The evolution law of fluid and solid motion in the initial stage of spiral separation process is critical for mineral particle separation. The current research intends to investigate the fluid and particle motion behaviors in the initial stage of a spiral separation process using a CFD-DEM approach. The results show that the motion behaviors of fluids and particles in the initial stage of spiral separation process could be separated into three states: the development state, the transition state, and the relatively stable state. In the development state, water flows outward with a rapid change of velocity and film thickness. Meanwhile, fluid forced particles to move outward rapidly with no obvious trajectory difference. In the transition state, a collision between the slurry and the outer trough sidewall happens with changing the moving direction of fluids and particles. Following the collision, the flow field distribution tends to be relatively constant, and the light particles progressively migrate to the spiral trough’s outer edge, while heavy particles gradually move to the spiral trough’s inner edge.

The dynamic migration of mineral particles within spiral separators and its control via structural parameters are not yet fully understood, hindering efficiency improvements. To this end, a set of spiral separators with systematically adjusted structural parameters was designed. Extensive sampling of a 1–0.25 mm coal slurry yielded 120 samples from 6 separators, across 5 turns and 4 radial streams. Sink-float analysis revealed a well-defined three-stage separation mechanism: the roughing stage involves rapid segregation of light and heavy particles, while intermediate-density particles remain widely distributed; the intensified cleaning stage governs the radial migration of intermediate-density particles while simultaneously enriching the high-density and low-density fractions; and the final cleaning stage stabilizes the particle distribution and redirects misplaced particles. The influence of key structural parameters was also quantified: the composite cross-section outperformed cubic parabolic and elliptical profiles, markedly enhancing the separation of high-density and medium-high-density particles from the lighter product; increasing the trough inclination angle significantly promoted the radial inward migration of medium-high-density particles; a reduced pitch-to-diameter ratio effectively concentrated high-density and medium-high-density particles within inner and middle regions. Based on these insights, a “process intensification” strategy was proposed and materialized in a novel spiral separator design featuring stage-optimized, multi-parameter coordination. Performance evaluation demonstrated a separation efficiency of 94.74% under equivalent product quality constraints, a substantial improvement over conventional design. This work provides a fundamental, stage-specific understanding of particle separation dynamics and establishes a practical basis for the advanced design of high-efficiency spiral separation systems.

Spiral separators commonly face the issue of particle misplacement during fine particle separation, which severely limits separation accuracy. This study employs a coupled CFD-DEM numerical simulation method to systematically investigate the influence mechanism of transverse inclination angle (10°, 15°, 20°) on particle moving behavior. The results show that the separation process exhibits distinct stage characteristics, which can be divided into an initial stage (first 1/3 turn), a transition stage (1/3 to 2 turns), and a quasi-steady stage (after 2 turns). A steeper angle (20°) optimizes the flow field, reducing the inner low-velocity zone and widening the high-velocity core, which promotes inward migration of particles. This enhances the enrichment of high-density particles while effectively suppressing their mixing into the clean coal product at the outer edge. For difficult-to-separate fine particles below 0.1 mm, although complete separation is challenging, increasing the transverse inclination angle still shows a clear reduction in the misplacement of high-density particles, providing a controllable approach for improving the quality of the outer edge product. This study offers theoretical insights and design guidance for optimizing spiral separator structures and enhancing fine coal separation efficiency.

The hydrothermal dewatering (HTD) of lignite results in noticeable variations in the low-temperature oxidation process. Consequently, this study was made on the gas release and temperature change characteristics to investigate the oxidation kinetics and mechanism of HTD coal samples. In this study, a lignite from Inner Mongolia in China was upgraded by HTD. N₂ adsorption, SEM, FT-IR, and chemical titration experiments were also carried out on raw and HTD coal samples to relate the physico-chemical structure properties with low-temperature oxidation characteristics. Results show that HTD coal samples have higher low-temperature oxidation activities and lower critical ignition temperatures compared with raw coal. According to the change in activation energy by kinetic analysis, the low-temperature oxidation process in the temperature range 35–140 °C could be divided into the stage I (oxygen adsorption stage) and stage II (accelerated oxidation stage). The correlation analysis indicates that the oxygen adsorption stage is controlled by the aliphatic and surface structures, while the accelerated oxidation stage is jointly affected by the competition of physico-chemical structures. The oxygen adsorption stage promotes the progress in accelerated oxidation stage.