Cadmium: the essentials
Cadmium is a soft, bluish-white metal and is easily cut with a knife. It is similar in many respects to zinc. Interestingly, aa characteristic cadmium "scream" is heard on bending a cadmium bar (such as that illustrated above). Cadmium and its compounds are highly toxic. Silver solder, which contains cadmium, should be handled with care.
Cadmium: historical information
Cadmium was discovered by Friedrich Stromeyer in 1817 from an impurity in some samples pf zinc carbonate, ZnCO3. He noted that these particular samples changed colour on heating, which pure zinc carbonate does not. He was persistent enough to follow this observation through and he eventually isolated some cadmium metal by roasting and reduction of the sulphide.
Cadmium: physical properties
Cadmium: orbital properties
Isolation: it is rare that preparation of cadmium in the laboratory should be required bacause of environmental concerns about cadmium. The isolation of cadmium is associated with zinc recovery as cadmium is an impurity in zinc ores. Most zinc production is based upon sulphide ores. These are roasted in industrial plants to form zinc oxide, ZnO. This may be reduced with carbon to form zinc metal, but in practice ingenious technology is required to ensure that the resulting zinc does not contain oxide impurities.
ZnO + C → Zn + CO
ZnO + CO → Zn + CO2
CO2 + C → 2CO
After this process, zinc may be refined by distillation under vacuum and this process also allows the separation of any cadmium present in the crude zinc.
The other type of extraction of zinc is electrolytic. Dissolution of crude zinc oxide, ZnO, in sulphuric acid gives zinc sulphate, ZnSO4 in solution. Before electrolysis to produce zinc, the cadmium impurity and is removed as a precipitate by the addition of zinc dust as cadmium sulphate.