Research On The Preparation Of Ti₅Si₃, TiSi₂ And Low Iron Al-Si Alloys By Recycling Industrial Wast

With the rapid development of the economy and technology,the reserves of Ti,Si,and Al resources in China are decreasing with each passing year,however,the demand for these resources is increasing year by year.Accordingly,the regeneration of Ti,Si,and Al resources is becoming increasingly important,which not only relieves the pressure on resources but also reduces environmental pollution.Ti-bearing blast furnace slag,spent SCR catalysts,diamond wire saw Si powder,and Al alloy scrap are typical Ti-,Si-and Al-rich industrial wastes,and their recycling has become an indispensable link for the sustainable development of Ti,Si,and Al resources in China.Based on the context of the“double carbon”target in China,this thesis proposes a new green process of Si/Al reduction-vacuum directional solidification for the recovery of Ti,Si,and Al resources from Ti-bearing blast furnace slag,spent SCR catalysts,diamond wire saw Si powder,and Al alloy scrap,and their preparation into high-value Ti₅Si₃,Ti Si₂,and low-Fe Al-Si alloys.The specific research and conclusions surrounding the new process are as follows,（1）The dissolution behavior of crucibles and the influence of slag composition on the slag-alloy separation and Ti extraction were investigated during the Si reduction of Ti-containing slag to produce Ti-Si alloys.Both Mg O and Al₂O₃ crucibles changed from direct to indirect dissolution during the Si reduction process,and the dissolution amount of the Mg O crucible was lower.The addition of extra Mg O and Al₂O₃ to the Ti-containing slag can decrease the dissolution amount of the Mg O and Al₂O₃ crucibles,however,the addition of Al₂O₃ was detrimental to Ti O₂ reduction and slag-alloy separation,whereas the addition of Mg O promoted Ti O₂ reduction（maximum reduction ratio of 98.4%）and slag-alloy separation.The electromagnetic separation technique can effectively remove oxide inclusions in Ti-Si alloys;the residual O content in the alloy was only0.03 wt.%after 40 min of electromagnetic separation at 1923 K.（2）The effects of slag composition and heating way on Al reduction,as well as the dissolution equilibrium of crucibles were investigated during the preparation of Ti-Si-Al and Ti-Al alloys from Al reduction of Ti-containing slag.The results showed that the alloy yield was lower in the graphite crucible compared to the Mg O and Al₂O₃crucibles.When the Mg O and Al₂O₃ crucibles reached dissolution equilibrium,the Al₂O₃ crucible showed a higher dissolution amount and more severe slag penetration.The addition of Ca O and Mg O to the Ti-containing slag promoted the extraction of Ti（maximum Ti extraction ratio of 94.7%）,while the addition of Al₂O₃ inhibited the extraction of Ti.Compared with resistance heating,induction heating provided better slag-alloy separation and higher reduction efficiency.The Ti alloys prepared by Al reduction mainly contained W,Fe,Mn,Cu,Ca,Mg,and Zn impurities,while the Ti alloys prepared by Si reduction mainly contained Fe,Mn,Ca,Mg,and Al impurities.（3）Whether Ti alloy can be purified by vacuum directional solidification is closely related to the thermodynamic properties of impurities,therefore,the key thermodynamic properties of Fe,Mn,Ca,Mg,Al,Cu,and Ni impurities in Ti-Si alloy were determined by high-temperature chemical equilibrium method.At 1773K,the activity of Ca,Al,and Mg in the Ti-Si alloy gradually decreased while their activity coefficients gradually increased with increasing Ti concentration.At1653 K,the equilibrium segregation coefficient of Al,Cu,and Mn in Ti Si₂ were 0.1,0.08,and0.07（approximate）,respectively.At 1733 K,the equilibrium segregation coefficient of Ni in Ti Si₂was 0.06（approximate）.At 1823 K,the equilibrium segregation coefficient for Al,Cu,Ni,Fe,and Mn in Ti₅Si₃ were 0.12,0.05,0.04,0.06,and 0.12（approximate）,respectively.At 1773K,the vapour pressures of Ca,Mg,and Al in the Ti-Si melt were all greater than 0,and all decreased with increasing Ti content in the Ti-Si melt.（4）The effects of the pull-down rate and the applied magnetic field on the separation and purification,the distribution pattern of impurities,and the phase separation mechanism were revealed during the vacuum directional solidification of Ti alloys.During electromagnetic directional solidification,Fe,Mn,Al,Ni,and Cu impurities were removed through the segregation effect,Ca,Mg,and Zn impurities were mainly removed through vacuum volatilization,and the most difficult to remove from Ti Si₂ and Ti₅Si₃were W and Al impurities.The removal ratios of impurities increased as the pulling-down rate decreased.The alloy phases were separated by precipitating in the longitudinal position of the alloy sequentially according to their precipitation temperature（from high to low）during directional solidification.Electromagnetic directional solidification promoted the separation and agglomeration of alloy phases compared to resistance directional solidification.This type of Ti alloy was prepared to obtain Ti₅Si₃ with 99.3%purity,Ti Si₂ with 99.4%purity,and low-Fe Al-Si（0.63wt.%Fe）after electromagnetic directional solidification.Adding Si（5-11.4wt.%）to the Fe-containing Al-Si alloys can effectively remove Fe（from 1.1 wt.%down to 0.32 wt.%）during electromagnetic directional solidification.（5）The kinetic behavior of Ca and Al impurities in the Ti-Si alloys melt during vacuum refining and vacuum directional solidification was investigated.The study found that the rate-determining step of Ca volatilization was the mass transfer of Ca in the melt boundary layer during the vacuum induction melting of Ti-Si alloy.The kinetic model of Al impurities content distribution in the longitudinal position of the Ti-Si alloy was established asC_S=C₀ k_eff（1-f_S）^{（keff+KLv-G-1）}during the vacuum directional solidification,and the kinetic model suggested that Al removal can be facilitated by reducing the value of(k_eff+_v^KL-G-1).（6）The properties of the Ti Si₂ and Ti₅Si₃ prepared by this research process were compared and analyzed.The heat transfer modes of Ti Si₂ and Ti₅Si₃ were phonon heat transfer and radiation heat transfer,respectively.The Al and W dissolved in Ti Si₂ and Ti₅Si₃ both reduced their thermal conductivity.The thermal conductivity of Ti Si₂ and Ti₅Si₃ prepared by this process was comparable to that of typical high-temperature alloy coating materials,and their thermal expansion coefficients also match that of typical high-temperature Ti alloys.Therefore,Ti Si₂ and Ti₅Si₃prepared by this process show great potential for the application of high-temperature alloy coatings.The above results show that the new process of Si/Al reduction-vacuum directional solidification has the advantage of being a green,economical,and short process for the recycling of Ti-,Si-and Al-rich industrial wastes,as well as the excellent properties of the Ti-Si alloy products thus prepared.Therefore,this thesis provides a new solution for recycling this type of resource.