SUPERCONDUCTING MAGNETS

In beneficiation plant, High magnetic fields (up to 2 tesla) are generated by passing current through a resistive coil or by permanent magnets. The development, through the use of finite element analysis techniques, of newer computer models has helped to achieve higher magnetic force. However, there is a logical maximum magnetic field for both the resistive coil and permanent magnet. Resistive coils are limited by the intrinsic resistance applied by the windings; the field strength of existing permanent magnets can be increased only marginally by modifying the magnet geometry. In the future, new magnetic materials may help to overcome this limitation.

Currently, superconducting magnets are the only economically and technically viable way to achieve field strengths as high as 5 tesla. Fundamental requirements of superconducting magnets are a suitable conductor and a cryogenic system. During the last decade, extensive research in material science has resulted in new alloys that are suitable candidates for superconducting magnets. Because of its reliability and favorable economics, a niobium and titanium alloy is the most suitable for low-temperature (about 4 K) industrial applications. High-temperature (about 20–30 K) superconducting magnetic separators have yet to be developed.

The cryogenic system is the most expensive component of the superconducting magnet, and it determines the economic viability and practicality of these machines. To date, three cryogenic systems have been successfully applied.

Closed-cycle Liquefier System. In a closed-cycle liquefier system, the superconductor resides in a bath of liquid helium, and boil-off gas is recirculated through a helium liquefier. Although the installation of such a system is quite complex, its performance has been good and reliable, provided there are no long-term interruptions in the supply of electrical power and cooling water.

Low-loss System. In a low-loss system the superconductor windings reside in a reservoir of liquid helium. A very efficient insulation system enables the magnet to operate for long periods, typically 1 year or more, between liquid helium refills. The salient feature of this system is its relative immunity to shortterm electrical failures. This feature has allowed this technology to be used where equipment is operated under difficult conditions.

Indirect Cooling. The advent of heat engines based on the Gifford McMahon cycle, which generate temperatures of 4 K or less, has made it possible to cool superconducting windings without the need for liquid helium. This technique offers great potential for small-scale systems in which the economics of helium supply or the cost of a liquefier cannot be justified. However, a constant power supply is essential for reliable operation.

In summary, the superconducting magnets have two main advantages:

_ Low power consumption resulting from zero resistance of the magnet winding

_ Generation of much higher magnetic fields

Super conducting High-gradient WetMagnetic Separator (HGMS)

In an HGMS, the magnetic particles are captured on a stainless steel–wool matrix contained within the bore of a high-intensity magnet. The high intensity is generated using a superconducting coil. Because these coils have essentially zero resistance, little electrical power is required to energize the magnet. Furthermore, once the magnet is energized, the coil ends can be shorted, leaving the magnet in a fully energized state without any additional power supply. This practice is called operating the magnets in persistent mode. Unloading the trapped magnetic particles from the matrix is an essential step that determines the separation efficiency and the capacity of the unit operation. The demagnetization is achieved either by de-energizing the magnet (a state commonly referred to as switch-mode) or by moving the matrix canister (referred to as a reciprocating canister HGMS). In reciprocating technology, captured magnetic particles are flushed using a ram to remove the trapping zone from the magnetic field regions. The ram operates on a magnetically balanced canister that houses a multisection separation region with unique and separate trapping zones (Figure 1). Units that combine reciprocating canister technology and a low-loss cryogenic system have been used in kaolin processing throughout the world. Figure 2 shows the installation of a typical large-scale reciprocating canister HGMS.

Superconducting Open-gradient DryMagnetic Separator (OGMS)

In a conventional OGMS, the magnet structure is arranged to provide a region in open space with a highly divergent field. Thus, the magnet geometry supplies both the magnetic field and the field gradient. Any paramagnetic material passing through this region will experience a force directly proportional to the field intensity and the magnitude of the field gradient. However, a superconducting OGMS offers not only higher magnetic force but also a deeper magnetic field, which in turn translates to larger separation volume than that obtained by conventional electromagnets and permanent magnets
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Sinonine technology team