carpco sms super conducting high gradiant magnetic separator

magnetic separation of clay

Conventional Magnetic Separator:An Eriez conventional high gradient magnetic separator (HGMS), model EL 20-4, with a 0.058 m canister diameter and a total volume of 1030 cm was used. The matrix was a fine grade stainless steel wool. The canister was packed to a 6 percent density with the matrix. The tests followed the recommended Eriez procedures.

After degritting, the kaolin slurry was stored in a feed tank and sent to a centrifuge. The centrifuge cut was adjusted to produce Coating A (90 % below 2 m) and Coating B (80 % below 2 m). The coarse fractions obtained were mixed, delaminated and again classified in the centrifuge to generate a delaminated fine fraction (80 % below 2 m).

The non-magnetics builtup in the matrix were removed by stopping the feed pump and rinsing the canister with water, while the magnetics particles were backflushed with water, after turning off the power supply.

The period of time for each test was calculated as a function of the feed flow rate, in order to assure the same duty factor for all the tests. The duty factor is defined as the ratio between time processing clay in one cycle and total time of one cycle. Three non-magnetics samples were collected in aproximately equal intervals of time.

The non-magnetics builtup in the matrix were removed by stopping the feed pump and rinsing the canister with water. The power supply was turned off and finally the magnetics were backflushed with entrained air rinse water.

magnetic separator - an overview | sciencedirect topics

As magnetic separators progress toward larger capacity, higher efficiency, and lower operating costs, some subeconomic iron ores have been utilized in recent years. For example, magnetite iron ore containing only about 4% Fe (beach sands or ancient beach sands) to 15% Fe (iron ore formations) and oxidized iron ore of only about 10% Fe (previously mine waste) to 20% Fe (oxidized iron ore formations) are reported to be utilized. They are first crushed and the coarse particles pretreated using roll magnetic separators. The magnetic product of roll magnetic separators may reach 2540% Fe and then is fed to mineral processing plants.

As shown in Figure5, slurry is fed from the top of an inclined screen in a low-intensity magnetic field, with the mesh size of screen sufficiently larger than those of particles in slurry. As the slurry flows down the above surface of screen, magnetic particles agglomerate with the size of agglomerations increasingly growing and roll down as magnetic concentrate at the lower end of screen. The less- or nonmagnetic particles pass through the screen as tailings. Figure5 shows the operation of screen magnetic separators for cleaning of magnetite.

Commercial magnetic separators are continuous-process machines, and separation is carried out on a moving stream of particles passing into and through the magnetic field. Close control of the speed of passage of the particles through the field is essential, which typically rules out free fall as a means of feeding. Belts or drums are very often used to transport the feed through the field.

As discussed in Section 13.4.1, flocculation of magnetic particles is a concern in magnetic separators, especially with dry separators processing fine material. If the ore can be fed through the field in a monolayer, this effect is much less serious, but, of course, the capacity of the machine is drastically reduced. Flocculation is often minimized by passing the material through consecutive magnetic fields, which are usually arranged with successive reversals of the polarity. This causes the particles to turn through 180, each reversal tending to free the entrained gangue particles. The main disadvantage of this method is that flux tends to leak from pole to pole, reducing the effective field intensity.

Provision for collection of the magnetic and nonmagnetic fractions must be incorporated into the design of the separator. Rather than allow the magnetics to contact the pole-pieces, which then requires their detachment, most separators are designed so that the magnetics are attracted to the pole-pieces, but come into contact with some form of conveying device, which carries them out of the influence of the field, into a bin or a belt. Nonmagnetic disposal presents no problems; free fall from a conveyor into a bin is often used. Middlings are readily produced by using a more intense field after the removal of the highly magnetic fraction.

Conventional magnetic separators are largely confined to the separation or filtration of relatively large particles of strongly magnetic materials. They employ a single surface for separation or collection of magnetic particles. A variety of transport mechanisms are employed to carry the feed past the magnet and separate the magnetic products. The active separation volume for each of these separators is approximately the product of the area of the magnetised surface and the extent of the magnetic field. In order for the separators to have practical throughputs, the magnetic field must extend several centimetres. Such an extent implies a relatively low magnetic field gradient and weak magnetic forces.

To overcome these disadvantages HGMS has been developed. Matrices of ferromagnetic material are used to produce much stronger but shorter range magnetic forces over large surface areas. When the matrices are placed in a magnetic field, strong magnetic forces are developed adjacent to the filaments of the matrix in approximately inverse proportion to their diameter. Since the extent of the magnetic field is approximately equal to the diameter of the filaments the magnetic fields are relatively short range. However, the magnetic field produced is intense and permits the separation and trapping of very fine, weakly magnetic particles (Oberteuffer, 1979).

The transport medium for HGMS can be either liquid or gaseous. Dry HGMS processing has the advantage of a dry product although classification of the pulverised coal is required to ensure proper separation. Small particles tend to agglomerate and pass through the separator. It has been shown that individual particles of coal in the discharge of a power plant pulveriser flow freely and hence separate well only if the material below about 10 m is removed (Eissenberg et al., 1979). Even then drying of that part of run of mine coal to be treated by HGMS may be required to ensure good flow characteristics.

A schematic representation of a batch HGMS process is shown in Figure 11.5 (Hise, 1979, 1980; Hise et al., 1979). It consists of a solenoid, the core cavity of which is filled with an expanded metal mesh. Crushed coal is fed to the top of the separator. Clean coal passes through while much of the inorganic material is trapped to be released when the solenoid is later deactivated.

Data from a batch HGMS process of one size fraction of one coal are plotted in Figure 11.6 as weight per cent of material trapped in the magnetic matrix, the product sulphur and the product ash versus the independent variable of superficial transport velocity. At low superficial transport velocities the amount of material removed from the coal is high partly due to mechanical entrapment. As the velocity is increased the importance of this factor diminishes but hydrodynamic forces on the particles increase. These hydrodynamic forces oppose the magnetic force and the amount of material removed from the coal decreases (Hise, 1979).

For comparison, Figure 11.7 shows data from a specific gravity separation of the same size fraction of the same coal. While the sulphur contents of the products from the two separation processes are similar the ash content of the HGMS product is considerably higher than that of the specific gravity product. It should be emphasised that this comparison was made for one size fraction of one coal.

More recently dry HGMS has been demonstrated at a scale of 1 t/h on carousel type equipment which processes coal continuously (Figure 11.8; Hise et al., 1981). A metal mesh passes continuously through the magnetised cavity so that the product coal passes through while the trapped inorganics are carried out of the field and released separately.

Wet HGMS is able to treat a much wider range of coal particle sizes than dry HGMS. The efficiency of separation increases with decreasing particle size. However, depending on the end use a considerable quantity of energy may have to be expended in drying the wet, fine coal product. Wet HGMS may find particular application to the precleaning of coal for use in preparing coal water mixtures for subsequent combustion as both pulverising the coal to a fine particle size and transporting the coal in a water slurry are operations common to both processes.

Work at Bruceton, PA, USA has compared the pyrite reduction potential of froth flotation followed by wet HGMS with that of a two stage froth flotation process (Hucko and Miller, 1980). Typical results are shown in Figures 11.9 and 11.10. The reduction in pyritic sulphur is similar in each case although a greater reduction in ash content is achieved by froth flotation followed by HGMS than by two stage froth flotation. However, Hucko (1979) concludes that it is highly unlikely that HGMS would be used for coal preparation independently of other beneficiation processes. As with froth flotation there is considerable variation in the amenability of various coals to magnetic beneficiation.

In the magnetic separator, material is passed through the field of an electromagnet which causes the retention or retardation of the magnetic constituent. It is important that the material should be supplied as a thin sheet in order that all the particles are subjected to a field of the same intensity and so that the free movement of individual particles is not impeded. The two main types of equipment are:

Eliminators, which are used for the removal of small quantities of magnetic material from the charge to a plant. These are frequently employed, for example, for the removal of stray pieces of scrap iron from the feed to crushing equipment. A common type of eliminator is a magnetic pulley incorporated in a belt conveyor so that the non-magnetic material is discharged in the normal manner and the magnetic material adheres to the belt and falls off from the underside.

Concentrators, which are used for the separation of magnetic ores from the accompanying mineral matter. These may operate with dry or wet feeds and an example of the latter is the Mastermag wet drum separator, the principle of operation of which is shown in Figure 1.43. An industrial machine is shown in operation in Figure 1.44. A slurry containing the magnetic component is fed between the rotating magnet drum cover and the casing. The stationary magnet system has several radial poles which attract the magnetic material to the drum face, and the rotating cover carries the magnetic material from one pole to another, at the same time gyrating the magnetic particles, allowing the non-magnetics to fall back into the slurry mainstream. The clean magnetic product is discharged clear of the slurry tailings. Operations can be co- or counter-current and the recovery of magnetic material can be as high as 99.5 per cent.

An example of a concentrator operating on a dry feed is a rotating disc separator. The material is fed continuously in a thin layer beneath a rotating magnetic disc which picks up the magnetic material in the zone of high magnetic intensity. The captured particles are carried by the disc to the discharge chutes where they are released. The nonmagnetic material is then passed to a second magnetic separation zone where secondary separation occurs in the same way, leaving a clean non-magnetic product to emerge from the discharge end of the machine. A Mastermagnet disc separator is shown in Figure 1.45.

The removal of small quantities of finely dispersed ferromagnetic materials from fine minerals, such as china clay, may be effectively carried out in a high gradient magnetic field. The suspension of mineral is passed through a matrix of ferromagnetic wires which is magnetised by the application of an external magnetic field. The removal of the weakly magnetic particles containing iron may considerably improve the brightness of the mineral, and thereby enhance its value as a coating or filler material for paper, or for use in the manufacture of high quality porcelain. In cases where the magnetic susceptibility of the contaminating component is too low, adsorption may first be carried out on to the surface of a material with the necessary magnetic properties. The magnetic field is generated in the gap between the poles of an electromagnet into which a loose matrix of fine stainless steel wire, usually of voidage of about 0.95, is inserted.

The attractive force on a particle is proportional to its magnetic susceptibility and to the product of the field strength and its gradient, and the fine wire matrix is used to minimise the distance between adjacent magnetised surfaces. The attractive forces which bind the particles must be sufficiently strong to ensure that the particles are not removed by the hydrodynamic drag exerted by the flowing suspension. As the deposit of separated particles builds up, the capture rate progressively diminishes and, at the appropriate stage, the particles are released by reducing the magnetic field strength to zero and flushing out with water. Commercial machines usually have two reciprocating canisters, in one of which particles are being collected from a stream of suspension, and in the other released into a waste stream. The dead time during which the canisters are being exchanged may be as short as 10 s.

Magnetic fields of very high intensity may be obtained by the use of superconducting magnets which operate most effectively at the temperature of liquid helium, and conservation of both gas and cold is therefore of paramount importance. The reciprocating canister system employed in the china clay industry is described by Svarovsky(30) and involves the use a single superconducting magnet and two canisters. At any time one is in the magnetic field while the other is withdrawn for cleaning. The whole system needs delicate magnetic balancing so that the two canisters can be moved without the use of very large forces and, for this to be the case, the amount of iron in the magnetic field must be maintained at a constant value throughout the transfer process. The superconducting magnet then remains at high field strength, thereby reducing the demand for liquid helium.

Micro-organisms can play an important role in the removal of certain heavy metal ions from effluent solutions. In the case of uranyl ions which are paramagnetic, the cells which have adsorbed the ions may be concentrated using a high gradient magnetic separation process. If the ions themselves are not magnetic, it may be possible to precipitate a magnetic deposit on the surfaces of the cells. Some micro-organisms incorporate a magnetic component in their cellular structure and are capable of taking up non-magnetic pollutants and are then themselves recoverable in a magnetic field. Such organisms are referred to a being magnetotactic.

where mpap is the inertial force and ap the acceleration of the particle. Fi are all the forces that may be present in a magnetic separator, such as the magnetic force, force of gravity, hydrodynamic drag, centrifugal force, the friction force, surface forces, magnetic dipolar forces, and electrostatic forces among the particles, and others.

Workable models of particle motion in a magnetic separator and material separation must be developed separately for individual types of magnetic separators. The situation is complicated by the fact that many branches of magnetic separation, such as separation by suspended magnets, magnetic pulleys, or wet low-intensity drum magnetic separators still constitute highly empirical technology. Hesitant steps have been taken to develop theoretical models of dry separation in roll and drum magnetic separators. Alternatively, open-gradient magnetic separation, magnetic flocculation of weakly magnetic particles, and wet high-gradient magnetic separation (HGMS) have received considerable theoretical attention. A notable number of papers dealing with the problem of particle capture in HGMS led to an understanding of the interaction between a particle and a matrix element. However, completely general treatment of the magnetostatic and hydrodynamic behavior of an assembly of the material particles in a system of matrix elements, in the presence of a strong magnetic field, is a theoretical problem of considerable complexity which has not been completed, yet. Detailed description of particle behavior in various magnetic separators can be found in monographs by Gerber and Birss (1983) and Svoboda (1987, 2004).

The brick material ratio was: Slag(1.0mm<): Grog (3.0mm<): Ceramic Gravel (1.0mm<): Clay (1.0mm<) at 20 : 35 : 25 : 20. To this mixture, 2% of pigment were added. Kneading and blending was done by a Mller mixer for 15 minutes. Molding was done by a 200 ton friction press, and the bricks were loaded onto the sintering truck.

This paper presents preliminary results using the Magnetic Micro-Particle Separator, (MM-PS, patent pending) which was conceived for high throughput isothermal and isobaric separation of nanometer (nm) sized iron catalyst particles from Fischer-Tropsch wax at 260 oC. Using magnetic fields up to 2,000 gauss, F-T wax with 0.30.5 wt% solids was produced from 25 wt% solids F-T slurries at product rates up to 230 kg/min/m2. The upper limit to the filtration rate is unknown at this time. The test flow sheet is given and preliminary results of a scale-up of 50:1 are presented.

Most loads for flap valves, conveyors, vibrating feeders, crushers, paddle feeders, magnetic separators, fans and trash screens generally are supplied at 415 V three-phase 50 Hz from the 415 V Coal Plant Switchboard, although 3.3 kV supplies may be used when the duty demands. Stacker/reclaimer machines are supplied at 3.3 kV. Electrical distribution is designed to safeguard the independent operational requirements of the duplicated coal plant facilities and to ensure that an electrical fault will not result in the total loss of coal supplies to the boilers.

The first step in any form of scrubbing unit is to break the lumpy materials and remove tramp elements by a magnetic separator. The product is then led into the scrubbing unit. The dry scrubbing principle is to agitate the sand grains in a stream of air so that the particles shot-blast each other. A complete dry scrubbing plant has been described in a previous book of this library in connection with sodium silicate bonded sands.* For clay-bonded sands the total AFS clay content in the reclaimed sand varies from 05% to 25% clay depending on the design of the plant.

superconductivity leaves the lab physics world

Superconducting magnets are a common tool in many physics and chemistry laboratories, and are used in a host of research applications, including solid-state physics, nuclear-magnetic-resonance chemistry and particle physics. Outside the research lab, the only truly widespread use of superconducting magnets is in magnetic resonance imaging (MRI) in medicine. However, applications of superconducting magnets are becoming more diverse. And nowadays, physicists and engineers might equally find themselves in remote mining sites in the Amazonian rainforest or in state- of-the-art hospitals. While such environments represent the extremes, they illustrate the movement of superconducting magnets out of the laboratory and into the wider world (see Industry warms to superconductors by Jeffery Tallon Physics World March 2000 pp27-31). Mineral processing Magnetic separation in the minerals industry is the largest application of superconducting magnets in industrial processing. Since the early 1990s Oxford Instruments has worked with the Carpco division of Outokumpu Technology Inc., developing magnets for laboratory and full industrial-scale use. Over 30 systems are now in operation across the world, in locations ranging from Cornwall in the UK to Australia, India and Brazil. A wide variety of materials can be purified using a magnetic-separation system with a high magnetic-field gradient. The technique relies on the fact that the magnetic-separation force is a product of the material susceptibility, the base magnetic field and the field gradient. The main application is in the improvement of kaolin (china clay), which is used in the ceramics and paper industries. Dark-coloured impurities, like iron oxide, tend to be paramagnetic and can be removed from the non-magnetic kaolin using high-gradient magnetic fields. This process increases the whiteness of the material, producing a higher grade, and therefore higher value, product. To separate the magnetic impurities, the raw material is suspended in a wet slurry, and passed through a filter canister in the magnetic field. The filter is typically a fine ferromagnetic matrix, which in its most simple form is similar to steel wool. This matrix creates the high localized magnetic-field gradients that attract and hold the impurities. The system contains a so-called reciprocating canister to avoid downtime while the filter is flushed once it is saturated with impurities. This works by building two filter matrixes into a single long canister such that the filters may be alternated in the magnetic field one filter processes the slurry while the other is being flushed. The superconducting magnets used produce a field of 5 tesla and are cooled to 4.2 K using liquid helium. A magnet with a 1 m bore, coupled with a reciprocating canister developed by Carpco, can process over 100 tonnes of material per hour. In general, magnetic-separation systems operate in demanding environmental conditions, so there is a clear need to have fully robust magnets and to save helium. For example, it can take three weeks to transport liquid helium by sea and river from Sao Paulo to a Brazilian mining site, losing over 30% of the liquid along the way. To avoid further losses, the system uses low-loss cryogenic techniques developed originally for MRI magnets. Specifically, so-called cryocoolers use a fixed volume of helium gas to cool the radiation shields between the room-temperature outer cryostat and the liquid- helium vessel. To allow the operating costs to be recouped, the magnet must be able to operate for at least 8000 hours per year (i.e. 92% of the time) without interruption. Modern magnets easily meet this standard even in the most hostile environments. While high-gradient magnetic separation can be applied to materials that can be suspended in slurry, this is not possible or desirable for many materials that could be purified using magnetic fields. An alternative method, known as open-gradient magnetic separation, segregates a stream of dry material falling from a conveyor belt through a field gradient in front of a magnet. Physicists at Oxford Instruments have designed a cryogen-free racetrack-coil magnet to produce the highest possible field and field gradient outside the system, with a geometry that matches the material feed. By removing the liquid-cryogen volume and using a closed-cycle refrigerator operating at 4 K to cool the system, the magnet coil and thus the region where the field gradient is highest can be brought close to the outer wall of the cryostat body and the flow of material. The high field gradients generated by the superconducting magnets can be used to separate weakly magnetic granular materials, such as sand, soda ash, marble and diamonds. In contrast, permanent-magnet separators do not provide a sufficient field and field gradient to separate these dry materials effectively. Magnets for surgery An exciting new application for superconducting magnets is a magnetic navigation system that could potentially allow surgeons to steer catheters through the brain and the vascular system. The device that is being developed by Stereotaxis Inc. in St Louis, USA, may allow instruments to be fed through these pathways to deliver drugs, carry out biopsies, close aneurysms and make electrical maps of the heart wall. The anticipated possible speed and ease of these procedures could potentially offer many benefits to patients and doctors, including more rapid surgical response, a lower level of surgical support and consequent cost savings. Faster procedures could potentially improve patient recovery times. Meanwhile, the number of procedures abandoned due to an inability to properly position the catheter may be greatly reduced. Oxford Instruments is building a series of magnets for these systems, the first of which was installed earlier this year at the Barnes-Jewish Hospital in St Louis (see figure). Meanwhile, Stereotaxis has developed a range of catheters, each with a small magnetic tip, designed to be oriented by changing the relative magnetic fields of three orthogonal superconducting coils positioned around the patients head or chest. Each coil generates a magnetic field of up to 5 tesla, giving a projected field of 0.3 tesla at the centre of the set of the three magnets. Very high stresses are generated in the magnet coils and surrounding structure due to the high rates at which the magnets are ramped in order to rapidly, and frequently, change the direction of the magnetic field. Intensive finite-element modelling played a key part in the successful magnet design. The magnets have also been designed so that patients may be positioned within the system easily, and so that X-rays can be taken to monitor the position of the catheter tip in real time. A hemispherical cryostat, quite unlike any conventional superconducting magnet, is used to provide a compact system (see figure). The current leads in the magnet are made from high- temperature superconductors so that the magnetic field can be increased quickly without generating high heating losses. The system also uses a closed-cycle cryocooler operating at 4 K that condenses the liquid-helium vapour so that it falls back into the liquid-helium vessel. This so-called recondensing system reduces the amount of liquid helium required and eliminates the need for a large helium reservoir in the operating theatre. When in use, the magnet should require only 30 litres of helium per month. Like many technologies, elements of the new superconducting-magnet designs draw heavily on previous experience. For example, the heavy iron shielding on the Carpco magnets helped us to develop stress-modelling and engineering techniques that are now being applied to the Stereotaxis magnets. Similarly, one of the first liquid-helium recondensing systems was used in a Carpco magnet, and this technology is similarly being extended to the Stereotaxis and other systems. As our engineering experience continues to grow, it is likely to enable more and more superconducting-magnet applications in the future. Want to read more? Register to unlock all the content on the site E-mail Address Register

Superconducting magnets are a common tool in many physics and chemistry laboratories, and are used in a host of research applications, including solid-state physics, nuclear-magnetic-resonance chemistry and particle physics. Outside the research lab, the only truly widespread use of superconducting magnets is in magnetic resonance imaging (MRI) in medicine.

However, applications of superconducting magnets are becoming more diverse. And nowadays, physicists and engineers might equally find themselves in remote mining sites in the Amazonian rainforest or in state- of-the-art hospitals. While such environments represent the extremes, they illustrate the movement of superconducting magnets out of the laboratory and into the wider world (see Industry warms to superconductors by Jeffery Tallon Physics World March 2000 pp27-31).

Magnetic separation in the minerals industry is the largest application of superconducting magnets in industrial processing. Since the early 1990s Oxford Instruments has worked with the Carpco division of Outokumpu Technology Inc., developing magnets for laboratory and full industrial-scale use. Over 30 systems are now in operation across the world, in locations ranging from Cornwall in the UK to Australia, India and Brazil.

A wide variety of materials can be purified using a magnetic-separation system with a high magnetic-field gradient. The technique relies on the fact that the magnetic-separation force is a product of the material susceptibility, the base magnetic field and the field gradient. The main application is in the improvement of kaolin (china clay), which is used in the ceramics and paper industries. Dark-coloured impurities, like iron oxide, tend to be paramagnetic and can be removed from the non-magnetic kaolin using high-gradient magnetic fields. This process increases the whiteness of the material, producing a higher grade, and therefore higher value, product.

To separate the magnetic impurities, the raw material is suspended in a wet slurry, and passed through a filter canister in the magnetic field. The filter is typically a fine ferromagnetic matrix, which in its most simple form is similar to steel wool. This matrix creates the high localized magnetic-field gradients that attract and hold the impurities. The system contains a so-called reciprocating canister to avoid downtime while the filter is flushed once it is saturated with impurities. This works by building two filter matrixes into a single long canister such that the filters may be alternated in the magnetic field one filter processes the slurry while the other is being flushed.

The superconducting magnets used produce a field of 5 tesla and are cooled to 4.2 K using liquid helium. A magnet with a 1 m bore, coupled with a reciprocating canister developed by Carpco, can process over 100 tonnes of material per hour.

In general, magnetic-separation systems operate in demanding environmental conditions, so there is a clear need to have fully robust magnets and to save helium. For example, it can take three weeks to transport liquid helium by sea and river from Sao Paulo to a Brazilian mining site, losing over 30% of the liquid along the way. To avoid further losses, the system uses low-loss cryogenic techniques developed originally for MRI magnets. Specifically, so-called cryocoolers use a fixed volume of helium gas to cool the radiation shields between the room-temperature outer cryostat and the liquid- helium vessel.

To allow the operating costs to be recouped, the magnet must be able to operate for at least 8000 hours per year (i.e. 92% of the time) without interruption. Modern magnets easily meet this standard even in the most hostile environments.

While high-gradient magnetic separation can be applied to materials that can be suspended in slurry, this is not possible or desirable for many materials that could be purified using magnetic fields. An alternative method, known as open-gradient magnetic separation, segregates a stream of dry material falling from a conveyor belt through a field gradient in front of a magnet.

Physicists at Oxford Instruments have designed a cryogen-free racetrack-coil magnet to produce the highest possible field and field gradient outside the system, with a geometry that matches the material feed. By removing the liquid-cryogen volume and using a closed-cycle refrigerator operating at 4 K to cool the system, the magnet coil and thus the region where the field gradient is highest can be brought close to the outer wall of the cryostat body and the flow of material.

The high field gradients generated by the superconducting magnets can be used to separate weakly magnetic granular materials, such as sand, soda ash, marble and diamonds. In contrast, permanent-magnet separators do not provide a sufficient field and field gradient to separate these dry materials effectively.

An exciting new application for superconducting magnets is a magnetic navigation system that could potentially allow surgeons to steer catheters through the brain and the vascular system. The device that is being developed by Stereotaxis Inc. in St Louis, USA, may allow instruments to be fed through these pathways to deliver drugs, carry out biopsies, close aneurysms and make electrical maps of the heart wall.

The anticipated possible speed and ease of these procedures could potentially offer many benefits to patients and doctors, including more rapid surgical response, a lower level of surgical support and consequent cost savings. Faster procedures could potentially improve patient recovery times. Meanwhile, the number of procedures abandoned due to an inability to properly position the catheter may be greatly reduced.

Oxford Instruments is building a series of magnets for these systems, the first of which was installed earlier this year at the Barnes-Jewish Hospital in St Louis (see figure). Meanwhile, Stereotaxis has developed a range of catheters, each with a small magnetic tip, designed to be oriented by changing the relative magnetic fields of three orthogonal superconducting coils positioned around the patients head or chest. Each coil generates a magnetic field of up to 5 tesla, giving a projected field of 0.3 tesla at the centre of the set of the three magnets.

Very high stresses are generated in the magnet coils and surrounding structure due to the high rates at which the magnets are ramped in order to rapidly, and frequently, change the direction of the magnetic field. Intensive finite-element modelling played a key part in the successful magnet design. The magnets have also been designed so that patients may be positioned within the system easily, and so that X-rays can be taken to monitor the position of the catheter tip in real time.

A hemispherical cryostat, quite unlike any conventional superconducting magnet, is used to provide a compact system (see figure). The current leads in the magnet are made from high- temperature superconductors so that the magnetic field can be increased quickly without generating high heating losses. The system also uses a closed-cycle cryocooler operating at 4 K that condenses the liquid-helium vapour so that it falls back into the liquid-helium vessel. This so-called recondensing system reduces the amount of liquid helium required and eliminates the need for a large helium reservoir in the operating theatre. When in use, the magnet should require only 30 litres of helium per month.

Like many technologies, elements of the new superconducting-magnet designs draw heavily on previous experience. For example, the heavy iron shielding on the Carpco magnets helped us to develop stress-modelling and engineering techniques that are now being applied to the Stereotaxis magnets. Similarly, one of the first liquid-helium recondensing systems was used in a Carpco magnet, and this technology is similarly being extended to the Stereotaxis and other systems. As our engineering experience continues to grow, it is likely to enable more and more superconducting-magnet applications in the future.

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