Pilot scale preparation of a-calcium sulfate hemihydrate from FGD gypsum in Ca–K–Mg aqueous solution under atmospheric pressure Baohong Guan ⇑ , Bao Kong, Hailu Fu, Jie Yu, Guangming Jiang, Li Yang Department of Environmental Engineering, Zhejiang University, Hangzhou 310027, China article info Article history: Received 27 September 2010 Received in revised form 8 February 2012 Accepted 15 March 2012 Available online 30 March 2012 Keywords: a-calcium sulfate hemihydrate FGD gypsum Salt solution method Pilot scale abstract Preparation of a-calcium sulfate hemihydrate (a-HH) from flue gas desulfurization (FGD) gypsum is one of the most promising alternatives and of great value in effectively utilizing the FGD gypsum. Based upon a salt solution method and the optimum operating conditions obtained in the laboratory scale experi- ments, pilot scale tests have been conducted to confirm the feasibility of the process. The pilot experi- ments were carried out in a Ca–K–Mg chloride solution medium at 94 ± 2 °C under atmospheric pressure on a 500–1000 kg/batch setup. The crystal water content, solid phases, crystal morphology, strength and chemical composition of the products were examined and compared. It was found that short prism-shaped a-HH was successfully prepared within 5–6 h, with the purity of a-HH up to 95%, the 2 h bending strength of 6.8–7.3 MPa, 2 h compressive strength of 12.2–23.3 MPa and dry compressive strength of 29.6–37.9 MPa. It can be concluded that the process is highly feasible in the pilot scale, with high quality of the products. This research can provide necessary and useful information for the industrial production in the future. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Worldwide, coal continues to be the most important fuel for power generation [1,2]. The application of wet flue gas desulfuriza- tion (FGD) project in coal power plants is still increasing to control the exhaust gas due to its high sulfur dioxide removal efficiency, reliability and cost-effectiveness [3]. Consequently, large quantity of the by-product called FGD gypsum has been produced and re- ceived great attention because of potential environmental impacts [4,5]. Management and disposal of FGD gypsum is an important issue for the power plants and the governments. In China, the quantity of FGD gypsum has been increasing dramatically in the past decade, causing a great demand to the utilization of FGD gypsum. Apart from beneficial uses including the production of wallboard, flowable fill material, cement additives, and autoclaved aerated concrete block [6–8], preparation of calcium sulfate hemihydrate (HH) is a very important reuse method. FGD gypsum is mainly composed of calcium sulfate dihydrate (DH) that is apt to dehydrate into HH, which has two forms of b- HH and a-HH. The former is usually used in the construction industry, whereas the latter, due to its better workability and higher strength values, is a very important material and has been applied in comprehensive fields. For the time being, a-HH is commercially produced by the autoclave method for which elevated temperatures and pressures are employed [9]. This process has a high demand due to the operating conditions and facilities, resulting in a high investment and running cost [10]. a-HH can also be prepared by the salt solution method, which realizes the transformation of DH to a-HH under atmospheric pres- sure and is attracting more interest for lower energy consumption. The mixed boiling or near-boiling solutions of inorganic acids or their salts (such as H 3 PO 4 ,H 2 SO 4 , KCl, and NaCl [11–14]) are used as the dehydration medium. During the past decades, much work has been done to investi- gate the preparation of a-HH in various salt solutions [15,16]. However it is restricted in the laboratory or in the bench scale and no report has ever been published regarding the preparation of a-HH from FGD gypsum with salt solution method in pilot scale. Therefore, it is desirable to develop a-HH production in pilot scale and investigate the process with the aim of technological industri- alization. In our previous work, a process preparing a-HH from FGD gypsum at atmospheric pressure using Ca–K–Mg chloride solution as the dehydration medium was successfully established [17]. The crystallization mechanism, kinetics [18], some important parameters [19] and crystal modifiers [20] were fully investigated. In this work, pilot scale experiments are conducted based upon the process and optimum operating parameters obtained in preli- minary laboratory scale investigations, focusing on the feasibility of the process. The crystalline, cementitious materials strength and chemical properties of the prepared a-HH are determined to elucidate the quality of the product. Such a work will contribute 0016-2361/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2012.03.032 ⇑ Corresponding author. Tel./fax: +86 571 88273687. E-mail address: guanbaohong@zju.edu.cn (B. Guan). Fuel 98 (2012) 48–54 Contents lists available at SciVerse ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel