• TECHNOLOGY
magnetic field. A stack of expanded metal discs are packed in the bore and induced by the magnetic field. These expanded metal discs, termed the matrix, provide the vehicle for separation. The matrix amplifies the background magnetic field, produces local regions of extremely high gradient and provides the collection sites for magnetic particle capture.
The magnetic force acting to capture a magnetic particle is proportional to the product of the magnetic field intensity and the magnetic field gradient. In equation form: where Fm is the magnetic force acting on a particle, H is the background magnetic field, and dH / dx is the magnetic field gradient( convergence of flux) generated on the matrix. Utilising a relatively high background magnetic field coupled with an extremely high magnetic field gradient results in a high-intensity high-gradient separator( see Schematic 1).
A matrix type separator substantially improves the capture of fine particles. In a matrix, the material must filter through several layers of highly induced magnetic grids, increasing the probability of capturing more contamination when compared to conventional plate, grate, trap or drum-type separators. Even with relatively large matrix spacing, the material is subject to this filtering effect, resulting in the capture of magnetic material.
The dry filters are rated by the magnetic field strength generated in the bore of the solenoid coil with the matrix removed. The background magnetic field, often termed the open bore field, represents the driving force that produces the amplified high magnetic gradient throughout the matrix. Depending on the matrix configuration, it is typically the case that a 5 000 gauss background field will result in an excess of 10 000 gauss in localised regions of the matrix.
The electromagnet is a solenoid coil completely sealed in a steel housing. Standard model filters generate background magnetic field strengths of 5 000 gauss. The solenoid coils are wound copper to dissipate heat and operate at relatively cool temperatures. They are oiled-cooled and utilise a heat exchanger where it is cooled with a 10GPM water flow.
Operating characteristics The background magnetic field is typically determined through an identification of the magnetic material or by quantitative testing. Eriez’ s experience has established some general guidelines for selection of the proper magnetic field selection.( Schematic 1)
The 5 000 gauss unit is perfect for very fine( minus 50 micron to sub-micron) ferromagnetic iron of abrasion, scale or paramagnetic contaminants such as ilmenite or chromite. It is specified when a high-purity product is required and where product specifications call for parts per million( ppm) contaminants levels. Again, the DVMF achieves a parts per billion( ppb) removed performance level.
Duty cycles, the operating time of the magnet between cleaning cycles, are typically determined by identifying the amount of magnetic material contained in the feed product. Materials containing up to 2 % contaminant may require frequent cleaning. In these applications, the duty cycle may approximate 10 to 20 minutes. In this scenario, an automated feeding valve and reject gate are recommended. Treating relatively pure materials, which may have only average ppm levels of contamination, allow relatively long duty cycles which can sometimes exceed an hour. •
References:
1. Thomas G. Goonan, 2012
About Eriez ® Eriez is recognised as a world authority in separation technologies. The company’ s magnetic lift and separation, metal detection, fluid recycling, flotation, materials feeding, screening, conveying and controlling equipment have application in the mining, aggregate, process, metalworking, packaging, plastics, rubber, recycling, food and textile industries. Eriez manufactures and markets these products through 12 international subsidiaries located on six continents. For more information, call visit www. eriez. com.
16 • African Mining • June 2025 www. africanmining. co. za