Gypsum and high quality binders derived from water treatment sediments and spent sulfuric acid: Chemical engineering and environmental aspects

Millions of tons of hazardous spent sulfuric acid and coagulation sediments from water treatment facilities are produced annually. In this work, a green approach for the synthetic gypsum and high-quality binders was proposed. The results obtained were confirmed by XRD, SEM, TEM, and DTA. Optimal parameters for the synthesis, including stirrer rotation speed, reactor temperature, acid concentration, acid feed rate, thickening time were investigated. Synthesized sample was identified as gypsum with a monoclinic structure with high degree of crystallinity and crystals of prismatic shape. The required fraction of more than 50 µm was 86.57 wt.%. Obtained gypsum binders had grades up to G23. Different options for the use of filtrate were proposed. High concentration of iron sulfate demonstrate a good coagulation result which was 5 % lower compared to commercial iron sulfate. Additionally, high concentration of iron made it possible of its use as a precursor for the synthesis of magnetic sorbents and photocatalysts. Neutralized filtrate contains sulfur, calcium, magnesium, and sodium and were tested as a complex fertilizer.