Most of the weakly magnetic minerals, such as garnet, ilmenite, and magnetic impurities in silica sand, can be effectively separated with a magnetic separator that has a flux density greater than 6,000 gauss. For nearly a century, induced-roll magnetic separators were the only economically viable unit operation in these applications. In spite of their considerable success, induced-roll separators have certain limitations in their selectivity and application. The development of permanent magnet technology during the last two decades has reestablished the importance of magnetic separation and has increased the efficiency of fine-particle separations that were not successful with induced-roll magnets.

Principle and Design of Permanent magnets

In the last decade, magnetic separation technology has undergone a revolution. Research in material science and ceramic technology has culminated in the development of new permanent rare-earth magnets and superconducting alloys that can be used to build high-gradient magnetic separators.

Successful adaptation of these new magnetic materials combined with the knowledge of magnet geometry has led to the design and development of a number of new magnetic separators. These separators have opened niche markets that were previously considered beyond the realm of magnetic separation. These new separators are capable of

_ Effectively removing magnetic impurities or reducing their concentration (even to ppm levels)

_ Producing high-grade mineral separates

_ Operating on virtually no energy, which makes them economical

_ Generating higher magnetic flux levels up to 21,000 gauss or 2.1 tesla

Dry Permanent Magnetic Separator

Recent improvements in magnet composition and design have led to the development of permanent magnetic separators. These improved rare-earth permanent magnets (e.g., NdFeB magnets) have a magnetic attractive force an order of magnitude greater than that of conventional permanent magnetic circuits. The two main types of dry permanent magnetic separators that have found wide industrial applications are the rare-earth drum (RED) and the rare-earth roll (RER). They are widely used to separate weakly magnetic materials, such as garnet, ilmenite, and chromite, and also to separate magnetic impurities present in low concentrations in silica sand washing plant.

Rare-earth Drum Separator.

In an RED separator, the NdFeB magnets are uniquely arranged to provide an intense (up to 9,000 gauss) and “deep” magnetic field perpendicular to the drum surface (Figure 1). Once the particles are on the drum surface, they experience uniform flux density that minimizes the misplacement of pinned particles to the middlings. The weakly magnetic particles pinned to the drum are carried to the region of no magnetic intensity and are released as magnetics. The centrifugal force of the rotating drum throws those particles not influenced by the magnetic field into the nonmagnetic hopper.

An industrial-scale RED separator usually has three drums (Figure 2). In general, the top drum is a low-intensity (up to 2,000 gauss) scalper magnet to remove ferromagnetic particles, and the nonmagnetic fraction is subsequently treated on the REDs. The main purpose of the scalper is to protect the bottom two REDs, as well as to increase their capacity. Some separators have a built-in internal aircooling system to protect the magnets from overheating when the feedstock is preheated, as in plants that process beach and silica sand.

Rare-earth Roll Separator.

The feed is fed onto a thin belt (usually 7.6 × 10–3 to 5.1 × 10–2 cm) that travels at a very high velocity. The unique aspect of these separators is the way in which the magnetic separator is configured as a head pulley. The feed material is passed through the magnetic field, and the magnetic (or weakly magnetic) particles are attached to the roll and separated from the nonmagnetic stream (Figure 3).

Drum separators can effectively handle coarse particles (12.5–0.075 mm), whereas roll separators are very effective in treating fine particles (<1 mm). The capacity of the deeper-field drum separators is generally higher than that of the roll separators at 400–500 lb/h/in. for drum separators but 100–300 lb/h/in. for roll separators. The major advantage of the drum separator is its low maintenance cost, because it does not contain a belt that must be replaced frequently. However, both separators have their own niche markets. Drum separators can treat coarser particles, such as garnet, ilmenite, and iron ore, at a higher throughput. Roll separators can be used in producing high-grade, high-purity glass sand products when the feed material is not preheated.

Eddy Current Separator.

In the case of the RED and RER magnetic separators, the outer drum or the belt rotates, while inside the shell, rare-earth magnets are mounted on a stator. However, in an eddy current separator not only does the nonmetallic outer drum rotate but also the inside shell, which is a faster-moving rotor containing rare-earth magnets arranged in alternating polarity to produce induced eddy currents. The induced eddy current sets up a repulsive force in good conductors and thus separates nonferrous electrically conductive metals, such as copper and aluminum, from nonconducting materials.

Wet Permanent Magnetic Separator

A wet permanent magnetic separator is shown in Figure 4. The permanent-magnet (NdFeB) bars are positioned more or less horizontally inside a revolving drum made of stainless steel. This separator provides a field strength of 0.7 tesla on the drum surface. The pulp tank is made of stainless steel with concurrent or semicountercurrent flow tank design and an adjustable discharge gap at the magnetics discharge end. The separator is equipped with an adjustable valve on the nonmagnetics discharge pipe. This valve helps to control the flow rate and overflow level. It has found industrial applications in processing low-grade (martitic) iron ore.