The Research On Formation Of Calcium Sulfate Whisker Using Phosphogypsum

At present, the total number of phosphorusgypsum storaged in our country is 200 million tons, and about 80 million tons of phosphorusgypsum newly increased with the development of compound fertilizer, the utilization rate is only 20%, and how to make effective use of the phosphorusgypsum is still a technical challenge. The use of phosphorusgypsum preparation environmental friendly and widely used calcium sulfate whisker is one of the high value utilization direction.In this thesis the influence of acid soak-roasting on the properties of gypsum (impurity content, whiteness, particle size) and the process optimization of hydrothermal formation of the CaSO4·0.5H2O whiskers with high aspect ratio. And mainly studied the influence of the impurities of phosphorous, iron and aluminum on hydrothermal formation of hemihydrate calcium sulfate whiskers, and at last the stability of calcium sulfate whisker prepared in this system.Firstly, the influence of the impurities of phosphorous, iron and aluminum on hydrothermal formation of hemihydrate calcium sulfate whiskers was studied. The resulted indicated that phosphorous and aluminum have significant effects on the morphology of whiskers, the addition of phosphorous accelerated the hydrothermal conversion of CaSO4·2H2O to via the formation of Ca3(PO4)2, producing CaSO4·0.5H2O whiskers with thinner diameter and shorter length. The addition of aluminum makes the [SO42-] more than [Ca2+] and suppresses the whisker growth on the C axis. In the presence of iron, the content of aluminum must be controlled within 0.5%.CaSO4·0.5H2O whiskers with the length of 100～300μm, a diameter of 1.0～10.0μm were achived after the pry-treatment in the condition of 20℃,5% HCl and 3.0 h. Then studied the possibility of acid soak-dry to prepare the CaSO·0.5H2O whiskers with high aspect. It turned out that CaSO4·0.5H2O whiskers with the length of 50～150μm, a diameter of 0.1～1.0μm were achived based on acid soak prtreatment and then in 150℃ dry 2.0 h.Secondly, the influencing factors of phosphorusgypsum whiteness were studied. The experiment work indicated that calcination temperature less than 600℃, carbon and aromatic organic compounds are the key factors that affect the whiteness, and inverse, is iron. The whiteness of phosphorusgypsum improved significantly after acid soak-roasting treatment. The dissolution rate of impuries phosphorous, iron and aluminum respectively are 58% (soluble phosphorous),28% and 65%, and the content of iron and aluminum decreased to 0.35% and 0.28% in the condition of 60℃,20% HCl pretreatment for 3.0 h, and the whiteness reached 89.0 roasted in the temperature of 800℃.Finally, the stability of calcium sulfate whisker prepared in this system were studied. The experiment result shows that the conversion of CaSO4·0.5H2O to CaSO4·2H2O occurred within 1.0 h, and after 4.0 h, all CaSO4·0.5H2O whiskers translated into CaSO4·2H2O plates. CaSO4 whiskers (aspect ratio is a little change compared with CaSO4·0.5H2O whiskers) can be stable in solution for 52.0 h without morphology and phase change were achieved via 6.0 h roasting in 600℃.