Master Magnets are a major global manufacturer of magnetic separators and metal detectors to the minerals and mining industry for the past 40 years. They are based in the United Kingdom at their manufacturing centre in Redditch, Worcestershire but have representation worldwide – supported by their recent merger with Bunting Magnetics of the USA. The manufacturing centre has a fully equipped mineral processing/magnetic separation laboratory for client testing and sample evaluation. This article investigates the use of high intensity magnetic separators in the minerals sector – specifically addressing units that process dry materials in the -15mm, +45 micron size range and the principles behind their design.
Principles of Magnetic Separation
There are 3 basic types of magnetism, which are utilised in industrial magnetic separation technology:
- Paramagnetic. Minerals that are only slightly affected by an applied magnetic field. They move towards concentration in lines of magnetic flux (e.g. hematite, ilmenite, and chromite).
- Ferromagnetism. Minerals capable of achieving a high degree of magnetic alignment (e.g. magnetite).
- A mineral (e.g. silica) that is very weakly repelled by the pole of a strong magnet. When a magnetic field is applied, a diamagnetic mineral will develop a magnetic moment through induction but in the opposite direction and is therefore repelled.
Magnetic separation of mineral deposits is based on a three-way competition between magnetic forces, other external forces such as gravitational or inertial forces, and inter-particle attractive and repulsive forces. The combination of these forces determines the outcome of any given magnetic separation and is much affected by the nature of the feed such as size distribution, magnetic susceptibility and other physical and chemical characteristics.
The control variables to optimise separation efficiency can be summarised as follows:
- Volume of Particle (V) (function of feed particle size distribution).
- Magnetic Susceptibility of Particle (K) (a function of the mineral type and liberation).
Machine Operation Variables
- Magnetic Field Strength of Separator (H) ( Machine Design Parameter)
- Magnetic Field Gradient of Separator (H/R) ( Machine Design Parameter)
The magnetic force F(m) generated on a paramagnetic particle in a magnetic separator is given by:
F(m) = V.K.H.H/R
For a successful separation, the magnetic force F(m) must be able to move the paramagnetic mineral from its natural path immediately after leaving a separator, or lift it from a belt (overcoming gravity) in the case of a Disc type magnetic separator.
These principles are used in the design and application of 3 types of magnetic separator used for the dry processing of mineral deposits.
Permanent High Intensity Magnetic Roll Separator
The Master Magnets’ Rare Earth MASTEROLL incorporates Neodymium Iron Boron permanent magnets, which are the most powerful magnets available and are built into a composite high-intensity magnetic head pulley.
High separation efficiencies are generated by the design of the magnetic roll assembly utilising high grade neodymium magnets and the optimum pole spacing to generate high magnetic field strengths and magnetic field gradients to maximise the magnetic force exerted on a paramagnetic particle as it passes over the roll.
Fig 1: Principle of operation of MASTEROLL Magnetic Separator.
As non-magnetic material is discharged forward of the roll in the natural trajectory, any magnetic particles present are influenced by the magnetic force generated by the roll and are discharged down a rear chute as the belt leaves the magnetic field on the underside of the roll. Separation trajectories are set by adjusting the conveyor speed using the inverter control setting on the control panel and adjusting the splitter chutes.
Magnetic rolls are available in 75mm, 150mm and 200mm diameters, up to a width of 1 metre. (Fig 2 shows double roll industrial unit). Multiple configurations of rolls are offered giving the non-magnetic fraction a further pass for improved product purity. The unit can process a wide size range of material ranging from 75 microns up to 15mm. Although, as with all physical separation processes, the narrower the size range the more efficient the separation will be.
Typical Applications include the removal of iron mineral contamination from silica sands, feldspar and other industrial minerals. Processing of granulated slag, ilmenite upgrading, beach sand processing, and recycling applications such as crushed glass. Typical capacities range from 2-4 tph depending on the specific application.
Fig 2: Industrial Scale (1 metre width) MASTEROLL Magnetic Separator (double pass unit)
Induced Roll Separator (IRS)
The Master Magnets Induced Roll Separator is used for the continuous extraction of small paramagnetic particles from material to produce mineral purification for a wide range of mineral and ceramic processing industries. The principal of operation of this unit can be seen in Fig 3.
Fig 3: Principle of operation of Induced Roll Separator
A splitter plate is interposed between the two product streams. The machine can also be set up to give a middlings stream by the addition of a second splitter plate. Two rolls can be mounted in series on the same unit to give a double pass machine for improved efficiency and process performance.
Fig 4: Induced Roll Separator
The roll also tends to generate very little static charge on the surface which means that there is minimal carryover of fine particles into the magnetics fraction – giving good grades and recoveries of valuable mineral.
Typical capacities for a metre wide unit vary based on mineral type, density and particle size distribution and are ideally determined by laboratory test trials:
Some examples are given below:
Concentration of ilmenite sands: 4tph
Chromite concentration: 3-5tph
Silica sand upgrading: 2-3tph
Disc Magnetic Separator
The Disc Separator has a very lengthy history, with its original designs dating back to the early 1900s. Although manufacturing techniques have significantly changed and more advanced machines have now been incorporated, the basic design still remains virtually the same.
Typically, a Disc Separator will feature up to three high-intensity electromagnetic discs, each set at a different height from a feed conveyor. The first disc will be set the furthest from the feed material, in order to extract only the most magnetically susceptible particles. The second and third discs are set at lower gaps, increasing the magnetic force at each disc and therefore separating different grades of magnetic material. Magnetic intensity can also be further adjusted by varying the current of each coil to suit each client’s specific mineral separation requirements.
Feed material is discharged from a hopper onto a vibratory feeder tray. A mono layer of material is continuously fed between the rotating high-intensity magnetic discs, where magnetic particles are attracted to the high-gradient zones on the discs. These captured particles are then carried by the rotating discs to the discharge chutes where they are released. Scrapers that are mounted on each of the chutes ensure the total discharge of the extracted magnetic particles. Any feed material that is non-magnetic will pass under each of the three discs and discharge at the end of the conveyor (See Fig 5).
Fig 5: Principle of operation of the Master Magnets Disc Separator
Typical applications for the disc magnetic separator include the removal of weakly magnetic minerals from high quality quartz sand, processing of ilmenite beach sands, monazite/zircon separation, garnet concentration and wolframite/ cassiterite separation.
A large number of these units are operating on columbite/ tantalite processing plants on the African continent.
Key process advantages of these units include a high magnetic field strength and gradient with variable magnetic field control at each the edge of disc (giving 6 different magnetic fractions). This gives highly selective mineral separations. It also offers flexible processing for complex mineral mixtures (e.g. rutile, zircon ilmenite, monazite, garnet, silica).
The unit has also variable disc rotation speed and belt speed optimum separation efficiency.
Fig 6: Master Magnets 3 disc magnetic separator
prior to shipment to West Africa.
Client Sample Testing Facility
Master Magnets Ltd has a mineral processing testing laboratory based in the UK with experienced staff to ensure that the most suitable and cost-effective machinery is recommended for each application. Our testing laboratory houses a range of laboratory equipment, representing smaller scaled versions of our industrial product range allowing accurate scale up to industrial capacities. X-Ray Fluorescence and X-Ray Diffraction analysis are available for chemical assay and mineralogical identification to aid the development of a viable process route for each application.
If you require further information regarding Master Magnets range of magnetic separators or wish to have a sample evaluated please contact:
Burnt Meadow Road,
North Moons Moat,
+44 (0)1527- 65858