Coordinated disposal of FGD gypsum and power plant concentrated brine via preparation of α-hemihydrate gypsum

Abstract

α-hemihydrate (α-HH), known for its exceptional mechanical properties, is a widely used building material. Despite its advantages, the traditional method of producing α-HH through regular pressure brine poses significant drawbacks. These include substantial industrial salt demands, high production costs, and water pollution. A more environmentally friendly α-HH preparation process has been developed to counteract these issues, combining FGD gypsum and concentrated brine. This study delves into the effects of the principal ions present in concentrated brine on the phase transition process and the microstructure of the final product. Our findings reveal that the reaction rate accelerates with increased concentrations of Ca2+, Mg2+, K+, and Na+ in concentrated brine. However, excessive Na+ concentration can alter the phase transition process, converting the product to β-HH. Additionally, an overly high concentration of KCl may prompt a part of the K+ to engage in the reaction, leading to the formation of isomeric potassium gypsum. This process can disrupt the original structure of hemihydrate gypsum. The addition of maleic acid to the salt solution was found to foster anhydrous gypsum formation. In contrast, succinic acid can decrease the crystals' aspect ratio, resulting in the growth of short columns, thereby enhancing the product's strength performance. Our research also identified that the reaction rate elevates as the pH value declines. However, an acidic environment diminishes the size of the crystal and counteracts the crystal modifier's effects. Lastly, incorporating CaCl2 and MgCl2 into concentrated brine with a low Na+ concentration can convert FGD gypsum into high-strength α-HH. This transformation results in a product boasting a 1-day compressive strength of 40.5 MPa.