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我国攀西钛渣是难处理的高钙镁低品位钛渣，目前尚缺乏将其升级为氯化钛白工艺所需高品位富钛料技术，无法满足未来我国氯化钛白产业发展的重大需求。氧化-还原-盐酸浸出工艺是最有希望将攀西钛渣升级为氯化钛白工艺原料的方法。然而该工艺对钛渣化学组成有较强的依赖性。由于化学组成对钛渣氧化、还原及浸出过程的影响规律尚不清楚，无法指导攀西钛渣升级提质，因此，开展钛渣氧化-还原-盐酸浸出过程物相结构演变规律研究，对攀西及其它低品位钛渣升级提质具有重要意义。针对此问题，本文系统地研究了攀西钛渣在不同工艺条件下氧化焙烧时的微观结构演变和还原焙烧时的物相转变，在此基础上重点研究了化学组成对钛渣氧化-还原焙烧-盐酸浸出过程的影响规律，为确定不同钛渣的最优升级工艺条件、全面调控钛渣升级结果提供指导。论文主要创新点如下：（1）系统地考察了钛渣在低温（800 °C）和高温（1000 °C）下氧化焙烧过程中的微观结构演变，发现钛渣在高低温下有不同的氧化反应历程。低温下，钛渣颗粒的氧化通过Fe离子向外迁移和O离子向内迁移实现，而Ti离子不发生迁移。Fe离子迁移到颗粒表面被氧化成一层晶须状Fe2O3。O离子向内迁移使反应界面处形成Fe元素富集带。随着氧化的进行，反应界面不断向颗粒内部缩进，颗粒内的Ti、Fe离子被氧化。但由于低温下离子迁移速率慢，且颗粒内部结构致密，焙烧后期反应界面的缩进速度十分缓慢，颗粒经长时间不能完全氧化。高温下，钛渣颗粒的氧化通过Ti、Fe离子向外迁移和O离子向内迁移实现。Ti、Fe离子的向外迁移及氧化使颗粒表面由内而外分布有Fe2TiO5和TiO2两层物相。由于高温下离子迁移速率快，且颗粒内部疏松多孔，反应界面快速向颗粒内部缩进，颗粒经短时间即可完全氧化。（2）研究了氧化焙烧过程氧气浓度对钛渣颗粒物相组成和微观结构演变行为，发现氧气浓度对钛渣的微观结构有重要影响并进而影响钛渣的酸溶性。低氧气浓度下（3 vol.% O2），Ti、Fe离子向外迁移程度低，导致钛渣颗粒结构致密，因此在后续浸出过程中盐酸内扩散成为钛渣除杂的限制步骤。升高氧气浓度促进Ti、Fe离子向外迁移，该迁移一方面促使钛渣颗粒内部形成多孔结构，另一方面降低颗粒中心(M3O5)2相的Fe/Mg比。因此，中氧气浓度下（6 vol.% O2），颗粒形成的多孔结构可保证盐酸足够快的内扩散，加之颗粒中难酸溶的(M3O5)2相尚具有足够高的Fe/Mg比，可被还原成易酸溶的M2O3相，所以该钛渣颗粒中的杂质元素可被大量浸出。高氧气浓度下（21 vol.% O2），钛渣颗粒中心的(M3O5)2相具有较低的Fe/Mg比，不能被还原成M2O3相，因此虽然钛渣疏松多孔，其中的杂质元素也难以浸出。（3）通过考察不同Fe/Mg比钛渣的氧化动力学和微观结构演变，探明了Fe/Mg比对钛渣完全氧化所需焙烧温度的影响规律及机理。对于Fe/Mg<2的钛渣，低温（800 °C）下氧化焙烧时其颗粒中的Fe离子向外迁移受到抑制，加之O离子由于温度低、颗粒致密而迁移速率慢，钛渣长时间焙烧不能完全氧化。升高温度（1000 °C）可使Ti、Fe离子少量向外迁移，同时O离子由于温度高、颗粒形成多孔而迁移速率加快，因此该Fe/Mg比钛渣需高温才能完全氧化。对于Fe/Mg≥2的钛渣，由于提高Fe/Mg比可促进Fe离子向外迁移，同时所形成的颗粒多孔结构减少O离子内迁阻力，该Fe/Mg比钛渣在高低温下（800-1000 °C）均可完全氧化。（4）研究了(M3O5)2相(Fe2TiO5)d(MgTi2O5)e还原焙烧时的物相转变过程，探明了Fe/Mg比对该过程的影响规律。研究结果表明，(M3O5)2相先被还原成M3O5晶型的中间相(M3O5)3相(FeTi2O5)h(MgTi2O5)i，然后(M3O5)3相进一步热分解成M2O3相(FeTiO3)f(MgTiO3)g。Fe/Mg比不影响(M3O5)2→(M3O5)3的还原过程，但影响(M3O5)3相向更易浸出的M2O3相的转变。当Fe/Mg≥1.8时，(M3O5)3相热稳定性差，可完全分解成M2O3相；降低Fe/Mg比使得(M3O5)3相热稳定性增加，当Fe/Mg<1.8时，(M3O5)3相仅部分分解成M2O3相，即便长时间还原仍会有部分富Mg相残留。（5）基于以上结果，明确了Fe/Mg比为影响低品位钛渣升级结果的核心因素，并建立了不同Fe/Mg比低品位钛渣升级工艺的指导性原则。对于Fe/Mg<1的攀西钛渣，氧化-还原焙烧-盐酸浸出工艺难以将其升级为氯化钛白工艺原料。Fe/Mg≥1时可实现攀西钛渣的成功升级。对于1≤Fe/Mg<1.8的攀西钛渣，需采用高温（1000 °C）氧化和加压浸出条件，TiO2 72 wt.%，CaO + MgO 5 wt.%的钛渣在该条件下升级其品位可提高至TiO2 89 wt.%，CaO + MgO 0.9 wt.%；进一步提高Fe/Mg可降低氧化温度，实现常压浸出。对于Fe/Mg>1.8的攀西钛渣，只需采用中低温（800-900 °C）氧化和常压浸出条件，TiO2>68 wt.%，CaO + MgO 5 wt.% 的钛渣在该条件下升级其品位可提高至TiO2 >85 wt.%，CaO + MgO<0.8 wt.%。 ;The Panxi titanium slag in China is a kind of intractable low-grade slag, which is rich in magnesium and calcium and difficult to be industrially upgraded to the feedstock suitable for the chloride process, making it difficult to meet the great demand for the future development of the chloride process in China. The oxidation-reduction-acid leaching process is the most hopeful method to upgrade the Panxi titanium slag to the chloride process feedstock. The upgrading process is greatly dependent on the chemical composition of the titanium slag. Since the influence of the slag chemical composition on the oxidation-reduction-acid leaching process has not yet been fully clarified, unable for people to obtain a guidance for upgrading the Panxi and other low-grade titanium slags, it is therefore very important to study the phase and microstructure evolutions during the oxidation-reduction-acid leaching process.To solve this problem, microstructure evolutions during the oxidation roasting and phase transition during the reduction roasting were systematically investigated based on the Panxi titanium slag in the present study, where the influence law of the slag chemical composition on the oxidation-reduction-acid leaching process was inspected, with the main focus on providing guidance for determining the optimum upgrading conditions and controlling the upgrading results of various titanium slags. The main findings of this thesis are summarized as follows:i) Microstructure evolutions of titanium slag particle oxidized at low (800 °C) and high (1000 °C) temperature were systematically investigated, and two different oxidation mechanisms were found. At 800 °C, the oxidation of slag particle was achieved by the outward migration of Fe2+ and inward migration of O ion, without the migration of Ti3+. Fe2+ migrated to the surface of the particle and was oxidized to whisker Fe2O3. The inward migration of O ion led to the formation of a Fe-rich belt in the reaction interface. The reaction interface kept shrinking towards the particle center, leading to the oxidation of Fe2+ and Ti3+. Due to the slow ion migration at low temperature and in the dense particle structure, the shrinking of the reaction interface slowed down drastically, resulting in that the slag particle could not be completely oxidized even in a long duration. At 1000 °C, the oxidation of slag particle was achieved by outward migration of Ti3+、Fe2+ and inward migration of O ion. Ti3+、Fe2+ migrated to the surface of the particle and was oxidized to the inner Fe2TiO5 and outer TiO2 layers separately. Due to the fast ion migration at high temperature and the porous particle structure, the reaction interface fastly shrank to the particle center, resulting in the fastly complete oxidization of the slag particle.ii) Phase transition and microstructure evolution of titanium slag particles oxidized under different oxygen volume percent (vO2 (%)) were investigated. It was found that vO2 (%) mainly affected the microstructure evolution of the slag particle and further changed its acid solubility. At low vO2 (%) (3 vol. % O2), no obvious Ti, Fe outward migration happened, resulting in a dense particle structure and the inward diffusion of hydrochloric acid became the rate controlling step during the following leaching process. Increasing vO2 (%) promoted Ti, Fe outward migration, which led to two microstructure changes. On one hand, the slag particle became more porous, on the other hand, Fe/Mg molar ratio of the (M3O5)2 phase in the particle center decreased. As a result, at moderate vO2 (%) (6 vol. % O2), the particle structure became porous, which ensured the fastly inward diffusion of hydrochloric acid. And the bad acid soluble (M3O5)2 phase still had a high enough Fe/Mg molar ratio to be transformed into the good acid soluble M2O3 phase. So most impurities in the slag could be acid leached. But at high vO2 (%) (21 vol. % O2), the (M3O5)2 phase in the particle center could be hardly transformed into the M2O3 phase due to its low Fe/Mg molar ratio. Therefore, although the particle presented a dense structure, the impurities could be hardly acid leached. iii) Oxidation kinetics and microstructure evolutions of the oxidized titanium slags with different Fe/Mg molar ratio were investigated. And the influence laws of Fe/Mg molar ratio on the oxidation temperature of titanium slag was clarified. For the slag with Fe/Mg<2, Fe outward migration was restricted at 800 °C. And O ion has a slow migration due to the low temperature and dense particle structure. So the titanium slag could not be completely oxidized even in a long duration. Raising temperature to 1000 °C promoted the outward migration of Ti3+ and a small quantity of Fe2+. O ion has a fast migration due to the high temperature and the formed porous particle structure. Therefore, the slag with Fe/Mg<2 needed high temperature (1000 °C) to complete the oxidation. For the slag with Fe/Mg≥2, since increasing Fe/Mg molar ratio promoted Fe2+ outward migration, and the formed porous particle structure released the inner diffusion resistance of O ion. So this titanium slag could be completely oxidized at both high and low temperature (800-1000 °C).iv) Phase transition process during the reduction of (M3O5)2 ((Fe2TiO5)d(MgTi2O5)e) with different Fe/Mg molar ratio was investigated. And the influence laws of Fe/Mg molar ratio on this process was clarified. The reduction process of (M3O5)2 underwent the formation of the intermediate (M3O5)3 (FeTi2O5)h(MgTi2O5)i in the M3O5-type and subsequently the (M3O5)3 further decomposed into the M2O3 phase (FeTiO3)f(MgTiO3)g. Fe/Mg molar ratio did not affect the reduction process of (M3O5)2→(M3O5)3, but affect the thermal decomposition of (M3O5)3 to the good acid soluble M2O3 phase. As Fe/Mg≥1.8, (M3O5)3 had a poor stability and could completely decompose into M2O3. Decreasing the Fe/Mg molar ratio enhanced the thermostability of (M3O5)3. Therefore, as Fe/Mg<1.8, (M3O5)3 could just partially decompose into M2O3, remaining some Mg-rich (M3O5)3 even after a long reduction.v) Based on the above results, it was experimentally demonstrated that Fe/Mg molar ratio was a core factor that obviously affected the upgrading results of titanium slag. And a guiding principle of upgrading titanium slags with different Fe/Mg molar ratio was established accordingly. For the titanium slag with Fe/Mg<1, the oxidation-reduction-acid leaching process cannot upgrade it to the chloride process feedstock. Increasing Fe/Mg to ≥1 could realize the successful upgrading. For the titanium slag with 1≤Fe/Mg<1.8, high temperature (1000 °C) oxidation and pressure acid leaching were necessary, the titanium slag with TiO2 72 wt.%, CaO + MgO 5 wt.% could be upgraded to the high-grade slag with TiO2 89 wt.%, CaO + MgO 0.9 wt.%. Further increasing Fe/Mg could lower the oxidation temperature and realize the atmospheric acid leaching. For the titanium slag with Fe/Mg>1.8, low/moderate temperature (800 °C-900 °C) oxidation and atmospheric acid leaching were enough, the titanium slag with TiO2>68 wt.%, CaO + MgO 5 wt.% could be upgraded to the high-grade slag with TiO2>85 wt.%, CaO + MgO<0.8 wt.%.
|张耿语. 钛渣氧化还原过程中物相和微观结构演变及其对钛渣浸出过程的影响[D]. 中国科学院大学,2017.|
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