Magnetic separators are used across a range of industrial sectors. For example, in waste processing, they help recover ferrous metals from other waste materials such as plastics, demolition waste and wood, by attracting them through magnetism. In food processing, they can remove ferrous and non-ferrous metals introduced with the raw material or following process equipment wear or damage. Other industrial sectors where this technology can be used include aggregates and quarries, ceramics, glass, mining, mineral processing and plastics.
SEPARATOR DESIGNS
There are an extensive range of different magnetic separator designs, according to equipment designer and manufacturer Bunting. There are essentially three types: permanent magnetic separators, electromagnetic separators and eddy current separators.
The first sort use permanent magnets to “attract, hold, deflect and separate ferrous and weakly magnetic particles,” a Bunting spokesperson explains. Those magnets can be standard strength ferrite or extra-strong neodymium rare earth.
Examples of smaller magnetic separators provided by Bunting include drawer filter magnets, bullet magnets, in-line magnets, liquid pipeline separators, magnetic liquid filters, plate magnets and tubes and grid magnets. The liquid pipeline magnetic separator, for example, extracts iron fines and other magnetically-susceptible materials from wet or viscous products conveyed in a pipeline. Additionally, special welds are available for corrosive applications, such as fruit juices (www.is.gd/cilaje).
Larger magnetic separators provided by Bunting include drum magnets, permanent overband magnets, stainless steel separators, pulley magnets, rare earth roll separators, suspension permanent magnets and wet drum magnetic separators. The pulley magnet, for example, replaces the head pulley of a conveyor. The magnetic field covers the entire 360°-circumference of the pulley, ensuring that conveyed material is fully exposed to the magnetic extraction force at all times. It is also commonly used as a second-stage metal separator (www.is.gd/micilu).
In contrast, electromagnetic separators only attract particles when powered. Then, “electromagnetic coils generate strong and deep magnetic fields to separate either larger or weakly magnetic particles,” the Bunting spokesperson says. Examples include electro drum magnets, electromagnetic filters, permanent overband magnets, induced roll separators, magnetic disc separators and suspension electro magnets. Overband magnets, for example, are suspended over conveyors to separate tramp ferrous metal.
The final type of separator uses eddy current generated by a changing magnetic field. In this case, “permanent magnets are used in a specially-configured, high-speed, high-strength magnetic rotor to repel and thus separate non-ferrous metals,” the spokesman says. This particular unit is said to be used widely in the recycling industry. Designs include both concentric and eccentric rotor designs (www.is.gd/cezobu).
Both permanent magnetic separators and electromagnetic separators are designed and manufactured to meet each customers’ specific separation requirements, says supplier Eriez-Europe, which has a manufacturing facility in South Wales (www.is.gd/ediduk).
Among its range of products are the CP (centre-pole) or TP (twin-pole) 20/120 SC2 HYD (permanent overband), which are said to typically be used in aggregate and quarrying applications, as they can be fixed on to a track-mounted mobile crusher for the removal of rebar from crushed concrete. For recycling applications, the Eriez Eddy Current Separator can be used to remove non-ferrous contaminants from a range of materials; for example, its RevX-E ST22-80 for removal of aluminum and copper from glass, plastic, wood and bottom-ash products.
Other products include the HI1000-65 magnetic filter, which is used for the purification of non-metallic minerals, such as silica sands and quartz sands, as well as the RE (rare earth) roll separator, model RE300-60-3V, most used in the ceramics and minerals industry as a purification process for feldspar.
HOW TO SPECIFY
According to Bunting, choosing the most suitable design of magnetic separator is governed by four objectives. The first is the separation objective: to remove or recover. The second step concerns the type of ferrous metal, including its size and shape. The third step considers the process involved, for example, liquid or solid state, conveyor or pipelines. The fourth step covers the installation process, including available space, as well as installation and maintenance requirements.
“These criteria will provide the data to select a suitable magnetic separator based upon magnetic strength at the point of separation,” the Bunting spokesperson explains. “For example, for an overband magnet suspended above a conveyor, the point of separation is in the conveyed material which will be at least 250-300mm away from the face of the magnet where there is the highest magnetic intensity. Therefore, a very deep magnetic field is required. However, for a simple tube magnet inside a magnetic liquid filter, the aim is to pass the liquid and the iron contamination over or at least very close to the surface of the magnet, where it is attracted and held. Therefore, this has a very strong but shallow magnetic field.”
As for servicing, testing and maintenance, both Bunting and Eriez agree that this varies by separator type. Eriez, for example, manufactures new rotors, and services and repairs older rotors for its eddy current separators. It also offers aftersales services, such as calibration and certification, as well as maintenance contracts. “Maintenance requirements depend on how arduous the application is,” explains the Eriez spokesperson. “Typically, any wear items, particularly in demanding recycling applications, like belts, rotors, bearings and drives, will require continuous monitoring. Eriez recommends that customers hold these items as strategic spares on site, should any parts need urgently replacing.” (Eriez-Europe does the same).
Meanwhile, Bunting often refurbishes old larger magnetic separators (especially electromagnets) and has a travelling service team to support customers at site. “For eddy current separators, we offer a health check,” the spokesperson adds, which sees adjustments and repairs made to keep equipment running smoothly. This inspection process often takes place during the annual plant maintenance shutdown, or when the plant is non-operational (www.is.gd/xakilu). Meanwhile, permanent magnetic separators, such as the ones used in the food industry, need checking every 12 months (see box).
BOX: CASE STUDY: CARRYING OUT A SEPARATOR AUDIT
Bunting engineers conducted a magnetic separator audit at the Jordans Dorset Ryvita plant in Poole, Dorset, in June, over a three days. The company had not experienced any problems due to metal contamination, but requested the audit as part of their continued improvement plan. The first day was spent assessing the location and recording the customer identity numbers of all the magnetic separators; 52 were identified.
The physical checks of the magnetic separators were conducted on day two and three, on the weekend to minimise any production downtime. Each separator was removed from its location and visually inspected. This includes checking welds, and assessing any wear and damage to the surface.
Stage two involved testing the strength of the separator by placing a magnetic ball or plate into the magnetic field and then measuring the force (in kgs) needed to remove that object from the surface. The test is repeated three times and an average reading taken. Similar techniques are used to test other designs of magnetic separators such as plate magnets. Reading the magnetic flux density, measured in Gauss, was avoided because of the difficulty of the measurement.
Results, and recommendations, were presented in a detailed report. “Having an external review of the magnetic separators really helps the client,” contends Adrian Coleman, Bunting’s general manager at the Redditch plant. “We conducted the audit without any assumptions and this freedom enabled us to highlight the areas where protection was good and also focus attention where they could be improvements.”
Following the audit, the engineering team at Jordans Dorset Ryvita have made several changes to improve metal removal during production.