Carbothermic reduction of scheelite (CaWO 4 ) doped with cobalt or nickel Elena Palmieri a , Andrea Marcucci a , GianCarlo Marcheselli b , Adriana De Stefanis c , Riccardo Polini a, ⁎ a Dipartimento di Scienze e Tecnologie Chimiche, Università di Roma “Tor Vergata”, Via della Ricerca Scientifica 1, 00133 Roma, Italy b Fabbrica Italiana Leghe Metalliche Sinterizzate (F.I.L.M.S.) SpA, Via Megolo 49, 28877 Anzola D'Ossola, VB, Italy c Istituto di Struttura della Materia (ISM), Unit of Montelibretti, Consiglio Nazionale delle Ricerche (CNR), Via Salaria km 29.300, 00015 Monterotondo Scalo, Roma, Italy abstract article info Article history: Received 16 March 2016 Received in revised form 7 May 2016 Accepted 8 May 2016 Available online 31 May 2016 Pure scheelite (CaWO 4 ) and carbon black mixtures, containing 0 or 2 wt% cobalt or nickel were prepared by 8 or 24 h planetary ball milling (PBM). The mixtures were studied by thermal analysis (TGA-DTA), isothermal anneal- ing at 950, 1000 and 1100 °C, X-ray diffraction (XRD) and scanning electron microscopy (SEM). Independently of the presence of transition metal (Co or Ni), the carbothermic reaction occurs through several steps, where the Ca:O atomic ratio goes from 1:4 to 1:1 according to the sequence: CaWO 4 → Ca 3 WO 6 → CaO, with concomitant devel- opment of gaseous CO and formation of metal tungsten. Finally, tungsten is carburized to give nanostructured WC. For the first time we here show that Co and Ni have a noteworthy catalytic effect on the carbothermic reduction of CaWO 4 . In particular, these transition metals catalyze both the two-step reduction of scheelite and the subse- quent carburization of tungsten. In the latter case, formation of intermediate η phases (Me x W y C z , with Me = Co or Ni) occurs. Doping with Co or Ni allows obtaining an almost quantitative (97%) conversion of scheelite into WC after 12 h at a temperature as low as 950 °C. The catalyst allows to reduce PBM duration as well, in that doped mixtures subject- ed to just 8 h PBM give WC yields larger than 90% after 1 h at 1100 °C, being W 2 C the balance. The doping of CaWO 4 :C mixtures with few weight percent of Co or Ni allows to produce nanostructured WC by reducing both the milling time and the annealing temperature. These results are particularly appealing from both industrial and sustainability point of view since they allow performing less energy-intensive syntheses of nano- structured WC powders from scheelite. © 2016 Elsevier Ltd. All rights reserved. Keywords: Carbothermic reduction Scheelite Catalysis Cobalt Nickel WC powders X-ray diffraction Nanoparticles 1. Introduction The carbothermic reduction of scheelite is an eco-friendly, energy saver and lower cost synthesis of tungsten carbide, which corresponds to more than 60% of the world tungsten demand [1–3]. In our recently published papers [4,5] we have shown that mixtures of scheelite (CaWO 4 ) and carbon black can be transformed into CaO and nanostructured WC by carbothermic reduction at T N 1100 °C. The carbothermic reduction starts via formation of metal tungsten and thermodynamically predicted Ca 3 WO 6 . The subsequent reduction of tricalcium tungstate leads, along with CaO, to W and W 2 C which even- tually form WC by reaction with residual carbon [4]. Eq. (1) represents the overall process, where one mole of scheelite reacts with four moles of carbon: CaWO 4s ðÞ þ 4C s ðÞ →CaO s ðÞ þ WC s ðÞ þ 3CO g ðÞ : ð1Þ This apparently simple reaction is a rather complex process. First, CaWO 4 reacts with two moles of carbon and is partly reduced to Ca 3 WO 6 and metal tungsten [4]: CaWO 4s ðÞ þ 2C s ðÞ →1=3 Ca 3 WO 6s ðÞ þ W s ðÞ þ 2CO g ðÞ : ð2Þ Then, tricalcium tungstate reacts with remaining carbon black to form additional W along with CaO [4,6,7]: 1=3 Ca 3 WO 6s ðÞ þ C s ðÞ →CaO s ðÞ þ 1=3W s ðÞ þ CO g ðÞ ð3Þ Finally, carbon diffusion in W leads to the desired product (i.e. WC) via W 2 C. Therefore, the carbothermic reduction of scheelite consists of a two-step reduction phase followed by the carburization of tungsten. Unlike the conventional production of WC, in which tungsten pow- der is reacted with carbon at 1300–1700 °C [1], the carbothermic reduc- tion of scheelite does not involve formation of an interfacial area between solid carbon and W during the milling phase; nevertheless, carbon and W must be in touch even after the consumption of a large part of the carbon initially present in the CaWO 4 :C mixture, and W is nucleated and grown in the two-step reduction phase. The residual Int. Journal of Refractory Metals and Hard Materials 59 (2016) 93–99 ⁎ Corresponding author. E-mail address: polini@uniroma2.it (R. Polini). http://dx.doi.org/10.1016/j.ijrmhm.2016.05.018 0263-4368/© 2016 Elsevier Ltd. All rights reserved. Contents lists available at ScienceDirect Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM