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New insights into the oleate flotation response of feldspar particles of different sizes: Anisotropic adsorption model

https://doi.org/10.1016/j.jcis.2017.06.009Get rights and content

Abstract

The anisotropic adsorption of sodium oleate (NaOL) on feldspar surfaces was investigated to elucidate the different flotation properties of feldspar particles of four different size ranges. Microflotation experiments showed that the feldspar flotation recovery of particles with sizes spanning different ranges decreased in the order 0–19 > 19–38 > 45–75 > 38–45 μm. Zeta potential and FTIR measurements showed that NaOL was chemically adsorbed on the Al sites of the feldspar surface. The anisotropic surface energies and broken bond densities estimated by density functional theory calculations showed that, although feldspar mostly exposed (0 1 0) and (0 0 1) surfaces, only the (0 0 1) surfaces contained the Al sites needed for NaOL adsorption. The interaction energies calculated by molecular dynamics simulations confirmed the more favorable NaOL adsorption on (0 0 1) than (0 1 0) surfaces, which may represent the main cause for the anisotropic NaOL adsorption on feldspar particles of different sizes. SEM measurements showed that the main exposed surfaces on coarse and fine feldspar particles were the side (0 1 0) and basal (0 0 1) ones, respectively. A higher fraction of Al-rich (0 0 1) surfaces is exposed on fine feldspar particles, resulting in better floatability compared with coarse particles. XPS and adsorption measurements confirmed that the Al content on the feldspar surface varied with the particle size, explaining the different NaOL flotation of feldspar particles of different sizes. Therefore, the present results suggest that coarsely ground ore should be used for the separation of feldspar gangue minerals. Further improvements in the flotation separation of feldspar from associated valuable minerals can be achieved through selective comminution or grinding processes favoring the exposure of (0 1 0) surfaces.

Introduction

As one of the most abundant minerals in the earth’s crust, feldspar is commonly associated with other silicate minerals in pegmatitic and feldspathic sand deposits, such as quartz (SiO2), spodumene (LiAl[Si2O6]) and mica (KAl2[AlSi3O10](OH)2) [1], [2]. Although feldspar is generally considered as a gangue mineral that must be separated from quartz or spodumene [3], [4], [5], it may also represent a valuable raw material for the glass, ceramic, and paper industries [6], [7]. Understanding the separation process of feldspar minerals is thus of significant importance for both the above purposes. Froth flotation is currently the most established method for the separation of feldspar from associated silicate minerals using fatty acid collectors [8], [9].

Because the flotation process is controlled by the chemical properties of the surfaces involved, flotation separation methods are based on the different behavior of the surfaces exposed by the minerals under study [10]. Anisotropic surface properties including wettability, energy, and charge will result in the different adsorption of a reagent on the surface of a mineral, which further affects its flotation behavior [11], [12]. For example, the different wettability and electrokinetic properties of the exposed surfaces of diaspore and aluminosilicate minerals enable their selective flotation separation using cationic collectors [13]. It is well known that the diverse surface properties of minerals reflect their different crystal structure [14]. Minerals with different crystal structure may exhibit substantially different exposed and cleavage planes, and even the properties of different surfaces exposed by the same mineral may be entirely different [15]. Previous reports showed that the density of broken bonds introduced at the surface of scheelite crystals, which is directly related to their structure, shows a positive correlation with the surface energies. Moreover, anisotropic adsorption of sodium oleate and dodecyl amine was observed on different scheelite surfaces [16]. Being framework structure minerals, feldspar crystals are basically composed of folded linear chains connected by a large number of square rings, consisting of silicate or alumina tetrahedra with alkali metal ions (M+) filling the interspace [4]. This unique crystal structure results in the presence of essentially three types of broken bonds (Alsingle bondO, Sisingle bondO, and Msingle bondO) on the cleavage surfaces of ground or crushed feldspar. The different strength of the Alsingle bondO, Sisingle bondO, and Msingle bondO bonds results in a different surface density of the corresponding broken bonds. It has also been found that the Al ions exposed by the Alsingle bondO bond breaking are the only adsorption sites of NaOL on the exposed aluminosilicate surfaces [17], [18], [19], [20].

Although the effects discussed above illustrate the correlation between the properties of the exposed surfaces and the crystal structure of the mineral, the relative number and area of the exposed surfaces on crushed or ground mineral particles is closely related to the size of the particles. Numerous studies emphasized the vital role of the particle sizes and size distributions in flotation separation processes [21], [22], [23]. It has been reported that the particle size and shape play a key role in the behavior of chalcopyrite, with angular particles exhibiting surface chemical characteristics favorable for chalcopyrite flotation [24]. Our previous work also showed that NaOL is mostly adsorbed on the (1 1 0) edges of fine spodumene particles with a size fraction [25]. There are no doubts, therefore, that a good understanding of the relationship between particle size and flotation recovery will not only improve the efficiency of flotation separation of silicate minerals, but also provide insight into the grinding process, which is the most energy-consuming stage of mineral processing. However, in order to accomplish this goal, a thorough understanding of the influence of the particle size on the relative number and area of the exposed surfaces is vital to enable the selective fabrication of specific exposed planes by controlling the crushing and grinding processes.

In recent years, significant advances have been made in the study of mineral-reagent interactions using molecular modeling tools [26], [27], [28]. For example, the surface-surfactant interaction energies calculated by molecular dynamics (MD) simulations were employed by Rai and coworkers to study the adsorption mechanisms of chemisorbed anionic oleate and physisorbed cationic dodecylammonium chloride molecules on two different crystal planes of spodumene and jadeite. The authors reported a very good agreement between their theoretical and experimental results [29]. Molecular modeling thus represents an elegant and rigorous quantitative methodology that could support the design of flotation reagents based on the crystal structure specificity of mineral-reagent interactions.

In the present study, MD simulations were used to illustrate the anisotropic adsorption behavior of NaOL on different feldspar surfaces. In particular, a series of experiments and analytical techniques were applied for directly and indirectly assessing the interaction between NaOL and feldspar particles of different size fractions. The first goal is to understand the influence of the particle size on the flotation behavior of feldspar; then, we aim at determining the anisotropic surface energies and broken bond densities of feldspar, and, finally, at elucidating the relationship between particle size, anisotropic crystal structure, and adsorption of NaOL on the feldspar surface. This will clarify the origin of the different flotation response of feldspar samples with different size fractions. The results of this study are expected to support the development of improved flotation separation methods of feldspar minerals.

Section snippets

Materials and reagents

Pure feldspar samples were obtained from the Jiajika lithium mine in the Ganzi district, Sichuan, China. The chemical composition (Table 1) and X-ray diffraction (XRD) results showed that the purity of as-prepared feldspar was ∼90%. After being handpicked, crushed, ground, and screened, the powder samples were screened to four size fractions (0–19, 19–38, 38–45, and 45–75 μm) and then used in flotation tests. Table 2 presents the size distribution of the four fractions and their corresponding

Flotation behavior of feldspar particles of different size fractions

The use of metal ions such as Fe3+ and Ca2+ as activators has great impact on the floatability of aluminosilicate minerals. The low (<10%) flotation recovery of feldspar without an activator implies that any meaningful study of the flotation behavior of feldspar should be performed in the presence of an activator. The flotation response of feldspar particles of different sizes (0–19, 19–38, 38–45, and 45–75 μm) in the presence of Fe3+ and using NaOL as collector is presented in Fig. 1 as a

Conclusions

The flotation recoveries of feldspar particles of different size fractions were found to be related to the anisotropic adsorption of NaOL on the surface of these particles. The underling mechanism was attributed to the anisotropic crystal chemistry characteristic of feldspar. The research results will, on the one hand, provide guidance to flotation separation of feldspar from associated valuable minerals through selective comminution or grinding processes, on the other hand, be conductive to

Acknowledgements

The authors would like to thank the National Natural Science Foundation of China (Grant Nos. 51674207, 51304162 and 51504224), the Key Foundation of Natural Scientific Research of the Education Department of Sichuan Province, China (Grant No. 16ZA0130), the Found of State Key Laboratory of Mineral Processing (Grant No. BGRIMM-KJSKL-2016-03), the Basic and Public Geology and Mineral Resources Survey Foundation of China Geological Survey (Grant No. DD20160074-6), the Doctoral foundation of

References (37)

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