Magnetic and Electrostatic Separation in Mineral Processing: The Complete Guide

Magnetic and electrostatic separation are two of the most versatile and selective techniques available to mineral processors. Where gravity and flotation rely on bulk physical properties and surface chemistry respectively, these methods exploit differences in the electromagnetic character of minerals — their response to magnetic fields, and their ability to acquire and retain electrical charge. Applied individually or in combination, magnetic separation mineral processing circuits recover iron ore and ilmenite at the scale of millions of tonnes per year, clean industrial minerals to optical brightness, and underpin the intricate flowsheets of the mineral sands industry. This comprehensive guide draws on Wills’ Mineral Processing Technology (8th Edition) and current industry literature to explain the principles, equipment types, and industrial applications of both magnetic and electrostatic separation, from low-intensity drum separators recovering magnetite to high-gradient superconducting machines capturing traces of paramagnetic impurity from kaolin clay.

Principles of Magnetic Separation

Magnetic separation exploits the interaction between mineral particles and an applied magnetic field. All minerals respond to a magnetic field to some degree, but the character and intensity of that response vary enormously among different mineral species — variations that create the basis for separation. The fundamental principle is straightforward: a magnetic force acts on a particle moving through a non-uniform magnetic field, and the direction of that force — toward regions of greater or lesser field intensity — depends on the magnetic character of the mineral.

The quantitative expression for the magnetic force F_x acting on a particle in the x-direction is given by:

F_x = V(χ_p − χ_m) H (dB/dx)

where V is the particle volume, χ_p and χ_m are the magnetic susceptibilities of the particle and the surrounding fluid medium respectively, H is the applied magnetic field strength (in A m⁻¹), and dB/dx is the magnetic field gradient (in T m⁻¹). This equation reveals three critical design parameters: a separator must provide sufficient field intensity H, a steep field gradient dB/dx, and must present the mineral in a particle volume V large enough for the total magnetic force to overcome competing forces — gravity, hydrodynamic drag, and particle-particle interactions.

The product H × dB/dx is sometimes called the force factor. Maximising this factor — either by increasing field intensity, sharpening the gradient, or both — is the central design challenge in magnetic separator engineering. As will be discussed, different separator classes achieve this in fundamentally different ways: low-intensity drum separators use conventional permanent magnets with open-gradient designs, while high-gradient magnetic separators (HGMS) pack ferromagnetic matrix elements into the separation volume to generate local gradients orders of magnitude steeper than the applied field.

The competing forces that act against magnetic capture define the effective operational range for a given separator and mineral. Considering only gravity and hydrodynamic drag as the principal competitors to magnetic force, the effective particle size range for magnetic separation mineral processing applications is approximately 5 µm to 1 mm. Below 5 µm, Brownian motion and surface forces dominate; above 1 mm, gravity and drag typically overwhelm the magnetic force for weakly paramagnetic minerals, though strongly ferromagnetic particles can be separated at much coarser sizes.

Magnetic Properties of Minerals: Ferromagnetic, Paramagnetic, and Diamagnetic

For mineral processing purposes, minerals are classified into three broad groups based on their magnetic character:

• Ferromagnetic minerals exhibit the strongest magnetic response. They carry permanent magnetic domains — regions of aligned magnetic dipoles — that align with an applied field rapidly and at low field strength, producing a very high effective susceptibility. The defining characteristic is exchange coupling between adjacent magnetic dipoles, which allows domain alignment to cascade across the mineral structure. Magnetite (Fe₃O₄) is the pre-eminent ferromagnetic mineral in mineral processing; pyrrhotite (Fe_1−xS) in its monoclinic form is also strongly ferromagnetic. These minerals saturate magnetically at moderate field strengths (approximately 0.63 T for magnetite) and are readily concentrated by low-intensity magnetic separators (LIMS) operating below 0.3 T.
• Paramagnetic minerals possess a positive but much smaller magnetic susceptibility arising from unpaired electron spins in their crystal structures. They are attracted toward regions of higher field intensity but require much greater field strengths to produce a separation force comparable in magnitude to that acting on ferromagnetic minerals. Important paramagnetic minerals that are routinely separated industrially include ilmenite (FeTiO₃), wolframite ((Fe,Mn)WO₄), monazite ((Ce,La,Nd,Th)PO₄), chromite (FeCr₂O₄), siderite (FeCO₃), and many manganese minerals. The susceptibility of paramagnetic minerals is approximately two to three orders of magnitude lower than that of ferromagnetic minerals, requiring high-intensity (WHIMS) or high-gradient (HGMS) separators for effective recovery.
• Diamagnetic minerals have a very small, negative magnetic susceptibility. They are weakly repelled from regions of high field intensity and in practice report to the non-magnetic product of any magnetic separator. Quartz, calcite, feldspar, zircon, and most silicate gangue minerals are diamagnetic. Though the diamagnetic force is too weak to exploit directly in most industrial applications, the effective discrimination between paramagnetic and diamagnetic species is the basis for separating, for example, ilmenite from rutile and zircon in mineral sands circuits.

It is important to recognise that real mineral samples rarely conform to ideal single-phase behaviour. Most paramagnetic mineral samples contain ferromagnetic inclusions or coatings — iron oxide films on grain surfaces being particularly common — that produce mixed magnetic responses detectable by vibrating sample magnetometry. Empirically, even quartz samples may show weak paramagnetic responses due to iron impurities. In industrial practice, the Frantz Isodynamic Separator is the standard laboratory tool for characterising the magnetic properties of mineral fractions and for predicting their behaviour in full-scale separators. For ferromagnetic minerals, the Davis tube tester is the corresponding standard laboratory device, providing wet-separation data directly applicable to scale-up of LIMS circuits.

Low Intensity Magnetic Separators (LIMS)

Low-intensity magnetic separators (LIMS) operate at magnetic flux densities below approximately 0.3 T and are designed to recover ferromagnetic minerals — principally magnetite — from process streams. Their applications in mineral processing are numerous and critical: concentration of magnetite iron ore from siliceous gangue, recovery of magnetite and ferrosilicon media from DMS circuits, removal of tramp iron from crusher and mill feeds, and recovery of magnetite or pyrrhotite from flotation tailings.

The standard LIMS machine is the wet drum separator, in which a rotating hollow non-magnetic drum encloses a stationary array of permanent magnets of alternating polarity. The drum surface conveys the magnetic product out of the feed pulp and out of the magnetic field, where it is discharged; the non-magnetic gangue drains away with the water stream. Modern drum separators use ceramic ferrite magnets or rare earth permanent magnet assemblies, which retain their intensity for an indefinite operational life. Field intensities of up to 0.7 T at pole surfaces are achievable, far exceeding the 0.12 T standard for ferrite-based LIMS drums.

The spatial arrangement of the magnets within the drum is configured to match the application:

• Concurrent configuration: feed slurry and the drum surface move in the same direction; the magnetic concentrate is compressed and dewatered as it leaves the field. Best suited for relatively coarse feeds (up to 6–8 mm) and widely used in DMS medium recovery systems.
• Counter-current configuration: tailings flow in the opposite direction to drum rotation, with lower pulp densities than concurrent units. Designed for finishing operations on fine material below approximately 800 µm.
• Counter-rotation configuration: feed flows opposite to drum rotation; suited to roughing operations where high solids loads and occasional feed surges are expected.

A key operational consideration in LIMS is the effect of feed solids content on recovery mechanism. At high feed solids (10–17% by weight), magnetic recovery occurs primarily through the formation of magnetic flocs — clusters of particles that capture both strongly and weakly magnetic particles. At low feed solids (around 2%), individual particle capture dominates, and fine ferromagnetic particles with lower individual susceptibility are more likely to be lost to tailings. Understanding this distinction is essential for optimising wash water flow rates and feed consistency.

At the Iron Ore Company of Canada’s Carol Lake operation, LIMS drum separators are used as an initial cobbing step on ball mill cyclone overflow to remove liberated magnetite, which is then combined with spiral tailings for further upgrading before pelletisation. At the Palabora copper mine (South Africa), LIMS separators recover 95% of the magnetite at 62% Fe grade from flotation tailings of the 1105 µm fraction. Both examples illustrate a recurring theme in magnetic separation mineral processing practice: LIMS circuits are often deployed downstream of flotation or gravity separation to recover residual magnetic values that were not captured by the primary circuit.

High Intensity and High Gradient Magnetic Separators (WHIMS, HGMS)

Weakly paramagnetic minerals cannot be captured in low-intensity separators. Separating them requires either high field intensity (B-fields of 2 T or greater) or high field gradient — or both. Two distinct engineering philosophies address this need: wet high-intensity magnetic separators (WHIMS) and high-gradient magnetic separators (HGMS).

Wet High-Intensity Magnetic Separators (WHIMS)

The Jones separator, developed in the 1960s, established the design template that most commercial WHIMS machines still follow. The separator consists of one or two large rotors mounted on a central shaft, with the rotors carrying plate boxes — grooved ferromagnetic plate assemblies — around their periphery in a carousel arrangement. As each plate box rotates into the magnetic field gap, slurry is fed through the grooved plates; the weakly paramagnetic particles are held in the grooves by the concentrated magnetic field while non-magnetic particles pass through. When a plate box rotates to a zone of low field between the magnetic poles, high-pressure scour water (up to 5 bar) washes the retained magnetic concentrate out of the plates. A low-pressure wash between feed and scour removes entrained non-magnetics as a middlings product.

Jones-type WHIMS machines can achieve field strengths exceeding 2 T, though the optimum field for a given application must be selected carefully. Increasing field strength does not necessarily improve separation — it can actually decrease selectivity by capturing weakly magnetic gangue particles and, at very high fields, by reducing the effective magnetic susceptibility of some minerals through saturation effects. The capital cost of WHIMS is high: a large machine may contain over 200 t of iron in its magnetic circuit. White (1978) showed that WHIMS capital costs for concentrating weakly magnetic iron ore were approximately five times those of equivalent flotation capacity, though operating costs were roughly one-third lower over a 10-year comparison period.

WHIMS applications span a wide range of duties in mineral processing:

• Concentration of hematite and goethite iron ore fines from siliceous gangue
• Concentration of ilmenite, wolframite, and chromite from mineral sands and heavy mineral deposits
• Removal of magnetic iron oxide impurities from cassiterite, scheelite, and other non-ferrous concentrates
• Purification of industrial minerals including quartz, feldspar, and kaolin from iron-bearing contaminants
• Removal of orthopyroxene gangue from PGM-bearing chromite (Merensky Reef, South Africa)
• Upgrading of gold and uranium from cyanidation residues

At the Cliffs-Wabush iron ore mine in Labrador, Canada, a scavenger WHIMS circuit using 100 t/h Jones separators operating at 1 T recovers fine hematite from spiral gravity tailings, complementing an upstream LIMS step that captures residual magnetite. The two-stage magnetic circuit allows the bulk of the ore to be processed by gravity separation at lower cost, while magnetic separation handles the fine fraction that gravity circuits cannot efficiently treat.

High-Gradient Magnetic Separators (HGMS)

HGMS addresses the fundamental limitation of WHIMS — the heavy iron circuits required to generate high field strengths — by separating the field generation (by a solenoid) from the field gradient generation (by a ferromagnetic matrix packed inside the solenoid). The matrix, which may consist of steel ball bearings, wire wool, or engineered rod arrays filling only approximately 10% of the working volume, creates a multitude of intense local field gradients of up to 14 T/mm around each matrix element. While the bulk field from the solenoid may be only 1–2 T, the local gradient within the matrix greatly exceeds what any conventional pole geometry can produce.

HGMS machines are available in batch and continuous carousel configurations. The carousel configuration (operating like a Jones WHIMS with a matrix-filled rotor) provides continuous operation with periodic flushing of the matrix in a low-field zone. HGMS is most effective for very fine particles (below approximately 75 µm) where the short reach of the local gradient — effective only within about 1 mm of a matrix element — is matched by the small particle-to-matrix spacing that results from fine particle size and close matrix packing.

SLon Vertically Pulsating HGMS (VPHGMS)

A major advance in HGMS design is the SLon VPHGMS (Vertically Pulsating High-Gradient Magnetic Separator), developed from the first SLon-1000 unit in 1988 and now widely deployed at over 30 mineral processing operations in China and internationally. The SLon design employs a unique matrix of steel rods oriented perpendicular to the applied field, combined with a pulsating feed mechanism that vibrates the slurry to prevent particle flocculation and matrix clogging. Captured magnetic particles are flushed from the matrix in the reverse direction to the feed flow, reducing particle momentum and maximising trapping efficiency. The rod diameter can be customised for each application (allowing passage of particles from 0.6 to 3.0 mm), and the averaged field intensity across the separator is no greater than 1.3 T, though local matrix surface intensities can reach 1.8 T.

The SLon separator has been successfully applied to the concentration of fine hematite and ilmenite, desulfurisation and dephosphorisation of iron ore feeds, and recovery of rare earth minerals. It represents the current state-of-the-art for treating fine (below 75 µm), weakly paramagnetic minerals at industrial scale.

Dry Magnetic Separators

Dry magnetic separation is applied when the feed is already dry, when subsequent processing requires a dry product, or when the economics of wet separation — slurry handling, dewatering, tailings disposal — are unattractive relative to the value of the product. The principal limitation of dry methods is that very fine particles (below about 75 µm) are difficult to process because air currents, electrostatic adhesion, and particle-particle cohesion all disrupt the separation. For most fine-grained applications, wet separation is preferred.

Dry Drum Separators

Low-intensity dry drum separators are used mainly for the concentration of strongly magnetic coarse sands — a process known as cobbing — and as guard magnets for tramp iron removal ahead of crushers and mills. The design is analogous to the wet drum but without the liquid medium; particles fall from a feed conveyor through the fringe of the magnetic field at the drum surface, with magnetic particles adhering to the drum and non-magnetics following a free-fall trajectory to the tailings bin. Careful control of feed moisture is critical: even at 90% passing 20 µm, the recommended maximum moisture for dry drum separators is approximately 3%.

Rare Earth Roll Separators

The rare earth roll (RER) separator uses alternating ferromagnetic and non-magnetic laminations on a rotating roll to generate very high field intensities (up to 2.1 T) at the roll surface. Feed is delivered by a belt that brings particles into the magnetic zone with a controlled horizontal velocity, minimising bounce and scatter. Magnetically susceptible particles are held to the roll as it rotates, while non-magnetic particles follow ballistic trajectories into the tailings compartment; a secondary splitter cuts off a middlings fraction. The key advantage of RER over the earlier induced roll magnetic (IRM) separator, which it has largely replaced, is substantially lower power consumption and greater throughput flexibility.

In mineral sands processing, RER separators are used to separate weakly paramagnetic leucoxene from diamagnetic rutile, and to clean ilmenite and garnet concentrates in multiple-stage circuits. Separation efficiency is primarily controlled by roll speed (which affects both centrifugal force and the charge decay time available for conductive particles), feed rate, and the position of the splitter plates cutting into the particle fan discharged from the roll.

Cross-Belt and Disc Separators

Cross-belt and disc separators, once common in the mineral sands industry, are now considered obsolete and have been largely replaced by rare earth roll and rare earth drum separators. Their relatively low field intensities, large footprints, and complex maintenance requirements make them uncompetitive against modern rare earth designs.

Applications: Iron Ore, Ilmenite, Chromite, and Industrial Minerals

The commercial importance of magnetic separation mineral processing technology is demonstrated across a range of major commodities:

Iron Ore

Iron ore is the largest single application of magnetic separation. At operations processing magnetite ores (typically 40–50% Fe in fine-grained banded iron formation), multi-stage LIMS drum circuits are the primary concentration method. Very fine grinding — often to below 45 µm — is required to liberate magnetite from the siliceous gangue, and the resulting fine particles are then concentrated through multiple LIMS rougher, cleaner, and scavenger stages producing pellet-plant feed at 66–69% Fe. The Carol Lake operation in Labrador processes low-grade magnetite ore through a combination of LIMS cobbing on mill discharge, spiral gravity concentration, and LIMS/WHIMS scavenging on spiral tailings.

For hematite-dominated iron ores, WHIMS and HGMS are needed. Hematite’s weak paramagnetism requires field strengths approaching 1.5–2 T for efficient recovery. The trade-off between WHIMS and flotation for hematite concentration is dominated by capital and operating cost considerations: flotation costs roughly one-fifth the capital of WHIMS but three times the operating cost per tonne, making WHIMS more attractive as plant life and throughput increase.

Ilmenite and Mineral Sands

Ilmenite (FeTiO₃) is paramagnetic and is a major target for high-intensity magnetic separation in mineral sands circuits. In a typical heavy mineral sand flowsheet, low-intensity drum separators first remove any residual magnetite, after which high-intensity wet magnetic separators (WHIMS or SLon) separate the paramagnetic ilmenite and monazite from the diamagnetic zircon and rutile. The SLon VPHGMS has been specifically developed for fine ilmenite recovery and is deployed at numerous mineral sands operations. Single-stage extraction of ilmenite from highly magnetic gangue has also been demonstrated using superconducting HGMS, which can generate field strengths up to 15 T.

Chromite and PGMs

Chromite (FeCr₂O₄) is weakly paramagnetic, with a magnetic susceptibility approximately 0.001 (similar to that illustrated for chromite in the magnetisation-versus-field-strength data in Wills’). WHIMS has been used on South Africa’s Merensky Reef to remove strongly paramagnetic orthopyroxene gangue from the PGM-bearing chromite fraction, illustrating the use of magnetic separation not to concentrate the target mineral but to clean it by removing a more strongly magnetic gangue species.

Industrial Minerals

HGMS has its largest single application in the kaolin (china clay) industry, where iron-containing particles — principally fine-grained ferruginous minerals — must be removed to achieve the high brightness required for paper coating and filler grades. Superconducting HGMS machines have been used commercially in the United States since 1986 for kaolin brightening, operating at 5 T with a power consumption of approximately 0.007 kW for the magnet itself, compared with approximately 250 kW for a conventional 2 T HGMS of similar throughput. Eriez WHIMS machines are also widely applied to the purification of quartz, feldspar, spodumene, and other industrial minerals from iron-bearing contamination.

Electrostatic Separation: Principles and Equipment

Electrostatic separation exploits differences in electrical conductivity between minerals to achieve separation in a dry state. It occupies a complementary niche to magnetic separation: where magnetic methods are applicable regardless of conductivity, electrostatic methods operate on a surface property, the ability of a particle to acquire and retain electrical charge in the presence of an applied electric field. The two methods are frequently combined in mineral sands flowsheets, where the combination of conductivity and magnetic susceptibility properties of the feed minerals provides a powerful and selective multi-step separation sequence.

Three distinct charging mechanisms are exploited in industrial electrostatic separators:

Corona Discharge (Ion Bombardment)

The most widely used mechanism is corona charging, employed in the high-tension roll (HTR) separator. A high-voltage electrode (up to 50 kV DC) generates a corona discharge — a flow of gaseous ions — that bombards all particles on the grounded rotating drum, giving them a surface charge. Conductive minerals rapidly dissipate this charge to the earthed drum and are thrown free by centrifugal force into the conductor bin. Non-conductive minerals retain their charge, which creates an image force pinning them to the drum surface; they are eventually dislodged by a brush. A splitter positioned in the fan of discharged particles divides the products into conductor, middlings, and non-conductor fractions.

HTR separators treat feed in the particle size range 60–500 µm. Separation efficiency is influenced by roll speed, electrode voltage and geometry, rotor diameter, and the coefficient of friction between particles and the drum surface. Larger rotors improve recovery of the conducting fraction; smaller rotors improve grade. Higher rotor speed increases centrifugal force, aiding rejection of conductors but also potentially dislodging poorly pinned non-conductors — an inherent trade-off in separator optimisation.

A critical requirement for effective HTR separation is the presence of at least one strongly conductive mineral in the feed. Very large differences in conductivity between two weakly conductive minerals will not produce a sharp separation. Conversely, two strongly conductive minerals can be separated with only a modest conductivity difference. This constraint reflects the fundamental physics: the image force that pins non-conductors is generated by the contrast between the charge-retaining non-conductor and the rapidly discharging conductor, and requires a sufficiently fast charge decay in the conducting species to create a meaningful force differential.

Recent innovations in HTR design have improved single-pass separation efficiency. The OreKinetics CoronaStat machine incorporates induction electrodes that simultaneously increase the pinning force on non-conductors and accelerate charge decay for conductors, widening the physical gap between the two particle streams. The Roche Mining Carara HTR and the Outokumpu eForce separator similarly incorporate additional static electrodes to improve trajectory control of the conducting fraction.

Conductive Induction

In electrostatic plate (ESP) separators, particles are charged by conductive induction rather than corona bombardment. An ungrounded particle entering an electric field polarises — developing opposite charges on its near and far surfaces. Conductive particles redistribute this induced charge when they contact a grounded surface, leaving a net charge on the particle that is then attracted toward an oppositely charged electrode. Non-conductive particles do not redistribute the induced charge and experience no net electrostatic force. This mechanism is most effective for separating strongly conductive from weakly conductive particles and is used principally as a cleaning stage following HTR roughing, removing final traces of non-conductor from the conductive product or vice versa. Modern ESP separators such as the OreKinetics UltraStat incorporate secondary induction electrodes and an additional cleaning roll to maximise the lifting force and surface decontamination.

Triboelectric Charging

Triboelectrification (contact electrification) occurs when two minerals of dissimilar work function come into frictional contact and exchange electrons at their interface. The mineral with the lower Fermi energy (higher work function) acquires a negative charge; the other becomes positively charged. Triboelectric separation does not require a strongly conductive species in the feed — virtually every binary mineral pair will exhibit some charge exchange — giving it potential breadth of application far beyond HTR separation.

A major commercial triboelectric separator is the Separation Technologies ST separator, which uses a high-speed open-mesh belt (5–20 m/s) running between positive and negative electrodes. Feed at rates up to 40 t/h enters from the top; tribocharging occurs through intense interparticle contact in the turbulent counter-current flow between belt and electrodes, with positively and negatively charged particles exiting from opposite ends of the unit. The ST separator processes particles from 1 to 300 µm — significantly finer than conventional HTR or ESP separators — and has been widely applied to removal of unburned coal char from power plant fly ash at 10–20 µm median particle size. Pilot plant results demonstrate effective separation of quartz from calcite (89% recovery, 99% grade) and magnesite from talc (77% recovery, 95% grade).

Applications of Electrostatic Separation

The primary industrial application of electrostatic separation is in the heavy mineral sands industry. Mineral sands deposits — beach or stream placers enriched in dense heavy minerals — contain a characteristic suite of minerals spanning the full range of magnetic and electrical properties, making them ideal candidates for sequential magnetic-electrostatic treatment. Table 13.3 in Wills’ summarises the classification of typical heavy mineral sand components:

• Magnetic conductors: magnetite (Fe₃O₄), ilmenite (FeTiO₃)
• Magnetic non-conductors: garnet, monazite
• Non-magnetic conductors: rutile (TiO₂), leucoxene
• Non-magnetic non-conductors: zircon (ZrSiO₄), quartz

This matrix of properties allows a sequential circuit to cleanly separate all major mineral components. In a typical Australian heavy mineral sands flowsheet:

1. Floating dredges feed floating concentrators at up to 2,000 t/h, producing a gravity pre-concentrate upgraded to approximately 90% heavy minerals using sluices, spirals, or Reichert cones.
2. Low-intensity drum LIMS separators remove magnetite from the gravity concentrate.
3. Wet high-intensity magnetic separators (WHIMS) separate the paramagnetic ilmenite and monazite from the diamagnetic zircon and rutile.
4. Both the magnetic fraction (ilmenite + monazite) and the non-magnetic fraction (rutile + zircon + quartz) are dried.
5. HTR separators split the dry fraction into conductors (ilmenite, rutile) and non-conductors (zircon, monazite).
6. Rare earth roll and rare earth drum separators on the conductor stream separate ilmenite (magnetic) from rutile (non-magnetic).
7. Screen and plate ESP separators provide final cleaning of the zircon and rutile products.

At the Tronox (formerly Tiwest) Chandala processing plant in Western Australia, this type of integrated circuit produces separate commercial-grade ilmenite, leucoxene, rutile, and zircon products from a mixed feed, using HTR, cross-belt, rare earth roll, and ESP separators in combination with wet magnetic stages.

Beyond mineral sands, electrostatic separation is applied to:

• Coal cleaning: HTR separators have been used to remove pyrite (conductive) from coal (non-conductive) in dry circuits, particularly where conventional water-based processing is unavailable or environmentally constrained.
• Potash beneficiation: Triboelectric separation is used commercially to upgrade potash from halite gangue.
• Fly ash processing: The ST separator removes unburned carbon from coal combustion fly ash to produce a pozzolanic material that meets ASTM standards for concrete admixtures — one of the most commercially successful recent applications of triboelectric technology.
• Recycling: HTR and ESP methods have been investigated for separating metallic from non-metallic fractions in shredded electronic waste and mixed plastics.

A consistent operational requirement for all electrostatic separation methods is strict moisture control. Excess humidity on particle surfaces alters surface conductivity, disrupts the charge differentials that drive separation, and can cause fine particles to agglomerate. Feed preheating — passing the feed through a rotary or fluidised-bed dryer before the separator — is standard practice in most mineral sands plants.

Circuit Design and Integration

In industrial practice, magnetic and electrostatic separation rarely operate in isolation. Both techniques are most powerful when integrated into a multi-stage circuit that sequentially removes each major mineral species, exploiting the differences in both magnetic and electrical properties that together uniquely characterise each mineral.

Key principles governing circuit design for magnetic separation mineral processing applications:

Staging and Recirculation

As with flotation and gravity circuits, the best metallurgical performance in magnetic and electrostatic circuits is achieved by multiple stages — typically a rougher stage for maximum recovery, followed by cleaner stages for grade improvement, and scavenger stages to minimise losses. HTR separators in mineral sands circuits commonly operate in banks of 50 or more rotors in rougher-cleaner-scavenger configurations, with middlings fractions recirculated to appropriate stages. The Cliffs-Wabush iron ore operation deploys 54 primary HTR units (288 rotors total) in rougher duty, followed by 6 scavenger units, with middlings recycle.

Sequence of Separation Steps

In combined magnetic-electrostatic circuits, the conventional sequence removes the most strongly responding material first. For mineral sands: LIMS removes magnetite (strongly ferromagnetic); WHIMS or SLon removes paramagnetic ilmenite and monazite; drying prepares the material for dry processing; HTR separates conductors from non-conductors; rare earth magnets refine the conductor fraction; ESP and UltraStat clean the final products. This stepped approach prevents each separator from being overwhelmed by extraneous material and maximises the sharpness of each individual separation.

Feed Preparation

Dry magnetic and electrostatic separators require carefully prepared feed. Particle size must be controlled to within the effective operating range of each machine: HTR and ESP separators operate on 60–500 µm particles; ST separators extend this to 1–300 µm; rare earth rolls are typically used on 75 µm to 2 mm material. Feed moisture must be controlled at less than 3% for dry operations. Pre-desliming to remove particles below about 10–20 µm reduces the tendency for fines to agglomerate on coarser particles and alter their effective conductivity or magnetic character.

Superconducting Separators

Superconducting magnetic separators represent the current frontier of magnetic separation mineral processing technology. By cooling niobium-titanium alloy coils to 4.2 K (the temperature of liquid helium), field strengths up to 15 T can be generated with a magnet power consumption of as little as 0.007 kW — versus 250 kW for a conventional 2 T HGMS of similar throughput. The main practical challenges are maintaining cryogenic temperatures, minimising heat leaks, and clearing the matrix of accumulated magnetic particles during cyclic operation. A superconducting drum separator has operated commercially since the 1980s with drum surface flux densities exceeding 4 T. High-temperature superconductors (operating at 35 K with ceramic oxide composites) offer the prospect of reduced refrigeration costs, but fabrication challenges and low current-carrying capacity have so far restricted industrial deployment to low-temperature niobium alloy designs.

Process Control and Optimisation

Advances in on-line sensing — vibrating sample magnetometry, conductivity measurement, and particle image analysis — are enabling tighter closed-loop control of magnetic and electrostatic circuits. In WHIMS and SLon operations, on-line measurement of magnetic concentrate grade allows field intensity to be adjusted dynamically to balance recovery against grade. In HTR circuits, humidity monitoring and feed temperature control are used to maintain optimal separation conditions. Combined with digital twin models calibrated from laboratory characterisation data, modern mineral processing operations are moving toward fully automated magnetic-electrostatic circuit optimisation.

References and Further Reading

1. Wills, B.A. & Finch, J.A. (2016). Wills’ Mineral Processing Technology, 8th Edition. Butterworth-Heinemann/Elsevier. Chapter 13: Magnetic and Electrical Separation.
2. Xiong, D. (2004). The latest application of SLon vertical ring and pulsating high-gradient magnetic separator. Minerals Engineering, 17(5), 649–656.
3. Mineral Technologies. (2024). Magnetic Separation Equipment. Mineral Technologies Product Guide.
4. Mineral Technologies. (2024). Electrostatic Separation. Mineral Technologies Product Guide.
5. ScienceDirect Topics. (2024). Intensity Magnetic Separation — Overview. Elsevier Engineering Topics.
6. Metso. (2024). Vertically Pulsating High Gradient Magnetic Separator (VPHGMS). Metso Product Portfolio.
7. Magquip. (2024). Wet High Intensity Magnetic Separator (HGMS / WHIMS). Magquip Equipment Guide.
8. Bunting Magnetics. (2024). Electrostatic Separation of Minerals. Bunting Magnetics Technical Blog.
9. STEQtech. (2024). Dry Triboelectrostatic Beneficiation of Mineral Sands. STEQtech Technical Literature.
10. Xiong, D. (1993). Pulsating high-gradient magnetic separation of fine hematite from tailings. Mining, Metallurgy & Exploration, 10(4), 227–233.