【作者】 王火生；

【导师】 傅高升；

【作者基本信息】 福州大学 ， 材料学， 2017， 博士

【摘要】 废铝再生相对电解铝具有低能耗、低排放等优势,发展再生铝资源是铝工业发展的必然趋势,但如何通过废铝熔体的高效排杂净化,提高再生铝材的品质,是废铝再生过程中必需解决的重要问题。本课题综合运用基于经典力学的分子模拟技术和基于计算流体力学的数值模拟技术,以及俄歇电子、DTA、XRD、SEM和EDS等材料分析方法,系统研究了铝熔体中的杂气行为及废铝排杂净化的热力学与动力学问题,包括铝熔体中氢的扩散行为及杂气作用机制,熔剂与A1203夹杂物的润湿性、溶解性,与铝液的界面张力,熔剂在铝熔体中扩散过程动力学,以及这些特性对废铝排杂净化效果的影响等,在此基础上设计高效的排杂净化熔剂,并进行废铝熔体的净化工艺试验。主要研究结果如下。(1)应用分子模拟技术研究了铝熔体中氢的行为,从原子层面探讨氢扩散行为的本质和杂气作用关系,运用Lennard-Jones势能函数揭示不同类型氢的扩散机理,并通过废铝熔体中的杂气关系验证试验,证实模拟与试验结果的一致性。分子模拟结果表明,普通氢原子(Hh)比较难扩散,H2中的H原子(Hhlh)和H20中的H+离子(Hhlo)可以较充分地扩散。铝熔体中的A1203会造成Hhlh在其周围聚集,且使Hhlh的扩散速率降低,从而形成气孔,但对Hh和Hhlo扩散的影响不显著。氢扩散行为的这些差别是由氢本身的力场半径和势阱深度决定的。废铝熔体中杂气关系试验表明,废铝箔和铝屑都会显著增加铝熔体的夹杂物和气体含量,二者呈正比关系,即杂多气多,且气孔依附在夹杂物上长大。试验验证结果与杂气关系的分子模拟是一致的,表明A1203夹杂物和气孔具有明显的寄生关系,因此铝熔体的高效排杂净化是除气的基础。从理论和实践上进一步表明了本课题组前期提出的“排杂为主,除气为辅”的铝液净化原则这一观点的正确性和实际应用价值。(2)在分析碱金属氯化物与氟化物的物理化学性质的基础上,建立了等摩尔NaCl-KCl+氟化物的熔剂组成结构,其中氯化物具有较好的化学稳定性和较低熔点,作为熔剂的基础组元,氟化物作为活性组元。并研究了该类熔剂的物理化学性质及与铝熔体中主要元素的化学反应特性,发现熔剂中的碱金属氟化物一方面可以降低熔剂的熔点,NaF和KF可以显著降低熔点,AlF3和Na3AlF6也可以较大程度降低熔点,而MgF2、CaF2、SrF2、BaF2等由于自身熔点较高,对熔剂的熔点影响较小。另一方面会与铝熔体中的主要合金元素发生化学反应,改变铝合金的组织与成分。其中含NaF、SrF2或BaF2的熔剂最容易与Al、Mg反生置换反应,在铝熔体中生成Na、Ba、Sr等金属单质,对共晶硅相有显著变质作用。含Na3AlF6熔剂也会少量增加铝熔体中Na含量,对共晶硅相有一定变质作用,而含AlF3、MgF2或CaF2的熔剂对铝合金组织和成分没有明显影响。(3)结合分子动力学模拟和试验,研究了熔剂排杂净化过程的热力学问题,揭示了碱金属氟化物对熔剂与Al2O3氧化膜的润湿性和溶解性、熔剂与铝液界面张力的影响规律和作用机理。废铝聚合试验结果表明,氟化物大多可以促进A1203氧化膜的溶解,使A356铝屑在750℃熔融熔剂中可以聚合为铝球,其中效果最显著的是 NaF、KF、Na3AlF6、CaF2,其次为 AlF3、BaF2、SrF2,而 MgF2最差。废铝聚合机理分析表明,熔融熔剂中游离态F-离子是溶解Al2O3夹杂物的必备条件,熔剂与氧化膜良好的润湿性可提高Al2O3氧化膜溶解速度。界面张力试验结果表明,添加NaF、KF可以显著降低界面张力,且随加入量增加,界面张力可持续降低;CaF2、BaF2、SrF2、MgF2也可降低界面张力,但随加入量增加,降低幅度减小;而AlF3和Na3AlF3会提高界面张力。(4)应用流体力学数值模拟的双流体分析技术研究了熔剂在铝熔体中的分散性与分离性,为改进熔剂加入方式,改善排杂净化的动力学条件提供科学依据。数值模拟结果表明,液态的熔剂在铝熔体中较难弥散分布,且界面张力越大,分散性越差。即使采用搅拌工艺也难以有效提高其分散性,因此,熔剂以固态加入到铝熔体时就应充分考虑其分散性,以保证熔解为液态时可以保持弥散状态,提高熔剂与夹杂物的接触几率。(5)通过废铝的排杂净化试验,系统研究了熔剂溶解氧化膜的能力和熔剂与铝液界面张力对排杂净化处理过程的影响规律,发现良好的氧化膜溶解能力和大的界面张力有利于排杂净化处理。试验结果表明,含NaF的熔剂由于界面张力低,不利于熔剂与铝液分离;AlF3虽然可增加铝液与熔剂的界面张力,但其润湿溶解Al2O3氧化膜的能力较差,净化处理效果不理想。而含Na3AlF6的熔剂,既有利于提高熔剂润湿Al2O3氧化膜的能力,也会增大熔剂/铝液界面张力,促进熔剂上浮,可获得最佳净化效果。(6)突破传统的依靠经验和试验优化熔剂组成的思路,在熔剂排杂净化热力学与动力学理论研究基础上,提出了熔剂组成的优化设计原则,再结合熔盐相图优化成分配比。分别设计了三种熔剂成分,加入方式和加入量,通过正交试验优化工艺,并应用于铝屑排杂净化。结果表明,最优熔剂成分为等摩尔NaCl-KCl(85%)+Na3AlF3(7.8%)+AlF3(5.7%)+CaF2(1.5%),最佳加入方式为铝屑和熔剂混合均匀后加入铝熔体,最佳熔剂加入量为1%。在最优净化工艺条件下,A356铝合金试样的含杂量、除气率和抗拉强度均优于市售精炼熔剂的净化处理效果。更多还原

【Abstract】 Recycling aluminum is more energy saving and environmental than primary aluminum and it is an inevitable trend to developing recycled aluminum.But at the same time it is important to remove inclusions and impurities in Al melt and improve the qualities of recycled aluminum.The behaviors of hydrogen in Al melt and the purification mechanism of recycled aluminum were studied in this paper by several materials analysis technologies,such as molecular simulation based on classical mechanics,numerical simulations of computational fluid dynamics(CFD),auger electron spectrometer,DSC,XRD,SEM,EDS etc.The research contents include the diffusion behaviors of hydrogen in A1 melt and its interaction with Al2O3,the wettabilities and solubilities between fluxes and Al2O3 inclusions,the interfacial tension between fluxes and A1 melt,the diffusion dynamics of fluxes in Al melt and their influences on the purification effects of recycled aluminum.Furthermore,a high effective flux was designed and the purification of recycled aluminum were carried out.The main results are summarized as follows.(1)The behaviors of hydrogen in Al melt were studied by molecular simulation and the nature of hydrogen diffusion and its interaction with Al2O3 were discussed on atomic level.Furthermore,Lennard-Jones potential function was used to reveal the diffusion mechanism of different types of hydrogen.The verifying tests of interaction between H and Al2O3 were carried out in recycled Al melt,and the tests results were consistent with that of simulation.The molecular simulation results showed that general H atoms(Hh)were hard to diffuse,but H atoms of H2(Hhlh)and H+ ions of H2O(Hhlo)diffused fully.Hhlh accumulated around Al2O3 and its diffusion velocity decreased,as a result,porosities formed on Al2O3.But the influences of Al2O3 on Hh and Hhlo were weak.The results of experiments on H/Al2O3 interaction in recycled aluminum showed that foils scraps and chips increased the inclusion content and porosities in Al melt,which were direct proportion.That was the higher the inclusion content,the higher the porosity was.Moreover,porosities developed on inclusions,which was consistent with the simulation results.The results proved the parasitic relationship between hydrogen and Al2O3 inclusions.So the removal of inclusions was the base of porosities deletion.The validity and practical value of aluminum purification principle developed by our team previously were also verified by present work,which was that inclusions removal being primary and degassing being auxiliary.(2)A flux component consisted of equimolar NaCl-KCl and alkali fluorides was established based on the studies of physical and chemical properties of several alkali fluorides and chlorides.The chlorides were basic components for higher chemical stability and lower melting point,and the fluorides were active components.The physical and chemical properties of this sort of fluxes and their reactivities with main elements of A1 alloys were studied.The results showed that alkali fluorides decreased fluxes melting point.NaF and KF decreased it most significantly,which were followed by AIF3 and Na3AlF6,and the influences of MgF2,CaF2,SrF2,BaF2 on fluxes melting point were weak because of the higher melting point of themselves.Moreover,some alkali fluorides reacted with main elements of A1 melt and changed the structures and constituent of Al alloys.NaF,SrF2 and BaF2 can react with Al or Mg and generated trace alkali metals,such as Na,Ca,Sr etc,which modified the eutectic silicon significantly.Na3AlF6 also generated trace Na in Al melt and modified eutectic silicon to some degree,but there were not influence of A1F3,MgF2 or CaF2 on the structures and constituent of Al alloys.(3)The thermodynamics of removing inclusions was studied by molecular dynamics simulation and experiments.The effects of alkali fluorides on the wettabilities and solubilities between fluxes and Al2O3 films and the fluxes/Al melt interfacial tension and their interaction mechanism were revealed.The coalescence experiments results showed that most of the fluorides improved the dissolve of Al2O3 films and the coalescence of A356 chips in molten fluxes at 750℃.NaF,KF,Na3AlF6 and CaF2 were the most effective,which were followed by AIF3,BaF2 and SrF2,and MgF2 was the worst one.The analysis of coalescence mechanism indicated that free F-ions were essential to dissolve Al2O3 films and better wettability improved the dissolving velocity.The experimental results of interfacial tension indicated that NaF and KF decreased the interfacial tension most significantly and with the increasing of fluoride contents,it decreased steadily.CaF2,BaF2,SrF2 and MgF2 also decreased the interfacial tension but the influences weakened with the increasing of addition amount of fluorides.However,AlF3 and Na3AlF3 increased interfacial tension because they did not generate Na in Al melt.(4)The dispensability and separability of molten fluxes in A1 melt were studied by numerical simulation results of CFD,which would provide scientific basis for improving feeding methods of fluxes and dynamics conditions of removing inclusions.The results showed that it is difficult for molten fluxes to disperse uniformly in Al melt and the dispensability decreased with the increasing of interfacial tension,moreover,the improvement of dispensability was tiny by stirring.So it is necessary to disperse the fluxes in solid state when charged into Al melt and improve the contacting probability.(5)The influences of the solubility of fluxes for oxide films and fluxes/Al melt interfacial tension on removing inclusions were researched by the purification experiments of aluminum foil scraps.The results showed that good solubility of fluxes on oxide films and big interfacial tension were helpful to remove inclusions.Salt fluxes containing NaF was difficult to separate from Al melt because NaF decreased interfacial tension.The purification effect of fluxes containing AlF3 were not good because AlF3 cannot dissolve oxide films effectively.Salt fluxes containing Na3AlF6 displayed the best purification effects because of its good wettability and solubility for oxide films and higher interfacial tension.(6)New principles of fluxes component design were proposed based on the research of thermodynamics and dynamics of removing inclusions,which broke through traditional methods of fluxes component design by experiences and numerous tests.The component ratios were optimized by molten salt phase diagrams.Three different fluxes components,feeding methods and addition amounts were designed and optimized by orthogonal tests.The purification experiments were carried out in A3 5 6 melt containing aluminum chips.The results showed that the optimal flux component was equimolar NaCl-KCl(85wt%)+ Na3AlF3(7.8t%)+ AlF3(5.7wt%)+ CaF2(1.5t%).The best feeding method was mixing aluminum chips and fluxes before charging into melt.The optimal addition amount was lwt%of Al melt weight.The inclusion content,porosity removal rate and tensile strength of Al alloy purified by optimal process were better than that of commercial fluxes.更多还原