| As a large gypsum resource country,China is rich in natural gypsum(calcium sulfate dihydrate),and its reserves rank first in the world.At the same time,a large amount of industrial by-product gypsum with the main component of calcium sulfate dihydrate is produced every year,which is an important bulk solid waste in China.A large amount of industrial by-product gypsum is piled up,which not only takes up land,but also wastes resources and causes environmental pollution.The natural gypsum and industrial by-product gypsum can be transformed into high economic value-addedα-calcium sulfate hemihydrate by atmospheric salt solution method,which has broad economic benefits and market prospects.However,when producing α-calcium sulfate hemihydrate with inorganic salt solution,it will corrode the equipment and easily produce double salt with α-calcium sulfate hemihydrate.Therefore,the preparation of α-calcium sulfate hemihydrate in common organic solvent aqueous solution is considered to explore a more economical and effective preparation process,improve the resource utilization of gypsum,and realize the generation of high added value of calcium based resources.In this paper,the solubility data of calcium sulfate dihydrate and α-calcium sulfate hemihydrate in five organic solvents(formamide,ethylene glycol,acetic acid,dimethyl sulfoxide and glycerol)were determined by isothermal dissolution equilibrium method in the temperature range of 303.15 K-363,15 K.Formamide can promote the dissolution of calcium sulfate dihydrate and α-calcium sulfate hemihydrate,with similar salt effect.The maximum solubility of calcium sulfate dihydrate andα-calcium sulfate hemihydrate increases about 1.8 times compared with that in aqueous solution,but it first reaches a maximum value in formamide solution and then gradually decreases.In glycerol sodium chloride aqueous solution,due to the salt effect of sodium chloride,the solubility of α-calcium sulfate hemihydrate first increased by 4.9 times than that without sodium chloride,and then decreased.The other three solvents inhibited the dissolution of calcium sulfate dihydrate.The density and viscosity of saturated solution were also studied.Secondly,the modified Apelblat-Jouyban-Acree model,Wilson model,Uniquac model,NRTL model and polynomial equation are used to correlate the solubility of calcium sulfate dihydrate andα-calcium sulfate hemihydrate.The calculated results are in good agreement with the experimental results,and the average RAD values are 0.8400(A-J-A),0,5907(Wilson),0.8711(Uniquac)and 2.1407(NRTL),respectively the mean 104RMSE was 1.1109(A-J-A),0.8396(Wilson),1.8455(Uniquac)and 5.2066(NRTL).According to the model parameters,the solubility data of calcium sulfate dihydrate andα-calcium sulfate hemihydrate were predicted,and their theoretical phase transition points at 353.15 K were obtained:formamide:8.3418 mol·kg-1;ethylene glycol:7.5184 mol·kg-1;acetic acid:7.7561 mol·kg-1;DMSO:7.9842 mol·kg-1.The results provide a theoretical basis for the subsequent preparation of α-calcium sulfate hemihydrate.Finally,calcium sulfate dihydrate was used as raw material to prepare α-calcium sulfate hemihydrate in four organic solvent aqueous solutions by normal pressure salt solution method.The order of the formation rate of α-calcium sulfate hemihydrate was formamide>ethylene glycol>dimethyl sulfoxide>acetic acid.In the reaction process,the number of particles with all sizes will increase when α-calcium sulfate hemihydrate appears,and the increase will stop at the end of the reaction.The morphology of products in each solvent is different.Needle like crystals are formed in ethylene glycol and acetic acid.The products in formamide aqueous solution are prismatic crystals with shorter length.The products in dimethyl sulfoxide are prismatic crystals with smaller aspect ratio.The research results of this paper provide a new way for the resource utilization of dihydrate gypsum and the generation of high added value of calcium based resources. |
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