Risk minimisation of FGD gypsum leachates by incorporation of aluminium sulphate

The incorporation of aluminiumsulphate to (flue gas desulphurisation) FGD gypsumbefore its disposal was investigated as a way tominimise the risk supposed by the high fluoride content of its leachates.Using a bathmethod the kinetic and equilibriumprocesses of fluoride removal by aluminium sulphate were studied at fluoride/aluminium molar concentration (F/Al) ratios in the range 1.75 10−2–1.75 under the pH conditions (about 6.5) of FGD gypsumleachates. Itwas found that fluoride removal was a very fast process at any of the (F/Al) ratios subject of study, with equilibrium attained within the first 15 min of interaction. High decreases in solution fluoride concentrations (50–80%) were found at the equilibrium state. The use of aluminium sulphate in the stabilization of FGD gypsum proved to greatly decrease its fluoride leachable content (in the range 20–90% for aluminium sulphate doses of 0.1–5%, as determined by the European standard EN 12457-4). Such fluoride leaching minimisation assures the characterization of this by-product as a waste acceptable at landfills for non-hazardous wastes according to the Council Decision 2003/33/EC on waste disposal. Furthermore, as derived fromcolumn leaching studies, the proposed stabilization systemshowed to be highly effective in simulated conditions of disposal, displaying fluoride leaching reduction values about 55 and 80% for aluminium sulphate added amounts of 1 and 2%, respectively.