Molten cryolite can dissolve alumina. Because alumina has a high melting point, synthetic cryolite is used as a flux to melt alumina.
Synthetic cryolite is also known as sodium hexafluoroaluminate or sodium aluminum fluoride. Its molecular formula is Na3AlF6. It is a small white crystal, odorless, and has a greater solubility than natural-synthetic cryolite. The specific gravity is 3, the hardness is 2-3, and the melting point is 1009°C. Absorbing water and damp, synthetic cryolite is mainly used as a flux for aluminum electrolysis, rubber, wear-resistant filler for grinding wheels, enamel polarizer, glass sunscreen, and metal flux, etc. The hall-Heroult method uses synthetic cryolite-based fluoride flux. This is because cryolite has other indispensable properties besides its ability to dissolve alumina, such as not containing more positive electricity than aluminum. It is a sexual element with good stability, does not decompose under normal conditions, does not volatility, does not deliquesce, has a higher melting point than aluminum, has good electrical conductivity, and saves electricity.
Synthetic cryolite can be divided into high molecular ratio cryolite and low molecular ratio cryolite according to the molecular ratio of sodium fluoride to aluminum fluoride. Most of the domestic aluminum electrolytic plants in China use cryolite with a molecular ratio of 1.8 to 2.2 as the electrolyte for aluminum electrolysis. Since the use of high molecular weight cryolite is superior to low molecular weight cryolite in terms of environmental protection and economic benefits, China's electrolytic aluminum plants generally use high molecular weight cryolite as the electrolyte for aluminum electrolysis. Cryolite can be divided into sand cryolite, granular cryolite, and powdered cryolite according to its physical properties.
(1) The melting point is low and the melting speed is fast, which can shorten the time to enter the normal working state.
(2) The molecular ratio can be adjusted in a larger range, which can adapt to the different requirements of the cryolite molecular ratio in different periods of the electrolytic cell.
(3) Low moisture content and low fluorine loss.
(4) Granular, good fluidity, good for conveying.
(5) The raw materials are easily available and the production cost is low.
(1) Good fluidity, dust pollution, suitable for mechanized blanking.
(2) The high actual yield in electrolytic production can reduce the cost of electrolytic aluminum.
(3) The molecular ratio is between 2.5 and 3.0, which is especially suitable for the opening of electrolytic aluminum.
(4) The particles are mostly 1-10mm.
(1) A higher particle size can be achieved, usually above 200 mesh.
(2) The molecular ratio can reach 1.75-2.5, with good adjustability.
(3) The ultra-fine product has a pass rate of over 98% for 325 mesh, which can meet the requirements of special industries for cryolite.
Before the end of 2014, the potassium fluoroaluminate produced by Xinxing Light Alloy was mainly used to make brazing flux and to add various alloying elements in the aluminum processing process.
At the end of 2014, after years of research, Xinxing Light Alloy found that when the frequency of the induction furnace is greater than 2,500HZ during the production process, adding a certain amount of anti-polymerization agent can reduce the hexafluoroaluminum produced in the production process of aluminum-titanium-boron alloy. Potassium acid is prepared into potassium tetrafluoroaluminate with a low molecular ratio. This low-molecular ratio potassium tetrafluoroaluminate can completely or partially replace the flux aluminum fluoride added in the aluminum electrolysis production process. Potassium tetrafluoroaluminate is used as aluminum. Electrolysis additives can reduce the electrolysis working temperature in the aluminum electrolysis production process, and cause the current efficiency to increase by at least about 2%. The increase in current efficiency makes the production of per ton of aluminum save at least 500 degrees compared with the traditional simple use of aluminum fluoride flux.
Li+ in the electrolyte easily reacts with AlF2-5 to form LiNa2AlF6, which reduces the solubility of alumina in the electrolyte. Adding an appropriate amount of KAlF4 to the high-lithium electrolyte can lower the primary crystal temperature, increase the solubility of alumina and achieve the purpose of low-temperature aluminum electrolysis.