LIU Hulin, WANG Jing, WU Yuanting, HAI Ou, XUE Yunlong, ZHANG Xinmeng
(School of Materials Science and Engineering, Shaanxi Key Laboratory of Green Preparation and Functionalization for Inorganic Materials, Shaanxi University of Science and Technology, Xi'an 710021, Shaanxi, China)

Extended Abstract: [Significance] Desulfurization gypsum is an industrial by-product obtained after desulfurization treatment of combustion flue gases from sulfur-containing fuels. Besides the major component which is calcium sulfate dihydrate, desulfurization gypsum also contains various metal elements and harmful substances. If desulfurization gypsum is treated by stacking or landfilling, it will not only lead to resource waste but also severely environment pollution. It is necessary to utilize desulfurization gypsum through reasonable means. Currently, desulfurization gypsum is mainly used as raw materials in the building materials industry, agriculture, etc. However, these approaches have relatively low added values. There is significant potential in realizing high-value utilization of desulfurization gypsum by fabricating calcium sulfate whiskers (CSWs). CSWs are one-dimensional inorganic materials, possessing advantages such as high strength and toughness, corrosion resistance, non-toxicity and high insulation properties, etc. They are widely used in industries, including paper-making, medicine and construction materials. Consequently, efforts have been made to prepare high-quality CSWs using desulfurization gypsum as raw materials. This paper was aimed to review the progress in the synthesis of CSWs via the salt solution method at atmospheric pressure. The reaction mechanism and process conditions are elucidated based on the characteristics of CSWs. Several key issues in the preparation of CSW from desulfurization gypsum are discussed based on the characteristics of desulfurization gypsum.[Progress] Firstly, the basic characteristics of desulfurization gypsum are introduced. There are dry, semi-dry and wet desulfurization gypsums. Dry desulfurization gypsum mainly consists of CaSO₃·2H₂O, CaCO₃, and small content of CaSO₄·2H₂O and CaO. Semi-dry desulfurization gypsum is primarily composed of CaSO₃·0.5H₂O and CaCO₃. Wet desulfurization gypsum mainly contains CaSO₄·2H₂O, with small content of CaCO₃ and SiO₂. In addition, desulfurization gypsum contains elements such as Na, Mg, Al, Si, P, Ti, Fe, K, Cl, and toxic elements like Pb and Cd. Compared with natural gypsum, wet desulfurization gypsum has similar composition and physicochemical properties, but finer and more uniform particle size distribution. Therefore, wet desulfurization gypsum can be used to replace natural gypsum as industrial raw materials. Secondly, the fundamental principles of preparing CSWs using the salt solution method at atmospheric pressure are discussed. The basic mechanism underlying this fabrication method is the dissolution-crystallization principle. The formation of CSW involves three processes: dissolution of calcium sulfate dihydrate, precipitation of calcium sulfate hemihydrate and grain growth of CSWs. The former two processes are driven according to the difference in the solubility between different calcium sulfate hydrates and the water activity. During the crystal growth process, hemihydrate calcium sulfate crystals preferentially grow along the c-axis, forming whiskers with the main crystal planes of (100) and (010). It is beneficial to the growth of CSWs by increasing the temperature, salt and slurry concentrations within certain ranges, as well as reducing pH value. Finally, several key issues in the preparation of CSWs using desulfurization gypsum as raw materials are summarized. Desulphurized gypsum requires pretreatment with physical and chemical methods to eliminate the impurities which are detrimental to the purity and quality of CSWs. Chloride or nitrate salts containing Na⁺ or Mg²⁺ ions can increase the solubility of calcium sulfate dihydrate, reduce the water activity and control the dehydration rate to promote the growth of CSWs. The morphology of CSWs can be tailored by the selective adsorption of crystal modifiers on different crystal planes of the whiskers. Metal cations can promote whisker growth and increase the aspect ratio, while organic anions inhibit whisker growth resulting in short columnar crystals.[Conclusions and Prospects] CSWs have been synthesized by using desulfurization gypsum as raw materials through salt solution method at atmospheric pressure. This method has the advantages of relatively mild reaction conditions, low equipment cost and high safety, which is potential for industrial production of CSWs in the future. There are two key factors in controlling the morphology of the whiskers, i.e., appropriate salt solution and phase transformation agent. The commonly used salt solutions are chlorides or nitrates. However, high concentrations of chloride ions can corrode equipment and produce a large amount of chlorinated wastewater. Therefore, the development of low-concentration salt solutions without or with less chlorides and the screening of appropriate phase transformation agents will be the important research directions for the preparation of CSWs using this method in the future.
Key words: desulfurization gypsum; calcium sulfate whiskers; salt solution method; grain growth mechanism