Editorial for the Special Issue “Advances in the Theory and Technology of Physical Separation”

1. Advances in Fundamental Understanding

Four papers in this Special Issue, from the fundamental perspectives of fluid dynamics, particle migration, interfacial behavior, and thermal treatment mechanisms, elucidate the key governing processes in physical separation.

Gao et al. (Contribution 1) explored the influence of cross-sectional geometry on the internal flow structure and particle separation performance of spiral concentrators. Via a computational fluid dynamics (CFD) approach, they systematically analyzed how the cross-sectional profile of the mixed-particle zone affected the secondary flow structure and separation behavior of 89.5 μm hematite and quartz particles, and they further optimized line segments through response surface methodology (RSM). The results showed that both the peak radial flux and the average radial velocity of the secondary flow increased with the slope of the corresponding line segments. Experimental validation demonstrated that employing the optimized cross-sectional profile improved the separation performance by nearly 5% when processing −100 + 75 μm hematite–quartz mixed feed. This work reveals the intrinsic connection between cross-sectional geometry, flow-field evolution, and particle migration, providing an important basis for the precise design of spiral concentrator cross-sections and for achieving efficient mineral separation.

Gao et al. (Contribution 2) investigated how wall roughness in the middle zone of a spiral concentrator influences the evolution of the flow field of the slurry and the associated particle separation behavior. By combining surface roughness measurement and numerical experimental methods, they analyzed the response of slurry film thickness, secondary-flow structure, and radial flux. The results show that increasing wall roughness suppresses the intensity of secondary flow, weakens the inward migration of particles toward the inner trough region, and shifts the enrichment zones outward. This study further demonstrates that mid-section wall roughness and feed particle size jointly determine the internal hydrodynamic characteristics and particle stratification in spiral concentrators, thereby providing a theoretical basis for trough material selection, surface-structure optimization, and roughness control during operation.

Guo et al. (Contribution 3) systematically examined how grinding parameters, including grinding concentration, media proportion, and filling ratio, influence the particle-size distribution, interfacial properties, and agglomeration behavior of galena. By integrating laboratory experiments with discrete element method (DEM) simulations, the study clarified how different grinding conditions regulate particle-size composition and showed that appropriate adjustments can significantly change the yields of the +25 μm and −10 μm fractions. Flotation tests demonstrated that excessive generation of −10 μm particles greatly reduces galena recovery because these ultrafine particles exhibit lower contact angles and smaller agglomerate sizes, which weaken their floatability. DEM simulations further indicated that moderately reducing grinding intensity can effectively limit the formation of −10 μm particles. The study establishes a mechanistic pathway in which grinding-induced abrasion intensity governs the generation of ultrafine particles, alters their interfacial behavior, and ultimately affects separation efficiency. This work provides new mechanistic insight into the longstanding challenge of poor physical separation performance for fine minerals and offers practical guidance for the coordinated optimization of grinding and separation processes in industrial applications.

Huang and Zhang (Contribution 4) focused on the development and underlying mechanisms of a combined process for producing high-purity quartz through impurity removal via pretreatment and subsequent metallurgical purification, with particular emphasis on thermal treatment mechanisms. Using river-sand–derived quartz with an SiO₂ content of 92.31% as the raw material, the authors proposed a multi-step flowsheet consisting of magnetic separation, reverse flotation, microwave roasting, and acid leaching, enabling the efficient upgrading and high-value utilization of low-grade quartz. Magnetic separation removed 87.24% of iron-bearing impurities, while flotation, microwave roasting, and leaching further reduced total impurity levels to below 100 ppm. Through SEM, TG–MS, and in situ XRD analyses, the study elucidated the key mechanisms of microwave-assisted impurity removal, including the rupture of fluid inclusions and lattice substitution. The results verify the effectiveness and feasibility of microwave thermal treatment for purifying low-grade quartz. In addition, the optimized magnetic separation and roasting parameters provide useful guidance for the industrial production of high-quality quartz.

Individually, these studies provide new scientific insights into how equipment structural parameters, grinding behavior, and impurity occurrence states influence separation performance.

2. Innovative Techniques and Applications

Five contributions in this Special Issue present technological advances and applied solutions that expand the capabilities and industrial relevance of physical separation processes.

Liu et al. (Contribution 5) demonstrated that a laboratory-scale reflux classifier (RC) could effectively preconcentrate ultrafine ilmenite tailings with a particle size D₅₀ of 13.8 μm and TiO₂ grade of 11.62%; a TiO₂ concentrate with a grade of 20.3% and a recovery rate of 82.8% could be obtained, and the separation efficiency reached 50%. The study elucidated the synergistic control mechanisms of fluidized-bed density, hindered settling, and inclined-surface flow on fine particle stratification. Comparative tests showed that magnetic separation performed poorly due to entrainment of magnetic gangue, whereas the RC achieved cleaner separation based on density disparity, significantly improving separation precision and economic efficiency. This approach provides a new, low-cost pathway for the preconcentration of ultrafine particles.

Liu et al. (Contribution 6) addressed the limited processing capacity of transverse-field high-gradient magnetic separators (HGMS) by systematically optimizing the structural parameters of the matrix box, including the diameter of the rod matrix, the filling ratio, the depth of the matrix in the direction of slurry flow. The study showed that these structural factors jointly determined the distribution of the magnetic capture zone, particle capture probability, and the ore unloading efficiency. Finally, an optimal double-layer matrix ring was applied to an industrial transverse-field HGMS, and its inner and outer rings were equipped with matrix boxes with φ3 mm and φ2 mm rods, respectively, which significantly enhanced ilmenite preconcentration efficiency and processing capacity, yielding higher TiO₂ grades and recovery than conventional longitudinal-field HGMS. This work highlights the critical role of structural optimization in improving the magnetic separation performance and provides a clear, engineering-feasible pathway for industrial matrix design.

Shi et al. (Contribution 7) proposed a 1 mm dry flip-flow screening technique to improve the screening of fine ilmenite particles, where conventional systems often suffer from clogged holes and reduced efficiency. Using a laser displacement measurement system, the researchers analyzed the dynamic characteristics of the flip-flow screen and evaluated its performance under different feed-size distributions and different ratios of obstructive particles. The results showed that the 1 mm screening efficiency reached 85.41%, demonstrating a significant improvement in fine-particle classification. The study also developed a pre-throwing tailings process based on 1 mm multi-stage screening, which does not use water and is particularly suitable for dry, water-scarce, or high-altitude permafrost regions. This provides strong potential for practical application.

Xu et al. (Contribution 8) evaluated the effects of smooth and humped flow channels on the recovery of industrial magnetic seeds in a drum magnetic separator. The results showed that under a range of operating conditions, the humped channel consistently achieved higher recovery than the smooth channel, with improvements of up to 3%. Simulations of the magnetic field, flow field, and particle trajectories revealed that the humped channel shifted impurity magnetic seeds from regions of weak magnetic force into those of stronger magnetic force, thereby significantly enhancing their capture efficiency. This work provides a clear structural optimization strategy for drum magnetic separators and offers practical value for improving magnetic flocculation performance in water-treatment applications.

Shafiee et al. (Contribution 9) developed an optimized and scalable synthesis route for high-purity barium sulfate using D-optimal methodology. By systematically varying BaCl₂ and sulfate concentrations, feed rate, and reaction temperature, the researchers established and validated the key parameters controlling product yield and density. Under the optimized conditions, the process produced barium sulfate with a density of 4.25 g/cm³ and an average purity of 96.08%. The method reduces reagent consumption, minimizes secondary waste, and demonstrates strong potential for sustainable and cost-effective scale-up, offering a practical approach for the green preparation of high-purity inorganic salts.

Collectively, these studies show how innovations in device structures, screening strategies, magnetic configurations, and process optimization can translate physical-separation principles into practical, scalable, and industry-ready solutions.

3. Future Directions

The contributions in this Special Issue reflect two major trends in physical separation research: (1) the continuous deepening of fundamental mechanistic investigations into multiphysics processes, such as flow-field evolution, particle migration, interfacial interactions, and impurity occurrence; (2) the effective translation of these mechanistic insights into engineering practice, driving structural optimization and performance enhancement of physical separation equipment, as well as the emergence of scalable and industrially applicable mineral processing technologies. Together, the presented studies demonstrate that physical separation, as a critical pillar of mineral processing, is continuously evolving. It not only retains its irreplaceable role in the beneficiation of traditional ores but also exhibits increasing strategic importance in the treatment of ultrafine and low-grade minerals, the efficient recovery of critical minerals, the revalorization of tailings, and environmentally responsible resource development.

Looking forward, the deep integration of computational multiphysics modeling, intelligent optimization algorithms, high-efficiency physical separation equipment, and green, low-carbon mineral processing flowsheets will serve as a central driving force for the continued advancement of physical separation and for empowering the broader mineral processing value chain. Meanwhile, the convergence of physical separation, multiscale multiphysics simulation, particle–fluid coupling models, intelligent sensing and process digitalization, data-driven process optimization, and environmentally sustainable processing technologies will represent a key direction for future development, guiding the industry toward higher efficiency, lower energy consumption, greater intelligence, and long-term sustainability.

We thank all the authors for their valuable contributions, the reviewers for their thoughtful evaluations, and the editorial team of Minerals for their support. We anticipate that the presented studies will serve as valuable references for researchers and practitioners, while also inspiring new directions in the theory and technology of physical separation.