Cadmium
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Background
Cadmium is a very soft, silvery-white metallic element. Its atomic number is 48 and its symbol is Cd. It is so soft that it can be cut with a knife. Cadmium has many chemical similarities to zinc, but is less reactive with acids than is zinc.
Cadmium is clearly toxic to animals, and during the past few decades has become familiar to the public mainly due to its undesirable presence in fertilizers and elsewhere, rather than for its positive industrial applications. Metallic cadmium is rarely used industrially in pure form.
Name
Cadmium was discovered in 1817 by the German chemist Friedrich Strohmeyer. He noticed that some samples of zinc carbonate (calamine) changed color when heated. Pure calamine, however, did not. He surmised there must be an impurity present and eventually isolated it by heating and reducing the zinc carbonate. What he isolated was cadmium metal. Strohmeyer coined the name cadmium, derived from the Latin word cadmia which means calamine.
Sources
| Previous Element: Silver Cadmium Next Element: Indium |
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| Physical Properties | ||
|---|---|---|
| Color | bluish-white | |
| Phase at Room Temp. | solid | |
| Density (g/cm3) | 8.65 | |
| Hardness (Mohs) | 2 | |
| Melting Point (K) | 594.1 | |
| Boiling Point (K) | 1038 | |
| Heat of Fusion (kJ/mol) | 6.109 | |
| Heat of Vaporization (kJ/mol) | 107 | |
| Heat of Atomization (kJ/mol) | 112 | |
| Thermal Conductivity (J/m sec K) | 96.9 | |
| Electrical Conductivity (1/mohm cm) | 146.413 | |
| Source | Sphalerite (sulfide) | |
| Atomic Properties | ||
| Electron Configuration | [Kr]5s24d10 | |
| Number of Isotopes | 44 (8 natural) | |
| Electron Affinity (kJ/mol) | --- | |
| First Ionization Energy (kJ/mol) | 867.7 | |
| Second Ionization Energy (kJ/mol) | 1631.4 | |
| Third Ionization Energy (kJ/mol) | 3616.2 | |
| Electronegativity | 1.69 | |
| Polarizability (Å3) | 7.2 | |
| Atomic Weight | 112.41 | |
| Atomic Volume (cm3/mol) | 13 | |
| Ionic Radius2- (pm) | --- | |
| Ionic Radius1- (pm) | --- | |
| Atomic Radius (pm) | 151 | |
| Ionic Radius1+ (pm) | --- | |
| Ionic Radius2+ (pm) | 109 | |
| Ionic Radius3+ (pm) | --- | |
| Common Oxidation Numbers | +2 | |
| Other Oxid. Numbers | --- | |
| Abundance | ||
| In Earth's Crust (mg/kg) | 1.50x10-1 | |
| In Earth's Ocean (mg/L) | 1.1x10-4 | |
| In Human Body (%) | 0.00007% | |
| Regulatory / Health | ||
| CAS Number | 7440-43-9 | |
| OSHA Permissible Exposure Limit (PEL) | TWA: 0.005 mg/m3 | |
| OSHA PEL Vacated 1989 | TWA: 0.005 mg/m3 | |
| NIOSH Recommended Exposure Limit (REL) | IDLH: 9 mg/m3 | |
| Sources: University of Wisconsin General Chemistry Mineral Information Institute Jefferson Accelerator Laboratory EnvironmentalChemistry.com | ||
Because cadmium is located just below zinc and above mercury in the Periodic Table, its physical and chemical properties are rather similar to those of zinc, and to a lesser degree, mercury. Most cadmium in nature occurs as an atomic substitution for zinc in zinc minerals, usually making up less than 1% of the mineral. Only a few relatively pure cadmium minerals are known. The best known of these is the mineral greenockite (cadmium sulfide, CdS), but even this mineral forms rare and rather small crystals.
In addition, cadmium can occur as an impurity in phosphate minerals. Some natural phosphate ores contain several hundred parts per million (ppm) of cadmium, and are thus undesirable to use as fertilizers.
Most cadmium used in industry is recovered from sphalerite (zinc sulfide), the principle ore of zinc where cadmium atoms replace some of zinc atoms in the sphalerite. On a worldwide basis, zinc ores around the world average about 1/400th as much cadmium as zinc. Although some zinc deposits have a higher cadmium/zinc ratio than others, those countries producing zinc from zinc ores also have the potential to produce significant quantities of cadmium. The cadmium is removed when zinc metal is purified in a refinery.
Cadmium is therefore produced in countries where zinc is refined, not necessarily in the countries where zinc ore is mined. China, Japan, and Korea are the world’s largest producers, with Mexico, the United States, the Netherlands, India, the United Kingdom, Peru, and Germany next. About 15 other countries produce smaller amounts.
Some cadmium is recovered from the recycling of nickel-cadmium batteries, which is required by law in some countries so that the cadmium is not discarded into the environment.
Uses
The single most important use of cadmium is in the production of nickel-cadmium ("Ni-Cad") batteries. About three quarters of the cadmium consumed annually is used to make batteries. Nickel-cadmium batteries are rechargeable and have found wide use in cellular phones, hand-held cordless power tools, cameras, portable computers, and a wide variety of household products. These applications account for the majority of the Ni-cad batteries produced. The remaining represents batteries used for emergency power supplies in hospital rooms, for emergency lights, telephone exchanges, etc.
Cadmium is useful in a small number of other applications. Cadmium sulfide (also called cadmium yellow) is used as a paint pigment. Cadmium is used to make low-temperature melting alloys, such as solder and Wood’s Metal for indoor sprinkler systems. The latter is an alloy of 50%Bi, 25%Pb, 12.5% Sn, and 12.5%Cd which melts at about 160 degrees Fahrenheit, the temperature of a very hot shower. Cadmium compounds are used both in black and white and color television tubes. It is used as a stabilizing compound in plastics.
Cadmium also has the physical property of being able to absorb neutrons. As a result, it is used in nuclear reactor control rods to dampen the nuclear reaction and keep the fission reactions under control.
China is the world’s largest consumer of cadmium, primarily for manufacturing batteries. Worldwide consumption of cadmium is stable or slightly declining, as its use is becoming more restricted due to environmental rules.
Soluble compounds of cadmium are poisonous, although the metallic and the sulfide forms are not soluble and therefore not very poisonous.
Substitutes and Alternative Uses
Due to the poisonous nature of cadmium, small Ni-cad batteries are being replaced by lithium-ion batteries and nickel-metal hydride batteries. This will obviously reduce cadmium consumption as this replacement increases. Presently, lithium-ion batteries are more expensive than Ni-cad, which will affect the pace at which this change occurs. Cerium sulfide can be used in place of cadmium sulfide as a paint pigment.
World reserves of cadmium are more than adequate for the foreseeable future, especially since the amount of cadmium produced depends on zinc smelter output, not the market for cadmium.
Further Reading
- Common Minerals and Their Uses, Mineral Information Institute.
- More than 170 Mineral Photographs, Mineral Information Institute.