| As one of the five largest production base of sulfur, Southern Sichuan has huge reserves of pyrite. But a few factors led large amounts of pyrite tailings produced, which were lack of awareness of environmental protection to producers, backward production techniques and uncontrolled mining. And pyrite tailings were piled up on farmland and dumped into rivers. As a result, large tracts of farmland were occupied, the river was polluted, and local people’s production and life were seriously affected, even their health. Thus, it is of realistic significance to study the comprehensive utilization of pyrite tailings in Southern Sichuan.Mullite whiskers performance excellent when used in composite materials as enhanced toughening materials. Fortunately, kaolin, the main mineral of pyrite tailing, can be transformed into mullite under certain conditions, which provides a possibility for the comprehensive utilization of pyrite tailings.In this paper, based on the mineral component and chemical composition analysis of pyrite tailings, mullite whiskers were successfully prepared using pyrite tailings by molten salt method at calcinations temperatures below 1000℃. Then, the factors affecting the synthesis of mullite whiskers were discussed, such as calcination temperature and time, heating-cooling system, tailings composition adjustiment, impurity content, the selection of aluminum additive and molten salts medium. Many test methods, such as XRF, ICP-OES, XRD, SEM, were used to characterize the products. And the following conclusions were drawn.(1) In south of Sichuan, the main mineral ingredients of pyrite tailings is kaolin. Its impurity contents are mainly in the form of pyrite, anatase for Fe and Ti, respectively. In addition, other impurities like Ca, K, Mg, C, S and organic matter also stored in pyrite tailing.(2) Using molten salt method, we can successfully prepared mullite whiskers by calcination pyrite tailings in 800～1000℃. Low temperature or short holding time is not good for the growth of mullite whiskers. Fourther more high calcination temperature may lead the formation of corundum impurity phase.Calcined at 950℃ in 2 h, we can obtain mullite nano whiskers with average diameter and length of 45 nm,500 nm, respectively. And the aspect ratio of mullite nano whiskers is about 10-20. Compared with slow warming processing, we can obtain more mullite crystal nucleus by rapid warming processing, and it is helpful to gain larger aspect ratio of mullite nano whiskers.(3)Composition adjustment can effectively reduce the Fe, Ca, K, Mg and other harmful impurity element in mullite whiskers, so to effective composition Al2O3. During the pickling process, the concentration and ratio of mixed acid can significantly influence chemical composition for mullite. In this paper, experiments shows that composition adjustment as pickled with mixed acid, which mixed concentration of 25% sulfuric acid and hydrochloric acid according to the volume ratio of 1:1, and calcined at 650℃ is conducive to the preparation of mullite whisker. Pickling treatment can not significantly reduce the content of TiO2. But acertain range of TiO2 helps the growth of mullite whiskers. Besides that,Low reactivity of raw materials will lead to produce other product than the mullite phase.(4)During the reaction, the quality of an aluminum source depends on its ability to provide highly reactivity of γ-Al2O3, and Al2(SO4)3 is suitable. Considering its low costing, bauxite is a preferred aluminum additive to carry out large-scale industrial production.(5)The melting point of molten salt medium and the viscosity of its liquid have an impact on the synthesis of mullite whiskers. Na2SO4 melts at 884℃ and could melt each other with Al2(SO4)3 at 950℃ to get lower viscosity liquid. The supersaturation of γ-Al2O3 and SiO2 is low in the this liquid, which makes large aspect ratio of mullite whiskers easy to be obtained. The viscosity of molten salt medium can be reduced by mixing Na2SO4 with K2SO4, but whiskers reunion phenomenon brought by K2SO4 is inevitable. Thus, Na2SO4 can be used as the preferred molten salt medium. |
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