Facile preparation of α-calcium sulfate hemihydrate whisker from by-product gypsum in chloride-free salt solution system

Abstract

Lactic acid gypsum (LAG), the by-product from lactic acid preparation, has caused land occupation and environmental pollution. Herein, hemihydrate gypsum (HHG) whisker, an inorganic material with excellent performance, prepared using LAG as the raw material via atmospheric pressure salt solution method was investigated. Nitrate enhanced the salt effect of the solution, which increased the solubility of dihydrate gypsum (DDG), and further accelerated the conversion into HHG. NaNO3 was proved to accelerate the formation of α- HHG whisker, while Mg(NO3)2 promoted its aspect ratio. The whiskers with an average length of 75.9 µm and aspect ratio of 17.8 were achieved under optimum conditions of NaNO3/Mg(NO3)2–0.5:1, mass fraction of LAG–15%, reagent concentration–40%, and the reaction temperature–100 °C. Mg2+ was found to bind with SO42- on the side of the whisker, promoting the growth of the whisker along the c-axis orientation and forming the morphology of high aspect ratios. Hence, it was feasible to achieve the recycling of salt medium by adding medicament containing Mg2+. By employing direct recycling, the salt medium was limited to be effective in two cycles. However, with the addition of the medicament, the salt medium can now be effectively recycled up to four times or more. Such technique realized the high-value utilization of industrial by-product gypsum, guided the HHG whisker preparation via the green and convenient method and reduced the environmental hazards.