, and is easily processed. Its uses include alloying materials as a metal, electroplating of iron and other metals, electrode plates of storage batteries, control rods of nuclear reactors, solder, silver solder, and cathode ray tubes of televisions. As a compound, cadmium yellow is used as a raw material for pigments, for example, in Van Gogh’s “Sunflowers.” Other uses include catalysts, batteries, stabilizers for vinyl chloride resin, and colorants for ceramics.
Properties of Cadmium
Cadmium has an atomic weight of 112.411, a melting point of 321 °C, and a boiling point of 765 °C. It has a specific gravity of 8.65 at 25 ºC and is insoluble in water. It is said to be contained in unpolluted river water at 0.02 to 0.1 μg/L and in seawater at 0.05 to 0.11 μg/L. The crystal structure of Cadmium is hexagonal.
Other information on Cadmium
1. Compounds of Cadmium
Cadmium compounds include the following.
Cadmium oxide (CdO)
It is a brown solid with a molecular weight of 128.41, a specific gravity of 8.15, and a CAS number of 1306-19-0. Applications include cadmium plating bath additives, pigments, catalysts, and alkaline batteries.
Cadmium chloride (CdCl2)
It is a solid with a molecular weight of 183.32, a melting point of 568 °C, and a CAS number of 10108-64-2. Applications include plating and catalysts.
Cadmium Sulfate (CdSO4)
It is a white solid with a molecular weight of 209.47, a specific gravity of 4.691, a melting point of 1000 ℃, and a CAS number of 10124-36-4. Applications include analytical reagents and cadmium batteries.
Cadmium sulfide (CdS)
It is a yellow solid with a molecular weight of 144.48 and a CAS number of 1306-23-6. It is known as the pigment cadmium yellow.
Cadmium Telluride (CdTe)
It is a black solid with a molecular weight of 240.00, a specific gravity of 6.2, a melting point of 1041 °C, and a CAS number of 1306-25-8. It is used in solar cells.
Cadmium Selenide (CdSe)
It is a black solid with a molecular weight of 191.37, a specific gravity of 5.81, and a CAS number of 1306-24-7. Its uses include pigments.
2. Cadmium Production Methods
The general production of cadmium involves the following steps
- The gas washing liquid generated from the exclusion equipment in the zinc and lead sintering process is separated from solid-liquid using a thickener or the like.
- The resulting overflow is brought into contact with an ion exchange resin to concentrate the cadmium to obtain a regenerated solution containing cadmium.
- The regenerated solution is neutralized with sodium carbonate, etc., and solid-liquid separated to recover cadmium as cadmium carbonate.
- Cadmium carbonate is dissolved in sulfuric acid to remove insoluble content, and then crude cadmium and electrolytic zinc are added to the cadmium sulfate solution to obtain sponge cadmium through cementation reaction.
- Sponge cadmium is dissolved to obtain crude cadmium, and crude cadmium is distilled to obtain distilled cadmium.
3. Health Effects of Cadmium
The oral lethal dose of cadmium to humans is estimated to be 350-3,500 mg. Chronic poisoning with cadmium is believed to be the main cause of Itai-itai disease, which causes osteomalacia. In terms of carcinogenicity, the International Agency for Research on Cancer (IARC) ranks cadmium as 2A (very likely to be carcinogenic to humans).