Duty of Magnetic Separators

Magnetic separators in dense medium separation (DMS) plants perform multiple critical functions beyond medium recovery. These machines regulate correct medium density, control viscosity, purge non-magnetic contaminants, and maintain circuit stability – yet their complete operational scope extends far beyond what most operators and engineers initially understand.

Multotec's webinar, The Duty of Magnetic Separators, presented by Willem Slabbert, explores advanced magnetic separator performance principles rarely covered in standard training. From the magnetic flocculation effect to resilience under extreme operating conditions, Slabbert demonstrates why magnetic separators function as the DMS circuit's regulatory hub and how understanding their complete duty optimises performance and reduces medium consumption costs.

What is the primary duty of a magnetic separator in a DMS plant?

The primary duty of a magnetic separator (magsep) in a dense medium separation (DMS) plant is medium recovery – reclaiming expensive magnetite or ferrosilicon (FeSi) from the process stream for recirculation. However, magnetic separators perform several critical functions beyond simple recovery that directly impact overall DMS performance and economics. These include:

Overdense medium production: The magsep’s discharge must deliver medium at higher density than the correct medium (CM) setpoint. This over dense stream allows operators to achieve precise density control simply by adding process water, eliminating the need for additional densification equipment and providing faster, more responsive density adjustment.

Non-magnetic fines purging: The separator continuously removes non-magnetic contaminants from the circulating medium inventory through the underflow stream, preventing build-up that would compromise separation efficiency.

Viscosity regulation: By bleeding degraded ultrafine medium particles to tailings via the underflow, the separator maintains optimal medium viscosity – critical for cyclone performance – without sacrificing medium stability.

Degrit water supply: The clarified overflow, free of fast-settling solids, can be recirculated to drain and rinse screens, reducing freshwater consumption and improving in-plant water utilisation.

The magnetic separator functions as the DMS circuit's kidney loop – continuously purifying, regulating, and optimising the circulating medium to ensure consistent separation performance and minimal medium losses.

How does a wet drum magnetic separator work?

A wet drum magnetic separator uses a rotating drum with an internal magnetic assembly to capture and recover magnetic particles from slurry streams. As slurry flows into the separator tank, the rotating drum's magnetic field attracts ferromagnetic particles (magnetite or FeSi) through the drum shell, holding them against the drum surface while non-magnetic material flows away as tailings.

The drum rotates continuously, carrying captured magnetic particles out of the slurry zone. As the drum moves beyond the strong magnetic field area, the particles release into a concentrate launder for return to the correct medium circuit. This self-cleaning action provides continuous, automated medium recovery without manual intervention. Key components include:

Rotating drum: Houses the internal magnetic assembly and provides the surface for particle capture and transport.

Magnetic assembly: Creates the magnetic field gradient necessary to attract and hold ferromagnetic particles. Field strength and pole configuration determine recovery efficiency across different particle sizes.

Feed distribution system: Ensures even slurry distribution across the drum width, maximising contact between magnetic particles and the magnetic field.

Discharge system: Separates recovered magnetic concentrate from non-magnetic underflow, with adjustable outlet orifices controlling the level in the tank.

The separator's effectiveness depends on maintaining optimal slurry level in the tank – the single most critical parameter for performance – and proper magnet angle to prevent medium loss through the magnetic field.

What factors affect magnetic separator performance and efficiency?

Magsep performance depends on multiple interrelated factors, with slurry level in the machine ranking as the most critical parameter for optimal recovery. Operating conditions must remain within recommended ranges to maintain the magnetic flocculation effect – the phenomenon where ferromagnetic particles form chains in the magnetic field, enabling aggregate recovery rather than individual particle capture.

Critical operating parameters (in decreasing order of importance):

Slurry level: Insufficient or excessive slurry depth prevents proper magnetic particle capture and transport. Maintaining design level ensures maximum contact time between particles and the magnetic field.

Magnet angle: Incorrect magnet shaft positioning causes dense medium losses. The magnetic assembly must be properly aligned to create the optimal field gradient for particle capture and release.

Feed rate: Overloading beyond recommended rates (90 to 120 m³/h per metre width) reduces recovery efficiency of 900mm nominal diameter drums. This includes complete machine overfeeding, sectional overfeeding from blocked pipes, or surging flow that prevents stable operation.

Magnetic solids concentration: Percentage of magnetic material in the total slurry flow outside the optimal 120 to 250 g/l range compromises magnetic flocculation. Too low prevents aggregate formation, too high causes excessive loading and reduced efficiency.

Non-magnetic material content: Exceeding 25% non-magnetic solids (Mnon-magnetics/Mtotal-solids) breaks magnetic particle chains, forcing individual particle recovery and decreasing overall efficiency.

Machine setup: Critical dimensions, level (both vertical and horizontal), and mechanical condition directly impact separation performance and medium recovery rates.

Understanding these factors enables operators to maintain optimal conditions and recognise when performance degradation indicates parameter deviation requiring correction.

What is magnetic flocculation and why does it matter?

Magnetic flocculation is the phenomenon where ferromagnetic particles become magnetised in an external magnetic field and form long, intertwined chains. These chains are recovered as aggregates rather than individual particles, dramatically improving recovery efficiency.

This effect requires magnetic solids concentration between 120 to 250 g of medium, per litre of slurry. Within this range, particles form robust chains that resist hydraulic forces. Below this threshold, chains cannot form adequately, forcing less efficient individual particle recovery.

Why it matters: Non-magnetic contamination above 25% breaks particle chains, reducing recovery efficiency. Ultrafine particles (below 10 microns) are displaced from chains by larger particles and report to tailings rather than being recovered. Understanding magnetic flocculation explains why maintaining recommended feed concentrations and controlling contamination are critical – these parameters directly enable the physical mechanism underlying efficient medium recovery.

What happens when you operate outside recommended parameters?

Operating outside recommended parameters reduces efficiency, yet magnetic separators demonstrate remarkable resilience. A case study on a FeSi DMS plant processing malachite copper ore operated at 90 to 100 tph despite 65 tph nameplate rating, with the separator receiving 400 m³/h feed (versus 150 m³/h design), 70% non-magnetic solids (versus 0 to 25% specification), and 120 g/l magnetic concentration at the lower boundary.

Despite these extreme deviations, the separator achieved 95.7% FeSi recovery, 98.2% non-magnetic solids in effluent (effective purging), and 3.1 over dense specific gravity (slightly below 3.2 target but sufficient for control). This proves magnetic separators remain functional under gruelling conditions, facilitating recovery, density control, and purging despite operational challenges – though operating within design parameters maximises efficiency, extends equipment life, and minimises medium consumption.

How can I reduce medium consumption in my DMS plant?

Medium consumption represents a significant operational expense in DMS plants, particularly with costly ferrosilicon. Reducing losses requires addressing multiple factors across the entire circuit, with housekeeping and leakage prevention ranking as the most important control measure.

Five critical factors affecting medium consumption (in decreasing order of impact):

Housekeeping and leakage control: Spillages and leaks of correct medium throughout the plant constitute the largest source of avoidable losses. Rigorous maintenance of pumps, pipes, valves, and tanks prevents medium escaping the circuit before magnetic recovery is possible.

Floor and sump design: Proper drainage systems and sump placement ensure spilled medium can be recovered rather than lost to waste streams. Well-designed collection systems capture and return medium to the circuit automatically.

Drain and rinse screens: Efficient screen operation maximises medium removal from separated product before it leaves the plant. Inadequate rinsing allows valuable medium to report with product and tails streams, representing direct economic loss.

Wet drum magnetic separator performance: Maintaining optimal separator operating conditions - correct slurry levels, feed rates, and magnetic solids concentrations – ensures maximum medium recovery from process streams and effective purging of non-magnetic contaminants.

Make-up circuit efficiency: The system supplying fresh medium to replace unavoidable losses must be properly calibrated and controlled to prevent over-addition whilst maintaining circuit stability.

Multotec's magnetic separation technology, combined with proper plant design and operational discipline, minimises medium losses and optimises DMS circuit performance for maximum economic efficiency.

Key Takeaways

• Magnetic separators perform multiple critical duties beyond medium recovery, including over dense production for density control, non-magnetic fines purging, viscosity regulation, and de-grit water supply.
• Slurry level is the single most important parameter affecting separator performance, followed by magnet angle, feed rate, magnetic solids concentration (120 g to 250 g/l optimal), and non-magnetic content (maximum 25%).
• Magnetic flocculation - where ferromagnetic particles form intertwined chains – is the fundamental mechanism enabling efficient aggregate recovery rather than individual particle capture.
• Magnetic separators demonstrate remarkable resilience, maintaining 95%+ recovery even under extreme conditions far exceeding design parameters, though optimal operation within specifications maximises efficiency and equipment life.
• Medium consumption is primarily controlled through housekeeping and leakage prevention, followed by floor design, drain and rinse screens, magnetic separator performance, and make-up circuit efficiency.

For expert guidance on optimising magnetic separator performance in your DMS plant, contact Multotec. Watch the complete webinar here.