**8.4. Magnetic separation**

In 1792, a patent was filed by WILLIAM FULLARTON describing the separation of iron minerals with a magnet.4 The early applications were based on the intrinsic magnetic properties of sediments for the separation. In 1852, magnetite was separated from apatite by a New York company on a conveyor belt separator. Later, a new line of separators was introduced for the separation of iron from brass fillings, turnings, of metallic iron from furnace products and of magnetite from plain gangue. The 1950s were the time of great expansion in the field of magnetic separations as the introduction of high-gradient magnetic separation (HGMS) systems permitted faster and more general magnetic separation processes. More recently, the separations using external magnetic fields have become common processes in biotechnology, where they are used for both protein purification as well as flow cytometry [18],[19].

Electromagnets almost completely replaced permanent magnets as the field-generating elements in drum separators [20]. Recent progress in magnet technology has realized eco‐ nomically and operationally favorable cryocooler-cooled5 [21] superconducting magnets, which can be used for commercial applications [22]. The first large superconducting6 [23] separator has been operating successfully in the USA since May 1986 and a larger system was installed with twice the capacity in 1989. A revolutionary design for the superconducting magnetic separator with a reciprocating canister system was installed and successfully operated for clay processing in May 1989. Following this, a number of other reciprocators have been installed for kaolin processing in the places as far apart as Brazil and Germany [24].

**Fig. 8.** Drum separator using a multi-pole superconducting magnet (Klochner Humbolt-Deutz Cologne, Germany) [22].

<sup>4</sup> The properties of magnetic materials were identified as early as 6th century BC, but the means by which magnets could move material remained only a curious phenomenon until the late 18th century. The background for electricity and magnetism, the reasons that magnets could move materials, were explained by GAUSS AND HELMHOLTZ [18].

<sup>5</sup> All superconducting devices share the need for sufficient refrigeration to overcome their low-temperature heat loading. This loading comes typically in two forms: (1) heal leaks from the surrounding and (2) internal heat generation in the device. In addition, the refrigeration system needs to bring the superconducting device from ambient temperature to its low operating temperature in reasonable length of time [21].

<sup>6</sup> After Kamerlingh Onnes's pioneering the demonstration in 1908 that the last so-called permanent gas helium could indeed be liquefied. His follow-up discovery of superconductivity in 1911 introduced the zero electrical current resistivity to the world. It was theorized that one could go beyond the resistive limit of a copper wire to develop a superconductor that could carry any amount of current but without the ohmic loss [23].

Despite all this progress, the majority of the commercial magnetic separators fulfill only the simple technological objective of the removal of magnetic substances without the ability to classify them. Only three classical separation products (tails, middlings and mags, **Fig. 9**) are usually obtained [25].

**Fig. 9.** Classical magnetic separation products [25].

**8.4. Magnetic separation**

with a magnet.4

[22].

5

6

In 1792, a patent was filed by WILLIAM FULLARTON describing the separation of iron minerals

sediments for the separation. In 1852, magnetite was separated from apatite by a New York company on a conveyor belt separator. Later, a new line of separators was introduced for the separation of iron from brass fillings, turnings, of metallic iron from furnace products and of magnetite from plain gangue. The 1950s were the time of great expansion in the field of magnetic separations as the introduction of high-gradient magnetic separation (HGMS) systems permitted faster and more general magnetic separation processes. More recently, the separations using external magnetic fields have become common processes in biotechnology,

where they are used for both protein purification as well as flow cytometry [18],[19].

nomically and operationally favorable cryocooler-cooled5

394 Apatites and their Synthetic Analogues - Synthesis, Structure, Properties and Applications

Rotating drum

Cryostat

low operating temperature in reasonable length of time [21].

that could carry any amount of current but without the ohmic loss [23].

Electromagnets almost completely replaced permanent magnets as the field-generating elements in drum separators [20]. Recent progress in magnet technology has realized eco‐

separator has been operating successfully in the USA since May 1986 and a larger system was installed with twice the capacity in 1989. A revolutionary design for the superconducting magnetic separator with a reciprocating canister system was installed and successfully operated for clay processing in May 1989. Following this, a number of other reciprocators have been installed for kaolin processing in the places as far apart as Brazil and Germany [24].

Superconducting

magnet

ingostatM <sup>S</sup>

Mags Tails Superconducting magnet

**Fig. 8.** Drum separator using a multi-pole superconducting magnet (Klochner Humbolt-Deutz Cologne, Germany)

<sup>4</sup> The properties of magnetic materials were identified as early as 6th century BC, but the means by which magnets could move material remained only a curious phenomenon until the late 18th century. The background for electricity and

 All superconducting devices share the need for sufficient refrigeration to overcome their low-temperature heat loading. This loading comes typically in two forms: (1) heal leaks from the surrounding and (2) internal heat generation in the device. In addition, the refrigeration system needs to bring the superconducting device from ambient temperature to its

 After Kamerlingh Onnes's pioneering the demonstration in 1908 that the last so-called permanent gas helium could indeed be liquefied. His follow-up discovery of superconductivity in 1911 introduced the zero electrical current resistivity to the world. It was theorized that one could go beyond the resistive limit of a copper wire to develop a superconductor

magnetism, the reasons that magnets could move materials, were explained by GAUSS AND HELMHOLTZ [18].

which can be used for commercial applications [22]. The first large superconducting6

The early applications were based on the intrinsic magnetic properties of

[21] superconducting magnets,

[23]

Unlike the conventional filtration methods that use the blocking-type filtration, the secon‐ dary waste is not produced in high-gradient magnetic separation (HGMS), which is also known as the magnetic or electromagnetic filtration. Furthermore, because HGMS systems use much higher magnetic forces than conventional magnetic separation techniques, it can also be used to separate rapidly large quantities of diluted suspension [22].

According to the applied separation method, two classes of magnetic separators are recog‐ nized [26]:


Although current separators usually achieve high grades of separation, they cannot classify the particles7 as they are being separated. The magnetic separator in which these two steps are performed at the same time and in the same machine was proposed by AUGUSTO and MARTINS [26].

Magnetic separation has been considered for many years a valuable method to achieve the purification of streams of particles (dry or wet) [26]. Magnetic separators have unrestricted industrial applications and are widely used in mineral beneficiation, food, textiles, plastic and ceramic processing industries. The separation efficiency of magnetic separator depends on the

<sup>7</sup> Differential magnetic classification and the selectivity are different definitions. The selectivity is defined as the ability to separate one certain kind of magnetic particles from all others, independently of how close their magnetic suscepti‐ bilities may be [25].

material characteristics and the design features of equipment along with the optimization of process variables [27].

The magnetic force (*F* → *<sup>m</sup>*) acting on weakly magnetic particle flowing in a fluid is given by the equation [19]:

$$
\vec{F}\_w = \frac{1}{\mu\_0} (\kappa\_p - \kappa\_{\gamma'}) V\_p B \nabla B \tag{7}
$$

where *B* is the magnitude of magnetic flux density at the particle position, *μ*0 is the magnetic permeability of vacuum, *κ*p is the volume susceptibility of the fluid and *V*p is the volume of the particle. The magnetic force on a particle is then proportional to the magnitude of magnetic flux density and the gradient. The magnetic field can be increased using a stronger magnet having more ampere turns, and the field gradient can be increased by changing the magnet‐ ic polarities and using a steel wool matrix. For sufficiently strong magnetic particles such as iron, magnetite and maghemite, it is advantageous, and **Eq. 7** can be written as:

$$
\vec{F}\_{\text{sr}} = \mu\_0 V\_{\rho} M \nabla H \tag{8}
$$

where *M* is the magnetization of the particle and *H* is the magnitude of magnetic field intensity at the particle position [19].

The basic principle behind magnetic separations is remarkably simple and remains un‐ changed from these early examples. It is based on a simple fact that materials with differing magnetic moments experience different forces in the presence of magnetic field gradients; thus, externally applied field can hand pick the components with distinctive magnetic characteris‐ tics out of physically similar mixtures [18]. When one of the major gangue constituents is magnetic, magnetic separators are used as one of the steps in the flow sheet to remove the magnetic constituents. This is mostly used in the beneficiation of igneous phosphate rocks. However, it was also used for the beneficiation of some sedimentary phosphate ores [1].

Paramagnetic minerals have higher magnetic permeability than the surrounding medium, usually air or water, and they concentrate the lines of source of an external magnetic field. The higher the magnetic susceptibility, the higher the field intensity in the particle and the greater the attraction up the field gradient toward increasing field strength. Diamagnetic minerals, on the other hand, have lower magnetic permeability than the surrounding medium and they repel the lines of force of magnetic field. These characteristics cause the expulsion of diamag‐ netic minerals down the gradient of the field towards decreasing field strength. This nega‐ tive diamagnetic effect is usually orders of magnitude smaller than the positive paramagnetic attraction. Thus, a magnetic circuit can be designed to produce higher field intensity or higher field gradient or both to achieve the effective separation [15].

Magnets are used in the mineral industry to remove the tramp iron that might damage the equipment and to separate minerals according to their magnetic susceptibility. According to the intensity of the magnetic field, two types of magnetic separators are recognized [15]:

material characteristics and the design features of equipment along with the optimization of

( )

where *B* is the magnitude of magnetic flux density at the particle position, *μ*0 is the magnetic permeability of vacuum, *κ*p is the volume susceptibility of the fluid and *V*p is the volume of the particle. The magnetic force on a particle is then proportional to the magnitude of magnetic flux density and the gradient. The magnetic field can be increased using a stronger magnet having more ampere turns, and the field gradient can be increased by changing the magnet‐ ic polarities and using a steel wool matrix. For sufficiently strong magnetic particles such as

=- Ñ <sup>r</sup>

0 <sup>1</sup> *Fm p fp* k k*VB B*

iron, magnetite and maghemite, it is advantageous, and **Eq. 7** can be written as:

*F VM H m p* = Ñ m0

where *M* is the magnetization of the particle and *H* is the magnitude of magnetic field intensity

The basic principle behind magnetic separations is remarkably simple and remains un‐ changed from these early examples. It is based on a simple fact that materials with differing magnetic moments experience different forces in the presence of magnetic field gradients; thus, externally applied field can hand pick the components with distinctive magnetic characteris‐ tics out of physically similar mixtures [18]. When one of the major gangue constituents is magnetic, magnetic separators are used as one of the steps in the flow sheet to remove the magnetic constituents. This is mostly used in the beneficiation of igneous phosphate rocks. However, it was also used for the beneficiation of some sedimentary phosphate ores [1].

Paramagnetic minerals have higher magnetic permeability than the surrounding medium, usually air or water, and they concentrate the lines of source of an external magnetic field. The higher the magnetic susceptibility, the higher the field intensity in the particle and the greater the attraction up the field gradient toward increasing field strength. Diamagnetic minerals, on the other hand, have lower magnetic permeability than the surrounding medium and they repel the lines of force of magnetic field. These characteristics cause the expulsion of diamag‐ netic minerals down the gradient of the field towards decreasing field strength. This nega‐ tive diamagnetic effect is usually orders of magnitude smaller than the positive paramagnetic attraction. Thus, a magnetic circuit can be designed to produce higher field intensity or higher

field gradient or both to achieve the effective separation [15].

m

396 Apatites and their Synthetic Analogues - Synthesis, Structure, Properties and Applications

*<sup>m</sup>*) acting on weakly magnetic particle flowing in a fluid is given by the

<sup>r</sup> (8)

(7)

process variables [27].

The magnetic force (*F*

at the particle position [19].

equation [19]:

→


Rotating-drum magnetic separators (**Fig. 10**) are mainly used in mines. The rotating disc magnetic separator is used in so-called ferritic processes [22].

**Fig. 10.** Typical magnetic pulley (a) and magnetic drum operating as lifting magnet (b) [15].

<sup>8</sup> The gauss (G or Gs) is a unit of magnetic field (magnetic flux density) named after CARL FRIEDRICH GAUSS (1 G = 10−4 T = 1 cm−1/2g1/2s1 ) [28].
