Zinc recovery from spent ZnO catalyst by carbon in the presence of calcium carbonate

Zinc recovery from the spent zinc oxide catalyst by carbon in the presence of calcium carbonate was studied using an X-ray diffractometer (XRD), an atomic absorption spectrometer (AAS), and a scanning electron microscope (SEM). The spent zinc oxide catalyst was determined to be composed of 87.5 wt pct zinc oxide and 3.1 wt pct zinc sulfide. The results of X-ray diffractometry revealed that calcium carbonate decomposed to calcium oxide and carbon dioxide; zinc oxide and zinc sulfide were reduced to zinc vapor and carbon monoxide evolving from solid sample; and sulfur content was scavenged as calcium sulfide remained in the solid. Steps involved in this reaction system were summarized to explain the overall reaction. The experimental results of atomic absorption spectrometry showed that the initial rate of zinc recovery and final zinc recovery can be increased by increasing either the sample height, the reaction temperature or the initial bulk density. Furthermore, they were found to increase with decrease in either the argon flow rate, the molar ratio of Zn-total/C, the molar ratio of Zn-total/CaCO3, the grain size of the spent catalyst, the agglomerate size of carbon, or the agglomerate size of calcium carbonate. Empirical expressions of the initial rate of zinc recovery and final zinc recovery have been determined.