Description
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Selenium is a nonmetal that has a lustrous gray black color in its natural solid state. Selenium is toxic in high concentrations, and it has electrical conductive properties. Selenium has an atomic weight of 78.96, a melting point of 430 °F, and a boiling point of 1265 °F. Some common uses of selenium include: as an electrical semiconductor, as a component in photocopying, as a stainless steel additive, as a component in the production of photocells and solar cells, and as the ruby red color in glasses and enamels.
Isolation
Selenium does occur free in nature, but only in very small quantities. Procedures used to isolate selenium are complicated and not usually performed in a laboratory.
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Selenium that is used for commercial purposes usually comes from such sources as selenide, and from sulfide ores such as copper, lead, or silver. Selenium that is isolated is collected as a byproduct from sulfide ore processing, mainly copper processing. Selenium containing ‘muds’ can be collected from the anode mud produced at copper refineries, and from the mud that is produced from lead chambers of sulfuric acid plants.
The ‘muds’ that are produced from ore processing begin their selenium isolation processes by using oxidation in the presence of sodium carbonate. This first reaction forms selenite, Na2SeO3. The next isolation step acidifies the selenite with sulphuric acid; this step results in any contaminants being precipitated out while selenous acid remains in solution. Sulfuric oxide and water is then added to the selenous acid to obtain pure selenium. Additional processes can be used to isolate selenium from the ‘mud’ produced in ore processing, but the procedure used depends on what type of ore the mud is produced from.