Synthesis Ofα-Hemihydrate Whiskers From FGD Gypsum And The Fate Of Heavy Metal(PB) During The Conversion Process In Alcohol-Water Systems

Calcium based wet desulphurization in coal-fired power plants produces bulk industrial solid waste–flue gas desulphurization gypsum（FGD gypsum）,which brings serious environmental safety risks and resource waste.Synthesis ofα-hemihydrate（α-HH）whiskers from FGD gypsum is a high value-added utilization,which is of great significance to ensure the environmental safety and solve the resource waste caused by FGD gypsum.In this paper,a new technique of preparingα-HH whiskers from FGD gypsum in ethylene glycol（EG）solutions was proposed,the non-lattice cations were used to optimize the preparation process and the role and fate of lead（Pb）during the conversion was clarified,so as to promote the high value-added harmless utilization of FGD gypsum.α-HH whiskers were successfully synthesized from FGD gypsum in EG-water solutions with the addition of NaCl.Increasing temperature,alcohol-water ratio,solid content and agitation rate could promote the conversion,mainly due to the shortening of induction time.The growth of whiskers was not fully developed and the average aspect ratio（about 20-27）was reduced when the conversion rate is increased.The sodium and potassium cations-mediated crystallization ofα-HH whiskers from gypsum（DH）in EG-water solutions was studied.The results showed that Na⁺could significantly regulate the transformation kinetics of DH toα-HH whiskers.In95 ^oC,25vol%EG-water solutions,Na⁺with low concentration（0-0.10 M）promoted the conversion rate because the formation of[NaSO4]^-ion pairs enlarged the driving force.With high concentration（0.10-0.30 M）,Na⁺inhibited the conversion rate result from Na⁺doping on the surface of the DH and the formation of solid solution-eugsterite,which blocked the nucleation sites ofα-HH.Na⁺could also dramatically shape the morphology ofα-HH whiskers.When the dosage of Na⁺increased from 0.075 M to 0.15 M,the aspect ratio ofα-HH whiskers decreased from105.07 to 36.98.The reason was that Na⁺bridged theα-HH nucleus by surface doping,resulting in high degree of agglomeration.As Na⁺dosage increased to 0.20M,the aspect ratio ofα-HH whiskers returned to 66.26,due to the decrease in the number ofα-HH nuclei and the weakening of agglomeration;K⁺monotonically promoted the conversion kinetics and boosted the conversion of DH to anhydrous gypsum（AH）with a high dosage（0.10-0.30 M）.With the addition of K⁺,the gypsum product presented the morphological evolution of"short needle-short column-shuttle".Na⁺with low dosage and EG-water with low volume ratio are suitable for the synthesis ofα-HH whiskers.The regulation effect of Mg²⁺and Cu²⁺was explored simultaneously to further optimize the transformation process.Mg²⁺showed a trend of"promoting（0-0.075M）-inhibiting（0.075-0.15 M）"in the regulation of transformation kinetics.The regulation of Cu²⁺（0-0.30 M）on conversion kinetics showed a monotonical promotion trend.α-HH whiskers presented slender needle-like morphology with excellent monodispersity in the presence of Mg²⁺and Cu²⁺with the aspect ratio of83.40-97.70 and 76.62-85.80,respectively.Combined with the above studies,Cu²⁺and Mg²⁺with low dosage are suitable for regulation.The fate of lead（Pb）during the conversion of DH toα-HH in EG-water solutions was studied.It was found that Pb²⁺inhibited the conversion rate by reducing the driving force ofα-HH,and reduced the aspect ratio by shortening the length ofα-HH whiskers.During the conversion,most of Pb²⁺distributed in the solids while only little moved free in solutions.In the induction period of conversion,the major portion of Pb²⁺was weakly adsorbed on DH,and the remaining Pb²⁺existed in anglesite precipitate（PbSO₄）.Once the conversion initiated,the anglesite phase was accumulated,and more of the adsorbed Pb²⁺transferred into the surface lattice ofα-HH(denoted as doping Pb²⁺).The adsorbed Pb²⁺onα-HH increased significantly from 0.055 to 1.63 wt%coupled with the doping Pb²⁺slightly decreasing from 0.82 to 0.76 wt%within initial Pb²⁺dosage 500-1500 mg/L.Kinetically controlled lattice incorporation accounted for the variation of Pb²⁺doping and lowering the reaction temperature could prevent Pb²⁺from doping intoα-HH.In summary,α-HH whiskers can be synthesized from FGD gypsum in EG-water solutions and the process parameters（temperature,alcohol-water ratio,solid content and stirring rate）are capable to regulate the conversion rate by affecting the nucleation;Non-lattice cations modulate the transformation kinetics by forming ion-pair couplers and blockingα-HH nucleation sites.The morphology of whiskers is determined by cationic selective adsorption under the action of kinetics;The distribution of Pb²⁺in solid increases within the conversion process,and regulating the conversion rate can minish the distribution.Relevant results provide theoretical and technical guidance for the high value-added harmless utilization of FGD gypsum.