Some of the most expensive failures in crushing and screening plants are caused by uncrushable metal entering the crusher: bucket teeth, drill bits, broken bolts, reinforcing bar, conveyor parts and steel pieces in recycled material. This metal can seriously damage the main shaft and liners of cone crushers, the blow bars and rotor of impact crushers, the rotor tips of VSI crushers and the belts of conveyors. Magnetic separators and metal detectors are protective equipment that prevent this risk at low cost.
In this guide we cover the types of magnetic separator, the working principle of the metal detector, correct layout, selection criteria, and maintenance and testing practice.
Why Is Metal So Dangerous?
- Cone crusher: When an uncrushable piece jams in the crushing chamber, the hydraulic tramp release system operates. Frequent tramp events fatigue the hydraulic system, mantle and concave; large metal pieces can damage the main shaft and head assembly. For details see our cone crusher hydraulic cylinders guide.
- Impact crusher: A metal piece can break blow bars, damage the rotor and overload the aprons (blow bars).
- VSI crusher: Rotor tips and rotor internal plates are badly affected by metal pieces (VSI rotor tips).
- Conveyor belt: Sharp metal pieces can jam at chute points and cause longitudinal rips in the belt (conveyor belt guide).
Types of Magnetic Separator
| Type | How It Works | Suitable Use |
|---|---|---|
| Overband magnetic separator (permanent magnet) | Suspended over the conveyor; lifts iron pieces and discharges them to the side on its own belt | Most crushing plants; low energy consumption |
| Overband electromagnet | Creates the magnetic field electrically; stronger and deeper field | Wide and fast belts, deep material burden |
| Manually cleaned suspended magnet | Collected metal is cleaned off by hand periodically | Small plants with little metal |
| Magnetic head pulley | The conveyor’s drive pulley is magnetic; iron separates at the discharge point | Fine and medium material, recycling |
| Plate and grid magnets | Installed in a chute or channel | Small iron pieces, fine material flows |
Magnetic separators only hold iron-bearing (ferromagnetic) metals. Austenitic materials such as manganese steel bucket teeth and wear parts are only weakly magnetic and may not be reliably removed by a magnet. Metals such as stainless steel, aluminium and copper are not held by a magnet either. A metal detector is therefore needed for full protection.
How Does a Metal Detector Work?
A conveyor metal detector creates an electromagnetic field with coils placed below and above the belt. Any conductive metal passing through this field (iron, manganese steel, stainless steel, aluminium, copper) disturbs the field and the detector signals. With the signal:
- The conveyor is stopped and the position of the metal piece is marked (paint or flag marker), and the operator removes it by hand; or
- A diverter flap sends the metal-bearing material to a separate chute.
The detector’s sensitivity is set according to the smallest metal piece the protected equipment regards as dangerous. Very high sensitivity can cause nuisance stops due to natural minerals in the material (e.g. magnetite-bearing ores) or belt fastener clips. On belts with metal clip splices, the detector must recognise the splice zone or vulcanised splices must be used.
Correct Layout
- Conveyors before the crusher: Protective equipment should be installed on the conveyor feeding the crusher to be protected. The primary crusher product belt, ahead of the secondary cone or impact crusher, is the most critical point.
- Magnet first, then detector: The magnet removes most iron pieces and reduces nuisance stops of the detector; the detector catches the manganese and non-ferrous metals the magnet misses.
- Distance: There must be enough distance between the detector and the belt’s discharge point to stop the belt. This distance is calculated from the belt speed and stopping distance.
- Magnet position: Overband magnets can be mounted across the belt (cross-belt) or in line at the belt discharge point. With in-line mounting at the discharge point, metal separates more easily while the material is in free flight.
- Height: The distance between the magnet and the belt should be set according to the burden depth and the largest particle size. Pull force falls rapidly as distance increases.
- Non-metallic idlers: Metal idlers and steelwork in the detector zone can affect the signal; non-metallic idlers or adequate clearance are needed according to the manufacturer’s instructions.
For the general structure of belt conveyors see our belt conveyor guide.
Selection Criteria
| Criterion | Point to Note |
|---|---|
| Belt width and speed | Magnet size and strength are chosen accordingly |
| Burden depth | A deep burden needs a deep magnetic field |
| Metal type and size | The smallest and largest metal pieces expected |
| Magnetic properties of the material | Detector setting needs special care with magnetite-bearing ores |
| Environment | Dust, moisture, temperature; the magnet’s protection rating |
| Energy | A permanent magnet uses no electricity; an electromagnet is stronger but needs power and cooling |
Importance in Recycling Plants
In construction and demolition waste recycling, there is a great deal of reinforcing bar and other metal in the concrete. In these plants the magnetic separator is not only protective equipment but also a process unit that secures product quality and scrap revenue. A powerful overband magnet is usually used at the impact crusher discharge. For the layout of recycling plants see our construction and demolition waste recycling guide.
Maintenance and Testing
- Magnet belt: The belt, drives, bearings and pulleys of self-cleaning magnets should be checked regularly. If the belt tears, collected metal can fall back onto the main belt.
- Magnetic field check: The strength of permanent magnets can decline over time, through excessive heat or physical damage. Checks should be made with periodic measurements and test pieces.
- Electromagnet: Coil temperature, cooling and the control panel should be checked.
- Detector sensitivity test: Test pieces of a set size should regularly be passed along the belt to check that the detector picks them up, and the results recorded.
- Stop records: The number of detector stops and the types of metal found should be recorded. These records help to work out where in the quarry the metal is coming from.
Common Mistakes
- Relying on the magnet alone and overlooking that manganese steel parts can get through.
- Bypassing the detector because of nuisance stops or reducing its sensitivity too far.
- Mounting the magnet too high above the belt.
- Leaving insufficient stopping distance between the detector and the crusher.
- Not investigating the source of metal in the quarry; not tracking lost bucket teeth.
Field Example
Suppose that at a basalt plant the secondary cone crusher experiences frequent tramp events, and problems soon develop in the main shaft seal and the hydraulic system. Although the plant has an overband magnet, the events continue. Investigation shows that the manganese steel bucket teeth of the excavator working in the quarry are breaking and the magnet cannot hold them. When a metal detector is added to the cone crusher feed belt and excavator tooth checks become part of the daily routine, tramp events fall markedly.
Frequently Asked Questions
Does a magnetic separator hold manganese steel?
Austenitic manganese steel is weakly magnetic and may not be held reliably. A metal detector is needed for such parts.
Permanent magnet or electromagnet?
Permanent-magnet self-cleaning separators are sufficient for most crushing plants. An electromagnet may be preferred on very wide and fast belts or with deep material burdens.
Why does the metal detector stop unnecessarily?
Sensitivity may be too high, the belt may have metal clip splices, the material may contain magnetic minerals or surrounding steelwork may affect the signal.
Do protective devices pay for themselves?
The parts and downtime cost of a single serious crusher failure often exceeds the cost of this equipment.
The Cost of Metal Damage: Which Parts Are at Risk?
| Equipment | Parts at Risk | Guide |
|---|---|---|
| Cone crusher | Mantle, concave, main shaft, hydraulic cylinders, accumulators | Main shaft and head assembly |
| Impact crusher | Blow bars, rotor, aprons, adjustment system | Impact crusher rotor |
| VSI crusher | Rotor tips, rotor plates, distributor plate | VSI rotor plates |
| Conveyor | Belt (longitudinal rip), skirt rubber, scrapers | Skirt rubber |
| Screen | Screen wire and panels | Screen surface selection |
For parts needed after metal damage, you can get a quick quote from our cone crusher spare parts, impact crusher spare parts, VSI crusher spare parts and belt conveyor spare parts pages. If a tramp event has loosened the mantle or concave, re-backing with our own Aurox backing compound restores full support.
Magnetic Separator Spare Parts
- Separator belt: The belt of self-cleaning overband magnets is specially made with cleats to carry the metal.
- Pulleys and bearings: The drive and tail pulleys and their bearings should be checked regularly.
- Drive motor and gearbox: The drive unit that turns the belt.
- Suspension chains and adjustment mechanism: For height adjustment and safe suspension of the magnet.
- Metal detector test pieces: For sensitivity verification.
- Marking system: Consumables for the paint gun or flag marker.
Glossary
- Overband: A magnetic separator suspended over a conveyor belt.
- Tramp metal: Unwanted metal pieces mixed into the material flow.
- Ferromagnetic: Materials strongly attracted by a magnet (iron, most steels).
- Austenitic manganese steel: A weakly magnetic, wear-resistant steel; may not be reliably removed by a magnet.
- Diverter: A flap that sends metal-bearing material to a separate chute.
- Tramp release: The cone crusher opening to let an uncrushable piece pass.
More Questions
How is magnet strength measured?
Measurements are taken at set distances with a magnetic field meter (gaussmeter) and compared with the original commissioning values.
Can a metal detector be installed anywhere along the belt?
No. A zone must be chosen away from metal idlers and steelwork, motors and sources of vibration, in line with the conditions specified by the manufacturer.
Which magnet is preferred in a recycling plant?
Usually a powerful self-cleaning overband magnet; with high iron content, in-line mounting at the discharge point is preferred.
CSP Mühendislik Support
CSP Mühendislik supports crushing and screening plants with planning the layout of magnetic separators and metal detectors, adapting conveyors for this equipment and supplying spare parts. See our belt conveyor spare parts page for conveyor parts, and our cone crusher spare parts and impact crusher spare parts pages for crusher parts.
