Immobilization by sulfuration of waste heavy metals

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The present study is concerned with a novel method to immobilize and deposit heavy metals contained in fly ash from waste incineration facilities.The toxic heavy metals such as lead and zinc are present as soluble compounds in fly ash such as metal chlorides and oxides. This study was based on the fact that heavy metals naturally exist as insoluble sulfides and that asphalt is water-repellent. We propose the physical and chemical immobilization of waste heavy metals by mixing with asphalt, sulfur and alkaline agents-sodium hydroxide and calcium hydroxide, in a bi-axial kneader heated at 403-523K.Calculations on the Gibbs' free energies showed that sulfuration of heavy metals resulted from the formation of either sodium sulfide or calcium sulfide via the reaction of sulfur with alkaline agents. Experimental results from XRD analysis of soluble metal compounds mixed with sulfur and alkaline agents also indicated that soluble metal compounds were convertable to insoluble metal sulfides only if either sodium sulfide or calcium sulfide is initially formed. It was found that the addition of sulfur and sodium hydroxide into fly ash at 403K led to the sulfuration of heavy metals. Similarly, the addition of sulfur and calcium hydroxide at 523K brought about sulfuration of heavy metals. The results of the leachability test for the treated fly ash by the proposed immobilization method indicated that leaching of heavy metals in fly ash was suppressible.