Cleaning Ceramic Feedstocks with Magnets
Particle and Separator CharacteristicsWhen subjected to a magnetic field, all particles respond in a particular manner classified as one of three groups: ferromagnetic, paramagnetic or diamagnetic. Particles that have a very high magnetic susceptibility and are strongly induced by a magnetic field are ferromagnetic. Particles that have a low magnetic susceptibility and a weak response to a magnetic field are termed paramagnetic, while particles with a negative magnetic susceptibility (i.e., non-magnetic) are termed diamagnetic.
In the design of a magnetic separator, the magnetic field intensity and magnetic field gradient are the two primary variables that affect the separation response. Separators generating a magnetic field strength of less than 2000 gauss or 0.2 Tesla, which is the practical limitation of conventional permanent ferrite magnets, are generally referred to as low-intensity magnetic separators. High-intensity separators typically operate in regions over 5000 gauss or 0.5 Tesla. The magnetic field gradient refers to the rate of change or the convergence of the magnetic field strength—the higher the rate of change or convergence, the stronger the magnetic attractive force.
The most common method for producing a magnetic gradient in a magnetic separator is to concentrate the lines of magnetic flux on a steel pole piece within the circuit. This can be accomplished simply by placing a steel pole piece between two magnets. The magnetic flux will be concentrated in the steel pole piece, resulting in an area of extreme magnetic field intensity.
The magnetic attractive force acting on a particle is proportional to (see Equation):
Separation MethodologyThe magnetic field on any given magnetic separator is generated either by a permanent magnet or an electromagnet. Two distinct types of permanent magnets exist. The first type is a “ferrite” magnet and is used in low-intensity magnetic separators. Ferrite is a very inexpensive material with a moderate energy product ranging up to ~4.5 million gauss-oerstads (MGOe). (The energy product of a permanent magnet is a relative measure of its intrinsic strength.) Ferrite magnets typically generate a magnetic field strength ranging up to 2000 gauss (0.2 Tesla).
The other type of permanent magnet is composed of rare earth elements. The advent of rare earth magnets has allowed for the design of high-intensity magnetic circuits that operate without an external energy source. The energy product of neodymium-based magnets now ranges up to 50 MGOe. Rare earth magnets are used in various types of magnetic separators that are effective for collecting paramagnetic minerals. Depending on the magnetic circuit, these separators generate a magnetic field strength ranging up to 24,000 gauss (2.4 Tesla).
Electromagnetic separators are typically designed using a solenoid electromagnetic coil. Some separators use the bore of the solenoid coil as the separating zone, while others use the solenoid coil to convey the magnetic flux through a steel or “C-frame” circuit. The magnetic field in the gap—the bore of the solenoid or the gap of the C frame—serves as the separation zone in a magnetic separator. Electromagnetic separators typically operate up to approximately 20,000 gauss (2 Tesla).
Types of SeparatorsStationary Rare Earth Permanent Magnetic Separators
Stationary permanent magnetic separators—specifically plates, grates and traps—represent the first industrial application of magnetic separation and are perhaps the most prevalent separators in use today. These separators are composed of rare earth permanent magnets arranged in a circuit and contained in a stainless steel enclosure. The process stream flows over, around or through the permanent magnets, and ferrous material is collected and held in the enclosures.
These separators are typically used to collect ferromagnetic iron particles from ceramic feedstocks to ensure high product quality. They also perform very well in removing fine iron of abrasion eroding from machine components, pipe lines, chutes, bins and process equipment in ceramic processing applications. Two different types of stationary permanent magnetic separators are shown in Figure 1.
The wide use of stationary permanent magnets can be attributed primarily to their low capital cost and virtually nonexistent operating costs. Additionally, their lack of moving parts virtually eliminates maintenance costs. However, these separators must be manually cleaned and are therefore best suited for applications where only a trace amount of ferrous material is present.
Two different types of self-cleaning rare earth magnetic separators provide peak separation efficiency for processing high-purity ceramic feedstocks: the rare earth drum and the rare earth roll. These separators generate high-intensity magnetic fields and rely on centrifugal force to impart the separation. Generally, feed material is introduced to the revolving magnetic rotor, and the nonmagnetic material discharges in a natural trajectory. The magnetic material is attracted to the rotor by the magnetic field and is rotated out of the nonmagnetic particle stream. The continuous feed and self-cleaning operation of these systems virtually eliminate the need for operator attention.
The rare earth drum magnetic separator consists of a stationary, shaft-mounted magnetic circuit completely enclosed by a rotating drum (see Figure 2). The magnetic circuit comprises segments of alternating rare earth magnets and steel pole pieces that span an arc of 120 degrees. The steel poles are induced and project a high-intensity, high-gradient magnetic field. The peak magnetic field intensity on the drum is approximately 7000 gauss and is effective in removing many paramagnetic constituents.
The rare earth drum is effective in treating relatively coarse (>75 micron) material at high capacity and lends itself to severe-duty applications. For example, a 15-in. diameter drum can treat 5 tons per hour (tph)/ft of drum width of -65 mesh +200 mesh zircon. The separator has been successfully used in a number of industrial applications, including treating quartzite, kyanite, feldspar, nepheline syenite and combined glass batch materials, as well as the demanding application of cleaning glass cullet. In most applications, unit capacities range from 3 to 5 tph/ft of drum width on a 15-in. diameter drum and from 5 to 8 tph/ft of drum width on a 24 in. diameter drum.
The rare earth roll (see Figure 3), which generates peak magnetic field strengths in excess of 24,000 gauss, is very effective for removing weak magnetic minerals from a dry process stream. The separator is designed to provide peak separation efficiency and is typically used when a high-purity product is required. The roll is constructed of discs of neodymium-boron-iron permanent magnets alternating with steel pole pieces that are magnetically induced to the magnetic saturation point. Magnetic roll diameters are typically 4 and 6 in.
In the rare earth roll, the circuit is configured as a head pulley in the separator. A thin belt is used to convey the feed material through the magnetic field. When feed material enters the magnetic field, the nonmagnetic particles are discharged from the roll in their natural trajectory. The paramagnetic, or weak magnetic, particles are attracted to the roll and are deflected out of the non-magnetic particle stream. A splitter arrangement is used to segregate the two particle streams.
The magnetic rolls are constructed up to a width of 60 in. Typical feed rates range from 100 to 400 lb/hour/in. of roll width. Sized silica sand or alumina can typically be treated at a rate of 200 to 300 lb/hour/in. of roll width.
The magnetic collection of micron-sized paramagnetic particles requires a high-intensity magnetic field combined with a high magnetic gradient. This can be accomplished with an electromagnetic matrix-type separator, also known as a magnetic filter.
A magnetic filter consists of a solenoid coil encased in steel. The coil generates a uniform magnetic field throughout the bore, which induces discs of expanded metal (the “matrix”) stacked in the bore of the coil. When induced, the matrix produces localized regions of extremely high gradients and provides the collection sites for paramagnetic particle capture. As feed material filters through the matrix, the paramagnetic particles are captured and subsequently removed from the particle stream. When the magnetic contaminants eventually build up on the matrix, the separator is de-energized and the matrix is automatically flushed clean. A schematic of a magnetic filter is shown in Figure 4.
The separator can be operated wet to treat a slurry or dry to treat a fine powder. In the wet mode, the fluid drag provides the separating force between the magnetic contaminants and the nonmagnetic medium. In the dry mode, the matrix is vibrated, and this vibration fluidizes the fine material as it flows through the matrix.
Wet Magnetic Filters. Wet magnetic filters are available in a wide range of bore diameters and magnetic field strengths to correspond with the production capacity and the desired level of magnetic collection. The magnetic field strength of wet magnetic filters ranges from 1500 gauss to collect ferromagnetic iron of abrasion to 20,000 gauss to collect fine paramagnetic contaminants where product specifications call for parts per million (ppm) or parts per billion (ppb) contaminant levels. Flow rates in the range of 10 to 50 gallons per minute (gpm) require bore diameters of 4 to 9 in. A bore diameter of 24 in. is capable of treating several hundred gpm.
The length of the duty cycle—the operating time of the magnet between matrix flushing cycles—is typically determined by identifying the amount of magnetic material contained in the filter feed. Materials containing up to 1% magnetic material will require frequent matrix flushing corresponding to duty cycles of 10 to 30 minutes. One magnetic filter installed to treat a ceramic slip used for substrate manufacturing containing ppm iron contamination requires matrix flushing every eight hours. The cycling of the electromagnet and the flushing of the matrix can be automated.
Recently, a magnetic filter to treat a very fine dry particle stream was developed. A high-frequency, low-amplitude vibration is imparted on the matrix, which fluidizes the fine powders and results in a high-capacity flow through the matrix.
Dry filters are available in a wide range of bore diameters and magnetic field strengths that correspond with the production capacity and the desired level of magnetic collection. The throughput capacity on a dry vibrating magnetic filter depends on the particle size, shape and density of the material being cleaned (see Table 1). The criterion for the necessary magnetic field strength parallels that of the wet filter, and duty cycles are typically determined by identifying the amount of magnetic material contained in the filter feed. Typical applications of dry magnetic filters are fine silica or quartzite, talc, cerium, zircon flour, alumina and magnesia. A dry vibrating magnetic filter used for treating glass batch materials is shown in Figure 5.