Recovery of Calcium Sulfate Hydrates from SDA Waste Sludge and Bag Filter Ashes

Abstract— SDAWS (Spray drying absorption waste sludge) and BFA (bag filter ash), which are the wastes generated in the SDA (pray drying absorption) process, are only partially recycled as cement additives, gypsum boards, building materials, and the rest are being landfilled in managed landfills. However, wastes that have not been disposed of at the landfill are accumulated around the landfill. Therefore, it is necessary to study the recycling method for SDAWS and BFA. In this study, the effects of various parameters, pH, reaction temperature, sulfuric acid concentration (or type) on the preparation of calcium sulfate have been investigated. The major parameters influencing the crystalline phase and recovery efficiency of calcium sulfate from the SDAWS and BFA were type of sulfuric acid and Ca(OH)2 content, respectively. There was little difference between the reagent grade sulfuric acid and the spent sulfuric acid in the preparation of calcium sulfate, only Ca(OH)2 contained in the SDAWS and BFS affected. Also, it can be concluded that the desired crystalline phase of calcium sulfate can be obtained by controlling the concentration of sulfuric acid during the sulfidation reaction. Also, when the impurities suspended in the upper part of the reactor are removed during the sulfidation reaction, calcium sulfate having a much higher chromaticity can be produced. The characterization of the raw waste and calcium sulfate was carried out by XRD.