Anisotropic surface energies and adsorption behaviors of scheelite crystal

Abstract

Anisotropic surface broken bond densities and surface energies of six surfaces of scheelite were calculated with density functional theory (DFT). The calculation results show that its surface energy is directly proportional to its surface broken bond density. {1 1 2} and {0 0 1} surfaces are the two predominant cleavage or exposed planes. The prediction of morphology is well consistent with our experimental observations based on XRD. Anisotropic adsorption behaviors and wettability of these two surfaces after being immersed in sodium oleate and DDA solutions were studied by means of contact angle measurement, AFM and flotation test. The contact angle values at a given NaOl concentration decreased in the order {1 1 2} > {0 0 1}. As for DDA, similar wettability for the two surfaces was observed. The results were interpreted by AFM and interaction energies for the adsorption of surfactants on each surface calculated in accordance with molecular dynamics (MD) simulation.

Introduction

As an important ore mineral of tungsten, scheelite CaWO₄ is commonly associated with other calcium-bearing minerals in mineral ore deposits, such as calcite CaCO₃, apatite Ca₁₀(PO₄)₆F₂ and fluorite CaF₂, from which it shall be separated by means of flotation as a result of the increasing need to process low-grade complex ores. It is well established that flotation is a surface-chemistry based process for separation of fine ores that takes advantage of the differences in wettability on mineral particle surfaces. However, most calcium minerals possess very similar surface properties, semi-soluble nature and similar responses to various known families of collectors such as fatty acids [1], [2]. To separate these calcium-containing minerals from each other by means of flotation still remains a thorny problem.

Most calcium minerals show a remarkable variety of habits in nature crystals [3], particularly calcite and scheelite, and hence exhibit several anisotropic exposed surfaces which may possess different physicochemical characteristics. Generally, the exposed mineral surfaces in the flotation slurry will be both cleavage planes because of the crushing of the mineral and expressed surfaces in its crystal morphology [4]. Since the challenge for flotation separation is that the separation has to be based on subtle differences on the exposed surfaces among various minerals, the anisotropic surfaces may have a significantly different wetting behavior from flotation reagents, which, in turn, determine the flotation separation performance [5]. Previous experimental and theoretical studies concerning the adsorption mechanisms of collectors on calcium-containing mineral surfaces concentrated mainly on the important structural differences of the minerals [1], [2], [6], [7], [8]. Therefore, the study of anisotropic surface chemistry of calcium-containing minerals on a detailed atomic scale may be of great importance in the flotation separation.

Some studies were reported on the anisotropic surface properties of calcium-bearing minerals. The interactions of water and a selection of organic surfactant molecules (i.e. methanoic acid, hydroxyl methanamide, methylamine and hydroxyl ethanol) on the surfaces of scheelite [4], [9], calcite [10], [11], [12], [13], [14], fluorapatite [15], and fluorite [16], [17] were calculated with the help of computer simulations, indicating that the mode of adsorption and the strength of the interaction of these model adsorbates on different mineral surfaces exhibit anisotropy. Up to now, limited experimental results have been reported on this topic in previous literature. Only recently did Rai and Pratip [6] report, by employing molecular dynamics (MD) simulations and contact angle measurements, the crystal structure specificity of oleate molecules with the different silicate minerals as well as its interactions with different crystallographic planes of a certain mineral. In addition, five families of slip systems and anisotropic microhardness and fracture in natural single scheelite crystal were studied using TEM, and interpreted using the anisotropic crystallographic structure of scheelite [18], [19].

In recent years, anisotropic surface chemistry of solids was studied in many other research fields, including bionic engineering [20], micro- or nano-scale surface materials [21], [22], [23], [24] and pharmaceuticals [25], [26]. Study on anisotropic surface chemistry of crystalline pharmaceutical solids suggested a direct relation between local surface chemistry and wettability. The research results of the anisotropic wettability or wetting phenomenon of surfaces with different micro- and nano-structures provided theoretical evidence for controlling the anisotropic wettability and the design of materials and devices with anisotropic wetting surface in microfluidics, micro- or nano-optics, antifouling, and so forth. Recent works by Ulusoy et al. [27], [28], [29] reported that the milling type, e.g. ball, rod and autogenous mills, influenced the shape properties (surface morphologies) of minerals which, in turn, affected their wettability and flotation behavior.

Inspired by previous studies, we are in a position to predict and confirm the most commonly exposed surfaces of the three calcium-bearing minerals, study the anisotropic adsorption behavior of collectors, depressants and modifiers on these surfaces, and magnify the hydrophobicity of these three minerals through change of crushing and/or grinding modes and adjustment of physicochemical conditions of the flotation slurry. It will be certainly conducive to the successful flotation separation of calcium minerals.

In this study, the anisotropic densities of surface broken bond and surface energies of scheelite crystal were calculated to analyze the relationship between them. With the aid of the calculation results we predicted the most commonly exposed surfaces of the morphologies of scheelite, which were then compared with the experimental cleavage planes. We selected these commonly exposed surfaces carefully from natural scheelite crystal samples, and then chose sodium oleate (NaOl), a well known and widely used flotation reagent for separation amongst calcium minerals [30], [31], [32], and dodecyl amine (DDA) to study the anisotropic wetting/adsorption behaviors on these surfaces through contact angle experiments. The inclusion of DDA gave us the opportunity to make a comparison between the two collector molecules with different functional groups. The anisotropic wettability was interpreted with the results of AFM and our molecular dynamics simulations (MD). This article aims at:

• Establishing the relationship between the surface broken bond density and surface energy of scheelite crystal.

• Establishing the validity and suitability of calculation of surface broken bond densities and surface energies for predicting the most commonly exposed surfaces of the morphologies of scheelite crystal.

• Studying the anisotropic wetting behaviors of NaOl and DDA on different scheelite surfaces and establishing a link between anisotropic wetting behavior and local surface chemistry.