Full Length ArticleInterfacial microstructures and adsorption mechanisms of benzohydroxamic acid on Pb2+-activated cassiterite (1 1 0) surface
Graphical abstract
Introduction
Cassiterite is a typical oxide mineral and has crucial applications in the tin extraction and advanced materials manufacture [1], [2], [3], [4], [5], [6]. It is brittle and is easy to be overgrinded in the process of crushing and grinding. Gravity separation process is difficult to effectively recover the produced fine cassiterite. Flotation is the preferred process for fine cassiterite recovery. Flotation is the most important and most widely used method in mineral processing and resource recovery [7], [8], [9]. The selective interactions of reagents with surfaces of different minerals are adopted to enhance the difference in hydrophobicity of these surfaces to recover the target mineral in flotation. Flotation is a typical interface process [7]. The interfacial microstructures of reagents on the minerals determine the flotation performance of reagents and the separation efficiency of minerals. A thorough understanding of the interfacial microstructure in flotation process is beneficial to better understanding the nature of flotation separation and the innovation of flotation technology.
Recently, the combination of benzohydroxamic acid [(BHA, C6H5-C = O(NH-OH)] as a collector and lead ions (Pb2+) as an activator has been extensively used in the flotation of cassiterite (SnO2) and scheelite (CaWO4) [1], [10], [11], [12], [13], [14]. Its excellent flotation performance should be attributed to the good balance between selectivity and collecting capacity of BHA on the Pb2+-activated oxide mineral surface [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]. Previous computational and experimental results consistently showed that BHA||Pb2+ (adding BHA after Pb2+) can be directly adsorbed on the mineral surface to control the hydrophilicity and hydrophobicity of the surface[15], [20], [21], [22]. These results have shown that BHA can strongly interact with the Pb2+-activated cassiterite suface. However, its interfacial bonding structures and adsorption mechanism are still unclear. It is still necessary to reveal the interface structures and the adsorption mechanisms of BHA on the Pb2+-activated cassiterite suface for the development of novel processing schemes and eco-friendly flotation reagents [22].
It should be mentioned that the natural surfaces of fine mineral particles usually possess complicated interfacial structures due to the different mineralisation conditions and the used processing approaches (i.e. grinding approaches, flotation) [23], [24], [25], [26], [27], [28]. It is challenging for a single characterization technique to effectively reveal the microstructure of the surface. Therefore, it is necessary to combine a variety of characterization techniques to better study the related systems. For example, Raman spectroscopy has been combined with scanning electron microscopy (SEM) and atomic force microscopy (AFM) to investigate pure cassiterite and complicated syenite rock, respectively [29], [30]. These attempts have obtained some insights into the interfacial interaction of the involved surface processing [31]. Recently, the Raman spectroscopy results in our previous work verified Raman characteristic peak of interfacial Pb-O bonds [22]. On the other hand, theoretical computation methods, such as density functional theory (DFT), have become powerful to describe the interfacial bonding reactions in details at the atomic scale [32], [33], [34]. DFT calculations have successfully reproduced fascinating electronic structures of molecules that can account for the properties of molecules at gas, aqueous and interfacial phases [35], [36], [37], [38], [39], [40]. The chemical shifts calculated from the molecular orbital energy have good consistency with the experimental binding energy (BE) obtained by X-ray photoelectron spectroscopy (XPS) [36], [41], [42]. The combination of theoretical calculations and experimental characterizations could be expected to give a more systematic and in-depth understanding of the interface microstructure of cassiterite.
The motivation of this work is to identify the interfacial microstructures and adsorption mechanisms of BHA on the Pb2+-activated cassiterite (1 1 0) surface. Accordingly, Raman spectroscopy, AFM and XPS were used to characterise the interfacial bonding structures, surface topography and chemical constituents, respectively. DFT calculations were then performed to further confirm the interfacial bonding structures by computationally reproducing the characteristic XPS peaks of interfacial species. High-resolution transmission electron microscopy (HR-TEM) was adopted to obtain the atomic-scale surface structures of cassiterite mineral particles. This work creatively used the natural cleavage surfaces of cassiterite as substrates to reveal the interface bonding structures in detail by experimental characterisations and theoretical calculations. This work deepens the understanding of interfacial microstructures and adsorption mechanisms of BHA on the Pb2+-activated cassiterite (1 1 0) surface, and might provide a new light on the understanding of the interface adsorption mechanism of flotation reagent and activation mechanism of metal ion on oxidized mineral.
Section snippets
Bulk-phase cassiterite
Natural cassiterite bulk minerals (size 50–200 mm, 2 kg) bought from a mine (Yunnan Province, China) were crushed into −10 mm bulk minerals. Most products lacking smooth cleavages were used for the preparation of powder-phase cassiterite particles. Only the products with good cleavages were used for the AFM and Raman experiments. The used natural bulk crystal was a brown, transparent and polyhedral crystal with smooth surfaces. The different surfaces were labelled with numbers from one to five
AFM results
The Raman and XRD spectra in Figures S1a and S1c (Refer to the ESI) consistently showed that the (1 1 0) surface was the most commonly exposed and the most reliable surface [52]. Thus, the cassiterite (1 1 0) surface was used to perform the AFM imaging tests.
Fig. 1(a)–1(c) show that the clean surface of cassiterite is imperfect and has many surface-attached particles and nanometer steps. The steps in 1(b) are not clearly visible due to the large height variation of the particle and surface that
Conclusion
In this work, the interfacial microstructures and adsorption mechanisms of lead ion-BHA on natural cleavage surface of cassiterite were systematically investigated by means of AFM, XPS, DFT calculations, Raman spectra and HR-TEM images. The Raman and X-ray diffraction spectra of different natural cleavage surfaces of cassiterite showed that the (1 1 0) surface was the commonly exposed surface. The AFM images revealed that the Pb-BHA adsorption can change the surface morphologies, and the trace
CRediT authorship contribution statement
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Jianyong He and Chenyang Zhang wrote the original paper. Jianyong He analyzed the data. Zhou Qi Qi and Shengda Chen visualized the data and prepared the samples. Jianyong He and Mengjie Tian prepared the samples and did the calculations. Chenyang Zhang and Wei Sun conceived the research idea, designed the experimental scheme and revised the original manuscript.
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.
Acknowledgement
The authors would to show many thanks to Hongliang Zhang and Tong Yue for their significant contribution in data processing and review of the final article. This work was financially supported by the National Key Research and Development Program of China (2019YFC0408300); Natural Science Foundation of China (No. 51704330, No. 52074356); the Natural Science Foundation of Hunan Province (No. 2020JJ5759); China Postdoctoral Science Foundation (No.2020T130188, No. 2018M642988); National 111 Project
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