WHIM is the short acronym for Wet High Intensity MagneticSeparation.At present, most U.S. iron raw materials are produced from magnetic taconites, which are ground to a nominal minus 270 mesh (53 micrometers), beneficiated by wet low-intensity magnetic separation, and pelletized. The taconite ore bodies generally contain 19 to 25 percent magnetic iron, defined as iron occurring in the mineral magnetite. Iron occurring in other minerals, such as iron silicates, siderite, and hematite, is generally lost in the tailings.
The iron-bearing materials intermediate in properties between the high-grade enriched iron ore deposits and the parent taconite are the oxidized taconites. The magnetite has been oxidized to hematite, and the iron silicates and siderite have been converted to goethite, a hydrated iron oxide. There may be some iron enrichment due to natural leaching of silica and migration and precipitation of iron ions in secondary iron oxide layers. Oxidized taconite is also called nonmagnetic taconite to distinguish it from the unaltered magnetic taconite. The western part of Minnesotas Mesabi Range contains in excess of 10 billion tons of oxidized taconite containing 30 to 40 percent iron. Typically, oxidized taconite surrounds and underlies an exhausted natural ore pit, while underlying the oxidized taconite is the parent taconite. In open pit mining of taconite, oxidized taconite is often removed as waste material. In some cases, oxidized taconite is being stockpiled for future use, in other cases, it is discarded with the overburden. The development of an economical method for beneficiating oxidized taconites would utilize a current waste material and convert the oxidized taconite from an iron ore resource to a reserve.
In the reduction roasting process, the hematite and goethite in the ore are converted to magnetite by heating in a reducing atmosphere at approximately 800 C. Although the first experimental plant was built on the Mesabi Range in 1934, the process has not been used commercially. The high operating cost of the energy consumed in roasting is partially compensated by reduced grinding costs and by energy returned in the conversion of magnetite to hematite during pellet induration. The capital cost, which is also high, would be reduced if roasted ore were processed in an existing magnetic separation plant. However, processing of reduction-roasted ore in a conventional taconite plant would require some modifications, because artificial magnetite has a higher remanence and coercivity than natural magnetite. After exposure to a magnetic field, very strong magnetite floccules, which entrain gangue particles, are formed. Demagnetization requires special high-frequency coils. Pilot plant results recently obtained by the Bureau of Mines show that the silica content of the artificial magnetite concentrate can be reduced to less than 5 percent while iron recovery is maintained at 80 percent by including a froth flotation step after the final magnetic separation stage.
Highly metamorphosed schistose iron ore from the Marquette range on Michigans Upper Peninsula has been successfully treated by froth flotation since 1956. The hematite in this ore occurs in the form of specularite, which is more easily beneficiated than the red or dull black hematite typical of oxidized taconites. The development of selective flocculation by the Bureau of Mines and research by the Cleveland-Cliffs Iron Co. made flotation a feasible beneficiation method for the bulk of the Lake Superior regions oxidized taconite resources. The research resulted in the opening of the Tilden plant (Marquette range, Michigan), which has a capacity of 4 million tons of pellets per year; long-term plans call for tripling the plants capacity. Mesabi Range oxidized taconites can also be beneficiated by selective flocculation-flotation; however, the much greater content of goethite in the Minnesota oxidized taconites, compared with the Tilden ore, makes it more difficult to maintain high iron recovery. A typical flowsheet for Mesabi Range oxidized taconite is shown in figure 1.
The application of WHIM separation to the beneficiation of oxidized taconites received impetus from innovative separator designs developed by G. H. Jones and J. H. Carpenter in the early 1960s. Beginning in 1964, application of WHIM separation to Mesabi and Marquette range iron ores was tested on the pilot plant scale at the Mines Experiment Station (now the Mineral Resources Research Center) of the University of Minnesota. The flowsheet developed involved WHIM separation on rodmill-ground ore to remove coarse quartz and primary slimes amounting to 20 to 30 percent of the feed weight. The preconcentrate obtained was reground in a ball mill, and the final concentrate was produced by flotation. The flowsheet development was influenced by the fact that the WHIM separators available used soft-iron spheres as a matrix and were not capable of selectively recovering the iron oxides present in particles finer than 500 mesh (25 micrometers). After flotation, overall iron recoveries ranged from 72 to 78 percent. In 1974, Kelland and Maxwell demonstrated that a continuous pilot plant WHIM separator with a matrix designed to produce high field gradients could be used to preconcentrate oxidized taconite ground to 90 percent passing 400 mesh (37 micrometers) before flotation. When a single-stage operation employing a 10-cm length of stainless steel wool matrix was used, preconcentrates containing 49 to 54 percent iron, with iron recoveries ranging from 68 to 79 percent, were obtained.
On the basis of direct operating costs, exclusive of amortization, WHIM separation, either alone or in combination with flotation, is less costly than either reduction roasting or selective flocculation-flotation. The cost advantages of WHIM separation over flotation are decreased reagent, grinding, and water treatment costs. For commercial-scale machines, the energy required to energize the electromagnets, typically in the range of 1 to 5 kwhr/ton of ore treated per pass, is small compared with the 30 kwhr/ton required for grinding. From the standpoint of capital costs, WHIM separation is significantly more expensive than flotation but probably less expensive than reduction roasting. Since WHIM separation is a relatively new and untried technology for the processing of oxidized taconites, high investment expense is a significant barrier to its application. Even at the research and development level, progress has been hindered by the high cost of effective pilot-scale machines. To minimize capital costs, the number of magnetic separation stages must be kept to a minimum.
The possibility of beneficiating the sample by WHIM separation alone was evaluated in the laboratory on 1-kg samples (fig. 2). A sample of rodmill discharge was separated in an Eriez Model L-4 WHIM separator with a 20-cm-deep matrix of 0.6-cm iron spheres and a 0.5-tesla applied field. A coarse tailing containing 31.5 percent of the feed weight, but only 3.8 percent of the iron in the feed, was obtained. To achieve sufficient liberation of the ore, the preconcentrate was reground to 80 percent passing 400 mesh (37 micrometers). Separation of the reground material was carried out with a matrix of coarse stainless steel wool and a 1.0-tesla applied field. Two stages of separation produced a finisher concentrate containing 63.5 percent Fe and 5.5 percent SiO2 with an accompanying iron recovery of 73 percent.
The laboratory results have not been duplicated in the pilot plant. The WHIM separators available for this research have not performed satisfactorily in continuous operation on rodmill discharge; the separators have either given much lower iron recovery than expected from laboratory tests or have shown a tendency to plug up during extended runs. (Design criteria for avoiding these difficulties are discussed in a subsequent section.)
The Bureau of Mines most successful application of WHIM separation on a pilot plant scale has been as a method of increasing iron recovery in the selective flocculation-flotation process. The unit operations of figure 3 were used in conjunction with the flowsheet of figure 1. The WHIM separator recovered iron values from the scavenger flotation tailings. The magnetic concentrate, after thickening, was returned to the scavenger flotation cells. During a continuous 48-hour pilot plant test in which the magnetic concentrate was recycled to the flotation circuit, the iron recovered in the cleaner flotation concentrate was increased by 2 percent with no adverse effect on concentrate grade. Average metallurgical results for a 48-hour period during which flotation was producing a low-iron tailing are shown in figure 4. Preliminary results from 6-hour pilot plant tests on higher grade flotation tailings have indicated that up to 10 percentage units of iron can be recovered by WHIM scavenging. The MEA-Sala Carousel magnetic separator used was operated at 0.6 tesla with flow through the 9-cm matrix of coarse stainless steel wool controlled by gravity.
The flotation process used was the cationic flotation of the gangue. To improve the concentrate grade, more amine collector is added to ensure flotation of coarse gangue particles. However, the added collector can also cause flotation of fine iron oxides, leading to a decrease in recovery. By providing a means of recovering the fine iron, magnetic scavenging allows positive control of the final concentrate grade while minimizing loss of iron values in the final process tailing. An effective means of controlling iron recovery while maintaining close concentrate grade control is significant in treating an ore as variable in its characteristics as oxidized taconite.
Because alternative processing methods and alternative sources of ore exist, WHIM separation will be commercially applied to oxidized taconites only if it is cost competitive. The key to minimizing costs is to optimize the separator design for a particular application. A rational optimization procedure requires a mathematical model of separator performance as a function of the basic design and operating parameters.
Most existing models of WHIM separation can be classified as highly theoretical or purely empirical. During the past few years, there has been much interest in the development of theoretical models of WHIM separation based on particle trajectory calculations. Recent computer-based models have shown some degree of correspondence with experimental results after the introduction of two adjustable parameters for each particle size. Application of a theoretical model to a practical optimization problem would require a very extensive ore characterization study followed by systematic testwork to determine the adjustable parameters. The calculations involved in finding an optimum separator design might be too cumbersome for practical design purposes, even when performed on a large electronic computer. Several empirical models relating WHIM separator performance to design and operating parameters have been published. Because the amount of testwork required expands rapidly with the inclusion of each additional independent variable, empirical models are rarely developed to the degree of generality required for optimizing separator design. Since neither purely theoretical nor purely empirical models are well suited to design optimization procedures, a significant opportunity for advancing WHIM technology lies in the development of phenomenological models, which combine the most useful features of the theoretical and empirical approaches. Typically, in this compromise approach, theoretical analysis is used to derive or suggest reasonable, uncomplicated forms of the descriptive equations, but statistical curve-fitting techniques are used to find the parameters that fit the data to the equations. With the proper form of the descriptive equations, the required number of adjustable parameters and the testwork required to establish their values can be decreased to a minimum. The following example will illustrate the development and use of a simple phenomenological model.
Concurrently with a 5-day pilot plant test during which magnetic scavenging was tested, samples of the flotation tailings were evaluated in bench-scale WHIM separation tests under controlled conditions. Over the 5-day period, a 24 factorial experiment was run. The effect of daily variations in the tailings was assessed by conducting two midpoint tests each day. The variables and levels used in the tests are shown in table 1. The dependent variable of most interest, the fractional iron recovery, was shown to be independent of the iron content of the tailings, which varied from day to day. Equations developed by Oberteuffer and Watson suggested the following phenomenological equation:
The properties of the feed, such as its particle size distribution, did not vary significantly; therefore, the model was not complicated unnecessarily by including the effect of particle size on recovery. The feed characteristics have a strong effect on the parameter and a lesser effect on the other parameters.
To obtain accurate values of the parameters, weighting factors determined by the variance of the transformed dependent variable should be used in the regression calculation. It is of interest to compare equation 1 with the strictly empirical equation of Dobby and Finch, in which recovery is expressed as a linear function of the logarithms of the independent variables. Since the left-hand side of equation 2 can be closely approximated by the linear expression 3.2 R 2 for 0.2 < R < 0.9, as long as recovery is restricted to that range, there is no practical difference between the empirical and the phenomenological equations.
A well-conceived phenomenological equation should behave properly for limiting values of the independent parameters. Although equation 1 gives correct results for limiting values of H, L, and V, it incorrectly predicts R 1 as F 0. To correct this defect, equation 1 was modified to
A plot of experimental and calculated values of R versus the appropriate function of the independent variables is shown in figure 5. As can be seen from the range of the midpoint values of recovery, equation 3 adequately represents the data within the limits of experimental error. Although the fit could be improved by a more sophisticated model, the precision of the data probably does not warrant the introduction of the additional adjustable parameters that would be required.
The utility of a simple phenomenological equation as an aid in separator design can be illustrated by an example requiring only the simplest mathematical analysis. A feature of equation 3 that makes it particularly useful for design purposes is that matrix loading is defined per unit cross-sectional area rather than per unit volume. This formulation of the independent variables allows the effects of matrix length and matrix cross section to be evaluated independently and avoids the implication that the effect on metallurgical performance of loading per unit matrix volume is independent of matrix canister shape. For example, it is unlikely that, for a given mass of retained magnetics, canister A of 10-m cross-sectional area (fig. 6) has retained over its 1-meter length the same distribution of magnetics that canister B of 1-m cross-sectional area has retained over its 10-meter length. In terms of the design and operating parameters, the retention time of the slurry in the matrix is equal to L/V. For a given rate of slurry flow through the matrix canister, the retention time is determined by the canister volume. If recovery is controlled by retention time, the shape of Device B (fig. 6) would be operated at 10 times the flow velocity of A to give the same capacity and recovery. For retention time to be a valid design concept, the exponents nl, and nv in equation 3 must be nearly equal. For the magnetic scavenging results that have been discussed, the exponent nv was more than twice nL. Thus, for a given magnetized volume, configuration A, with a large cross-sectional area and small bed depth, gives optimal recovery. In fact, even if nL and nv were equal, configuration A would give better performance, because, for a given volume of feed slurry treated, matrix loading per unit area would be less. In principle, device B could be converted to give the same performance as A if the matrix were divided into 1-meter segments, as shown in C, and a suitable flow distribution system were provided.
Models developed from bench-scale tests are valuable for designing pilot- or full-scale WHIM separators only when the larger machines offer the same degree of control over operating parameters that laboratory units do. One important feature lacking in many continuous units is control over flow velocity by means other than the natural resistance of the matrix bed. The Bureau of Mines pilot plant experience has indicated several other important features of effective separator design. The applied magnetic field should be reasonably uniform across the cross section of matrix through which the feed slurry and rinse water flow. Adequate provision should be made for rinsing the matrix with clean water in the high magnetic field region. The nonmagnetic , rinse, and magnetic products should be effectively separated. The design should avoid areas where particles can settle out and accumulate, such as flat-bottomed sections, regions of stagnant flow, and high-gradient regions outside of the matrix.
Probably the most important problem encountered with pilot- or full-scale WHIM separators is permanent retention of particles in the matrix. The effects may range from a slight decrease in recovery to complete clogging of the system. The first step in attempting to eliminate this problem is matching the matrix to the feed. The matrix elements should be spaced widely enough not to capture large particles by purely physical means. So as not to adversely affect magnetic capture of fine particles by excessive spacing of the matrix elements, it is advantageous to adapt the feed to the matrix by removing the oversized particles. Permanent retention of ferromagnetic particles in the matrix can be minimized by reducing magnetic flux leakage into the matrix flush zone and by choosing a matrix material of low coercivity and remanence. If a large proportion of ferromagnetics is present in the feed, it is desirable to use a conventional low-intensity magnetic separator before high-intensity separation. A critical factor for maintaining a clean matrix is a flush water velocity much higher than that of the feed slurry. For a given quantity of flush water, pulsating flow is preferable to steady flow; back-flushing is preferable to forward flushing. One of the most effective methods of assuring a clean matrix bed is to provide a method of agitating and rearranging the matrix elements.
WHIM separation, either alone or in combination with flotation, is a technically feasible method of processing the enormous resources of oxidized taconite in the Lake Superior region. Commercial feasibility depends upon continuing progress in the design and construction of effective, reliable, yet economical magnetic separators. Mathematical models of separator operation can be valuable aids to the development of optimal designs. However, over dependence on unconfirmed theoretical descriptions, extrapolation of purely empirical results, and neglect of certain practical aspects of separator design can all lead to costly errors. Rational design of a full-scale separator for a particular application requires a realistic phenomenological model, supplemented by a systematic program of laboratory and pilot plant experimentation.
To help assure an adequate supply of minerals to meet the Nations economic and strategic needs, the Bureau of Mines is conducting bench- and pilot-scale research using wet high-intensity magnetic (WHIM) separation to recover iron from oxidized taconites of the Lake Superior region. The advantages and disadvantages of WHIM separation in relation to other separation methods, the most recent innovations in WHIM flowsheet design, and the role of mathematical models in process optimization are discussed.
The Jones wet high-intensity magnetic separator (WHIMS) was developed in 1956. The structure of the Jones separator is shown in Figure 9.6 and consists mainly of an iron-core electromagnet, a vertical shaft with two (or more) separating rings, a driving system, and feeding and product collection devices. Grooved plates made of magnetic conductive iron or stainless steel serve as a magnetic matrix to enhance the field gradient of the electromagnet. The plates are vertically arranged in plate boxes that are placed around the periphery of the rotors. When the Jones magnetic separator is operating, its vertical shaft drives the separating rings with the matrix plates rotating on a horizontal plane.
When a direct electric current passes through the energizing coils, a high magnetic field with a high gradient is established in the separating zone located in the electromagnetic system, with the focused magnetic field at the teeth top of the grooved plates reaching 0.82T, which is adjustable. The slurry is gravity fed onto the matrix at the leading edge of the magnetic field where the magnetic particles are captured on the teeth top of the grooved plates, while the nonmagnetic fraction passes through and is collected in a trough below the magnet. When the plate boxes reach the demagnetized zone half-way between the two magnetic poles, where the magnetic field changes its polarity, the magnetic field is essentially zero and the adhering magnetic particles are washed out with high-pressure water sprays.
In the past, cross-belt and rotating disc high-intensity magnetic separators were used for concentration of relatively coarse weakly magnetic particles such as wolframite and ilmenite, etc., under dry conditions. In the operation of these two magnetic separators, material is distributed onto the moving conveyor belt in a very thin layer, through a vibrating feeder. Such magnetic separators are not effective even inapplicable for the treatment of fine materials.
With the increasing reduction in liberation size of valuable components in magnetic ores, the conventional cross-belt and rotating disc high-intensity magnetic separators are almost replaced by gravity and flotation, particularly by high-gradient magnetic separators, as a result of its effectiveness to fine materials and high solids throughput. In the recent years, however, a wet permanent disc high-intensity magnetic separator as shown in left Figure7 seems applicable in recovering fine magnetic particles from tailings. In this disc separator, slurry is fed across a round tank, in which vertically rotating discs with permanent magnet blocks pick up fine magnetic particles, and they are brought up and scraped down by rotating scrapers, near the top of discs. Nonmagnetic particles are discharged at the bottom of tank.
And, a dry high-intensity roll magnetic separator as shown in right Figure7 is replacing the conventional roll magnetic separators and is used for concentration of relatively coarse magnetic particles. The design of such a roll magnetic separator is similar to that of the conventional roll magnetic separator, but it achieves a higher magnetic induction and its installation requires a much smaller occupation for space.
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.
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.
Various classification schemes exist by which magnetic separators can be subdivided into categories. Review of these schemes can be found in monographs by Svoboda (1987, 2004). The most illustrative classification is according to the magnitude of the magnetic field and its gradient.
Low-intensity magnetic separators (LIMS). They are used primarily for manipulation of ferromagnetic materials or paramagnetic of high magnetic susceptibility and/or of large particle size. These separators can operate either in dry or wet modes. Suspended magnets, magnetic pulleys, and magnetic drums are examples of these separators. Operation of a dry drum separator is shown in Fig. 3.
High-intensity magnetic separators. They are used for treatment of weakly magnetic materials, coarse or fine, in wet or dry modes. Induced magnetic rolls (IMR), permanent magnet rolls and drums, magnetic filters, open-gradient (OGMS) and wet high-intensity magnetic separators (WHIMS) are examples of this class of separators.
Weakly paramagnetic minerals can only be effectively recovered using high-intensity (B-fields of 2T or greater) magnetic separators (Svoboda, 1994). Until the 1960s, high-intensity separation was confined solely to dry ore, having been used commercially since about 1908. This is no longer the case, as many new technologies have been developed to treat slurried feeds.
Induced roll magnetic (IRM) separators (Figure 13.19) are widely used to treat beach sands, wolframite and tin ores, glass sands, and phosphate rock. They have also been used to treat weakly magnetic iron ores, principally in Europe. The roll, onto which the ore is fed, is composed of phosphated steel laminates compressed together on a nonmagnetic stainless steel shaft. By using two sizes of laminations, differing slightly in outer diameter, the roll is given a serrated profile, which promotes the high field intensity and gradient required. Field strengths of up to 2.2T are attainable in the gap between feed pole and roll. Nonmagnetic particles are thrown off the roll into the tailings compartment, whereas magnetics are held, carried out of the influence of the field and deposited into the magnetics compartment. The gap between the feed pole and rotor is adjustable and is usually decreased from pole to pole (to create a higher effective magnetic field strength) to take off successively more weakly magnetic products.
The primary variables affecting separation using an IRM separator are the magnetic susceptibility of the mineral particles, the applied magnetic field intensity, the size of the particles, and the speed of the roll (Singh et al., 2013). The setting of the splitter plates cutting into the trajectory of the discharged material is also of importance.
In most cases, IRM separators have been replaced by the more recently developed (circa 1980) rare earth drum and roll separators, which are capable of field intensities of up to 0.7 and 2.1T, respectively (Norrgran and Marin, 1994). The advantages of rare earth roll separators over IRM separators include: lower operating costs due to decreased energy requirements, less weight leading to lower construction and installation costs, higher throughput, fewer required stages, and increased flexibility in roll configuration which allows for improved separation at various size ranges (Dobbins and Sherrell, 2010).
Dry high-intensity separation is largely restricted to ores containing little, if any, material finer than about 75m. The effectiveness of separation on such fine material is severely reduced by the effects of air currents, particleparticle adhesion, and particlerotor adhesion.
Without doubt, the greatest advance in the field of magnetic separation was the development of continuous WHIMSs (Lawver and Hopstock, 1974). These devices have reduced the minimum particle size for efficient magnetic separation compared to dry high-intensity methods. In some flowsheets, expensive drying operations, necessary prior to a dry separation, can be eliminated by using an entirely wet concentration system.
Perhaps the most well-known WHIMS machine is the Jones separator, the design principle of which is utilized in many other types of wet separators found today. The machine has a strong main frame (Figure 13.20(a)) made of structural steel. The magnet yokes are welded to this frame, with the electromagnetic coils enclosed in air-cooled cases. The separation takes place in the plate boxes, which are on the periphery of the one or two rotors attached to the central roller shaft and carried into and out of the magnetic field in a carousel (Figure 13.20(b)). The feed, which is thoroughly mixed slurry, flows through the plate boxes via fitted pipes and launders into the plate boxes (Figure 13.21), which are grooved to concentrate the magnetic field at the tip of the ridges. Feeding is continuous due to the rotation of the plate boxes on the rotors and the feed points are at the leading edges of the magnetic fields (Figure 13.20(b)). Each rotor has two feed points diametrically opposed to one another.
The weakly magnetic particles are held by the plates, whereas the remaining nonmagnetic particle slurry passes through the plate boxes and is collected in a launder. Before leaving the field any entrained nonmagnetics are washed out by low-pressure water and are collected as a middlings product.
When the plate boxes reach a point midway between the two magnetic poles, where the magnetic field is essentially zero, the magnetic particles are washed out using high-pressure scour water sprays operating at up to 5bar. Field intensities of over 2T can be produced in these machines, although the applied magnetic field strength should be carefully selected depending on the application (see Section 13.4.2). The production of a 1.5T field requires electric power consumption in the coils of 16kW per pole.
There are currently two types of WHIMS machines, one that uses electromagnetic coils to generate the required field strength, the other that employs rare earth permanent magnets. They are used in different applications; the weaker magnetic field strength produced by rare earth permanent magnets may be insufficient to concentrate some weakly paramagnetic minerals. The variables to consider before installing a traditional horizontal carousel WHIMS include: the feed characteristics (slurry density, feed rate, particle size, magnetic susceptibility of the target magnetic mineral), the product requirements (volume of solids to be removed, required grade of products), and the cost of power (Eriez, 2008). From these considerations the design and operation of the separator can be tailored by changing the following: the magnetic field intensity and/or configuration, the speed of the carousel, the setting of the middling splitter, the pressure/volume of wash water, and the type of matrix material (Eriez, 2008). The selection of matrix type has a direct impact on the magnetic field gradient present in the separation chamber. As explained in Section 13.4.2, increasing magnetic field can in some applications actually cause decreased performance of the magnetic separation step and it is for this reason that improvements in the separation of paramagnetic materials focus largely on achieving a high magnetic field gradient. The Eriez model SSS-I WHIMS employs the basic principles of WHIMS with improvements in the matrix material (to generate a high field gradient) as well as the slurry feeding and washing steps (to improve separation efficiency) (Eriez and Gzrinm, 2014). While this separator is referred to as a WHIMS, it is in fact more similar to the SLon VPHGMS mentioned in Sections 13.4.1 and 13.5.3. Further discussion on high-gradient magnetic separation (HGMS) may be found in Section 13.5.3.
Wet high-intensity magnetic separation has its greatest use in the concentration of low-grade iron ores containing hematite, where they are an alternative to flotation or gravity methods. The decision to select magnetic separation for the concentration of hematite from iron ore must balance the relative ease with which hematite may be concentrated in such a separator against the high capital cost of such separators. It has been shown by White (1978) that the capital cost of flotation equipment for concentrating weakly magnetic ore is about 20% that of a Jones separator installation, although flotation operating costs are about three times higher (and may be even higher if water treatment is required). Total cost depends on terms for capital depreciation; over 10 years or longer the high-intensity magnetic separator may be more attractive than flotation.
In addition to recovery of hematite (and other iron oxides such as goethite), wet high-intensity separators are now in operation for a wide range of duties, including removal of magnetic impurities from cassiterite concentrates, removal of fine magnetic material from asbestos, removal of iron oxides and ferrosilicate minerals from industrial minerals such as quartz and clay, concentration of ilmenite, wolframite, and chromite, removal of magnetic impurities from scheelite concentrates, purification of talc, the recovery of non-sulfide molybdenum-bearing minerals from flotation tailings, and the removal of Fe-oxides and FeTi-oxides from zircon and rutile in heavy mineral beach sands (Corrans and Svoboda, 1985; Eriez, 2008). In the PGM-bearing Merensky Reef (South Africa), WHIMS has been used to remove much of the strongly paramagnetic orthopyroxene gangue from the PGM-containing chromite (Corrans and Svoboda, 1985). WHIMS has also been successfully used for the recovery of gold and uranium from cyanidation residues in South Africa (Corrans, 1984). Magnetic separation can be used to recover some of the free gold, and much of the silicate-locked gold, due to the presence of iron impurities and coatings. In the case of uranium leaching, small amounts of iron (from milling) may act as reducing agents and negatively affect the oxidation of U4+ to U6+; treatment via WHIMS can reduce the consumption of oxidizing agents by removing a large portion of this iron prior to leaching (Corrans and Svoboda, 1985).
At the CliffsWabush iron ore mine in Labrador, Canada (Figure 13.22), the cyclone overflow from the tailings of a rougher spiral bank is sent to a magnetic scavenger circuit utilizing both low-intensity drum separation and WHIMS. This circuit employs the low-intensity (0.07T) drum separators to remove fine magnetite particles lost during the spiral gravity concentration step, followed by a WHIMS step using 100th1 Jones separators which are operated at field strengths of 1T to concentrate fine hematite. Cleaning of only the gravity tailings by magnetic separation is preferred, as relatively small amounts of magnetic concentrate have to be handled, the bulk of the material being essentially unaffected by the magnetic field. The concentrate produced from this magnetic scavenging step is eventually recombined with the spiral concentrate before feeding to the pelletizing plant (Damjanovi and Goode, 2000).
The paramagnetic properties of some sulfide minerals, such as chalcopyrite and marmatite (high Fe form of sphalerite), have been exploited by applying wet high-intensity magnetic separation to augment differential flotation processes (Tawil and Morales, 1985). Testwork showed that a Chilean copper concentrate could be upgraded from 23.8% to 30.2% Cu, at 87% recovery.
By creating an environment comprising a magnetic force (Fm), a gravitational force (Fg), and a drag force (Fd), magnetic particles can be separated from nonmagnetic particles by MS. Magnetic separators exploit the differences in magnetic properties between particles. All materials are affected in some way when placed in a magnetic field.
where V: particle volume (determined by process); X: magnetic susceptibility; H: magnetic field (created by the magnet system design) in mT; GradH: magnetic field gradient (created by the magnet system design) in mT (mT: milli Tesla, 1kGauss=100mT=0.1T). Materials are classified into two broad groups according to whether they are attracted to or repelled by a magnet. Non/diamagnetics are repelled from and ferro/paramagnetics are attracted to magnets. Ferromagnetic substances are strongly magnetic and have a large and positive magnetism. Paramagnetic substances are weakly magnetic and have a small and positive magnetism. In diamagnetic materials, the magnetic field is opposite to the applied field. Magnetisms are small and negative. Nonmagnetic material has zero magnetism. Ferromagnetism is the basic mechanism by which certain materials (such as Fe) form permanent magnets, or are attracted to magnets. Ferromagnetic materials can be separated by low-intensity magnetic separators (LIMSs) at less than 2T magnetic intensity. Paramagnetic materials can be separated by dry or wet high-intensity magnetic separators (HIMSs) at 1020T magnetic intensities. Diamagnetic materials create an induced magnetic field in the direction opposite to an externally applied magnetic field, and are repelled by the applied magnetic field. Nonmagnetic substances have little reaction to magnetic fields and show net zero magnetic moment due to random alignment of the magnetic field of individual atoms. Induced roll separators, with field intensities up to 2.2T, and Permroll separators can be used for coarse and dry materials (>75m). Fine materials reduce the separation efficiency due to particlerotor and particleparticle agglomeration. For wet HIMS, Gill and Jones separators are used at a maximum field of 1.4 and 1.5T, at 150m size . Dry LIMSs are used for coarse and strongly magnetic substances. The magnetic field gradient in the separation zone (approximately 50mm from the drum surface) ranges between 0.1 and 0.3T. Below 0.5cm, dry separation tends to be replaced by wet LIMS. Concurrent and countercurrent drum separators have a nonmagnetic drum containing three to six stationary magnets of alternating polarity. Separation depends on the pick up principles. Magnetic particles are lifted by magnets and pinned to the drum and then conveyed out of the field. Field intensities up to 0.7T at the pole surfaces can be used. Coarse particles up to 0.56mm can be tolerated. The drum diameter is 1200mm and the length 6003600mm. Concurrent operation is normally used as a primary separation (cobber) for large capacities and coarse feeds. Countercurrent operation is used as a rougher and finisher for multistage concentration.
Moderately magnetic dry substances on a conveyor/belt can be collected by overhead, cross-belt, or disc separators using magnetic field intensities between 0.8 and 1.5T. Very weakly paramagnetic substances can only be removed if field intensities are greater than 2.0T. At 5200mm size fractions, overhead permanent magnets are used to remove ferromagnetics. Magnetic separators, such as dry low-intensity drum types, are widely used for the recovery of ferromagnetic materials from nonferrous metals (Al and Cu) and other nonmagnetic materials (plastic and glass) at 5mm in size. The magnetic field may be generated by permanent magnets or electromagnets. There have been many advances in the design and operation of HIMS due mainly to the introduction of rare-earth alloy permanent magnets with the capability of providing high field strengths and gradients. There are, however, some problems associated with this method. One of the major issues is agglomeration of the particles, which results in the attraction of some nonferrous fractions attached to the ferrous fractions . This leads to low efficiency of this method. Through the process of MS, it is possible to obtain two fractions: the magnetic fraction, which includes Fe, steel, Ni, etc., and the nonmagnetic fraction, which includes Cu . For WEEE, MS systems utilize ferrite, rear-earth or electromagnets, with high-intensity electromagnet systems being used extensively. Veit et al.  employed a magnetic field of 0.60.65T to separate the ferromagnetic elements, such as Fe and Ni. The chemical concentration of the magnetic fraction was 43% Fe and 15.2% Ni on average. However, there was a considerable amount of Cu impurity in the magnetic fraction as well. Yoo et al.  used a two-stage MS for milled PCBs. The milled PCBs of particle size >5.0mm and the heavy fraction were separated from the <5.0mm PCB particles by gravity separation. In the first stage, a low magnetic field of 0.07T was applied, which led to the separation of 83% of Ni and Fe in the magnetic fraction and 92% of Cu in the nonmagnetic fraction. The second MS stage was conducted at 0.3T, which resulted in a reduction in the grade of the NiFe concentrate and an increase in the Cu concentrate grade.
Magnetic separations depend on a particle's magnetic susceptibility in a magnetic field. Based on magnetic susceptibility, materials can be one of two types: paramagnetic (those attracted by a magnetic field) and diamagnetic (those repelled by a magnetic field). It is usual to consider strongly magnetic materials as being in a separate category called ferromagnetic.
Magnetic separators are divided into low-intensity and high-intensity separators, the former being used for ferromagnetic minerals (and some paramagnetic minerals of high magnetic susceptibility) and the latter used for paramagnetic minerals of (lower) magnetic susceptibility. (In effect, a third category of separator exists: that used for removing tramp iron from process streams.) High- and low-intensity separation can be carried out wet or dry: tramp separators operate only on dry streams.
The most common separator, the wet low-intensity, consists of a revolving drum partly submerged in a suspension. An arc of magnets within the drum pulls the magnetically susceptible material against the drum, lifting it out of the slurry and over a discharge weir. Permanent ceramic magnets are now typical in these units.
Dry high-intensity separators use powerful electromagnets that induce a magnetic field in a comparatively small diametered roll, against which the magnetically susceptible particles are held until they pass a suitable discharge point.
where r=radial distance; V=particle volume; p, m=magnetic susceptibility of particle and medium, respectively. This shows that the force depends on both the strength and the gradient of the magnetic field. The latter component is especially significant in WHIMS, where high curvature ferromagnetic surfaces (e.g., wire, balls) are used to produce very high gradients.
An indication of the lower limits of particle size that can be treated in a magnetic separator can be obtained by balancing the magnetic force against the likely opposing forces (usually fluid drag and gravitation), but with the addition of centrifugal forces in drum separators. Mechanical considerations usually determine the upper particle size limit.
In principle, separability and performance curves can be used to predict separator performance. However, difficulties arise in determining properties independent of experimental conditions, so the approach has not been widely used.
Two simple models of wet low-intensity drum separators have been described. One uses a probability concept while the other empirically correlates losses of magnetic material near the drum take-up and discharge with feed rate and drum speed.
Nanomaterials have also been prepared by ball milling the parent materials. High-energy ball milling not only prepares nanoparticles quickly but it also uses little chemicals as compared to the sol-gel methods. However, it has low energy efficiency because it dissipate a lot of energy in form of heat.
Planetary ball mill was used to synthesize iron nanoparticles. The synthesized nanoparticles were subjected to the characterization studies by X-ray diffraction (XRD), and scanning electron microscopy (SEM) techniques using a SIEMENS-D5000 diffractometer and Hitachi S-4800. For the synthesis of iron nanoparticles, commercial iron powder having particles size of 10m was used. The iron powder was subjected to planetary ball milling for various period of time. The optimum time period for the synthesis of nanoparticles was observed to be 10h because after that time period, chances of contamination inclined and the particles size became almost constant so the powder was ball milled for 10h to synthesize nanoparticles . Fig. 12 shows the SEM image of the iron nanoparticles.
The next step involved the crushing of the pyrite particle by high-energy ball milling at a rate of 320rpm for various periods of time, that is, 2, 4, and 6h which ultimately resulted in the formation of pyrite nanoparticles.
The process of ball milling was employed under controlled parameters about 298K temperature and 760 torr pressure. Stainless steel made ball and bowl were utilized for the process. In the process, ball:pyrite ratio of 10:1 was selected and at varying time periods of 2h, the samples were removed. The method was named as interrupted milling. The synthesized nanoparticles were washed with ethyl alcohol thrice to eradicate contamination. The nanoparticles were dried in an oven for 4h at 50C [12,13]. Fig. 13 shows the SEM image of the nanoparticles.
Jaw and cone crushing was performed on the martite ore until they became within the size range of 0.52cm. The sample was further crushed by ball and rod milling until the particles size was reduced to 3050lm. Ultimately, the particles were subjected to interrupt high-energy planetary ball milling for different time durations, that is, 2, 4, and 6h to get the nanoparticles of martite. The ball:martite ratio of 10:1 was selected and a rotation speed of 320rpm was chosen . Fig. 14 shows the SEM images of martite nanoparticles.
Caron onions preparation was carried out by employing graphite carbon having high purity. A reported method was used to synthesize AlCuFe quasicrystal. The synthesis of alloy was carried out under ambient environment. The percentage composition of alloy material was set to be Al64Cu24Fe12. The alloy was solidified under ambient conditions. Annealing of the synthesized alloy was performed under argon environment at 700C for 96h. The synthesized composite material is brittle and inclines to be fractured when subjected to ball milling process. In this typical procedure, the reaction of moisture with aluminum in the composite results in the formation of aluminum oxide film over the surface but simultaneously, the release of atomic hydrogen incites cleavage fracture of the composite material and occasionally it was observed that the whole material got converted into fine powder after a few days. Graphite and the composite materials were mixed in 1:1 ratio and then high-energy ball milling was performed on the mixture under ambient environment. Ball milling was performed for various time periods of 1.5, 3, 6, and 10h. The ball milling media was composed of hard steel vials and balls having a ratio of powder:ball to be 1:7. The mixture was grinded using a grinding medium size of 12.7mm. The synthesized nanocomposites were characterized using various techniques including XRD, Raman spectroscopy, TEM, and the size of the nanoparticles was observed to be within 412nm . Fig. 15 shows the TEM image of the nanoparticles.
A modified ball milling device having assistance of ultrasonication was employed in the synthesis of zinc oxide nanoparticles. The synthesis of nanoparticles involved analytical grade zinc acetate dihydrated salt as the zinc precursor material. Ball milling medium of stainless steel with diameter of balls of 2mm was employed. The ratio of milling balls to the zinc precursor was set to be 1:100. The frequency and power of the microwave were 2450MHz and 0.8kW, respectively. The synthesized nanoparticles were characterized by UV-Visible spectroscopy, XRD, TEM, fluorescence measurements, and electroconductivity detections. The average size of the nanoparticles was observed to be 15nm . Fig. 16 shows the TEM image of the nanoparticles.
The synthesis of Na3MnCO3PO4 nanoparticles involved dry ball milling of the precursors. The precursors of the nanoparticles were Mn(NO3)2.4H2O (A), Na2HPO4.2H2O (B), and Na2CO3.H2O (C). The concentrations optima were evaluated by doing extensive preliminary experiments and the amount of 8mmol of A and B and 12mmol of C. Planetary ball milling of the mixture was performed by keeping a ball: mixture ratio of 30:1. The mixture was ball milled for different time periods, that is, 15, 30, 60, and 180min at a rate of 300rpm. The synthesized billed nanoparticles were then added into deionized distilled water under continuous stirring so that the nanoparticles can be separated from impurities. The nanoparticles were separated and characterized by various techniques . SEM image of the nanoparticles are provided in Fig. 17.
Direct reduction of iron ore to produce electric furnace feed for steelmaking is becoming economically important on a worldwide scale. For this application an iron ore superconcentrate containing less than 2% silica is demanded. Pilot plant and commercial operations have shown that high-intensity wet magnetic separation is well adapted to meet this specification with high recovery and attractive capital and operating costs. Tests with the Jones separator have produced superconcentrates containing less than 1.5% silica with over 90% iron recovery from preconcentrates produced by sizing or by gravity or low-intensity magnetic separation. Estimated total capital and operating costs to produce a ton of superconcentrate by high-intensity wet magnetic separation are $0.43 for retreatment of specular hematite spiral concentrate, $0.47 for Brazilian hematite fines, and $0.49 for magnetite.