Aluminum Electrolysis Principle and Application

Author: Qiu Zhuxian
Editor-in-Chief: Li Qiang
Publisher:
Publish Date: 1998-07-01
Features:
Fragment: Chapter 1: The History and Development of Aluminium Metallurgy
1.1 The History of Aluminium Metallurgy
Aluminium is widely distributed in nature, with an approximate content of 8% in the Earth's crust, ranking third after oxygen and silicon among all metal elements. However, among all metal elements, aluminium ranks first. Due to its highly reactive chemical properties, elemental aluminium is rarely found in nature [1–3]. There are over 250 types of aluminium-containing minerals, with the main ones being bauxite, kaolin, and alunite. China has a long history of using aluminium ores, and it was among the earliest to extract alum (known as fangshi in ancient times) from alunite for medicinal and industrial applications. The Han Dynasty's Bencao Jing (1st century BC) recorded 16 mineral drugs, including fangshi, lead oxide, lime, and niter. The Ming Dynasty's Tian Gong Kai Wu (1637 AD) documented the production and uses of fangshi. The term "Aluminium" derives from alum, as the ancient Romans called it Alumen. In 1746, Pott produced a metal oxide from alum. Marggraf believed that clay and alum contained the same metal oxide. In 1876, Morveau referred to this oxide as alumina (Alumine in English). In 1807, British Davy attempted to separate metal from alumina using electrolysis but failed. In 1808, he named this hypothetical metal Aluminium, a name that has been retained ever since. Metal aluminium was first produced chemically. In 1825, Danish Oersted reduced anhydrous aluminium chloride with potassium amalgam, obtaining a grey metal powder that exhibited a metallic luster when ground, though it was not identified at the time. In 1827, German Wohler reduced anhydrous aluminium chloride with potassium, producing small, fine metal particles. In 1845, he passed gaseous aluminium chloride over the surface of molten potassium, obtaining metal aluminium beads, each weighing 10–15 mg. This allowed for the preliminary determination of some of aluminium's physical and chemical properties. In 1854, French Deville used sodium to replace potassium in reducing the NaCl–AlCl3 complex salt, thereby producing metal aluminium. Sodium and potassium are both monovalent alkali metals, but sodium has a smaller atomic weight than potassium. Producing 1 kg of aluminium requires approximately 3.0–3.4 kg of sodium, while potassium requires about 5.5 kg, making sodium a more economical choice. At the time, aluminium was called "silver in the soil." In 1855, Deville exhibited 12 small aluminium ingots, with a total mass of about 1 kg, at the Paris World Expo. In 1854, the world's first aluminium smelting plant was built near Paris. In 1865, Russian Beketov proposed using magnesium to reduce cryolite for aluminium production. This method was later adopted by the German Gmelingen aluminium-magnesium plant. After electrolytic aluminium production plants began operating between 1887 and 1888, chemical methods gradually fell out of use. Over the preceding 30 years, chemical methods produced approximately 200 tons of aluminium. During this period, German Bunsen and French Deville, following British Davy, continued to research electrolytic methods. In 1854, Bunsen published a report on his experiments, claiming to have produced metal aluminium by electrolyzing NaCl–AlCl3 complex salt. He used carbon anodes and cathodes during electrolysis. Deville, in addition to electrolyzing NaCl–AlCl3 complex salt, also electrolyzed mixtures of this complex salt and cryolite, both of which yielded metal aluminium. Deville may have been the first to recognize that alumina could dissolve in molten fluorides. At that time, batteries could not provide large currents and were very expensive, making electrolysis impractical for industrial production. Only after the invention of the generator in 1867 and its improvement in 1880 could electrolysis be applied to industrial production.