Environmental Availability Assessment Of Heavy Metal-containing Wastes In The Hydrothermal Sulfidation-floatation Process

Non-ferrous smelting sludge contains a large amount of heavy metals. On one hand, these metals can be recycled as a second resources; one the other hand, these metals would release form wastes though physical, chemical, biological and physiochemical approaches to the surrounding environment, causing persistent hazardous to the ecological system. Thus, discovering and development a technology that can have both sound of metal recovery and environmental protection has been one of the hot topics in the heavy metal-containing waste management. The environmental friendliness of hydrothermal sulfidation-floatation process received attention because its potential metal stabilization as well as metal recovery.Therefore, the environmental availability of heavy metals of this new process was systematic studied in this paper using neutralization sludge as raw materials, which generated from the treatment of nonferrous heavy metal containing waste water by lime. Besides, a new evaluation method was proposed for the potential ecological risk assessment, which combined the features of the content, leaching properties, chemical speciation and the ecological toxicity of heavy metals in tailings based on the former researches. The research can be divided into the following three parts:the environmental availability assessment of heavy metals in neutralization sludge; effects of hydrothermal process on the environmental availability of heavy metals; the environmental risk assessment of heavy metals in hydrothermal-floatation tailings. Innovative conclusions of this research are as follows:(1) The environmental risks of neutralization sludge was systematic studied, and finally this risks was quantitatively described. Zn, Pb, Cd, Cu and As were the main metal elements of the sludge, of which Zn(13.6%) and Pb(0.95%)had the highest content. Pb, Cd, Cu and As exist in the more stable forms, the content in residue forms of them were80.12%,87.98%,91.2%and74.99%, respectively; whereas Zn had a high potential release risk, its content in acid exchangble forms was as high as 30.43%. The leaching toxicity concentration of Zn and Cd were higher than the threshold standard of EPA regulations. Results of the potential ecological risk assessment demonstrated that neutralization sludge had a serious ecological risk (RI=6130.2>>600), and Cd was the main pollution factor, which occupied95.56%of the total risk index.(2) The main hydrothermal sulfidation conditions on the environmental availability of heavy metals were systematic studied and the variation rules of environmental availability of heavy metals in the floatation tailings along with the change of hydrothermal sulfidation parameters was revealed. Sulfidation temperature had a positive effect on the stability of heavy metals in tailings, whereas the variation tendency would slow down when the temperature was higher than240℃. Sulfidation time has little effect on the stabilization of heavy metals in tailings. Sulfur content has an important effect on the stabilization of heavy metals in tailings.When the sulfur content was less than1.4times of the zinc content, metal stability was enhanced with the increase of sulfur content; when the sulfur content is higher than1.4times of the zinc content, metal stability was decreased with the increase ofsulfur content. However, Cd shows a different rule, the stability of Cd was enhanced with the increase of sulfur content. In addition, natural cooling was more benefited to the stabilization of heavy metals in the tailings.(3) The potential environmental risk of heavy metals in the neutralization sludge (NS) and the flotation tailings (FT) was comparative studied, and ultimately the risks of heavy metals in FT was comprehensive described. Compared to NS, the heavy metal content and leaching toxicity(TCLP method) of FT were significantly decreased, especially for Zn and Cd. The leaching concentration of Zn decreased significantly from846.85mg/kg (NS1) to2.586mg/kg (FT1), from646.88mg/kg (NS2) to1.454mg/kg (FT2); while Cd decreased significantly from10.175mg/kg (NS1) to0.124mg/kg (FT1), from8.275mg/kg (NS2) to0.043mg/kg (FT2), respectively. In addition, all of the metal concentration in the leacate of FT were under the threshold standards of EPA regulations and the GB5085.3-2007standards. Results of the BCR analysis confirmed that most of metals were transformed into more stable phases after hydrothermal sulfidation process.(4) Results of potential ecological risk index (RI) assessment indicated that the overall risks caused by heavy metals decreased significantly from6627.59and7243.66(very serious risk) in NS1and NS2, respectively, to80.26and96.27(low risk) in FT1and FT2, respectively. According to the risk assessment code (RAC), all of the heavy metals in FT imposed little risk to the natural environment. Results of simulated acid rain sequential leaching process demonstrated that the long term stability of heavy metals was high. All of metal concentrations in the leacate were lower than Integrated Wastewater Discharge Standard(GB8978-2002). In general, hydrothermal sulfidation-floatation process has a positive effect on the stabilization of heavy metals in wastes and it is a new potential environmental friendly technology.There are25figures,35tables and137references in this thesis.