Selenium is an element that is found in association with tellurium in a few rare minerals. Most of the world’s selenium is recovered from the anode muds from electrolytic copper refineries. The element is a member of the sulfur family and resembles sulfur both in its various forms and in its compounds. Selenium finds its most important applications in the photocopying, glass making and pigments industries; it is also a component of rectifiers and photocells. Another application of selenium is as an additive in the production of electrolytic manganese metal.
The electrowinning of manganese metal results in a relatively low current efficiency due to the low reduction potential of the element manganese and the subsequent high potential for the evolution of hydrogen. In order to suppress the hydrogen evolution reaction selenium can be added to the electrolyte. The improvement in current efficiency increases yields and results in a substantial saving in production costs. Unfortunately the majority of the selenium added to the electrolyte will report to the cathode product thus contaminating the manganese.
Manganese Metal Company (MMC) employs a selenium free process since the use of selenium in manganese production has the following detrimental effects:
1. The selenium contamination of electrolytic manganese results in a product with a metal purity of 99.7% Mn, whereas selenium free manganese has a metal purity of 99.9% Mn. The selenium content in manganese metal produced using the selenium technology can be as high as 0.15%.
2. Elemental selenium is practically non-toxic and is considered to be an essential trace element; however, hydrogen selenide and other selenium compounds are extremely toxic, and resemble arsenic in their physiological reactions. Hydrogen selenide in a concentration of 1.5 ppm (parts per million) is intolerable to man, and exposure can result in irritation of the respiratory tract, skin or eyes. Repeated or prolonged exposure has been reported to cause a metallic taste in the mouth, a garlic odour of the breath and sweat, bronchial and nasopharyngeal irritation, pallor, coated tongue, nervousness, depression, fatigue and gastrointestinal disturbances. The toxicity of selenium compounds will be a hazard in each of the following areas:
- The employees of MMC will be exposed to the toxic effects of selenious acid if selenium technology were used in MMC’s hydrometallurgical production process.
- The employees of manganese consumers, mainly steel and aluminium manufacturers, could be exposed to the toxic effects of selenium compounds. For example, in a metal smelting operation selenium fumes will be released (boiling point of selenium is 690°C) and inhalation of such fumes, the most probably form being selenium dioxide that forms selenious acid with water or sweat, is extremely hazardous.
- The end user of a product containing selenium could be exposed to its toxic effect. For example flux cored welding wire is manufactured using electrolytic manganese powder and selenium contamination would have occurred if the manganese supplier used the selenium technology.
For more information on the toxicity of selenium compounds refer to the attached Material Safety Data Sheet.
3. Selenium compounds have a hazardous effect in the environment and disposal of selenium compounds is regulated. Material Safety Data Sheet attached gives ecological information related to selenium, ecotoxicity levels, disposal considerations and regulatory information. A number of USA state drinking water standards are 10 µg/litre maximum – this gives an indication of the severe ecological toxicity of selenium.
Selenium is unique among trace elements in that toxicity can occur at concentrations only slightly higher that those required for normal metabolism.
Selenium free manganese (99.9% Mn) is purer, safer and more environmentally friendly than selenium containing manganese (99.7% Mn).
Download: Selenium MSDS.pdf