Hydrothermal Synthesis And Stabilization Of Calcium Sulphate Whiskers

As an environmental friendly and cheap material, calcium sulfate whiskers havegreat potential to be widely used as polyurethane, plastics, ceramics and cements,owing to their special properties of toughness, high electrical insulation, high strength,resistance to temperature, chemical stability, acid and alkali resistance and otheradvantages. The study of the technology and mechanism of producing stable anduniform calcium sulphate whiskers from the industrial by-product desulphurizationgypsum is to favor the full multi-utilization of the industrial wastes as well as toprovide the theoretical basis for the formation and application of calcium sulphatewhiskers.In this work, hydrothermal method was applied in the preparation of hemihydratecalcium sulfate whiskers using industrial by-product desulphurization gypsum as rawmaterial. Effects of additives, reaction temperature and granularity on the calciumsulfate whiskers were systematically studied. Moreover, the stabilization treatment tothe prepared hemihydrate calcium sulfate whiskers has also been carried out. Themain work and results of the thesis are as follows:1. The effects of different additives, including magnesium chloride, citric acid,sodium citrate, hexadecyl trimethyl ammonium bromide (CTAB), dodecyl benzenesulfonate (SDBS) and sodium oleate on the morphology of hydrothermal product wereinvestigated with industrial by-product desulphurization gypsum as a raw material.Results show that the magnesium chloride has no obvious promote role on the growthof whiskers. Citric acid, sodium citrate and CTAB will restrain the growth of thewhisker; SDBS has a certain promoting role on the growth of whiskers. The optimumadding amount of SDBS is3%, but the whisker radial relative to the shaft has somegrowth, and the product is not uniform. Sodium oleate has better promotion on thewhisker growth. The addition of4%sodium oleate can obtain the whiskers uniform inmorphology, smooth in surface,1μm in diameter and70in length to diameter. Themechanism of sodium oleate on the growth of half water calcium sulfate crystals is:the sodium oleate in hydrothermal system with oleic acid dihydrate gypsum reactionof calcium, promote the dissolution of dihydrate calcium sulphate, and oleic acidcalcium absorption on the (002) face of calcium sulphate whikers, promoteing thegrowth along the C axis.(2) The conditions of hydrothermal process including reaction temperature and time, concentration of slurry, the granularity of raw material, stirring speed, fillingamount were determined for the preparation of half water calcium sulfate whiskerwith sodium oleate as additives. The optimum parameters is: at temperature130℃for3h under a concentration of5%slurry, and1.15μm particle size, stirring speed of60r/min and a filling rate of62.5%.(3) The stabilizing treatment of half water calcium sulphate whikers were carriedout through calcining at high temperature or surface modification on the whiskers.Results show that the half water calcium sulfate whisker calcined at700℃for4htransformed into anhydrous calcium sulphate whikers, but no change in themorphology of whiskers. In several kinds of modifier, the stearic acid is the best tothe modification of calcium sulfate whisker, with an activation index of0.64and acontacting angle of49.8°, and without change in the compositions and ingredients ofwhiskers. Stearic acid exists on the surface of calcium sulfate whisker by bothphysical and chemical adsorption. The modification mechanism of stearic acid andadsorption model are Briefly established.