Surface chemistry of magnesite and calcite flotation and molecular dynamics simulation of their cetyl phosphate adsorption

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Abstract

Recovery of magnesite requires a selective separation process from its associated carbonate minerals like calcite present in the same ore deposits. As part of our previous research, potassium cetyl phosphate (PCP) was identified as a promising flotation collector for magnesite. In the current work, we report results of flotation experiments, measurements of zeta potential and contact angle with these minerals in a range of pH from about 5 to 10.5. In addition, we carried out molecular dynamics simulations to delineate further characteristics of this flotation system. The experimental results obtained have provided clear evidence for excellent separation between magnesite and calcite. These observations indicate that PCP is selectively adsorbed on magnesite, which is responsible for the development of a significant hydrophobicity level that is sufficient for its separation from calcite. The results from the molecular dynamics simulating the adsorption process indicate a large PCP concentration profile that is much closer to the magnesite surface compared to the case with calcite. It has been found that the diffusion coefficient for water, Dwater out of the hydrated interface of magnesite is larger by about 34 % than that of calcite. Furthermore, the interaction energy, -ΔE of the magnesite-PCP adsorption system is much higher than that of the calcite-PCP system, supporting the selectivity for direct flotation of magnesite.

Introduction

Magnesite is a magnesium carbonate mineral with a composition of MgCO3, which geochemically forms together with other carbonates such as calcite (CaCO3). These two represent the main alkali earth carbonates that are also part of many commercially important ore deposits [1,2]. The global magnesite market was valued at $7.4 billion in 2017 [3]. In comparison, the global calcite market was valued at $10.1 billion in 2018 [4]. Magnesite (47.8 % MgO) is the most important mineral of magnesium and has been widely utilized in many fields including new battery applications [[5], [6], [7]]. However, the impurity minerals coexisting in the magnesite-bearing ores can seriously affect the potential applications in these fields. Calcite is one of the most common carbonate minerals in natural magnesite ore deposits. Thus, a common problem in the processing of such ores involves magnesite-calcite separation. The selectivity of the separation is important to obtain value-added mineral products for various applications.

As a method with high efficiency, froth flotation is usually conducive to the industrial separation of magnesite from its associated impurities [[8], [9], [10]]. Calcite often causes significant difficulties, and its soft nature can generate slime problems and coat particles of valuable target minerals during processing [11,12]. The mechanism of this slime coating is attributable to electrostatic interactions between dissimilar minerals, as reported for the inhibition of galena separation by alumina slimes [13]. Moreover, magnesite and calcite have considerable solubility in slurries, particularly at relatively low pH levels. The chemical composition of the aqueous phase and the interfacial characteristics are controlled by their solubilities [9,14]. Studies have shown that the surface properties of apatite can be affected by the dissolved species from calcite, which will lead to changes in the flotation behaviour of apatite [15].

The key to obtaining good flotation separation results is, to a very marked degree, decided by using sufficiently long chain collectors with high selectivity [16]. For the flotation of magnesite ores, the main collectors are fatty acids and dodecyl amines [17,18]. Nevertheless, the selectivity issues with such collectors remain a challenge for magnesite ores. Therefore, there is a need for more selective collectors for the selective separation of magnesite and calcite and a better understanding of the separation mechanisms. Investigations on the surface chemistry of mineral flotation and adsorption of surfactants focus on the phenomena at the solid-aqueous interface [16,19]. Recent research progress in the separation of magnesite is confined to cationic collectors, which have been applied in the reverse flotation of magnesite ores [[20], [21], [22]].

As part of our research in this area, we have identified potassium cetyl phosphate (PCP) as a potential selective collector for magnesite recovery from calcite [23]. We have been intrigued by the high level of selectivity observed, which has been a motivation for further investigation. Some aspects of surface chemistry giving rise to the development of selective hydrophobicity through adsorption of PCP on magnesite are still incompletely understood and these features are the subject of this paper. To obtain an improved understanding of the adsorption features of PCP on magnesite and calcite surfaces, a series of micro flotation tests, zeta potential and wettability measurements were carried out. Also, molecular dynamics modelling and simulations have been conducted to get an improved understanding of the flotation chemistry of magnesite and calcite using the PCP collector, which can be a basis for further investigations in this area or related flotation systems.

Section snippets

Materials and reagents

High-purity crystalline magnesite blocks were acquired from Refratechnik Group, Canada. Optical-grade pure calcite blocks, obtained from Changsha, China were used in this research. Typical pieces of magnesite and calcite samples were selected for the contact angle measurements. Other hand-picked samples were crushed, dry ground and wet sieved (with deionized water) to -150 + 75 μm and −5 μm for the micro flotation tests and analytical determinations, respectively. X-ray powder diffraction

Microflotation results

The flotation behaviours of magnesite and calcite using PCP as the collector were investigated, and the corresponding results are reported in Fig. 4, Fig. 5. It should be pointed out that no fronting agent was required in these experiments due to the long hydrocarbon part of this surfactant. The effect of PCP concentration on the floatability of magnesite and calcite at natural pH is shown in Fig. 4. As may be noted, the recovery of magnesite increases with increasing PCP concentration from 20

Summary and conclusions

The surface chemistry of magnesite and calcite was studied using PCP as a collector in reference to the flotation behaviour of each mineral. The experimental work involved microflotation, zeta potential, and contact angle tests. These series of experiments were followed by a theoretical analysis of MD simulations. Although some aspects of this flotation system are still under investigation, the following conclusions can be made based on the results presented:

  • (1)

    PCP provides an excellent

CRediT authorship contribution statement

Yuan Tang: Investigation, Formal analysis, Validation, Visualization, Writing - original draft, Funding acquisition. Sadan Kelebek: Conceptualization, Supervision, Writing - review & editing, Resources. Wanzhong Yin: Project administration, Supervision, Methodology, Funding acquisition, Software, Data curation, Writing - review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work is financially supported by the National Natural Science Foundation of China (51874072) and also through Yuan’s fellowship provided by the China Scholarship Council (No. 201806080082) to support part of his PhD studies as a visiting research fellow at Queen’s University, in Canada. Some of the research expenses has been covered through a discovery grant from NSERC.

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