Recycling industrial byproduct gypsum for use as plastering materials by the tandem pyro-hydro process: impurities removal, whiteness improvement, and regularity of phase evolution

In China, large amounts of industrial byproduct gypsum (IBG) end up as waste and excessive stockpiling due to its low whiteness and high impurity content. In this study, two typical IBG including phosphogypsum (PG) and flue gas desulfurization gypsum (FGDG) were the research object. The impurities removal and the whiteness improvement by the tandem pyro-hydro process were investigated. Species of impurities, mechanism of whiteness improvement and the evolution of the gypsum phase during the tandem process were revealed. The results indicate that the impurities responsible for the poor whiteness of IBG included organic matter, carbon particles, and silicates. The temperature of the pyro process is the critical factor. The IBG is calcinated at 400–500 °C to fully remove organic matter and carbon particles, while decompose silicates to silica. Following, in the hydro step, the calcined IBG was treated with dilute sulfuric acid (0.2–1.0 M) to remove ion impurities such as Fe(III), the higher temperature and higher acidity are favorable to the production of valuable CaSO₄. After treatment, the whiteness of IBG was improved from below 40% to above 80%, and the soluble impurity content can meet the standards. Moreover, both PG and FGDG products exhibited a compressive strength exceeding 10.0 MPa of curing for 7 d. This work provides theoretical guidance to promote the resource utilization of IBG, especially as plastering materials after purification and whitening.