Study on the Electrosynthesis of Titanium Alloys/Carbides/Composites from Ti-Bearing Compound Ores

Author:Li Shang Shu

Supervisor:lu xiong gang

Database:Doctor

Degree Year:2019

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Pages:167

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Although there exist abundant titanium resources in China,these titanium resources have not been efficiently utilized due to the complex composition of mineral facies,the diverse symbiotic element,and the high content of alkaline earth elements.The conventional method for extraction of titanium typically involves series of tedious processes,such as enrichment of titanium ores,chlorination and reduction,etc.These processes are always faced with high energy consumption,serious pollution,complicated technology and other issues,which leads to the high prices of titanium products and consequent application restriction of titanium.It is also not conductive to the green sustainable development.Molten salt electrolysis process is a simple and green method which can efficiently reduce the production cost by direct electrolyzing the solid oxides to prepare metals or alloys.In this paper,molten salts electrolysis process was used to prepare binary titanium alloys and corresponding ternary composites/carbides from Ti-bearing compound ores based on the features of element interaction and element composition features of different Ti-containing ores.This work firstly focuses on the reaction behaviors of Ti and impurity elments(such as Si and Al),and studies on the electrosynthesis of binary TisSi3 and TiAl3 alloys by adjusting the elements contents of the ores.Then,further researches on the electrosynthesis of ternary carbide(such as Ti3AlC2)and composites(such as Ti5Si3/TiC,Ti5Si3/Ti3SiC2 and Fe-TiC)are carried out by the addition of carbon in the precursors.Finally,the preparations of binary titanium carbides with special structures using the "molten salt electrolysis-chemical/electrochemical etching" process are also investigated in this work.The major conclusions are drawn as follows:1)Ti-bearing blast furnace slag is first used as the main raw material,and TisSi3 can be electro-synthesized from Ti-bearing blast furnace slag/TiO2 precursor.The main reactions during the electroreduction process involve three parts,i.e.,the decomposition of initial diopside(Ca(Mg,Al)(Si,Al)2O6),the reductions of intermediate phases(such as CaTiO3,Ca12Al14O33,CaSi2O5 and Mg2SiO4),and the formations of final products(i.e.,TisSi3,TiC and Ti3SiC2).The variations of impurity elements have also been analyzed,and the results show that elements Ca and Mg can be efficiently removed,and the removal speed of Ca is faster than that of Mg.However,element Al cannot be easily removed from the pellet because of the formation of Ti-Al phases during the electroreduction process.Therefore,the further investigations including theoretical consideration and experiment researches focus on the electrosynthesis of TiAl3 alloy.The different precursors including TiO2/Al2O3 precursor and titanium-rich slag/Al2O3 precursor were successively used to synthesize TiAl3.The main reactions occurred during the electroreduction process involve three parts,i.e.,the formations of CaTiO3 and Ca12Al14O33,the reductions of titanium-containing oxides(such as CaTiO3 and TiO2)and aluminum-containing oxides(such as Ca12Al14O33 and Al2O3),as well as the formation of TiAl3.The formation of Ti is prior to the formation of Al,thus the Ti-Al alloy formed during the electrolysis change from Ti-rich phase to Al-rich phase,until TiAl3 finally forms.During the electroreduction of titanium-rich slag/Al2O3 precursor,impurity element Fe cannot be completely removed due to the high content of FeOx in titanium-rich slag.The formed Fe will react with Al and Ti to form Tio 75Fe0.25Al3,and the residual Ti will react with Si to form Ti5Si3.The final product mainly consists of TiAl3 and Ti0.75Fe0.25Al3 along with trace amount of TisSi3.In addition,the preparation of porous TiAl3 from TiO2/Al2O3 precursor with addition of NaCl as space-holder material has been proved by using molten salt electrolysis process.2)On the basis of the research on electrosynthesis of Ti-Al binary alloy,the synthesis of Ti-Al-C ternary carbides using molten salt electrolysis process has been further investigated.Aiming at the electrosynthesis of Ti3AIC2,the electroreduction of TiO2/Al2O3/C precursor was first studied.The results show that the composition of precursor can influence the phase composition of final product and the structure of the formed T13AIC2.The increase of carbon content will result in a linear change in the lattice parameter c of Ti3AlC2.The optimum compositions of precursors are TiO2:Al2O3:C=3:0.5:1.5 and 2:0.5:1,respectively.During the electroreduction process,the reactions mainly include four parts,i.e.,the formations of CaTiO3 and Ca12Al14O33,the reductions of titanium-containing oxides and formation of TiCxOy,the reductions of TiCxOy and aluminum-containing oxides,as well as the formation of Ti3AlC2.The electro-synthesized Ti3AIC2 typically exhibits a flake-like morphology.The electroreduction of titanium-rich slag/Al2O3 precursor was then investigated.The results show that both electrolysis temperature and molten salt system have a certain influence on the final products.The impurity elements Fe and Si can be respectively enriched into Fe-Al alloys and Ti3(Al1-xSix)C2 by controlling the electrolysis conditions.The Fe-Al impurity phases can be removed through the leaching process.The purity of Ti3AlC2 can reach 98.16 wt%through the electroreduction of titanium-rich slag/Al2O3/C precursor and subsequent leaching process.In addition,the loose layered Ti3C2Tx and multi-layered TiCx can be prepared from the electro-synthesized Ti3AIC2 by using the chemical etching process and the electrochemical etching process,respectively.3)The study also demonstrates that carbide composites can be synthesized from Ti-bearing compound ores with addition of carbon.The results show that Ti5Si3/TiC and Ti5Si3/Ti3SiC2 can be electro-synthesized from Ti-bearing blast furnace slag/TiO2/C precursors,as well as Fe-TiC can be electro-synthesized from ilmenite/C precursor.The phase variations,product characteristics and behavious of the impurity elements during the eletroreduction of Ti-bearing blast furnace slag/TiO2/C precursors are same as that of Ti-bearing blast furnace slag/TiO2 precursor,and the final products possess a porous nodular micro-morphology.The electroreduction of ilmenite/C precursor can be divided into three stages,ie.,the formations of Fe and CaTiO3,the reduction of CaTiO3 and the formation of TiCxOy,as well as the reduction of TiCxOy and the formation of TiC.Fe formed in the early stage of electrolysis has good electrical conductivity,which can improve the electroreduction of the cathode pellet.During the electroreduction process,the preferential formed Fe and post-generated TiC are wrapped or isolated from each other,thus preventing the particles from sintering and controlling the morphology and particle size of final products.Therefore,nanometer TiC can be prepared from the ilmenite/C precursor through the electroreduction process and subsequent etching process.The average particle size of the prepared TiC is approximately 89 nm.The comparative investigation of electrolysis with different anodes shows that SOM-based anode process has faster reduction speed and higher current efficiency than graphite-based anode process,because higher voltage and temperature are applied during SOM-based anode process.Moreover,SOM-based anode process can avoid the problem of low current efficiency caused by carbon pollution occurred in the graphite-based anode process.4)The reaction behaviors of oxides in Ti-bearing ores has been investigated by theoretical consideriation.The results show that deoxidation behaviors of various oxides are different.The oxides can inevitably react with Ca2+and O2-to form complex intermediate phases,and these intermediate phases are relatively stable until the concentration of oxygen ions becomes lower.The kinetic model has been established based on the three-phases interlines(3PIs)reaction mechanism.It reveals that the diffusion of oxygen ions in the cathode is the main rate-determined step,and the diffusion of oxygen ions is affected by various factors,such as porosity,concentration of oxygen ions,as well as area of the cathode.