The effect of laboratory synthesized polyaluminum chloride (PAC), which was prepared by alkali titration method, on the flotation performance of graphite was systematically studied in this work. Flotation tests using natural graphite ore demonstrated that PAC could enhance flotation yield while maintaining concentrate quality. Further flotation tests using artificial mixed graphite-kaolinite minerals demonstrated that the addition of PAC at around 30 mg/L can improve graphite recovery and separation efficiency, and has no significant negative effect on the loss on ignition in concentrate. The possible improvement mechanism of PAC on graphite flotation including water entrainment, slime coating, bubble-graphite attachment was revealed by characterization methods such as zeta potential measurements, focused beam reflectance measurement (FBRM), particle vision and measurement (PVM), single bubble loading tests and contact angle measurements. Zeta potential measurements show that PAC at 30 mg/L neutralized the negative charge on the surface of kaolinite, while graphite was positive charged. The real-time FBRM results show that the average chord length of kaolinite particles increased significantly when PAC concentration was 30 mg/L and decreased at PAC concentration of 90 mg/L, while graphite particles remained in a dispersion state. However, the PVM results indicated that the slime coating between kaolinite and graphite surface was aggravated when PAC was 30 mg/L and then got diminished at 90 mg/L. The single bubble loading tests and contact angle measurements proved that PAC at 30 mg/L significantly increased the attachment probability between bubble and graphite particles. Meanwhile, the contact angle of graphite remained stable without significant reduction, effectively maintaining the surface hydrophobicity of graphite and ultimately promoting graphite flotation recovery. This work is expected to provide theoretical understanding and technical support for graphite flotation by the adjustment of PAC concentration.

As a strategic non-metallic mineral resource in nature, graphite has excellent properties such as high thermal conductivity, excellent lubricity and remarkable chemical stability. It is widely used in metallurgy, machinery, aerospace and other fields [1,2]. The application of graphite in industry is determined by its crystal morphology [3]. Natural graphite can be roughly divided into crystalline graphite and cryptocrystalline graphite according to the degree of crystallization, geological origin and properties. Crystalline graphite is usually divided into lump graphite and flake graphite. Flake graphite possesses inherent hydrophobicity, granting it superior floatability compared to other graphite varieties, thereby ranking among the most selectively separable minerals. Through beneficiation processes, its fixed carbon content can exceed 90 %. In contrast, cryptocrystalline graphite exhibits non-uniform particle size distribution and complex impurity composition, resulting in significant challenges for purification [4].

The economic potential of graphite ore mainly depends on the purity of graphite [5]. Conventional mineral processing techniques for graphite enrichment include gravity separation, electrostatic separation, flotation, and magnetic separation [6]. In flotation, the equilibrium between hydrophilic and hydrophobic components on mineral surfaces plays a critical role in separation efficiency [7]. Owing to its inherent hydrophobicity and natural floatability, graphite can be effectively separated from common gangue minerals (e.g., feldspar, quartz, mica, and carbonate minerals), which are predominantly hydrophilic. Consequently, froth flotation has become the standard industrial method for the primary concentration of graphite ore and remains one of the most efficient and widely adopted techniques for graphite purification [8]. Therefore, flotation enables effective separation of graphite from gangue minerals, thereby achieving significant purification.

With declining ore grades and increasingly complex mineral dissemination, direct flotation has become more challenging for recovering valuable minerals, necessitating fine grinding to achieve sufficient liberation of target minerals [9]. However, the fine grinding process tends to simultaneously reduce both valuable and gangue minerals to fine/ultrafine particles. Among these fine-grained minerals, gangue minerals, due to their small particle size and strong surface hydrophilicity, are highly prone to mechanical entrainment into the concentrate [10]. Mechanical entrainment refers to the non-selective process whereby mineral particles suspended in the pulp are carried upward by fluid into the flotation froth. Li et al. [11] demonstrated that sericite exhibits significant entrainment behavior in graphite flotation, with its entrainment degree strongly dependent on particle size. Xu et al. [12] further confirmed that gangue minerals in graphite flotation concentrates primarily originate from mechanical entrainment. Additionally, hydrophilic fine gangue minerals can coat the surfaces of valuable minerals, reducing their hydrophobicity and hindering bubble-particle attachment, ultimately leading to lower recovery of valuable minerals [13].

Mechanical entrainment poses a significant challenge in the flotation of fine mineral particles [14,15]. Research indicates that there is strong size dependence in particle entrainment behavior [16,17]. Typically, ultrafine gangue minerals are easily dragged into the froth phase by fluid forces. However, their low inertia prevents them from overcoming the hydrodynamic resistance within the froth, hindering drainage back into the pulp and resulting in severe gangue carryover. To mitigate this issue, flotation researchers have proposed employing polymeric additives to induce selective aggregation of gangue minerals, thereby suppressing their entrainment [18,19]. For instance, Li et al. [20] proposed that the use of polyethylene oxide (PEO) could selectively flocculate quartz, which reduced the entrainment of quartz in hematite flotation, thereby improving the grade and recovery of final concentrate. Chen et al. [21] pointed out that PAC can selectively aggregate cryolite, reduce its entrainment in the flotation process, and improve the flotation efficiency of spent carbon cathode (SCC). The method markedly decreases gangue entrainment into the froth while improving the settling behavior of entrained gangue aggregates.

PAC is widely used in the field of wastewater treatment due to its advantages of easy solubility in water, wide adaptability to pH value, fast floc formation, low cost and simple use [22,23]. Previous studies have demonstrated that PAC as a flocculant can selectively flocculate gangue minerals, reduce contamination to concentrate and increase concentrate grade [24]. However, the PAC applied in the existing research is mostly industrial product, and its composition is not clear. In this work, laboratory synthesized PAC prepared by alkali titration method was used for the first time in the flotation of the artificial mixed graphite-kaolinite minerals. The structural characteristics of PAC were characterized using Al-Ferron timed complexation colorimetry and Fourier transform infrared spectroscopy (FTIR). The selective aggregation behavior of kaolinite and its effect on graphite flotation performance were systematically investigated by flotation tests, zeta potential analysis, FBRM, PVM, single bubble loading tests and contact angle measurements. It was found that besides reducing the water entrainment of kaolinite, PAC could also increase the bubble-graphite attachment probability and the recovery of graphite. In addition, the slime coating between kaolinite and graphite was aggravated when kaolinite was aggregated by PAC, but this negative effect on graphite flotation could be offset by other positive effects. These findings not only provide a new technical path for efficient separation of graphite resources, but also offer a new research perspective for selective flocculation behavior in mineral flotation.

80 of graphite and kaolinite are 48 μm and 14 μm, respectively. Flake graphite and kaolinite were uniformly mixed at a ratio of 4:1 to obtain graphite-kaolinite artificial mixed minerals. Kerosene was used as collector and sec-octyl alcohol (AR, 99 %, obtained from

Fig. 6 demonstrates the effects of PAC on both yield and loss on ignition of the flotation concentrate using natural graphite ore under different collector and frother dosages (0 and 100 g/t versus 300 and 150 g/t). As shown in Fig. 6(a), in the absence of collector, the control group (without PAC) exhibited a flotation yield of merely 3.41 % with concentrate loss on ignition of 66.97 %. Although PAC addition at 30–50 mg/L slightly increased the yield, the overall yield remained unsatisfactory

Flotation tests using both natural ore and artificially mixed samples demonstrated that PAC improved the key indicators such as concentrate yield, graphite recovery, and separation efficiency. Zeta potential analysis showed that PAC exhibited selective flocculation effect on kaolinite at 30 mg/L PAC concentration. FBRM and PVM tests further showed that 30 mg/L PAC could promote the selective aggregation of kaolinite, and kaolinite would attach on the surface of graphite in the form of flocs.