Cell measurements of the reduction potentials of gas-phase emanating from PbS/CaO/C at elevated temperatures
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I.
INTRODUCTION
A single-step direct reduction process that produces lead from lead sulfide without sulfur oxide emission is based on the following reaction: PbS(s) + CaO(s) + yC(s) -- Pb(1) + CaS(s) + ( 2 y - 1)CO(g) + (1 - y)CO2(g) The value o f y , the stoichiometric coefficient for the carbon reductant, depends upon a number of factors including the reactivity of carbon, particle size o f solid reactants, temperature, and the presence of catalytic additives. A value close to unity for y implies that the product gas is virtually all CO(g), a condition thermodynamically feasible only at high temperatures ( > 8 0 0 °C). The foregoing reaction, involving as it does three solid reactants, necessarily occurs through gaseous intermediates. A plausible mechanistic scheme consisting o f two consecutive heterogeneous reaction steps is given below: PbS(s) + CaO(s) + CO(g) = Pb(l) + GaS(s) + CO2(g) [1] C(s) + COz(g) = 2CO(g)
[2]
Clearly, the reduction potential, i . e . , the CO/CO2 ratio, o f the gas-phase in the P b S / C a O / C system depends upon the relative rates of these two reactions. When reaction [1] occurs quite rapidly as compared to reaction [2], the gasphasetends to contain excess CO2 with the result the reduction potentials remain small. When the reverse condition prevails, i . e . , reaction [2] is significantly faster than reaction [1], the gas-phase reduction potentials tend to be large. Between these two extremes lies the vast range of possible CO/CO2 ratios which correspond to situations when neither of the two reaction steps exercises a dominant rate-controlling influence. In a recently concluded study,~ a detailed investigation was made o f the kinetics of reduction processes in the S.K. EL-RAHAIBY, Postdoctoral Research Associate, is with the School of Materials Engineering, Purdue University, West Lafayette, IN 47907. Y.K. RAO is Professor, Department of Materials Science and Engineering, FB-10, University of Washington, Seattle, W A 98195. Manuscript submittedMay 10, 1983. METALLURGICAL
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heterogeneous system PbS/4CaO/4C at elevated temperatures. In this system a large excess o f CaO and C, over that required stoichiometrically, was maintained to minimize the loss of PbS species through vaporization. In the course of the investigation it was found that certain catalytic reagents significantly enhance the reduction kinetics. It was thought that the key to understanding the catalysis mechanism lies in the nature o f the gas-phasegenerated within the PbS/4CaO/4C system. Since the reduction system itself is kept immersed in a purified inert gas atmosphere (N2 or He), clearly the pore space within the PbS/4CaO/4C mixture will contain significant concentrations o f N2 or He as the case may be. It is well to note that it is the remaining constituents, i . e . , CO and CO2, o f the gas-phase that play a vital role in bringing about the overall conversion o f PbS(s) to Pb(1). At any given temperature, it is expected that the reduction potential (CO/CO2) ratio ofthe gas-phase changes cont
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