| Potassium dihydrogen phosphate(KDP),an important phosphate product,has been widely used in industry,agriculture,medicine and food industry.At present,KDP is mainly prepared by neutralization method,which is difficult to be popularized in agriculture due to its high production cost.The preparation of potassium dihydrogen phosphate by calcium dihydrogen phosphate and potassium sulfate has attracted much attention for its advantages,such as low raw material cost and easy separation of products.However,the resource utilization of calcium sulfate produced by the reaction has not been mentioned in the existing studies.In this paper,potassium dihydrogen phosphate is prepared by the reaction of phosphate ore acidolysis solution,decomposed by phosphoric acid and mainly composed of Ca(H2PO4)2,and potassium sulfate.To obtain high strength?hemihydrate gypsum,the crystal form and morphology control of calcium sulfate in the reaction system of Ca(H2PO4)2-H3PO4-K2SO4 were emphatic studied.The crystal rule of calcium sulfate without impurities in the reaction system of Ca(H2PO4)2-H3PO4-K2SO4 was studied firstly.Based on the reaction parameters of suitable reaction conditions,high strength?hemihydrate gypsum was prepared from actual acidolysis solution and potassium sulfate reaction.Firstly,the one-step regulation of crystal form and morphology of calcium sulfate in Ca(H2PO4)2-H3PO4-K2SO4 system was studied.Calcium dihydrogen phosphate solution and potassium sulfate solution preheated to about 95?for reaction.Then the effects of reaction time,reaction temperature,SO42-excess coefficient?,Ca O mass fractions and P2O5 concentration on the crystal form and morphology of calcium sulfate,the yield of phosphorus and potassium and decalcification rate were studied.The results show that the crystal form and morphology of gypsum in Ca(H2PO4)2-H3PO4-K2SO4system can be controlled by controlling the reaction parameters,so as to the short columnar?-Ca SO4·0.5H2O can be prepared.With long reaction time and large SO42-excess coefficient,it is easy to form K2SO4(Ca SO4)5·H2O.As the reaction proceeds,K enters the crystal lattice,causing crystal defects,and mainly starts from the crystal cone to form K2SO4(Ca SO4)5·H2O,showing a cone canine shape morphology.Under the optimum conditions of reaction time of 10 min,reaction temperature of 95?,SO42-excess coefficient of 1.2,Ca O mass fractions of 5.0%and P2O5 concentration of 40%,the short columnar?-Ca SO4·0.5H2O crystals with length of 42?70?m and diameter of 13?24?m are prepared,the compressive strength and flexural strength of which are4.83 MPa and 33.74 MPa,respectively.The yields of potassium and phosphorus and decalcification rate of the filtrate are 94.23%,83.31%and 83.80%,respectively.Since there are some problems in the one-step method,such as too fast reaction,the two-step method of crystal form and morphology of calcium sulfate in Ca(H2PO4)2-H3PO4-K2SO4 system was studied next.The Ca SO4·2H2O was obtained by mixing calcium dihydrogen phosphate solution and potassium sulfate solution at low temperature,and then Ca SO4·2H2O was transformed into?-Ca SO4·0.5H2O by increasing the temperature.The effect of different reaction conditions on the crystal form and morphology of calcium sulfate in the system was studied.The results show that when the reaction temperature and P2O5 concentration are too low,and the Ca O mass fractions is too high,the solid phase is still Ca SO4·2H2O after the reaction.With the increase of reaction temperature and P2O5 concentration,the crystal shape of?-Ca SO4·0.5H2O developed from short columnar shape to long rod shape.When the SO42-excess coefficient is greater than 1.4,the concentration of SO4 is higher than 12%and the content of Ca O is 5.5%,K2SO4(Ca SO4)5·H2O is easily formed,and a small part or most of the crystal forms are hedgehog shaped.Under the experimental conditions,the appropriate reaction conditions are as follows:initial reaction temperature of 70?,initial reaction time of 60min,reaction temperature of 102?,K2SO4 concentration of10%,SO42-excess coefficient of 1.2,Ca O concentration of 5%,P2O5 concentration of25.0%,reaction time of 5 h.Under these conditions,the?-Ca SO4·0.5H2O with compressive strength of 4.25 MPa and flexural strength of 35.07 MPa are prepared and the yield of potassium,phosphorus and decalcification are 98.35%,91.43%and 89.74%,respectively.To study the phase transition process of Ca SO4·2H2O into?-Ca SO4·0.5H2O,XRD and SEM were used to analyze the phase composition and morphology of the samples at different times in the two-step method.The conversion of Ca SO4·2H2O to?-Ca SO4·0.5 H2O follows the dissolution-crystallization mechanism.As the temperature rises,the system concentration is a supersaturated solution for Ca SO4·0.5 H2O.After reaction for 2.2 h,?-Ca SO4·0.5 H2O crystals appear,and then the dissolution of Ca SO4·2H2O and the precipitation of?-Ca SO4·0.5H2O occurred alternately and repeatedly.It is until the reaction reached 5.0 h when all Ca SO4·2H2O is transformed into?-Ca SO4·0.5H2O.When the reaction continues to 7 h,a hedgehog-like morphology crystal appears,and K2SO4(Ca SO4)5·H2O begins to form.Finally,according to the suitable reaction conditions of?-Ca SO4·0.5H2O prepared by Ca(H2PO4)-H3PO4-K2SO4 system,the?-hemihydrate gypsum was prepared by one-step and two-step reaction of phosphoric acid decomposition of phosphate ore acidolysis solution and potassium sulfate solution,respectively.The phase composition and morphology of the gypsum were analyzed by XRD and SEM.The results show that the suitable reaction conditions for the preparation of?-Ca SO4·0.5H2O based on the Ca(H2PO4)-H3PO4-K2SO4 system,after fine-tuning the reaction parameters,the morphology of?-Ca SO4·0.5 H2O prepared by acidolysis solution and potassium sulfate solution is basically the same as that of?-Ca SO4·0.5H2O prepared by Ca(H2PO4)-H3PO4-K2SO4 system. |
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