Magnetic drum separator technical and maintenance information from IMS Magnetic Mining

INTRODUCTION

This page has been compiled by International Magnetic Solutions (IMS) to give operations personnel a greater understanding of the functions magnets have in drum separators. Having an understanding will assist in the correct care and maintenance of separators. Figure 1.1 shows a typical set-up for a concurrent unit.

Figure 1 - General arrangement of a typical wet drum separator - concurrent drum type

The magnet assemblies are fixed to a central shaft allowing the stainless steel outer skin to rotate freely around them as pictured below (click on either photo for a larger view). A common arrangement for the magnet assemblies is shown in figure 1.2

Magnet assembly in a drum separator with the skin partially removed

In this case each individual bank of magnets is arranged in brick formation and enclosed in a stainless steel skin to form a single magnet assembly. The picture on the left shows the magnet assembly with the skin partially removed. When these individual magnet assemblies are fitted together they form a sub-assembly as shown in figure 1.2.

Basics of Magnetic Separation

The translational magnetic forces on magnetisable particles is of the form:

F =	V k grad(B²)	(1)

	2 µ₀

Where the volume susceptibility is k and the particle volume V. Formula (1) holds for weakly magnetic particles but is also valid - with some restrictions (demagnetisation factor, saturation) - for more or less strongly magnetic particles.

To obtain high forces, the factor grad (B²) or |B| has to be optimised in the separation volume. This volume ranges from drum radius to the bottom of the trough.

What we need is high flux density and high gradient. But if we aim at a high gradient at the drum surface, then the flux density will decrease rapidly in radial direction. To obtain high forces at the outer region of the working volume, we have to carefully adjust the gradient depending on the specific application (grain size, throughput rate, gap width, etc). Once attracted to the drum surface, particles must remain there while being transported to the magnetic discharge. Therefore field gradients parallel to this path (ie variations of the magnetic field which are identical to transverse forces) should be minimised. Such transverse forces hold particles in the regions of relatively higher fields and give rise to clogging and losses of magnetic particles.

On the other hand, many pole changes on the path along the drum surface are advantageous. Strongly magnetic particles rotate with changing polarity, and non- magnetic particles - which might be captured by the clogging of some magnetic particles - can be freed. This leads to cleaner products and better selectivity.

We now can state what we think to be best:

High flux density at the drum surface
High field gradient according to the requirements of the specific application
Many pole changes
Minimum variations of the absolute field value on the drum surface

The basic principles

Magnetic separation is used for the concentration of minerals and for the removal of suspended magnetisable particles in a fluid. As a method it depends on the behaviour of different minerals under the influence of magnetic force. Broadly minerals can be classified into three groups - whether they are attracted to, repelled by, or unaffected by a magnet.

Paramagnetic minerals are attracted in the direction in which the field intensity increases. In almost all cases this results in attraction aligned with magnetic flux urging particles towards a magnetic surface. They can be concentrated in high intensity magnetic separation.

Ferromagnetic minerals are generally included in this classification since they are attracted in the same way. Ferromagnetics can be strongly magnetised by a low external field.

Diamagnetic minerals are repelled in the direction of decreasing field intensity. The forces involved are small and diamagnetic substances are not concentrated magnetically except by bench scale equipment.

The capacity of a magnet to lift a mineral is not only dependant on the field intensity but on the field gradient ie the rate at which the field intensity increases towards the magnet surface. Paramagnetic minerals have higher permeabilities than the surrounding fluid; water or air and therefore concentrate the lines of force of an external magnetic field. Thus the higher the magnetic susceptibility is the higher the density in the particle and the greater its attraction up the field gradient towards increasing magnetic strength. Diamagnetism results from induction in a substance of magnetic moment opposite in direction to the external field.

All magnetic separators are designed to provide a magnetic field gradient, either in the gap between the poles of a magnetic circuit or in the air space near the magnetised surface such as that of a permanent magnet. An effect of the field gradient is to induce stronger magnetism in one pole than the other pole. The particle then experiences a translational force that urges it in the direction of increasing field intensity. Simple means for producing a field gradient are a V-shaped pole above a flat pole. Alternate magnetic and non-magnetic laminations also produce field gradients. By placing ferromagnetic elements in a magnetic field to serve as secondary poles, a number of separate regions in which field gradients effective for separation can be provided. The shape and orientation of the ferromagnetic elements determine their degree of magnetisation and the field intensity gradient adjacent to their surfaces. For continuous machines the speed at which material passes through machines is also important. Flocculation of particles is also often avoided by passing material through consecutive magnetic fields, usually arranged with successive reversal of the polarity. This causes the particle to turn through 180° with the reversal releasing entrained particles. The main disadvantage of this is flux leakage from pole to pole thus reducing the effective field intensity.

There are several different separation techniques encompassed under the umbrella of magnetic separation including:

dry low intensity magnetic separators
wet low intensity magnetic separators
high intensity magnetic separators
wet high intensity high gradient magnetic separators
superconducting magnetic separators

Dry low intensity magnetic separation

Low intensity magnetic separation is well established technology and some of the basic techniques are reviewed here.

Low intensity drum separators date from the turn of the century and are still the most common form. Typically the rotating drum contains from three up to ten magnets of alternate polarity. These are arranged radially on the shaft covering approximately 180° of the drum. Initially the magnets were electromagnets but now have all been replaced in designs by permanent magnets using, for example, strontium-ferrite magnets.

The drum cover is non-magnetic, and the drum rotates about the hub at typically 40 rpm. Material is fed onto the top of the drum and separated as shown in the accompanying diagram. The alternating polarity aids the rotation of magnetic particles and the release of non-magnetics.

Typical uses for this type of separator includes upgrading of blast furnace slag, iron ore, and sponge iron and commercial separators are available over a wide range of sizes. Drum diameters vary from 300 to 1500mm and drum lengths from 300 to 4000mm. The performance of a drum dry low intensity magnetic separator can be predicted by the use of four parameters - magnetic field at the drum surface, angular spacing between the magnets, drum radius and the revolution rate. By correct adjustment a fairly high degree of control is offered over the concentrated grade. The typical radial magnetic induction values for current drum separators are of the order of 0.1 - 0.15 Tesla (1000 - 1500 Gauss) at the drum surface.

Dry low intensity magnetic separation is mainly applied to strongly magnetic coarse sands, typically 5 - 8mm material economic feed rates of 150 tph per metre of length can be obtained.

In other forms dry low intensity magnetic separators are often used as pulleys installed over the end of conveyor belts which draw the tramp iron from the material and carry it to the underside of the conveyor to be discharged. Over band separators and suspension magnets are installed over conveyor belts to remove tramp iron. Suspension magnets are used to extract iron form the material being transported and are used on conveyor widths up to 2200mm with an operating gap up to 850mm.

Wet low intensity magnetic separators

This is today by far the most widely used type of magnetic separation. But it can only be used for strongly magnetic minerals, so its primary use is for magnetite and ferrosilicon recovery in dense media circuits. Other applications include iron ore concentration and removal of highly magnetic material prior to high intensity wet magnetic separation. Other low intensity wet magnetic separations take the form of matrices through which the product percolates, such as clayslip in order to remove any ferrous contaminants.

Several industrial mineral processes use dense media recovery as a preconcentrating stage including fluorspar, barytes, petalite and diamonds. The economic recovery of the medium is an important integral part of the process.

Typical wet drum separators are available in diameters from 600 to 1200mm and in lengths of up to 3000mm.

There are three basic designs of wet low intensity magnetic separation - concurrent, countercurrent and counter rotation.

Concurrent: This type of drum separator is almost exclusively used for the treatment of particles of 5mm and below. The ore is carried forward by the drum since it flows in the same direction as the drum rotation. It then passes through a gap where it is compressed and dewatered before leaving the separator. Magnetic material is picked up by the drum and the non-magnetics are discharged at the bottom of the tank. This method is widely used in heavy media recovery systems since it offers a magnetic concentrate from relatively coarse material with high throughput rates.
Counter rotation: In the counter rotation design of wet low intensity magnetic separation the feed flows in the opposite direction to the drum rotation. This type is often used in roughing operations where occasional surges in feed must be accommodated without a minimum loss of magnetic material and where a high quality concentrate is of secondary importance. This is because the tailings flow along the entire magnetic arc of the drum. The feed size for a counter rotation drum separator is nominally less than 1mm for optimum results.
Counter current: The term counter current is derived from the fact that the tailings flow counter to the rotation of the drum when leaving the tank. Sometimes known as the Steffenson Tank the separator is often used for the separation of finely ground particles of approximately 100µ and less in size. Coarser ores cause settling problems. The feed is introduced near the bottom of the drum and the magnetic particles are picked up by the drum and agitated by wash water jets. The tailings discharge through an overflow at the opposite end of the tank, which also acts as a pulp level control. The counter current design ensures a high recovery of magnetic material and a high quality concentrate.

What does IMS test?

Our current standard testing procedure is based on the assumption that the current manufactured drum separators have optimised the field strength and gradients in their equipment designs. As this is controlled by the geometry of the design the relationship between field strength and gradient is fixed. So simply by measuring changes in field strength it is obvious that the effective gradients are also changing in a similar way.

It is also obvious that the fields and gradients experienced by the flow of magnetite change with the density of magnetite in the flow but again the total field effect on the magnetite is directly related to the fields measured on a clean drum.

Hence IMS’s test reports give you comparative information on the magnets in your drum by:

Comparing the magnetic characteristics of other drums of the one type in service and
Identifying changes in your drums magnetic characteristics between successive tests.

The test results tell the story in two ways:

Is your drum as strong as it should be?
Is your drum’s performance deteriorating?

If the drum strength is significantly low or is deteriorating then we look for the cause.

CARE OF DRUMS

Owing to a minimal gap between the outer stainless steel skin and the magnet assemblies any dings in the outer skin can cause damage to the magnets within.

Some of the major causes of dings in the drum’s skin are:

bolts, washers etc. entering the system during operation
incorrect lifting of the drums when replacing or installing
pressure applied to the skin during storage

These pictures show how a magnet can be worn away when rubbing on a damaged skin, and the resulting wear on the magnets (click on either image for a larger view).

The damaged skin of a separator drum. This can be caused by bolts or washers entering the separator or by careless lifting storage or handling of the drum.

Separator magnet worn away by rubbing on a damaged skin.

PROBLEMS THAT MAY AFFECT THE EFFICIENT OPERATION OF YOUR SEPARATOR

Magnetite leaking internally is a problem that will reduce efficiency. Internal leaking means the magnetite is able to enter the inside of the drum through the shaft seal, damaged outer skin or via the seal between the end cap and outer skin.

Magnetite laying on the magnet assembly short-circuits and absorbs the magnetic flux resulting in reduced effectiveness.

This photo shows a large build-up of magnetite on the drum. Magnetite laying on the magnet assembly short circuits and absorbs the magnetic flux, resulting in less magnetic influence on the outside of the skin.

The incorrect installation of magnets within the individual assemblies will also reduce the drum's efficiency. As previously shown the magnets are stacked in brick formation. It is vitally important that the magnets should be facing the same direction.

Arrangement of magnets in a drum separator. Shown left is correct with all magnets aligned. On right is an incorrect arrangement which results in greatly reduced effectiveness

The diagram above shows this (correct arrangement on left). If some are stacked in reverse as shown on the right then their effectiveness will be greatly reduced.

Excessive heat directed onto magnetic material will decrease its magnetic properties. Heat from an oxy torch being used when the drums are being refurbished will decrease the magnetic properties of the magnets in the drums.

For further maintenance information and details on correct settings for your drum separators consult your manufactures handbook.

CONCLUSION

In conclusion it is of great importance that care and maintenance of your drum separators be of a high standard. Any defect will influence your magnetite recovery.