New developments in magnetic separation technology offer highly effective and efficient methods for magnetically cleaning pharmaceutical feedstocks. Today’s innovative magnet materials and circuit design methodology enable the production of separators that operate at substantially higher field strengths than ever before. Accordingly, many new techniques for cleaning and purifying of pharmaceutical materials through magnetic separation have evolved. With all of these new opportunities and a demand for increased purity, it is vital for pharmaceutical manufacturers to fully understand the technology and realize key considerations when looking to install such systems in their facilities.
Particle CharacteristicsWhen exposed to a magnetic field, particles will exhibit a specific response that classifies them in one of three groups: ferromagnetic, paramagnetic, or diamagnetic.
Ferromagnetic particles have a very high magnetic susceptibility and are strongly induced by a magnetic field. Paramagnetic minerals have a low magnetic susceptibility and a weak response to a magnetic field. Lastly, minerals with a negative magnetic susceptibility are termed diamagnetic and, for all practical purposes, are considered nonmagnetic.
Ferromagnetic, and to a lesser degree, paramagnetic materials, will become magnetized when placed in a magnetic field. The amount of magnetization induced on the particle relies on the mass and magnetic susceptibility of the particle and the intensity of the applied magnetic field. This is expressed as:
Where is the induced magnetization of the particle, is the mass of the particle, is the specific magnetic susceptibility, and is the magnetic field intensity.
Separator CharacteristicsMagnetic field intensity and the magnetic field gradient are key variables that affect separation response. High intensity magnetic separators usually operate in regions over 5,000 gauss or 0.5 Tesla. Low intensity separators usually generate a magnetic field strength of less than 2,000 gauss or 0.2 Tesla.
Figure 1 illustrates two different magnetic field configurations. Case A has a very consistent pattern of flux lines without gradation. Therefore, a magnetic particle entering this field will be attracted to the lines of flux and remain stationary without migrating to either pole piece.
Case B illustrates a converging pattern of flux lines displaying a high gradient. As these lines pass through a smaller area, there is a significant increase in the magnetic field intensity. A magnetic particle entering this field configuration will not only be attracted to the lines of flux, but will also migrate to the region of highest flux density.
From the earlier equation for magnetization, the magnetic attractive force acting on a particle is the product of the particle magnetization and the magnetic field gradient and can be expressed as:
Where is the magnetic attractive force, and is the magnetic field gradient. Maximum magnetic force results only when both the magnetic field intensity and field gradient are maximized.
There are two commonly used methods for producing a magnetic gradient in a magnetic separator. The first, which is typical of magnetic circuits utilizing permanent magnets, is to concentrate the lines of flux on a steel pole piece within the circuit. This can be accomplished easily 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 second involves positioning a steel matrix, such as a metal mesh, directly in a uniform magnetic field that is generated by an electromagnetic solenoid coil. Consequently, this matrix amplifies the magnetic field and converges the lines of flux to produce localized regions of extremely high magnetic field intensity.
Magnetic Field GenerationAll magnetic separators utilize either permanent magnets or an electromagnet to generate the magnetic field.Permanent Magnets There are two distinctive types of permanent magnets. A “ferrite” magnet is used in low-intensity magnetic separators. These 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 this type of magnet allows for the design of high-intensity magnetic circuits that operate energy free. Rare Earth magnets are used in various types of magnetic separators and are effective for collecting paramagnetic particles. Dependent on the magnetic circuit, these separators generate a magnetic field strength ranging up to 24,000 gauss (2.4 Tesla).
ElectromagnetsElectromagnetic separators are typically designed utilizing a solenoid electromagnetic coil. Some separators use the bore of the solenoid coil as the separating zone. Other separators use the solenoid coil to convey the magnetic flux through a steel circuit or a “C” frame circuit. The magnetic field in the gap, either the bore of the solenoid or the gap of the C frame, is the separation zone in a magnetic separator. Most electromagnetic separators operate up to approximately 20,000 gauss (2 Tesla).
Stationary Rare Earth Permanent Magnetic SeparatorsStationary permanent magnetic separators, in particular, plates, grates and traps, are often used to collect ferromagnetic iron particles and ensure product quality. (Stationary permanent magnetic separators are illustrated in Figure 2.)
Plates, grates and traps are simply 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.
Stationary permanent magnets have a low capital cost and no operating cost. There are no moving parts, which virtually eliminates maintenance costs. These separators are manual clean and are best suited for applications where only a trace amount of ferrous material is present.
Magnetic FilterThe 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.A magnetic filter consists of a solenoid coil encased in steel. The coil generates a uniform magnetic field throughout the bore. Discs of expanded metal (termed matrix) are stacked in the bore of the coil and are induced by the magnetic field. 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 consequently removed from the particle stream. When the magnetic contaminants eventually buildup on the matrix, the separator is deenergized and the matrix is flushed clean. (A schematic of a magnetic filter is shown in Figure 4.)
The separator can be operated when wet treating a slurry or dry treating 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, fluidizing the fine material as it flows through the matrix.
Wet Magnetic FilterMagnetic filters are available in a wide range or 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 range from 1,500 gauss to collect ferromagnetic iron of abrasion to 20,000 gauss to collect fine paramagnetic contaminants where product specifications call for ppm or ppb contaminant levels. Duty cycles, the operating time of the magnetic between matrix flushing cycles, are typically determined by identifying the amount of magnetic material contained in the filter feed. Materials containing up to 1 percent magnetic material will require frequent matrix flushing corresponding to duty cycles of 10 to 30 minutes.
Dry Magnetic FilterOperation of high-intensity magnetic separators for dry applications, including the Rare Earth Drum and Rare Earth Roll, always balances the magnetic attractive force with a counter-acting force. With these types of separators, separation efficiency decreases as the particle size decreases. Finer particles react to electrostatic forces and other adhesion forces, resulting in a deterioration of the separation. When there is no longer a balance between the magnetic attractive force and the counteracting forces, a separation based on magnetic susceptibility is possible.
A high-frequency low-amplitude vibration is imparted on the matrix, which fluidizes the fine powders resulting in a high-capacity flow through the matrix. Dry filters are available in a wide range or bore diameters and magnetic field strengths to correspond with the production capacity and the desired level of magnetic collection. (A dry vibrating magnetic filter treating glass batch materials is shown in Figure 6.)
Particle size, shape, and density are all major factors affecting throughput capacity on the dry vibrating magnetic filter.
ConclusionRecent advances in magnetic separation technology have resulted in a variety of separators specifically developed for the treatment of fine, high-purity materials. The ongoing development of rare earth permanent magnets and the sophistication of electromagnetic circuit design is credited for the evolution of magnetic separators.
It is difficult at best to predict the separation response of finely sized particles to magnetic separation. Theoretical determinations balancing particle size to the magnetic force is of little practical value below a particle size of 50 to 75 microns. The natural variability of most materials, and specifically the characteristics of ferrous contaminants, often necessitates laboratory or pilot scale magnetic separation testing to determine capacity and quantify separation efficiency.
(General specifications of the various magnetic separators are provided in Table 2.)