Thermal treatment of contaminated de-inking sludge in a low-high-low (LHL) temperature reactor was investigated. Experiments were conducted in a Cahn TGA-171 furnace coupled with a Mattson Galaxy 5020 FTIR. Sludge samples (70 g) were heated up to 1770 K (50 K/min.), kept at that temperature for 5 min. and were then quenched (60 K/min.). During the LHL process the sludge underwent structural transformations from a mixture of fibers, through skeleton-like structures (45-110 mu m), porous particles, to compact spheres (5-30 Gun). It was shown that the LHL treatment results in a safe encapsulation of heavy metals (Pb, Cd) inside the ash and in a 78.2% decrease in the amount of mutagenic polycyclic aromatic hydrocarbons. The LHL method developed could help to solve waste management and environmental protection problems.