| Phosphogypsum(PG)is a solid waste in the production of industrial wet-process phosphoric acid,and its main component is CaSO4·2H2O(DH).The annual discharge of phosphogypsum in my country is about 75 million tons,and more than 50%of it is stored for a long time,causing water resources and soil pollution and increasing environmental pressure.Alpha hemihydrate gypsum has the advantages of high strength and strong corrosion resistance;It has a wide range of uses in the fields of gypsum relief,gypsum ceiling board,high-grade medical gypsum,and mold gypsum.Expanding the use of phosphogypsum is the only way to improve the quality of gypsum and the comprehensive utilization rate of phosphogypsum.In view of the problem of large alcohol consumption in the preparation of hemihydrate gypsum from a single alcohol system,potassium sulfate and zinc nitrate are introduced into the reaction solution.On the one hand,the low content of potassium sulfate and zinc nitrate can reduce the alcohol consumption,and on the other hand,the mixing of potassium sulfate into can accelerate the reaction rate to change the crystal form of gypsum.This thesis systematically studies the single-factor study of the alcohol-free chlorine salt system phosphogypsum for the preparation of?-hemihydrate gypsum,the study of the crystal conversion agent and the crystal surface modification of the coupling agent,and the basic physical properties of the gypsum are explored;Three different types of modifiers,namely naphthalene-based water-reducing agent(FDN),polycarboxylic acid water-reducing agent(PS),and melamine water-reducing agent(SMF)were used to investigate the different incorporation methods of this type of modifier,The effect of the preparation of?-hemihydrate gypsum and the performance of hydration test block was investigated.The influence of the amount of water reducing agent on the water-reducing rate of?-hemihydrate gypsum,water consumption of standard consistency and absolute dry strength was investigated.The conclusions obtained are as follows:1.Investigation on the conditions for preparing?-hemihydrate gypsum:The experiment explored the dehydration and conversion conditions of dihydrate gypsum in the alcohol-free chloride salt system.The obtained dehydrat ion conditions were:zinc nitrate concentration 15%,alcohol-water ratio 15%,temperature 95?,The liquid-solid ratio is 5:1,and the pH of the solution is 6.In the alcohol-salt dehydration system,as the content of potassium sulfate increases,the dehydration rate of dihydrate gypsum increases.When the content of potassium sulfate is 3%,it takes 4 hours for the crystal water content to reach 6.31%(close to the theoretical crystal water content of 6.20%).On the basis of determining the dehydration conversion conditions,the orthogonal experiment was used to optimize the process conditions of the experiment,and the experimental results were analyzed by the range:the order of influence of each factor is liquid-solid ratio,temperature,alcohol-water ratio,zinc nitrate content,potassium sulfate dosage,pH of the solution;liquid-solid ratio and temperature have a greater impact on the experiment.The optimal process conditions for the reaction are:liquid-solid ratio of 5:1,temperature of 105?,alcohol-water ratio of 25%,zinc nitrate content of 25%,potassium sulfate content of 3%,solution pH of 6;The basis for optimizing the conditions here.The conversion rate of the dehydrated dihydrate gypsum to?hemihydrate gypsum is 98%.The amount of potassium sulfate has a certain effect on the microscopic morphology of?-hemihydrate gypsum.With the increase of the amount of potassium sulfate,the crystals change from needle-like bodies to long columnar bodies.The reaction time is 4 h when the amount is 0.5%.The ratio of length to warp is 15:1,and the reaction time is 4 hours when the content is 3%,and the ratio of length to diameter is shortened to 5:1.2.The influence of inorganic salt crystal conversion agent on the basic physical properties of?-hemihydrate gypsum is sodium citrate?magnesium sulfate?aluminum sulfate,and the influence of organic acid crystal conversion agent is sodium citrate?sodium acrylate?ethylenediamine.The order of influence of disodium tetraacetic acid and organic salts is potassium hydrogen phthalate<phthalic anhydride.Through the SEM and XRD analysis between aluminum sulfate,phthalic anhydride,and disodium edetate in the three types of crystal conversion agents,0.45%of edetate is obtained disodium has the best effect and the highest crystallinity in the preparation of?-hemihydrate gypsum in the alcohol-free chlorine salt system.Phosphogypsum is used as the dehydration raw material of?-hemihydrate gypsum.No matter how the FDN is incorporated,it has little effect on the basic physical properties of phosphogypsum-based?-hemihydrate gypsum.It is not suitable for use as a crystal conversion agent and later modification during hydration.Sex agent.The effect of modifiers mixed in the dehydration reaction solution on cryst al growth:PS?SMF?FDN.Comprehensive comparison of the mixing method with the reaction solution as the hydration solution has the best effect(mode 3).When the PS content is 0.4%,compared with the first doping method,the dry strength of the third doping method is increased by 14.07%,the third method of blending increased 30.0%compared to the second method of absolute dry strength.Among them,the better effect is the polycarboxylic acid water-reducing agent added in the third mixing method.The polycarboxylic acid water-reducing agent can be used as an effective crystal conversion agent and modifier during hydration,and the mixing amount is 0.4%.3.KH792 was used as the silane coupling agent.The increase of KH792 content did not change the crystal water content of gypsum after conversion.The KH792produced by the oil-free body did not inhibit the dehydration and conversion process of dihydrate gypsum to?hemihydrate gypsum.Through the research on the basic physical properties of?-hemihydrate gypsum,when the content is 1.0%,the 2h flexural strength reaches 6.23MPa and the absolute dry strength reaches 33.15MPa.From the perspective of the strength improvement,it exceeds most of the crystal conversion agent,so it can be used as alcohol-free chlorine salt system phosphogypsum is used to prepare?-hemihydrate gypsum crystal conversion agent.Coupling agent KH792 changes the crystal morphology of?-hemihydrate gypsum after potassium sulfate transformation.The special nature of its oil-free body makes it not mixed with impurities during the reaction and can act as a crystal surface modifier;compared with traditional chlorine in the salt system,the calcium sulfate hemihydrate prepared by the non-chlorine salt medium and the glycerol system contains less impurities;the alcohol solution can be recycled to avoid resource waste and is more conducive to the preparation of high-purity hemihydrate gypsum.Three types of coupling agents:phthalate coupling agent TMC-201,aluminate coupling agent F-1,and silane coupling agent KH792 are used to prepare?hemihydrate gypsum under the alcohol-free chloride salt system,and the content of crystal water after the reaction is detected.And the changes of basic physical properties,it is concluded that the effect size is silane coupling agent KH792?aluminate coupling agent F-1?phthalate coupling agent TMC-201. |
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