Production of multiphase plaster and anhydrite from Mae Moh flue-gas desulphurised gypsum

Precleaning of flue-gas desulphurised gypsum (FGD gypsum), a waste product from the Mae Moh power plant, by a hydrocyclone classifier, acid leaching and sedimentation significantly reduced the content of impurities in the FGD gypsum, such as fly ash, quartz and iron compounds, resulting in a lighter colour and higher purity product (95·60% dihydrate). Subsequently, the semidry acid-anhydrite technique was adapted to the calcining of the precleaned FGD gypsum to reduce the transition temperature of the anhydrite. Accordingly, low-temperature multiphase plaster was prepared using a two-step calcination. The FGD gypsum with sulphuric acid was first calcined at 60°C for 1 h and then recalcined at 150°C for 3 h. The obtained multiphase plaster consisted of 29% hemihydrate, 60% anhydrite II, 10% anhydrite III and 1% dihydrate. At a water to plaster ratio of 0·6, its viscosity and initial and final setting times were 3633 centipoises and 16·28 and 27·83 minutes, respectively. The flexural strength and hardness of the plaster specimens were 5·07 and 10·79 N/mm2. However, the remaining impurities imparted a buff colour to the multiphase plaster. Anhydrite (95% anhydrite II) with crystal morphologies ranging from prismatic to plate-like shapes was achieved by calcining at 100°C for 3 h. Its whiteness measured according to the CIE system was 92·64%.