Acid leaching technology for post-consumer gypsum purification

Abstract

Background: Contaminants and water-soluble salts present in mechanically recycled gypsum from refurbishment and demolition (post-consumer) plasterboard waste limit its use as a secondary raw material in plasterboard manufacturing.  This research addresses this limitation, developing a novel acid leaching purification technology combined with an improved mechanical pre-treatment for post-consumer gypsum valorization.  Methods: Laboratory-scale acid leaching purification was performed with a borosilicate beaker, hot plate, and overhead stirrer.  Stuccos were produced after calcination of gypsum at 150 °C for 3 hours.  Samples were characterized through X-ray fluorescence, X-ray diffraction, thermal gravimetric analysis, scanning electron microscopy and particle size analysis.  Results: Acid leaching at 90 °C for 1 h using a 5 wt% sulfuric acid solution was revealed to be the optimum purification conditions.  Stuccos produced from purified gypsum under optimum conditions had similar initial setting times to that of a commercial stucco but with higher water demand, which could be reduced by optimizing the calcination conditions.  A magnesium-rich gypsum was precipitated from the wastewater. Conclusions: Purified post-consumer gypsum with > 96 wt% chemical purity and calcium sulfate dihydrate content was produced.  The research recommends acid neutralization prior filtration, use of gypsum particles < 2 mm in size, and stirring speed of 50 rpm to reduce the economic and environmental impacts of the acid leaching purification process at industrial scale.  The magnesium-rich gypsum could potentially be marketed as soil fertilizer.