How is aluminum extracted from the earth and processed?

Answer 1

Aluminum ore is called Bauxite, which is a heterogenous mixture of aluminum hydroxide minerals (mainly gibbsite, with some boehmsite and diaspore) bound in a matrix of clay, silica, silt and other metallic hydroxides. It is usually found near the Earth's surface, and so strip-mining and pit mines are the typical methods of extraction. The production of metallic aluminum requires two steps. First the Bauxite ore has to be converted to a mineral called Alumina (aluminum oxide Al2O3), by a method called the Bayer process. In this process, the Bauxite ore is treated with hot sodium hydroxide solution (2NaOH + 3H2O), which dissolves only the Alumina from the Bauxite. The process is completed by cooling the "Bayer liquor", drying the residue, and then reheating (calcining) the residue to obtain nearly pure Alumina. Aluminum metal is obtained by electrolysis of the Alumina using the Hall process (Hall-Héroult process). The Alumina is dissolved in liquid Cryolite (Na3ALF6) at a temperature of over 1000oC, and then a low-voltage high-current electrical field is used to separate the aluminum from the oxygen and Crylolite. The voltage in a Hall cell is only 3-5 volts, but the current is typically 200,000-300,000 amperes. Because the liquid pure aluminum metal is denser than the Cryolite, the metal collects at the bottom of the cell where it can be periodically drained out and cooled. In the early 20th century, over half of the world's Bauxite was mined in Arkansas. Now the USA produces virtually no domestic Bauxite and is an importer of the ore. Nearly 50% of the Alumina used in the US in 2008 was imported from Australia. Other major sources of Bauxite and Alumina are Guinea, Jamaica, Brazil, China and India. The production of aluminum metal uses a tremendous amount of energy. The recycling of existing aluminum products is the most cost-effective way of obtaining aluminum metal for future use, being far less expensive than processing Bauxite and Alumina.