| China is the world's largest producer of lead and zinc. The amount of lead and zinc produced in 2009 reached 3.71 and 4.36 million tons, respectively. Lead and zinc smelting process generates a great deal of sulfur dioxide gas, which is mainly used for the production of sulfuric acid. This smelting gas also contains mercury and other heavy metals. High concentration of acidic wastewater containing heavy metals (shorten as acidic wastewater) was generated in the washing process before the production of sulfuric acid. This acidic wastewater has high acidity and contains various toxic heavy metal ions of high concentration and in complex form. The traditional sulfide approach is difficult to achieve stable discharge standard. If acidic wastewater discharged into the environment without effective treatment, it will bring serious pollution to the water system.This study was aiming at acidic wastewater generated from the gas washing process of lead and zinc smelting in Zhuzhou Smelter, its character was very complex. The content of sulfuric acid in acidic wastewater was 2-4%. It also dissolved high concentration of acidic SO2 gas, which mainly contains heavy metals such as Hg, Cu, Pb, Zn, Cd, As and high concentration of F, Cl, sulfate ions. Treatment of acidic wastewater containing mercury by sulfide method is difficult to stably meet the discharge standards, thus the properties of acidic wastewater was studied and the novel technique was invented to treat acidic wastewater by biologics. The main results are as follows:Based on the properties of acidic wastewater, it can be confirmed that the speciation of mercury in acidic wastewater was complex, which mainly includes three kinds of forms, i.e., granule state, colloid state and ion state. Moreover, thermodynamic studies about treatment of acidic wastewater using sulfide method revealed the reasons why the acidic wastewater treated by sulfide can not meet the emission standards stably. The theoretical foundation was established to solve the difficult problem of removal colloidal mercury from acidic wastewater.The structure of colloidal mercury was studied by electrocapillary curve and the model of colloidal mercury in acidic wastewater was established. When the atomic mercury was entered into the acidic wastewater, it preferentially adsorbed HgCl42- to form three-electron-layer structure. Moreover, its colloidal bond energy was analyzed. The result indicated that the bond energy between the surface of mercury and HgCl42- was weak. The zeta potential of colloidal mercury was calculated according to Gouy-Chapman-Stern model. In addition, effect of metal cations in solution and cationic compounds of organic polymer on the stability of colloidal mercury were also investigated. Thus, a new method for destroying the structure of colloidal mercury in acidic wastewater was developed.Novel technology of "biologics complex-hydrolysis" for treatment of acidic wastewater containing mercury was invented. Water treatment reagent—biologics and a kind of cationic compounds of organic polymer—demercuration were developed to deeply remove mercury and other heavy metal ions from acidic wastewater. Biologics containing various functional groups can be used to remove ionic mercury and other heavy metal ions by forming complexes. Demercuration was used to destroy the structure of colloidal mercury by compressing double-layer. Demercuration was also used to make colloidal mercury sedimentation in flocculation via fishing and bridging effect. Single-factor experiment combined with orthogonal experiment were carried out to determine the optimal conditions for removing mercury and other heavy metals in acidic wastewater using biologics. Under the following optimal conditions:biologics to mercury ratio was 16, the dose of demercuration was 16mg/L, complexing reaction time was 30min, hydrolysis pH was 10.0 and hydrolysis time was 30min, heavy metals in treated water reached Integrated wastewater discharge standard (GB 8978-1996).In order to transfer the laboratory research into productive forces and make the chemical reaction suitable for industrial production, the pilot test (15-30m3/h) was carried out continuously in acidic wastewater section of Zhuzhou Smelter. A multi-level spill response equipment was invented; moreover, the pipe reactor was designed with the help of computer simulation and optimization. The efficient reaction of biologics and demercuration agent with acidic wastewater was realized and the optimal parameters for industrial production were determined as follows: biologics to mercury ratio was 40, the dose of demercuration was 20 g/m3, hydrolysis pH was 10-11. Trial operation of this new technology was carried out in Zhuzhou Smelter based on the original facilities for sulfide method. The results was continuously tracked and monitored for 48 hours by Hunan Environmental Monitoring Station, which indicated that heavy metals in treated water meet Integrated wastewater discharge standard (GB 8978-1996).Based on the characteristics of different species of mercury in acidic wastewater, the new idea of increasing the homogenization process to purify particulate mercury was proposed. In order to offer the design parameters for process modification of acidic wastewater treatment by biologics and provide evidence for the comprehensive utilization of the slag generated from the acidic wastewater treatment, industrial experiment was carried out. The results indicated that simultaneous and deep removal of mercury and heavy metals from acidic wastewater was realized via homogenization, complex and hydrolysis processes of biologics method. Moreover, the content of mercury in the recovered homogenization slag and in the recovered complex slag was 28.31% and 45.08%, respectively. Thus, the recovered homogenization slag and the recovered complex slag can be used as raw materials for smelting to recover the heavy metals. However, the low content of heavy metals in the recovered hydrolysis slag led to its safe disposal.On the basis of the industrial experiment, the system of acidic wastewater treatment in Zhuzhou Smelter was comprehensively upgraded according to the flow of biologics method. The demonstration project of 100m3/h was established. This technology was listed in the advanced pollution control technology of 2009, which provides an available technology for treating domestic acidic wastewater containing mercury. |
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