Implications of moisture content determination in the environmental characterisation of FGD gypsum for its disposal in landfills

The leachable contents of elements of environmental concern considered in the Council Decision 2003/33/EC on waste disposal were determined in flue gas desulphurisation (FGD) gypsum. To this end, leaching tests were performed following the standard EN-12457-4 which specifies the determination of the dry mass of the material at 105 °C and the use of a liquid to solid (L/S) ratio of 10 l kg−1 dry matter. Additionally, leaching tests were also carried out taking into account the dry mass of the material at 60 °C and using different L/S ratios (2, 5, 8, 10, 15 and 20 l kg−1 dry matter). It was found that the dry mass determination at 105 °C turns out to be inappropriate for FGD gypsum since at this temperature gypsum transforms into bassanite, and so, in addition to moisture content, crystalline water is removed. As a consequence the moisture content is overvalued (about 16%), what makes consider a lower L/S ratio than that specified by the standard EN-12457-4. As a result the leachable contents in FGD gypsum are, in general, overestimated, what could lead to more strict environmental requirements for FGD gypsum when considering its disposal in landfills, specially concerning those elements (e.g., F) risking the characterisation of FGD gypsum as a waste acceptable at landfills for non-hazardous wastes.