In the melting process of aluminum and aluminum alloy containing cryolite, in order to prevent the oxidation of the melt and remove the inclusions, the purpose is often achieved by adding flux. The former is to remove a layer of flux called covering agent from the melt surface, and form a dense liquid layer with a certain thickness after melting, which can prevent the infiltration of oxygen, prevent the oxidation of aluminum and significantly reduce the burning loss of aluminum; The latter is to add a certain amount of refining agent to the melt, which can separate the inclusions such as Al2O3 from the aluminum melt and float on the melt together to form slag, and can adsorb a certain amount of hydrogen in the melt. Therefore, the refining agent has the dual function of impurity removal and hydrogen removal.
The covering agent and refining agent of aluminum and aluminum alloy melt are mostly composed of chlorine salt and fluorine salt. Some also contain cryolite (Na3AlF6), but do not contain potassium cryolite (kalf4), because of its high price. Common No. 1 covering agent composition (mass%): 40-50kcl, 25-35nacl, 18-26na3alf6. It is a powdered flux containing cryolite, with a density of about 2.3kg/m?, The melting point is about 670 ℃. Due to cryolite, the interfacial tension between aluminum melt and flux increases, the volatility and hygroscopicity of flux decreases, the covering layer becomes more dense and has a certain refining ability. Therefore, it has the comprehensive process performance of covering, separation and refining, and is widely used in all aluminum and aluminum alloys except aluminum alloys with high magnesium content.
In principle, the cryolite in the flux can be replaced by potassium cryolite (kalf4), which can achieve better coverage and refining effect. However, the price of potassium cryolite is high, so it is better not to use potassium cryolite in terms of cost performance. Generally, the melting ability of chloride salt to alumina is not large, which is 1% - 2%. If cryolite is added, the melting ability of flux to alumina will be greatly strengthened.
