Dense Medium Separation (DMS) in Mineral Processing: The Complete Guide

Dense medium separation (DMS) stands among the most precise and powerful pre-concentration methods available to mineral processors. By suspending crushed ore in a fluid medium engineered to a specific density, operators can cleanly divide valuable heavy minerals from lighter waste rock — often at a coarse particle size, long before expensive grinding and flotation circuits are engaged. Whether processing coking coal in South Africa, recovering diamonds from kimberlite in Botswana, or rejecting siliceous gangue from a lead-zinc deposit in Australia, dense medium separation mineral processing circuits deliver sharp, repeatable separations that improve downstream efficiency and reduce operating costs. This guide draws on the authoritative treatment in Wills’ Mineral Processing Technology (8th Edition) and current industry practice to explain every aspect of the process — from the physics of buoyancy to the design of modern DMC circuits and the economics of medium recovery.

What is Dense Medium Separation (DMS)?

Dense medium separation, also referred to as heavy medium separation (HMS) or the sink-and-float process, is a gravity-based beneficiation technique in which a feed of crushed ore is immersed in a liquid or pseudo-liquid medium whose engineered density falls between the densities of the mineral species to be separated. Particles denser than the medium sink; particles less dense float. Unlike conventional gravity separation on shaking tables or spirals, DMS achieves this split not by exploiting differences in particle settling velocity across a flowing film of water, but by controlling the absolute buoyancy forces acting on each particle. The result is a separation that is largely insensitive to particle shape and surface texture and that can be controlled to within a relative density (RD) of ±0.005 under stable operating conditions.

The process has two principal industrial applications. The first is the pre-concentration of metalliferous ores, in which DMS rejects low-grade or barren gangue after a suitable degree of liberation by coarse crushing, thereby reducing the tonnage that must pass through energy-intensive grinding and flotation circuits. The second is coal preparation, where DMS separates clean coal from the heavier shale, sandstone, and high-ash material that dilutes it, producing a saleable product of specified ash content directly from the run-of-mine feed.

DMS is preferred over cheaper jig separators when the feed contains a significant proportion of near-density material — particles whose density is very close to the desired cut point — because the precise control of medium density allows the operator to maintain a sharp, well-defined separation even as feed characteristics fluctuate. Particles should ideally be coarser than approximately 4 mm, where settling rates are fast enough to achieve clean separation within the residence time of the vessel. Centrifugal dense medium cyclones extend the effective lower particle size limit to about 0.5 mm, and with careful circuit design even finer separations are possible.

The Physics of DMS: Relative Density and Separation

The operating principle of dense medium separation mineral processing applications rests on Archimedes’ principle: a body immersed in a fluid experiences an upward buoyant force equal to the weight of fluid it displaces. When the relative density of a particle exceeds that of the surrounding medium, the net gravitational force is downward and the particle sinks. When the particle is less dense than the medium, the net force is upward and the particle floats. At exactly the medium density, the net force is zero and the particle neither rises nor falls — it is in neutral equilibrium and will ultimately be distributed between the float and sink products in proportion to the hydrodynamic conditions inside the vessel.

In practice, the medium is never perfectly still. Turbulence, agitation, and the movement of material through the vessel all impose hydrodynamic drag on settling particles. The viscosity of the medium is therefore a critical parameter. A medium that is too viscous imposes excessive drag, slowing particle movement and increasing the proportion of near-density material that does not reach the correct product stream in the available residence time. Medium viscosity must be managed through careful selection of medium particle size, operating density, and the removal of fine contaminants — particularly clay slimes — that dramatically increase apparent viscosity at high concentrations.

The efficiency of any DMS unit can be characterised by the partition curve (or Tromp curve), first introduced by Tromp in 1937. The partition curve plots the fraction of feed material at each density increment that reports to the sink product against the nominal particle density. An ideal, perfect separation produces a vertical step at the cut-point density. In practice, the curve has an S-shape whose steepness reflects separation efficiency. The width of the transition zone is quantified by the Ecart probable (E_p), defined as half the difference between the densities at which 75% and 25% of the feed partition to the sinks product:

E_p = (D₇₅ − D₂₅) / 2

For well-operated gravitational vessels treating coarse material, E_p values of 0.02–0.05 are typical; DM cyclones treating fine material may show E_p values of 0.03–0.08. The lower the E_p, the more efficient the separation and the greater the confidence that near-density material reports to the correct product. The density at which 50% of the feed reports to sinks is termed the RD₅₀ or effective cut-point density. In centrifugal separators this can differ from the nominal medium density because centrifugal force shifts the effective density of separation upward relative to the medium density fed to the cyclone.

Laboratory heavy liquid testing is the standard method for evaluating the suitability of DMS for a given ore and for determining the optimum separation density. Incremental density fractions are prepared using heavy liquids such as tetrabromoethane (s.g. 2.96), bromoform (s.g. 2.89), or diiodomethane (up to s.g. 3.3), and each fraction is weighed and assayed. The resulting data, tabulated as yield and metal distribution at each density cut, allow the metallurgist to construct washability curves and to identify the economically optimal separation density — the point at which the saving in downstream milling costs exceeds the value of valuable mineral lost in the float reject.

Dense Medium Vessels: Drum and Bath Separators

Several types of separating vessel are used in industrial DMS circuits, broadly classified as gravitational (static-bath) vessels and centrifugal (dynamic) vessels. Gravitational vessels rely on buoyancy and gravity alone to drive particles to the appropriate product stream; they are robust, well-suited to coarse feeds, and have been used in industrial mineral processing for more than a century.

The DM drum separator is one of the most widely deployed vessels, available in sizes up to 4.3 m diameter by 6 m long with capacities up to 450 t/h, treating feed particles up to 300 mm. Inside the rotating drum, fixed lifters continuously elevate the sink product and discharge it into a launder when each lifter passes the horizontal position. The float product overflows a weir at the feed end. The comparatively shallow pool depth in the drum minimises settling of medium particles and delivers a uniform apparent density throughout the vessel. For applications requiring two products at different density cut-points, a two-compartment drum separator connects two drum units in series on a common rotating shell, allowing the float from the first compartment to be re-separated at a lower medium density in the second compartment.

The Wemco cone separator, with diameters up to 6 m and capacities up to 500 t/h, accommodates feeds up to 100 mm. Free-fall feed introduction allows the ore to plunge several centimetres into the medium, aided by gentle agitation from central rakes that maintain medium suspension. Sinks are removed by internal pump or air-lift, and the float simply overflows a peripheral weir. Cone separators provide a deep medium pool, which gives a greater height for buoyancy-driven particle movement but also increases the risk of medium density stratification at high throughputs.

The Drewboy bath and the Norwalt washer (developed in South Africa and widely used in Southern Hemisphere coal preparation plants) are further examples of gravitational vessels optimised for high float capacity. In the Norwalt design, raw coal enters the centre of an annular vessel while stirring arms carry floats over a peripheral weir; sinks are scraped to a central outlet and removed by sealed elevator. These vessels find particular application in treating the large, low-grade coal deposits of the former South African Transvaal province, where high middlings fractions demand robust DMS.

All gravitational vessels are broadly limited to feeds coarser than approximately 5 mm because of the slow settling velocity of fine particles. Below this size, centrifugal separators become necessary.

Dense Medium Cyclones: Design and Operation

The dense medium cyclone (DMC) is by far the most widely used centrifugal DMS device and arguably the most important single equipment item in modern coal and hard-rock pre-concentration flowsheets. Its operating principle is essentially that of a conventional hydrocyclone: feed slurry — the ore particles suspended in the dense medium — is introduced tangentially under pressure, generating a centrifugal field many times the acceleration due to gravity. Dense particles (the reject in coal washing, the valuable pre-concentrate in ore processing) are flung to the cyclone wall and exit via the underflow apex. Light particles migrate to the low-pressure core and exit via the vortex finder overflow. The centrifugal field dramatically accelerates settling rates relative to gravitational vessels, enabling effective separation of particles as fine as 0.5 mm and, under optimised conditions, as fine as 0.1–0.2 mm.

Cyclone diameters for coal processing have grown from early 0.5 m units to modern units of 1.4 m diameter capable of treating throughputs exceeding 250 t/h on feed particles up to 75–90 mm. This increase in unit size has allowed single DMC installations to replace multiple smaller units, reducing the number of cut-points to manage, minimising surge effects between units, and simplifying circuit control. In metalliferous applications the largest cyclones currently deployed are approximately 0.8 m in diameter, but experience at lead-zinc operations such as Mount Isa demonstrates that even at this scale the benefits of pre-concentration — reduced Bond Work Index, lower grinding energy, and coarser product size distributions — are compelling.

Feed can be introduced to a DMC either by pump or by gravity head. Pump feeding is more compact and less costly in terms of building height, but subjects the feed to wear and the risk of size degradation. Gravity feeding, requiring a taller structure, achieves more consistent flow and lower maintenance on feed preparation equipment. The cone angle of DMC units is typically fixed at 20°, with manufacturers having found no significant benefit from varying this angle across the range of applications.

The LARCODEMS (Large Coal Dense Medium Separator) is a cylindrical vessel inclined at 30° to the horizontal, designed to treat a wide size range — up to 100 mm — at high capacity in a single vessel using medium introduced under pressure at the lower involute inlet. Clean coal exits at the lower end while dense rejects are expelled upward via a vortextractor. The LARCODEMS has also been applied to iron ore concentration: a 1.2 m unit at the Kumba Sishen operation treats up to 800 t/h. The Dyna Whirlpool and the Tri-Flo separator (effectively two Dyna Whirlpools in series) offer further options for two-density separation, the Tri-Flo being configured to produce distinct sink products at two controlled densities or to apply two-stage scavenging on metalliferous ores.

The Dense Medium: Magnetite, Ferrosilicon, and Medium Stability

The choice of medium material is fundamental to the performance and economics of any DMS circuit. An effective dense medium must achieve the required suspension density, maintain low viscosity, resist degradation during recirculation, be recoverable by magnetic separation, and be chemically stable in contact with the ore being processed.

Magnetite (s.g. approximately 5.0) is the standard medium for coal preparation. It is widely available, relatively inexpensive, and readily recovered by low-intensity magnetic separation. Its principal limitation is that it cannot achieve the very high medium densities required for metalliferous ore separations: practical medium densities using magnetite are up to approximately 2.3 RD, and work on spheroidised magnetite aims to extend this to 2.8. For coal, where cut-point densities typically lie between 1.3 and 1.8, magnetite is entirely adequate.

Ferrosilicon (s.g. 6.7–6.9) is the preferred medium for metalliferous ore applications requiring higher separation densities. It is an alloy of iron and silicon, specified to contain at least 82% Fe and 15–16% Si. If the silicon content falls below 15%, the alloy tends to corrode; above 16%, magnetic susceptibility and density are significantly reduced. Milled ferrosilicon is produced in a range of size distributions from 30% to 95% passing 45 µm; finer grades are used with centrifugal separators and at higher operating densities. Coarser, lower-viscosity grades achieve medium densities up to approximately 3.3. Atomised ferrosilicon, consisting of rounded particles that pack less densely in suspension, produces media of lower viscosity and can sustain operating densities up to approximately 3.8.

In some applications, blends of magnetite and ferrosilicon are used to achieve intermediate density ranges between about 2.2 and 2.9 RD. For the highest-density metalliferous separations — for example, iron ore processing where medium densities may exceed 4.0 — ferrosilicon alone is used, and the management of medium viscosity at these extreme densities requires particular attention.

Medium stability — the ability of the suspension to maintain a uniform density throughout the vessel — is critical for separation efficiency. Below approximately 15% solids by volume, fine-particle suspensions behave essentially as Newtonian fluids and particle settling is primarily a function of medium viscosity. Above this concentration, the suspension becomes non-Newtonian, exhibiting a yield stress that must be overcome before flow occurs. Near-density particles that cannot generate sufficient force to overcome the yield stress are distributed between products in proportion to medium flow patterns rather than density difference. Agitation by paddles, air sparging, and the movement of ore through the vessel all reduce the apparent yield stress by shearing the suspension, but the fundamental solution is to maintain medium concentration in the correct operating range and to remove fine contaminants aggressively.

Ferrosilicon losses from a dense medium circuit range from as little as 0.1 to more than 2.5 kg per tonne of ore treated. The main loss mechanisms are adhesion to product surfaces, losses in magnetic separator tailings, and, to a lesser extent, corrosion. Corrosion of ferrosilicon is effectively suppressed by ensuring passive oxidation, typically through maintaining dissolved oxygen in the medium circuit or adding small quantities of sodium nitrite. Medium losses represent 20–40% of total DMS operating costs, making aggressive medium recovery essential to circuit economics.

Medium Recovery and Cleaning Circuits

The recovery, cleaning, and recirculation of the dense medium is the single most complex and expensive element of any DMS plant. Because the float and sink products leave the separating vessel wetted with medium, effective recovery is critical both to process economics and to maintaining the quality and density of the medium in circulation.

In a typical DMS circuit, the float and sink fractions from the separating vessel pass onto separate vibrating drainage screens, which recover more than 90% of the adhering medium and return it directly to the medium sump via gravity launders. The partially drained products then pass to wash screens, where fresh water sprays substantially complete the removal of residual medium and fines. The combined underflows of the drainage screens are split: a portion is returned directly to the medium sump at process density, while the remainder passes to a densifier — a centrifugal or spiral device — to increase medium concentration before returning it to the circuit.

The wash screen underflows are too dilute and contaminated with ore fines to be returned directly as medium. They are treated by wet drum magnetic separators that capture the magnetic medium particles (magnetite or ferrosilicon) from the non-magnetic ore fines and water, both recovering the medium and densifying it before it is returned to the main sump. The recovered medium passes through a demagnetising coil prior to re-entering the separating vessel; this step is essential because residual remanent magnetism causes medium particles to flocculate, dramatically increasing apparent viscosity and degrading separation efficiency.

Automatic density control is standard on most large DMS plants. Medium density is measured continuously using a gamma-ray attenuation gauge on the feed to the separating vessel. When density deviates from the set-point, the control system adjusts the addition of fresh water or directs more medium to the densifier, restoring the target density. This closed-loop control is essential when processing feeds with variable sink-to-float ratios, where the withdrawal of dense material from the circuit would otherwise cause medium density to drift upward.

Medium rheology is one of the most important and difficult-to-manage aspects of circuit operation. The accumulation of fine ore particles — particularly clay minerals — in the circulating medium raises apparent viscosity, widens the partition curve, and increases the E_p. Management strategies include rigorous pre-screening of the feed to remove fines below approximately 0.5 mm, provision of a medium bleed circuit to divert a fraction of the circulating medium to the cleaning circuit when contamination builds up, and the use of correctly specified medium grades matched to the ore characteristics and operating density.

DMS for Coal Processing

Coal preparation is the dominant application of dense medium separation mineral processing technology worldwide, accounting for the largest installed capacity and the greatest diversity of equipment types. The objective is straightforward: clean coal (low ash content) is less dense (typically 1.30–1.50 RD) than the shale, sandstone, pyrite, and high-ash coal fractions (typically above 1.80 RD) that dilute it. By engineering a medium density between these populations, DMS allows efficient removal of the waste fraction in a single pass, producing a saleable product of specified ash content and calorific value.

DMS is preferred over cheaper Baum jig separation when the feed contains more than approximately 7% near-density material in the ±0.1 RD band around the separation density. For feeds with less near-density material, jigs are often competitive; as near-density content rises above 10%, and especially above 25%, DMS with close automatic density control becomes the only practical separation method.

Modern coal preparation plants employ tiered circuits tailored to different particle size ranges. A typical US plant might apply static dense medium vessels (drums or baths) to the coarser fraction above 10 mm, DMCs to the intermediate 1–10 mm fraction, spiral concentrators to the 0.15–1 mm fraction, and froth flotation to the sub-0.15 mm fines. British coals, which are generally relatively easy to wash, often use drum or Drewboy separators for coarse fractions and DM cyclones or Vorsyl separators for fines. Southern Hemisphere coals — particularly the large low-grade deposits of the South African highveld — typically contain very high middlings fractions that demand DMS throughout. At the Landau Colliery (Anglo Coal), a two-density operation using Norwalt baths at 1.6 RD followed by 1.4 RD baths produces a low-ash metallurgical coke feed (approximately 7.5% ash) and a power-station grade product (approximately 15% ash) from the coarse fraction, with parallel Dyna Whirlpool circuits treating the 0.5–7 mm fraction.

The ash content of each density fraction from heavy liquid testing is determined by combustion at 815°C, and the results are plotted as washability curves that relate cumulative yield to cumulative ash content at each density cut. From these curves the metallurgist identifies the density of separation that achieves the target product specification at maximum yield. For a given ash requirement, the yield and required cut-point density can be read directly from the washability curve, allowing both plant design parameters and commercial contracts to be established before plant construction begins.

A key advantage of DMS over conventional froth flotation in coal preparation is that DMS can treat oxidised coal that responds poorly to flotation. DMS is also effective at removing pyritic sulfur, which floats with the coal in conventional flotation but sinks in DMS when the desired cut density is set correctly. This makes DMS particularly attractive for producing low-sulfur coals for metallurgical applications.

DMS for Diamonds, Iron Ore, and Other Minerals

Beyond coal, dense medium separation mineral processing circuits play an essential pre-concentration role across a wide range of commodities, exploiting density contrasts developed by coarse crushing before energy-intensive downstream processing is applied.

Diamond recovery represents one of the most demanding DMS applications in terms of both the value of the target mineral and the required concentration ratio. Diamonds have a relatively high specific gravity of 3.52 but occur at very low grades — sometimes less than one carat per hundred tonnes of ore — meaning concentration ratios of several million to one must ultimately be achieved. DMS produces an initial enrichment of 100 to 1,000 fold by exploiting the density contrast between diamonds and the lighter kimberlite gangue (typically 2.6–2.8 RD). Ferrosilicon medium is used at separation densities between 2.6 and 3.0. Both gravitational vessels and centrifugal separators are deployed, with care taken to minimise turbulence that could damage diamonds. Clay-rich ores present particular challenges: swelling clays increase medium viscosity, reducing recovery to the sinks and risking diamond loss to the float fraction. Pre-scrubbing and de-sliming before DMS are essential in these cases. DMS produces a dense-mineral pre-concentrate that is subsequently processed by X-ray luminescence sorting, grease tables, or optical sorters for final diamond recovery.

Iron ore upgrading by DMS is practiced at a number of operations where the ore contains sufficient density contrast between the iron minerals (hematite, magnetite) and the siliceous gangue to allow effective gravity pre-concentration. Medium densities may exceed 4.0 RD for high-density separations, and LARCODEMS units up to 1.2 m diameter are used at the Kumba Sishen operation in South Africa to treat feeds up to 800 t/h. The economics of DMS in iron ore are particularly attractive when the ore is coarsely banded, allowing liberation of ore-rich and gangue-rich particles at a relatively coarse crush size, thus deferring fine grinding until after a substantial waste rejection step.

Lead-zinc ores were among the earliest metalliferous ores to benefit from DMS pre-concentration. At Mount Isa Mines in Queensland, Australia, a DMS plant installed in 1982 treats approximately 800 t/h of 1.7–13 mm material at a separation density of approximately 3.05 RD using DM cyclones. The plant rejects 30–35% of the run-of-mine ore as tailings while recovering 96–97% of the lead, zinc, and silver to the pre-concentrate. Beyond the direct upgrading benefit, the pre-concentrate has a 25% lower Bond Work Index than the full ore — a significant downstream grinding energy saving. The removed siliceous gangue is returned underground as fill.

Other metalliferous applications include tin and tungsten ores, fluorite, barite, and chromite, wherever a suitable density contrast exists between the valuable mineral assemblage and the host gangue. The key pre-condition for DMS economic viability is that liberation of the density contrast must occur at a size coarser than approximately 4 mm; if values are finely disseminated, the density difference between crushed particles cannot be developed at an economical crush size.

DMS Circuit Design, Cut-Point Optimisation, and Efficiency

Designing an effective DMS circuit requires integrating knowledge of the ore’s density characteristics, the separation equipment’s performance envelope, the downstream process requirements, and the economics of medium recovery and consumption. The starting point is always heavy liquid testing, which defines the washability curves and the density distribution of the feed. From these data, the optimum separation density is determined on economic grounds: the density at which the saving in downstream milling costs per tonne of ROM ore exceeds the smelter revenue lost by rejecting a small fraction of valuable mineral to the float discard.

Particle size is the most important determinant of separator selection. Gravitational vessels (drums, cones, Drewboys, Norwalt) treat particles from approximately 5 mm up to 300 mm. Centrifugal separators (DMC, LARCODEMS, Dyna Whirlpool) extend the range down to 0.5 mm; under optimised conditions, fine DMC circuits can treat down to 0.1–0.2 mm. Below this size, water-based gravity methods — spirals, water-only cyclones — are employed. The partition curve efficiency, expressed as E_p, generally degrades as particle size decreases because the slower settling velocity of fine particles reduces the probability of reaching the correct product stream within the residence time of the vessel.

The amount of near-density material in the feed is the key operational challenge. For feeds with less than 7% near-density material (within ±0.1 RD of the separation point), almost any gravity process works well. Between 7% and 25%, an efficient DMS process with careful operation is required. Above 25%, only DMS with close automatic density control and well-maintained equipment can achieve commercially acceptable organic efficiencies.

The E_p is used both as an equipment performance benchmark and as a commercial contract specification. Well-operated drum and bath separators processing coarse coal (above 10 mm) routinely achieve E_p values of 0.02–0.03. DM cyclones processing intermediate coal (1–10 mm) typically deliver E_p values of 0.03–0.06. These figures can be verified using density tracers — colour-coded plastic tracers of known density fed to the separator and hand-sorted by colour in the float and sink products — providing a rapid, direct measurement of the partition curve without the need for heavy liquid testing.

Automatic density control is standard practice in modern plants, with gamma-ray attenuation gauges monitoring medium density on-line and PLC systems adjusting water addition to maintain the target cut-point. In two-density circuits, independent density control loops operate in each stage. Computational fluid dynamics models of DMC internals are increasingly used in circuit design and troubleshooting, revealing the density gradients that develop radially and axially inside the cyclone and explaining the dependence of RD₅₀ and E_p on cyclone diameter, feed pressure, and medium-to-ore ratio. Mathematical models developed over the past four decades — including the Rosin-Rammler and Whiten partition curve functions — allow plant performance to be simulated from a combination of ore characterisation data and separator performance parameters, supporting both design optimisation and operational troubleshooting.

Major costs in DMS are power (for medium pumping and screen drives), medium consumption (20–40% of total operating cost), and maintenance of wear items in cyclones and pumps. Minimising medium losses requires correct sizing of drainage and wash screens, controlled rinsing water volumes, high-efficiency magnetic separator operation, and careful management of medium degradation. The energy advantage of DMS relative to the alternatives it displaces — grinding, flotation — is compelling: energy requirements for DMS are often only one-tenth of those for the equivalent mass of material processed through a fine-grinding and flotation circuit, providing a strong thermodynamic argument for maximising the particle size at which pre-concentration is applied.