The behavior of selected trace metals during and after thermal treatment of paper-mill sludge

The influence of thermal treatment conditions and structural modifying additives (CaO, CaCO₃, and Na₂CO₃) on the behavior of cadmium, chromium, and lead during and after the thermal treatment of paper-mill sludge was assessed. Cadmium retention by the ash obtained under oxidizing and oxygen-starved conditions decreased significantly with increased treatment temperature. However, equilibrium predictions suggest that CdO•SiO ₂ dominates cadmium speciation under oxidizing conditions for the temperature range investigated (450°C to 1050°C). It is surmised that localized conditions occurring at the particle surface of the sludge resulting in low O₂ concentrations and a high organic content near elevated regions of cadmium are controlling cadmium behavior. Chromium also exhibited decreased ash retention with increased sludge treatment temperatures under both oxidizing and oxygen-starved conditions. Maximum chromium leachability was observed for the 650°C ash. Results from the alkaline extraction indicate that essentially all of the acid-extractable chromium in the 650°C ash is Cr(VI), suggesting that changes in speciation may be controlling chromium release from the ash. Chromium in the ash appeared to oxidize from a relatively insoluble form to a more soluble form as the treatment temperature of the sludge was increased under oxidizing conditions. Under oxygen-starved conditions, chromium oxidation did not appear to occur and extraction from the ash was limited by the low solubility of Cr(III) compounds. Lead did not exhibit appreciable vaporization under oxidizing conditions and equilibrium speciation predictions for lead (PbAl₂Si₂O₈ and PbO•SiO ₂) support the experimental data reasonably well. Acid-extractable lead decreased significantly with increased sludge treatment temperature and residence time. The decrease in acid-extractable lead was significant since the ash also exhibited a decrease in acid neutralization potential across this same temperature range. Changes in particle morphology did not account for the decrease in acid-extractable lead since significant structural changes were not observed until a treatment temperature of 1050°C and particle size reduction did not increase lead extraction. It appears that the lead may have reacted with one or more mineral phases of the high-temperature (≥650°C) ash forming an insoluble lead compound, such as a lead silicate or lead aluminosilicate. The addition of structural modifying compounds did not increase metals retention by the residual ash and, in the case of Na₂CO₃, actually decreased metals retention at various sludge treatment temperatures. Additionally, the addition of CaO, CaCO₃, or Na₂CO₃ to the sludge altered cadmium and lead behavior during thermal treatment, resulting in a cadmium species more susceptible to leaching in acidic environments and a lead species more susceptible to leaching in alkaline environments.