Mechanism Of Transforming Agent In The Semi-liquid Method For Preparation Of α-Hemihydrate Gypsum By Phosphogypsum

Phosphogypsum is a solid waste produced in the process of industrial production of phosphoric acid.The annual discharge of phosphogypsum is large and the utilization rate is low in China.Using phosphogypsum to replace natural gypsum to prepareα-hemihydrate gypsum is one of the research directions for resource utilization of phosphogypsum.This topic uses phosphogypsum semi-liquid method to prepareα-hemihydrate gypsum,mainly studies the key technical parameters of water-solid ratio and slurry p H value,the effect of the type and amount of crystal conversion agent on the preparation ofα-hemihydrate gypsum,and uses Materials Studio software establishes the molecular dynamics model of the crystal face of the crystal conversion agent and hemihydrate gypsum,and explores the mechanism of the crystal conversion agent in combination with the macroscopic experiment and the microscopic characterization.The main results achieved are as follows:（1）The effects of water-solid ratio and slurry p H value on the crystal morphology and basic properties ofα-hemihydrate gypsum prepared by semi-liquid phase method have been deeply studied.The results show that too high or low water-solid ratio is not conducive to crystal growth.The p H of the slurry has little effect on the crystal morphology ofα-hemihydrate gypsum.Theα-hemihydrate gypsum prepared with water-solid ratios of 23%,33%,43%,53%,80%and slurry p H of 2,4.4,6.7 all meet the requirements of the strength grade ofα30 in the specification for high-strength gypsum.When the water solid ratio is 33%,when the p H of the slurry is 4.4,the energy consumption is small,the production rate is high,and the compressive strength of theα-hemihydrate gypsum is 38 MPa.（2）Compared with the blank group without crystal conversion agent,the incorporation of organic acid crystal conversion agent makesα-hemihydrate gypsum crystals change from long rods to short columns,and the standard consistency of water consumption is reduced from 45%to 40%.Significantly increased.Succinic acid has the best effect in the organic acid group.The strength first increases and then decreases with the increase of succinic acid content.The best content is 0.15%,and the flexural/compressive strength is 7MPa/49.9MPa.If the diacid content is too large,the dehydration reaction of phosphogypsum will be inhibited.The incorporation of inorganic salt crystal conversion agent has little effect on the crystal morphology and standard consistency ofα-hemihydrate gypsum,but it can promote the dehydration of phosphogypsum and increase the content ofα-hemihydrate gypsum,so the strength is significantly improved.In the inorganic salt group,potassium sulfate has the best effect,and the strength increases with the increase of potassium sulfate content.The flexural/compressive strength is 5.2MPa/44MPa when the content of potassium sulfate is0.2%.XRD patterns,FTIR spectra and crystal plane parameters confirmed that the crystal conversion agent did not change the chemical composition of the crystal.（3）The results of molecular dynamics model of crystal conversion agent and hemihydrate gypsum show that succinic acid and potassium sulfate have adsorption effect on the main crystal faces of hemihydrate gypsum crystals,but the end faces of succinic acid and hemihydrate gypsum crystals（002）interact with each other.It has the greatest effect,so it has a strong inhibitory effect on growth in the c-axis direction.The interaction energy between potassium sulfate and the（110）crystal face of hemihydrate gypsum crystal is the largest,so potassium ions will preferentially adsorb to the negatively charged SO₄^2-exposed on the side of the crystal to inhibit the radial growth of the crystal.（4）Mechanism of organic acid crystal conversion agent:succinic acid can adjust the crystal ofα-hemihydrate gypsum crystals by adsorption.The carboxyl group in the organic acid crystal conversion agent preferentially adsorbs to the（002）surface of the crystal.It is complexed with Ca²⁺to inhibit the growth of crystals along the c-axis,and finally short columnarα-hemihydrate gypsum crystals are obtained,and the water consumption for standard consistency is reduced.At the same time,the carboxyl group ionized by the organic acid crystal conversion agent interacts with Ca²⁺to form a small amount of insoluble matter,which promotes the dissolution of phosphogypsum in the solution,increases the content of the productα-hemihydrate gypsum,and significantly improves the strength.（5）The mechanism of the inorganic salt crystal converter:the inorganic salt crystal converter has no effect on the crystal morphology ofα-hemihydrate gypsum.However,the incorporation of inorganic salt crystal conversion agent will promote the dissolution of phosphogypsum and increase the content ofα-hemihydrate gypsum in the product,so the strength is significantly improved.