A magnetic separator, which creates a powerful magnetic flux, is used to protect vital process equipment downstream from damage while it produces a clean, separated final product. Vital process equipment may include primary jaw crushers, cone crushers, hammer mills and other tertiary crushers. At the same time, it can effectively prevent long sharp metal shards from cutting, ripping or tearing conveyor belts.
APPLICATION IN THE INDUSTRY
The type and style of magnets used in the industry will depend on their location within the process. Larger materials and deeper burden depth require larger magnets to be operational. Magnets that are suspended over the belt are typically designed to remove large ferrous material such as hand-held tools,iron scrap and machinery tips, but can also remove nails, wire, nuts and bolts from the process. Rare earth magnets are employed to remove fine metals such as iron flakes, pinsand bits of wire. These rare earth magnets have now been improved to the point that they even remove materials such as stainless steel which are only weakly magnetic.
To eliminate the possibility of downstream equipment damage, it is critical to remove ferrous impurities as early as possible. Therefore, magnetic separators should be installed at the beginning of the conveyor transport system. Magnetic separators can typically be designed to retrofit on any existing conveyor system, without the necessity of relocating major equipment. Magnetic separators are also used in combination with metal detectors to offer complete peace of mind. This will ensure that any metal material embedded or trapped in or under the aggregate that may escape the magnetic separator does not pass the metal detector. Once a metal detector is tripped, the conveyor will stop and human intervention is required. This will ensure no metal components end up in the process that will cause costly problems and downtime.
THE ORIGIN OF TRAMP METAL
Operators of plant often wonder about the origin of tramp metal. To fully explain the origin, we must begin with the ore mining and quarry process. Ore can be mined from open cut and underground mines. In order to gain access to these valuable minerals either excavation or explosives are required. Parts can often be left behind, such as metal tool tips from jack hammers, heavy duty excavation bucket tips and drill parts. This tramp iron can escape and is difficult to be removed during the mining phase.
Another example includes concrete recycling companies that collect used concrete from demolished buildings. The used concrete is crushed into small pieces to be reused to make new concrete. Magnetic separators are used to remove metal reinforcing structures found within the used concrete.
These ferrous impurities not only affect production rate, but also product purity. Therefore, the use of magnetic separators becomes critical.
TYPES OF MAGNETS USED FOR PROTECTION OF CONVEYORS
In recent years, in order to satisfy the ever changing needs and requirements of the mining industry, new and improved magnetic separators had to be developed. Depending on the type of mineral being mined, the size of tramp irons occurring will vary. Hence, magnetic separators require varying degrees of magnetic flux power. These are separated into electromagnets and permanent magnets.
These powerful electromagnets require rectifiers, which are more commonly used insuspended applications. Suspended magnetsneed an overhead supporting structure to fix the magnet above a belt conveyor.
A rectifier changes alternating current (AC) to a direct current (DC). Generally, electromagnets require DC current in order to generate a magnetic field. These electromagnets contain two coils, as power is applied to the coils, this creates a powerful magnetic field which can now be used to remove tramp iron from conveyor belts.
Electromagnets have the ability to remove large pieces of ferrous material from conveyor belts. If a permanent magnet were used, removal of this would be very difficult. But with electromagnets, removal of captured tramp metal is as simple as turning off the power source.
A permanent magnet is made from a materialthat is magnetised and creates its own persistent magnetic field. The materials thatcan be magnetised (that is, are strongly attracted to a magnet) are called ferromagnetic. Ferromagnetic materials include iron, nickel, cobalt, some rare earth metals and some of their alloys, as well as some naturally occurring minerals such as lodestone.
Permanent magnetic separators are used on conveyor belts where only small levels of tramp ferrous material are expected. The ferrous material is captured by the magnet. Plant operators will monitor the amount of ferrous metals captured. Once the magnet requires cleaning, a non-metallic stainless steel tray scraper is used to remove the ferrous metals off the magnet.
These magnets are based on the existing and improved technology of permanent magnets and electromagnets.
Self-cleaning magnetic separators are employed to automatically remove and separate metallic and non-metallic materials (in most cases, the product). This is only used if we expect large volumes of metallic materials to require separation.
Self-cleaning magnetic separators include motors to drive belts, a speed reducer and a non-metallic sweeper belt. These can be mounted either in line with the conveyor belt or in cross-belt configuration.
In the past 20 years there has not been a lot of research in the magnetic separator area, but most industries have increased their standards of waste ferrous material separation from the end product.
The Chinese magnet industry now contributes to the world market of permanent magnets and electromagnets. High quality sintered ferrites and rare earth permanent magnets are available at reasonable prices. An increasing number of overseas magnet users turn their purchase orders to Chinese permanent/electromagnet manufacturers. This sea change has resulted in high quality magnets with drastic reduction of operation and ongoing cost. Nevertheless, due to increased demand in the global market, long lead times and quality consistency in some instances become an issue.
In a competitive society, cost and quality become very important issues. There are afew methods to keep costs low. One is fromthe design perspective to select the specificgrade magnets for the appropriate protectionapplication. Another is from the manufacturing perspective, to limit the unnecessary manufac-turing process of magnets.
Technology advances in material and manufacturing enables us to build magnetic separators that are tailored to a specific application. The magnets are built to specifications with the right strength for the application. If the magnets have been correctly engineered, these powerful separators can remove almost any ferrous material that dares to pass through their magnetic fields.
As technology and research advances, conveyor belts become wider (>2400 mm) and operate at much faster speeds (>5.7 m/s). In turn, the belts are required to handle at a much greater capacity (7200 tph). This puts more stress on magnetic separators to keep up with the increased trends in width, speed and overall bulk material handling capacity. The future of magnetic separator design shall have the following criteria:
? Power saving features, with advanced rectifiers.
? Cooling of electromagnets.
? Automatic control of rectifier.
? Development of an ultra high magnetic field, able to have increased effective height.
? Increased purification ability.
THE TRUE VALUE OF MAGNETIC SEPARATOR PROTECTION
With the use of magnetic separators there are many areas where cost saving can be realised. These will include reduction of equipment maintenance and repair. Equipment will be able to run for much longer periods of time with minimal downtime and loss of production hours. Another obvious point would be the reduction of labour costs to maintain and repair equipment.
In order to get the full benefits from magnetic separators, the plant supervisor will authorise a complete survey of the plant by a magnetic separation specialist who has extensive knowledge of magnetic separation. This will allow the magnetic suppliers to determine the practical point where magnetic separators can be installed with minimal downtime, cost and ensure maximum protection of downstream equipment.
Peter Wei is a sales engineer for Kinder & Company Pty Ltd.