The magnetic collection of fine particles requires a high-intensity, high-gradient magnetic field. An electromagnetic matrix-type separator, referred to as a magnet filter, has proven to be the most effective method of separation. This type of separator utilizes a high-intensity electromagnet and a flux-converging matrix. The matrix amplifies the magnetic field and provides high-gradient collection sites for the magnetic contaminants.
A line of permanent magnetic filters is also available.
The electromagnet filter consists of a solenoid coil encased in steel. The coil generates a uniform magnetic field throughout the bore. A stack of expanded metal discs (termed matrix) is packed in the bore of the coil and is induced by the magnetic field. A typical matrix will consist of 20 to 40 medium-grid (1/4 inch [6mm), 18 gauge) expanded metal discs reaching the height of 6 inches (152 mm). The matrix amplifies the background magnetic field, produces localized regions of extremely high gradient, and provides the collection sites for paramagnetic-particle capture. As feed material filters through the matrix, the paramagnetic particles are captured and consequently removed from the particle stream.
The filters are rated by the magnetic field strength generated in the bore of the solenoid coil with the matrix removed. This background magnetic field represents the driving force that produces the amplified high gradient throughout the matrix. Dependent on the matrix configuration, it is typically the case that a 2500-gauss background field will result in an excess of 10,000 gauss in localized regions of the matrix. Filters are available with background magnetic fields ranging up to 10,000 gauss.
The permanent magnetic filter is constructed with conventional-ferrite magnetic material. These magnets are attached to the filter housing requiring removal of the matrix for cleaning. A permanent magnetic filter is rated by its magnetic field strength in the open-air gap (between the poles). The field strength of these filters measures 1,600 gauss.
The required background magnetic field for effective particle collection is typically determined through an identification of the magnetic contaminant or by quantitative testing. Some general guidelines for magnetic field requirements are as follows:
Relatively coarse (+50 micron) ferromagnetic iron of abrasion in a medium not exceeding 1000 centipoise.
Fine (-50 micron) ferromagnetic iron of abrasion or scale in a medium not exceeding 5000 centipoise.
Very fine (sub-micron) ferromagnetic iron of abrasion or scale, or paramagnetic contaminants such as iron-bearing minerals or nickel and cobalt compounds.
Fine paramagnetic contaminants. Often used to provide maximum product brightness or where product specifications call for ppm or ppb contaminant levels.
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