Behavior of metals during combustion of industrial organic wastes in supercritical water

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 ru ns were carried out under supercritical water conditions: 30.6 MPa, 450 or 525 degreesC, 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 precipit ated to ash, with leachability varying from 0.5% to 7.3% and decreasing wit h increasing run time and temperature. In runs at 450 degreesC, the soluble Cd concentration showed little or no change, but its ash's leachability dr opped when more oxygen was added. At 65.75 excess oxygen, when temperature was increased from 450 to 525 degreesC, the Cd concentration and ash leacha bility declined, and a downward trend appeared with longer run time. In run s at 450 degreesC and 17.1% excess oxygen, the soluble Cr concentration inc reased 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 th e temperature went up from 450 to 525 degreesC, the soluble Cr concentratio n rose to 26.1% at 5 min and subsequently showed a trend similar to that ob served for the runs at 450 degreesC. At 30.6 MPa, 525 degreesC, and a 30-mi n run time, 100% of the Pb, 97.6% of the Cd, and 87.3% of the Cr were conve rted 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 wer e conducted under the same conditions (pressure/temperature/time). Only 0.0 3% of the Pb but 82.0% of the Cd and 79.4% of the Cr remained soluble. It w as 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. E lectron 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" o f DISR in supercritical water showed an effective removal of heavy metals.