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.
Primary production of selenium
Roasting with soda ash
The following are the major reaction steps:
- Roasting of anode mud from which copper has been removed, with soda ash, to yield the selenite and the selenate of sodium
- Leaching out of the selenium with water
- Neutralization of the leached solution in order to remove tellurium as a precipitate
- Sparging of the leached solution with sulfur dioxide to yield precipitated selenium
This involves the following steps:
- Anode mud is mixed with silica and soda ash after the removal of copper, and smelted
- The slag contains various metallic impurities such as iron and silica, which are thus removed
- Air is forced through the molten charge to produce volatile selenium oxide, which can be separated from the solid residues. These are further treated to extract various precious metals.
- The soda ash is now leached out using water, and the leached solution filtered to remove the solid waste.
- Neutralization of the filtered leached solution removes tellurium as a precipitate
- Acid is added to the remaining solution to precipitate selenium
- The precipitate which contains selenium is now boiled, washed and dried, and finally powdered to yield selenium.
Purification of selenium
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.