The use of secondary ion mass spectrometry for investigating oxygen in pyrometallurgical reactions
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Communications The Use of Secondary Ion Mass Spectrometry for Investigating Oxygen in Pyrometallurgical Reactions M.A. RHAMDHANI and G.A. BROOKS Oxygen has an important role in pyrometallurgical systems because many of the reactions involve the transfer and reaction of oxygen at an interface. In the case of reactions involving liquid metal, adsorption of oxygen also occurs, resulting in oxygen preferentially residing along the interface due to its surface active nature. In molten metals such as iron and copper, the presence of oxygen at the interface significantly lowers the interfacial tension and may create interfacial tension gradients along the interface. These interfacial tension gradients can enhance the overall kinetics of reactions in these systems.[1,2] Despite the importance and complexity of the role of oxygen in many reactions, there is only limited knowledge of the distribution of oxygen in liquid metal during reactions. An analytical technique for investigating oxygen concentration gradients in metal is essential for better fundamental understanding. However, a major constraint of oxygen characterization in pyrometallurgical systems, for example, in ferrous systems reacting with slag, is that the concentration of oxygen is very low and may range from few ppb to a maximum of 2800 ppm, which is the maximum solubility of oxygen in liquid iron at 1923 K. To the authors’ knowledge, secondary ion mass spectrometry (SIMS) is the only available technique at present that can be used for characterization of oxygen in this range of concentration.[3,4] This article discusses the use of dynamic SIMS for investigating oxygen distribution in samples generated from reactions between Fe-Al droplets and CaO-SiO2-Al2O3 slag. Secondary ion mass spectrometry is a characterization technique based on the mass analysis of either positive or negative ions ejected from the top few monolayers of a solid surface resulting from a sputtering process using a primary ion beam. One of the strengths of SIMS is its detection limits. The detection limit is down to the parts per billion range and applicable for light elements analysis such as oxygen. These capabilities are not possessed by other surface analytical techniques such as Auger electron spectrometry, X-ray photoelectron spectrometry, and Rutherford backscattering spectrometry.[3,4] A drawback of SIMS is the “matrix effect,” which refers to a wide variation of sensitivity with different elements and with the same element in different matrices. Thus, an empirical approach must be used for accurate quantitative analysis of SIMS, for example, by calculating the relative sensitivity factor or developing calibration curves from standard samples of similar composition as the subject M.A. RHAMDHANI, Graduate Student, and G.A. BROOKS, Associate Professor, are with the Department of Materials Science and Engineering, McMaster University, Hamilton, ON Canada L8S 4L7. Contact e-mail: [email protected] Manuscripts submitted December 6, 2002.
METALLURGICAL AND MATERIALS TRANSACTIONS B
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