Basic Nickel Carbonate: Part I. Microstructure and Phase Changes during Oxidation and Reduction Processes
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THIS article describes a series of fundamental studies to identify the elemental reactions and phenomena taking place during oxidation and reduction processes used in production of solid Ni metal. A great deal of fundamental research has been undertaken on the factors influencing the gas/solid reduction processes; this includes thermal decomposition reactions and heterogeneous reactions involving gas/solid systems.[1–6] These studies have formed the basis for the use of these reactions in metallurgical and materials production on an industrial scale. The evolution of materials characteristics during these processes is complex, since a number of elemental reactions and fundamental phenomena occur simultaneously; these may include, but are not limited to, solid-precursor decomposition, reduction reactions, oxidation reactions, gas- and solid-phase mass transfer, and sintering. Identifying and describing these fundamental phenomena is important for understanding the underlying science, as well as for improved control of technological applications. Differences in the relative contributions of these phenomena will lead to differences in the properties of the final product. Despite the accumulated knowledge to date, the complexities of the processes mean that it is still necessary to characterize individual systems through M.A. RHAMDHANI, formerly Postdoctoral Research Fellow, Pyrometallurgy Research Centre, School of Engineering, University of Queensland, is Lecturer, Faculty of Engineering and Industrial Sciences, Swinburne University of Technology, Hawthorn VIC 3122, Australia. E. JAK, Associate Professor, and P.C. HAYES, Professor, are with the Pyrometallurgy Research Centre, School of Engineering, University of Queensland, Brisbane QLD 4072, Australia. Contact e-mail: [email protected] Manuscript submitted October 18, 2007. Article published online February 14, 2008. 218—VOLUME 39B, APRIL 2008
systematic experimental studies. An important industrial example of the class of gas/solid-reaction processes is the production of nickel. Basic nickel carbonates (NiCO3ÆxH2O, NiCO3ÆxNi(OH)2ÆyH2O, NiCO3ÆxNiOÆ yH2O), or BNC, are used in the commercial production of nickel-metal powders and compacts through calcination and reduction with hydrogen.[7] In the case of nickel production from nickel oxide, residual oxygen and overall reduction rate are the key parameters determining the specification and value of the final Ni product and the production rates, respectively. The European Union’s new chemical policy regulations[8] require that oxygen concentrations in the nickel product are less than 0.1 wt pct as nickel oxide. These restrictions are driven by the need to minimise the generation of residual NiO dust, which has been shown to be carcinogenic.[9] From the point of view of workplace health and safety, and marketability of nickel products, it is important to characterize the form of any residual nickel oxides present, and to determine how they were formed, and therefore, how they might be further reduced. These processes, a
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