Preparation Of Dehydrated Calcium Sulfate By Microwave-assisted Microemulsion Method

Calcium sulfate,as a traditional inorganic material of green and environmental protection,has a wide range of applications with large natural mineral deposits and industrial by-products production,and flexible synthesis methods.So,it has a good availability.Calcium sulfate has 5 phases and 7 variants,and each phase has its own different morphology,and its performance and application methods are greatly different.Therefore,it is of great significance to develop and prepare calcium sulfate with specific phase state and crystal structure,and to study the effective means of morphology regulation and control,for the purpose of expanding the resource utilization of calcium sulfate mineral phase.At present,the traditional heating method is used to prepare the dehydrated phase of calcium sulfate,which has a long time and high energy consumption.By microwave assisted synthesis method of calcium sulphate microemulsion,this paper studies the influence of the reaction conditions on the phase formation and CTAB and SDS regulation function of calcium sulphate morphology.The effects of microwave irradiation and conventional heating,on the formation of calcium sulfate crystal in the micro-emulsion system were discussed,which may serve for providing valuable reference for the development of efficient new method for calcium sulfate mineral phase controlled synthesis.Firstly,calcium sulfate hemihydrate was synthesized in oil-in-water microemulsion system by microwave heating.The effects of reaction time,temperature,raw material concentration,the amount of surfactant SDS,stirring rate and other factors on the phase state and structural morphology of calcium sulfate were studied.The results showed that the formation of β-hemihydrate calcium sulfate could be promoted by increasing the reaction time,temperature and the amount of SDS.Conventional heating takes 2h at 140℃ to form hemihydrate calcium sulfate,while microwave heating only takes 15 min at 100℃ to complete,greatly reducing the reaction time.The morphology of calcium sulfate hemihydrate is mainly long columnar,and the ratio of crystal length to diameter can be reduced by increasing the dosage of SDS.Different from α-hemihydrate calcium sulfate formed in hydrothermal system,β-hemihydrate calcium sulfate was synthesized in oil-in-water microemulsion system assisted by microwave,and the formation mechanism of β-hemihydrate calcium sulfate was discussed.Secondly,calcium sulfate hemihydrate was synthesized in CTAB/n-octane/water microemulsion by microwave irradiation assisted reversed microemulsion method.The effects of reaction temperature,time,concentration of raw material,amount of surfactant and ratio of oil to water on the phase and morphology of calcium sulfate were studied.The results show that calcium sulfate hemihydrate can be formed under the condition of microwave heating at 140℃ for 5min,and it is difficult to judge whether it belongs to α or β type.With the increase of CTAB/oil molar ratio,the crystal diameter decreases and the aspect ratio increases,and the morphology changes from long columnar to needle-like.When SDS is used as surfactant,the transformation temperature of the dehydrated phase of calcium sulfate can be reduced,that is,HH transformation can be completed at 100℃ for 5min,and nearly anhydrous calcium sulfate can be prepared at 120℃ for 5min.The crystal size of the product is larger,the diameter increases,and the length decreases.Contrastive analysis showed that the different selective adsorption of the two surfactants on the crystal surface of calcium sulfate resulted in the different crystal morphology.Finally,the effect of traditional heating method and microwave irradiation heating on the formation of calcium sulfate in microemulsion system was compared.By microwave irradiation,the crystal size was smaller,the length and diameter were larger,the crystal shape was more complete,and the transformation rate was faster.The whisker shape calcium sulfate could be prepared.Using the traditional heating method,the crystal size is larger,the surface is rough,the defect is more,the transformation rate is slow,and the transformation time is extended several times.Therefore,microwave heating can not only improve the reaction rate,promote the nucleation and growth of crystal,but also refine the crystal and improve the integrity of crystal.By measuring the temperature rise of each component of microemulsion heated by microwave,it is preliminarily revealed that the dominant microwave response and absorption of water molecules may be the main reason that microwave irradiation promotes the rapid crystallization of calcium sulfate in microemulsion micelles.