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magnetic separation principle

magnetic separation method

magnetic separation method

Magnetic separation is a process used to separate materials from those that are less or nonmagnetic. All materials have a response when placed in a magnetic field, although with most, the effect is too slight to be detected. The few materials that are strongly affected (magnetised) by magnetic fields are known as Ferromagnetics, those lesser (though noticeably) affected are known as Paramagnetics.

Ferromagnetics require relatively weak magnetic fields to be attracted and devices to separate these materials usually have magnets that are permanently magnetised (Permanent magnets do not require electricity to maintain their magnetic fields). Paramagnetics require stronger magnetic fields and these can only be achieved and maintained by electro magnets (large wire coils around an iron frame current is continuously passed through the coils creating the magnetic field within the iron. The field is concentrated across an air gap in the circuit).

Both ferromagnetic (low intensity) and paramagnetic (high intensity) separation devices (Laboratory Magnetic Separator) may be operated with dry solids or with solids in pulp form. (A complete classification of magnetic separating devices is given in Wills Mineral Processing Technology, pp. 338-356).

(*The units given are kilogauss (kG). These are the units most commonly used. The equivalent S.I. unit is the Tesla (T) * 1 Tesla = 10 kilogauss). The extremes of field strength used are based on experience from a magnetic separation testing laboratory over many years.

magnetic separation - an overview | sciencedirect topics

magnetic separation - an overview | sciencedirect topics

Magnetic separation takes advantage of the fact that magnetite is strongly magnetic (ferromagnetic), hematite is weakly magnetic (paramagnetic), and most gangue minerals are not magnetic (diamagnetic).

The current research and development initiatives and needs in magnetic separation, shown in Fig. 7, reveal several important trends. Magnetic separation techniques that have been, to a greater extent, conceived empirically and applied in practice, such as superconducting separation, small-particle eddy-current separation, and biomedical separation, are being studied from a more fundamental point of view and further progress can be expected in the near future.

In addition, methods such as OGMS, ferrohydrostatic separation, magnetic tagging, and magnetic flocculation of weakly magnetic materials, that have received a great deal of attention on academic level, are likely to enter the development and technology transfer stages.

The application of high-Tc superconductivity to magnetic separation, and novel magnetism-based techniques, are also being explored, either theoretically or empirically. It can be expected that these methods, such as magnetic flotation, magnetic gravity separation, magnetic comminution, and classification will take advantage of having a much wider control over these processes as a result of the presence of this additional external force.

Magnetic separation takes advantage of the fact that magnetite is strongly magnetic (ferromagnetic), hematite is weakly magnetic (paramagnetic), and most gangue minerals are not magnetic (diamagnetic). A simple magnetic separation circuit can be seen in Figure 1.2.5 [9]. A slurry passes by a magnetized drum; the magnetic material sticks to the drum, while the nonmagnetic slurry keeps flowing. A second pass by a more strongly magnetized drum could be used to separate the paramagnetic particles from the gangue.

Magnetic separation can significantly shorten the purification process by quick retrieval of affinity beads at each step (e.g., binding, wash, and elution), and reduce sample dilution usually associated with traditional column-based elution. The method can be used on viscous materials that will otherwise clog traditional columns and can therefore simplify the purification process by eliminating sample pretreatment, such as centrifugation or filtration to remove insoluble materials and particulates. The capability of miniaturization and parallel screening of multiple conditions, such as growth conditions for optimal protein expression and buffer conditions for purification, makes magnetic separation amenable to high-throughput analysis which can significantly shorten the purification process (Saiyed et al., 2003).

Paramagnetic particles are available as unmodified, modified with common affinity ligands (e.g., streptavidin, GSH, Protein A, etc.), and conjugated particles with specific recognition groups such as monoclonal and polyclonal antibodies (Koneracka et al., 2006). In addition to target protein purification, they can also be used to immobilize a target protein which then acts as a bait to pull down its interaction partner(s) from a complex biological mixture. See Chapter 16.

Magnetic separation of cells is a simple, rapid, specific and relatively inexpensive procedure, which enables the target cells to be isolated directly from crude samples containing a large amount of nontarget cells or cell fragments. Many ready-to-use products are available and the basic equipment for standard work is relatively inexpensive. The separation process can be relatively easily scaled up and thus large amount of cells can be isolated. New processes for detachment of larger magnetic particles from isolated cells enable use of free cells for in vivo applications. Modern instrumentation is available on the market, enabling all the process to run automatically. Such devices represent a flexible platform for future applications in cell separation.

IMS play a dominant role at present but other specific affinity ligands such as lectins, carbohydrates or antigens will probably be used more often in the near future. There are also many possibilities to combine the process of cell magnetic separation with other techniques, such as PCR, enabling the elimination of compounds possibly inhibiting DNA polymerase. New applications can be expected, especially in microbiology (isolation and detection of microbial pathogens) and parasitology (isolation and detection of protozoan parasites). No doubt many new processes and applications in other fields of biosciences and biotechnologies will be developed in the near future.

Magnetic separation methods are widely used for isolation of a variety of cell types. Magnetic particles with immobilized antibodies to various antigens have been employed for the rapid isolation of populations T-(CD4 +, CD3 +, CD8+) and B- (CD19+) of lymphocytes, NK cells, and monocytes. Similarly, immobilization of glycoconjugates on magnetic beads allows the isolation of cell populations expressing a particular carbohydrate-recognizing molecule [19, 20]. Glycosylated magnetic beads can be prepared by loading biotinylated probes onto streptavidin-coated magnetic beads. The glycoparticles are then incubated with a cell suspension and the subpopulation of interest is fished out by means of a magnetic device [20].

When these materials are used in the biological field, special restrictions should be considered and all possible reactions with the biological materials should be predicted. Magnetic properties should be maintained for a specific time during the test. Some applications can be classified as follows:

Magnetic separation is used for clinical application, such as in the separation of proteins, toxemic materials, DNA, and bacteria and viruses. This is also used for real time detecting of viruses. The most important stage in this field is the labeling of molecules with magnetic materials by a reliable connection. Magnetic beads from iron oxide are typically used for biological separation. The main properties of iron oxide are super paramagnetic properties (Meza, 1997).

Effective drug delivery can greatly improve the process of treatment and reduce side effects. In this method, while the amount of drug decreases, the concentration of the drug in the target area increases. Protecting the drug before its gets to the target area is one of the most important factors, because after releasing the drug in the blood stream, white cells detect the drug and swallow them in a short time. An ideal nanoparticle for drug delivery should have the potential to combine with a relatively high-weight drug and disperse uniformly in the blood stream (Shultz et al., 2007).

Also, while chemotherapy is one of most effective methods for cancerous tissues, many of the other healthy cells are destroyed in the process. So the conventional thermotherapy has many side effects. In hyperthermia treatment, after delivering the drug to the target area, an AC magnetic field is used to generate controllable energy and increase temperature. Heat transfer in this process is a balance between blood flow, heat generation, and tissue porosity and conductivity (Sellmyer and Skomski, 2006).

Magnetic Resonance Imaging (MRI) is considered a great help in the diagnoses of many diseases. The advantages of this imaging are high contrast in soft tissue, proper resolution, and sufficient penetration depth for noninvasive diagnosis. In fact, in MRI imaging magnetization of protons is measured when exposed to the magnetic field with radio frequency (Corot, 2006).

Magnetic separation: based on the generation of magnetic forces on the particles to be separated, which are higher than opposing forces such as gravity or centrifugal forces. This principle is used to separate ferromagnetic particles from crushed scrap mixtures.

Eddy current separation: is a particular form of magnetic separation. An alternating magnetic field induces electrical eddy currents on a metal particle. This results in a magnetic field whose direction is opposite to the primary magnetic field. The exchange interactions between the magnetic fields result in a repulsive force on the metallic particle; the net effect is a forward thrust as well as a torque. This force and hence the efficiency of separation is a function of the magnetic flux, or indirectly of the electrical conductivity and density and the size and shape of the metallic particles.

Air separation/zigzag windsifter: Air-based sorting technique, which separates the light materials from the heavier. The most prominent application is in shredder plants producing the shredder light fraction, or in fridge recycling, removing among others the polyurethane (PUR) foam from the shredded scrap.

Screening: Separation of the scrap into different particle size classes is performed to improve the efficiency of the subsequent sorting processes and/or to apply different processing routes for different size fractions (based on material breakage and hence distribution over various size fractions).

Fluidized bed separation: A fluidized bed of dry sand is used to separate materials based on density. This technology is in principle a dry sink-float separation, which is still hampered by several difficulties (tubular or hollow particles filling up with sand and tend to sink; formation of unsteady current due to the use of high velocity air, etc.). The fluidized bed could also be heated for simultaneous de-coating and combustion of organic material.

Image processing (including colour sorting): Colour sorting technologies, which sense the colour of each particle and use computer control to mechanically divert particles of identical colour out of the product stream (red copper, yellow brass, etc.). A complicating issue is that shredding results in mixtures of particles that show a distribution in composition, size, shape, texture, types of inserts, coatings, etc. The variance of these properties complicates identification that is solely based on this principle.

X-ray sorting: Dual energy X-ray transmission imaging (well known for luggage safety inspections at airports) identifies particles based on the average atomic number, particle shape, internal structure (e.g. characteristic variations of thickness) and presence of characteristic insert material. It is rather sensitive to particle thickness and surface contaminations.

LIBS (laser induced breakdown spectroscopy) sorting: A series of focused ablation laser pulses are delivered to the same spot on each particle. A pulse of an ablation laser vaporizes only the first nanometres of the surface, i.e. the first pulses are necessary to clean the surface of oxide layers (different composition than the mother metal), the last pulse vaporizes a tiny amount of metal generating a highly luminescent plasma plume. The light from the plasma is collected and analysed to quantitatively determine the chemical composition. This determines to which bin the particle is directed (e.g. by air pulse).

Iron ore processors may also employ magnetic separation for beneficiation of classifier output streams. Wet high-intensity magnetic separators (WHIMS) may be used to extract high-grade fine particles from gangue, due to the greater attraction of the former to the applied magnetic field.

In addition to beneficiating the intermediate middlings streams from the classifier, WHIMS may be used as scavenger units for classifier overflow. This enables particles of sufficient grade to be recovered that would otherwise be sacrificed to tails.

Testwork has been performed on iron ore samples from various locations to validate the use of magnetic separation following classification (Horn and Wellsted, 2011). A key example was material sourced from the Orissa state in northeastern India, with a summary of results shown in Table 10.2. The allmineral allflux and gaustec units were used to provided classification and magnetic separation, respectively.

The starting grade of the sample was a low 42% Fe. It also contained significant ultrafines with 58% passing 20m. This is reflected in the low yield of allflux coarse concentrate; however, a notable 16% (abs) increase in iron grade was eventually achieved. The gaustec results for the middlings and overflow streams demonstrate the ability to recover additional high-grade material. With the three concentrate streams combined, an impressive yield of almost 64% was achieved with minimal decline in iron grade.

The automatic separation system, developed by Magnetic Separation System of Nashville, Tennessee, uses X-ray, IR, and visible spectra sensors for separating the post-consumer recyclate bottles or flakes into individual plastics and into different color groups. X-ray sensors, used for separating PVC, are very accurate and can operate at as high as 99% or better efficiency. IR and visible sensors are used to separate the colored bottles into individual polymers and color groups.

The separation system (Figure 4) essentially consists of a metering inclined conveyer, air knife, special disk screen, singulating infeed conveyor, and sensor module. A motor control system provides operator interface screens which control the sorting functions, including the number of bottles sorted into each fraction, ejection timing, and sort positions. Individual systems currently in use in Germany, Switzerland, and the United States are described in a paper by Kenny and Vaughan.16 The systems are customized, based on the composition of the post-consumer recyclate and the end application of the separated streams. Some systems use X-ray and IR sensors in two locations to achieve better separation. In addition to sorting equipment, some systems also use equipment for breaking the bales and splitting the bottles into more than one stream for smooth operation. Grinders are used when the bottles have to be ground into flakes for further processing. Whereas PVC separation is accomplished at 99%. HDPE and PET separation is between 80 and 90%, depending on the level of contamination.

Automated separation provides two advantages: improved quality and lower labor cost for sorting. The automatic separation system at Eaglebrook Plastics uses the Magnetic Separation System (MSS), which detects and separates the bottles into different categories based on the type of the resin and color, and eliminates impurities such as broken pieces of plastics, rocks, aluminum cans, and other contaminants.17 Metering the feed is critical to obtain maximum throughput at Eaglebrook. This is accomplished by a special debaling device and an incline metering system. Factors contributing to proper operation include clear height, width, spacing, belt speed, and incline angle. Proper presentation of the bottle to the sensor is critical. The bottles are split into four streams and two to three bottles are presented to the sensor per second, one at a time.

The primary identification sensor uses a multibeam, near-IR array to identify the bottles into three classes: Class 1, PVC, PET; Class 2, natural HDPE, PP; Class 3, mixed color HDPE and opaque containers. This sensor is also capable of separating colored PET from clear PET and PP from milk jug HDPE. The X-ray sensor identifies PVC, and a machine vision sensor system provides up to seven color classifications of the plastic bottles. After identification, the containers are ejected from the conveyors into appropriate collection stations using high-speed pulsed air nozzles. The motor control center (MCC) of the separation system controls motor protection, sequential slant up for the system, fault indication, and operation control. In addiiton, a touch screen input panel allows the operator to select any available sort to be directed to any ejection station. Visible light color sensors have been added which sort pigmented HDPE into different colors. The system also includes a decision cross-checking device between the primary sensor and the color sensor. This compares the decisions of the two sensors by comparing them with a logic file. The latter then provides correct identification in case there are discrepancies between the two decisions. The system has successfully operated for the last three to four years at a capacity of 5000 bottles h1.

The debaling system designed for Eaglebrook requires that the bales be presented to the debaling equipment in the same orientation as the original compression. This design feature requires less horsepower, reduces bottle clusters, and requires minimum energy. The debaling and declumping system incorporates a surge bin and metering conveyor to feed the screening system. The improved capacity and higher separation accuracy, due to increased metering efficiency, reduces bottle clusters and provides a more uniform feeding system. The separation efficiency depends on several factors. Timing and catcher bounceback accounts for 12% accuracy loss; contamination, container distortion, and loose labels contribute to about 34%, and nonsingulation of the bottles 510% of accuracy loss.

Asoma Instrument of Austin, TX, is a leading manufacturer of automated bottle sorting equipment. The company uses an X-ray fluorescence spectrophotometer sensor. The identification is completed in 10ms and the separation takes about 20s per bottle. The sorted PET streams have less than 50ppm PVC. National Recovery Technology of Nashville, TN, uses a proprietary electromagnetic screening process which can handle the bottles either in crushed or whole form and does not require any special positioning or orientation of the bottle to achieve high efficiency. Chamberlain/MCR, Hunt Valley, MD, and Automation Industrial Control of Baltimore, MD, offer a paysort bottle sorting system, which uses a sophisticated video camera and color monitor incorporating a strobe to detect and distinguish colors of post-consumer bottles following a near-IR detection system which also determines the primary resin found in each bottle.

A substantial amount of research is focused on microseparation techniques and on techniques which can reject bottles with trace amounts of harmful contaminant. Near-IR spectrometry is being used to separate bottles for household chemicals and ones with hazardous waste residues.

Sorting of automotive plastics is more difficult than sorting of plastics from packaging recyclates. Whereas only five to six polymers are used for packaging, post-consumer automotive plastics contain large numbers of engineering and commodity plastics, modified in various ways, including alloying and blending, filling, reinforcing, and foaming. Hence, sorting of automotive plastic recyclate poses several challenges. Recently, a systematic study, PRAVDA, was undertaken by a German car manufacturer and the plastic suppliers in Europe to investigate the potential of various analytical techniques in separating post-consumer automotive plastics.18

The techniques examined in this study include near-IR spectroscopy (NIR), middle-IR spectroscopy (MIR), Fourier transform Raman spectroscopy (FTR), pyrolysis mass spectrometry (PY-MS), pyrolysis IR spectroscopy (PYIR), and laser-induced emission spectral analysis (LIESA). X-ray methods were excluded because they have insufficient sensivitity to polymers, other than ones containing chlorine. Since commercial spectrophotometers were not available for most techniques except NIR, either laboratory models (MIR, FTR) or experimental stage instruments (PY-MS, PY-IR, and LIESA) were used in this study. A large number of parts (approximately 7000) were analyzed. The techniques were compared in respect to their success in identification, fault rate, time for identification, degree of penetration, and sensitivity to surface quality. The fault rate is the number of wrong identifications, given as percent. If the sum of the identification and fault rate is less than 100, the difference gives the rate of incomplete correct identification. The biggest stumbling block was the identification of black samples which could not be analyzed by NIR and FTR. MIR is the only technique which not only identified the black samples, but gave the highest identification rate. Some difficulties were experienced, however, in MIR analysis in the case of blends of two similar polymers such as PP/EPDM or nylon 6/nylon 66. The pyrolytic methods showed poorer identification rates and higher fault rates. The LIESA method is very fast and a remote technology, particularly for fast identification of heteroatoms. It is therefore suitable for identifying fillers, minerals, reinforcing fibers, pigments, flame retardants, and stabilizers specific to the individual plastic. The difficulty with MIR is that it is sensitive to surface micro-roughness and, hence, the samples need to be very smooth. Also, paint or surface coats on the part have to be removed for correct identification of the resin used for making the parts. Further, at this stage, no fiber optic or separated probe is available with MIR technology and, hence, the part has to be brought close to the spectrophotometer instead of the probe reaching the part. Another method of measuring efficiency is the level of contamination. Contamination of parts sorted by the MIR method was less than 1%, whereas contamination of parts sorted manually, using a Car Parts Dismantling Manual, is greater than 1015%. When the level of contamination is high, further separation by swim-sink or hydrocyclone techniques are necessary.

The cost of a MIR spectrophotometer is approximately DM 100000. The cost calculated for small dismantlers (dismantling less than 25 cars per day) is approximately DM 0.34 per kg and that for large dismantlers is somewhat less than DM 0.19. Manual sorting, on the other hand, would cost DM0.71 and DM0.23 per kg for small and large dismantlers, respectively. Spectrophotometric identification of plastics in automotive plastics waste therefore makes substantial economic sense.

magnetic separator - an overview | sciencedirect topics

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.

principles of magnetic separation - minerallurgy

principles of magnetic separation - minerallurgy

Magnetic separation has been used since 1955 and has proven to be one of the most effective processes for beneficiating magnetically susceptible materials (AI-Wakeel and EI-Rahman, 2006; Yavuz et al., 2006; Dobbins et aI., 2007, Dobbins et al., 2009; Das et aI., 2010; Angadi et al., 2012).

Over the last three decades, the production of good quality concentrates from iron ore has been sharply increasing as a result of the high demand in the steel industry (Svoboda and Fujita, 2003; Mohanty et ai, 2010).

This has motivated countries like India, China and South Africa with an increasing depletion in high grade iron ore reserves to improve their beneficiation techniques for run of mine (ROM), fines and slimes (AI-Wakeel and EI-Rahman, 2006; Yavuz et al., 2006; Das et al.; 2010; Kumba, 2012).

Magnetic separation is based on the difference in the magnetic susceptibility of materials. To some degree, all materials respond to an external applied magnetic field, which is the basis for achieving separation between particles. The separation may be aimed at purifying feed materials like kaolin from iron impurities, or beneficiating materials such as iron ore from quartz (Dobbins et at., 2007; Linkun and Yun, 2010, Chen et at., 2012).

A separation is achieved when the magnetic force attained is greater than other competing forces, for example, the force of gravity largely acts upon coarse particles, while frictional force, attractive or repulsive force, surface and hydrodynamic drag force predominantly act upon fine particles. These forces are shown in the Figure below. The magnitude of these forces, their nature or the characteristic of the material to be treated together with the design of the equipment determines the efficiency in separation.

The nature of the material includes its particle size and magnetic susceptibility, while the equipments variable parameters include the magnitude of the magnetic field and its capacity, matrix material and type, and rotation speed of the rotor. These form part of the equipments design.

The magnetic force or field gradient used in the separation of materials can be generated through different methods. It can be achieved through the application of a permanent magnet, an electromagnet with an iron yoke, a solenoid or a superconducting magnet which differs in magnetic field geometries and magnitude.

Based on the difference in the mechanism by which magnet magnetic fields are generated, the efficiencies of separation also differ. One way to measure the efficiency is through the determination of grade quality and the quantity equated to recovery, achieved under the different techniques (Oberteuffer, 1974; Chakravorty, 1989).

The principles of magnetic separation are such that when particles of different magnetic susceptibility are placed in a magnetic field, they tend to disrupt the direction or the flow of the magnetic field and at the same time lead to the particles being magnetically induced. Hence, the induced field experienced by the particles determines which direction each of the particles will be deflected, leading to a separation.

The symbol Fm refers to a magnetic force exerted onto a particle, J is the magnetic polarisation of a particle in Telsa (T), V is the volume of the particle in (m3), if is the magnetic field strength in Am-1 and is the magnetic field gradient operator with 0 is a constant at 4*(10) -7*Hm-1

Equation 6 shows that the magnetic force Fm is directly proportional to magnetic field strength H and the magnetic field gradient. An increased magnetic field strength H will cause an increase in the magnetic field in the direction of the magnetic gradient and, as a result of that, increased magnetisation of the particle.

The figure above shows that there are other forces exerted on a particle competing with Fm and that the dominance of a particular force is dependent on the particle characteristics and the type of magnetic separator used.

For wet high intensity magnetic separators, the theoretical equations are limited to two fundamental factors, namely the force of gravity and the hydrodynamic drag force as indicated below in equation 2.7 and 2.8, respectively. For a spherical particle with a density p the force is given by:

In equation 8, represents the viscosity of the fluid medium, dr/dt is the velocity of the fluid and v is the velocity of the particle relative to the stream at position r. All the forces have different dependence on the particle size S, and thus the forces will vary with particle size. The force of gravity Fg will be dominant on coarse size particles whilst Fd will be more dominant on small particles respectively (Svoboda, 1987; Alp, 2007).

Materials are classified into ferromagnetic, paramagnetic and diamagnetic categories based on their strongly or weakly susceptible characteristics when experiencing the intensity of an external applied magnetic field (Svoboda, 1987; Dwari and Rao, 2009).

Ferromagnetic and paramagnetic materials are known to be magnetically attracted to a magnetic field, whilst diamagnetic materials are repelled once passed through a magnetic field. The difference in the two types is that paramagnetic and ferromagnetic materials have positive susceptibilities while diamagnetic substances have negative susceptibility.

Ferromagnetic materials are regarded as a special case of paramagnetism with very high susceptibility to the magnetic forces, and may possess permanent magnetism. The direct proportionality of the magnetic field and induced field causes a strong interaction between atoms in a nucleus and results in a parallel alignment between atoms and against the force of thermal motion. Thus, ferromagnetic materials are more susceptible to a magnetic field compared to paramagnetic materials (Svoboda, 1987).

Paramagnetic response to magnetism is as a result of the competing aligning effect of the applied field and the random effect of the thermal vibrations. If, for an instant, an atom with a free electron in its outermost shell experiences an applied magnetic field, it will tend to react by moving towards the highly magnetic field regions. Particles are magnetised to some degree when they enter into the magnetic field and act as a magnetic dipole (Svoboda, 1987).

In the case of diamagnetic particles, the electrical charges tend to shield the internal shell of the atom. While a particle is under the influence of the applied field, it will move in the opposite direction, thus repelled from the magnetic field. It is for this reason that diamagnetic with weak magnetic attributes are not processed magnetically (Svoboda, 1987; Chakravorty, 1989; Yves et al., 2009).

Coal is also classified as a weakly diamagnetic material which contains minerals associated within the organic matrix, some of which may be iron minerals. Magnetic separation may be used for coal beneficiation when the gangue minerals contain such iron phases, however their very weak magnetic susceptibilities would require strong magnetic field strength (Dwari and Rao, 2009).

Previous works on the magnetic separation of pyrite from coal have shown pyrite to be a weakly paramagnetic mineral. However, with the transformation into another form called pyrrhotite through caustic microwave treatment and microwave pre-treatment (Rowson and Rice, 1989; Butcher and Rowson 1994), pyrrhotite can be converted into a strongly paramagnetic form and can easily be separated from coal in only moderate magnetic field strengths.

In India the demand for quality steel has been estimated to be between 56 Mt to 200 Mt in the next decade and, currently, known reserves were estimated to be able to supply steel plants with only 13 Bt to 14 Bt of iron ore concentrate in the next 35 to 40 years (Das et al., 2010).

In South Africa the Postmasburg mine located in the Northern Cape Province stockpiled or discarded 3.48 Mt iron ore as waste (AI-Wakeel and EI-Rahman, 2006; Zogo, 2009, Li et al., 2010; Angadi et al., 2012; Kumba, 2012). In terms of coal, the Grootegeluk colliery was reported as contributing 18 Mtpa of reactive coal discard. This is prone to spontaneous combustion and is therefore a specific environment hazard (An independent competent persons report on the mining assets of Exxaro Resources Limited accessed 16 Jan 2013).

These findings have motivated researchers to explore new beneficiation techniques for treating these high quantities of fine ores and discarded slimes as secondary resources worldwide. Treating these fines and slimes provides the potential for recycling discards, environmental pollution control and extending the resources of both ferrous and non-ferrous low grade ores (Oberteuffer, 1974; Das et al., 2010; Monhaty et al., 2010).

Recent studies conducted by researchers have shown that magnetic separation is a technique that could be used for pollution control, for waste water recycling and for improving beneficiation of low grade ferrous ores. Chen et al., (2012) conducted an investigation using a Vibrating High Gradient Magnetic Separator (VHGMS) for the removal of ferrous minerals (hematite and limonite) from kaolin (clay). The results reported a kaolin product grade of 0.50% Fe203 with an 84.56% mass yield, and at a 42.08% iron removal rate and the results were found to be acceptable for commercial application.

Jaimeson et al. (2006) conducted a magnetic separation investigation on Darling Range Red Sand using a combination of Low Intensity Magnetic Separator (LIMS) and Wet High Intensity Magnetic Separator (WHIMS). The results reported ~56% as Fe203 to the magnetic fraction and a relatively clean non-magnetic fraction composed of <4% Fe203. This technology was seen as having the potential to convert large volumes of hematite from stockpiles and slimes dams into commercial products.

Li et ai., (2011) used a magnetic separator for the recycling of red mud tailings, known as by-product in the aluminium industry. The feasibility study showed that it was possible to separate red mud tailings into high iron content and low iron content products with the former possessing a grade appropriate in iron-making and the latter with the potential for being recycled in a sintering process for alumina production for use as a construction material.

Furthermore, the application of magnetic separation for the concentration of diamagnetic material such as colemanite (CaB304(OH)3.H20) from weakly magnetic material was investigated by Alp, (2006). The results obtained showed a mass reduction of 31.4 7% in plant tailing disposal, and produced a colemanite concentrate with a commercially acceptable grade of 43.74% B203 at a high recovery rate of 95.06%. Economic and environmental benefits were thus attained.

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