| Summary: | 碩士 === 國立臺灣科技大學 === 化學工程系 === 91 === Carbothermic reduction of zinc sulfide in the presence of calcium oxide was investigated in this study. A crucible containing powder mixture of zinc sulfide, calcium oxide and carbon black was loaded in an argon flowing thermogravimetic analysis system (TGA). The reaction was proceeded in the temperature range of 1,180 K to 1,353 K. After a certain reaction time, the solid sample was removed and analyzed by using an X-ray diffractometer (XRD), an atomic absorption spectrometer (AAS), a scanning electron microscope (SEM) and a surface area meter (BET). The variations of composition and physical properties of the solid sample during reaction could be known through the analyses mentioned above. The results of TGA gave us the instantaneous weight of the solid sample. Zinc yield could be calculated based on the weight change of the solid sample. Consequently, the relationship between zinc yield and reaction time can be drawn. From these results of the effects of seven parameters on zinc yield were studied.
Results of XRD indicated that the crystal structure of zinc sulfide was first transformed from β type to α type, then reacted to an intermediate product of zinc oxide and finally reduced to zinc vapor evolving from solid sample; sulfur content in zinc sulfide was scavenged as calcium sulfide remaining in the solid sample; carbon black was gasified to carbon monoxide and escaped from solid. The SEM photographs showed that the zinc containing particles and carbon grains disappeared gradually; the calcium containing grains swelled and sintered during reaction. The results of surface area measurement indicated that the surface area of the solid sample decreased drastically in initial stage and then increased with reaction time; the pore volume of the solid sample was also reduced fast initially and then increased slowly; the average pore diameter, however, increased remarkably in initial stage, decreased and then leveled off. These results were explained by considering the phase transformation of zinc sulfide, escape of zinc vapor, gasification of carbon, expansion and sintering of calcium sulfide. If reaction time was fixed at 10,800 s, the variations of the surface area and the pore volume of the solid sample were found to increase and the average pore diameter of the solid sample decrease with temperature. A reaction mechanism and a reaction model were proposed to explain the variations in chemical composition and physical properties of solid sample during reaction, respectively.
Results of TGA indicated that the rate of zinc yield could be increased by increasing the argon flow rate, the reaction temperature, the initial molar ratio of C/ZnS, or the initial molar ratio of CaO/ZnS. Furthermore, the rate was also found to be increased with a decrease in sample height, the agglomerate size of carbon black or the initial bulk density. Based on these experimental data, an empirical rate equation of zinc yield has been determined.
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