Study On Novel Sulfidation Technologies In Managing Zn&Pb Smelting Waste

Tons of heavy metal containing wastes are generated every year by zinc-lead smelters, whose total amount is more than100million tons. The management of these wastes is one of the great subject in environmental protection field. However, current methods are difficult to realize the resourcelization and stablization of heavy metals. For example, the widely-used direct extraction technique leads to a large quantity of wastewater and residue containing unstable state heavy metals; while cement solidification often causes the losses of heavy metal. Sulfidation treatment is a novel method to deal with heavy metal containing wastes. Through this approach the heavy metal can be converted into metal sulfide, the metal in which, subsequently, can be separated and recycled by means of floatation technique, and moreover the managed wastes are fairly stabilized after the treatment. According to the survey and the analysis of the heavy metal containing wastes from zinc-lead smelters, the stockpile wastes—namely volatilization kiln slag (VKS), atmospheric enriched-oxygen leaching residue (AELR) and neutralization (NZ)—are determined as research contents. In this research, three methods of dry-milling sulfidation, wet-milling sulfidation and hydrothermal sulfidation are firstly applied to deal with these wastes and the reaction mechanism of the three methods and the characteristics of reaction process are discussed.The analysis on chemical composition and on environmental activation of the three categories reveals that the total composition of Zn&Pb in VKS is up to2%-3%and its heavy metal leaching concentration exceeds the national limits; the AELR contains5%of Zn and5%of Pb, in which the leaching concentration is also far beyond the national requirement. Though in NS, the contents of zinc is as high as15%-20%, the leaching concentration, however, can stay under the national limits since the heavy metal remains a stable property. Based on the results gained above together with the exploratory experiments on sulfidation of heavy metal oxide, three sulfidation technologies are introduced respectively to manage the three kinds of stockpiles. The dry-milling sulfidation is suitable for VKS with sulfur as sulfidizer, the wet-milling sulfidation is for AELR with sodium sulfide (Na2S) as sulfidizer and the hydrothermal sulfidation for NS with sulfur as sulfidizer.The mechanism of the mechanically induced self-propagating sulfidation reaction (MSR) is systematically investigated in this research and the conversion of VKS is stabilized. The result of the dry-milling method shows that the optimum parameter is:sulfur:4.5%; milling time:1hour; ball-to-material ratio:20:1, and after the treatment95.4%of Zn and94.2%of Pb can be vulcanized, and the heavy metal concentration in leachate of the treated waste is under the allowable limit. The research also goes to study the crystal oxide structure variation of the heavy metal compounds as well as the features of other components. It is found that the vital factor to determine the mechanically induced self-propagating sulfidation reaction is adiabatic temperature (Tad) in the reactor, while the variation of crystal structure during the milling process can promote or hinder the reaction. The existence of iron powder (Fe) can induce the self-propagating sulfidation reaction, whose mechanism can be explained by the increasing of the Tad. The treated VKS can be used to make prefabricated sulfur materials for building. The optimal approach to solidification is heating step by step with40%of sulfur addition, particle size less than150μm and15%of aggregate in the filler. Under the conditions above, the compressive strength of solidified stuff can reach up to35MPa, and water absorption up to4%.Heavy metal in AELR can be efficiently converted into metal sulfide when the surface active effect is caused by wet-milling sulfidation. The optimum parameters in operating process of the wet-milling sulfidation are established as the following:the reagent dosage of Na2S:8.4%, B/M:10:1, ball-milling time:1hour. Under these experimental conditions,73.2%of Pb in leaching residue can be converted to PbS. The shrinking core model discloses the cause that mechanic force can promote the heavy metal sulfidation, and the research shows that the sulfidation of PbSO4is controlled by inner diffusion of the reactants and the intensive mechanical stressing would result in faster kinetics by using fine particles to minimize diffusion problems. Flotation test indicates that up to50%of Pb, Cu and Cd could be recovered from the sludge. But the amount of sulfur in the sludge and the size of sulfide particles generated will determine the recovery during the process of flotation. TCLP and continuous leaching test reveal that the heavy metal concentration in leachate from the treated sludge is lower than the allowable limit. And the heavy metal left in the tailings is in a long-term stable state.The Zn in the NS can efficiently be converted to zinc sulfide through dissolution-recrystallization process under hydrothermal conditions. The optimum parameters of operating process for hydrothermal sulfidation are: reaction time:2hours; temperature:200℃; sulfur concentration:15%; pulp density:300g·L-1; pH value:10plus. The result of the above shows that85%of Zn and75.4%of Pb in the sludge can be vulcanized. The entire course of hydrothermal sulfidation consists of the following stages: disproportionation reaction of sulfur, dissolution of metals, formation of CaSO4, synthesis of complexes with ligands containing sulphur atoms, and crystallization.33.3%of Zn,58.9%of Pb and68.8%of Cu can be recovered from the sludge by floatation. The lower recovery of ZnS is due to the small ball-shaped and heavily clustered particles with rough surface, which decline the floatability. But the poor floatability of ZnS can be improved by crystal modification. With an increase of temperature to260℃, reaction time to4hours, and adjusting initial Zn concentration to10%, the recovery is raised from33.3%to72.8%. The TCLP results indicate that all the leached heavy metal concentrations form floatation tailings are under the allowable limit. And the heavy metal in the tailings possesses a property of a long-term stability.During the research, the "Hydrothermal Sulfidation and Floatation Treatment of Heavy-Metal-Containing Waste Pilot Experimental System" is developed and the pilot sulfidation experiment on NS is implemented as well. A series of experiments have witnessed that this pilot system is very well-functioning in terms of operative stability, a high rate of sulfdation and low secondary pollution. After floatation, the Zn sulfidation extent can reach up to87.34%. NS can produce Zn&Pb sulfide concentrate with a grade about40%and a recovery above65%.