Linear structures of Nd-Fe-B magnets: Simulation, design and implementation in mineral processing – A review

Highlights

• Basic properties of Nd-Fe-B magnets and their use in drum and roll separators.

• Computer simulation and optimization of the assembly of magnets for separators.

• Linear and Halbach structures of Nd-Fe-B magnets and their use.

• An influence of dimensions of linear structures on resulting magnetic field.

• A use of linear structures of magnets in processing of suspensions and raw materials.

Abstract

This paper presents the different types of linear structures from mainly large blocks made from permanent Nd-Fe-B magnets with a high value of the maximum energy product and their implementation in mineral processing facilities. The methods of the assemblage of these large blocks which allow control the speed of the attraction of magnets during this assembly are described and discussed. The most serious problem in this case is the safe handling of large magnetic forces, both during the assembly of the magnets into the large blocks and subsequently during the insertion of these blocks into the iron circuit for final implementation. Linear assemblies can be of various designs, both in the individual and opposing arrangements, depending on the purpose for which they were designed and constructed. In technological practice, linear magnetic structures find significant application in the purification of raw materials using the methods of magnetic separation. Specifically, magnetic filtration using Nd-Fe-B magnets is a very effective method of eliminating undesirable metal impurities from the feedstock. Another method is the use of suspended magnetic separators, designed for the removal of undesirable ferromagnetic particles from the layer of the material transported by a conveyor belt. All the methods are described and discussed here in this critical review.

Introduction

The purpose of this work is to summarize the knowledge of the use of linear assemblies of Nd-Fe-B magnets in practical implementations, gained at the Institute of Rock Structure and Mechanics (Prague) over the past two decades. At the Institute, functional models of magnetic facilities were created, which were subsequently installed in technological lines. This knowledge builds on our previous experience with the use of classic ferrite magnets in industrial installations. The main goal of introducing Nd-Fe-B magnets was to significantly increase the efficiency of industrial equipment using linear assemblies of these magnets, moreover, with the relatively simple design of different magnetic separators. This goal has been achieved and therefore the authors considered it useful to present both the methods of the assembly of Nd-Fe-B magnets into linear structures and the use of the resulting structures for processing raw materials and wastes by magnetic separation.

Permanent magnets from sintered neodymium-iron-boron (Nd-Fe-B) material find extensive application in practice. They are an important part of a number of industrial facilities, means of transport and various devices; they are used in electrical, power and mechanical engineering and the automotive industry, telecommunications, in the treatment of raw materials, in human and veterinary medicine, etc. Therefore, their research and development continue to receive great attention, and the same applies to the assembly of these magnets into large blocks. This is a way to increase the resulting magnetic field significantly in comparison with the magnetic field of a single magnet. The considered materials are ferromagnetic, magnetically hard, with a typical hysteresis curve (Fig. 1), determining the most important magnetic properties of permanent magnets: remanent magnetic induction (remanence B_r), coercive magnetic field intensity (coercivity H_cB, H_cJ) and the maximum energy product (B.H)_max, which expresses the highest magnetic energy flow achievable with the material considered. It determines the optimal working point of a permanent magnet, which means that the magnet operating at this point generates the most power per unit volume. (The values of remanence, coercivity and the (B.H)_max are measured according to DIN EN 10 332 or ASTM A977/A 977 M – 97.)

What is important for practical work with Nd-Fe-B magnets is their density (~7.5 g/cm³), determining their mechanical properties and workability, but mainly their operating temperature and temperature coefficients. The operating temperatures are e.g. 80 °C or 120 °C, in the case of advanced materials even 220 °C. Because of the temperature coefficients, which are always negative for Nd-Fe-B magnets (i.e. induction decreases with growing temperature), the value of the operating temperature is of essential importance, especially when working at higher temperatures, e.g. above 100 °C. The temperature coefficients T_k are purely empirical and express the percentage of the decrease of magnetic induction or remanence with a temperature increase of 1 K (T_k = ΔB/ΔT or T_k = ΔB_r/ΔT, %/K). The values of ΔB or ΔB_r are related to induction at room temperature (the initial value B or B_r is considered at room temperature). The temperature coefficients are material parameters that can be changed by doping with suitable metal elements. There are substantial differences between the temperature coefficients of Nd-Fe-B materials, e.g. −0.05 and −0.13%/K. It is necessary to take them into account especially for magnets working at higher temperatures, e.g. 150 °C.

The magnetic properties of a sintered magnetic material are closely related to the interactions between the grains (Gao et al., 2000), while the magnetic field of a given magnet is a function of the physical structure of the magnet, its chemical composition, material homogeneity, and dimensions. In the simplest case, the structure consists of two different phases, namely a magnetically hard Nd₂Fe₁₄B phase with domains of a volume of 0.001–1 mm³, and a non-magnetic granular phase from practically pure neodymium (Fig. 2). Since the presence of Nd results in susceptibility to corrosion, it is necessary to coat the magnets, preferably with nickel or plastics. The chemical composition is determined by the content of key elements, Fe = 58–61 wt%, Nd = 26–28 wt% and B = 3–4 wt%, further by the content of carbon (1–3 wt%) and oxygen (2–4 wt%) and can be modified by admixtures of selected metallic f-elements from the group of lanthanides, namely praseodymium or dysprosium at concentrations of e.g. ~5 wt% of Pr or ~1 wt% of Dy. These admixtures significantly increase

the coercivity of Nd-Fe-B magnets (Corfield et al., 2010, Yao et al., 2015, Liu et al., 2017) and thus enable both their further development and the development of magnetic devices. The usual way to increase coercivity is to grind the feedstock into very fine particles (Nakamura, 2018), because the resulting sintered magnets from particles of e.g. 3 μm have substantially higher coercivity than those from particles of e.g. 9 μm. However, an increase in coercivity is often accompanied by a decrease in remanence as a result of the growing friction between particles during compression, which changes the magnetic orientation of the particles. In general, higher-quality magnets have a lower average size of the sintered particles (Diez-Jimenez et al., 2018).

Improving the magnetic characteristics of Nd-Fe-B magnets, namely increasing coercivity, involves the application of such admixtures as Ga and Cu and post-sinter annealing at a temperature lower than the temperature of the actual sintering (Sasaki et al., 2016), another way to increase coercivity is to anneal sintered magnets at 1000 °C in vacuum for 24 h and then to cool them slowly (Cortfield et al., 2010), which has a positive effect on the coercivity of Pr-Fe-B magnets while preserving or slightly decreasing the remanence of the resulting magnets in comparison with the original magnets. Another possibility is the diffusion source method (Liu et al., 2017), which has been applied to Ga-doped Nd-Fe-B magnets exposed to heat. The surface of the original Nd-Fe-B material was covered with strips of Nd₆₂Dy₂₀Al₁₈ alloy (diffusion source). Subsequently, the entire sample was heated at 700 °C for 1 h. The resulting material exhibited an increase in coercivity from 0.91 to 2.75 T (at laboratory temperature) accompanied by a relatively small decrease of remanence from 1.50 to 1.30 T.

With dysprosium-containing Nd-Fe-B magnets, an increase in coercivity is always accompanied by a decrease in remanence, but coercivity decreases with growing temperature. When Nd-Fe-B based magnets are used in traction engines of hybrid vehicles or wind turbines, it is necessary to design magnets (or their assemblies) with a coercivity of e.g. 0.8 T at an operating temperature of ca 200 °C, therefore, their coercivity at laboratory temperature must be higher than 3 T (Liu et al., 2017).

Since commercial Nd-Fe-B materials are sintered (Fig. 3), an important factor is the homogeneity of the material, in particular the homogeneity of the magnetically hard phase.

If the material is not homogeneous, neither will the resulting magnetic field be homogeneous, which may lead to inaccurate operation of the magnetic device. Physically, the material is inhomogeneous because it contains two phases, as already mentioned. Chemically, however, it can be considered homogeneous as the non-magnetic Nd-rich phase is in the clear minority and does not necessarily have to cause significant changes in the chemical composition of the magnet. In other words, individual magnet volume units will not differ much in their chemical composition. Therefore, the authors have measured the concentrations of the key elements in N35 and N52 magnetic materials using an X-ray microanalyzer with a scanning electron microscope (Quanta 450 with an Apollo X energy dispersive analyzer and the EDAX Genesis evaluation system). Element concentrations were determined in a large number of randomly selected magnet locations, with the measure of homogeneity being standard deviations of the concentrations of the 6–7 elements forming the magnet. The elements monitored in the N35 material included boron, carbon, oxygen, niobium, praseodymium, neodymium and iron, in the case of N52 boron, carbon, oxygen, dysprosium, neodymium and iron. Table 1 shows that the relative standard deviations of the key elements, Fe, Nd and B, are very small, between 0.27 and 3.22%, of the admixtures (Nb, Pr and Dy) then 1.45–5.52%, which is very acceptable considering their small amount, and of the other elements (O and C) between 1.14 and 3.14%, hence very low again. On the whole, it can thus be said that currently produced sintered Nd-Fe-B materials have good chemical homogeneity. Simultaneously, Table 1 shows the proportions of rare earths elements (Nd, Pr and Dy) in magnets used for the design of the magnetic separators with linear structures. It is evident that Nd-Fe-B magnets are suitable for assembly into large blocks. Nevertheless, it must be added that the magnetic properties at the corners and edges of the magnet, normally in the shape of a cuboid, are rather different from the properties in the center of the sample and near the center. This is caused by different cooling rates in the central and peripheral areas of the magnet after sintering. This circumstance has been taken into account in the construction of magnetic filters and separators based on the developed methods of the assembly of magnets into linear structures. But the most important task, both when assembling individual magnets into larger units and compact magnetic plates into a large block, and to gradually insert these large blocks into an iron magnetic circuit of a magnetic separator, is to find a solution to safely handle these magnetic forces, which increase sharply during magnet attraction.

Permanent magnets based on the rare earths (especially Nd-F-eB) have been applied in various types of magnetic separators already for a longer time while the popularity of neodymium-based permanent magnets has grown greatly over the last two decades. Beside separators with linear assemblies, roll and drum separators are further mentioned for their practical relevance. The use of Nd-Fe-B magnets in roll and drum separators enhanced the technical performance and economic viability of high-intensity magnetic separation (Arvidson and Henderson, 1997, Svoboda and Fujita, 2003). They have also found use in magnetic grids and rods and in suspended magnetic separators. However, in these cases assemblies from magnets of the smaller dimensions have mostly been employed, ultimately from one layer of magnets only. The reason is the rapidly increasing large forces by which the large magnets in the process of their assembly interact with each other and affect the surrounding ferromagnetic objects.

The active part of the roll separator is a roll consisting of disks or rings of permanent magnets, sandwiched between mild steel rings. The adjacent permanent rings are arranged with the same polarity facing one another. Maximum steel magnetization (near saturation) could be obtained if the rings are stacked to make a roll using a 4:1 ration of magnet to steel thickness. Very high magnetic fields and field gradients are generated on the surface of the roll, often 1 T in the case of Nd-Fe-B magnets; the peak magnetic field 1.9 T can be generated on a surface of the rolls. The magnetic roll itself forms a part of a short conveyor fed by a vibratory feeder. The conveyor belt is made of a thin high-tensile material and is supported by second roll (idler) (Svoboda, 2001, Svoboda, 2004).

Magnetic drum separators are one of the oldest types of magnetic separators and are certainly the most widely used in various industrial applications. In the case of this separator, large blocks of ferrite magnets or possibly small blocks of Nd-Fe-B magnets are mounted (fixed) on the surface of a solid steel segment inserted into a rotating drum. The outer surface of the magnets copies the inner surface of the drum with as little gap as possible. When the material is fed to the drum surface, the magnetic particles (impurities, objects) are trapped in the area of the magnetic field on the drum surface and fall off when leaving the field, the non-magnetic particles are falling loosely (Svoboda, 2001, Svoboda, 2004).

Separators can operate with a low-intensity or high-intensity magnetic field and can work with dry or wet material. The choice of type depends mainly on the distribution of particle size, the magnetic properties of the particles to be separated and the throughput of the equipment.

Regarding drum separators, the main use of dry low-intensity separators is to concentrate strongly magnetic particles; the wet low-intensity separators are used mainly for the recovery of heavy media used in dense medium separation while the production of concentrates of strongly magnetic ores is also a possible application. Drum separators work in a radial or axial configuration of magnets. A tumbling motion of the separated particles over rows of magnets with alternating polarity allows the removal of non-magnetic particles and concentration of the magnetic ones. In a radial configuration, the polarity of magnets alternates along the width of drum – this configuration is often used for the recovery of strongly magnetic material. In an axial configuration, the polarity alternates along the perimeter of a stationary magnet cylinder – this arrangement is preferred to obtain a high-quality magnetic fraction.

Regarding roll separators, the construction using Nd-Fe-B magnets generates an intense magnetic field that extends that produced by electromagnets, moreover, it is possible to design rolls for processing materials of different particle size and distribution of magnetic susceptibility. The roll separators are therefore of high-intensity; usually they operate in dry mode. Using dry high-intensity separators, materials with a large particle size and those with medium or weakly magnetic minerals can be processed.

The introduction of a matrix into a circuit of magnetic separators has made significant progress in the magnetic separation of fine and weakly magnetic materials. In separators designed in this way, a metal matrix placed between magnetic poles is combined with a high magnetic field (wet high-intensity high-gradient separators). The ferromagnetic matrix (from grooved plates, steel balls, steel rods, expanded metal, woven wire mesh, steel wool) not only generates the high gradient of magnetic field to increase the magnetic force acting on magnetic particles, but also ensure the surface sites to collect a magnetic fraction of the separated material, namely from suspensions. In applications, a suspension flow through the matrix and a distribution of magnetic field are affected by (a) the volume of magnetic particles and magnetic susceptibility, (b) how the matrix is placed in the separation chamber and (c) how the matrix elements are arranged. These three factors are closely related to the performance of the separation (Pasteur et al., 2014). While the conventional separators described above are limited to separate strong magnetic material with a particle size greater than 50 μm (Svoboda and Fujita, 2003), wet high-intensity high-gradient separators can handle very fine and weakly magnetic particles in different suspensions. At present, this technique is widely used for the purification of ores, quartz, clay, coal and water, also for recovering weakly magnetic minerals, e.g. ilmenite (Zeng et al., 2015). Newly – in the biotechnological field – for the purification of cells, proteins and DNA (Bucak et al., 2003, Ge et al., 2017). Furthermore, micro-magnetic separation techniques to capture superfine magnetic particles for biomedical applications are being intensively developed (Lund-Olesen et al., 2007, Bu et al., 2008, Earhart et al., 2009, Dong et al., 2010); methods in bio-analysis for separation of biological cells and molecules (Zborowski and Chalmers, 2008) and in water purification from pollutants (Ambashta and Sillanpää, 2010) are evaluated.

A way of optimizing roll separators’ design is mentioned by Svoboda and Fujita (2003). This option is based on the availability of electromagnetic modeling using unspecified software, but, on the whole, it seems that the optimization paths for roll separators appear to be either too complex or underutilized.

Since 2006, four main efforts can be observed in magnetic separation literature: (a) beneficiation of rare earth ores, (b) separation of weakly magnetic minerals, (c) purification of non-metallic ores, and (d) processing of slags and slurries. For these purposes, it is suggested to use advanced drum separators and roll separators with rare earth magnets (typically from Nd-Fe-B or Nd-Fe-B based materials). Moreover, a newly designed high gradient magnetic separator employing Nd-Fe-B magnets was constructed especially for the purification of non-metallic ores.

Use of advanced drum separators. The growing demand for rare earth elements on the one hand and the depletion of their ores on the other underscore the production of these elements from their sources through the development of efficient and environmentally friendly processes. Advanced drum separators can be used in the processing of rare earth ores to remove strongly magnetic gangue and concentrate rare earth minerals such as monazite or xenotime containing desired rare earth (paramagnetic) elements (Kumari et al., 2015). The combination of magnetic separation with other methods is also useful (flotation, Xiong et al., 2018); electrostatic separation (Kumari et al., 2015), e.g. when significantly paramagnetic xenotime is concentrated along with ilmenite, xenotime can be separated from ilmenite electrostatically after drum magnetic separation, because ilmenite, unlike xenotime, is conductive.

Regarding the beneficiation of rare earth ores, Jordens et al., 2016a, Jordens et al., 2016b described a separation process involving a gravity centrifugation to reject a silicate gangue and pre-concentrate rare earth minerals, and subsequent low-intensity drum magnetic separation resulting in a total rare earth oxide recovery of 11.75% and a total rare earth oxide grade of 7.50%. Finally, a froth flotation can be employed. The tested ore originated from the Nechalacho Deposit (Avalon Rare Metals, Northwest Territories, Canada); a gravity pre-concentration was conducted using a Knelson concentrator; magnetic separation was carried out using a model WD(20) wet drum magnetic separator with permanent magnets (Carpco Inc., USA). The process is proposed to concentrate the rare earth minerals through a combination of gravity step and magnetic separation prior to froth flotation. Similarly, Xiong et al. (2018) developed a method combining flotation with wet high-intensity magnetic separation. Using flotation, rare earth particles of bastnaesite/hydroxylbastnaesite are first agglomerated and separated from the tailings, then, the obtained flotation concentrate is directly fed into a magnetic separator. As a result, an average rare earth oxide recovery of 55% was reached. Dalucao rare earth ore from Sichuan Province, China, was tested; an ERIEZ L-4-20 magnetic separator was used. It is significant that flotation was carried out on a commercial scale. After completing the magnetic separation for large-scale use, the method can probably be introduced in industrial practice.

An innovative high gradient drum magnetic separator (referred to as ZCLA HGPMS) was developed for recovering weakly magnetic minerals (Zeng et al., 2015). It uses high performance rare-earth (apparently Nd-Fe-B based) permanent magnets to generate the required magnetic field while these magnets are fixed on the circular periphery of the separating cylinder. The magnet poles’ shape is concave producing a concave magnetic field. The separating cylinder uses the permeability of stainless-steel material and the magnetism permeates through the separating cylinder; the rotation speed and slope of the cylinder are controlled variables that affect the concentrate grade and the recovery rate of the magnetic material. This innovative drum separator is suitable, among others, for the separation of ilmenite, hematite, limonite and other weakly magnetic minerals.

To recover high-grade iron from slag from the steel industry, Menad et al. (2014) suggested a combination of low-intensity drum magnetic separation and high-intensity drum magnetic separation of ground slag (at ~63 μm) in a wet process. This combination may recover ferromagnetic particles (iron oxides, mainly wüstite) at low-intensity separation and paramagnetic particles (ferrites) at high-intensity separation; the non-magnetic fraction then provides mostly calcium silicates. Whereas ferromagnetic particles were recovered in two stages using a SALA separator, paramagnetic ones were separated through a BOXMAG 18,000 Gauss one. As a result, a high-grade iron for recycling in metallurgical processes and calcium silicates for the cement industry were obtained.

Jamieson et al. (2006) demonstrated the drum magnetic separation of red sand slurry (a by-product of alumina extraction from bauxite) through the combined techniques of low intensity drum magnetic separation for pre-treatment, i.e. to remove the highly magnetic fraction of the red sand (using an Eriez L8 LIMS unit operating at up to 1,000 G) and subsequent wet high intensity drum magnetic separation (using a pilot plant Humboldt–Jones P40 WHIMS, 11,200 G) for the treatment of the remaining magnetic fraction. As a result, three products are obtained: the first is oxides high in iron, typically about 40% as Fe; another is high in silica content, ca 93% as SiO₂, which has a potential use within concrete manufacture; a third fraction, comprised of a mixture of iron and silica, is suitable as general fill. The application of this combined magnetic separation technology has the potential to utilize a large volume of resources from different repositories for tradable commodities.

Use of rare earth roll separators. To separate the hematite fines from a low-grade siliceous iron ore, Tripathy et al. (2017) tested a rare earth roll magnetic separator in a dry process. Separations were carried out on a roll separator L/P10-30, Outotec with Nd-Fe-B magnets while the diameter of the roll was 100 mm and the width 300 mm; the maximum induction was 1.2 or 1.6 T. Parameters such as magnetic field strength, roll speed, feed rate and splitter position were varied. It was found that (a) with a given magnetic field, the roll speed is decisive for particle separation, (b) a Nd-Fe-B magnetic separator can be successfully used as a pre-concentrator to remove the maximum amount of tailings in one stage of dry operation, and (c) the method allows the effective separation of hematite in the next step.

Varela et al. (2006) tried to improve the quality of natural carbonates using a roll separator with Nd-Fe-B magnets. The equipment used generates a strong gradient of magnetic field with magnetic field strengths of up to 2 T (on the surface of the magnets). Due to the belt conveyor thickness of 0.5 mm, magnetic field strengths are lowered to about 0.9 T. Fractions above 0.25 mm were tested, the feed rate was of 20.5 kg/h. This type of magnetic separation was used to reduce undesirable color impurities as well as the iron content, however, to introduce this method into industrial practice, it is necessary to modify the parameters.

To remove undesirable admixtures from the manufactured fine aggregates, Miceli et al. (2017) conducted a magnetic separation using a roll rare-earth magnetic separator. The reason is that the produced fine aggregates have become an important alternative to natural sands in the construction industries. Granites and gneisses are among the rocks that are most commonly used for fine aggregate production, but use of these rocks largely depends on the removal of contaminating minerals (especially mica/biotite) which have a negative effect on both the rheology and the strength of mortars and concrete. The authors carried out a magnetic separation of these minerals using a roll rare-earth magnetic separator, model R.E.-ROLL, INBRAS-ERIEZ, fitted with a 8 cm diameter roll containing Erium-3000® rare-earth magnets and a 16-cm-wide belt. The results demonstrate that high-intensity dry magnetic separation is a very good alternative for the removal of undesirable admixtures (biotite, hornblende, muscovite, chlorite and others) from manufactured fine aggregates. This technology can be successfully used on a commercial scale, however, before its introduction on this scale it is necessary to determine the optimum specific throughput and to take into account the sensitivity of the separation to moisture content.

Use of a high gradient magnetic separator with Nd-Fe-B magnets. To purify feldspar and quartz ores from unwanted iron oxides and fine iron scraps, a pilot-scale high gradient magnetic separator with Nd-Fe-B magnets working in wet process was introduced by Chen et al. (2016). This separator is designed as a driven shallow belt beneath which Nd-Fe-B magnets of the plate-type are placed, between which there are narrow poles acting as a matrix. With the belt, the rotation speed and inclination angle can be varied. On the belt surface, perpendicularly to the belt rotation, the protruding strips used as transverse stops are arranged at a suitable interval. During operation, an inclined flow of ore slurry that is several cm in thickness flows downward while iron particles are captured on the belt surface by magnetic forces and carried upward by the transverse stops. The iron particles are then rinsed by water sprays and stored, while the non-magnetic particles flow downward with the slurry as the non-magnetic fraction. Thus, ~96% of the undesired iron admixtures are removed, moreover, the method makes it possible to avoid matrix clogging. The processing capacity is up to 500 kg/h, the magnetic induction on the belt 1.0 T, the rinsing water consumption 0.2–0.5 m³/h.

At present, special importance are separators for recovery of magnetic nanomaterials which offer great potential in biotechnology, medicine and environmental remediation. Nanomaterials have significant properties as high specific surface area, chemical stability, low intraparticle diffusion and high loading capacity and serve as magnetic carriers or adsorbents. To recover them, magnetic separators are used. These separators are often based on Nd-Fe-B magnets and their linear structures that achieve high magnetic induction and can efficiently operate even in small scale applications. An overview of design and applications provides the work Gómez-Pastora et al. (2017).

From the previous overview it can be deduced that the considered and further described linear structures can be successfully used in mineral processing, since (a) the devices (including those with a matrix) made of them have, unlike the above mentioned separators, a relatively simple construction, (b) the prediction of the magnetic properties, especially magnetic induction, is quite real, because the mathematical background is sophisticated and the software available (see below), and hence optimization of the design for obtaining target magnetic field intensity is also quite real, and (c) Halbach linear magnet assemblies can be used for magnetic separation. Computer simulation can be fully utilized, both for conventional linear assemblies and Halbach linear assemblies.

The criteria for the selection of the design can be set as follows:

• scale: commercial, pilot plant, macro-laboratory, laboratory?

• material for separation: wet, dry, suspension?

• magnetic fraction: weakly or strongly magnetic? approximate amount?;

• particles: grain size greater than 50 μm or less?

• process: low-intensity, medium-intensity, high-intensity high-gradient?

• Nd-Fe-B magnets: N35, N45, N52?

The linear structures of Nd-Fe-B magnets were chosen for review because (a) they showed high magnetic induction, (b) experience with ferrite magnets could be used, (c) separators from these structures installed in technological lines (see Section 3) showed high efficiency of the separation of undesirable metal impurities, (d) separators had relatively simple construction, and, last but not least, (e) their magnetic induction could be predicted from a computer simulation. Thus, the linear structure of the magnets seems to be beneficial to the mineral processing and progress in this area was therefore chosen as appropriate for the review.