Preparation Of α-high Strength Gypsum From Phosphogypsum Under Atmospheric Hydrothermal And Its Crystal Regulation Mechanism

The accumulation of phosphogypsum occupies land resources and damages the ecological environment.Therefore,how to accelerate the resource utilization of phosphogypsum is the focus of current research.The preparation ofα-hemihydrate gypsum（α-HH）from phosphogypsum is a hot topic in the application and development of phosphogypsum in China.In this paper,phosphogypsum was used as raw material to prepareα-HH by atmospheric pressure hydrothermal method.The influence of process conditions,salt medium,and crystallization agent on the crystal morphology,crystallization rate,and mechanical properties ofα-HH was studied.The mechanism of crystal morphology regulation ofα-HH was revealed by molecular dynamics simulation combined with macroscopic experiment and microscopic characterization.A theoretical method for efficient screening of crystallization agents is proposed.The main work and achievements of this paper are as follows:（1）The influence of process parameters on the preparation ofα-HH was investigated and the best washing method of the product was given.The theoretical salt content of the product was 0.003%.The decrease in the p H value of the salt solution can accelerate the crystallization rate of phosphogypsum,increase the length-diameter ratio ofα-HH,and reduce the mechanical properties of the product.The increase in phosphogypsum slurry concentration will slow down the crystallization rate of phosphogypsum and improve the mechanical properties of the product.Therefore,when the initial p H value of the salt solution is controlled at 5.5-7 and the concentration of phosphogypsum slurry is 30%,the production efficiency,equipment corrosion,and mechanical properties of products are the best.（2）The effect of salt on the preparation ofα-HH was comprehensively evaluated.The order of crystallization rate of phosphogypsum under different cations was as follows:Na⁺>K⁺>Mg²⁺≈Ca²⁺,phosphogypsum in Mg Cl₂ and Ca Cl₂ salt solution does not occur crystallization,the product after heat treatment isβ-hemihydrate gypsum,phosphogypsum in KCl solution dehydration crystallization product is potassium gypsum（K₂Ca₅（SO₄）₆·H₂O）,the cation effect on mechanical properties of the product is not obvious.The order of crystal conversion rate of phosphogypsum under different anions is SO₄^2->Cl^->NO₃^-.A high concentration of sodium salt should not be used to prepareα-type high-strength gypsum,and the anion has no significant effect on improving the mechanical properties ofα-HH.Phosphogypsum was dehydrated and trans-crystallized in the 25%Ca Cl₂-15%Mg Cl₂-0.2%KCl composite salt solution for 2h,and the 2h flexural/absolute dry compressive strength was3.24/28.6MPa.（3）In Ca-Mg-K composite salt solution,maleic acid,succinic acid,and sodium citrate can effectively regulate the crystal morphology ofα-HH,which changes from a long rod and needle to a hexagonal short column.The effect of EDTA on regulating the crystal morphology ofα-HH is not obvious.The dehydrated crystallization rate of phosphogypsum under different crystallization agents was as follows:EDTA≥blank group≥sodium citrate>succinic acid>maleic acid.Theα-HH crystal prepared by 0.03%maleic acid and succinic acid has the smallest particle size and the best mechanical properties.The 2h flexographic/absolute dry compressive strength ofα-HH crystal is 4.65/47.5MPa and 4.15/42.3MPa,respectively.The compressive strength ofα-HH crystal can reach the grade ofα40 in the high-strength gypsum specification.（4）The molecular dynamics results show that the absolute value of the interaction energy on the（002）plane ofα-HH in an aqueous solvent is the smallest,so the inhibition effect on the（002）plane is the weakest.α-HH in aqueous solvent is hexagonal columnar with a high aspect ratio.The regulating mechanism ofα-HH crystal morphology is different with different crystallization agents.The mechanism of maleic acid regulation is that two carboxyl groups coordinate with a Ca atom of the three crystal faces at the end ofα-HH crystal to inhibit the growth rate ofα-HH in the C-axis direction.The crystal regulation mechanism of succinic acid is mainly through the coordination of two carboxyl groups of succinic acid molecules with two Ca atoms on the（002）plane and the formation of the stable ring structure.The crystal regulation mechanism of EDTA is mainly through the coordination between two O atoms of an adjacent carboxyl group and two Ca atoms of theα-HH（002）plane,forming a stable ring of organic calcium complex,which inhibits the growth rate ofα-HH along the C-axis.The crystal regulating mechanism of sodium citrate is mainly through the stable adsorption of two adjacent carboxyl groups on the（002）surface to inhibit the growth ofα-HH crystals along the C-axis.（5）In this paper,the morphology regulation coefficient of the crystallization agent is introduced,which is calculated as the ratio of the average interaction energy of the rotating agent on the important crystal surface of theα-HH end to the average interaction energy of the rotating agent on the important crystal surface of theα-HH crystal side.The morphology regulation coefficient of the different rotating agents in the order from large to small is Maleic acid>succinic acid>Sodium citrate>EDTA.Maleic acid has the strongest effect on inhibiting the growth ofα-HH crystals along the C-axis,followed by succinic acid and sodium citrate,and EDTA.The results are in good agreement with the actual experimental results,and the results have a certain guiding significance for screening efficient crystallization agents.