Selenium has, from being an obscure element, become an intriguing component of health as well as disease, and a vital part of many industrial processes. It is found in 75 minerals, such as crooksite and clausthalite, but hardly ever in the pure form.
Thus most of the selenium in use today comes from the anode slime generated during the electrolytic refining of copper. Anywhere from 30 – 80% of the contained selenium may be retrieved, either by smelting the residue with soda ash, or roasting with soda ash.
There are various other techniques, including roasting with fluxes in order to convert selenium into its volatile oxide, which is then recovered from the flue gas. Alternatively, the selenium is calcined into a soluble form which may be leached out. More than 80% of the selenium in the world is produced in refineries in Belgium, Japan, Canada and the United States. Commercial grade selenium is 99.5% pure.
Selenium is also recovered from industrial scrap, such as from discarded photocopy machine photoreceptor drums, and this accounts for 15% of the production of refined selenium. Secondary sources of selenium are responsible for the production of 250 tons a year.
The following are the major reaction steps:
This involves the following steps:
Selenium is prepared from a solution of the impure selenium in sodium sulfite which is then filtered, to remove the solid waste. Acidification of the filtrate precipitates the selenium, which then undergoes purification by distillation to produce high-grade elemental selenium.
In these processes, elemental selenium in red amorphous form is removed from the settling tanks and elsewhere, and coked using steam and hot water, resulting in a gray crystalline form which is called coked selenium. This is straightaway used to prepare commercial grade selenium by drying, grinding and grading by size.
For higher grades of purity, distillation is used. High-grade selenium is more than 99.999% pure, and is used in thermoelectric devices.