Measurement of oxygen potentials in Ag-Pb system employing oxygen sensor
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NTRODUCTION
THE lead bullion is produced from the smelting of lead ores, traditionally carried out by the sintering of lead concentrate and by the smelting of the resulting sinter in a blast furnace.[1] More modern direct smelting methods, in which these operations are carried out in one step,[2–5] are now being used. The lead bullion produced is a complex alloy containing many of the common metallic impurities that are present in the ore, such as Sb, As, Sn, Cu, and Bi, as well as gold and silver, and, often, platinum-group metals. It is important to refine lead bullion to recover precious metals such as silver, gold, and platinum. The silver concentration in lead bullion could be around 0.4 wt pct, which is equivalent to 4 kg of silver per ton of lead bullion. Therefore, the economics of the process depends entirely on the recovery of the silver. The desilvering of lead bullion is carried out by the Parkes process, during which silver is recovered as an Ag2Zn3 intermetallic compound called Parkes crust.[6] This still contains a significant amount of Pb and Cu, which is removed by oxygen injection using the bottom blown oxygen cupelling (BBOC) process.[7] Lead is removed in the first step, followed by copper; the latter is aided by lead addition. Once lead and copper are removed from silver, nitrogen is blown to deoxidize silver. Currently, the end point of the process of removing Pb, Cu, and O2 from silver is determined by periodic sampling. It is possible to determine not only the end point but also to monitor continuously the removal of Pb, Cu, and O2 from silver using an online oxygen sensor. As part of a larger research program on solid-state sensors and their applications in metallurgical processes,[8,9] studies have been conducted to determine the oxygen potential in molten silver as a function of lead concentration. The concentration of lead was varied between 0.02 and 10.20 wt RAJNISH KURCHANIA, Research Fellow, and GIRISH M. KALE, Lecturer in Mineral Engineering, are with the School of Process, Environmental and Materials Engineering, Department of Mining and Mineral Engineering, University of Leeds, Leeds LS2 9JT, United Kingdom. Manuscript submitted July 19, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS B
pct. The current composition range has been chosen to investigate the effect of decreasing lead content on the electromotive force (EMF) of the oxygen sensor. The solid-state oxygen sensor used for measuring the oxygen potential in a Ag-Pb system at 1323 K can be schematically represented as (⫺)Pt, Ni-NiO//8 mol pct Y2O3-ZrO2(YSZ)//OAg-Pb, Mo(⫹) [I] In the following sections, we report the design of the oxygen sensor and its testing in a molten Ag-Pb system at 1323 K. II. EXPERIMENTAL The physical and electrical properties of a yttia-stabilized zirconia (YSZ) solid electrolyte tube for the fabrication of an oxygen sensor has been reported earlier.[9] In this article, we report on the design and testing of an oxygen sensor for molten silver employing YSZ as the solid electrolyte. The EMF as a function
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