Environmentally responsible production of lime from recycled gypsum and weakly alkaline wastewater

Abstract

Lime (which exists as calcium oxide and calcium hydroxide) is typically obtained by calcinating limestone, which consists of calcium carbonate. The process releases large amounts of carbon dioxide (CO₂) into the atmosphere. Calcium hydroxide (Ca(OH)₂) may also be obtained by treating calcium carbonate or calcium sulphate with an alkaline aqueous solution. These processes generate Ca(OH)₂ without calcinating limestone, thereby avoiding the emission of CO₂. However, the use of alkaline chemicals may still be accompanied by a carbon footprint associated with the manufacturing process (for example, using electricity generated through the combustion of fossil fuels). In this study, we produced Ca(OH)₂ using gypsum (calcium sulphate dihydrate) powder and wastewater containing a small amount of sodium hydroxide (NaOH). Our method constitutes an environmentally responsible process for producing lime, using resources harvested from demolishing buildings and returnable bottle-washing processes. Our results demonstrate that a small amount of NaOH in an aqueous solution can effectively transform gypsum into Ca(OH)₂ and that the composition of Ca(OH)₂ and gypsum can affect the NaOH content and reaction time. We establish a process for transforming wastewater treatment into resource recovery, producing lime in an environmentally responsible manner and augmenting the construction of a carbon-neutral society.