Heavy Metal Residue Sulfur Fixed Stabilization

Aiming at developing a novel technology which can both separate and recycle valued metals in the heavy-metal containing waste residues and comprehensively and economically utilize solid wastes after separation of metals, the sulfur immobilization process of solid residues, through synthesizing man-made sulfuration minerals and exploring sulfur-containing construction material, was proposed in this research. Sulfur, abundant in our country, is used to sulfurize heavy metals in the residues to form metal sulfide, which can then be separated by flotation process, and the remaining heavy metals in the residues will be immobilized by encapsulation and solidification. Furthermore, the immobilized solid wastes can be developed into a novel construction material which has several incomparable advantages.As the initial work of this large project, the paper was focused on confirming the optimal technical conditions for sulfur immobilization process, developing effective additives which can enhance the immobilization effect of heavy metals, and studying the promoting and strengthening process of metal sulfuration to serve for the further heavy metals recovery. The following results were gained:(1) The optimal technical conditions for immobilizing heavy metals in the waste residues using elemental sulfur were confirmed. In the experiment, sulfuration-neutralization sludge produced in a smelter factory, with the Cd leaching toxicity much higher than the identification standard, was used as the experimental material. The heavy metals Cd and Zn could be immobilized effectively by using elemental sulfur, and the blend of big and small particles of residue was propitious to Cd immobilization. The effect of sulfur immobilization on heavy metals was improved with the amount of sulfur addition increasing. The Cd concentration in the leachate of the solidified sludge with a sulfur addition rate of 55% was 0.248 mg/L, which was lower than 0.3 mg/L regulated in the identification standard. A satisfying effect of sulfur immobilization could be achieved in a very short heating and agitating time at the optimal temperature of 140℃. The cooling manner of the solidified body had minor influence on both the immobilization of heavy metals and the surface morphology of the solidified body. Moreover, the long-time leaching test showed that heavy metals in the sulfur-solidified sludge were encapsulated and immobilized so effectively that no longer leached out.(2) Effective additives which can enhance the immobilization effect of heavy metals were developed. Among the seven investigated additives, via NaOH, Na₂SO₃, Ca(OH)₂, NaNO₂, Na₂S₂O₃·5H₂O, Na₂S·9H₂O and Na₂CO₃, only NaOH, Ca(OH)₂ and Na₂CO₃ were effective concerning to decrease the leaching toxicity of the solidified body. Both NaOH and Ca(OH)₂ could greatly improve the performance of sulfur immobilization, Na₂CO₃ could also enhance the immobilization effect, however, to a less extent. NaOH was the only additive which could decrease the leaching toxicity of the sulfur immobilized solidified body in acidic leaching environment. Moreover, with the presence of certain amount of effective additives, the sulfur addition amount required in the sulfur immobilization process to put the sulfuration neutralization sludge passing the identification standard for hazardous materials could be reduced. In the sulfur immobilization process without additives, the immobilization effect of heavy metals was achieved by macroencapsulation and microencapsulation in the solidified body. Ca(OH)₂ and Na₂CO₃ modified the S/S process by increasing the pH value of the leachate and converting heavy metals into insoluble metal hydroxides at high pH value, whereas NaOH gained the modification effect by promoting metal sulfuration reaction and forming stable and insoluble metal sulfides, thus is effective in both neutral and acidic medium.(3) To serve for the further heavy metals recovery work using sulfuration and flotation technology, the promoting and strengthening process of metal sulfuration was studied. Both thermodynamic calculation and probative experiment showed that the sulfuration tendency of different heavy metals was in order of Pb>Cd>Zn. When using mechanic activation to promote metal sulfuration, the X-ray diffraction peaks of experimental samples were shortened and widened with increasing ball milling time. New crystals of PbS and PbSO₄ were produced in the mixture of PbO and S after ball milling for 5h whereas the diffraction peaks of CdS was only observed in the mixture of CdO and S after ball milling for 60h. However, no new diffraction peak was found in the ball milling products of ZnO and S even after 60h. For heavy metals Cd and Zn, mechanical activation alone could not effectively realize their metal sulfuration, however, with promoting process of heating and adding additive, the sulfuration of Cd and Zn could be realized more easily. Moreover, when adopting pyrite (FeS₂) as a sulfur source instead of elemental sulfur, metal sulfuration reaction could occur in a relatively short ball milling time, and excessive pyrite could further promote the sulfuration process of CdO and ZnO.