Properties and Overview of Cadmium

Overview:

Cadmium (Cd) is a chemical element with the atomic number 48 and Cd symbol on the periodic table. It is a soft, bluish-white metal chemically similar to zinc and mercury, often associated with these elements in nature. Discovered in 1817 by German chemists Friedrich Stromeyer and Karl Samuel Leberecht Hermann, Cadmium was first identified as an impurity in zinc carbonate. Since its discovery, cadmium has been used in various industrial applications due to its unique properties, although its use has become more regulated due to concerns over its toxicity. Physically, cadmium is a soft metal that can be easily cut with a knife. It has a relatively low melting point of approximately 320°C and a boiling point of 770°C. Cadmium is also noted for its excellent corrosion resistance, particularly in alkaline environments, which contributes to its use in protective coatings. When exposed to air, cadmium forms a thin oxide layer that protects the metal from further oxidation, giving it good durability in certain conditions. Its density is about 8.6 g/cm³, making it heavier than zinc but lighter than lead. Cadmium's ductility allows it to be drawn into wires or hammered into thin sheets, and its electrical conductivity is moderate, though less than that of copper.
Chemically, cadmium exhibits properties typical of a transition metal. Cadmium forms a variety of inorganic compounds, including cadmium oxide (CdO), cadmium chloride (CdCl₂), and Cadmium sulfide (CdS). Cadmium sulfide is particularly important as a pigment, cadmium yellow, due to its bright yellow color and stability. Cadmium compounds also produce phosphors, contributing to light emission on television screens and other displays. In addition, cadmium reacts with acids to produce hydrogen gas and soluble Cadmium salts, but it is resistant to attack by alkalis.
Safety concerns surrounding cadmium are significant because the metal is highly toxic to humans and the environment. Cadmium and its compounds are carcinogenic and pose serious health risks if inhaled, ingested, or absorbed through the skin. Long-term exposure to cadmium can lead to a variety of health problems, including lung and kidney damage, bone weakening, and an increased risk of cancer. Because of its toxicity, cadmium is classified as hazardous, and stringent safety measures are required when handling it. Workers in industries that involve cadmium must use personal protective equipment and adhere to strict occupational safety guidelines to minimize exposure. In the environment, cadmium can accumulate in soils and aquatic systems, leading to contamination of food sources and posing risks to wildlife and human populations.

Production:

Cadmium production is primarily a byproduct of zinc refining. The metal is typically extracted during the processing of zinc ores, such as sphalerite (zinc sulfide), which often contain small amounts of cadmium. During zinc smelting, cadmium is separated from zinc in the form of cadmium vapor, which is then condensed and purified. This production process also applies to lead and copper smelting, where cadmium may be present in trace amounts. Depending on the intended application, the purified cadmium is then cast into ingots or processed into various forms, such as cadmium oxide or cadmium sulfide. Major producers of cadmium include China, South Korea, and Japan, with production being closely regulated due to environmental and health concerns.

Applications:

One of the most significant applications of cadmium is in the manufacture of rechargeable nickel-cadmium (NiCd) batteries. These batteries were once widely used in portable electronic devices, power tools, and emergency lighting due to their ability to deliver high currents and relatively long life cycles. However, due to environmental and health concerns, NiCd batteries have declined in favor of more environmentally friendly alternatives, such as nickel-metal hydride (NiMH) and lithium-ion batteries. Despite this, NiCd batteries are still used in some applications where their specific characteristics are advantageous.
Cadmium is also used as a corrosion-resistant coating for metals, particularly in the aerospace and marine industries. Electroplating with cadmium provides a protective layer that prevents corrosion in harsh environments, such as saltwater. However, due to cadmium's toxicity, alternatives like zinc-nickel and tin-zinc coatings are increasingly used to reduce environmental impact.
Cadmium telluride (CdTe) is an important material used in producing photovoltaic cells for solar panels. It is a highly efficient semiconductor for converting sunlight into electricity, and it has enabled the development of thin-film solar technology, which is more cost-effective than traditional silicon-based solar cells. While CdTe solar panels offer advantages in production costs and efficiency, concerns over cadmium's toxicity and potential environmental impact have spurred research into alternative materials.
Cadmium is also employed in the manufacture of pigments, particularly in plastics, ceramics, and glass. Cadmium-based pigments are valued for their bright and stable colors, which include shades of yellow, orange, and red. These pigments are used in artist paints and plastics, where their colorfastness and resistance to high temperatures are essential. However, due to regulatory restrictions, cadmium pigments have declined, especially in consumer products, with safer alternatives being developed.
Another application of cadmium is in the nuclear industry, where cadmium rods are used as control rods in nuclear reactors. Cadmium has a high neutron absorption cross-section, making it effective at controlling the rate of nuclear fission in reactors. By absorbing excess neutrons, cadmium control rods help to regulate the chain reaction and maintain the stability of the reactor. This application highlights cadmium's critical role in the safe operation of nuclear power plants, though alternatives such as hafnium and boron are also used.

Summary:

Cadmium is a versatile metal with a range of industrial applications, from batteries and coatings to pigments and nuclear reactors. However, its toxicity poses significant health and environmental risks, leading to increased regulation and the development of safer alternatives. The unique physical and chemical properties of cadmium continue to make it valuable in specific specialized applications, even as its use is being reduced in response to safety concerns. Ongoing research into the environmental impact of cadmium and the search for less harmful substitutes reflect the balancing act between utilizing cadmium's beneficial properties and protecting human health and the environment.

See a comprehensive list of atomic, electrical, mechanical, physical and thermal properties for cadmium below: