Micaceous Iron Oxide Prepared From Pyrite Cinders By Hydrothermal Process And Its Fundamental Theory

In this dissertation, micaceous iron oxide (MIO) particles have been prepared from pyrite cinders by hydrothermal process for the first time. Its formation mechanism, surface modification and corrosion resistance are investigated.MIO particles were prepared from sulphuric acid leaching solution of pyrite cinders using ammonia as a precipitant by hydrothermal process in neutral condition. MIO particles can be successfully prepared when the optimal parameters of reaction temperature, n(Fe2+)/n(Fe3+), total iron concentration, the amount of red iron oxide seed, and reaction time are set to be 230?,0.07?0.1,1.25 mol·L-1,5 g·L-1 and 30 min, respectively. As-synthesized MIO particles show regular?-Fe2O3 hexagonal flakes which are about 0.5?m in diameter and with a diameter-to-thickness ratio of 5.0. Besides, the content of Fe2O3 of the prepared MIO is 98.54% and its quality meets the requirements of MIO pigments for paints (ISO 10601-2007).The formation mechanism of MIO has been investigated under the presence of Fe2+ and Al3+ in neutral conditions. Fe2+ could accelerate the dissolution of Fe(OH)3 gels and promote the growth of?-Fe2O3 crystals. The formed?-Fe2O3 particles are cube in shape, and its particle size increased gradually with higher Fe2+ concentration. The cubic morphology of?-Fe2O3 particles is attributed to the SO42- adsorption on {012} planes of?-Fe2O3 crystal. In the presence of Al3+, the cell parameters of?-Fe2O3 (a and c) decrease linearly with the increasing of Al3+ concentration. And the formed?-Fe2O3 particles are platelet-type, which is attributed to the adsorption of Al3+ ions onto (0001) planes, retarding the crystal growth along z axis. The preparation of MIO from simulated pyrite cinders lixivium, demonstrates that the formation of MIO is ascribed to Al3+ ions adsorption on (0001) planes and SO42-adsorption on {012} planes, which induces growth restriction for?-Fe2O3 crystals. The MIO formation process is dominated by dissolution re-precipitation, though solid-state transformation also happens. MIO particles were also prepared from pyrite cinders lixivium by hydrothermal process in highly concentrated alkaline medium. The concentration of NaOH has played a crucial role in the morphologies of MIO particles. The morphology of?-Fe2O3 changes from spheres into flakes, and its particle size increases with the increasing of NaOH concentration. The optimal conditions for MIO preparation are concluded as follows:CNaOH(?)7.0 mol·L-1; CFe(?)=0.94?2.2 mol·L-1; T(?)200?; t= 0.5?1.0 h; stirring rate should be 200r·min-1 and there is no need to add red iron oxide seeds. As-prepared MIO particles are quite uniform with flake shape of metallic gray color, the mean particle size and aspect ratio of which are?7.0?m and?7.0, respectively. Furthermore, the content of Fe2O3 in the prepared MIO is 99.34%, and its quality fulfills the standards of MIO pigments for paints (ISO 10601-2007).Effects of Al and Si on MIO crystals growth in highly concentrated alkaline medium have been investigated. The thickness of MIO decreases in the presence of Al and its particle size gradually reduces with the increasing Al content. Al in this system is in the form of [Al(OH)4]-, which is adsorbed on the (0001) planes of?-Fe2O3 crystals. Si inhibits the phase transformation of Fe(OH)3 gels into Fe2O3 and the inhibition becomes stronger when the content of Si is increased.Based on the crystal habit of?-Fe2O3, the crystal growth system of MIO was established. It comes to the conclusion that the formation of MIO is due to strong adsorption of OH- on the (0001) planes of?-Fe2O3 crystal, which reduces surface energy of (0001) planes and retards the crystal growth along z axis, leading to the exposure of (0001) planes.Effects of surface modification on the lipophilicity and hydrophilicity of MIO have been systematically studied. The lipophilic degree of MIO is 43.5% when the optimal modification conditions of sodium stearate, ball to powder and milling time were 5.0%,8:1 and 2 h, respectively. And the modified MIO changes from hydrophilic into hydrophobic. Under the optimal modification conditions of KH5501.5%, ball to powder ratio of 8:1,and milling time of 1h, the initial turbidity and turbidity after standing 60min for modified MIO are the largest, which shows an excellent hydrophilic dispersion. According to IR and TG-DSC analysis, when MIO is modified by sodium stearate, the surface hydroxyl groups of MIO are esterified with stearic acid and -CH2- groups are grafted on its surface, showing oil-water-repellent; when MIO is modified by KH550, the surface hydroxyl groups of MIO are held together with silanol groups from silane hydrolysis by hydrogen bond, and further form Si-O-Fe bond by dehydration. The formed polymer chains of silane coupling agents make MIO disperse well in aqueous media because of its steric hindrance. XRD and TEM analysis results show the crystalline phase and morphology of modified MIO have no change compared with that of unmodified MIO.Corrosion resistance of MIO epoxy coatings was investigated by electrochemical impedance spectra (EIS) and coating adhesion testing. And comparison of corrosion resistance of different morphology and particle size of iron oxide epoxy coatings was conducted. Open-circuit potential measurements show that epoxy coating with 50% MIO possesses the maximum open-circuit potential when they were immersed in 3.5% NaCl solution for 72 h, exhibiting the best corrosion resistant. EIS results show that the coating resistance are higher than 3.5×106?·cm2 when epoxy coating with 50% MIO immersed in 3.5% NaCl solution for 32d, which fulfills the requirements of coating impedance >106?·cm2 for common organic coatings, showing a better corrosion resistant. Machu test and boiling water immersion test indicate that no peeling and blistering of epoxy coating with amount of 40% and 50% MIO are observed, showing a fine coating adhesion. When epoxy coating with different particle size of MIO and spherical iron oxide immersed in 3.5% NaCl solution for 10 days, EIS results show the impedance at high frequency of 6.10×106?·cm2,1.89×105?·cm2 and 1.01×105?·cm2 for 7.0?m MIO,1.0?m MIO and spherical iron oxide, respectively. Their corrosion resistant decreased one by one. Machu test and boiling water immersion test indicate that the coating adhesion of MIO epoxy coatings is stronger than that of spherical iron oxide epoxy coatings.