Contents lists available at ScienceDirect Minerals Engineering journal homepage: www.elsevier.com/locate/mineng Short communication Observations of the varied reactivity of xenotime and monazite in multiple systems Daniel E. Lazo, Laurence G. Dyer ⁎ , Richard Diaz Alorro, Richard Browner Department of Mining Engineering and Metallurgical Engineering, Western Australian School of Mines, Curtin University, Australia ABSTRACT In the present study, a comparison of the amenability of a monazite and xenotime (both rare earth phosphates) sample to treatment via alternative extraction techniqueswasconducted.Monaziteandxenotimeconcentrateswerestudiedundertheefectsoforganicacidsandmechanochemicalpre-treatment.Maleic,tartaric, citric and oxalic acids have shown the ability to increase the dissolution of phosphorus in the monazite concentrate. Oxalic acid demonstrated to enhance the release ofphosphorus(>30%)atpH < 1.3,roomtemperatureupto72h(Lazoetal.,2017)asopposedtoxenotime,whichunderwentverylittledissolutionundersimilar conditions (0.2 M, pH 2 for 24 h and 25 °C). Using mechanochemical activation prior to sulphuric acid leaching signifcantly increased the dissolution of rare earth elements (REEs), P and Fe in monazite (milled 300 RPM for 15 min) whereas in xenotime increased only slightly from a very low starting point. The diference in crystal structure and electronic confguration may play an important role in the more refractory nature of xenotime. 1. Introduction While both minerals are rare earth phosphates, monazite contains thenaturallightrareearthelements(ceriumgroup)andoftenhaslarge quantities of thorium and uranium (Clavier et al., 2011), while xeno- timecontainsthenaturalheavyrareearthelements(yttriumgroup)(Ni et al., 1995). Currently, conventional hydrometallurgical processing is the sole method to extract REEs from monazite and xenotime. Con- centrated lixiviants, high temperatures and corrosive reagents are uti- lized in the conventional methods. The most common methods for processing monazite (Kim and Osseo-Asare, 2012) and xenotime are acid roasting with H 2 SO 4 followed by a water leach, or alkali treatment (Franken, 1995).Aswasmentionedbefore,theseprocessesrequirehigh temperature, pressure and high concentration of corrosive reagents (H 2 SO 4 , 98% or NaOH, 70%, respectively) (Franken, 1995). Moreover, these techniques involve high cost maintenance (associated with cor- rosion resistant liners), high energy demand and the potential to create hazardous by-products. Earlier research performed by the authors addressed the initial successofusingorganicacidsintheextractionofrareearthelementsin monazite(Lazoetal.,2017).Itwasshownthatoxalicacidenhancesthe dissolution of phosphorus and the creation (and precipitation) of RE oxalate. This can be conducted under ambient conditions but is ac- celerated by moderately elevated temperature (65 °C). Lazo et al. (2018) demonstrated the second part of the process in which dissolu- tionofREEsfromthepretreatedmaterialwasachieved.Itwasexpected that a similar approach may be useful in the treatment of xenotime given the similarity between the two minerals. Mechanochemicalactivationwasalsoconsideredasanothermethod for achieving a chemical conversion of the rare earth phosphate mi- nerals to a species that could be readily leached. It involves physico- chemical changes in the material produced by applying mechanical energy (Baláž, 2003; Baláž et al., 2005). When several substances are milled together, solid phase reactions may arise during welded inter- facing(Kimetal.,2009).Thecombinationofincreasedsurfaceareaand enforced high-energy interaction of the materials under this treatment isabletoinducereactionsthatdonototherwiseoccur.Thiscanprovide a basis for conversion reactions that are only achievable over long time scales or under extreme conditions in a conventional hydro- metallurgical system. Such reactions can include pretreatments for ex- traction,suchascreatingasolublecompound.Thereducedparticlesize thenalsoaidsinfurtherextractionprocessesthroughincreasedreactive surface area. This paper describes the attempts to use these processes to treat both monazite and xenotime concentrates and the observation of the signifcantly more refractory nature of xenotime. 2. Materials and methods 2.1. Mineral characterization The concentrations of key components in both the monazite and xenotime concentrate samples used are given in Table 1. The particle size distributions of the samples were very similar and returned P 80 https://doi.org/10.1016/j.mineng.2020.106633 Received 19 February 2019; Received in revised form 16 August 2020; Accepted 8 September 2020 ⁎ Corresponding author at: Locked Bag 30, Kalgoorlie, Western Australia, 6430, Australia. E-mail address: Laurence.Dyer@curtin.edu.au (L.G. Dyer). Minerals Engineering 159 (2020) 106633 0892-6875/ © 2020 Elsevier Ltd. All rights reserved. T