Study On Preparation Of Calcium Sulfate Hemihydrate Whiskers By Autoclaved Phosphogypsum

The massive accumulation of phosphogypsum has caused serious environmental pollution and increased land costs.How to efficiently use industrial waste phosphogypsum has become an urgent problem to be solved.In this paper,solid waste phosphogypsum is used as the raw material,and the high-value calcium sulfate hemihydrate whiskers are prepared by the autoclave method.By studying the autoclave temperature,autoclave time,average particle size,water-solid ratio and other key technical parameters.The influence of calcium sulfate whiskers in water determines the key technical parameters for the optimal preparation of calcium sulfate hemihydrate whiskers.On this basis,the effects of autoclave time,p H value,and different impurity phosphogypsum on calcium sulfate hemihydrate whiskers were further studied.With the help of micro-technical methods such as XRD,XPS,FTIR,etc.,the inorganic crystal conversion agent（K₂SO₄,Mg SO₄and Na₂SO₄）and organic crystal conversion agents（sodium citrate,sodium stearate and EDTA-tetrasodium）on the influence of calcium sulfate hemihydrate whiskers.Finally,molecular dynamics simulation was used to study the microscopic mechanism of the effect of the crystal conversion agent on the different crystal faces of calcium sulfate hemihydrate whiskers.The main conclusions of the paper are as follows:1.Using phosphogypsum as raw material,the best key technical parameters for preparing calcium sulfate hemihydrate whiskers by autoclave are:autoclave time of 3h,autoclave temperature of 150℃,average particle size of 0.30mm and water-solid ratio of 5:1,that is,the average length of calcium sulfate hemihydrate whiskers is 42.2μm and the average aspect ratio is 34.32.2.The phosphogypsum with different impurities in Yichang area can be used to prepare calcium sulfate hemihydrate whiskers meeting the"Hunan Provincial Local Standard"（DB43/T 1155-2016）;when the optimal autoclave time is 3h,it is conducive to the preparation of high and long diameters.Compared with calcium sulfate hemihydrate whiskers;acidic solutions are beneficial to the growth of calcium sulfate hemihydrate whiskers,while alkaline solutions are not conducive to the preparation of calcium sulfate hemihydrate whiskers.3.Research on the effect of key technical parameters on calcium sulfate hemihydrate whiskers shows that different metal ions can produce calcium sulfate hemihydrate whiskers with high aspect ratio,and the whiskers are fully developed and their lengths are increased,that is,different metal ions.Key technical parameters have a certain promotion effect on the growth of calcium sulfate hemihydrate whiskers;sodium citrate,sodium stearate and EDTA-tetrasodium all have a certain promotion effect on the axial growth of calcium sulfate hemihydrate whiskers,but citric acid Sodium inhibits the radial growth of calcium sulfate hemihydrate whiskers relatively weakly.The order of the effect of metal ions on calcium sulfate hemihydrate whiskers is:K⁺<Mg²⁺<Na⁺;the order of the effect of different carboxyl key technical parameters on calcium sulfate hemihydrate whiskers is:sodium citrate<EDTA-tetrasodium<Sodium stearate.4.XRD analysis shows that different metal ions have a certain promotion effect on the growth of calcium sulfate hemihydrate whiskers along the C axis,but sodium ions and potassium ions have a greater effect on magnesium ions,and along the B axis,different metal ions have a positive effect on the growth of calcium sulfate hemihydrate whiskers.The effect of calcium sulfate hemihydrate whiskers is basically the same;the effect of different carboxyl key technical parameters on the crystal plane of calcium sulfate hemihydrate whiskers along the C axis is opposite to the number of carboxyl groups,and different carboxyl key technical parameters all have the same effect on calcium sulfate hemihydrate whiskers along the C axis.The growth of calcium sulfate has a certain promoting effect,and only sodium citrate is conducive to the growth of calcium sulfate hemihydrate whiskers along the B axis.XPS analysis shows that all metal ions can replace the calcium ions in the calcium sulfate hemihydrate crystal lattice in a relatively small amount;different carboxyl key technical parameters have basically the same effect on the growth of calcium sulfate hemihydrate whiskers.FTIR analysis shows that stearate ions only chemically react with calcium ions and do not produce adsorption;the amount of citrate ions adsorbed on the surface of calcium sulfate hemihydrate whiskers is very small or the citrate ions complexed with calcium ions on the surface are extremely small;EDTA acid radical ions may participate in the chemical reaction of calcium ions,and may also participate in the complex reaction of calcium ions on the surface,but the amount of adsorption is relatively small.5.The results of the study on the microscopic mechanism of key technical parameters acting on different crystal faces of calcium sulfate hemihydrate crystals show that potassium ions can replace calcium ions in a small amount and enter the crystal lattice of calcium sulfate hemihydrate,and have little effect on the growth of calcium sulfate hemihydrate whiskers.Both magnesium ions and sodium ions can promote the growth of calcium sulfate hemihydrate whiskers along the C axis.The order of interaction energy between different carboxyl key technical parameters and calcium sulfate hemihydrate whiskers in the C-axis direction crystal planes is:ΔE_{（EDTA-tetrasodium）}<ΔE_{（sodium citrate）}<ΔE_{（sodium stearate）},It shows that the adsorption of different carboxyl key technical parameters on the crystal plane of calcium sulfate hemihydrate whiskers along the C axis has a positive correlation with the number of carboxyl groups.