Trace metals behavior during the thermal treatment of paper-mill sludge

The objective of the present study is to investigate trace metals (Cd, Cr, and Pb) behavior during thermal treatment of sludge wastes. Paper mill sludge enriched with Cd, Cr, and Pb was subjected to different thermal regimes using two different bench-scale reactors. Metals retention in the resultant ash is discussed as is the influence of structural modifying additives. Ash was characterized using leachability tests to determine the environmental availability of the trace metals and provide insight as to the extent of metals immobilization within the ash matrix. The bulk of the ash appears to be supermicron particles composed of individual fragments kept together by an irregular framework. Ash morphology appears unchanged throughout the temperature regime investigated with the exception of the 900 °C ash which exhibited signs of localized melting. Cadmium and chromium partitioning behavior is similar for both reactors, with metal retention by the ash strongly influenced by treatment temperature. Lead behavior exhibits significant differences between the two reactors. Under oxidizing conditions, 95 to 100% of the lead is retained by the ash and occurs as an immobilized form resistant to leaching using a strong mineral acid (HNO3). Structural modifying additives did not increase metals retention or decrease metals leachability from the ash matrix. Alternatively, these additives appear to have a detrimental effect upon metals immobilization by the ash.