Working with chemicals – be they fine or coarse, in small or bulk amounts – demands precision and accuracy of scale in order to achieve the best efficiencies for the process. In this article, Dan Norrgran discusses the use of magnetic filters when working with chemicals, especially in the production of ceramic and metallurgical components.

Introduction
The advancements in magnetic separation technology have resulted in new types of magnetic separators specifically designed for treating fine materials. These new separators are termed “magnetic filters” and are effective in treating fine materialsin either a wet or dry process stream. As a result new opportunities for magnetic separation have evolved in the production of high purity feedstocks. Typical applications are the magnetic cleaning of raw materials used in the manufacture of ceramic, glass, chemical, pharmaceutical and metallurgical components to name but a few.

The magnetic force produced by a magnetic separator is equally proportional to two first order variables. One variable is the magnetic field strength generated in the separation zone. The magnetic field strength refers to the flux lines generated by the magnet. The common unit of magnetic field strength is gauss and refers to the number of magnetic flux lines passing through a square centimetre. The more concentrated the lines of magnetic flux, the higher the magnetic field strength. High intensity magnetic separators typically operate in regions over
5,000 gauss.

The other variable is the magnetic field gradient. The magnetic field gradient refers,

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Schematic diagram of a magnetic filter. This unit is for treating a slurry of fine particles.

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Magnetic filter matrix. This particular matrix consists of 42 fine expanded metal discs.

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Dry vibrating magnetic filter. This separator is providing a final cleaning stage on
specialty glass batch materials delivered in “super-sacks”.

to the rate of change or the convergence of the magnetic field throughout the separation region. The effective design of any magnetic separator maximises these two variables.

A magnetic filter consists of an electromagnetic solenoid coil encased in steel. The electromagnetic coil generates a uniform magnetic field throughout the bore.

Precise magnetic circuit modelling and optimisation is now carried out on the computer using sophisticated multidimensional finite element analysis. The input to the computer is a scale design of the magnetic circuit; the output is a contour,

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Application of magnetic field strength in magnetic filters

plot of the generated magnetic field with the associated magnetic field intensity and the magnetic field gradient. Any change in the design input is immediately reflected by changes in the contour plot of the magnetic field. It is these “what if” changes that provide the design criteria for specific applications (such as magnetic field strength and operating temperature. This technique can be applied to the design of both permanent and electromagnetic circuits. As a consequence, essentially any type of magnetic separator can be developed (or redesigned) with a very high level of confidence and predictability.

Magnetic filter
The magnetic filter uses a high-intensity electromagnet and a flux converging matrix to produce a magnetic force that collects ultrafine paramagnetic particles from a process stream. A magnetic filter consists of an electromagnetic solenoid coil encased in steel. The electromagnetic coil generates a uniform magnetic field throughout the bore. Discs of expanded etal (termed matrix) are stacked in the bore of the coil and are induced by the magnetic field. The matrix produces localised regions of extremely high gradients and provides the collection sites for magnetic particle capture. As feed material filters through the matrix, the magnetic particles are captured and consequently removed from the particle stream.

When the magnetic contaminants eventually build up on the matrix,the separator is de energised and the matrix is flushed clean. A schematic of a magnetic filter. A typical matrix of 400 series stainless steel, representing the magnetic filter medium,

Typical applications [of magnetic separators] are the magnetic cleaning of raw materials used in the manufacture of ceramic, glass, chemical, pharmaceutical and metallurgical components.

The separator can either be operated 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. This fluidises the fine material as it flows through the matrix.

Wet magnetic filter
Magnetic filters are available in a wide range or bore diameters and magnetic field,

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General characteristics for magnetic filters treating high purity feedstocks

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Eriez’s dry vibrating magnetic filter

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. Flow rates in the range of 10 to 50 GPM require bore diameters of 4 to 9 inches. A bore diameter of 24 inches is capable of treating several hundred GPM. The required background magnetic field is typically determined through identification of the magnetic material or by quantitative testing. Some general guidelines for magnetic field requirements are provided in Table 1.

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. A magnetic filter installed to treat a ceramic slip used for substrate manufacturing contain ppm iron contamination requires matrix flushing every eight hours. The cycling of the electromagnet and the flushing of the matrix can be automated.

Dry magnetic filter
The magnetic treatment of dry fine particles has typically been problematic due to the difficult material handling aspects. Fine particles react to electrostatic forces and other adhesion forces resulting in a deterioration of the separation. A magnetic filter to treat a very fine dry particle stream has now been developed. A high-frequency low-amplitude vibration is imparted on the matrix, which fluidises the fine powders resulting in a high-capacity flow through the matrix. Dry 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 criterion for the necessary magnetic field strength parallels that of the wet filter. Duty cycles again are typically determined by identifying the amount of magnetic material contained in the filter feed. A dry vibrating magnetic filter treating glass batch materials Particle size, shape, and density are all major factors affecting throughput capacity on the dry vibrating magnetic filter. Examples of capacity in the dry vibrating magnetic filter are provided in Table 2.

Summary
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. A list of the general characteristics required for the magnetic filters is provided in Table 3.

New opportunities for magnetic separation have evolved. Magnetic separation is now a
viable option in areas that have not been previously considered.

There have been significant advancements in the design and application of magnetic separators in the recent past. New magnet materials and circuit designs have allowed for the manufacture of separators that operate at substantially higher field strengths. As a result, new opportunities for magnetic separation have evolved. Magnetic separation is now a viable option in areas that have not been previously considered. These separators are essential in view of new advances in minerals and materials specifications. Increasingly, there is a demand for higher-purity mineral and chemical products and feedstocks.

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