ABSTRACT
Commercially, elemental selenium is produced from selenide anode mud as a by-product in the electrolytic refining of sulphide ores such as copper, nickel and lead. The industrial production of selenium is via the extraction of selenium dioxide from the residues during the purification of copper. Bauxite may be processed to synthetically manufacture abrasive corundum in the Verneuil process that allows the production of flawless single-crystal sapphires, rubies and other corundum gems. Thermal activation of kaolin-clays to obtain metakaolin produces artificial pozzolans which may also be waste or by-products from high-temperature process such as fly ashes from coal-fired electricity production. Carbonatites and metamorphosed magnesium rich limestone may also be the source of vermiculite with associated mineral phases that include corundum, apatite, serpentine and talc. Zeolites are found in low-temperature metamorphic rocks and are often contaminated by other minerals such as ferrous, sulphates, quartz, other zeolites, and amorphous glass.
