Synergistic depression mechanism of Ca2+ ions and sodium silicate on bastnaesite flotation☆
Graphical abstract
Both Ca2+ and sodium silicate individually has little depression effect on bastnaesite flotation, but their combination depresses bastnaesite more severely due to synergistic effect.
Introduction
Rare earth elements (REE) have important application value in many high-tech fields such as permanent magnets, wind turbines, electric cars, nickel-metal hydride batteries, lasers and fluorescent lamps due to their unique optical, electrical and magnetic properties.1,2 There are approximately 250 REE-containing minerals in the earth's crust, but only a dozen of them have been exploited. Bastnaesite ((REE)FCO3)), containing approximately 75% rare earth oxide (REO) with primarily Ce, La, Pr and Nd, is one of the most important industrial produced rare earth mineral.3,4 Flotation is the most common industrial method to recover bastnaesite.5 Nonetheless, bastnaesite and its associated calcium-bearing gangue minerals such as fluorite and calcite all belong to semi-soluble salt minerals and contain the similar anions in their crystal lattices. Metal ions dissolution and transformation will lead to the formation of similar surface properties between different minerals and make them difficult to separate, which presents a major challenge to bastnaesite flotation process.3,6,7
Over the past decades, hydroxamate has been used to promote the separation efficiency of bastnaesite and calcium-bearing gangue minerals because hydroxamate can form more stable complexes with rare earth elements than alkaline earth metal cations such as Ca2+ .2,8,9 However, Ca2+ ions are easily dissolved from the semi-soluble minerals including fluorite (CaF2, solubility 10−4.07 mol/L) and calcite (CaCO3, solubility 10−4.18 mol/L) and can transform and adsorb/precipitate on the surface of bastnaesite, thus affecting the adsorption behavior of collector on bastnaesite and deteriorating its separation from calcium-bearing gangue minerals.
Ca2+ ions are usually used to activate the flotation of non-metallic minerals such as quartz and aluminosilicate.10, 11, 12 However, the inevitable Ca2+ ions in pulp have adverse effects on bastnaesite flotation. For example, Ca2+ ions can drastically reduce bastnasite recovery when salicylhydroxamic acid was used as a collector at pH 8 .13 Ca2+ ions dissolved from calcite and fluorite can significantly reduce the recovery of bastnaesite with sodium oleate as a collector by forming Ca-hydroxy and carbonate compounds and hindering the chemisorption of oleate species on bastnaesite surface.14 Ca2+ ions affected the flotation of monazite through competitive adsorption of CaOH+ with hydroxamic acid on monazite surface.15 Moreover, it was found that Ca2+ ions from dolomite supernatant can consume collector, by forming complexation with Ca2+ ions from supernatant, and thus cause the loss of collector for rare earth minerals flotation.16,17
Sodium silicate is commonly used to depress calcium-bearing and silicate gangue minerals in tungsten and rare earth ores flotation.18, 19, 20, 21, 22, 23 It was found that Ca2+ ions could increase the unfavorable adsorption and depression effect of sodium silicate onto rare earth mineral such as monazite.24 However, few studies have been conducted to understand the effect of Ca2+ ions on bastnaesite flotation when sodium silicate was used as depressant. The aim of this work is to study the synergistic depression mechanism of Ca2+ and sodium silicate on bastnasite, which will provide a premise foundation for the elimination of the unintentional depression effect of sodium silicate on bastnaesite flotation.
Section snippets
Materials and reagents
The bastnaesite sample was collected from Jining City, China. The sample was first ground to −74 μm and then purified through shaking table gravity separation and magnetic separation with magnetic field of 2.0 T to remove impurities such as calcite, barite and silicate minerals. Table 1 shows the chemical compositions of the purified bastnaesite sample by using gravimetry and inductively coupled plasma atomic emission spectrometry (ICP-AES) methods according to Chinese standard GB/T18114-2010.
Micro-flotation results
pH is a key factor to mineral flotation because it can influence the existence form of collectors and ions in pulp. Fig. 2 shows the variation of bastnaesite recovery with pulp pH with 1 × 10−4 mol/L OHA as collector. It can be seen that the recovery of bastnaesite increased with the increase of pH to the maximum recovery at around pH 9.0, beyond which the recovery decreased with the further increase of pH to 12.0. The optimal pH 9.0 has been also reported when processing minerals such as
Conclusions
In the current study, the effect of Ca2+ ions and sodium silicate individually or in combination on bastnaesite flotation was investigated through micro-flotation, zeta potential, fluorescence spectroscopy and X-ray photoelectron spectroscopy (XPS) tests. The micro-flotation results show that Ca2+ ions and sodium silicate both have slight depression effect on bastnaesite when they are present individually; however, their co-existence exhibits stronger depression on bastnaesite flotation than
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Foundation item: Project supported by the National Natural Science Foundation of China (51764045), the Open Project of State Key Laboratory of Mineral Processing (BGRIMM-KJSKL-2020-23), the Excellent Youth Foundation of IMUST (2017YQL05), the Inner Mongolia Natural Science Foundation (2020LH05027, 2019MS05039) and the Inner Mongolia Major Basic Research Open Project (0406091701).