| Summary: | 碩士 === 國立臺北科技大學 === 材料及資源工程系所 === 94 === Due to the increase of steel consumption in Taiwan, the generation of domestic scrap steel also increases. Since the scraps are recycled by electric arc furnace (EAF) process, the production of hazardous waste EAF dust is also increased. Therefore, the recycling of EAF dust becomes an important issue. According to the EPA regulation, EAF dust must be recycled before proper disposal. However, the current capacity of processing EAF dust cannot satisfy the increasing amount of EAF dust. In addition, a series of environmental events, such as illegal landfills and dioxin pollution; further impede the treatment of EAF dust. In this study, the current problem of EAF dust treatment in Taiwan was addressed and the current available resource recycling processes were surveyed. Although the Waelz Rotary Kiln Process had successfully solve the EAF dust recycling problem, the problems of dioxin pollution and the low commercial value of recycled crude zinc oxide still need to be overcome.
In this study, we adopt theory of halide metallurgy as the basis of dechlorization of dust and valuable technology for preparation of nano ZnO particle. According to the Gibbs free energies（ΔG0）of metal chloride and metal oxide, the variables that could affect the dechlorization reaction were investigated. A process combined low temperature roasting with caustic leaching was adopted to selectively extract the zinc, followed by Na2S precipitation to reduce the lead content in the solution. Finally, a process was proposed to prepare nano ZnO particles. In order to find the optimal process to prepare nano ZnO particles from Zn (OH)42-, the effects of concentration, surfactant and the type of acid were investigated. In this study, the current performance of Waelz process was investigated and the suggestion and strategy of EAF dust treatment was also proposed, in order to find economically feasible solutions for both recycling and reduction of EAF dust.
The dust was first mixed with NaBH4 powder to adjust the H/Cl ratio. Then, the sample was roasted at low temperature to promote the efficiency of dehalogenation/hydrogenation reaction. By way of Cl- titration, it was found that the roasting temperature (300℃~500℃), roasting time (5 to 25 minutes) and H/Cl ratio (6.0 to 20.0) would change the dissolution of Cl- from 0.0% to 17.5%. After caustic roasting and leaching of the dust, the zinc (17.4 g/l) and lead (1.6 g/l) recovery ratio were found to be 82.7% and 61.8% respectively. Followed by sodium sulfide precipitation of the lead , the purified solution contained zinc (12.8 g/l) and lead (0.002 g/l). This purified solution was then used for the preparation nano zinc oxide. The ZnO particles were prepare by direct precipitation transformation method at room temperature. Through the adjustment of the acid and alkali concentration to control the pH value within 11.5~12.5 and the addition of dispersing agent, the nanosize ZnO (~ 20 nm) particles can be obtained at proper reaction time.
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