Phosphogypsum Carbonation-precipitation-hydrothermal Preparation Of Calcium Sulfate Whisker Process

With the rapid growth of compound fertilizer' production in China,emissions of the phosphogypsum(PG)also increased.The annual emission of PG had reached 74 million tonnes by 2014,and Yunnan province as one of richest in phosphorus resources,which accounted for 1/5 of the country's total emissions of PG,However,the comprehensive utilization rate of PG was less than 20%.Improving the comprehensive utilization rate of PG is not only the pressing issue that phosphate and compound fertilizer industry must face,but also an effective solution to environmental and resource problems.Since preparation of calcium sulfate whisker by PG is an efficient way to use solid waste initiatives of phosphate and compound fertilizer enterprises,it becomes a hot topic.In this paper,phosphogypsum as raw material,a new process of carbonation-coprecipitation-hydrothermal was used to prepare calcium sulfate whiskers.Firstly,the effects of the conditions of carbonation-coprecipitation-hydrothermal process on whiskers9 morphology were studied.Optimal conditions of PG carbonation conversion process:sodium carbonate as carbonating agent,excess coefficient of carbonate 1.2,conversion time 1.5 h and conversion temperature 45 ?5 PG carbonation conversion rate exceeded 99%under above conditions;Optimal conditions of coprecipitation process:precipitating agent chloride calcium 0.4 mol·L-1,reaction temperature 25 ?,molar ratio of calcium to sulfur 2,and reaction time 3 h,the main component of gypsum precursor obtained under this conditions is calcium sulfate dihydrate with particle size about 20 ?m;calcium sulfate hemihydrate whiskers with aspect ratio 150,98.9%purity,whiteness 97.0%could be prepared under optimum conditions of hydrothermal temperature 150 ?,hydrothermal time 4h,pH 3~4 adjusting by carboxylic acid and solid holdup 2%.Secondly,the effects of soluble phosphorus,iron and aluminum impurities on hydrothermal formation of calcium sulfate whiskers were researched.Soluble phosphorus content less than 1.0%favored in synthesis of calcium sulfate hemihydrate whisker;the presence of iron impurities have little effect on whisker morphology;but the presence of aluminum impurities have significant effect on whisker morphology,the larger content of Al2O3 exist in system solution,the smaller[Ca2+]/[SO42-]became,furthermore,Al(OH)3 would precipitate,which is not conducive to axial growth of calcium sulfate hemihydrate whisker.So aluminum impurities should be avoided as much as possible in the hydrothermal preparation process of calcium sulfate hemihydrate whisker.Finally,the influence and mechanism of formic acid compounded with metal ions(Mg2+,Fe3+,Cu2+)on whisker morphology were investigated.The product with diameter 2.0(im,mean length 388 ?m,the maximum aspect ratio 194 was obtained when systems pH 3.42 and addition amount of MgClh 0.5%(wt)5 for preliminary conjecture,the mechanism of Mg2+ impacting on whisker morphology involved the selective adsorption mechanism and common ion effect;the mean diameter of crystals was 1.9 ?m,and the maximum aspect ratio could reach 185.26 with addition amount of FeCl3 1.0%wt,the mechanism of Fe3+ affecting on whisker morphology essentially represented iron into the whisker lattice for displacement type;the product with diameter 2.0 ?m,mean length 408 ?m,the maximum aspect ratio 204 was obtained when addition amount of CuCl2 1.0%(wt),the initial guess that the mechanism of Cu2+ affecting on whisker morphology registered as selective adsorption mechanism of crystal face was concluded.