Flotation is one of the most used methods to upgrade natural graphite resources. However, the efficiency is usually decreased due to the entrainment of undesirable fine gangue minerals. In this work, the impact of different factors such as particle size, pulp density, and flotation reagent on the entrainment of mica and quartz in a flake graphite flotation was studied. The single gangue mineral flotation results showed increased gangue entrainment when reducing the particle size of gangue minerals and increasing the pulp density. Moreover, the flotation of artificial mixtures indicated enhanced entrainment of mica and quartz in the presence of graphite particles. The collector and frother also strongly affect gangue entrainment by modifying the structure and stability of the froth. Kerosene, which has a prominent deforming feature, can reduce gangue entrainment more effectively than diesel.

This investigation aims to demonstrate the effects of hydrophobic tails on the affinity and relevant flotation response of sulfonate-based collectors for fluorite. For this purpose, a series of alkyl sulfonates with different hydrophobic tails, namely sodium decanesulfonate (C10), sodium dodecylsulfate (C12), sodium hexadecanesulfonate (C16), and sodium dodecylbenzenesulfonate (C12B) were applied. The flotation tests showed that C12 and C12B had a better collecting performance than C10 and C16 at pH < 10, and the flotation recovery of fluorite was higher when adopting C12B as a collector compared with C12 with a strong base. The adsorption behaviors of collectors on the fluorite surface were studied through zeta potential, Fourier transform infrared (FTIR), and X-ray photoelectron spectroscopy (XPS) analyses. It was found that the affinity of alkyl sulfonates for fluorite was enhanced with the increase of the alkyl chain length from C10 to C16. The existence of phenyl in the hydrophobic tail of sulfonates could improve its activity for fluorite by reducing its surface tension. The abnormal phenomenon C16 with a high affinity for fluorite had a low collecting performance for fluorite mainly due to its overlong alkyl chain, resulting in low solubility in pulp, which restrained its interaction with fluorite. We concluded that C12B was the most applicable collector for fluorite among these reagents due to its high activity, high solubility, and low cost, which was further substantiated by calculating their molecular frontier orbital energy.

Using sodium oleate (NaOL) as a collector, the flotation characteristics of natural colorless fluorite (CF), green fluorite (GF), and purple fluorite (PF) were investigated through micro-flotation tests, collector adsorption measurements, and surface tension measurement. The micro-flotation results indicated that CF had a much better flotation response than both GF and PF, and had higher floatation recovery. The results demonstrated a considerable discrepancy in the interfacial properties between colorless and colored fluorite, even though all the samples were obtained from the same deposit, holding a similar high purity of CaF₂. The adsorption capacity of CF, GF, and PF for NaOL was 2.27, 4.18, and 8.21 × 10⁻⁶ mol/g under neutral conditions, respectively. Fourier transform infrared (FT-IR) measurements revealed that the carboxyl groups of NaOL reacted with Ca sites on the surface of fluorites by chemical adsorption. From the zeta potential analyses, PF exhibited a lower positive potential than CF and GF, mainly due to its surface carbonation. In the presence of NaOL, the surface potential of fluorites changed from positive to negative because the NaOL collector had been adsorbed onto the mineral surface and changed their surface potential, which was consistent with the flotation results at different pH values. We found that the floatability of the fluorite samples was influenced by their surface roughness, measured by an atomic force microscope (AFM) and scanning electron microscope (SEM). PF can be floated with adding more reagent dosage than CF and GF to compensate for its higher surface roughness.

In this paper, a surfactant, atlox4862 (formaldehyde condensate of methyl naphthalene sulfonic sodium salt) (MF), was introduced as a depressant for reverse flotation separation of sericite from graphite. Natural flake graphite has a strong hydrophobic property. After interacting with MF, the graphite became moderately hydrophilic. The flotation results showed that MF had a depressing ability for both sericite and graphite and that the flotation separation of sericite from graphite was attributed to the different declining levels of recovery between graphite and sericite with increased MF concentration. For a pulp pH of 8 and a MF concentration of 250 mg/L, the recovery rates of sericite and graphite were 89.7% and 11.3%, respectively. The results of the FTIR spectra and zeta potential measurements demonstrated that the interaction of MF with graphite and sericite is mainly through electrostatic attraction. MF was preferred to adsorb on the surface of graphite, decreasing its zeta potential and improving its hydrophilicity more than that of sericite.

The dispersion and agglomeration behavior of flaky graphite particles in an aqueous solution was studied. The small particles of flaky graphite naturally formed a large and compact agglomeration in an aqueous solution at pH 7.0 and 9.0, respectively. The theoretical calculation result based on typical DLVO theory was opposite the phenomenon observed by optical microscope, indicating that the interaction force between the flaky graphite particles exceeded the van der Waals and electrostatic double-layer forces. When the extended DLVO theory was used, the total interaction energy (U_T = U_A + U_R + U_HI) was negative over most of the distance range, indicating that the agglomeration of the small particles was attributed to the attraction. This result indicated that hydrophobic interactions dominated the dispersion and agglomeration behavior of the hydrophobic flaky graphite particles. GRAPHICAL ABSTRACT

In the flotation of semi-soluble minerals such as calcite and fluorite, Ca²⁺ is one of the common dissolved components influencing the collector adsorption behaviors on the mineral surfaces. However, there is very limited research focusing on how the dissolved Ca²⁺ affects the separation of fluorite and calcite. In the study, with sodium oleate (NaOL) as a collector and tannic acid (TA) as a depressant, a flotation test, zeta potential measurement, and adsorption experiment in the presence of Ca²⁺ were conducted. Flotation tests indicated that in the presence of Ca²⁺, fluorite and calcite were both significantly depressed by TA, leading to difficulty in the separation of the two minerals. It was also found that the depression effect on fluorite flotation was minimized with a high concentration of NaOL. Zeta potential measurement and the adsorption experiment results are consistent with the flotation results, revealing that it is only in the low concentration of NaOL that the flotation of fluorite was depressed by TA in the presence of Ca²⁺. Specifically, in the low concentration of NaOL, TA adsorbed on the fluorite surface and depressed the flotation of fluorite due to the preferential interaction between NaOL and Ca²⁺ in the solution leading to a shortage in the effective NaOL in the solution. In the high concentration of NaOL, the adsorbed TA on the fluorite surface was displaced by the excessive NaOL in the solution; hence, the flotation of fluorite was recovered. In contrast, TA always hinders the interaction of NaOL with calcite regardless of the presence and absence of Ca²⁺ and NaOL, hence, depressing the flotation of calcite. The study presented that a high concentration of NaOL may mitigate the negative effect of Ca²⁺ on the fluorite surface and improve the separation of fluorite from calcite.