Study On Purification Mechanism Of Fluxes In Aluminum Melt

The oxide inclusions and dissolved impurity elements in aluminum melt are essential factors affecting the performance of aluminum alloy.The purification treatment is one of the critical steps to producing high-quality aluminum alloy.At present,the most widely used and effective melt purification method in aluminum alloy production is solid flux purification.Recycling aluminum scraps as a return charge is an effective way to reduce energy consumption and carbon footprint.Improving the cleanliness of the aluminum melt prepared using high fraction of aluminum scraps and producing thereby high-quality aluminum alloy is the key to realizing the reclamation of aluminum scraps.In this dissertation,the surface oxide film of 1235 alloy aluminum foil,A356 turning chips,and 5083 milling chips were used as the purification subject.The purification mechanism of applying salt fluxing in the aluminum melt was studied from the existing form of oxide film on the surface of the return charge in the aluminum melt,and salt fluxes melt.The influence of fluxes on impurity elements in the aluminum melt was analyzed.The purification mechanism of fluxes was systematically studied,the result of which provided a theoretical basis for the purification treatment of aluminum melt and the compositional design of fluxes.The evolution behavior of the surface oxide film of return charge in the aluminum melt was studied in a binary system of aluminum melt and return charge.The results showed that the surface oxide film of 1235 aluminum foils,A356 scraps,and 5083 scraps could be transformed into micron-sized oxide particles after being kept in the industrial pure aluminum melt for 30 min;In the aluminum melt containing magnesium alloying element,the surface oxide film of the return charge was thickened and difficult to be broken and presented in the form of large flakes.The thickness of the surface oxide film of three kinds of return charges were characterized by Auger electron spectroscopy,and the content of oxide inclusion introduced by different return charges into aluminum melt were calculated.In the binary system of salt flux melt and return charge,the coalescence behavior of return charge in different fluxes melt was studied,and the effect of flux on the surface oxide film of return charge was analyzed.The NaCl-KCl,NaCl-MgCl2,and KCl-MgCl2 fluxes enable coalesce 1235 aluminum foil into a big aluminum droplet in the melt.For the A356 alloy scraps and 5083 alloy scraps,all three chloride salt fluxes failed to remove the surface oxide film and promote thecoalesce.Adding fluoride in the NaCl-KCl system can effectively promote the coalescence of return charge droplets in the flux melt.The existing form of the surface oxide film of the return charge in the fluxes melt was analyzed,and the peeling mechanism of the oxide film was studied.In the ternary system of aluminum melt,flux,and return charge,the interaction between flux and the surface oxide film of return charge in the aluminum melt was studied.It was observed experimentally that the interaction between the flux and different forms of oxide inclusions,such as small-size particles,flocculent oxides,large-size flaky oxide film,and agglomerated oxide film.The thermodynamic and kinetic analysis was carried out on the flux wetting,coating the oxide film,and the floating of oxide film.The wetting model of the flux on the oxide film was established.The removal mechanism of oxide inclusion by flux was analyzed,and the quantitative relationship between the amount of flux and the content of oxide inclusion was established.In the binary system of aluminum melt and purification agent,the influence of sodiumcontaining fluxes on sodium content in Al-Mg and Al-Si alloys was studied,and the present form of sodium introduced by the flux in the alloys was analyzed.The NaF and Na3AlF6 in the sodium-containing flux can cause a significant increase in sodium content in the aluminum melt.The alloying element magnesium in the aluminum melt is the critical factor driving the sodium content increase.The sodium introduced by the flux existed as sodiumrich particles in the Al-Mg and Al-Si alloys.The impurity elements such as calcium,magnesium,and lithium in aluminum melt were purified by different fluxes,and the removal mechanism was analyzed.With the addition of 0.75 wt%NaCl-KCl+AlF3 flux,the removal efficiency of calcium and magnesium elements in the aluminum melt was 96.2%and 87.0%,but the removal effect of the lithium element was not significant.Adding the flux in small amounts and several times can effectively improve the removal of impurity elements.The holding time had the most significant effect on the burning loss of the lithium element,followed by the calcium element,and had less impact on the magnesium element.