Oxygen Permeation of Partly Molten Slags
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UCTION
MOLTEN oxides or slags are important to metal production.[1,2] They protect the metal and remove undesirable impurities. Usually, a liquid slag layer seals off the liquid metal from oxygen and prevents oxidation. However, some slags have not the protective properties. A high oxygen permeation rate through mixed-conducting partly molten oxide scales, formed on metals during high temperature oxidation, is established.[3–8] The driving force for such permeation is an oxygen electrochemical potential gradient. Under the oxygen electrochemical potential gradients, a high ambipolar conductivity of oxygen ions and electrons results in the high oxygen permeation rate.[9] The solid/liquid interfaces affect the transport properties of partly molten slags.[10–12] In particular, the space charge regions with an enhanced ionic conductivity are formed at the interfaces.[13,14] Electrical and mass transfer along the interfaces is accompanied by change in interfacial tension, which causes Marangoni flow[15] and easy transfer. At the present time, techniques for measuring the oxygen permeation rate through molten slags become a critical tool to either develop novel processes or optimize existing ones. The measurements are frequently difficult because of the high temperatures involved and the consequent slag/container reactions.[16–18] A major
VALERY V. BELOUSOV, Head of Department, is with the A.A. Baikov Institute of Metallurgy and Materials, Russian Academy of Sciences, Leninskii pr. 49, 119991 Moscow, Russia. Contact e-mail: [email protected] Manuscript submitted February 3, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
problem in performing the measurements is the selection of a container material, which is inert to the slag. The partly molten slag method is developed to alleviate this problem.[19–22] The oxygen permeation rates of some extremely reactive oxide melts have been measured by using this method under various oxygen partial pressure gradients and temperatures.[22–25] The transport property data are needed to (i) solve industrial problems with process control and improve product quality and (ii) develop mathematical models of the process. This article summarizes recent laboratory work involving electrical and mass transport measurements aimed at determining conductivity, oxygen ion transference number, and oxygen permeation rate in Bi2O3- and V2O5-containing mixed-conducting partly molten oxide systems.
II.
MATERIALS, SAMPLES AND CHARACTERIZATION
The slag compositions were separated into two groups those containing the Bi2O3 and V2O5 extremely reactive melts: (i) NiO-Bi2O3, In2O3-Bi2O3, and ZnO-Bi2O3 and (ii) ZrO2-V2O5 and Bi2O3-V2O5. Source materials of our samples were NiO (99.9 pct purity), In2O3 (99.9 pct), ZnO (99.9 pct), ZrO2 (99.9 pct), Bi2O3 (99.9 pct), and V2O5 (99.9 pct) powders. In order to prepare the NiO-30, 36, 42, and 48 wt pct Bi2O3, In2O3-30, 36, 42, and 48 wt pct Bi2O3, ZnO-15, 20, 25, and 30 wt pct Bi2O3, ZrV2O7-16, 20, 24, and 28 wt pct V2O5, and BiVO4-5, 7, 10, and 12 wt pct V2O5 ceram
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