Study On Contronlling The Transformation Process From Dihydrate To Hemihydrate Gypsum

Gypsum is an important industrial ore raw material and is also produced in the form of by-products in some industrial processes.The production of industrial by-product gypsum of China is increasing day by day,and its stacking can cause certain environmental problems.Most of the industrial by-product gypsum is mainly composed of dihydrate calcium sulfate?DH,Ca SO₄·2H₂O?,which can be economically and efficiently converted into hemihydrate calcium sulfate?HH,Ca SO₄·0.5H₂O?.That is helpful to solve the environmental problems of solid waste,and has become a hot topic in driving the technological progress and innovation of high-value utilization of gypsum resources.Therefore,the corresponding theoretical and application research is very urgent.In this paper,the phase transformation and morphology control law of DH to HH in glycerol-water solution were studied.In addition,the conditions and effects of microwave irradiation on the phase transformation of gypsum were preliminarily discussed.Under the condition of constant temperature reaction in oil bath,the phase transformation of gypsum in glycerol solution was studied.The results show that?-and?-HH crystals could be obtained at 110?by adjusting the concentrations of glycerol,Na₂EDTA and NaCl.Low concentration of glycerol and NaCl could reduce the activity of water and increase the supersaturation of solution,which is beneficial to the formation of?-HH block crystal.However,when the content of glycerol increases from 50 wt%to 70 wt%or the concentration of NaCl increases from 0.14mol to 0.26 mol,the local relative supersaturation may increase sharply,it tended to form?-HH with defect morphology.Na₂EDTA mainly severd as a crystal regulator in this experiment.Two kinds of HH formed in the same system,which is helpful to make a profound study of the crystal structure of gypsum.Further results indicated that DH had a slower conversion speed,which provided more sufficient nucleation and growth time for HH crystals and only resulted in?-HH products with complete crystallization when Mg Cl2?6H2O was added in glycerol-water solution system,compared with the effect of NaCl.Under the condition of 0.14 mol/L Mg Cl2?6H2O,Na₂EDTA was replaced by CTAB,SDS and succinic acid respectively.Then,the morphology of?-HH would change from whiskers with large aspect ratio to columnar,layered and shuttle shape,etc.The influence mechanism of various additives was discussed and the composite additive effect on the morphology of?-HH in the glycerol-water system was analysed.Compared with the traditional heating method,microwave heating has the merits of energy saving,high efficiency and selectivity.The preliminary experimental results displayed that too small?solid-liquid ratio<1:3?or too large?solid-liquid ratio>2:3?moisture content of DH in the sealed vessel with microwave irradiationwould be not conducive to phase transformation.When the controlling temperature is higher than120?,?-HH could be produced.Compared with the autoclave method by traditional heating,microwave can not only greatly increase the reaction speed,but also reduce the crystal size of?-HH.Under the condition of microwave irradiation,the formation mechanism of the?-HH crystal morphology was considerd as a process of DH dissolution,HH crytallization,and the directional aggregation of small crystal.