A methodologic approach based on hydrocyclone separation and acid leaching to purify gypsum from refurbishment plasterboard waste

One of the main challenges in recycling gypsum from refurbishment plasterboard waste for the production of new plasterboards comes from the difficulty to achieve consistent purity values higher than 92 wt% via current treatment technologies. These treatments generally rely solely on magnetic and manual physical separation combined with grinding. Although gypsum purity values ranging from 75 wt% to 90 wt% can be achieved, this depends mainly on the quality of the refurbishment plasterboard waste received, as current treatments have limited efficiency and customisation possibilities. As a result, the highest practical rate of recycled gypsum in new plasterboards is currently around 20 wt%. Therefore, physical and chemical treatments have the potential to increase the content of recycled gypsum in new plasterboards. As part of the ICEBERG project from the European Union's Horizon 2020 Research and Innovation Framework Programme under grant agreement No. 869336, hydrocyclone separation and acid purification have been evaluated in this work to achieve purity values above 96 wt% in gypsum from refurbishment plasterboard waste. The plasterboards were crushed and sieved to obtain gypsum particles less than 250 µm in size. For hydrocyclone separation testing, a centrifugal pump was used to produce a gypsum slurry in a mixing tank and to feed the slurry to a commercial mini-hydrocyclone of 2 inches in diameter. Acid leaching was performed at atmospheric pressure on the initial waste gypsum and on a sample collected at the hydrocyclone, using different temperatures, residence times and sulphuric acid solutions. The purity of gypsum obtained from individual and sequential hydrocyclone separation and acid leaching tests was determined through X-ray fluorescence. Furthermore, the calcination behaviour dihydrate phase content of the sample with the highest purity was determined through thermal gravimetric analysis. Hydrocyclone separation was not effective for gypsum purification but acid leaching at 90 °C for 1 hour using a 5 wt% H2SO4 solution produced recycled gypsum with more than 96 wt% chemical purity and dihydrate content.