Contents lists available at ScienceDirect Minerals Engineering journal homepage: www.elsevier.com/locate/mineng Floatability of molybdenite and chalcopyrite in artificial seawater Gde Pandhe Wisnu Suyantara a , Tsuyoshi Hirajima b, ⁎ , Hajime Miki b , Keiko Sasaki b a Graduate School of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan b Department of Earth Resources Engineering, Faculty of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan ARTICLE INFO Keywords: Flotation Artificial seawater Molybdenite Chalcopyrite Atomic force microscopy ABSTRACT Seawater has been reported to depress the floatability of molybdenum in copper-molybdenum (Cu-Mo) flotation circuits under alkaline conditions (pH > 9.5). However, the seawater used in the process contains various mi- nerals and flotation reagents, which make it difficult to investigate the depression mechanism. This paper presents a fundamental study into the effect of artificial seawater as a seawater model solution on the floatability of molybdenite and chalcopyrite, which are the main minerals in the Cu-Mo flotation process. Floatability tests in the absence of flotation reagents (i.e., frothers and collectors) reveal that artificial seawater adversely affects the floatability of molybdenite and chalcopyrite at pH > 9. This phenomenon can be attributed to the adsorption of hydrophilic Mg(OH) 2 precipitates formed under alkaline conditions on the mineral surfaces, which increases the surface wettability of the mineral particles, as shown by contact angle measurements and atomic force microscopy (AFM) images. The effect of kerosene as a molybdenite collector has also been investigated to assess its potential in the selective flotation of molybdenite and chalcopyrite in artificial seawater. 1. Introduction Flotation is a water-intensive process; thus, to minimize the use of freshwater, most mining operations use recycled water, underground water, saline water, or seawater, which contain various inorganic electrolytes (Wang and Peng, 2014). Seawater is used in the Las Luces copper-molybdenum (Cu-Mo) beneficiation plant in Taltal, Chile (Moreno et al., 2011), in which the copper ores are depressed by adding sodium hydrosulfide (NaHS) and molybdenum ores are collected as froth products. Other flotation plants use saline or seawater to process sulfide minerals (Drelich and Miller, 2012; Wang and Peng, 2014), such as the Michilla Project (Antofagasta), Chile, and the KCGM Project (Barrack/Newmont), Australia. The Batu Hijau concentrator (New- mont) in Sumbawa Island, Indonesia (Castro, 2012) also uses seawater to process gold-rich porphyry copper ore. Bore water is used in the nickel flotation plants operated by BHP Billiton in Mt Keith mine, Leinster mine, and Kambalda Nickel Concentrator, Australia. Flotation units are controlled at an alkaline pH using lime (CaO or Ca(OH) 2 ) because this is cost effective. Moreover, lime can act as a strong depressant for pyrite and arsenopyrite when xanthate collectors are used (Wills and Napier-Munn, 2006) because the hydroxyl and calcium ions form Fe(OH), FeO(OH), CaSO 4 , and CaCO 3 on the mineral particle surfaces. In freshwater, pyrite is traditionally depressed at pH 11.5–12.0 (Castro, 2012). However, in seawater, pyrite should be de- pressed at a lower pH (pH < 9.5) to avoid the depression of molybdenum and minimize excessive lime consumption. Alkaline con- ditions are also beneficial in preventing the generation of toxic hy- drogen sulfide gas (H 2 S) from the added NaHS. The typical electrolyte composition of seawater is listed in Table 1 (U. S. Department of Energy, 1994). Seawater exerts a buffering effect, which increases the lime consumption at pH values traditionally used for Cu-Mo flotation in freshwater (Castro, 2012; Jeldres et al., 2015b, 2015a). The buffering effect is caused by the presence of bicarbonate/ carbonate ions (HCO 3 − /CO 3 2− ) and boric acid and borate ions (B (OH) 3 /B(OH) 4 − )(Pytkowicz and Atlas, 1975). As shown in Table 1, seawater contains secondary ions (i.e., Ca 2+ , Mg 2+ , SO 4 2− , and CO 3 2− ), which can form colloidal precipitates. For example, calcium and magnesium ions can form colloidal hydroxides, carbonates, and sulfates based on the following reactions (Castro, 2012; Hirajima et al., 2016; Jeldres et al., 2016): Speciation of CO 2 + ↔ CO H O H CO 2(g) 2 2 3(aq) (1) ↔ + − + H CO HCO H 2 3(aq) 3(aq) (aq) (2) ↔ + − − + HCO CO H 3(aq) 3(aq) 2 (aq) (3) Speciation of Mg and Ca + ↔ + − + Ca HCO CaHCO (aq) 2 3(aq) 3(aq) (4) http://dx.doi.org/10.1016/j.mineng.2017.10.004 Received 22 February 2017; Received in revised form 28 September 2017; Accepted 7 October 2017 ⁎ Corresponding author. E-mail address: hirajima@mine.kyushu-u.ac.jp (T. Hirajima). Minerals Engineering 115 (2018) 117–130 0892-6875/ © 2017 Elsevier Ltd. All rights reserved. MARK