high efficiency high gradient magnetic separators with capacity 10 500tph

gradient magnetic separation - an overview | sciencedirect topics

gradient magnetic separation - an overview | sciencedirect topics

High gradient magnetic separation (HGMS) (Trindale et al., 1974, and Liu, 1982) is based on coal being diamagnetic (repulsed by a magnet), whereas pyrite is paramagnetic (attracted to a magnet). The magnetic susceptibility of pyrite is 0.3 10-6 G/g (G = gauss), compared to coal at -0.4 10-6 to 0.8 10-4 G/g. If pyrite is not altered to the more magnetic pyrrhotite, perhaps via microwave heating, the separation can be enhanced through control of the particle size and/or through control of the field strength.

With the primary purpose of HGMS being the removal of pyrite from fine coal, experiments on high-S Illinois Basin coals by Murray (1977), Hise et al. (1979), Harris and Hise (1981), and Hower et al. (1984) were a logical test of the concept. Harris and Hise (1981) noted an increase in inertinite in the magnetic fraction, perhaps a function of coarse pyrite with fusain, while Hower et al. (1984) did not see much maceral difference between the clean and the refuse. Hower et al. (1984) noted that most of the pyrite in the clean coal was very fine (98% <10 m in one case).

High-gradient magnetic separation has achieved remarkable progress and wide applications since Jones in 1955 achieved a high-gradient magnetic field in a magnetized matrix (Svoboda, 2004). By using magnetic matrix, magnetic force upon magnetic particles is remarkably increased, resulting in the significantly improved recovery for fine weakly magnetic particles.

However, it has undergone a long period in the development of high-gradient magnetic separators, from cyclic to continuous operation, and from the horizontal design philosophy of separating ring to the vertical one. At the same time, magnetic matrix, as carrier for the capture of magnetic particles in a high-gradient magnetic separator, has evolved from the early used fibers and balls to meshes, and to the present rod poles. Now, the rod matrix made of rod poles has proven the most applicable medium in the magnetic capture of fine weakly magnetic particles from slurry in a high-gradient magnetic separator, due to its high operational reliability, simplified combinatorial optimization, and resistance to clogging (Chen etal., 2013a).

Physical processes (e.g., physical retrieval; over-packing/re-packaging/re-drumming; screening; soil washing; high gradient magnetic separation; solidification; vitrification/ceramics; incineration; filtration/ultra-filtration; reverse osmosis/membrane processes; solar evaporation); Fig. 8.5 shows a general view of the soil washing plant at Kurchatov Centre (Russia).

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.

Mssbauer spectroscopy was used to document the efficiency of pyrite and clay removal from raw coal by float-sink methods using organic solvents to simulate washing in coal-preparation plants. For coal from the Upper Freeport seam, it was found that pyrite could be removed more efficiently than clays by such procedures. This result is not surprising because clay minerals have densities considerably closer to that of coal than pyrite, and the efficiency of removal can be expected to be a function of the density difference unless the particle-size distributions of the minerals are quite different.

Similar tests of the efficiency of high-gradient magnetic separation (HGMS) were also made by Mssbauer spectroscopy and showed that the removal of siderite (FeCO3) was more efficient than removal of either pyrite or clay minerals. Detailed magnetic studies of this process have suggested that pyrrhotite formation may be responsible for the necessary enhancement of the magnetic properties of pyrite in order for pyrite to be removed efficiently by HGMS. High localized temperatures generated upon crushing of the coal were suggested to be responsible for converting a surface layer on pyrite particles to pyrrhotite.

One further application of Mssbauer spectroscopy, examined by Jacobs et al. (1978), was to use the technique to investigate mineral transformations during coal-liquefaction processes. The objective was to find the procedure for liquefaction that also generated the most highly magnetic form of pyrrhotite (Fe1-xS) for possible application of HGMS. Both Mssbauer and magnetic studies showed that most of the pyrite is converted during liquefaction of coal to pyrrhotite, which may be weakly or strongly ferrimagnetic, depending on the temperature-time pathways of the processes. By slow cooling from 225C, a highly ferrimagnetic form of pyrrhotite, monoclinic Fe7S8, can be obtained, which is the preferred form for removal by HGMS.

This process is being investigated in the separation of coal and pyrite. The difference in magnetic susceptibility of coal and pyrite is very small; therefore, the most promising application is for high-gradient magnetic separators (HGMS) or the pretreatment of coal, which could increase the susceptibility of coal mineral matter.

Because pyrite liberation usually requires very fine grinding (e.g., below 400 mesh), high-gradient magnetic separation, the process developed to upgrade kaolin clays by removing iron and titanium oxide impurities, seems to be particularly suitable for coal desulfurization. Wet magnetic separation processes offer better selectivity for the fine dry coal that tends to form agglomerates. Experiments on high-gradient wet magnetic separation have so far indicated better sulfur rejection at fine grinding.

Perhaps the most interesting studies are on the effect of coal pretreatment, which enhances the magnetic properties of the mineral matter, on the subsequent magnetic separation. It has, for example, been shown that microwave treatment heats selectively pyrite particles in coal without loss of coal volatiles, and converts pyrite to pyrrhotite, and as a result, facilitates the subsequent magnetic separation.

In the Magnex process, crushed coal is treated with vapors of iron carbonyl [Fe(CO)5], followed by the removal of pyrite and other high-ash impurities by dry magnetic separation. Iron carbonyl in this process decomposes on the surfaces of ash-forming minerals and forms strongly magnetic iron coatings; the reaction with pyrite leads to the formation of pyrrhotite-like material.

In 1970, Krukiewicz and Laskowski described a magnetizing alkali leaching process, which was applied to convert siderite particles into magnetic -Fe2O3 and Fe3O4. A similar process has recently been tested for high-sulfur coal, and it was reported that, after pretreatment of the coal in 0.5-M NaOH at 85C and 930kPa (135psi) air pressure for 25min, more than 50% of the coal sulfur was rejected under the same conditions in which the same high-gradient magnetic separation had removed only 5% sulfur without the pretreatment.

where 0 is the permeability constant of vacuum, Vp and Mp are the particle volume and magnetization, respectively and H is the strength of the magnetic field gradient at the location of the particle. Mp is given by Mp=H where is the magnetic volume susceptibility, which changes with particle size and shape, and H is the magnetic field strength.

High-gradient magnetic separation (HGMS) is an established concept where a magnetic field is typically applied across an array of magnetically soft metal wires to establish a high-field gradient.99,100 Magnetic nanoparticles travelling through such a field are attracted to the surfaces of the wires allowing effective collection. However, the operation of such a separator is strongly dependent on the size and magnetic properties of the particles involved and the strength of the magnetic field must be high enough to overcome possible competing colloidal forces acting on a flowing particle suspension arising from fluid drag, gravity, inertia and diffusion.99

The gravitational force may be expressed as Fg=(pg)Vpg where p and g are the spherical particle and fluid densities, respectively, g is the acceleration due to gravity and Vp is the particle volume. Hydrodynamic fluid drag results in a force according to Stokes Law: Fd=6b(vfvp) where is the dynamic viscosity of the fluid, b is a fluid-dependent constant and vf and vp are the fluid and particle velocities, respectively. In many of the laboratory-based ex situ demonstrations of removal of a contaminant from its suspension, a handheld magnet is usually strong enough to demonstrate magnetic separation over length scales of a few centimetres. This range may be sufficient for effective separation in, for example, bioseparation, but for industrial-scale in situ environmental remediation, a much greater separation distance is usually required necessitating the use of a stronger magnetic field as the dependence of field strength on distance follows an inversecube relationship (for a dipole source). Other significant factors acting against magnetic separation include buoyancy and random Brownian motion, both of which are enhanced as the particle becomes smaller. For Fe oxide, the critical diameter of a nanoparticle at which its volume becomes large enough for the magnetic force exerted on it to overcome Brownian motion is ~50nm, assuming large magnetic field strengths typically used in HGMS are applied.101

Yavuz et al. first demonstrated magnetic separation with very low fields (<100Tm1) using monodisperse 12nm Fe3O4 nanoparticles that had previously been applied to the removal of As from water.101 They demonstrated multiplexed separation of an initially bimodal mixture through the application of different magnetic fields and noted that aggregation, caused by high-field gradients at the surface of the nanoparticles, actually helped the separation process (Figure 7.12).

Figure 7.12. (A) Magnetic separation of 16-nm water-soluble Fe3O4 nanoparticles within several minutes using a low-field gradient of 23.3Tm1. (B) and (C) Adsorption isotherms comparing the removal of As3+ (A) and As5+ (B) using Fe3O4 nanoparticles of different diameters. Concentrations were measured using inductively coupled plasma-mass spectrometry (ICP-MS).

The magnetizing force, which induces the lines of force through a material, is called the field intensity, H (or H-field), and by convention has the units ampere per meter (Am1) (Bennett et al., 1978).

The intensity of magnetization or the magnetization (M, Am1) of a material relates to the magnetization induced in the material and can also be thought of as the volumetric density of induced magnetic dipoles in the material. The magnetic induction, B, field intensity, H, and magnetization, M, are related by the equation:

where 0 is the permeability of free space and has the value of 4107NA2. In a vacuum, M=0, and M is extremely low in air and water, such that for mineral processing purposes Eq. (13.1) may be simplified to:

so that the value of the field intensity, H, is directly proportional to the value of induced flux density, B (or B-field), and the term magnetic field intensity is then often loosely used for both the H-field and the B-field. However, when dealing with the magnetic field inside materials, particularly ferromagnetic materials that concentrate the lines of force, the value of the induced flux density will be much higher than the field intensity. This relationship is used in high-gradient magnetic separation (discussed further in Section 13.4.1). For clarity it must be specified which field is being referred to.

For paramagnetic materials, is a small positive constant, and for diamagnetic materials it is a much smaller negative constant. As examples, from Figure 13.1 the slope representing the magnetic susceptibility of the material, , is about 0.001 for chromite and 0.0001 for quartz.

The magnetic susceptibility of a ferromagnetic material is dependent on the magnetic field, decreasing with field strength as the material becomes saturated. Figure 13.2 shows a plot of M versus H for magnetite, showing that at an applied field of 80kAm1, or 0.1T, the magnetic susceptibility is about 1.7, and saturation occurs at an applied magnetic field strength of about 500kAm1 or 0.63T. Many high-intensity magnetic separators use iron cores and frames to produce the desired magnetic flux concentrations and field strengths. Iron saturates magnetically at about 22.5T, and its nonlinear ferromagnetic relationship between inducing field strength and magnetization intensity necessitates the use of very large currents in the energizing coils, sometimes up to hundreds of amperes.

The magnetic force felt by a mineral particle is dependent not only on the value of the field intensity, but also on the field gradient (the rate at which the field intensity increases across the particle toward the magnet surface). As paramagnetic minerals have higher (relative) magnetic permeabilities than the surrounding media, usually air or water, they concentrate the lines of force of an external magnetic field. The higher the magnetic susceptibility, the higher the induced field density in the particle and the greater is the attraction up the field gradient toward increasing field strength. Diamagnetic minerals have lower magnetic susceptibility than their surrounding medium and hence expel the lines of force of the external field. This causes their expulsion down the gradient of the field in the direction of the decreasing field strength.

The equation for the magnetic force on a particle in a magnetic separator depends on the magnetic susceptibility of the particle and fluid medium, the applied magnetic field and the magnetic field gradient. This equation, when considered in only the x-direction, may be expressed as (Oberteuffer, 1974):

where Fx is the magnetic force on the particle (N), V the particle volume (m3), p the magnetic susceptibility of the particle, m the magnetic susceptibility of the fluid medium, H the applied magnetic field strength (Am1), and dB/dx the magnetic field gradient (Tm1=NA1m2). The product of H and dB/dx is sometimes referred to as the force factor.

Production of a high field gradient as well as high intensity is therefore an important aspect of separator design. To generate a given attractive force, there are an infinite number of combinations of field and gradient which will give the same effect. Another important factor is the particle size, as the magnetic force experienced by a particle must compete with various other forces such as hydrodynamic drag (in wet magnetic separations) and the force of gravity. In one example, considering only these two competing forces, Oberteuffer (1974) has shown that the range of particle size where the magnetic force predominates is from about 5m to 1mm.

The normal particles of benign wear of sliding surfaces. Rubbing wear particles are platelets from the shear mixed layer which exhibits super-ductility. Opposing surfaces are roughly of the same hardness. Generally the maximum size of normal rubbing wear is 15m.

Break-in wear particles are typical of components having a ground or machined surface finish. During the break-in period the ridges on the wear surface are flattened and elongated platelets become detached from the surface often 50m long.

Wear particles which have been generated as a result of one surface penetrating another. The effect is to generate particles much as a lathe tool creates machining swarf. Abrasive particles which have become embedded in a soft surface, penetrate the opposing surface generating cutting wear particles. Alternatively a hard sharp edge or a hard component may penentrate the softer surface. Particles may range in size from 25m wide and 25 to 100m long.

Fatigue spall particles are released from the stressed surface as a pit is formed. Particles have a maximum size of 100m during the initial microspalling process. These flat platelets have a major dimension to thickness ratio greater than 10:1.

Laminar particles are very thin free metal particles between 2050m major dimension with a thickness ratio approximately 30:1. Laminar particles may be formed by their passage through the rolling contact region.

There is a large variation in both sliding and rolling velocities at the wear contacts; there are corresponding variations in the characteristics of the particles generated. Fatigue particles from the gear pitch line have similar characteristics to rolling bearing fatigue particles. The particles may have a major dimension to thickness ratio between 4:1 and 10:1. The chunkier particles result from tensile stresses on the gear surface causing fatigue cracks to propagate deeper into the gear tooth prior to pitting. A high ratio of large (20m) particles to small (2m) particles is usually evident.

Severe sliding wear particles range in size from 20m and larger. Some of these particles have surface striations as a result of sliding. They frequently have straight edges and their major dimension to thickness ratio is approximately 10:1.

The size and position of the particles after magnetic separation on a slide indicates their magnetic susceptibility. Ferromagnetic particles (Fe, Co, Ni) larger than 15m are always deposited at the entry or inner ring zone of the slide. Particles of low susceptibility such as aluminium, bronze, lead, etc, show little tendency to form strings and are deposited over the whole of the slide.

High-gradient magnetic separation (HGMS) with B>1000T/m [37] has been successfully used for the separation of microalgae from lakes for more than thirty years [42]. Recently, a lab-scale HGMS was applied for the magnetic separation of both freshwater algae and marine microalgae, and demonstrated the potential of HGMS for efficient microalgae harvesting using magnetic particles [50]. Toh et al., investigated the performance of both HGMS and low gradient magnetic separation (LGMS, B<80T/m) with varying dosages of magnetic particles. At a low particle dosage, the HGMS resulted in a higher RE compared with that of LGMS due to the high magnetophoresis kinetics under the high field gradient. However, the LGMS achieved an equal RE with that of HGMS when a high particle dosage was tested [37]. In HGMS, the high power consumption is necessary for magnetic power generation while the magnetic field can be simply generated by permanent magnet arrays in LGMS. Therefore, the LGMS system was more cost-effective due to its low energy consumption, and a system can be easily designed [37]. Although HGMS has been widely used in manufacturing, LGMS is more widely used in biotechnology processes [25,6062]. Furthermore, the LGMS has been used in magnetic harvesting of microalgae and shows good potential [46,49]. The magnetic field in LGMS can be generated without extra power and the separation time is even less than 3min using nanorod particles, and it may be concluded that the LGMS technique is more energy and time-efficient and is a better option for microalgae harvesting compared with HGMS technology [46].

gradient magnetic separator - an overview | sciencedirect topics

gradient magnetic separator - an overview | sciencedirect topics

SLon pulsating high-gradient magnetic separator as shown in Figure12 has been innovatively developing since 1986, based on the principle of pulsating high-gradient magnetic separation, and it is now the mostly applied high-gradient magnetic separator over the world, widely used for concentration of fine weakly magnetic ores such as hematite, ilmenite, etc., and for purification of nonmetallic ores such as quartz, feldspar, etc.

Figure12. SLon pulsating high gradient magnetic separator: 1 = pulsating mechanism, 2 = energizing coils, 3 = magnetic yoke, 4 = separating ring, 5 = feed box, 6 = wash water box, 7 = concentrate flush, 8 = concentrate boxes, 9 = middling chute, 10 = tailings boxes, 11 = slurry level box, 12 = ring driver, 13 = support frame, F = feed, W=water, C = concentrate, M = middling, T = tailings.

It can be seen from Figure12, SLon pulsating high-gradient magnetic separator mainly consists of pulsating mechanism, energizing coils, magnetic yoke, separating ring, and feed and product boxes. In most cases, rods matrix made of magnetic stainless steel is used as magnetic matrix.

While working, a direct current flows through the energizing coils and a magnetic field is built up in the separating zone. The separating ring with magnetic matrix rotates around its horizontal axis. Slurry is fed from feed box and enters into the matrix moving through the separating zone. Magnetic particles are attracted from slurry onto the surface of matrix and brought to the top of separating ring, where magnetic field is negligible, and is flushed into concentrate boxes. Nonmagnetic particles flow through the matrix and enter into the tailings boxes under the combined actions of pulsating, gravity, and hydrodynamic drag. As the flushing direction of water is opposite to that of feed for each matrix pile, coarse particles will be flushed out without necessity to pass through the matrix depth, so that the clogging of matrix is avoidable. Moreover, the pulsating mechanism below the magnetic yoke drives the slurry in the separating zone up and down, keeping particles in the zone in a loose state, thus magnetic particles will be more easily captured by matrix and nonmangetic particles will be more easily dragged out as tailings.

Obviously, the vertical rotation of ring prevents matrix from being clogged and the pulsation of slurry improves the selectivity of separation. Due to these two innovative designs, SLon magnetic separators possess such merits as high efficiency, high reliability and flexibility, etc., in comparison to conventional high-gradient magnetic separators.

There are more than 3000 of SLon magnetic separators in operation in the field of mineral processing. It treats materials in a wide range of solids throughput from 100g per batch in SLon-100 cyclic pilot-scale magnetic separator to several hundred tons per hour in SLon full-scale magnetic separators. The technical specifications of SLon magnetic separators are listed in Table 6.

Figure13 shows the largest SLon-4000 full-scale pulsating high-gradient magnetic separator and its operation for concentration of fine ilmenite from tailings at Panshihua Iron and Steel Company in southwest Sichuan province of China. It achieved a much superior performance to conventional separators, at a solids throughput reaching 570 t/h.

The SLon vertical ring and pulsating high-gradient magnetic separator (SLon magnetic separator or SLon) was developed in 1981 (Xiong et al., 1998). It has the advantages of high efficiency, high reliability, and low operating cost. Nowadays, it is one of the most popular magnetic separators used for upgrading oxidized iron ores (Xiong, 2006, 2008, 2010, 2012).

The SLon magnetic separator utilizes the combined forces of magnetic fields, pulsating fluids, and gravity to beneficiate weakly magnetic minerals. It consists of 10 main parts, including an energizing coil, a magnetic yoke, and a separating ring, as shown in Figure 9.7. The pulsating frequency and stroke are adjustable. The energizing coil is made of hollow copper tube and cooled internally with water. Along the periphery of the separating ring, a number of rod matrix piles made of magnetic stainless steel are located. The rods in the matrix have diameters varying from 1 to 5mm depending on the characteristics of the iron ores to be processed. When the SLon magnetic separator is in operation, the ring with the matrix rotates around its horizontal axis.

Figure 9.7. SLon vertical ring and pulsating high-gradient magnetic separator. (1) Pulsating mechanism, (2) energizing coil, (3) magnetic yoke, (4) separating ring, (5) feed box, (6) wash box, (7) magnetic flushing device, (8) magnetic collecting box, (9) middling chute, (10) nonmagnetic box. F, feed, W, water, C, concentrate (magnetic), M, middling, T, tails (nonmagnetic).

When a direct electric current passes through the energizing coils, a high-intensity magnetic field of high gradient is established in the separating zone in the lower portion of the separating ring located in the electromagnet system, where the background field intensity is 01.3T and adjustable. The intensity of the focused magnetic field on the surface of the matrix rods can reach up to 2T. The slurry fed from the feeding boxes (on both sides of the ring) enters the matrix located in the separating zone. Magnetic particles are attracted from the slurry onto the surface of the matrix, while nonmagnetic particles pass through the matrix and enter the nonmagnetic product collecting box. When the rod matrix reaches the top of the ring where the magnetic field is close to zero, the adhering magnetic particles are washed out into the magnetic product boxes.

Since the ring rotates in the vertical plane, the flushing direction of the magnetic particles is opposite to the direction of the slurry feed relative to each matrix pile, so coarse particles can be washed out without having to pass through the entire depth of the matrix pile. The pulsating mechanism drives the slurry up and down and keeps the particles in a loose state in the separating zone all the time. Thus, magnetic particles can be more easily recovered by the matrix, and the nonmagnetic particles can be more easily dragged out through the matrix.

The opposing flushing and pulsation help to prevent matrix clogging, and the pulsation helps to purify the magnetic product. These measures guarantee that SLon magnetic separators offer the advantages of higher grade and higher recovery of the magnetic product and more flexibility.

This process is being investigated in the separation of coal and pyrite. The difference in magnetic susceptibility of coal and pyrite is very small; therefore, the most promising application is for high-gradient magnetic separators (HGMS) or the pretreatment of coal, which could increase the susceptibility of coal mineral matter.

Because pyrite liberation usually requires very fine grinding (e.g., below 400 mesh), high-gradient magnetic separation, the process developed to upgrade kaolin clays by removing iron and titanium oxide impurities, seems to be particularly suitable for coal desulfurization. Wet magnetic separation processes offer better selectivity for the fine dry coal that tends to form agglomerates. Experiments on high-gradient wet magnetic separation have so far indicated better sulfur rejection at fine grinding.

Perhaps the most interesting studies are on the effect of coal pretreatment, which enhances the magnetic properties of the mineral matter, on the subsequent magnetic separation. It has, for example, been shown that microwave treatment heats selectively pyrite particles in coal without loss of coal volatiles, and converts pyrite to pyrrhotite, and as a result, facilitates the subsequent magnetic separation.

In the Magnex process, crushed coal is treated with vapors of iron carbonyl [Fe(CO)5], followed by the removal of pyrite and other high-ash impurities by dry magnetic separation. Iron carbonyl in this process decomposes on the surfaces of ash-forming minerals and forms strongly magnetic iron coatings; the reaction with pyrite leads to the formation of pyrrhotite-like material.

In 1970, Krukiewicz and Laskowski described a magnetizing alkali leaching process, which was applied to convert siderite particles into magnetic -Fe2O3 and Fe3O4. A similar process has recently been tested for high-sulfur coal, and it was reported that, after pretreatment of the coal in 0.5-M NaOH at 85C and 930kPa (135psi) air pressure for 25min, more than 50% of the coal sulfur was rejected under the same conditions in which the same high-gradient magnetic separation had removed only 5% sulfur without the pretreatment.

Future developments and applications of magnetic separation in the mineral industry lie in the creation and use of increasingly higher product of field and field gradient, that is, the force factor. Matrix separators with very high field gradients and multiple small working gaps can draw little benefit from field strengths above the saturation levels of the secondary poles (~2T for an iron/steel matrix material). As discussed in Section 13.4.1, the alternative to HGMS is OGMS, where separators with large working volumes deflect coarser particles at high capacity, rather than capture particles, as in HGMS. As the gradient in OGMS is relatively low, these separators need to use the highest possible field strengths to generate the high magnetic forces required to treat weakly paramagnetic particles. Field strengths in excess of 2T can only be generated economically by the use of superconducting magnets (Kopp, 1991; Watson, 1994).

Certain alloys have the property of presenting no resistance to electric currents at extremely low temperatures. An example is niobiumtitanium at 4.2K, the temperature of liquid helium. Once a current is established through a coil made from a superconducting material, it will continue to flow without being connected to a power source, and the coil will become, in effect, a permanent magnet. Superconducting magnets can produce extremely intense and uniform magnetic fields, of up to 15T. The main problem, of course, is in maintaining the extremely low temperatures. In 1986, a Ba/La/Cu oxide composite was made superconductive at 35K, promoting a race to prepare ceramic oxides with much higher superconducting temperatures (Malati, 1990). Unfortunately, these materials are of a highly complex crystal structure, making them difficult to fabricate into wires. They also have a low current-carrying capacity, so it is likely that for the foreseeable future superconducting magnets will be made from ductile niobium alloys, embedded in a copper matrix.

The main advantage of superconducting separators is that elevated magnetic field strength increases the maximum feed slurry velocity with a corresponding increase in capacity (Kopp, 1991). In order to fully utilize this capacity, downtime for removal of accumulated magnetic particles from the working volume of the separator must be minimized through the use of a reciprocating or continuously cycling matrix (Kopp, 1991). Another advantage of these separators is the reduced weight of the separators (smaller coils and windings along with much less iron required compared to the heavy frames and matrix materials used in HGMS) (Gillet and Diot, 1999). The factors limiting the adoption of superconducting separators are the difficulties in maintaining the very low temperatures necessary for the material to retain its superconducting properties against heat leaks, and the high energy costs associated with maintaining this refrigeration (Kopp, 1991). Superconducting magnets are generally only viable when large field volumes and magnetic fields greater than 2T are required (Kopp, 1991).

In 1986, a superconducting HGMS was designed and built by Eriez Magnetics to remove magnetic (and colored) contaminants from kaolinite clay for operations in the United States. This machine used only about 0.007kW in producing 5T of flux, the ancillary equipment needed requiring another 20kW. In comparison, a conventional 2T high-gradient separator of similar throughput would need about 250kW to produce the flux, and at least another 30kW to cool the magnet windings.

The 5T machine is an assembly of concentric components (Figure 13.27). A removable processing canister is installed in a processing chamber located at the center of the assembly. This is surrounded by a double-walled, vacuum-insulated container that accommodates the superconductive niobium/titaniumtantalum winding and the liquid helium coolant. A thermal shield, cooled with liquid nitrogen to 77K, limits radiation into the cryostat. In operation, the supply of slurry is periodically cut off, the magnetic field is shut down, and the canister backwashed with water to clear out accumulated magnetic contaminants.

An open-gradient drum magnetic separator with a superconducting magnet system has been operating commercially since the 1980s (Unkelbach and Kellerwessel, 1985; Wasmuth and Unkelbach, 1991). Although separation is identical to that in conventional drum separators, the magnetic flux density at the drum surface can reach over 4T.

The development of HGMS and superconducting separators capable of concentrating very fine or very weakly magnetic mineral particles has prompted the application of magnetic separation techniques to treat many waste streams from mineral processing operations. Fine (<10m), weakly magnetic hematite and limonite have been recovered by a combination of selective flocculation using sodium oleate and kerosene followed by HGMS (Song et al., 2002). HGMS has been used to recover fine gold-bearing leach residues from uranium processing, and fine Pb minerals containing V and Zn from a mining waste dump (Watson and Beharrell, 2006). A single-stage extraction of ilmenite from highly magnetic gangue minerals has been developed using a superconducting HGMS system (the difference in magnetic susceptibility between ilmenite and gangue is only significant at very high magnetic field strength). However, this process is still faced with the typical challenges associated with an industrial installation of a superconducting separator (Watson and Beharrell, 2006). Another interesting, and potentially significant, application of HGMS is in the treatment of wastewater streams containing heavy metal ions. Multiple authors have developed processes where the metal ions to be removed are coprecipitated with Fe ions to form a fine, dispersed magnetite phase which can be easily extracted through the use of HGMS (Gillet et al., 1999; Karapinar, 2003).

In recent years there has been a growing recognition that imposed magnetic, electric, ultrasonic and vibration force fields can be used to improve separation processes. Although the mechanisms of operation are not always clear, their use has shown considerable promise in flux enhancement and process intensification (see, for instance, Bollinger and Adams, 1984; Birss and Parker, 1981; Gundogdu et al, 2003; Kyllnen et al, 2005; Lin and Benguigui, 1983; Murkes and Carlsson, 1988; Park, 2005; Saveyn et al, 2003; Tarleton, 1992; Wakeman, 1982b; Wakeman and Tarleton, 1991b; Watson, 1990). Vibration assisted filters are described in Section 1.6.3.

Ferromagnetic solids of high magnetic permeability can be separated in a Low Intensity Magnetic Separator (LIMS) using permanent magnets of less than 2 T (see Figure 1.56). A typical unit operates continuously and comprises a rotating non-magnetic drum inside which four to six stationary magnets are placed. The wet or dry feed contacts the outer periphery of the drum and the magnetically susceptible particles are picked up and discharged leaving the weakly or non-magnetic material to pass by largely unaffected. Alternative designs include the disc separator and the cross-belt separator where dry solids are conveyed towards a cross-belt which moves across a series of permanent magnets.

The efficiency of magnetic separation is generally improved by maximising both the intensity and the gradient of an applied non-uniform field. By doing so paramagnetic material of low magnetic permeability can be separated in a High Intensity Magnetic Separator (HIMS). Electromagnets, with intensities in excess of 2 T, are used in continuous equipment such as the Jones rotating disc separator to affect separations of dry feeds down to 75 m and wet feeds to finer sizes. Very weakly paramagnetic material cannot usually be separated satisfactorily with a HIMS, and a High Gradient Magnetic Separator (HGMS) must be used (Figure 1.56). In these units a matrix of fine stainless steel wool is placed between the poles of either electromagnetic or superconducting magnets, the latter generating magnetic intensities up to 15 T. Very high magnetic gradients are produced adjacent to the wool fibres and this allows for the separation of very fine particulates. Although the capital cost of HGMS can be relatively high compared with more conventional equipment, commercial units are readily available.

Dielectrophoretic separators utilise 1025 kV non-uniform DC and AC electric fields to remove particles from dilute, generally non-aqueous suspensions. Particles moving through the electric field are polarised by redistribution of their surface and/or internal charge and (usually) move towards the region of highest field intensity where they concentrate. Small scale dielectrophoretic separators employ relatively simple wire and plate or wire and cylinder electrode arrangements. Larger scale units employ a high porosity dielectric matrix between two, generally insulated, electrodes to form a High Gradient Dielectrophoretic Separator (HGDS, see Figure 1.57). The feed suspension flows through the matrix and field gradients up to 10 kV cm1 induce sufficiently large dielectrophoretic forces to capture fine particles at the fibre surfaces. Cleaning of the clogged matrix is achieved by simply switching off the electric field. Although dielectrophoretic separators have found uses in the petroleum and biotechnology industries, operational problems can arise with suspension decomposition, current leakage and electric field generation.

Uniform and non-uniform DC electric fields with field gradients less than 100 V cm1 can be combined with more conventional filtration and deliquoring apparatus to improve separation rates and reduce overall operating costs (see Figure 1.58). Electrokinetic phenomena such as electrophoresis (movement of particles) and electroosmosis (movement of liquid) can be observed when electric fields are applied to filtering suspensions. It is a prerequisite that the majority of particles in the feed are less than 5 m in size and exhibit an average (absolute) zeta potential greater than 20 mV. Rates of separation can typically be improved by more than an order of magnitude with the application of a suitable electric field. This has facilitated the construction and commercial operation of modified leaf and belt filters and diaphragm filter presses for finer particle separations. Moreover, laboratory scale investigations with very low crossflow velocities (0.1 m s1) and uniform DC fields have shown significantly reduced fouling in membrane systems. So-called electrofilters show considerable promise in the processing of colloidal suspensions where the potential for enhancements in separation rates is greater.

Figure 1.58. Application of low voltage electric force fields to the dead-end vacuum filtration of 0.01% v/v aqueous bentonite suspensions (top) and the crossflow filtration of 1.4% v/v aqueous anatase suspensions (bottom).

Ultrasound, with a sound wave frequency in excess of 16 kHz, is known to induce particle agglomeration, particulate dispersion, enhanced reaction rates and enhanced separation rates when conditions allow. The ultrasound is either applied prior to separation to condition the feed or during separation to reduce/prevent particle deposition at a filtering surface. However, as relatively little work has been done in applying ultrasound directly within solid/liquid separation processes, the enhancements of separations rates claimed to date have been relatively modest. It is realised that ultrasound and low voltage electric fields combine together in a synergistic manner and that the phenomenon could potentially offer significant processing advantages.

Related Equipments