Spiral concentrators are widely used for the concentration of coal and heavy minerals. The design for the spiral concentrator is full of challenge because of the sensitivity of design parameters to water flow field. The study aims to reveal the rule of the design parameters on water flow field in spiral concentrators. The pitch, the transverse angle, and the flow rate were taken as the variables. A suggested simulation method based on grid independence investigation, four types of turbulence models, and Volume of Fluid (VOF) approach was proposed. The water flow field on spirals with different design parameters was determined by the validated model. The predicted water flow depth show agreement with the measured results. The Reynolds-Stress Model (RSM) turbulence model is superior to other turbulence models. The water depth and the primary velocity in spiral concentrators increase smoothly outward. The secondary velocity is relatively small compared with the primary velocity. No obvious secondary flow exists in the inner trough. The reversal position of the secondary flow is approximate at the fractional depth of 0.2 ~ 0.5. The reduction of the pitch and the flow rate, together with the increase of the transverse angle, can strengthen the secondary flow in spiral concentrators.

This study examined the incompatibility of high-intensity conditioning (HIC) conditions required for fine and coarse coal particles. The effects of speed and time of stirring on the recovery of coarse and fine particles were investigated by using three different flotation processes. The results showed that the coarse particle recovery of surface cleaning (SC) flotation reached the maximum value of 69.71% at the stirring condition combination of ‘1400 rpm +2 min.’ However, the coarse particle recovery reached the maximum value of ~69.37% at a stirring condition combination of ‘1600 rpm +3 min’ or ‘2000 rpm +2 min’ in HIC flotation. Achieving the same coarse particle recovery, SC flotation consumes less energy than HIC flotation. The maximum recovery of fine particle 65.37% was achieved at a stirring condition combination of ‘1600 rpm +2 min’ in HIC flotation. SC failed to promote the recovery of fine particles and even produced some negative effects in some cases. Fine particles consume less energy than coarse particles with respect to their maximum recovery. In addition, the optimum stirring speed and time for SC were 1400 rpm and 2 min, respectively, while 2000 rpm and 3 min for HIC, respectively. The optimum energy requirements involving stirring speed and time for SC do not match that for reagent dispersion and particle-reagent collision. The best flotation result achieved by HIC is a compromise of the above three aspects. It is suggested that SC, reagent dispersion, and particle-reagent collision should be carried out separately.

The mechanism of high-intensity conditioning (HIC) has not been thoroughly revealed, and therefore this work investigates the effect of HIC on the surface hydrophobicity of coal with different particle sizes and the possible formation of particle–bubble clusters. The results show that different HIC conditions are required for coarse and fine particles. Coarse particles (+75 μm) require a higher turbulence intensity to increase collector dispersion, thereby increasing the adsorption of the collector. Fine particles (−75 μm) require a lower turbulence intensity to reduce the desorption of the collector. In this study, the optimum HIC conditions for coarse and fine particles are “2200 rpm + 1 min” and “1300 rpm + 1 min”, respectively. Interestingly, it seems that the adsorption capacity between fine particles and the collector is weaker than that for coarse particles. A non-enclosed HIC system produces up to 1.78 × 10⁴/g bubbles in coarse particle–bubble clusters, and the mean bubble diameter is approximately 87 μm. The cluster achieves pre-mineralization and increases the apparent particle size, which is expected to improve flotation.

A stable attachment between bubbles and solid particles is essential for flotation. Therefore, it is particularly necessary to study the dynamic process that occur in the attachment of bubbles to a solid surface. In this paper, Teflon and plexiglass plates were used as hydrophobic and hydrophilic solid surfaces, respectively, and solid surfaces with roughness of 0.018 μm to 5.33 μm were prepared by polishing with sandpaper. The influence of roughness on the dynamic process in bubble attachment to solid surfaces with different hydrophobicity was studied via a high-speed camera (750 frames per second). It was found that roughness played a positive role in the attachment to the hydrophobic Teflon surface while a negative role in the attachment to the hydrophilic plexiglass surface in terms of the bubble’s attachment to the solid plates. For a smooth Teflon surface, the formation of three–phase contact (i.e., the drainage of wetting film) took up to 95 ms, whereas for a very rough Teflon surface it took only 5 milliseconds. On the contrary, the high roughness prevented the bubble from attaching to the hydrophilic plexiglass surface. It was concluded that the increased roughness of Teflon plates was conductive to air entrapment in surface irregularities, inducing the rapid rupture of the wetting film on a very rough Teflon surface, while the increased roughness of the plexiglass plates resulted in “water pockets” in surface grooves, making the wetting film on the plexiglass surface stable.

Washery reject is a by-product discharged during coal preparation, in which there are still certain amount of valuable combustibles. The recovery of clean coal from washery rejects is of great significance for protecting China’s scarce coking coal resources. The main purpose of this work was to design a deep process that can maximize the recovery of clean coal from a coking coal washery rejects with about 50% ash and test the technical feasibility of the process. Based on fundamental analyses, the high ash fractions of +0.2 mm and +1.80 kg/cm³ were firstly removed from the raw coal sample via hydrocyclone and spiral separator. Then, the remained parts having an ash of 27.57% were ground, pre-conditioned, and separated by flotation. The results showed that, under the optimum process parameters, a maximum yield of 24.47% clean coal with 12.23% ash can be recovered from this washery rejects. The ash content of the washery rejects increased from 50.10% to 62.37% after this treatment. This study indicated that it was technically feasible to achieve deep recovery of clean coal from washery rejects, which was of significance for the treatment of coking coal tailings.

1. Froth flotation is widely used to selectively separate valuables from gangue minerals through the hydrophobicity differences between mineral surfaces (Cheng et al. 2024; Nunna et al. 2022). In t...