Kinetics of zinc oxide formation from zinc sulfide by reaction with lime in the presence of water vapor
- PDF / 439,758 Bytes
- 6 Pages / 584 x 778 pts Page_size
- 62 Downloads / 200 Views
I.
INTRODUCTION
C O N V E N T I O N A L processes for roasting sulfide minerals often suffer from the problem of sulfur dioxide emissions into the atmosphere. As an alternative, Haver and Wong l and Bartlett and Haung 2 investigated the use of lime to remove sulfur dioxide in copper concentrate roasting. These investigators reported difficulties associated with this process such as the likelihood of extreme local temperatures and ferrite formation. A novel reaction scheme for transforming certain metal sulfides to the corresponding oxides without pollution has recently been developed in this laboratory. 3.4 In this process, water vapor is used as the oxidizing gas in the presence of lime. The reaction of water vapor alone with most metal sulfides has very small equilibrium constants, which make it very unattractive. The presence of lime, however, can effectively lower the partial pressure of hydrogen sulfide produced from the reaction. The overall reaction in this process consists of two coupled gas-solid reactions as given below:
overall:
MexS(s) + H20(g) = MexO(s) + H2S(g)
[1]
CaO(s) + H2S(g) = CaS(s) + H20(g)
[2]
MexS(s) + CaO(s) = MexO(s) + CaS(s)
[3]
It is seen from the above that, although water vapor is the oxidizing gas, it is regenerated in stoichiometric quantity by the second reaction. The overall net reaction is between the metal sulfide and lime. There is no net consumption or regeneration of gaseous species, and, therefore, it is not necessary to maintain a flow of gas into or out of the reactor. The possibility of emitting sulfur-containing gaseous pollutants out of the reactor is thus eliminated. The gaseous species H20 and H2S serve as carriers of oxygen and sulfur between the two solid reactants without H.Y. SOHN is Professor in the Department of Metallurgy and Metallurgical Engineering, University of Utah, Salt Lake City, UT 84112-1183. DAESOO KIM, formerly Graduate Student at the University of Utah, is Head of Specialty Metals Laboratory, Korea Institute of Energy and Resources, Daejeon, Korea. Manuscript submitted May 14, 1987. METALLURGICALTRANSACTIONS B
themselves being consumed or generated. Although the overall reaction is between the two solids producing two solid products, the overall reaction does not proceed at any measurable rate without the gaseous intermediates. 5 In this paper, we describe the results of an investigation on the overall kinetics of reaction [3] for ZnS proceeding through the gaseous intermediates H20 and H2S according to reactions [1] and [2].
II.
THERMODYNAMIC CONSIDERATIONS
If the overall reaction is to proceed according to Eq. [3] involving only the pure solid phases, it must have a negative standard free energy of reaction. Table I summarizes the standard free energies of relevant reactions for the oxidation of ZnS in the temperature range studied. As can be seen, the overall reaction between ZnS and CaO has a negative standard free energy of reaction. It is also seen that the gas phase may contain minor amounts of SO2 and H2. During the actual expe
Data Loading...
