Electronic and ionic transport in liquid PbO-SiO 2 systems
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I.
INTRODUCTION
U N L I K E gas/solid and gas/liquid metal reactions very little information has been published on gas/slag reacuons. While our knowledge of gas/slag reactions is far from being complete, it is known that the reaction rates are influenced by several complex factors involving transport of several species and chemical reactions at different reacnon sites. Different views have been expressed regarding the transport of oxygen through liquid slag. For instance. Goto I has suggested that the physical dissolution of oxygen seems more likely than chemical dissolution in PbO-SiO2 melts. He argued that oxygen ions do not contribute to the transport of oxygen through the oxide melt as long as there is no electronic conductivity in the melt. The oxygen permeabihtles experimentally obtained by Goto were greater by a factor of fifty than the permeability obtained theoretmany assuming ionic transport of oxygen anions. Therefore. the major mechanism which supported Goto's results seemed to be the diffusion of physically dissolved oxygen. Caley and Masson 2 investigated the oxidation of liquid lead covered by a layer of PbO-SiO2 melt. They associated the diffusion of Pb 4+ with the oxidation of liquid lead underneath the melt and proposed the following interfacial reactions: 2 P b 2+ + 89O2 -'-+ Pb 4~ + O 2-
Pb + Pb 4+ ---> 2Pb 2+
Gas/slag interface Slag/metal interface
One of the main difficulties in the study of such reacUons is that, due to requirements of electroneutrality, the reactions are generally coupled to other steps in the overall mechanism. Therefore, it is difficult to isolate a single reaction for detailed investigation. For instance, the transport of Pb 4+ is equivalent to electrons hopping between lead ion sites which is the same as transport of electron holes. Alternatively, the oxidation of liquid lead may also be associated with the counter transport of oxygen ions and electrons. In
UDAY PAL, formerly with the Department of Materials Scmnce and Engineering, The Pennsylvama State Umverslty, is now x~lth the Allegheny Ludlum Steel Corporatmn. T DEBROY, Associate Professor of Metallurgy, and G. SIMKOVICH, Professor of Metallurgy. are with the Department of Materials Scmnce and Engineering, The Pennsylvama State University, Umversit3, Park, PA 16802 Manuscript submitted January 26, 1984 METALLURGICAL TRANSACTIONS B
this paper such possibilities are considered and it is found that the oxidation of liquid lead covered by the melt occurs through counter transport of Pb 2' and electron holes. The work involved measuring the oxidation rates of the hquid Pb covered by slags of different depths. The oxidauon measurements were also performed as a function of Po, and Fe203 content in the slag. These results were combined with conductivtty measurements on the slag to determine the transport mechanism involved in the oxidation of liquid lead. Such an analysis also allowed us to estimate the transport number of electrons m the slag.
I1.
EXPERIMENTAL DETAILS
A. Oxidation The oxidation experiments were c
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