The electrolytic Al2O3 is added with cryolite to reduce the operating temperature of the electrolyte.
At present, the only method of industrially producing primary aluminum is the Hall-Elu aluminum electrolytic method. Invented by Hall of the United States and Eroux of France in 1886. The Hall-Elu aluminum electrolytic method is an electrolyte composed of alumina as a raw material and cryolite (Na3AlF6) as a flux, and the alumina in the electrolyte melt is decomposed into aluminum and aluminum by electrolysis at 950-970 ° C. Oxygen, aluminum is precipitated in liquid form at the carbon cathode, and oxygen escapes as carbon dioxide gas at the carbon anode.
The melting point of cryolite is 1009 ° C, and the melting point of alumina is 2050 ° C. Cryolite is used as a flux. Molten cryolite can dissolve alumina, so that electrolysis can work at a temperature lower than the melting point of alumina, saving energy.
To date, no other compound has been found to replace cryolite. This is because cryolite has other indispensable properties besides being able to dissolve alumina. For example, it does not contain elements that are more electrically conductive than aluminum, has good stability, does not decompose, does not volatilize, does not deliquesce under normal conditions and has a high melting point For aluminum, it has good conductivity and saves electricity.