Oxidation and reduction of tungsten alloy swarf A.A. Alhazza * Department of Chemical Engineering, Loughborough University, Loughborough, LE11 3TU, UK article info Article history: Received 18 August 2008 Accepted 13 November 2008 Keywords: Oxidation Heavy metal Reduction Swarf Tungsten abstract Oxidation of a heavy metal alloy (swarf) followed by reduction in dry hydrogen atmosphere was studied. The swarf was oxidised at 750 °C to 1000 °C and then reduced at 800 °C. Analysis of the resulting powder was studied using thermogravimetry, X-ray diffraction, and scanning electron microscopy. The average particle size of the reduced powder was 1–3 lm. The chemical composition of the reduced powder was the same as the primary heavy alloy swarf. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Transition metals show outstanding properties, and tungsten oxide is an important material for applications, such as electrical devices, catalysis and chemical sensors [1–4]. The wide range of electrical conductivity is one characteristic property of tungsten oxides, from that of semiconductors (WO 3 ) to that of conductors (WO 2 ). The range of tungsten oxides includes stoichiometric oxi- des, WO 3 , WO 2.9, WO 2.7, and WO 2 , in addition to non-stoichiome- tric structures. The non-stoichiometric systems consist of ordered or partially ordered defect structures of the oxygen-rich oxide, in which the central W atom is octahedrally surrounded by six oxy- gen atoms [9–11]. In WO 3 , neighbouring octahedra are in contact only at the corners, which increases oxygen deficiency, progres- sively forming common edges and surfaces. The aim of this work is to produce a homogeneous powder from ‘‘heavy metal swarf” for recycling or reuse. The process route envisaged was a controlled oxidation reaction to breakdown the swarf followed by temperature-controlled reduction with dry hydrogen to remove the oxide and form a heavy metal powder. The microstructure of the swarf was characterised using optical metallography, scanning electron microscopy (SEM) and X-ray dif- fraction (XRD). Characterisation was followed by a controlled oxi- dation reaction to mechanically breakdown the oxide. In this step, thermogravimetry (TGA) was used to determine the optimal oxida- tion temperature and to give an indication of the process kinetics. A reduction process was then performed in a Carbolite furnace (Vecstar Furnace) under dry hydrogen atmosphere. XRD and SEM were used to chemically analyse and determine the morphology of the powder. 2. Experimental procedure 2.1. Sample preparation This work was carried out using heavy metal (tungsten) swarf obtained from cutting machine waste. Samples were prepared by washing the swarf in acetone in order to remove any residues of the oil used as cooling fluid in the cutting machines. After wash- ing with acetone, the swarf was dried at room temperature for 5 h. It was then washed several times with distilled water to re- move any other deposits, and dried at room temperature for an- other 5 h. The swarf was mounted in cold-setting resin and polished to 6 lm for 10 min and then etched by immersing the sample in ‘‘Nital” for 30 s (Nital is an acid that removes the lighter elements from the surface of the polished samples to uncover two or more layers on the surface, revealing the underlying microstructures). The samples were then washed and dried for observation with an optical microscope. For XRD analysis, samples were ground and sieved, resulting in a heavy metal powder of less than 150 lm. 2.2. Materials characterisation The samples were characterised using XRD and SEM. Character- isation was performed on the materials present before and after each step of the homogenising process, including the starting 0263-4368/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2008.11.006 * Tel.: +44 (0) 7501769751; fax: +44 (0) 9654989059. E-mail addresses: cgaaa@lboro.ac.uk, alhazza2@yahoo.com Int. Journal of Refractory Metals & Hard Materials 27 (2009) 705–710 Contents lists available at ScienceDirect Int. Journal of Refractory Metals & Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM