氢气协同镁热还原TiO2制备低氧金属钛粉的基础研究

Titanium metal and its alloy are highly coveted functional and structural materials that have a broad range of applications. Most applications require oxygen content in titanium to be extremely low because oxygen can be very detrimental to the mechanical properties of titanium and its alloys. However, titanium has a very strong chemical affinity to oxygen, thus reducing oxygen content in Ti has been a difficult challenge for the industry. Currently, titanium primary metal has been commercially produced by using the Kroll process, which uses purified titanium tetrachloride (TiCl4) as the precursor to avoid the introduction of oxygen from the raw material. But the Kroll process is normally carried out in a batch production mode at high temperatures, giving rise to high energy consumption. Using Mg metal to reduce TiO2 is one of the approaches to produce Ti metal with lower energy consumption, however, it was deemed thermodynamically unfavorable to produce Ti metal with low oxygen content by magnesiothermic reduction of TiO2. This proposal outlines a novel method to produce Ti metal with low oxygen by hydrogen assisted magnesiothermic reduction of TiO2 (HAMR), based on a recent discovery by the PI of this proposed project that Ti-O solid solutions can be destabilized by temporarily alloying it with hydrogen represented by the increased oxygen potential of the corresponding Ti-O-H solid solutions. Based on the thermodynamic equilibrium between Ti-O and Mg-O, Mg cannot reduce oxygen content in Ti to low levels because the solid solutions of titanium with oxygen can be more stable than MgO at relatively low oxygen contents. However, when the hydrogen is introduced into the system, the increased oxygen potential of the Ti-O-H causes the reducing capability of Mg meal to increase relatively, thus enabling the reduction of lattice oxygen content in Ti by Mg. The proposed methodology also controls the surface oxygen content by decreasing of metal surface area. This proposed project will further focus on the reaction thermodynamics, kinetics and reduction mechanism relevant to the hydrogen assisted magnesiothermic reduction, in order to determine the thermodynamic limit of removing lattice oxygen in titanium by Mg and to obtain the means to enhance the reduction kinetics. It is expected that the proposed work will greatly help to optimize the process and make it more mature. Moreover, we also plan to apply the HAMR process methodology to prepare other reactive metals, including Zr, Ta, et al. A general method for the production of reactive metals with ultralow oxygen contents by thermochemical reduction will be established, which will certainly provide a technical foundation for the development and growth of relevant industries.

钛及钛合金是性能优异的功能与结构材料。为保证其机械性能，钛金属对氧含量要求极为严格。钛与氧反应能力极强，致使用Mg还原TiCl4的Kroll法是现行唯一工业生产法，但高温及间歇操作导致工艺能耗高。用Mg还原TiO2是节能途径之一，但镁热还原TiO2制备低氧金属钛在热力学上被似为不可行。本项目基于Ti-O固溶体固溶氢而提高其相应Ti-O-H固溶体氧势的原创性发现，突破了Mg还原TiO2时钛中晶格氧高的关键难题，并结合表面氧的控制，形成了氢气协同镁热还原TiO2制备低氧金属钛新方法—HAMR。本项目将通过研究HAMR的反应热力学特性、动力学规律与过程机理等关键科学问题，获取新方法镁脱除钛中晶格氧的热力学极限及反应强化方法，优化新工艺，建立HAMR新过程。本项目还将HAMR方法拓展至锆/钽等其它钛/钒副族金属的制备，形成热化学法制备低氧活泼金属的普适性方法，为相关产业技术进步和革新提供技术支撑。

钛与氧反应能力极强，致使用Mg还原TiCl4的Kroll法是现行唯一工业生产法，但高温及间歇操作导致工艺能耗高。用Mg还原TiO2是节能途径之一，但镁热还原TiO2制备低氧金属钛在热力学上被似为不可行。本项目基于Ti-O固溶体固溶氢而提高其相应Ti-O-H固溶体氧势的原创性发现，突破了Mg还原TiO2时钛中晶格氧高的关键难题，并结合表面氧的控制，形成了氢气协同镁热还原TiO2制备低氧金属钛新方法HAMR。结果表明，一定O/Ti摩尔比的钛氧固溶体吸H可调整氧势，与MgO氧势相等的Ti-O-H固溶体组成表现为高H低O的特点，奠定了间隙H强化间隙O脱除的热力学基础；镁氢结合具有强大的脱氧能力，700~800°C下纯钛粉中的氧含量可降低至425ppm以下，750~850°C下TC4粉中的氧含量可降低至750 ppm以下；温度是影响还原动力学的重要因素，还原致密前驱体时，其影响更大，但温度对多孔前驱体还原效果的影响没那么显著，可能是还原反应被激发，体系本身释放大量的反应热，促进了还原反应的进行，且还原产物中镁含量随时间的变化趋势与TiO2前驱体致密化程度相关，决定于钛酸镁的形成和消耗速率；TiO2还原过程伴随着多相转变过程，其中MgTiO3相在不同还原温度下的寿命不同，提高还原温度可推动MgTiO3相的还原进程；基于活泼金属溶解于相同阳离子卤化盐中的离子化效应，建立了少量熔盐介质辅助的反应动力学强化方法，Mg与Mg2+盐相互作用为还原剂提供了多状态、多路径的传输方式；发现了氢致钛粉体表面钝化层厚度与构成调控的规律，确定了钛表层H对表面氧增长的抑制效应；针对TC4合金粉的HAMR法制备，重点研究了(Ti-6Al-4V)-O拟二元相在高氧时的烧结致密化/合金化/组织均匀化规律，确定了因铝与钛、钒扩散速度的不同步，导致铝源单独引入是降低烧结致密化的关键因素，据此建立了优化的致密化烧结流程；采用优化的合金粉制备流程，完成了百克级Ti-6Al-4V合金粉的制备，获得颗粒内部致密、主元素分布均匀、氧含量在0.15wt%以下、真密度99.4%的粉末。本项目形成的热化学法制备低氧钛及其合金粉的方法，将为钛产业技术进步和革新提供技术支撑。