Elsevier

Minerals Engineering

Volume 186, August 2022, 107724
Minerals Engineering

Effect of calcium ions on bentonite network structure

https://doi.org/10.1016/j.mineng.2022.107724Get rights and content

Highlights

  • The network structure of bentonite played an important role in flotation.

  • The stability, association mode and particle size were investigated.

  • These features were varied with Ca2+ concentration in salt water.

  • The evolution mode of bentonite network structure was proposed.

Abstract

In this study, the network structure of bentonite was characterized systematically in deionized water and salt water containing calcium ions. It was found that the stability of bentonite network structure firstly increased and then gradually decreased, as the concentration of calcium ions increased from 0 to 1.0 mol/L. Cryo-SEM observed that bentonite platelets formed in three-dimensional E-F mode in deionized water, which were developed into E-E association and then much denser and more compact aggregates, with increasing the concentration of calcium ions. Meanwhile, the size of the bentonite network structure increased firstly and then slightly decreased. Based on these characterizations, the evolution modes of bentonite network structure with increasing the concentration of Ca2+ was proposed, which could be selected and used as a guideline to control clay minerals in flotation.

Introduction

Clay minerals are phyllosilicate minerals, which are basically composed of layers comprising silica tetrahedral (T) sheets and alumina octahedral (O) sheets joining together in certain proportions (Theng, 2012). Two structural units are involved in the atomic lattices of most clay minerals, 1:1 (T-O) and 2:1 (T-O-T). Kaolinite belongs to the 1:1 structural group. Bentonite is a typical 2:1 clay mineral with montmorillonite as the main component. Generally, the basal surface of clay mineral carries a permanent negative charge due to the isomorphous substitution, while the edge face carries either a positive or negative charge depending on the type of metal ions and the pH of the solution (Schoonheydt and Johnston, 2006). To balance the negative charges on the basal surface, the exchangeable cations are located at the external basal surfaces or near the edges of kaolinite. For montmorillonite, the negative charge on basal faces is neutralized by an interlayer of exchangeable cations, such as Na+, K+, Ca2+ and Mg2+, to make it chemically stable (Lagaly and Dékány, 2013).

In relatively concentrated suspensions, clay particles may associate by face-to-face (F-F), edge-to-face (E-F) and edge-to-edge (E-E) interactions, according to their relative positions and the relative magnitude of their surface and edge potentials (Luckham and Rossi, 1999). These network structures, dominated by the interactions between particles, play a major part in the flow behavior. It has been found that the pulp rheology modified by clay minerals was strongly correlated with the flotation performance. Forbes et al. (2014) found that the pulp rheology in the presence of kaolinite adversely affected chalcopyrite flotation due to the anisotropic property of kaolinite face and edge. Zhang and Peng (2015) found that bentonite increased the pulp viscosity more significantly than kaolinite, and poorly crystallized kaolinite increased pulp viscosity more than well crystallized kaolinite. The copper recovery strongly depended on the pulp viscosity: the higher the pulp viscosity, the lower the copper recovery. It was further found that for bentonite, as pulp viscosity increased, the flotation kinetics was adversely affected and the amount of froth was reduced, leading to the lower copper recovery. Increasing the kaolinite content mainly decreased the copper grade due to the fine size of kaolinite and the low-density aggregate structure, instead of the modification of pulp viscosity (Wang et al., 2015). Similar results were found by Chen’s study, where the detrimental effect of clay minerals on the flotation of pyrite increased as follows: montmorillonite > kaolinite > illite, with montmorillonite significantly increasing the pulp viscosity Chen et al., 2020. The mechanism was the E-E and E-F structure formed by montmorillonite, while F-F mode formed by kaolinite and illite platelets.

To modify the pulp rheology caused by clay minerals, sea water and saline water have been used in flotation. In the study of Zhang et al. (2015a), sea water was used to enhance the copper and gold flotation performance significantly in the presence of bentonite, which was attributed to the reduction of pulp viscosity. Cryo-SEM observation shown that association of bentonite platelets in the pulp transferred from E-F mode in tap water to F-F mode in sea water. The composition of seawater included Na+, K+, Mg2+, Ca2+, Cl and SO42-. Although it has been found that divalent cations, Mg2+ and Ca2+, in sea water have a detrimental effect of divalent cations on flotation (Laskowski and Castro, 2015, Laskowski et al., 2019, Uribe et al., 2017), a previous study found that at the same salt concentration, divalent cations, Mg2+ and Ca2+, were more effective than monovalent cations, Na+ and K+, to reduce bentonite viscosity and increase copper recovery of a practical ore (Wang et al., 2016). The prior study further found that instead of removing Ca2+ and Mg2+, within an appropriate range of divalent cations concentration, both copper grade and copper recovery can be enhanced through manipulating pulp rheology, entrainment and slime coating behaviour (Song et al., 2021).

On the other hand, in the presence of kaolinite, using sea water exacerbated the entrainment, resulting in lower copper and gold grades, with the association of kaolinite platelets transferring from E-E mode in tap water to the formation of cross-linked network structures through E-E and E-F linkages in sea water (Zhang et al., 2015b). This result together with the previous findings indicates that although the pulp viscosity in the presence of kaolinite was relatively lower, their network structure still affected the flotation results significantly, either in tap water or sea water.

The above investigations lead to the following hypothesis: the network structure of clay minerals may have a more direct relationship with the flotation performance. It is therefore that the objective of this study is to investigate how salt water containing calcium ions modifies the network structure of bentonite. The characterization of bentonite network structure includes stability, association mode, size distribution and so forth. Based on the results in this study, the role of bentonite network structure in flotation will be further understood.

Section snippets

Material and reagents

Sodium bentonite with P80 less than 33 μm was purchased from Sibelco Group, Australia, which consists of 63% sodium montmorillonite, 12% quartz and 25% albite (Wang et al., 2016). Deionized water was used to eliminate the effects of ions in tap water. Salt water was made up of calcium chloride dihydrate (CaCl2·2H2O) with different concentrations as required with analytical grade.

Rheological measurements

The density of bentonite suspension sample was maintained the same with the flotation experiment (Song et al., 2021).

The stability of bentonite network structure

The rheograms of bentonite suspension in salt water with different concentrations of calcium ions are shown in Fig. 1. The experimental data, as shown in Fig. 2, were fitted to the Herschel-Bulkley model:τ=τy+K(γ̇)nwhere γ̇ (s−1) is the shear rate; τy (Pa) is the yield stress; τ (Pa) is the shear stress; K (Pa·sn) is the consistency index and n is the flow behaviour index.

In combination with Fig. 1, Fig. 2, it can be seen that the bentonite suspension in deionized water behaved as a gel with

Discussion

The above results indicate that the stability, complex mode of association, pore size and particle size distribution of the network structure varied according to the concentration of Ca2+ in salt water. In combination with the findings, the evolution modes of bentonite network structure with increasing the concentration of Ca2+ is proposed, as illustrated in Fig. 6.

In deionized water, water molecules are able to penetrate into clay platelets, and cause the hydration of exchangeable cations and

Conclusion

The stability, association mode, size distribution of bentonite network structure all varied with the salt concentration. In deionized water or salt water with low calcium ions concentration, bentonite associated into E-F or E-E mode with smaller pore size and smaller size of network structure, which increased the yield stress of bentonite suspension significantly. When the concentration of calcium ions was high in salt water, bentonite aggregated into F-F compacts and flocculated into E-E or

CRediT authorship contribution statement

Shiya Du: Methodology, Formal analysis. Tiefeng Peng: Formal analysis. Siyu Song: Methodology, Investigation. Guohua Gu: Supervision, Project administration, Funding acquisition. Yanhong Wang: Conceptualization, Supervision, 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.

Acknowledgement

The authors gratefully acknowledge the financial support from National Natural Science Foundation of China (No. 52104288), Natural Science Foundation of Hunan Province, China (No. 2019JJ50835) and Key Program for International S & T Cooperation Projects of China (2021YFE0106800). The authors would also like to thank Dr Chunli Li (Institute of Microbiology, Chinese Academy of Sciences) for his contributions to Cryo-SEM measurements in this paper.

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