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PERMAX separator



Wet drum separators are used in Dense Media Separation (DMS) plants for the recovery of magnetic particles from the dilute medium. They must recover the maximum amount of magnetic particles at the highest possible density.

A wet drum separator consists of a stationary magnet arc around which a drum rotates. The drum is partially submerged in a tank which follows the contour of the drum.

When magnetic particles in the slurry are introduced into the magnetising field produced by the magnet, they first align themselves with the lines of force. Then, the individual grains act as secondary magnets and they begin to gather together to form flocs; the form of which will be long chains or clusters depending on the amount of magnetic particles in the slurry. When the magnetic force acting on the flocs exceeds the opposing forces, the flocs attach themselves to the drum surface.

Flocculation occurs more readily with a 'thick' slurry than with a 'dilute' slurry, where the magnetic particles are far apart from each other.

The magnetic media used is – Magnetite in Coal washing plants and Ferrosilicon in Mineral Processing plants.

Wet drum separators usually perform better with Ferrosilicon. Magnetite medium used in Coal washing plants has a much finer size distribution. The magnetic properties are also different - the magnetic susceptibility is lower and the coercive force higher.


A general arrangement of a typical wet drum separator is shown in Fig. 1.

Permax wet drum magnetic separator

The magnet assembly is made up of permanent magnets enclosed in stainless steel cans. The arc is typically 115°.

The drum comprises of a shell carried on end flanges fitted with sealed, self-aligning bearings. The shell is made from non-magnetic stainless steel, typically 3mm thick. The shell is covered with a wear wrap, typically 2mm thick, of non-magnetic stainless steel or polyurethane. The drum end flanges are machined from non-magnetic stainless steel plate.

The feed box and tank (underpan) are an integral construction, fabricated from non-magnetic stainless steel, typically 3mm thick. The inside of the feed box and tank may be lined with polyurethane.

The support frame is fabricated from mild steel sections and plate; painted or galvanised.

A geared motor drives the drum; either through a roller chain drive arrangement, or direct coupled to the end flange via a stub shaft. The latter requires minimum maintenance and spare parts.


Two tank designs are used. They are called 'concurrent type' and 'counter rotation type'.

Concurrent Type

A schematic representation is shown in Fig. 2.

Permax wet drum magnetic separator

The slurry is introduced into the feed box and flows in the direction of drum rotation. Magnetic particles attach themselves to the drum and are transported by the rotation of the drum, to a point outside of the influence of the magnetising field where, they are discharged into a chute.

Non-magnetic particles and water which are unaffected by the magnetising field, leave the tank through the tailings discharge outlet and weir overflow.

Pulp level control is extremely important with the concurrent tank. Efficiency falls quickly as the pulp level drops.

Counter Rotation Type

A schematic representation is shown in Fig. 3.

Permax wet drum magnetic separator

With this type, the slurry is introduced into a specially designed feedbox and flows against the drum rotation. The magnetic particles are discharged into a chute under the feed box.

Claimed advantages for the counter rotation tank are [1].

  1. Maximum recovery, the magnetic particles are recovered and transported a short distance into the discharge chute. Particles missed in the initial pick-up are scavenged out by the remaining part of the magnet assembly.
  2. High Magnetics loading can be handled without serious reduction of efficiency due to the short transportation and scavenging features.
  3. Efficiency is maintained at high volume flow rates.
  4. Magnetic concentrate is discharged on the feed side of the tank. This may be an advantage in plant layout design.

Low intensity (~2000 gauss) magnet assemblies are made up from Strontium Ferrite permanent magnet blocks. The standard block size is 150mm long, 100mm wide, 25mm thick. The preferred direction of magnetisation is through the thickness. To construct an assembly, the blocks are stacked on top of or against each other.

Typically two grades of permanent magnet material are used.

Coercive Force
Energy Product
Ceramic 5 2400 3950 3,6
Ceramic 8 3200 3900 3,5

Assemblies built with Ceramic 8 magnets produce approximately ten percent stronger magnetising field.

The force acting on a particle in a magnetic field is proportional to the product of the magnetising field H and the rate of change (gradient) of the field dH/dx.

The requirements for a wet drum magnet assembly are:

  1. A strong magnetising field at the feed point.
  2. A high gradient at the feed point.
  3. A magnetising field profile on the drum surface which allows easy transfer of magnetics from pole to pole to the discharge point.

Two common types of magnet assemblies used in wet drum separators, are the 'High Gradient Type' and the 'Interpole Type'.

High Gradient Type

This comprises of a series of magnet blocks stacked on top of each other, arranged in alternating north-south polarity. The width of the block decreases with the stack height.

The profile and intensity of the magnetising field on the drum surface is shown in Fig. 4.

Permax wet drum magnetic separator

This type does not allow easy transfer of the magnetics. The magnetics loading is limited, particularly at low drum speed. The high gradient type is not recommended for primary separators, particularly in Coal washing plants.

Interpole Type

In this type, small magnets are interposed between the main magnet stacks. They have the polarity of the stack to which they are adjacent.

The profile and intensity of the magnetising field on the drum surface is shown in Fig. 5.

Permax wet drum magnetic separator

Transfer of magnetics is easier with this type. Magnetic loadings are higher even at low drum speeds. Efficiency with very dilute slurries, (such as in secondary separator duty) is less. It is not recommended to use this type as a secondary separator in a Coal washing plant.


Drums are made in nominal diameters of 300mm, 380mm, 600mm, 750mm, 900mm and 1200mm. The rated capacity increases with drum diameter.

The 300mm and 380mm diameter drums are only installed in small modular DMS plants, such as those used for diamond exploration.

The 600mm and 750mm diameter drums are now only made for spares in existing plants. Normally they are not installed in new plants.

Maximum capacities per effective metre drum length (used by International Magnetic Solutions Magnetic Mining) for 900mm and 1200mm diameter drums are shown in the table below.

Slurry m3/hour Magnetic solids t/h
Magnetite Ferrosilicon
900mm Φ 5 stack Interpole Type CC Tank 90 16 21
900mm Φ 6 stack Interpole Type CC Tank 100 18 24
900mm Φ 6 stack Interpole Type CR Tank 120 24 32
1200mm Φ 6 stack Interpole Type CC Tank 125 25 33
(CC=Concurrent tank, CR=Counter rotation tank)

Drums available from International Magnetic Solutions Magnetic Mining are from 690mm up to 3130mm long in increments of 305mm. The effective drum length is 80mm less than the overall length.


Typical drum speeds are 11r/min for the 900mm and 9r/min for 1200mm diameter drums.


Adjusting screws are provided on the support frame to move the drum up or down and forward and backward in the tank.

The magnet assembly is moved to the working position, via a special spanner fitted over 'flats' machined on the end of the magnet assembly support shaft.

International Magnetic Solutions Magnetic Mining wet drum separators are despatched with 50mm tank – drum gap, and 30mm discharge lip – drum gap.


Recovery efficiency and concentrate density are the measures of performance.

Recovery Efficiency

This depends on the concentration of magnetic material in the feed slurry. Most manufacturers provide a curve of efficiency against magnetics concentration (at maximum volume flow rate).

Wet drum separator manufacturers define efficiency as - feed magnetics concentration less tailings magnetics concentration, divided by feed magnetics concentration, times one hundred percent.

Typical efficiencies at 100 - 120g/l magnetics loading and maximum flow rate are 99,8% with Magnetite and 99,9% with Ferrosilicon.

Concentrate density

This also depends on the concentration of magnetics in the feed slurry. Typical values are 2,1 – 2,7 for Magnetite, and 2.8 – 3.2 for Ferrosilicon.


In the mid 1980’s, a French company (FCB) introduced to South Africa a wet drum separator with higher capacity and better performance than existing separators. These separators featured a counter rotation tank and a unique magnet assembly.

The Permax is an improved version of the FCB separator. The price per m3/h of flow rate is competitive with other separators.


Counter rotation, with facility to overflow up to 50% of the water for recirculation back to the drain and rinse screens.

Magnet assembly type. The magnets are 'sandwiched' between steel poles. Steel operates at much higher flux densities than permanent magnets. This is a common construction method with magnetic devices and is known as 'flux concentration'.

The profile and intensity of the magnetising field on the drum surface is shown in Fig. 6.

Permax wet drum magnetic separator

The magnetising field and gradient is higher than that of the other types. Only the Permax magnet assembly has the three requirements referred to previously. Comparison 'gauss curves' are shown in Fig. 7.

Permax wet drum magnetic separator

The drum is 900mm Φ; the rotational speed is 7 rpm.

High Volume flow rate up to 160m3/h/metre
High magnetics loading up to 30 t/h/metre Magnetite
up to 40 t/h/metre Ferrosilicon
High efficiency more than 99,8% Magnetite
more than 99,9% Ferrosilicon
High concentrate density up to 2,7 Magnetite
up to 3,2 Ferrosilicon
Ability to recover magnetic particles from slurry with magnetics concentration of less than 0,1g/l.
Magnetic particles in the overflow are not lost from the dilute medium circuit.
Direct drive requiring little or no maintenance.

The first Permax separator (a double drum arrangement) was supplied in 1996 to Dowding Reynard & Associates (DRA) for installation in a 200t/h Pre-treatment feed plant at a Namibian Diamond Mine.

The guarantees for efficiency and concentrate density were achieved on the primary separator. The performance of the secondary separator was remarkable. The feed magnetics concentration of 0,09g/l was reduced to 0,03g/l in the tailings at a flow rate of approximately 100m³/h metre.

In 1996, Queensland Cement asked for the supply of a (strong) wet drum with rare earth magnets for use in the recovery of magnetite derived from Power Station ash. So called Fly Ash Magnetite, (FAM). The magnetic susceptibility of this material is less than that of ultra fine magnetite and the existing separators could not provide the required recovery efficiencies.

Instead of a rare earth wet drum, a Permax drum was proposed. This was purchased by Queensland Cement and installed in an existing counter rotation tank at the Australian Coal Industry research Laboratories.

Tests were performed with the Permax drum and drums of other manufacturers in concurrent and counter rotation tanks. Baird et al [3] has reported the results of these tests in detail. The recovery of FAM with the Permax drum in a counter rotation tank was greater than 99,9% with a single treatment. This could not be achieved with the other drums in a two-stage treatment.

The success with the Permax drum led to the installation of two drums at Burton Coal, Northern Queensland, for full-scale FAM tests.

Realising the potential of the Permax separator, Australian Magnetic Solutions, embarked on a marketing campaign.

Following from this, in 1997, Hamersley Iron asked International Magnetic Solutions Magnetic Mining to quote for the supply of wet drum separators for installation at their Tom Price Concentrator, Western Australia. This plant has three drum and three cyclone modules. At the time, the dilute medium circuit for each module comprised three primary separators, inter-stage cyclones and one secondary separator.

The installation of new separators formed part of a project to reduce the Ferrosilicon adhesion losses.

Specifications for the new dilute medium circuit were:

  1. Only one separator to be installed in each module, due to the difficulty in controlling the flow to more than one unit.
  2. Flow rate to the separator to be increased.
  3. Inter-stage cyclones and secondary separator to be dispensed with.

Other manufacturers offered separators with 1200mm diameter drums.

The order was placed with International Magnetic Solutions Magnetic Mining for six, three metre long, double drum separators. Installation and commissioning took place in 1998.

Initial test results at approximately 400m3/h flow rate to the primary drum and 200m3/h flow rate to the secondary drum, gave Ferrosilicon concentrations in the secondary drum tailings, of less than 0,04g/l in the drum plant and 0,02g/l in the cyclone plant.

In the middle of 1999, the secondary separators were removed. It was found that the small amount of Ferrosilicon recovered on the secondary separator built up on the concentrate discharge lip and wore a hole in the drum. After they were removed, the Plant Metallurgist reported that primary separator Ferrosilicon tailings concentration varied between 0,03g/l and 0,08g/l. This is at a feed rate of 350 – 400m³/h and a magnetics loading of 78t/h. Concentrate density is 3,1 – 3,2 on the cyclone plant separators. Ferrosilicon grade used is 100d in the cyclone plant and 65d in the drum plant.

As part of the ongoing Ferrosilicon adhesion loss project, improved spray nozzles will be installed on the screens and the volume flow rate to the separators will increase to 470m3/h.

There are now several installations of Permax wet drum separators in DMS plants. A list of these is shown below.

Installation list Permax Wet Drum Separators – 1996 - 2000
Quantity/Size Owner Product
6 x 3m double drum Hamersley Iron W.A. Iron Ore
1 x 2,7m SAMREC Krugerspost Andalusite
1 x 2,4m double drum Namdeb Diamonds
1 x 1,8m Namdeb Diamonds
2 x 1,5m Namdeb Diamonds
2 x 1,5m Mayer Parry Scrap Recycling
2 x 0,9m Transhex Diamonds
1 x 1,2m Transhex Diamonds
2 x 2,7m double drum Middelburg Mine Coal
3 x 2,4m Kleinkopje Colliery Coal
2 x 2,4m United Collieries
N.S.W. Australia
2 x 2,1m Rietspruit Colliery Coal
1 x 3m ISCOR Sishen Iron Ore
On order
1 x 1,5m Samancor Ferrosilicon
1 x 1,2m De Beers Kooingnaas Diamonds

Four years of operational experience have been gained with the Permax separator. Performance data has been collected from several plants. A curve of percentage efficiency against Ferrosilicon concentration in the feed slurry is shown in Fig. 8.

Permax wet drum magnetic separator

With feed rates up 18t/h/m a single stage separator will recover at least 99% of the Ferrosilicon from the slurry. At the optimum Ferrosilicon concentration, the recovery will be 99,98%.

It has been demonstrated that the performance required by Plant Operators can be achieved without inter-stage thickening and secondary separators.

The high capacity of the Permax separator will normally enable only a single drum to be installed. The cost per m³ of flow rate is comparable with conventional 900mm diameter drum separators. It is approximately 20% less than that of 1200mm diameter drum separators operating at the same flow rates.

The Permax separator enables the simplification with consequent reduction in capital and operating costs of the dilute medium circuit.


Recently a Permax magnet assembly was retrofitted into a drum installed in a modular coal preparation plant. It replaced a five stack interpole type magnet assembly.

The separator efficiency increased to 99,9% and the concentrate density to 2,4 (at 11r/m drum speed).

This has prompted International Magnetic Solutions Magnetic Mining to introduce a concurrent tank separator with Permax magnet assembly. This will be of benefit to constructors of modular plants whose layout includes a concurrent tank type separator. It will replace both the five and six stack magnet assemblies, either high gradient or interpole type.

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[ 1 ] Ceramic permanent wet magnetic drum separator
Stearns Magnetics Inc. Bulletin 2017

[ 2 ] Goffinet et al, Development of Large Capacity Dense Medium Cyclone Circuits
An Australian Designer’s Experience
13th International Coal Processing Congress
Brisbane, October 1998

[ 3 ] Fly Ash Magnetite alternative update
Australian Coal Preparation Society Seminar
Emerald, Queensland, August 1997