Study On Phase Change Behavior And Physical And Chemical Properties Of Phosphogypsum During Heat Treatment

One of the characteristics of the phosphoric acid sulfate process is that calcium sulfate precipitates directly during the acid hydrolysis process,and the liquid-solid phase separation is easy to achieve,which greatly simplifies the production method of phosphoric acid,but a large amount of phosphogypsum is produced during the production process（the main component is CaSO₄·2H₂O）.A large amount of phosphogypsum is not only occupying land and polluting the environment,but also a waste of gypsum resources.The preparation of phosphorus construction gypsum powder from phosphogypsum is an effective way to utilize phosphogypsum resources.Due to the fluctuation of raw materials and process conditions during the production of wet-process phosphoric acid,the composition and physicochemical properties of phosphogypsum are not stable,which affects the frying conditions of phosphorus building gypsum and the physical and chemical properties of the product.In this paper,rotary tube furnace and fluidized bed device were used to heat treat phosphogypsum,and the heat treatment process parameters,as well as the influence of P₂O₅ and F^-on the physical and chemical properties of fried gypsum samples were studied.Focus on studying the heat treatment conditions,and the effects of P₂O₅ and F^-on the phase distribution,solubility,relative crystallinity and micro-morphology of the fried gypsum samples.（1）The rotary tube furnace was used to heat treat the phosphogypsum,the effects of heat treatment temperature and time,heating rate and rotation speed,aging time and raw material particle size on the soluble phosphorus and fluoride content,phase distribution,solubility,relative crystallinity and micro-morphology of the fried gypsum samples were studied.The results show that,with the change of heat treatment temperature,constant temperature time,heating rate,rotation speed and raw material particle size,the phase distribution of the fried gypsum sample also changes,in which the influence of heat treatment temperature and constant temperature time on the phase distribution is particularly significant.The change of heat treatment temperature and constant temperature time will affect the content of soluble phosphorus and fluoride in the fried gypsum sample,while the change of heating rate and rotation speed will not affect the content of soluble phosphorus and fluoride in the fried gypsum sample.Changes in heat treatment conditions will cause changes in the solubility and relative crystallinity of the fried gypsum samples,and the change trends of the two are basically the same.When the heat treatment temperature is increased from 100℃to 400℃,the two tend to increase first and then decrease.When the constant temperature time was extended from 1 h to 3 h,the two showed an increasing trend overall.With the change of heat treatment conditions,the microscopic morphology of fried gypsum changes only as cracks appear on the surface and between layers of plate-like crystals.（2）Using the method of mixing H₃PO₄ and NaF into the fully washed phosphogypsum,the effects of soluble phosphorus and fluorine impurities in the phosphogypsum on the conditions of the heat treatment process and the phase distribution,solubility,relative crystallinity and micro-morphology of the fried gypsum sample were studied.The results show that,under the same heat treatment conditions,impurities will affect the phase distribution and solubility of the fried gypsum,but will not affect its microscopic morphology.Under the conditions of heating rate of 10℃/min,rotation speed of 20 r/min,and constant temperature of 130℃for 3 h,the doping of H₃PO₄ has little effect on the phase transition of CaSO₄·2H₂O,but the effect on the transition of CaSO₄·0.5H₂O to III-CaSO₄ is obvious.Compared with washed phosphogypsum,when the content of P₂O₅ increases to 0.54%,the content of CaSO₄·0.5H₂O phase of fried gypsum decreases by 32.60%,and the content ofⅢ-CaSO4 phase increased by 32.93%.After that,the increase of P₂O₅ content makes the effect of CaSO₄·0.5H₂O to III-CaSO₄ tend to be gentle.When the content of P₂O₅ increases to1.04%,the content of CaSO₄·0.5H₂O phase decreases by 27.67%,and the content of III-CaSO₄ phase increases by 26.34%.Incorporating F^-into the washed gypsum will inhibit the conversion of Ca SO₄·2H₂O to CaSO₄·0.5H₂O and promote the conversion of CaSO₄·0.5H₂O to III-CaSO₄,which will increase the CaSO₄·2H₂O phase content and the III-CaSO₄ phase content of the fried gypsum sample,and the CaSO₄·0.5H₂O phase content decreased.And when the F^-content is 1.01%,the effect on the content of each phase is most significant.Compared with washed phosphogypsum,the CaSO₄·2H₂O phase content of the fried gypsum sample increased by 3.49%,the CaSO₄·0.5H₂O phase content decreased by19.81%,and the III-CaSO₄ phase content increased by 16.31%.With the increase in the amount of impurities,the solubility of the sample increases after heat treatment.With the addition of P₂O₅,the relative crystallinity of the fried gypsum samples showed irregular changes.However,when the content of F^-varies from 0.03%to 1.01%,as the content of F^-increases,the relative crystallinity of the fried gypsum samples becomes better.（3）The fluidized method was used to heat treat phosphogypsum with a particle size of 95～106μm,and the effects of fluidization temperature and fluidization time on the soluble phosphorus and fluoride content,pH value and phase distribution of the fried gypsum samples were investigated,as well as the relationship between the solubility of the fried gypsum sample and its phase distribution and relative crystallinity.The results show that,the changes of the soluble phosphorus and fluorine content,phase distribution,solubility,relative crystallinity,micro-morphology,etc.of the samples prepared by fluidization method with temperature and time are basically consistent with the changes of the samples prepared by heat treatment using a rotary tube furnace.The fluidization method has high heat transfer efficiency and short time-consuming,but compared with the rotary tube furnace,the fluidization method has higher requirements on the particle size range of the raw material phosphogypsum.And because the fluidization method belongs to"flash firing",the phenomenon of"under-firing"easily occurs during the heat treatment process.When the temperature reached 700℃,the sintered gypsum sample still contained a small amount of CaSO₄·2H₂O phase.