Aluminum Production Wastes
Waste muds created by the extraction of alumina from its ore, bauxite, may contain low levels of Technologically-Enhanced, Naturally-Occurring Radioactive Materials (TENORM), usually:
- their radioactive decay products.
The table below describes typical concentrations in ore, product material, and production wastes:
|Source||Radiation Level [pCi/g]|
The Radiation in TENORM Summary Table provides a range of reported concentrations, and average concentration measurements of NORM associated with various waste types and materials.
Most bauxite is imported from countries in Africa and South America and from Jamaica. Only a few surface mines in Arkansas have been responsible for all recent bauxite mining in the United States. Bauxite refineries produce alumina (Al2O3) which is used primarily as a feedstock for the aluminum reduction industry.
A study conducted by EPA indicates that the refinery processes generate about one ton of solid waste during the production of a ton of aluminum. This includes a small amount of waste rock, waste "muds," and a small amount of other scrap and solid wastes coming from the smelter.
Wastewater from Mining
The only ore beneficiation (particle size reduction for separation of mineral and waste) operations performed at bauxite mines are crushing and grinding. Water used for dust suppression, water drained from the mine, and surface runoff generate a small volume of wastewater. This water is neutralized by lime and then discharged into nearby streams.
Refinery "Muds" from Processing
Bauxite ore is processed by mixing dried bauxite with hot caustic solutions to dissolve the alumina from the bauxite. The bauxite residue, called "red mud," is removed in settling tanks and placed in a tailings impoundment near the plant. In some plants, red mud is further processed to produce aluminum oxides. The waste product from this process is called "brown mud." The impoundments that receive the muds typically have surface areas between 45 and 105 hectares, ranging from 10 hectares to almost 1,300 hectares (EPA90). The depth of the impoundments range from 1 to 16 meters, with an average impoundment depth of 7 meters. The quantity of muds accumulated on site at five facilities studied ranged from 500,000 to 22 million MT, with an average of 9.7 million MT per facility.
The refinery muds (both red and brown mud) dry to a solid with very fine particle size (about 1 μm) and contain significant amounts of iron, aluminum, calcium, and sodium. They may also contain trace amounts of TENORM and various elements, such as barium, boron, cadmium, chromium, cobalt, gallium, lead, scandium, and vanadium. (The types and concentrations of minerals present in the muds depend on the composition of the ore and processing conditions.)
Disposal and Reuse
Red and brown muds are caustic and no secondary use has been made of the waste from the tailings impoundments. The EPA has identified elevated arsenic (16μg/g) and chromium (374 μg/g) concentrations in some mud samples. However, muds might be used for land reclamation, for the construction of site dams or embankments, or as a feed material for other extraction processes because of the high iron content (20 to 50 percent).