Study On The Pretreatment Of Phosphogypsum And The Preparation Of Calcium Sulfate Whiskers

In view of the huge demand for industry and agriculture,the production capacity of phosphate fertilizer will not decrease too much in a short period of time,which will lead to huge production of its solid waste phosphogypsum.As of 2017,the national phosphate gypsum stockpiles have exceeded 500 Mt.The massive accumulation of phosphogypsum not only occupies a large amount of land resources,increases the maintenance cost of enterprises,but also exerts tremendous pressure on the ecological environment.With the contraction of national environmental protection policies,research on the efficient use of phosphogypsum has become a hot topic.Since the impurities existing in phosphogypsum affect its application,it is of great practical significance to carry out the impurity removal of phosphogypsum and to prepare high value-added calcium sulfate whiskers.In this paper,industrial phosphogypsum was used as raw material,the phosphogypsum was purified by hydraulic separation pretreatment and sulfuric acidtributyl phosphate combined treatment,and the preparation of calcium sulfate whiskers from the removing impurities phosphogypsum was studied.(1)The hydraulic separation pretreatment of phosphogypsum was studied,in which the liquid-solid ratio is 40:3(water consumption is 400 g,phosphorus gypsum dosage is 30 g),the yield of hydraulic separation impurity removal products reached the highest(70.13%),the whiteness reached 75.78%,the removal rates of silicon,phosphorus,potassium and aluminum reached respectively 39.0%,39.2%,60.9% and 53.7%,and the impurity removal effect was better than that water washing removing impurity.The hydraulic separation impurity removal phosphogypsum was used as raw material,the effect of the precursor prepared by the grinding-drying process on the preparation of whiskers by hydrothermal was studied.The suitable process conditions were obtained: the particle size of the grinding material ?59.04 ?m,the drying temperature ?150 ?,and the drying time ?2 h.The phosphogypsum by hydraulic separation-grinding-drying pretreatment is used as a precursor,the hydrothermal preparation of calcium sulfate whiskers was carried out.Under the suitable conditions of solid content of 0.71%,hydrothermal temperature of 150 °C,hydrothermal system pH 3.5 and hydrothermal time of 4 h,prepared high quality calcium sulphate hemihydrate whisker which has an average length of 388.6 ?m,an average aspect ratio of 62.68,a whiteness of more than 88%,and a uniform morphology.The effects of adding magnesium chloride and ethanol on the whiskers of hydrothermal products were investigated,and it was found that none of them could effectively improve the quality of calcium sulfate whiskers.(2)Treated phosphogypsum by sulfuric acid-TBP combining.After combined impurity removal,only a small amount of silicon,phosphorus and aluminum impurities remain in the product,and the corresponding removal rates reached respectively 99.33%,96.7% and 95%.The combined treatment has very good removal effect on the organic and inorganic impurities of phosphogypsum,an anhydrous calcium sulfate powder having a high purity could be obtained.On the basis of combined removing impurity,the technological experiment was carried out on treating cyclically phosphogypsum with combination agent(sulfuric acid-TBP)and by-producing calcium sulfate whisker.The effect of the combination agent recycling on phosphogypsum impurity removal remained basically unchanged.The whiteness of the impurity removal product of the cycle 8 times was still greater than 94%.Treatment under acidification conditions could by-produce calcium sulphate dihydrate whisker: whisker length between 100~1814 ?m,diameter between 8~40 ?m,whiteness >91%.(3)Using combined impurity removal phosphogypsum as solid raw material,sulfuric acid as acidic solution,prepared calcium sulfate whiskers by solid-liquid cyclic acidification.Acidification treatment under the condition of solid-liquid ratio=1:10,acidification temperature=90 ?,time=30 min and hot filtration were carried out,and the filtrate was aged for 4 hours,and then separated by filtration to obtain calcium sulfate whiskers.Performing nine times solid-liquid cycles,it could prepare calcium sulfate dihydrate whisker with an average length greater than 700 ?m,an average aspect ratio greater than 34,a whiteness greater than 95%,and less surface defects.