Minerals Hub / Mining & Production / A spiral sorts by density, and grain size gets in the way
Mining & Production · 5 min read
A spiral sorts by density, and grain size gets in the way
A spiral concentrator separates heavy grains from light ones on a film of flowing water, but the same flow also responds to grain size, which is one reason spiral feed is sized, and slimes are removed first.
Reviewed by Peter Uppal
The short version
A spiral concentrator is a kind of thin-film flow gravity separation equipment in which, as Metso describes its own design, adjustable splitters take heavy mineral concentrate from near the inside of the spiral. The separation is not by density alone, since Metso describes it as based on the size and specific gravity difference present in a mineral suite. On this publication's reading, that is one reason spiral plants size their feed before it reaches the trough; Metso also advises removing slimes, which can act like a heavy medium.
How the trough works
Researchers who study spirals describe two forces acting at once. Spiral separation rests on the combined action of gravity and centrifugal force, with the slurry rotating downwards along the trough under gravity while it moves outward across the trough under centrifugal force. The water itself is uneven, with thin inner and thick outer film-type flow.
Within that flow, the grains part company. A 2023 study of beach sands by Singh and colleagues says that particles with the greatest specific gravity rapidly settle to the floor of the trough, while the lighter, faster flowing material is forced outward, and separation is enhanced by the differences in centrifugal forces between the two. Metso's description of its own spiral says that the lighter minerals are more readily suspended by the water and climb toward the outer rim, while heavier grains that the water does not suspend migrate by saltation along the lowest part of the trough, a rolling and bouncing motion.
A 2023 laboratory study describes the primary downward rotating flow and the lateral secondary flow across the trough, and Gao and colleagues define part of that secondary flow as an inward flow.
Splitters take the cut
What leaves the spiral is decided by splitters. The width of the concentrate band is determined by the radial coordinates of the splitter location, and Gao and colleagues report products with different distribution widths by adjusting the splitter position. Metso states that adjustable product splitters take the heavy mineral concentrate from near the inside of the trough.
Where concentrate is drawn off at several points down the spiral, Metso's operating advice is that splitters near the top should be opened wider than those near the bottom, because the concentrate band is wider at the top. The result, it says, is that the splitters collect concentrate at a relatively uniform grade rather than uniform rate.
Where size gets in the way
Size affects where a grain goes, as well as density. In a laboratory spiral treating hematite and quartz, Gao and colleagues found that separation reflected both the density effect and the particle size effect, and that fine hematite and coarse quartz form a large amount of misplaced material. In that study, the misplaced heavy grains were the fine ones; the misplaced light grains were the coarse ones. A 2025 paper by Gao and colleagues describes a zone where low-density coarse particles and high-density particles readily mix, and reports that the migration amount and distance of particles show apparent differences in density and size.
"Fine hematite and coarse quartz form a large amount of misplaced material." Gao and colleagues, on a laboratory spiral, 2023
On beach sands, the same pattern appears. Singh and colleagues report that the performance of the spiral depends on both the feed grade and the texture of the material fed to it. In their study of lean and off-grade Indian beach sands, the designed spirals were not suited for low grade samples with finer heavy particles, and the spirals performed better at higher grade and coarser feed. They also cite earlier work by Güçbilmez and Ergün reporting that separation performance decreased dramatically in very fine particles.
Why the feed is sized and deslimed
Spiral makers' own guidance turns on feed size and cleanliness. Metso states that the spiral works best on closely sized feed, with some tolerance for wide size ranges in select cases. It advises that spiral feed be relatively free of slimes, which can act like a heavy medium, and says desliming will almost certainly improve performance where slimes exceed 10% by weight. It also says that, in general, a specific-gravity difference of at least 1 between light and heavy particles is required for successful gravity separation.
Multotec, describing chromite plants in a 2024 trade-magazine interview, says there has to be a feed preparation process of some sort prior to separation, and that ultrafine material in the slurry causes viscosity problems. In that chromite context, it says, the feed is deslimed to remove the ultrafine particles prior to spiral concentration.
The working ranges the makers publish differ. Metso's August 2026 page gives, in general, a coarse size limit of about 1 mm and a fine limit of approximately 45 µm. Metso's 2011 handbook (Edition 8) gives spiral ranges of 75 µm to 1.0 mm for light particles and 75 µm to 0.5 mm for heavy ones. Mineral Technologies says its FM series of fine-mineral spirals is used for valuable heavy mineral particles in the range of 30 to 150 µm. Each figure describes one maker's equipment.
On this publication's reading, sizing exists to narrow the range of grain sizes the spiral has to handle, so that density can do more of the sorting; desliming removes slimes, which Metso says can act like a heavy medium and which Multotec, for chromite, says cause viscosity problems.
Where spirals sit
At the mineral-sands operations described here, spirals sit in the wet concentration plant. Kenmare states that at its Moma mine, after oversize is rejected, progressive stages of gravity spirals separate a heavy mineral concentrate from the tailings. Kerala Minerals & Metals states that low-grade beach-washing sand is upgraded using spiral concentrators at various stages. Singh and colleagues, citing Sunita and Rao, describe spirals as the primary unit to recover total heavy minerals from lean and off grade beach placer sand deposits, and note they have been successfully applied to recovery of rutile, ilmenite and zircon.
One pass is often not enough. Metso states that "a spiral will normally achieve a 3:1 upgrading ratio" and that a multi-pass flowsheet is often required. Multotec, describing chromite plants, names the stages: rougher spirals for initial separation, scavenger spirals for recovering misplaced ore, and cleaning and recleaning spirals that remove misplaced gangue from the value stream.
Related
- Processing Technology
- Mining Methods
- Rehabilitation
- Water
- Exploration
Sources
- PRIMARYGao, S., Zhou, X., Li, B., Wang, Q. and Liu, C., 2025, 'Effect of Wall Roughness in the Middle Zone of Spiral Concentrator on the Flow Field Evolution of Hematite–Quartz Slurry and Particle Separation Behaviour', Minerals 15(3), 208, doi:10.3390/min15030208. Introduction and abstract: the spiral as thin-film flow gravity separation equipment; thin inner and thick outer film; the easy mixing zone; particle migration by density and size.
- PRIMARYGao, S., Wang, Q., Zhou, X., Liu, C., Shen, Y. and Cui, B., 2024, 'Effect of the Cross-Sectional Geometry of the Mixed Particle Zone on the Spiral Separation Process and Its Structural Optimization', Minerals 14(12), 1251, doi:10.3390/min14121251. Introduction: gravity and centrifugal inertia force; s. 2.4: inward secondary flow; splitter position and product width.
- PRIMARYGao, S., Zhou, X., Meng, L., Zhao, Q. and Liu, W., 2023, 'Variation of Flow Hydrodynamic Parameters and Prediction of Particle Separation Indices in the Spiral Concentrator with the Regulation of Pitch-Diameter Ratio', Separations 10(7), 410, doi:10.3390/separations10070410. Laboratory spiral, hematite–quartz. s. 3.1.2: primary and secondary flows; s. 2.4: splitter location and concentrate band width; abstract: density and particle size effects, misplaced material.
- PRIMARYSingh, D., Mishra, B. and Bhima Rao, R., 2023, 'Recovery of Total Heavy Minerals Using Gravity Concentrators on Lean and Off Grade Beach Sands', Journal of Mining and Metallurgy 59A(1), 15–24, doi:10.5937/JMMA2301015S. Indian beach sands. pp. 15–16: settling of heavy particles; spirals as the primary unit for total heavy minerals (citing Sunita and Rao) and their applications; feed grade and texture; very fine particles (citing Güçbilmez and Ergün). pp. 21, 23: performance by grade and feed size.
- SECONDARYSingh, Mishra and Bhima Rao (2023), as above, reporting findings of earlier authors (Sunita and Rao; Güçbilmez and Ergün) that this publication has not read in the original.
- PRIMARYMetso, 'Spiral Concentrator' product page, last updated August 2026 (equipment maker). Principle of separation; splitter positioning; product take-off; specific-gravity difference; closely sized feed; slimes and desliming; upgrading ratio and multi-pass flowsheets; size limits.
- PRIMARYMetso Corporation, Basics in Minerals Processing, Edition 8, 2011 (equipment maker's handbook, hosted in the AusIMM minerals processing toolbox). p. 5:5: spiral size ranges for light particles (75 µm to 1.0 mm) and heavy particles (75 µm to 0.5 mm). p. 5:8: the spiral as a gravity separator.
- PRIMARYCPG Resources – Mineral Technologies, 'Fine Gravity Separation' (spirals product sheet), undated, hosted in the AusIMM minerals processing toolbox (equipment maker). FM series particle range; grade bands of other series.
- PRIMARYMechChem Africa, 'Spiral separation for optimised recovery', May–June 2024, interview with Multotec (equipment maker), chromite context. Feed preparation, desliming and viscosity; rougher, scavenger, cleaner and recleaner spirals.
- PRIMARYKenmare Resources plc, 'Moma's operating process' (web page, published 24 September 2024, modified 15 May 2026). The company's description of its own wet concentrator plants.
- PRIMARYThe Kerala Minerals & Metals Ltd (Government of Kerala undertaking), 'Manufacturing Facility: Mineral Separation Unit', web page, undated. Wet concentration plant.
- ANALYSISANALYSIS: the reading that sizing the feed exists to narrow the range of grain sizes the spiral has to handle, so that density can do more of the sorting. This publication's framing.Non-public document · no public URL
