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Ind. Eng. Chem. Res. 2000, 39, 4536-4542
Behavior of Metals during Combustion of Industrial Organic Wastes in Supercritical Water Zhen Fang, Si-kun Xu, and Janusz A. Kozin ´ ski* McGill University Energy & Environmental Research Group, Department of Minerals & Materials Engineering, 3610 University Street, M.H. Wong Building, Room 2160, Montreal, QC, Canada, H3A 2B2
De-inking solid residue (DISR) doped with nitrates of 2000 mg/kg of Pb, Cd, and Cr was burned in supercritical water in a batch reactor. Combustion runs were carried out under supercritical water conditions: 30.6 MPa, 450 or 525 °C, and 17.1% or 65.7% excess oxygen. The run time varied between 5 and 30 min. In all runs, more than 99.2% (up to 100%) of the Pb precipitated to ash, with leachability varying from 0.5% to 7.3% and decreasing with increasing run time and temperature. In runs at 450 °C, the soluble Cd concentration showed little or no change, but its ash’s leachability dropped when more oxygen was added. At 65.7% excess oxygen, when temperature was increased from 450 to 525 °C, the Cd concentration and ash leachability declined, and a downward trend appeared with longer run time. In runs at 450 °C and 17.1% excess oxygen, the soluble Cr concentration increased with time from 4.1% (5 min) to 19.2% (30 min). When 65.7% oxygen was applied, it declined to 12.6% at 30 min, which was followed by an increase from 15.0% (5 min) to 37.5% (15 min). In the runs with 65.7% oxygen, as the temperature went up from 450 to 525 °C, the soluble Cr concentration rose to 26.1% at 5 min and subsequently showed a trend similar to that observed for the runs at 450 °C. At 30.6 MPa, 525 °C, and a 30-min run time, 100% of the Pb, 97.6% of the Cd, and 87.3% of the Cr were converted to an insoluble substance. Only 0.5% of the Pb, 0.6% of the Cd and 0.8% of the Cr in ash were leached. Tests with 20,000 mg/kg Pb, Cd, and Cr were conducted under the same conditions (pressure/temperature/time). Only 0.03% of the Pb but 82.0% of the Cd and 79.4% of the Cr remained soluble. It was found that CO2 and acetate from organics combustion could help to remove heavy metals via formation of insoluble carbonate salts. X-ray diffraction spectra indicated the presence of PbCrO4 and Al2Si2O5(OH)4 in the ash. Electron microprobe results showed a close connection between Pb and Cr but no relation between Pb and Cd in the ash. The main solid products were CdO, CdCO3, CrO2, HCrO2, PbCrO4, PbCO3 and PbOx. In general, the “combustion” of DISR in supercritical water showed an effective removal of heavy metals. 1. Introduction De-inking solid residue (DISR) is an organic waste generated by the pulp and paper industry. It contains heavy hydrocarbons and inorganics (ash ≈ 20%) including heavy metals. Recently, Kozinski et al.1 and Zheng et al.2 found that, when DISR was burned in a multitemperature reactor, a safe encapsulation of heavy metals (Pb, Cd) inside ash could be achieved. However, this method requires a high temperature of about 1500 °C. Therefore, a less energy consuming approach based on a low-temperature (
