Jan Svoboda. Magnetic Techniques for the Treatment of Materials

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520 CHAPTER 6. INDUSTRIAL APPLICATIONS

Magnetic

carrier

Target

species

Target (non-magnetic)

species

Magnetic tags

(i) Magnetic carrier

(ii) Magnetic tagging

Figure 6.44: Schematic representation of magnetic carrier and magnetic tagging

techniques.

by a magnetic separator operating at 0.2 T. It can thus be seen that a volume

fraction of the magnetic carrier material in the range 0.05% to 1% is enough to

make the target material su!ciently magnetic for easy separation.

There are two basic mechanisms for using a magnetic particle so that it

renders a non-magnetic particle magnetic, namely as a magnetic carrier or as a

magnetic tag. These mechanisms are schematically shown in Fig. 6.44.

Magnetic carriers are usually 10 to 1000 times larger than the target species

[M40]. By varying the surface characteristics, selective recovery of non-magnetic

colloidal and macromolecular species can be achieved by attaching the desired

species onto the surface, or entrapped within, magnetic carrier particles, as is

illustrated in Fig. 6.44. Fine magnetite and polymer-based beads coated with

magnetite are frequently used as magnetic carriers, depending on application.

Magnetic tags, on the other hand, are usually smaller than the target non-

magnetic particles. These particles, often very ﬁne magnetite or ions (e.g. Y

coat non-magnetic species in order to allow them to be manipulated by an

external magnetic ﬁeld. In view of their small size the mean magnetization of

the cluster is low and large magnetic ﬁelds are often required for their recovery.

6.4.2 E"uent processing and metal ion removal

Magnetic seeding

The earliest examples of the use of magnetic carriers are in e"uent treatment

using ground magnetite to remove organic impurities from industrial waste wa-

ters. In this process a coagulant (e.g. alum) was added to the feed stream,

6.4. MAGNETIC CARRIER TECHNIQUES 521

Pressurized air

Water tank

Non-magnetic

sludge

Magnetic sludge

Seed recovery

HGMS

Floc breaking

Feed

Seeds and

flocculant

Clean water

Figure 6.45: Magnetic ﬁltration process with a magnetic seeding technique

(adapted from [K32]).

at concentrations ranging from 10 to 200 mg/c, followed by the addition of the

magnetite seed in a concentration from 50 to 5000 mg/c. The magnetic particles

coagulated with the non-magnetic impurities, thereby allowing the ﬂocs to be

recovered by a magnetic separator. The ﬂowsheet of the process is shown in

Fig. 6.45.

De Latour and Kolm [D19, D20] and Bitton and Mitchell [M41, B42] con-

ducted some pioneering work on magnetic ﬁltration of river water using the

magnetite seeding technique. The results demonstrated the e!ciency of this

technique for the removal of bacteria, viruses, algae and non-magnetic solids

and for improving turbidity and colour of the water. The removal e!ciency was

around 95% and was strongly dependent on the pH.

The development of an industrial-scale process was carried out in the late

nineteen seventies at Sala Magnetics. The applicability of magnetic seeding

ﬁltration to sewage and combined storm and sewage overﬂow was studied on

pilot-plant scale and a comparison between HGMS and other sewage treatment

processes is shown in Table 6.24. [A40, A41].

Selective removal of oil, phosphorus and heavy metals from waste water

by a combination of magnetic seeding, chemical coagulation and high-gradient

magnetic separation has been demonstrated in several reports [B43, O9,T12].

In order to overcome limitations of a cyclic operation of high-gradient mag-

netic ﬁlters and high energy consumption, Isogami et al. [I5] developed a contin-

uous superconducting ﬁlter for the treatment of sewage water. By the applica-

tion of the magnetite seeding technique, the level of biological oxygen demand

was reduced by 90%, chemical oxygen demand by 79% and concentration of

suspended solids by 97%. Figure 6.46 shows the pilot plant at Hitachi, Ltd.

522 CHAPTER 6. INDUSTRIAL APPLICATIONS

Table 6.24: Comparison between HGMS and other sewage treatment processes

[O8].

Process Removal [%]

Solids COD Bacteria

HGMS 88 - 95 60 - 75 99 - 99.9

Chemical clariﬁcation 60 - -

Activated sludge treatment 55 - 95 70 - 80 90 - 98

Physical chemical treatment 99 80 99

Figure 6.46: A pilot plant for magnetic ﬁltration of waste water, using a con-

tinuously operating superconducting magnet (courtesy of Hitachi,

Ltd.).

6.4. MAGNETIC CARRIER TECHNIQUES 523

adjustment

Water +

magnetite

Slurry +

coagulant

Magnetic

flocculation

Magnetite +

NaOH

Solid/liquid

separation

Wash

magnetite

Solid/liquid

separation

Feed

Demag

coil

Coagulant

Magnetite

Liquid effluent

Clean water

Loaded

magnetite

Figure 6.47: Flowsheet diagram for water treatment with reusable magnetite

particles (adapted from [A42]).

(Japan).

Magnetic coating

This process relies on the selective adhesion of ﬁne magnetic particles to the

surfaces of target materials, as is shown in Fig. 6.43(ii). Fine particles, usually

smaller than 2 m, are required to form resilient coatings on the minerals [P20,

P21]. The selective removal of calcite and dolomite, from mixtures of apatite,

barite and scheelite, by selective magnetic coating, or tagging, has been demon-

strated at laboratory and pilot plant scale [P20]. Other mixtures that were

separated are quartz/ﬂuorite and metallic lead/copper.

Coating of particles with magnetic tags has not yet been exploited to any

signiﬁcant extent on industrial scale. However, tags can be potentially used

in recovering suspended particulates from a wide spectrum of materials and

magnetic coating can provide a basis for selective separation of minerals, e"uent

particles and organic wastes [M40].

Siroﬂoc

process

The Siroﬂoc

process developed at CSIRO (Australia) [K33] employs the

magnetite seeding technique. It is established that a surface of magnetite parti-

cles activated in acid environment is positively charged, which enables negatively

charged particles of the impurities contained in the water to be captured on the

surface of the magnetite particles. On the other hand, in an alkaline environ-

ment, the surfaces of the magnetite particles are charged negatively and thus

repulsion of charged particles from the magnetite surface takes place. Magnetite

thus acts as an impurity adsorbent in acid environment and as a desorbent in

alkaline medium.

524 CHAPTER 6. INDUSTRIAL APPLICATIONS

Table 6.25: River water treatment by chemically precipitated magnetite and

HGMS [H33].

Parameter Untreated water Treated water

Colour [mg/c Pt] 40 8

Turbidity [ZF] 11.5 1.5

COD [mg/c] 11.5 1.5

Fe total [mg/c] 0.23 0.01

No. of psychrophylic bacteria/mc 1000 130

No. of coliform bacteria/mc 13 0.02

Kolarik’s results [K33] were tested on pilot-plant scale, the process ﬂowsheet

of which is shown in Fig. 6.47. It was demonstrated in several plants that have

been in operation in Australia and UK that the process can produce water which

complies with the EC Drinking Water Directive [B40].

The magnetite particles used in the Siroﬂoc process are rather coarse (5 to

10 m), which results in their low speciﬁc surface area and in the need to use

a high concentration of magnetite (e.g. 10 to 12 g/c). Moreover, preparation

of magnetite in this size range is rather costly and complex. In order to over-

come this drawback, Hencl and Mucha [H33] proposed to employ chemically

precipitated magnetite [O9] prepared from waste material, such as acid pickling

steel liquor or waste ferrous sulphate arising from production of titanium white.

Magnetite prepared in this manner, with 90% of particles smaller than 1 m

and 54.3% smaller than 0.4 m, was used to purify the Vltava River (Czech

Republic) water. The impurity-coated magnetite was concentrated by HGMS

operating at 0.5 T at a ﬂow velocity of 0.1 m/s. As can be seen from Table 6.25,

colour was improved by 79%, while turbidity and COD were reduced by 85%

and 76.4%, respectively. The concentration of coliform bacteria was reduced by

99.9%.

6.4.3 Biomagnetic extraction of heavy metals

The phenomenon of bioaccumulation of metal ions from solution by a wide va-

riety of micro-organisms has been well established for some time. A number

of micro-organisms can precipitate heavy metals onto their surfaces as a conse-

quence of their metabolism and growth. This, therefore, gives them the ability

to accumulate such metals in large quantities from external surroundings.

Microbial accumulation is achieved by growing a variety of micro-organisms

in solutions containing the ions of the metal to be extracted and some added

nutrients. As the microorganisms grow, they accumulate the metallic species

onto their surfaces with a corresponding decrease in the concentration of the

metal ion in solution. If the ions are paramagnetic, the resultant coating on

a microorganism makes it magnetic and magnetic separation can be used to

extract the biomass from the solution. A block diagram of the process is shown

in Fig. 6.48.

6.5. MAGNETIC CARRIERS AND SEPARATION IN BIOSCIENCES 525

Chemostat

HGMS

Organism growth

on specific medium

Medium

Effluent

(metals ions)

Decontaminated

effluent

Metal-coated

organisms

Figure 6.48: Block diagram of growth and separation of microorganisms loaded

with heavy metals (adapted from [B44]).

Watson et al. [W34, B44] investigated a large number of magnetic ion species

and of microorganisms and their ability to ingest or precipitate ion species onto

their surfaces. The work was directed towards a study of e"uents from nuclear

and other industries. In most cases, starting concentrations of heavy metals

of the order of 10 ppm were reduced to the 1 ppb level. Successful tests were

carried out on solutions containing U, Cu, Pt, Pd, Rh, Rd, Fe, Ni and other

metals.

6.5 Magnetic carriers and magnetic separation

in biosciences

6.5.1 Principles of biomagnetic separation techniques

With the exception of a few naturally occurring paramagnetic or ferrimagnetic

molecules or cells, such as ferritin, deoxygenated erythrocytes and magneto-

tactic bacteria, biological material is diamagnetic and has to be magnetically

labelled or immobilized on magnetic particles. The magnetic labelling or im-

mobilization can be performed using magnetic carriers, magnetic nanoparticles,

magnetoliposomes and molecular magnetic labels (such as erbium ions) [S88].

Magnetic carriers, in the form of either magnetite or magnetic polymer par-

ticles, or beads, are being increasingly used to separate, analyze and diagnose

cells or biomolecules such as antibodies, DNA molecules, proteins, hormones or

antigens. Drug and reagent delivery are another area where magnetic carriers

or tags are ﬁnding their application.

526 CHAPTER 6. INDUSTRIAL APPLICATIONS

Magnetic beads in

Capture of target cells

1 on beads

Magnetic separation of

target cells 1

Removal of cells 2

Magnetic bead

Cell 1

Cell 2

Magnet

Figure 6.49: Schematic diagram of magnetic separation of cells using magnetic

beads.

Figure 6.50: A scanning electron microscope image of 2.8 mDynabeads

su-

perparamagnetic monosized polymer magnetic carriers (courtesy of

Dynal Biotech ASA).

6.5. MAGNETIC CARRIERS AND SEPARATION IN BIOSCIENCES 527

Table 6.26: Physical characteristics of magnetic carriers Dynabeads.

Head Dynabeads

M-450 M-280 MyOne

Diameter 4.5 m 2.8 m 1.05 m

Density 1600 kg/m

1400 kg/m

1800 kg/m

Fe content 20% 12% 26%

The magnetic beads are usually spherically shaped particles of diameter

ranging from 1 to 20 m, and consist of a polymer matrix, often polystyrene,

polyderivatives and polyvinylalcohol, in which a magnetic colloid, such as mag-

netite, is encapsulated. Speciﬁc a!nity ligands are chemically coupled, often

selectively, to the polymer matrix to separate biomolecules or cells. These lig-

ands display complementary structures to speciﬁc sites of biomolecules or cell

receptors so that these target substances speciﬁcally bind to the magnetic parti-

cles. These target structures can then be separated from the substance mixture

in a magnetic ﬁeld. Many magnetic carriers are biocompatible and some of them

are biodegradable. The basic principle of cell separation is shown schematically

in Fig. 6.49. A scanning electron microscope image of 2.8 m magnetic carriers

Dynabeads

is shown in Fig. 6.50, while Table 6.26 lists physical properties of

Dynabeads

In addition to magnetic carriers in the micrometer size range, magnetic

nanoparticles in the form of ferroﬂuids are often used for the labelling of tar-

get structures (usually cells) and as biodegradable or biocompatible carriers

for biomedical applications. In contrast to micrometer-sized magnetic carri-

ers, high-gradient magnetic separators are required to manipulate systems of

magnetic nanoparticles.

In addition to its current progressive role in biosciences, novel developments

in the biocompatible magnetic carrier technique are expected to have an en-

abling eect on capabilities in biological warfare defence, personnel monitoring

and diagnostic and therapeutic treatments for military personnel. For instance,

the Department of Defence (USA) believes that magnetic carriers are suited to

provide mechanisms for monitoring and controlling biological activity [B45].

Magnetic separation, such as isolation and/or removal of speciﬁc cells, iso-

lation of nucleic acids, proteins and heavy metals, is a common approach to

exploit magnetic principles in biosciences. Biomagnetic techniques can be used

in a variety of other applications. Magnetic support can be applied to immobi-

lization of biologically active compounds and cells when magnetic carriers enable

simple manipulation with immobilized structures.

Magnetic separation can also be used for modiﬁcation of standard immunoas-

say analytical methods to determine concentrations of analytes in medical di-

agnostics. Ferroﬂuids have been studied in biomedical and clinical applications,

often in connection with manipulation of drugs, viruses and tumours.

The separation of erythrocytes (red blood cells) from the remainder of the

528 CHAPTER 6. INDUSTRIAL APPLICATIONS

Figure 6.51: A batch magnetic separator for biomedical applications (courtesy

of Dynal Biotech ASA).

blood components using high-gradient magnetic separation has received con-

siderable attention during the last two decades. The same technique has been

successfully applied to the concentration of malarial parasites and bone marrow

cells.

Magnetic separation has several advantages in comparison with standard

separation procedures. Separation can often be performed directly in raw sam-

ples containing suspended solid material such as body ﬂuids, food and clinical

samples and waste water. As a result of their magnetic properties, target struc-

tures captured to speciﬁc surface-modiﬁed magnetic particles or magnetically

modiﬁed a!nity materials can be relatively easily and selectively removed from

the sample in an external magnetic ﬁeld.

A review of the current status of the application of magnetic methods in

biosciences can be found, for instance, in an article by Šafa

ríková and Šafa

rík

[S88].

6.5.2 Magnetic separators in biosciences

In recent years numerous magnetic separators have become available for appli-

cations in biosciences, ranging from small laboratory units based on permanent

magnets, to automated cell sorters that are able to separate up to 10 million

magnetically labelled cells per second. There are two basic types of laboratory

magnetic separators for biomedical applications, namely batch and ﬂow-through

units [S88].

6.5. MAGNETIC CARRIERS AND SEPARATION IN BIOSCIENCES 529

Figure 6.52: Quadrupole and hexapole batch magnetic separators for biomedical

applications (courtesy of Immunicon, Inc.).

Batch magnetic separators

In most cases the isolation of magnetically labelled structures is performed in a

batch mode. Test tube magnetic separators, an example of which is shown in

Fig. 6.51, equipped with rare-earth permanent magnets, are used to separate

magnetic microparticles from volumes ranging from 5c to 50 mc. Magnetic

nanoparticles require a higher magnetic force and can thus be separated in

quadrupole or hexapole magnetic ﬁelds. Such QMGS and HMGS separators

are shown in Fig. 6.52.

Flow-through magnetic separators

Flow-through separators are cyclic units consisting of a column packed with

magnetizable stainless steel wool, placed between the poles of a permanent mag-

net or electromagnet. Magnetically labelled structures are passed through the

column and are captured in the steel wool matrix. After the removal of the

column from the magnetic ﬁeld, the captured structures are removed from the

matrix. High-gradient magnetic separators are used when working with sub-

colloidal particles, particularly when separating magnetically labelled cells. De-

vices that can separate 10

to 10

cells in one cycle have become commercially

available [S88].

Improved selectivity of separation of biological objects can be obtained when

multipole d.c. magnetic ﬁelds or a.c. magnetic ﬁeld are used. A ﬂow-through

quadrupole magnetic separator [Z2] generating a ﬁeld gradient of 164 T/m and

a hybrid a.c. magnetic device [G23] have been applied in various a!nity-based

separations.