Kinetics of vaporization of lead sulfide
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I.
INTRODUCTION
MOST sulfides dissociate into component atoms during vaporization. For instance, zinc sulfide when vaporized splits into Zn(g) and S2(g). Lead sulfide is unusual in the sense that it retains its molecular form PbS, i . e . , it does not dissociate upon vaporization. Furthermore, the vapor pressure of PbS(s) is appreciable even at moderate temperatures like 900 ~ The feasibility of reducing lead sulfide vapor to metallic lead using a hydrogen reductant was investigated by Cole and co-workers.~ In the direct reduction of lead sulfide to metallic lead using a carbon reductant and a lime flux the vaporization step, PbS(s) ~ PbS(g) plays a significant role. The present investigation was undertaken to determine the kinetics of vaporization of particulate lead sulfide from PbS/4CaO solid mixtures. Lime behaves essentially as a chemically inert material; it does help preserve the structural characteristics, i . e . , porosity, pore size, e t c . , of the vaporizing mixture intact. The vaporization kinetics were investigated in the range 785 to 993 ~ using a thermogravimetric apparatus. The effect of ambient (nonreactive) atmosphere on the vaporization kinetics was determined.
II.
PREVIOUS WORK
Much of the published work on vaporization deals with the behavior of single crystals. Typically, all faces save one of a single crystal specimen are covered with platinum foil and the uncovered surface is allowed to vaporize freely into vacuum. Somorjai 2 reported data on the vaporization rates of CdS and CdSe single crystals in the temperature range 600 to 860 ~ and 10 -6 Torr pressure. A mechanism was formulated 2 for the vaporization of these crystals. Activation energies of 210 kJ 9 mole -~ for CdS-vaporization and 234 kJ 9 mole -1 for CdSe-vaporization were reported. The evaporation coefficient was found to be of the order of 0.01
S.K. EL-RAHAIBY, Graduate Research Assistant, and Y.K. RAO, Professor, are both with Metallurgical Engineering, FB-10, University of Washington, Seattle, WA 98195. Manuscript submitted October 13, 1981.
METALLURGICAL TRANSACTIONS B
to 0.1 for both these compounds. Ewing and Stern 3'4 studied the kinetics of vaporization of NaC1, KBr, CsI, LiF, and Ag in the temperature range 440 to 760 ~ and 10 -5 Torr pressure. At temperatures up to 660~ the evaporation coefficient was found to be nearly unity and at higher temperatures it decreased progressively with temperature. The activation energy values ranged from a low of 182 k J . mole -1 for CsI to a high of 265 kJ 9 mole -~ for LiE Howleh et al s reported an activation energy of 279 kJ 9 mole -~ for the vacuum vaporization of LiF single crystals in the range 690 to 820 ~ They also reported 5 a value of 0.17 for the evaporation coefficient which is smaller than that found by Ewing and Stern. 4 Halle and Stern 6 investigated the vaporization and decomposition of Na2SO4 in the temperature range 880 to 1200 ~ by means of thermogravimetry and mass spectrometry; they found the evaporation coefficient to be 0.251. Lam and Munir 7 formulate
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