There are some factors that affect the molten aluminum flux refining effect, includes the flux composition, flux dryness, completeness of flux treatment, and refining process parameters.
Molten Aluminum Flux Refining Influence Factors
1) The nature of the salt that makes up the flux. Generally, chloride flux can wet oxides and melt well, and adding fluoride to chloride flux can improve the refining ability of the flux. The reason why fluoride improves the refining capacity is as follows:
- It increases the interfacial tension between the flux and the melt, and enhances the adsorption of the flux to the oxide.
- The fluoride has a higher reactivity with the aluminum melt, which is in the liquid aluminum oxide Surface-active reaction products such as aluminum fluoride gas and liquid sodium are formed on the interface, which promotes the elimination of oxides. When only treated with chloride, only a small amount of gaseous product of aluminum chloride is produced, so the refining effect is not as good as that with fluoride.
2) Dryness of the flux. Chloride flux has high hygroscopicity. The flux must be carefully dried or remelted before use to remove adsorbed water and crystal water to improve the refining effect.
3) The completeness of the flux treatment along the entire melt thickness. That is, the flux droplets evenly penetrate the melt, fully contact, evenly distribute, and leave no dead corners during refining.
4) Refining process parameters. That is to choose the appropriate refining temperature, refining agent dosage and correct adding method. When the refining temperature is increased, the surface tension of the flux is reduced, and the wettability of the oxidized inclusions is better, so the adsorption and slag removal effect is stronger; at the same time, the ability of the flux to dissolve the inclusions is improved, and the chemical interaction between the flux and the melt is enhanced. The decrease in bulk viscosity makes the ups and downs of inclusions easier, so it is beneficial to improve the refining effect. In addition, when the flux is added to the melt, not only a large amount of heat of dissolution must be absorbed, but the decomposition of some fluorides (such as cryolite) and the formation of aluminum fluoride are endothermic, so the refining temperature is also better. Of course, too high temperature will increase the oxidation and gas absorption tendency of the melt, so it should be selected appropriately. Usually, the temperature of the flux refining is 730～750℃.