Preparation And Application Of Nano Calcium Sulfate From Flue Gas Desulfurization Gypsum

Flue gas desulfurization gypsum（FGD gypsum）is a kind of solid-phase waste mainly coming from the flue gas desulfurization.The high value-added utilization of FGD gypsum is a current research focus.As a high value-added inorganic material,nano calcium sulfate shares the characteristics of conventional calcium sulfate of satisfactory mechanical properties and compatibility with organic materials,as well as the ones of nanoparticles of high dispersion,large specific surface area,and good affinity.Currently in the synthesis of calcium sulfate crystals from FGD gypsum,it is difficult to effectively control the process of dissolution-recrystallization in the reaction.Therefore,the difficulty lies in the synthesis of nano calcium sulfate crystals from FGD gypsum.To solve this problem,this study proposes novel technique for the synthesis ofⅡ-CaSO₄nanosheets andα-CaSO₄0.5H₂O nano whiskers from FGD gypsum,and applies them to the reinforcement of polymer materials and the removal of trace water from water-in-oil emulsions.In this paper,FGD gypsum pretreated by using concentrated sulfuric acid（H₂SO₄）was further hydrothermally reacted in a glycerol-water system to obtainⅡ-CaSO₄nanosheets.The effects of glycerol,hexadecyl trimethyl ammonium bromide（CTAB）,and concentrated H₂SO₄on the products were studied,respectively.By adjusting the addition amount of concentrated H₂SO₄,the formation mechanism ofⅡ-CaSO₄nanosheets was illustrated.The synthesizedⅡ-CaSO₄nanosheets exhibit a regular morphology featuring a length and a width of less than 500 nm and a thickness of about 50–80 nm.Taking advantage of the dehydration effect of concentrated H₂SO₄,the crystal water was removed,thus resulting in the destruction of the original crystal structure of the FGD gypsum.The concentrated H₂SO₄was also involved in the process of dissolution-recrystallization in the reaction,affecting the morphology of the product.Additionally,the obtainedⅡ-CaSO₄nanosheets were added to PVA as inorganic fillers.The effect ofⅡ-CaSO₄nanosheets addition on the mechanical properties ofⅡ-CaSO₄/PVA composite films was investigated.It was found that with the addition ofⅡ-CaSO₄nanosheets of 2%,the strength and elongation at break of the synthesizedⅡ-CaSO₄/PVA composite films were simultaneously improved by31.86%and 26.57%,respectively,in comparison with pure PVA.The enhancement mechanism ofⅡ-CaSO₄nanosheets for the mechanical performance of PVA was explored.The microwave-ultrasonic technique was used to synthesizeα-CaSO₄·0.5H₂O nano whiskers from FGD gypsum in a glycerol-water system with Mg Cl₂as a crystal form control agent.The effects of reaction temperature and duration,power of ultrasonic,the volume ratio of glycerol to water,as well as the addition amount of Mg Cl₂on the products were investigated.Compared with the variation of concentration of Ca²⁺in conventional hydrothermal reaction,the enhancement mechanism of the microwave-ultrasonic technique on the synthesis ofα-CaSO₄·0.5H₂O nano whiskers was studied.The obtainedα-CaSO₄·0.5H₂O nano whiskers features a diameter of ca.200 nm and an aspect ratio of about 30.The microwave-ultrasonic could dissolve a large amount of FGD gypsum in a short period of time,resulting in generation of a large number of nuclei,and forming products with smaller size,Meanwhile,the dispersion effect of ultrasonic could avoid the directional gathering ofα-CaSO₄·0.5H₂O nano whiskers along[100]direction.By means of the easy hydration property ofα-CaSO₄·0.5H₂O,α-CaSO₄·0.5H₂O nano whiskers were used for the separation of trace water from water-in-oil emulsion,and the separation efficiency could reach 97.89%.The isolated solid phase was reacted under the synthesis conditions of nano whiskers,and could be re-synthesized intoα-CaSO₄·0.5H₂O nano whiskers,which could still be used for the separation of water-in-oil emulsions.The recycling process ofα-CaSO₄·0.5H₂O nano whiskers was designed.