Sulfur determination in carbon-saturated iron by solid-state electrochemical sensor
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I.
INTRODUCTION
IN the iron-making and steelmaking industry, there is a continuous effort to increase the productivity and to improve the quality of the final product. One of the key points for achieving such objectives is a careful process control. In this context, monitoring chemical and physicochemical parameters involved in the process is of fundamental importance. In this framework, the research on specific sensors to monitor chemical species is in full development, because of the increasing demand of automation coming from the plants. One of the most important species to be monitored is sulfur. The importance to reach a low sulfur content in order to have good steel quality with respect to brittleness is well known. Currently, sulfur content is measured by conventional chemical analysis on samples taken up from the metal bath during process operations. In general, this operation is time-consuming both for sampling and analysis. An in situ continuous sulfur measurement would then be convenient in conventional batch processes; it becomes mandatory in the development of continuous processes, which represent the latest trend in the steelmaking industry. Note that continuous refining treatments have recently received novel attention in the European steelmaking industry in the frame of the continuous steel production project [see for reference European Direct Steel Feasibility Study, ECSC Agreement 7215-AA/401 (92-T1.2)]. DANIELE GOZZI, Professor of Theory of Electrolytes and Physical Chemistry, is with the Department of Chemistry, University of Rome. "La Sapienza," Rome, Italy. PAOLO GRANATI, Researcher, is with the Ironmaking Steelmaking Department, Centro Sviluppo Materiali, Rome, Italy. Manuscript submitted November 12, 1993. METALLURGICALAND MATERIALSTRANSACTIONS B
Electrochemical probes were already extensively tH used in steelmaking for oxygen monitoring, and the electrochemistry-based techniques seem at present the most promising way to measure in situ and, possibly, in continuous mode the content of chemical species dissolved in liquid iron. Some examples of sulfur probes based on CaS solid electrolytes were reported in the literature, ~21but the high electronic conductivity of CaS at steelmaking temperatures strongly reduced the possibility to use such probes in the steel industry. The difficulty of finding a true sulfur ionic conductor has led us to search for an indirect electrochemical probe using an alkaline earth ff'-alumina as an ionic conductor coupled to a semiconductor containing both sulfur and the cation of the ff'-alumina. A possible choice is Ca/3"-alumina coupled to CaS. The purpose of this article is to show some results on the determination of sulfur in iron carbon-saturated melts obtained by making use of solid-state electrochemical sensors having Caff'-alumina as the solid electrolyte. Probes were, in all their parts, designed, produced, and assembled at the Centro Sviluppo Materiali, Rome, Italy, where this work was carried out. II.
BASIC PRINCIPLE
The basic principle of the sensor
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