Abstract
Montmorillonite is always a troublemaker for the dewatering in coal processing since its existence can decrease the rates of sedimentation and filtration of coal slurry. To eliminate the adverse effect of montmorillonite, adjusting the slurry pH and adding electrolytes are always the key methods. However, the underlying mechanism still needs to be further studied. The dewatering of Na-montmorillonite (Na-Mt) suspensions has been studied as a function of NaCl concentration (10−3, 10−2, and 10−1 M) at different pH values (6.0, 7.7, 8.1, 9.2). The point of zero charge of edge surface of Na-Mt (pHPZC,edge) appeared at the pH value of 6.8. The sedimentation and rheology experiments described the coagulation and flow behaviors of Na-Mt suspensions, respectively. The Na-Mt suspension coagulated spontaneously at low salt concentration with the pH ~ 6.0. For the pH > pHPZC,edge, the height of the sediment bed reduced and apparent viscosity increased with the increase of the electrolyte concentration. The filtration properties were evaluated on the basis of Darcy’s law. The obtained result clearly demonstrated that the filtration rate was accelerated with the increase of pH and electrolyte concentration. The modes of particle association and its effect on filtration performance were discussed. Moreover, a comparison with related results from the literature was performed. At pH ~ 6 and low electrolyte concentration, the positively charged Edge surfaces and negatively charged Face surfaces coagulate rapidly to form a sealed structure by electrostatic attraction. Furthermore, inside this sealed structure, the water molecules cannot be removed in the filtration process easily. However, by increasing the electrolyte concentration at pH > pHPZC,edge, the gradually formed Face/Face structure increases the filtration rate sharply because of the inhibiting effect of the electric double layer (EDL) and the osmotic expansion. Therefore, adjusting solution conditions of the aqueous suspension to tune the particle coagulation behavior is one of the effective methods to solve the problem of montmorillonite dewatering.
Highlights
With the scarcity of high-grade mineral resources, the separation process of lower-grade clay-bearing ore brings about more challenges in mineral processing plants
The points of zero charge of the edge surface of Na-Mt were sites at pH = 6.8, which is consistent with the reported value pH ~ 6.5–7 [9,15,16,17]
Theintroduction, dispersed and montmorillonite closely related to the in the thecoagulated main contributions to the surface charges ofare montmorillonite are the negative charges on the basal plane by the isomorphic substitution and pH-dependent charges on the surface charges, which in turn are a function of the electrolyte concentration and pH
Summary
With the scarcity of high-grade mineral resources, the separation process of lower-grade clay-bearing ore brings about more challenges in mineral processing plants. Raw ore containing montmorillonite leads to adverse effects on the process performance, such as floatation, dewatering, and wastewater treatment. Understanding of surface properties, rheological characteristics, and particle interaction of montmorillonite are essential when dealing with the aforementioned difficulties in the dewatering process [1]. Yield stress and viscosity are the essential factors to evaluate rheological properties of aqueous suspensions of montmorillonite. The increased solid concentration of montmorillonite suspension leads to an increase in yield stress and apparent viscosity. It affects the rheological behaviors, which are transformed from Newtonian to Bingham plastic to shear thinning material with yield stress. The divalent cation salts were more effective in reducing the yield stress and apparent viscosity than monovalent cation salts [3]
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