| As an environmental friendly and cheap material, calcium sulfate whiskers have great potential to be widely used as polyurethane, plastics, ceramics and cements, owing to their special properties of toughness, high electrical insulation and resistance to temperature, chemical corrosions, and acidic or basic media. The study of the technology and mechanism of producing calcium sulfate whiskers from the industrial wastes is to favor the full multi-utilization of the industrial wastes as well as to provide the basic data for the formation and application of calcium sulfate whiskers.With the aim of developing a new method to synthesis the advanced calcium sulfate whiskers from the rich and cheap Ca-bearing and S-bearing resources in China, this thesis studied the the controllable synthesis and the preferential growth mechanism of calcium sulfate whiskers. The contents include the influence of feed kind on the formation of the dihydrate gypsum precursor (CaSO4·2H2O), the hydrothermal formation and preferential growth of the hemi-hydrate calcium sulfate (CaSO4·0.5H2O) whiskers, and the thermal conversion of hemi-hydrate calcium sulfate whiskers and the sintering technology to anhydrous calcium sulfate(CaSO4) whiskers.Firstly, using the various gypsums (including the gypsum reagent, the phosphogypsum, desulfurization gypsum, the gypsums formed from the systems of carbide slag-sulfate acid, calcium oxide-sulfate acid, calcium chloride-sodium sulfate) as the raw materials, the influence of gypsum properties (the particle size, the purity and crystallinity) on the morphology of hemi-hydrate calcium sulfate whiskers was studied. The influence of the formation conditions (temperature, the mixing way and the molar ratio of the raw material, the solution composition) of the gypsums, esp. in Na2SO4-CaCl2 system, on the morphology of hemi-hydrate calcium sulfate whiskers was investigated. The influence of the super-saturation and [Ca2+/[SO42-] on the properties of gypsum and the morphology of hemi-hydrate calcium sulfate whiskers was discussed. It was shown that hemi-hydrate calcium sulfate whiskers can be formed from the mentioned various gypsums. The small particle size and the high purity favored the formation of hemi-hydrate calcium sulfate whiskers with high aspect ratios. In Na2SO4-CaCl2 system, hemi-hydrate calcium sulfate whiskers with the comparatively high aspect ratio (>100) were formed from the fine dihydrate gypsum precursor particles with the small average particle size and the precursor was formed at the following condition with the speciality of comparatively low supersaturation and high [Ca2+/[SO42-] value:ambient temperature, adding Na2SO4 solution into CaCl2 solution, molar ratio of Na2SO4 to CaCl2 being 0.5:1. The existence of the surface modification agent-SDBS reduced the particle size of the precursor but inhibited the hydrothermal formation of hemi-hydrate calcium sulfate whiskers with high aspect ratios. The presence of the complex agent-EDTA or NaCl led to the increase of the supersaturation and the decrease of [Ca2+/[SO42"] value, forming the precursors with high crystallinity which inhibited the formation of hemi-hydrate calcium sulfate whiskers with high aspect ratiosSecondly, the influence of typical hydrothermal parameters (heating rate, solid content, pH, the filling ratio) and the hydrothermal solution surroundings on the morphology of hemi-hydrate calcium sulfate whiskers was studied. The suitable condition was confirmed as follows for the hydrothermal formation of hemi-hydrate calcium sulfate whiskers with high aspect ratios:6℃/min of heating rate,3% solid content, initial pH=6.8,62.5% of filling ratio, low supersaturation, high [Ca2+/[SO42-] value. The presence of Ca(OH)2 in aqueous solution increased the pH, leading to the formation of hemi-hydrate calcium sulfate whiskers with short length and agglomeration. The existence of Fe3+ promoted the formation of Fe-bearing hemi-hydrate calcium sulfate whiskers with high aspect ratios. The complex of EDTA led to the increase of the total concentration of soluble Ca2+ and the decrease of the free Ca2+, which unfavored the growth of hemi-hydrate calcium sulfate whiskers. The existence of NaCl, CaCl2 and Na2SO4 led to the increase of the supersaturation and the change of [Ca2+/[SO42"] value. The hemi-hydrate calcium sulfate whiskers with the length of 1000-2000μm, the diameter of 1-10μm and the aspect ratio> 100 were formed in water medium without any additives.Thirdly, the hydrothermal dissolution-precipitation process and the one-dimensional growth of hemi-hydrate calcium sulfate whiskers were studied in detail. The variation of the composition and morphology of the hydrothermal products formed in water medium with reaction time (0-6 h) and temperature (25-200℃) was studied and the experimental results indicated that the hydrothermal conversion of calcium shlfate phases were carried out via the following dissolution-precipitation route:dihydrate calcium sulfate granules (<110℃)→hemi-hydrate calcium sulfate whiskers (110-170℃)→nhydrous calcium sulfate spindles (>170℃). The dissolution of precursor and the formation of whiskers were slow at low temperature (130℃), the dissolution rate of precursor and the formation of whiskers were fast at high temperature (200℃), and shortened the formation time of whiskers. The growth rates of the planes were different from each other due to the corresponding different atom densities and the varying arrangement styles of Ca2+ and SO42- of calcium sulfate crystals. The comparatively faster overlying speeds of Ca2+ and SO42- in (002) plane for dihydrate calcium sulfate, hemi-hydrate calcium sulfate and anhydrous calcium sulfate promoted the preferential growth of calcium sulfate crystals alone c-axis. Hemi-hydrate calcium sulfate whiskers can be formed at proper solution environments. The presence of the solution composition as Cl-, SO42", Fe3+ and EDTA in the solution surroundings can change the growth style and the final morphology of calcium sulfate crystals.Finally, the thermal conversion of hemi-hydrate calcium sulfate whiskers and the sintering technology to anhydrous calcium sulfate (CaSO4) whiskers were studied. TG and DSC analysis for the hydrothermal products formed at different temperature and time indicated that in heating process, the hemi-hydrate calcium sulfate was first converted to anhydrous calcium sulfate by removing the crystalline water; the anhydrous calcium sulfate was then converted to glass states and then to several crystalline states, even decomposed to calcium oxide at high temperature. The’ temperature range for the formation of the pure stable calcium sulfate whiskers from the hemi-hydrate calcium sulfate whiskers was established. The thermal conversion kinetic of the hemi-hydrate calcium sulfate whiskers to anhydrous calcium sulfate whiskers was studied and the corresponding kinetic model was established. The research work indicated that the above removing water conversion process was carried out according to the random shape-nuclear growing control mechanism. On the basis of studying the thermal conversion process, the influence of the typical sintering parameters such as the heating rate, temperature, time on the morphology and structure of anhydrous calcium sulfate was studied and the following sintering condition was suggested for formation of the anhydrous calcium sulfate whiskers with stable properties and uniform morphology:heating rate:4.5℃/min, temperature:600℃, time:2h. |
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