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Phase and microstructure changes of Panzhihua ilmenite during oxidation-reduction pretreatment and their influence mechanisms on the hydrochloric acid leaching

Publication Date
February 2014
Source
Institutional Repository of Institute of Process Engineering, CAS (IPE-IR)
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Abstract

Upgrading ilmenite to obtain synthetic rutile is a key step of chlorination process to produce titanium dioxide. Hydrochloric acid leaching is a preponderant process to produce high quality synthetic rutile. Ascertaining appropriate roasting pretreatments to alter the phase compositions and microstructure of ilmenite and solve the problems that occur during the leaching process, e.g. low leaching rate and serious pulverization of leaching product, has been a necessitated problem for the development of hydrochloric acid leaching process. Based on the research findings of predecessors, we cannot fully understand the effects of phase and the microstructure changes during roasting pretreatments on the hydrochloric acid leaching or systematically optimize the roasting-leaching process. Therefore, this paper systematically studied the phase and microstructure changes during the oxidation and reduction roasting and their influence mechanisms on the following leaching process. The main obtained innovative conclusions are described as following: (1) The results of study on the oxidation roasting process for Panzhihua ilmenite at 600-1000 ℃ show that there are two different reaction paths for the oxidation process of ilmenite. At temperatures below 800 ℃(low and moderate temperature ranges), only oxidation reaction of ilmenite happens and the main products are Fe2O3 and TiO2. In the initial stage, ferrous irons of ilmenite are oxidized through the out-diffusion of Fe cations. A Fe2O3 layer is therefore formed on the particle surface (another product is Fe2Ti3O9). However, when the thickness of Fe2O3 layer grows to 1-2 μm, the in-diffusion of O anions replaces the Fe diffusion to lead to the oxidation of residual ferrous irons. Needlelike TiO2 grains intermingled with Fe2O3 grains are therefore formed inside of particle. The oxidation products constantly form and grow during the roasting process. Finally TiO2 grains become conterminous and form a network structure intermingle with Fe2O3 grains. While when the temperature is increased to 800 ℃ or greater (high temperature range), ilmenite undergoes the same oxidation reaction first and a TiO2 network is formed inside of particle. However, due to thermodynamic instability, Fe2O3 and TiO2 combine with each other to form Fe2TiO5, which becomes the main phase finally. Residual TiO2 and Fe2O3 grains disperse in the Fe2TiO5 matrix with island and spot shapes respectively. At the two temperature ranges, oxidation temperature only affects rates of reaction and crystal growth. (2) The results of study on the weak reduction roasting process at 600-800 ℃ for two typical thermodynamic stable (oxidation temperatures: T<800 ℃ and T≥800 ℃) pre-oxidized ilmenites show that the weak reduction roasting of pre-oxidized ilmenite has two competitive reaction paths. At high reduction temperatures e.g. 750 and 800 ℃, TiO2 grains formed by the pre-oxidation take part in reduction reactions and are consumed. Ilmenites pre-oxidized at T <800 ℃ and T≥800 ℃ are both reduced to FeTiO3. The former obtains FeTiO3 with network structure, while the latter obtains micron sized FeTiO3 grains scattered by micro-pores. At low reduction temperature, e.g. 600 ℃, TiO2 grains do not participate in the reduction reactions. For ilmenite pre-oxidized at T<800 ℃, Fe2O3 is reduced to metallic iron; and the TiO2 network is reserved well; while for ilmenite pre-oxidized at T≥800 ℃, Fe2TiO5 is reduced independently to FeTiO3, and a little island-like TiO2 grains inside of particle are also reserved. (3) The results of investigation on influence mechanisms for pre-oxidation and oxidation-high temperature weak reduction on the leaching process show that the leaching rate is controlled by both the phase and the microstructure of ilmenite. Leaching kinetics of pure FeTiO3、Fe2O3 and Fe2TiO5 has been characterized to clarify the phase effect on the rate of iron leaching and it is found that the rate of iron leaching is in the orde

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