Jan Svoboda. Magnetic Techniques for the Treatment of Materials

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530 CHAPTER 6. INDUSTRIAL APPLICATIONS

Figure 6.53: Bench-top BeadRetriever

instrument which performs auto-

mated immunomagnetic separation using Dynabeads

magnetic

carriers (courtesy Dynal Biotech ASA).

6.5.3 The application of magnetic separation

Cell separation and isolation of nucleic acids

The isolation and removal of cells can be achieved using magnetic carriers as

has been described above. The principle of separation is based on the fact

that practically all body cells bear a speciﬁc cluster of dierentiation (CD)

receptors on their surface. These receptors determine speciﬁc functions of the

cells in the body. By coating the magnetic particles with antibodies against

respective CD receptors all types of cells can in principle be isolated from the

corresponding body ﬂuids or cell cultures. The magnetic carrier technology

thus opens particularly interesting perspectives for the therapy and diagnosis

of tumours. The quantitative isolation and determination of tumour cells from

tissue samples can provide information on the progress of therapy. Magnetic

carriers can also play a role in the separation of tumour cells during bone marrow

transplants [M41].

The magnetic a!nity separation is used particularly in molecular biology

for isolation of nucleic acids (both RNA and DNA). The separated nucleic acids

can be eluted from the beads and used for further applications [S88].

6.5. MAGNETIC CARRIERS AND SEPARATION IN BIOSCIENCES 531

Immunomagnetic separation Immunomagnetic separation is the technique

that uses antibodies immobilized on magnetic carriers, for capturing target cells.

Immunomagnetic particles speciﬁc for a target organism are usually suspended

in an aliquot of the mixed cell suspension. After an appropriate incubation pe-

riod the immunomagnetic particles with bound target cells are separated from

the suspension in a magnetic separator. The remaining suspension is removed

and the magnetic particles are washed with a suitable solution. An important

advantage of immunomagnetic separation is the ability to capture and detect

dead or severely damaged microorganisms, which cannot be detected by stan-

dard cultivation processes [S89, Y10].

Immunomagnetic separation is often used in microbiology, cell biology and

parasitology. Magnetic particles are used for the isolation of various cell sub-

sets according to their CD receptors directly e.g. from tissue homogenates.

Immunomagnetic removal of cancer cells from the blood and bone marrow of

patients suering from cancer is being practiced in numerous laboratories [S88].

A bench-top BeadRetriever

instrument that performs automated immuno-

magnetic separation is shown in Fig. 6.53.

Drug and radionuclide targeting

Although selective delivery of drugs to speciﬁc targets within an organism would

be beneﬁcial to many patients, conventional treatment is not able to achieve sig-

niﬁcant drug concentration in a target without distributing the drug throughout

most other, usually healthy body parts and without developing drug resistance.

To circumvent these pitfalls, magnetic microspheres or nanospheres have

been used for immobilization or encapsulation of drugs or radionuclides. These

magnetic particles are then transported to a target position using an external

magnetic ﬁeld. Localized delivery of drugs at high concentration, for instance

to tumours, a signiﬁcant reduction of the toxic eects of most conventional

anticancer drugs to healthy tissue has been achieved [P22, L26, J8, A43].

Transport of magnetic particles loaded with drugs can be achieved by mag-

netically guiding the particles using a high-gradient magnetic ﬁeld. As a result

of a short reach of such a magnetic ﬁeld it is rather di!cult to achieve su!cient

focus and drug accumulation in a tissue that is located deep inside a body. A

dierent principle of localizing a drug into the tumour area is illustrated in Fig.

6.54. A suspension of magnetic particles can be injected through a catheter

into the artery feeding the tumour while a magnetic ﬁeld is imposed onto the

tumour area. The magnetic ﬁeld can be conveniently generated by permanent

magnets located outside the patient’s body close to the tumour [K34].

Tumour regression through selective magnetic delivery of cytotoxic agents

to the tumour site has been observed in numerous clinical cases, such as brain

tumour and kidney and colon carcinoma. In order to increase the rate of drug

delivery, the size of magnetic carriers should be as large as possible, compatible

with the fact that the smallest blood vessels have a 5 m inner diameter. Iron

oxide particles are well tolerated and magnetite-based agents are approved for

human use.

532 CHAPTER 6. INDUSTRIAL APPLICATIONS

Magnet

Catheter

Artery

Tumour

Figure 6.54: Schematic diagram of the introduction of drug-loaded magnetic

carriers into a tumour (adapted from [K34]).

Another technique used to achieve tumour regression is based on the applica-

tion of magnetorheological ﬂuids that change their viscosity when exposed to an

external magnetic ﬁeld. Such a ﬂuid can be injected into the blood vessels that

supply the tumour. When a magnetic ﬁeld is applied to the tumour site, the

viscosity of the ﬂuid increases and the ﬂuid seals the blood vessels and inhibits

blood supply and tumour necrosis develops [S94].

Hyperthermia

Local heating of certain organs or tissues to temperatures between 42

Cand

C, particularly for cancer therapy, is called hyperthermia. The heating of

the target tissue to this temperature range generally reduces the viability of

cancer cells and increases their sensitivity to chemotherapy and radiation. The

heating can be achieved by means of various external sources such as radio

frequencies and microwaves. In order to optimize thermal homogeneity in the

target volume, inductive heating of magnetic materials has been explored.

The use of ferroﬂuids, exposed to an external alternating electromagnetic

ﬁeld, for the localized non-invasive hyperthermia, has been thoroughly stud-

ied. Magnetic particles conﬁned in the target volume dissipate the energy of

the external ﬁeld in the form of heat through several types of relaxation mecha-

nisms. Multidomain particles exhibit hysteretic and Rayleigh losses while single-

domain nanoparticles lose energy through Néel and Brownian relaxation [H34,

R26]. Multidomain magnetic particles thus require large ﬁeld amplitudes for

extensive heating, while single-domain particles have large speciﬁc absorption

rates at already lower ﬁeld amplitudes. Under typical conditions of ferroﬂuid

hyperthermia a magnetic induction of 0.04 T at 100 kHz is applied to a tumour

for a period of 30 minutes.

6.5. MAGNETIC CARRIERS AND SEPARATION IN BIOSCIENCES 533

Although hyperthermia with magnetic ﬂuids has been studied for several

decades, its performance has been demonstrated only on laboratory scale. Ap-

plications such as brain tumour, prostate carcinoma [J9], and AIDS therapy

[M41] have been proposed and successfully investigated. Nevertheless, it ap-

pears that several problems must still be eliminated before this technique can

be introduced into clinical practice [S88].

6.5.4 Separation of red blood cells by HGMS

The separation of erythrocytes (red blood cells) from whole blood using mag-

netic means has received considerable attention during the last twenty years. An

innovative approach by a group of researchers from the Southampton University

and other organizations [M42, M43,G24, G25, R27, R28] resulted in the devel-

opment of a novel method of erythrocyte separation by high-gradient magnetic

separation. This separation process is based on the fact that the paramagnetic

properties of the reduced haemoglobin of erythrocytes allow capture of these

cells under suitable conditions. By diluting fresh, anticoagulated blood in an

isotonic reducing agent, the iron of haemoglobin is reduced to the paramagnetic

form. The magnetic force exerted on these particles allows the red blood

cells to be separated from the whole blood.

Red blood cells are ﬂexible biconcave discoids with a diameter of 8.5 mand

a thickness of 2.3 m. The volume magnetic susceptibility of red blood cells

is estimated to be 3.9×10

6

(SI) when the haemoglobin is in the completely

deoxygenated state. It can be shown [S90] that under conditions typical for

HGMS of red blood cells, where the velocity of the red blood cells relative to

the ﬂuid of y =3×10

4

m/s was usually employed [M42], a magnetic induction

of 2 T and a ﬁeld gradient greater than 10

T/m are required to render the

magnetic force greater than the hydrodynamic drag. Such a magnetic ﬁeld

gradient can be obtained, for instance, using ferromagnetic stainless steel wire

matrix of diameter smaller than 400 m.

It was also demonstrated by Melville et al. [M43] that it was possible to

collect selectively diamagnetic white blood cells and platelets onto the regions

of diamagnetic capture of the matrix wire. By magnetically labelling various

cell subpopulations, such as lymphocytes, in human blood and bone marrow

it was possible to separate and concentrate magnetically these labelled cells at

99% purity and 68% recovery [R27].

Concentration of malarial parasites by HGMS

The malarial parasite, on entering the human body, spends an important part

of its life cycle in the red blood cell. During this cycle the parasite progressively

digests haemoglobin to leave oxidized haem product (pigment). The oxidized

haem product is paramagnetic, in contrast to the diamagnetic oxygenated group

found in oxyhaemoglobin.

This magnetic property can be exploited to separate malaria-parasitized red

cells from a suspension of oxygenated normal cells at a short processing time (?

534 CHAPTER 6. INDUSTRIAL APPLICATIONS

Figure 6.55: Comparson of the energy input in primary and secondary metal

production (adapted from [N12]).

10 min) at a purity of the concentrate in excess of 90% [P23]. It was observed

that in order to achieve percentage capture similar to the normal red cells case,

the ﬂow rate for parasitized cells had to be reduced by one order of magnitude.

6.6 Recovery of metals from wastes

Resource conservation by re-use, reclamation and recycling of materials has

been practiced for millennia, and the recovery of metals and alloys is a prime

example of such activities. The recovery of materials from waste and recycling of

materials are of increasing importance due to a wide spectrum of factors [V11].

Increased scarcity of economically viable deposits of ores, energy saving, and

increased emphasis on environmental control are some of the more important

ones. Signiﬁcant reduction of energy input in metal recycling compared to

primary production is illustrated in Fig. 6.55, while Fig. 6.56 shows the relative

costs of mining and extraction of various metallic products.

It is argued [V11] that it is the economic cost, rather than actual physical

depletion, that will provide constraints on resource use in the future. It is likely

that the only structural metals which will not be in short supply in the future

are magnesium (from sea water), aluminium and iron [V11]. It is thus clear that

recycling of materials is strongly associated with the economics of producing raw

materials. This implies that for a ﬁnancially self-sustaining system, the value of

the recycled materials must exceed the cost of producing them [N13]. Recycling

6.6. RECOVERY OF METALS FROM WASTES 535

Figure 6.56: Comparison of relative costs of mining and extraction of various

metallic products (adapted from [N12]).

will, therefore, take place only where the economic incentive is great enough.

6.6.1 The separation processes in metal recycling

In broad terms physical separation methods can be divided into dismantling,

liberation, dry and/or wet separation and melting [N13, D21]. After liberation,

a metal must be separated from a variety of materials. Materials have many

unique physical and chemical properties, which may be applied in separation

processes, ranging from colour and density to magnetic and electrical properties.

Various dry and wet separation methods are used in practice, and their size

ranges are listed in Tables 6.27 and 6.28.

Magnetic separation

Suspended magnets, magnetic pulleys and dry low-intensity drum magnetic sep-

arators are frequently used in metal recycling processes for concentration of fer-

romagnetic materials. These magnetic separators are usually situated directly

after the primary shredder, or just after the air classiﬁer, if used. Occasionally,

secondary low-intensity magnetic separators are utilized to retrieve the 10% to

20% of ferrous metal missed by the primary separator. It is often necessary to

re-shred and then air-classify a magnetic concentrate to free contaminants such

as the contents of tin cans [V11]. A schematic diagram of major process steps

536 CHAPTER 6. INDUSTRIAL APPLICATIONS

Table 6.27: Dry separation techniques for recycling (after Dalmijn et al. [D21]).

Dry separation Particle size range [mm]

Gravity or density sorting

Screening and sizing 1 - 100

Centrifuges and cyclones 0.05 - 20

Air classiﬁcation 5 - 100

Friction separators, shaking tables 0.05 - 5

Magnetic and electrostatic separation

Magnetic separation 0. 5 - 100

Eddy-current separation 1 - 100

Corona discharge separators 0.1 - 10

Triboelectrical separators 0.1 - 10

Particle-for-particle separation

Sensors for material recognition 1 - 100

Image processing 1 - 100

Multi-sensor systems 1 - 100

Table 6.28: Wet separation techniques for recycling (after Dalmijn et al. [D21]).

Wet separation Particle size range [mm]

Gravity or density separation

Classiﬁcation 1 - 100

Rising current separators 1-50

Centrifuges 1-50

Cyclones 0.05 - 0.5

Sink-ﬂoat or DMS 1 - 100

Jigging 1-50

Flotation 0.05 - 0.5

Electromagnetic separation

Magnetic separators 0.1 - 1

Eddy current or Magnus separators 0.05 - 5

6.6. RECOVERY OF METALS FROM WASTES 537

Decontamina-

tion/Partial

dismantling

Size

reduction

Air

classification

Magnetic

separation

Non-ferrous

separation

Obsolete cars

Batteries, fuel, oil

ASR, 25%

65%

Heavy ASR, 5%

Pb, Cu, Zn,

brass,

ss steel

Figure 6.57: Major process operations in the mechanical processing of auto-

mobile scrap. ASR - autmotive shredder residue (adapted from

Dalmijn et al. [D21]).

is shown in Fig. 6.57.

Dry drum magnetic separators equipped with rare-earth magnets are used

for recovery of feebly magnetic stainless steel.

Wet low-intensity drum magnetic separators are used to recover magnetite

and ferrosilicon from dense media plants prior to feeding scrap material to an

eddy-current separation plant. High-intensity magnetic separators are also used

to remove iron-aluminium, a complex material consisting of aluminium and

magnetic parts mechanically connected, such as steel bolts [N13].

Eddy-current separation

Eddy-current separators are used in recycling in combination with a dense me-

dia separation plant to improve the grade and recovery of the metal products.

Magnesium and aluminium are the ﬁrst products from a dense media plant fed

primarily with material from automobile shredders. This product still contains

materials that responded to the same cut-point density at which DMS operated.

These materials, such as stones, glass, copper wire and complex materials rep-

resent approximately 10% of the aluminium fraction with a size range of 10 to

65 mm [N13]. The aluminium concentrate can be obtained in a process shown

in Fig. 6.55. The feed to an eddy-current separator is usually screened into two

fractions, for instance - 32 + 8 mm and - 65 + 32 mm.

The heavy non-ferrous fraction obtained in DMS can also be processed by

eddy-current separators. The current technology cannot, however, separate the

heavy non-ferrous mixture into the ﬁnal concentrates of individual components,

such as zinc, lead and copper [N13]. However, eddy-current separators are useful

to produce pre-concentrates for further sorting, for instance by image processing.

538 CHAPTER 6. INDUSTRIAL APPLICATIONS

Aluminium float, 16 - 65

Dryer

LIMS

Ferrous

metal

Screen

ECS

Al product

Friction

separator

ECS

Cu wire

Reject

Cu wire

Reject

Al product

Figure 6.58: Schematic diagram of the processing of the aluminium ﬂoat frac-

tion with eddy-current (ECS) and friction separators (adapted from

Nijkerk and Dalmijn [N13]).

Rotating

magnet

Non-ferrous

metal particle

Deflection

Particle

trajectory

Non-metal

particle

Figure 6.59: Principle of separation of non-ferrous metal particles from non-

metal particles using the Magnus eect. FL = lift force.

6.6. RECOVERY OF METALS FROM WASTES 539

Separation by Magnus-Robins Eect

It was demonstrated by Robins and later by Magnus that a particle, rotating

either in water or air, does not fall strictly vertically, but, as a result of asymme-

try of the ﬂow and pressures around the rotating particle, a lift force is created

that aects the particle trajectory [M45, B51, T13].

It has been suggested by Rem et al. [F36] that this eect can be applied to

the separation of ﬁne non-ferrous metal particles from a mixture. By exposing

a mixture of vertically falling particles to a rotating magnetic ﬁeld, the non-

ferrous metal particles, in contrast to non-metallic particles, also start rotating.

The Magnus lift force thus created deﬂects the metal particles from the vertical

direction, as shown in Fig. 6.59.

A wet Magnus separator was applied, on pilot scale, to the separation of

copper and aluminium particles from the bottom ash of a municipal waste in-

cinerator [F36]. A production-size plant with a throughput of 50 t/h has been

proposed [R31]. The wet Magnus separator can treat particles in the size range

from 0.5 to 20 mm, while in a dry mode the size range is from 0.2 to 5 mm.

6.6.2 Sorting of heavy non-ferrous metals

Heavy non-ferrous metals include copper, lead, zinc, titanium, cobalt and pre-

cious metals such as silver, gold and platinum, and metal alloys such as brass

(Cu+Zn) and bronze (Cu+Sn+Zn). Approximately 75% of the mixture of heavy

non-ferrous metals comes from automobile scrap, the source of the rest being

electronic and domestic waste. Brass and copper make up approximately 70%

of the value of the mixture, as is illustrated in Fig. 6.60.

Hand sorting is generally the most cost-eective method for sorting of heavy

metals. Labour rates for sorting in Asia are low and the costs of shipping scrap

materials, from most parts of the world, to Asia are presently very attractive.

Moreover, the Asian markets for non-ferrous metals are more attractive than,

for instance, the European markets.

A rather recent entry into the sorting of heavy non-ferrous metals is colour

sorting. High-speed, real-time processors interpret images from high-resolution

cameras. The processors are pre-programmed to recognize certain metals by

colour and geometry and compressed air is used to eject the recognized metal

fragments. One metal type can be recovered per cycle per unit. The e!ciency of

this technique is limited by, for instance, insu!cient liberation, contamination,

dirt and corrosion. Recovery ranges from 80% for lead to 93% for zinc, while

the purity of the concentrates is approximately 95% to 98%.

Sorting in a magnetic ﬂuid

Separation in magnetic ﬂuids, and particularly in ferroﬂuids (FHS) is a natural

choice for sorting of non-ferrous metals into individual metal concentrates. Con-

siderable eort was expended to develop the FHS for the treatment of automo-

bile scrap [K16, K17, F21, S50, N4, R9, H35, A44], followed by the application