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The presence of large amounts of Ca2+ in the flotation water can cause a poor flotation index of low-rank coal (LRC), and its influence mechanism has not been reported. In this paper, the mechanism of the Ca2+ effect on LRC flotation was analyzed for the first time from the perspective of the interaction between Ca2+ and the LRC surface oxygen-containing functional groups using flotation tests, solution chemistry calculations, ICP, Contact angle, Zeta potential, Fourier transforms infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) detection. The results of solution chemistry calculations and flotation tests show that the calcium in flotation water with pH = 6–8 is mainly in the form of Ca2+, and it has a certain inhibitory effect on the LRC flotation. The results of Contact angle, Zeta potential, FTIR, and XPS show that Ca2+ can adsorb on the LRC surface and chemically react with phenolic hydroxyl and carboxyl groups of the LRC surface, leading to an increase in the hydrophilicity of the LRC particles, weakening the adsorption effect of collectors on the LRC surface. These will further deteriorate the LRC flotation index. [ABSTRACT FROM AUTHOR]

Calcium ions are a kind of unavoidable ions in water. It has the deleterious effect on molybdenite flotation. High-calcium flotation wastewater (HCFW) was reused for flotation circuits after the pretreatment removing Ca2+ in from HCFW. The high cost of wastewater treatment limits HCFW reuse. In this paper, an efficient, innovative, low-cost and environmental-friendly flotation wastewater reuse technology was introduced. XLM, as a composite collector for molybdenite, is a mixture of diesel oil (DO) and polycyclic aromatic hydrocarbons (PAHs). It could reduce the deleterious effects of Ca2+ on the flotation of molybdenite in HCFW. Therefore, this was used to replace the pretreatment removing Ca2+ in from high Ca2+ wastewater and saved the cost of wastewater treatment. When XLM consists of 4 wt % PAHs and 96 wt % DO, it has better adaptability than DO in the different Ca2+ concentration of flotation water. The contact angle measurements indicated that PAHs, as a synergistic component of a composite collector, could adsorb on the edges of molybdenite in the presence of Ca2+ by forming PAHs-Ca2+-MoO42− structure to increase the contact angle of fine molybdenite particle and reduce the deleterious effects of Ca2+ on the flotation of molybdenite. The industrial-scale test further that demonstrated XLM can improve the molybdenite roughing recovery and grade by 1.8% and 3.46% compared with DO as the collector in high Ca2+ flotation wastewater. It is feasible and effective to replace high-cost wastewater treatment for molybdenum plants. [ABSTRACT FROM AUTHOR]

[Display omitted] • AH as a co-collector improve molybdenite flotation recovery in high-calcium water. • The interaction of AH with Mo atoms of MS100 relies on cation-π bonds. • AH cannot interact with Ca2+ on MS100 relying on cation-π bonds. • AH can promote the adsorption of HO in MS100. • Synergistic adsorption of AH and HO improves molybdenite surface properties. The poor molybdenite flotation behavior in high-calcium water is mainly caused by the fact that Ca2+ ions can inhibit the adsorption effect of hydrocarbon oils on the molybdenite surface. Eliminating the negative influence of Ca2+ ions on molybdenite flotation is of great significance. In this work, aromatic hydrocarbon (AH) was selected as a synergistic collector to reduce the deleterious effect of Ca2+ ions on the molybdenite flotation using hydrocarbon oils (HO) as the collector. Various methods including flotation tests, contact angle measurements, zeta potential, X-ray photoelectron spectroscopy (XPS), density functional theory (DFT), and molecular dynamics (MD) simulations were carried out to understand the adsorption behavior of AH on molybdenite surface and its synergistic mechanism with HO. The flotation results showed that AH as a synergistic collector could improve molybdenite flotation in high-calcium water. The zeta potential and contact angle measurements indicated AH can adsorb on molybdenite (1 0 0) surface (MS100), weaken the MS100 hydrophilicity, and promoted more HO adsorb on the MS100. The XPS showed that the adsorption is mainly physical adsorption. Moreover, DFT calculation results indicated that AH can form a cation-π bond with the Mo-atom of MS100. MS simulations results suggested that the adsorption of AH on the MS100 can enhance the adsorption of HO molecules in this environment, thus weakening the adverse influence of Ca2+ ions on molybdenite flotation. The results provide a basis for understanding and improving the separation effect of molybdenite from other minerals in the presence of Ca2+ ions. [ABSTRACT FROM AUTHOR]