How is aluminum smelted?

Aluminum and its alloys are among the most widely used metallic materials today. Currently, aluminum ranks second only to steel in terms of production and consumption (measured in tons), making it the second most utilized metal by humans. Moreover, aluminum is abundant in resources, being the most prevalent metallic element in the Earth's crust.

However, due to its reactive chemical nature, aluminum exists naturally in a compound state, and it is difficult to reduce it using common reducing agents, thus rendering aluminum smelting a challenging process.

How can aluminum be extracted from aluminum ore?

Hans Christian Oersted, a Danish scientist, was the first to smelt aluminum. In 1825, Oersted passed chlorine gas through a mixture of red-hot charcoal and bauxite (primarily composed of alumina) to produce aluminum chloride. Subsequently, he reacted potassium amalgam with aluminum chloride to form an "aluminum amalgam." By evaporating the mercury from the "aluminum amalgam" under an inert atmosphere, he obtained aluminum with impurities.

In 1827, German chemist Friedrich Wöhler published a method for refining aluminum: reacting hot potassium carbonate with boiling alum solution to obtain aluminum hydroxide. After washing and drying the aluminum hydroxide, it was mixed with charcoal powder, sugar, oil, etc., into a paste and heated in a sealed crucible, yielding a sintered mixture of alumina and charcoal. By heating this sintered material to a red-hot state and passing dry chlorine gas through it, anhydrous aluminum chloride was obtained. Then, a layer of excess anhydrous aluminum chloride was placed over a small amount of metallic potassium in a platinum crucible, which was covered. Upon heating the crucible to a white-hot temperature, the reaction completed, and upon cooling, the crucible was submerged in water. It was found that the mixture in the crucible did not react with water, and the aqueous solution was not alkaline, indicating that all the potassium had reacted, leaving behind gray powder—metallic aluminum, albeit in small granular form. Today, while it is possible to successfully smelt aluminum with high purity, the cost remains significant.

In 1884, Charles Martin Hall, a young student at the Oberlin College Chemistry Department in the United States, applied electrolytic smelting of molten alumina to produce aluminum. Given alumina's high melting point (2050°C), Hall added cryolite (Na3AlF6), a material that dissolves alumina and lowers its melting point, to the alumina. This mixture had a melting point between 930°C and 1000°C, and cryolite remained stable and fluid at electrolytic temperatures, facilitating the process. Using a porcelain crucible, a carbon rod (anode), and a homemade battery, Hall observed gas bubbles during electrolysis but no metallic aluminum precipitation. He hypothesized that the current decomposed silica in the crucible, so he modified the battery by lining the crucible with carbon and using carbon as the cathode, resolving the issue. Meanwhile, French chemist Paul Héroult invented the same aluminum smelting method in the same year.

Electrolysis significantly reduced the cost of aluminum smelting, enabling its widespread industrial application.

Regarding the electrolytic smelting of aluminum, it is crucial to note that aluminum primarily exists in bauxite ore in nature. To extract aluminum from bauxite, high-purity alumina must first be prepared. The preparation process involves removing impurities such as iron oxide and silica from the bauxite ore. Then, concentrated sodium hydroxide solution is used to treat the bauxite, yielding a sodium aluminate solution. By passing carbon dioxide through this solution, aluminum hydroxide precipitates, which is heated to decompose into high-purity alumina.