Comparison of immobilisation of air pollution control residues with cement and with silica

Cement as agent for immobilising Pb from air pollution control residues is compared with the use of different silica-containing materials. The DIN 38414-S4 leaching test was used to control Pb leachability and to compare obtained Pb leachate concentrations with the landfill limit of 2 mg/l for Pb. Firstly, one scrubber residues was treated with cement and micro-silica. With cement, the Pb leachability could be reduced with a factor ranging from 3 to 50 depending on the type and amount of cement used and depending on the curing time. The landfill limit of 2 mg/l was, however, never attained. From all tested silica-containing additives, aerosil could reduce the initial Pb leaching (101.3mg/l) to below the detection limit at a dosage of 0.13 g aerosil/g residue. Second best and an economically preferable silica-containing additive was micro-silica: a reduction from 101.3 to 0.7 mg/l was observed at a dosage of 0.4 g micro-silica/g residue. The formation of Ca-silicates was found to be responsible for the decreased Pb leachability. To generalise the findings, the Pb leachability of five cement-treated and five micro-silica-treated air pollution control residues were compared. For three scrubber residues, 2-20 times lower Pb leachate concentrations were measured for micro-silica-treated samples (cured for 5 weeks) than cement-treated samples. For a fly ash and a boiler ash the difference was, respectively, 48 and 17 times. pH-dependent leaching tests showed that at pH=2.5, Pb leaching is 250 times lower for the micro-silica-treated residue than for the cement-treated residue and almost seven times lower at pH 12.4.