Process optimization for preparation of anhydrite-Ⅱ via lime neutralization coupled with medium-temperature calcination

DING Yanbo; FENG Shuxia; XIA Xiao; TIAN Yeshun; WU Changliang; DUAN Guangbin

doi:10.13732/j.issn.1008-5548.2026.02.017

Abstract：Objective Phosphogypsum(PG)is one of the predominant solid wastes generated by the phosphorus chemical industry.It con-tains harmful components,including phosphorus compounds,sulfates,fluoride,organic matter,trace metals,and radioactive substances,posing severe environmental threats by contaminating soil,surface water,and groundwater.Therefore,the resource utilization of PG is an urgent environmental challenge.Anhydrite Ⅱ(A Ⅱ),a gypsum-based cementitious material with superior properties,is primarily produced through high-temperature calcination to remove crystallization water.This method offers advantages such as reasonable costs and controllable processing,making it widely applicable in the building industry.However,such traditional processes have significant limitations,as the required high temperatures cause high energy consump-tion and costs and reduce the reactivity of anhydrous gypsum,thereby impairing its hydration rate and application performance.To address these issues,this study aims to optimize the calcination process of A Ⅱ and explore a technical approach for produc-ing high-performance A Ⅱ at moderate temperatures.By lowering the calcination temperature and shortening the holding time,this study achieves the dual goal of reducingenergy consumption and enhancing product performance.This provides a scientific foundation and technical support for the efficient utilization of PG. Methods A combined approach of lime neutralization and medium-temperature calcination was adopted.Lime neutralization was employed to eliminate soluble phosphorus and fluorine,thereby reducing firing temperature.Medium-temperature calcination was utilized to remove organic matter.First,A Ⅱ was synthesized at various calcination temperatures.The synthesized samples were characterized using aseries of advanced analytical techniques,including X-ray diffraction(XRD),X-ray fluorescence spectroscopy(XRF),scanning electron microscopy(SEM),and Fourier-transform infrared spectroscopy(FTIR).Phase com-position,microstructure,water requirement for normal consistency,setting time,compressive strength,and eutectic phospho-rus content were comprehensively evaluated.The impact of calcination temperature on the mineral composition,morphology,performance,and mechanical properties of A Ⅱ was systematically analyzed to determine the optimal calcination temperature.Subsequently,the influence of different holding times on the properties of A Ⅱ was investigated,and the optimal duration was identified. Results and Discussion As the calcination temperature increased,the phase composition of the calcined product transitioned from hemihydrates to anhydrous gypsum.At 450 ℃,XRD patterns revealed no diffraction peak in hemihydrate gypsum.At 500 ℃,the microstructure evolved from grains with distinct boundaries into a denser plate-like form.This transition was accom-panied by a gradual reduction in water requirement and an extended setting time,correlating with the increasing proportion of A Ⅱ in the system.The 3-day compressive strength initially increased,peaking at 500 ℃,but then declined when A Ⅱ became the dominant.This reduction in strength at higher temperatures was due to the decreased reactivity of the material.In the FTIR spectrum,the absorption peak of eutectic phosphorus gradually faded and disappeared at 500 ℃,indicating its complete decom-position.As the holding time increased,the phase composition of the calcined products remained as anhydrous gypsum.After 1.5 hours of calcination,the microstructure transformed into a plate-like form.The water requirement for normal consistency initially decreased and then increased,while the 3-day compressive strength first increased and then decreased,both reaching their respective lowest and highest values at 1.5 hours of calcination.These changes were attributed to the complete decomposi-tion of eutectic phosphorus at 1.5 hours,eliminating its suppression of hydration.The FTIR analysis further validated the pro-cess,showing a significant reduction in phosphorus absorption peaks after 1.5 hours.Experiments showed that the optimal calci-nation temperature for A Ⅱ preparation was 500 ℃,and the ideal holding time was 1.5 hours.Compared to traditional pro-cesses,this optimized method reduced the calcination temperature by 38％and the holding time by 25％.Additionally,the set-ting time was reduced by 78％,and the 3-day compressive strength was enhanced by 55％,demonstrating significant improve-ments in both energy efficiency and material performance. Conclusion Both calcination temperature and holding time significantly influence the properties of A Ⅱ.Insufficient tempera-tures or duration result in the incomplete decomposition of eutectic phosphorus,causing impurities.Conversely,excessive tem-peratures or prolonged holding time reduce the reactivity of the material,thereby compromising its application potential.The combination of lime neutralization and medium-temperature calcination can effectively remove harmful eutectic phosphorus impurities from PG,yielding high-performance A Ⅱ.The optimal calcination conditions are determined to be 500 ℃ with a hold-ing time of 1.5 hours.