Industrial Magnet Primer

The magnetic collection of fine ferrous particles requires a high-intensity high-gradient magnetic field. Some basic information about magnets is necessary to fully understand magnetic particle filtration. There are three basic types of magnets used in industrial applications: Alnico, standard ceramic, and rare earth.

Alnico. One of the earliest magnet materials used in the magnetic separator industry, it is commonly recognized by the horseshoe shape often used in the early years. Consisting of aluminum, nickel, cobalt and iron, its hardness makes it difficult to machine or drill. Comparable in strength to ceramic materials, it's used to remove relatively large pieces of ferrous metals, such as nuts, bolts, and staples.

Ceramic. In the late 1960s, the price of cobalt began to rise and manufacturers began looking for a substitute for Alnico. Many manufacturers went to a ceramic-ferrite material in their magnetic separators. These materials became the standard until the early 1980s. Ceramic was easy to work with because it could be easily cut and assembled into a circuit. Its strength in tube magnets is comparable to Alnico castings (1000 to 2000 gauss in a circuit), and its basic limitation was greater reversible loss at high temperatures. Ceramic-type magnet material is best for removing relatively large pieces of ferrous metals. It has minimal holding strength on small particles, which, even if captured, are easily washed off.

Rare earth. Originally thought to be rare, the metallic elements with an atomic number between 57 and 71 are classified as "rare earth," or the lanthanide elements. Samarium (number 62 on the Periodic Table) was the first material used to make these magnets. A samarium-cobalt magnet can produce more than 4100 surface gauss in a tube magnet circuit, compared to 1000 to 2000 gauss for standard Alnico and ceramic.

The newest generation of rare-earth magnets consist of neodymium, iron, and boron (neodymium is number 60 on the periodic table). The first neodymium-iron- boron magnets on the market developed a surface gauss of approximately 4800 in a tube-type circuit. Strength levels have been increasing over the past ten years, and these magnets are now producing more than 10,000 surface gauss in a tube-type circuit.

Rare-earth magnets are used to remove small ferrous particles from materials ranging from liquid streams to powdery products. Although magnet manufacturers make many different strengths of rare earth separators, all are generally seven to ten times stronger than ceramic magnets.