Study On Preparation Of Structure And Morphology Of Calcium Sulfate Hemihydrate Based On Phosphogypsum Calcium Sulfate Dihydrate

The main component of phosphogypsum as a by-product of the acid industry of phosphate rock leaching is CaSO₄·2H₂O,and its resource utilization is a worldwide problem today.Taking phosphogypsum raw materials as the research object and using the utilization of phosphogypsum as the research background,the research on the growth process of CaSO₄·2H₂O,?-CaSO₄·0.5H₂ O crystals and the controlled preparation of ?-CaSO₄·0.5H₂ O by the transformation of CaSO₄·2H₂O have been carried out which has important scientific value and application potential.In this paper,we study the controlled preparation of ?-CaSO₄·0.5H₂ O by the conversion of CaSO₄·2H₂O without chloride salt at atmospheric pressure.The preparation strategies of ?-CaSO₄·0.5H₂ O with different shapes such as rod,column,granule,flake and flower were studied.The relationship between CaSO₄·2H₂O,?-CaSO₄·0.5H₂ O crystal structure,morphology,size and performance was also studied.In the preparation of CaSO₄·2H₂O system by CaHPO₄-H₂SO₄ reaction,the effect of Al3+ addition on CaSO₄·2H₂O crystal morphology,size and process regulation was studied.The average crystal length decreased from 71.4 ?m to 46.7 ?m,the average crystal width increased from 5.2 ?m to 32.3 ?m,and the aspect ratio decreased from 14.3: 1 to 1.4: 1 which causing the morphology of CaSO₄·2H₂O crystals changed from rod-shaped to thick flaky by Al3+.The separation rate of CaSO₄·2H₂O slurry prepared with the assisted of Al3+ increased from 4.56×10-4 m3/?s·m2?to 23.51×10-4 m3/?s·m2?,which was 5.2 times higher than the original.Al3+ is incorporated into the CaSO₄·2H₂O lattice gap.In the preparation of CaSO₄·2H₂O system by Na₂SO₄-CaCl₂ reaction,the influence of the doping of amorphous cation Cu²⁺ on the morphology,size and process regulation of CaSO₄·2H₂O crystal was studied.Studies have shown that Cu²⁺ doping significantly inhibits the growth of CaSO₄·2H₂O crystals in two dimensions,forming CaSO₄·2H₂O crystals with smaller dimensions and lower aspect ratios,whose average length is reduced from 197.5 ?m to 35.2 ?m,and the average width from 20.3?m to 6.1?m,the aspect ratio is reduced from 9.7: 1 to 5.7: 1.At the same time,the equilibrium solubility of the CaSO₄·2H₂O crystal was increased due to the presence of Cu²⁺,which increased the interfacial tension of the crystal,reduces the nucleation rate of the crystals,thereby prolonging the nucleation induction time of the CaSO₄·2H₂O crystals and induce heterogeneous nucleation of crystals.Cu²⁺ can be doped into the interstitial space of CaSO₄·2H₂O,causing the unit cell volume of the crystal to increase and the unit cell to be distortedUsing phosphogypsum?I-PG?as raw material,a small amount of potassium tartrate?PT?was used as a crystal phase transition agent and morphology directing agents to prepare ?-CaSO₄·0.5H₂ O crystals.The morphology and size of ?-CaSO₄·0.5H₂ O crystals could be effectively controlled.The average length of the crystals decreased from 105.3 ?m to 21.8 ?m,the average width increased from 2.1 ?m to 20.5 ?m,and the aspect ratio decreased from 52: 1 to 1: 1.When the ?-CaSO₄·0.5H₂ O crystals transformed from CaSO₄·2H₂O,?-CaSO₄·0.5H₂ O crystals nucleation process occurred with CaSO₄·2H₂O crystals dissolution process together.The tartaric acid ion in the water phase preferentially complexes Ca²⁺ on the?001?and?002?crystals plane of the top of ?-CaSO₄·0.5H₂ O crystal,and inhibits the ?-CaSO₄·0.5H₂ O crystal growth along the c-axis direction,which the preparation of controllable morphology and aspect ratio ?-CaSO₄·0.5H₂ O crystals were realized.Using phosphogypsum as the raw material,Na₂SO₄ as the phase transition agent,and a small amount of trisodium citrate?TSC?as the crystal morphology directing agent to prepare ?-CaSO₄·0.5H₂ O crystals.In a 1.5 M Na₂SO₄ aqueous solution at 98 ° C and 200 m M TSC,regular hexagonal flake ?-CaSO₄·0.5H₂ O crystals were prepared,and the yield was close to 100%.The study found that ?H₂O was reduced due to the increased of Na₂SO₄ concentration,and caused the increased of S DH-?-HH which promotes the conversion of CaSO₄·2H₂O crystals to ?-CaSO₄·0.5H₂ O crystals in the aqueous solution and accelerates the nucleation of ?-CaSO₄·0.5H₂ O crystals;With the increase of TSC concentration,the growth of ?-CaSO₄·0.5H₂ O crystals along the c-axis in the aqueous solution was inhibited,which resulted in the transformation of ?-CaSO₄·0.5H₂ O crystals from rod-shape to regular hexagonal plate-like crystals.The control of crystal morphology was attributed to the strong adsorption of citrate ions to the?001?and?002?crystal planes by chemical adsorption;The evolution of the morphology of ?-CaSO₄·0.5H₂ O crystals from rods to regular hexagonal flakes increased its 3d compressive strength to 46.1MPa,182.8% increased in contrast.Using Na₂SO₄ and CaCl₂ as crystalline system and TSC as the morphology directing agent,a two-dimensional flower-shaped ?-CaSO₄·0.5H₂ O crystal was prepared,which average width is 45.1 ?m,and the yield is close to 100%;By simply adjusting the TSC concentration in the crystal system,?-CaSO₄·0.5H₂ O crystals with various morphologies?rod-shaped,column,granular,plate-shaped,and flower-shaped?and sizes were prepared.?-CaSO₄·0.5H₂ O crystals could be used for the adsorption and separation of Pb²⁺ in the aqueous solution.Compared with rod-shaped and plate-shaped crystals,flower-shaped ?-CaSO₄·0.5H₂ O crystals have better adsorption effect on the Pb²⁺.Meanwhile,flower-shaped ?-CaSO₄·0.5H₂ O crystals could be used as an efficient separation of trace amounts of water in water-in-oil microemulsions which the fix the efficiency?IE?values were over 92% and the separation efficiency?SE?values were over 99%.