| In the sulfate process for the production of TiO2 white pigments,proper crystal composition and uniform particle size distribution are the key factors limiting the pigment performance.As phase transformation from anatase to rutile and the grain growth take place during calcination,there is no doubt that the calcination process has become a crucial process of determining these properties.In this study,changes in granule morphology and the crystal composition during the process of crystallization from amorphous metitatinic acid to rutile and the phase transformation from anatase to rutile was fully observed.Other factors like raw material properties,calcination temperature,calcination conditions including time,heating rate,calcined seeds and dopants and their influences on phase transition and crystal growth were also investigated.Rsearch results would provide a theoretical basis for optimizing the calcination process,uniforming grain distribution and improving product quality.It was found that metatitanic acid was amorphous with free SO42-ions,asymmetric S=O and diverse T-O structure attached to the surface.TG-DTA study shows that during the temperature-programmed process,non-hydrogen bonded water absorbed would firstly remove at a low temperature below 100℃ from metatitanic acid,and then bridged hydroxyl group and terminal hydroxyl group remove at 400℃ or above.Desulfurization occurred at 500-850℃and the desulfurization velocity reached a maximum at 550℃.Phase composition and crystallinity of samples were analyzed by X-ray diffraction(XRD)and the morphology was observed by high power field scanning electron microscopy.Grain size distribution and the average size achieved from SEM were used to characterize the conditions of crystal growth and agglomeration.During crystallization process of metatitanic acid,temperature has a significantly influence on crystallinity increase which provides enenery for atoms orientation from an an amorphous state,higher than that caused by dehydration and desulfurization.By means of SEM,pore structure test and XRD analysis,changes of metatitanic acid in continuous calcination process can be summarized as follows:metatitanic acid before calcined and the samples obtained under tempertures below<500℃ are mostly flaky stacking particles in the form of agglomerates connected by the chemical bond of sulfur.As the calcination temperature increases,water and SOx in the metatitanic acid gradually remove,causing the shrink of microporous structure,a sharp decrease of specific surface area of the samples and the collapses of agglomerates along with the crystallization from amorphous metatitanic acid to anatase.The phase transformation from anatase to rutile won’t take place until metatitanic acid has completely converted into anatase form.Once rutile appears,grains grow rapidly,and the fusion phenomenon becomes severe at high temperature.The van der Waals force is established between the metatitanic acid primary agglomerate particles by SO42-connection to form a larger secondary agglomerate.The desulfurizing experiment was carried out by using aqueous ammonia to neutralize SO42-in industrial metatitanic acid.After the removal of S element,not only were the adsorption and binding force of H2O on the surface of the sample weakened,making it easier to desorpt at lower temperatures,but also onset temperature of phase transformation was reduced.Furthermore,compared to the undesulfurized samples under same conversion,the grain size distributions are more concentrated and the degree of dispersion is smaller.When it comes to the influence of various calcination conditions on the temperature range of phase transformation,the original sample TiO2(without nuclei and dopants)began at 900℃and ended at 1000℃.WSTiO2’s(S element removal)phase transition temperature range was 850～1000℃;NTiO2 with 3%rutile seeds/nuclei is phase-transformed at 800-900℃,while three kinds of single salt treatment agents of K,P and Al(KNTiO2,PNTiO2,and AlNTiO2)all underwent a crystal transformation at 800℃,and completely converted to rutile phase at 1000℃.The phase transition temperature range DNTiO2(3%rutile seeds,0.28wt%K2O,0.20wt%P2O5 and 0.28wt%Al2O3)was 850～1050℃.The addition of rutile seeds makes the phase transformation coincide with the crystallization in the range of nearly 100℃.Moreover,dopants show an inhibition function of grain growth in the temperature range of 800-900℃ and are manifested by the extension of the phase transition interval and the adjustment of the crystal morphology after 900℃.As a result,grain sintering is effectively reduced,and the particle size distribution are greatly improved. |
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