Old dumps of mercury waste sludges from chlor-alkaline industry are an environmental threat if not properly secured. Thermal retortion can be used to remove mercury from such wastes. This treatment reduces the total mercury content, and also may reduce the leachability of the residual mercury. The effects of treatment temperature and treatment time on both residual mercury levels and mercury leachability according to the US EPA TCLP leaching procedure, were investigated. Treatment for 1 h at 800 degrees C allowed to quantitatively remove the mercury. Treatment at 400 degrees C and above allowed to decrease the leachable Hg contents to below the US EPA regulations. The ultimate choice of treatment conditions will depend on requirements of further handling options and cost considerations. (C) 2010 Elsevier B.V. All rights reserved.