| Titanium,as a rare metal,due to low density,high specific strength,fatigue resistance,corrosion resistance and biocompatibility,is the hottest candidate in some advanced technology areas as the structural materials,such as aerospace,petrochemical and biomedical fields.Currently,Ti metal is produced by a mature commercial technology named as Kroll process.However,the Kroll process is considered as a complex,expensive,high energy consumption and heavy pollution process.Therefore,developing a new method charactered with short procedure,low cost and environmentally friendly for preparing Ti and its alloys is significant to broader applications.In present article,two new processes have been studied,an approach integrating carbochlorination and electrolyzation for preparing Ti and preparation of Ti/Ti-Al alloys by a two-stage aluminothermic reduction process.Through the preliminary studies of different electrolysis technique,an approach integrating carbochlorination and electrolyzation was identified to produce metallic titanium.The effects of electrolysis voltage,electrolysis temperature,electrolysis time and TiO2/C ratio on the electrolysis products were studied during the electrolysis.The electrolysis mechanism for producing of Ti was discussed in this paper.Research results show that there are two main stages occurred.At first,titanium ions,prepared by carbochlorination of TiO2,enter into the melts and form complex chlorides.Then,complex chlorides achieve electrons at the cathode,and Ti metal is obtained.In NaCl-CaCl2 melts,the content of Ti in the cathodic production was increased as the cell voltage arising;with extended period,the content of Ti in the cathodic production was increasing,which also favored the generation of TiC;the effect of TiO2/C ratio on the cathodic production was obvious.The amount of C with a higher favorable generated metallic titanium,but could also lead to the generation of TiC.Ti powder contained a little TiC and CaCO3 could be obtained at 3.5V for 6h with a TiO2/C mass ratio of 10:1.5 in 850℃ NaCl-CaCl2 melts.Furthermore,cyclic voltammetry indicates that Ti3+ was reduced to Ti by a two-step mechanism,corresponding to the reduction pathway:Ti3+→Ti2+→Ti.The reduction of Ti2+ was controlled by diffusion process,and the minimum diffusion coefficient value of Ti2+ in NaCl-CaCl2 melts was 9.04×10-5 cm2/s.In NaCl-KCl melts,purity 95.54%titanium powder could be obtained after electrolysis at 4.0V for 5h at 850℃ with a TiO2/C mass ratio of 10:1.5.TiC nanopowder could be prepared in the anode chamber with the cell voltage up to 4.5V,and the cathodic production was the mixture of Ti and TiC.The result of cyclic voltammetry shows that Ti3+ was reduced to Ti by one step mechanism in NaCl-KCl melts,corresponding to the reduction pathway:Ti3+ + 3e → Ti.In the study of aluminothermic reduction process to prepare Ti and Ti-Al alloys using Na2TiF6,the effects of reduction temperature,holding time,the ratio of A1 reducing agent on the reduction process were researched.Calculations for thermodynamic and dynamic parameters of the reduction process were carried out.The mechanism for reduction reaction was discussed.The results show that the first stage of aluminothermic reduction for preparing Ti was an exothermic process and occurred at about 660℃.The activation energy and the fractional order of reaction were obtained by Freeman-Carroll method as 3.26×103 kJ/mol and 0.81 respectively.With the increase of reduction temperature,holding time and the added amount of reducing agent decreasing,the content of A1 element contained in Ti production was decreased,and the content of Ti element contained in the distillated product was increased.Ti powder with O content lower than 0.4%could be obtained after reduction at 1000℃ for 1.5h,which carried out at the excess coefficient of A1 as-3%and heating up time as 45min.The distillated product obtained after the first stage reduction process was mainly consisted of Na3AlF6 and Na5Al3F14,while a little of Na3TiF6,low-titanium compound and Ti might be presented.The Ti content of the distillated product was 3%-10%.It is shown by the results of TG-DSC analysis that the first stage of aluminothermic reduction for preparing Ti3Al,TiAl and TiAl3 were all exothermic processes and occurred at approximately 660 ℃.The activation energy obtained by Freeman-Carroll method was 6.32x 103 kJ/mol,7.85 x 103 kJ/mol and 3.28 x 103 kJ/mol respectively,and the fractional order of reaction was 1.79,4.46 and 1.02 respectively.The TG-DSC result of aluminothermic reduction of the Ti-containing cryolite shows that the reaction occurred at about 780.5℃.With the increase of reduction temperature and the added amount of reducing agent,the Ti recovery ratio of the Ti-containing cryolite was improved.Holding time over than 30min had no significant effect on the Ti reduction rate.The Ti-containing cryolite mixed with A1 powder under a mass radio to be 4:1 was used as the raw feed,the secondary stage reduction was carried out at 1050℃ and sintered for 0.5h.After that,bleached cryolite with Ti content lower than 0.01%and Al-Ti master alloy could be obtained.The bleached cryolite has met the national standard of cryolite standards for aluminum industry(GB/T 4291-2007).The purity of Ti powder obtained by electrolysis integrating carbochlorination was about 95%.Much more systematic experimental studies for carbochlorination process are essential for improving the purity of the metal titanium powder by electrolysis.Ti/Ti-Al alloys powders(particle size of less than 74μm),with O content lower than 0.4%,could be obtained by aluminothermic reduction of Na2TiF6.Due to low energy consumption,high efficiency,short process,low cost and non-polluting,this reduction process is an energy-saving and environmental protection method for preparation of Ti/Ti-Al alloys. |
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