Se - Selenium
|Chemical Element||Selenium||Melting Point °C||217|
|Chemical Symbol||Se||Boiling Point °C||685|
|Atomic Number||34||Density g/cm3||4.8|
|Atomic Weight||78.96||Oxide||SeO2 and SeO3|
Selenium (Se) is a member of Group 16 of the Periodic Table and is the 67th most abundant element within the Earth’s crust. Selenium is chemically similar to sulphur (S) and tellurium (Te). It exists in two forms: a silvery metal or a red powder, the former being produced from the latter by heat. Red selenium is an amorphous, glass-like solid, whereas grey selenium is a soft, bluish-grey metal. Selenium’s ability to give and take electrons explains why it is different from sulphur. When selenium gives electrons it is acting as a metal and when it accepts them it is acting as a non-metal. Selenium can be toxic, but is an essential nutrient in small amounts. Too much or too little of this substance is harmful to plants and animals alike.
Selenium was discovered by Swedish chemist Jons Jacob Berzelius in 1817, the same year as cadmium (Cd). Whilst visiting a sulphuric acid works in which Berzelius had shares, he noticed that at the bottom of the chambers where the acid was produced, a red-brown sediment was gathering. He took some of this sediment home to study. At first he thought it was tellurium, but then came to realise that although it had very similar properties to tellurium, it was in fact a new element. He named it Selenium, from the Greek word ‘selene’, meaning Moon, to match the name tellurium, from Latin ‘tellus’, meaning Earth.
The global production of selenium reached 1,550 mts in 2007 and nearly 1,560 in 2008. Global demand for selenium was predicted to rise to 2,800mts by 2010, due to the rapid take-off in the photovoltaics market. One of the main applications for selenium is for pigmentation in glass manufacture to colour and decolourise glass, and also in paint, which comprises approximately 40% of the selenium demand. Other areas of demand are as follows: metallurgy (approx 30% of the market) where it is used as an additive in manganese (Mn) production, in agriculture (where it is used as a supplement for animal feed-stock for countries such as China where there is little selenium-rich soil – comprising 15% of the market) and in electronics (such as the CIGS (copper-indium-gallium-diselenide) photovoltaic technology – 10%) with a similar amount for use in pigments.
Consumption of selenium is not always representative of the success of its applications. For example, CIGS only requires low volumes of selenium, but is one of the more successful uses of selenium, likely to grow in demand in the coming years. A relatively large proportion of selenium (25%-75%) is wasted in the process of sputtering onto thin polymer sheets or foils for photovoltaics, however, much of this can be recovered and re-used for future solar-cell production. In glass production, selenium is used for infrared technology, ultra-violet lights and as a red colourant. In the production of electrolytic manganese (Mn), selenium is used as an additive, in order to improve efficiency, increase yields and reduce production costs. Due to this, the prices of selenium are closely related to those of manganese.
Selenium can be traded in several forms: selenium metal powder, granules and flakes (Se 99.5% min), also as selenium dioxide or, from mine sources as it arises at refineries, as ‘selenium crude’ (Se 85% approx).
- Emsley, John. Nature’s Building Blocks, An A-Z Guide to the Elements, New Edition, Oxford University Press, 2011
- Gray, Theodore. The Elements, A Visual Exploration of Every Known Atom in the Universe, Black Dog & Leventhal Publishers, Inc, NY, 2009
- Stwertka, Albert. A Guide to the Elements, 3rd Edition, Oxford University Press, 2012