Rheological investigations on the hetero-coagulation between the fine fluorite and quartz under fluorite flotation-related conditions
Graphical abstract
Introduction
Fluorite (CaF2) is the main source of fluorine in the nature. It has wide application in the modern industry, such as producing hydrofluoric acid in chemical plants and acting as a flux in steel making. Normally, 99% or higher purity for fluorite concentrate is required in fluorite market. At present, most of the fluorite concentrate is produced by flotation in which the fluorite is separated from the gangue minerals such as quartz, calcite and other silicates. Typically, fatty acids and their derivatives are used as the fluorite collectors [1,2], among which the most conventional one is the sodium oleate [3]. The collecting mechanism of these collectors for fluorite is the chemisorption deriving from the chemical interactions between the exposed Ca2+ on the fluorite surface and carboxyl (–COO–) in the collectors [4,5].
Fluorite deposits are classified to fluorite-quartz type, fluorite-calcite type, fluorite-sulfide type and other types based on the gangue minerals associated to it. For the fluorite-quartz type deposit, it is theoretically easy to selectively separate fluorite from the quartz because the latter usually exhibits poor reactivity to the fatty acids collectors [6]. However, due to the mineral liberation of the low grade, finely disseminated fluorite ores, fine particles (mostly less than 10 μm) containing both fluorite and quartz are often generated in the size reduction process. The fine particles in flotation feed are hard to handle and are often regarded as the main reason for the low flotation recovery and the low grade of flotation concentrate [7]. Commonly, the negative effect of the fine particles on flotation is ascribed to the fine particle characteristics [8], such as small mass and large specific surface area, which often lead to low collision probability and non-selective reagent adsorption in flotation process [9,10]. The published papers about the treatment of fine particles are mostly focused on increasing the particle size [11,12], decreasing the bubble size [13] and surface modification [14,15]. Unfortunately, only limited improvements in the separation efficiency of fine particles were obtained in these papers. For the fine fluorite and quartz, the efficient separation remains a challenge. The mechanism of the quartz coming into the fluorite concentrate is still not clear.
Particle interactions in mineral pulp are very important for studying the flotation separation process, especially for the flotation separation of fine particles. The particle interactions are usually influenced by the flotation conditions such as solid concentration, pH, electrolyte concentration, shear environment and reagent concentrations [16]. Van der Waals force, electrostatic force and hydrophobic force are the main factors that determine the particle interaction under flotation conditions. It has been proved that the hydrophobic force between the collector-adsorbed particles are beneficial for recovering the fine particles (less than 10 μm) in the case of shear flocculation flotation practice [10,17]. The Van der Waals force and the electrical double layer force between the valuable particles and the gangue particles also exhibit a non-negligible effect on the separation sub-process. When these forces between valuable particles and gangue particles are attractive, flotation of the valuable particles is often depressed by the phenomena of “slime coating” [18,19]. However, these forces in flotation pulp are difficult to be precisely measured and used for quantifying the particle interactions, which limits our full understanding of the particle interactions in the flotation process.
Rheological measurement on the flotation slurries offers quantitative measurements for these forces and provides the direct insight into the particle interactions [20]. The rheological parameters, such as yield stress and apparent viscosity, have been reported to be the most important determinant rheological properties for evaluating the particle interactions and can be correlated to the flotation performance. The use of the rheological properties of mineral pulp as the determination of the particle interactions and flotation performance has been applied in the flotation of oxide minerals [21,22], sulfide minerals [23,24], coals [25,26] and clays [27,28]. However, to date, there are few published papers referring to the particle interactions between fine fluorite and fine quartz under flotation related conditions, especially in the field of rheological investigations.
This study aims to study the particle interactions between the fine fluorite particles and fine quartz particles under fluorite flotation conditions and reveal how the fine quartz comes into the fluorite concentrate. Sodium oleate was used as fluorite collector. The yield stress value of mineral slurry was utilized to evaluate the aggregation/dispersion degree of the particles in the flotation pulp. The yield stress of single mineral and mixed minerals slurry as a function of solid concentration, pH and sodium oleate dosage was measured. The flotation performance of the fine fluorite in the presence of different ratios of fine quartz was investigated. A possible action model of the particle interaction between fluorite and quartz in slurry was proposed based on the rheological investigations and flotation tests results. The model was used to describe how the fine quartz comes into the fluorite concentrate in flotation process.
Section snippets
Materials and reagents
The fluorite and quartz samples used in this research were all obtained from Hunan, China. The hand-selected crystals were crushed to −1 mm in a laboratory roll crusher. The crushed products were ground in a planetary mill to get the −10 μm particles for rheological studies and flotation tests. A portion of the −10 μm particles was further ground to −2 μm with an agate mortar for zeta potential and FT-IR spectroscopy studies, of which the results are shown in Supplementary Material. The
The hetero-coagulation between the fine fluorite and quartz in aqueous suspension
Fig. 4 shows the yield stress of fluorite and quartz slurry at pH 7.0 as a function of the solid concentration by volume. It can be seen that the yield stress of the fluorite slurry was much higher than that of the quartz in the whole solid concentration range tested. As the solid concentration increased, the yield stress of fluorite slurry increased remarkably, but that of quartz kept nearly unchanged at around 0.04 Pa. When the solid concentration was 20 vol%, the yield stress of the fluorite
Conclusions
Given the rheological investigations, optical observations and the flotation results, the following conclusions can be drawn:
- (1)
Fluorite particles could aggregate in the pH range of 5–10 and form net-work structures in suspension. The yield stress of the net-work structures increased as the solid concentration increased but decreased as pulp pH increased. Quartz particles did not aggregate across the pH range of 5–12 regardless of the change of solid concentration.
- (2)
There existed strong
Acknowledgement
The authors acknowledge the support of the Major State Basic Research Development Program of China (973program) (2014CB643402), the Project funded by China Postdoctoral Science Foundation (Grant No. 2018 M640964 and 2019T120884), the General Project (Youth) of Natural Science Basic Research funded by Shaanxi Science and Technology Department (Grant No. 2019JQ-368), the Special Research Project of Shaanxi Education Department (Grant No.18JK0473), the Natural Science Project of Shaanxi Education
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