Jan Svoboda. Magnetic Techniques for the Treatment of Materials

Подождите немного. Документ загружается.

510 CHAPTER 6. INDUSTRIAL APPLICATIONS

Dry OGMS of coal The application of a split-coil superconducting magnet

to the beneﬁciation of coal was investigated by numerous authors [H28, C25,

P17]. The e!ciency of removal of ash and sulphur was generally lower compared

to that obtained in wet HGMS, at a similar level of losses of the combustible

matter. The selectivity of separation improved considerably when the ﬁnes

smaller than 40 m were removed, while with wet HGMS the limiting minimum

particle size was found to be 20 m [P17].

Enhancement of magnetic properties

The e!ciency of the magnetic separation process could be improved by increas-

ing the magnetic susceptibility of pyrite. Numerous means of enhancing the

magnetic properties of pyrite have been proposed over the years, few of the

them e!cient enough, and probably none of them economically justiﬁable on

an industrial scale.

In the Magnex process [K30], dry pulverized coal is heated to 170

Cand

then subjected to iron pentacarbonyl vapour. The iron is selectively deposited

on the mineral matter, which is removed by a magnetic separator. Reductions of

up to 85% of pyritic sulphur and 26% of ash were reported. Distinct drawbacks

of the process are toxicity and high consumption of the chemical agent.

Conversion of pyrite to ferrimagnetic pyrrhotite takes place during semi-

coking and pyrolysis of coal, in the temperature range from 500

C to 700

Pyrolysis followed by dry magnetic separation is thus a possible mechanism for

coal cleaning [Y8, K31]. Reduction of 61% of sulphur with 57% of combustible

recovery was obtained after a 30 minute exposure of coal to a temperature of

450

C [K31].

To avoid heating coal and wasting energy, it is possible to convert pyrite se-

lectively to pyrrhotite by dielectric heating [B38, B39]. As a result of dierences

in dielectric properties of coal and pyrite, pyrite heats more rapidly than coal

[B38]. Although signiﬁcant increases in the magnetic susceptibility of pyrite can

be expected, the results of the microwave treatment of coal are often disappoint-

ing. Although ash removal of the order of 20%, at the loss of caloriﬁc value of

17% has been obtained, sulphur tends to report into the non-magnetic product

of clean coal. However, when coal was mixed with aqueous NaOH solution and

treated with microwaves, sulphur reduction as high as 40% was achieved [B39].

6.2 Nuclear industry

6.2.1 Removal of solids from fuel dissolver liquor

The reprocessing of the spent irradiated fuel from a nuclear reactor consists of

separating the fuel into three ingredients: unburnt uranium, highly radioactive

ﬁssion products and plutonium. Concentrated nitric acid is usually used to

dissolve the ﬁssion products; however, not all of these are soluble in nitric acid.

In the dissolution of oxide fuels, a signiﬁcant amount of insoluble matter arises

6.2. NUCLEAR INDUSTRY 511

Table 6.17: Mass magnetic susceptibilities of insoluble ﬁssion products [M37].

Product "

/kg]·10

8

" at 1.2 T [m

/kg]·10

8

fuel 1.29 6.43

15% PuO

fuel 10

40% PuO

fuel 6.13 6.39

(0.1% to 0.3% by mass) which contains the ﬁssion products Ru, Rh, Mo, Te,

Zr, Pd, with particle size ranging from 0.1 mto10m.

Although the centrifuging of the dissolution streams is a well-established and

e!cient technique [W31], as a result of some mechanical and thermal problems,

HGMS was proposed [W32] as an alternative method for removing insoluble

ﬁssion products from the reprocessing streams.

Almost all solids that arise at dierent stages in the reprocessing of nuclear

fuels are transition elements or compounds thereof, and have moderately high

magnetic susceptibilities, as shown in Table 6.17. The removal e!ciency of

ﬁssion products with mean particle size of 2 m, by HGMS operating at 1.5 T

was found to range from 85% to 98% [W33].

6.2.2 Removal of ferric hydroxide ﬂocs

Waste-water streams from nuclear power plants often contain signiﬁcant por-

tions of iron. If the pH is adjusted to a value greater than 3, the iron precipitates

out as ferric hydroxide ﬂocs. It has been found that about 60% of total radioac-

tivity and approximately 99% of the -emitting radioactive contaminants are

precipitated with ferric ﬂocs and can therefore be removed with it [W32].

Precipitates of this type are di!cult to separate with conventional equip-

ment, such as ﬁlters or centrifuges, and the mechanical complexity of such

equipment does not make it suitable for operating in a radioactive environment.

It has been shown by Harding and Baxter [H29] that iron and radionuclides can

be removed using HGMS with an e!ciency of 93% to 94%.

6.2.3 Soil decontamination

DOE Defence Complex, Nevada, USA

It is estimated that more than two million cubic meters of soil are contaminated

with plutonium, uranium and ﬁssion products in the Department of Energy

(DOE) Defence Complex (USA). The US Department of Defence has additional

contaminated sites. In order to remediate these sites, several physical techniques

for extraction of radioactive materials were tested by Lockheed Environmental

Systems and Technologies Company (LESAT), in cooperation with Los Alamos

National Laboratory (LANL).

Since many ﬁssion products and all plutonium and uranium compounds are

paramagnetic, while most soil components are diamagnetic, magnetic separation

512 CHAPTER 6. INDUSTRIAL APPLICATIONS

Table 6.18: Volume magnetic susceptibility of selected compounds and elements

[A38].

Compound or element Susceptibility ×10

[SI]

1204

O 3844

Pd 805

Am 707

Pu 636

U 411

PuO

384

RuO

107

ThO

-7

ZrO

-8

MgO -11

SiO

-14

Graphite -14

-18

of paramagnetic contaminants from soil is possible. Table 6.18 summarizes

volume magnetic susceptibilities of selected compounds and elements.

In many situations, the highest concentration of contaminants is found in size

fractions of soils smaller than 50 m and often smaller than 10 m. While most

physical separation techniques are ine!cient in this size range, HGMS can treat

particles down to quasi-colloidal size. High-gradient magnetic separation was

thus identiﬁed as a technique that can selectively extract numerous radioactive

materials from soils while producing a minimum of secondary waste [A38].

Under a detailed experimental programme at LANL, tests were conducted

with slurries of CuO with 4 m mean diameter and volume magnetic suscepti-

bility of 242×10

6

(SI), at a concentration of 400 ppm. The slurry was passed

through an ultraﬁne steel wool matrix 300 mm long placed in the warm 150

mm diameter bore of a cyclic superconducting magnetic separator. The magnet

generated a background magnetic induction of 2 T or higher. The separation

e!ciency of CuO particles was found to be generally greater than 98% [P18]. In

addition, less than 6% of the soil smaller than 45 m reported into the magnetic

concentrate, so that the volume of soil that must be stored in a repository, at

very high cost, is considerably reduced. The results are shown in Table 6.19.

In spite of promising results, the soil remediation project was terminated, as

a result of budget and schedule constraints [P25].

Chernobyl Nuclear Power Plant, The Ukraine

After the Chernobyl Nuclear Power Plant accident in 1986, large areas in the 30-

km zone were uninhabitable as a result of radioactive contamination. In order

6.2. NUCLEAR INDUSTRY 513

Table 6.19: Mass yield into the HGMS magnetic concentrate of the - 45 um

fraction of various soil samples. The applied magnetic induction: 2

T [A38].

Soil sample Soil fraction

(?45 m) [%]

Magnetic frac-

tion (? 45 m)

[%]

Fernald (0.5 T) 66.1 4.9

INEL (Idaho) 63.9 5.2

Los Alamos 26.2 3.4

Oak Ridge 53.2 5.5

Rocky Flats 47.8 5.8

Savannah River Site 21,8 5.0

Superconducting

net

Separation

chamber

Expanded metal

matrix

Figure 6.40: A dry magnetic separator for decontamination of radioactive soils

at the Chernobyl Nuclear Power Plant (adapted from [P19]).

514 CHAPTER 6. INDUSTRIAL APPLICATIONS

Table 6.20: Decontamination of Chernobyl soil samples by dry magnetic sepa-

ration. K = initial activity/activity of non-magnetics, E = activity

of magnetics/initial activity [P19].

Soil Radionuclide K E[%]

Clay

137

Cs 2.1 52

134

Cs 2.3 56

144

Ce 3.5 71

106

Ru 3.1 68

Sand

137

Cs 7.1 86

134

Cs 7.7 87

144

Ce 12.1 92

106

Ru 10.3 90

to investigate the possibility of soil decontamination, a planar superconducting

magnetic system shown in Fig. 6.40. was tested [P19]. A separation chamber

ﬁlled with an expanded metal matrix was placed on the surface of the magnet,

which generated a magnetic induction of 1.2 T inside the chamber. In order

to propel dry material through the matrix and to eliminate entrainment of

non-magnetic particles in the matrix, the inclined chamber was vibrated at a

frequency of 100 Hz.

It was observed that the e!ciency of the removal of contaminants depended

on the types of soil and decontaminants and not on the degree of contamination.

Typical results of removal of radionuclides

137

Cs,

134

Cs,

144

Ce and

106

Ru, which

contribute approximately 98% of the total radioactivity, are listed in Table 6.20.

6.3 Waste water treatment

6.3.1 Steel industry

In modern steel plants, the volume of water used for metal production varies

from 100 to 200 m

per tonne of raw steel produced. About one half of the total

volume of dirty water produced originates in hot mills, the principal contami-

nants being mill scale and oils. Because many of the ﬁne suspended particles

in these waters are magnetic, high-gradient magnetic separation oers an alter-

native to conventional sedimentation and ﬂocculation in clariﬁers. The early

studies by McNallan et al. [M38] and Harland et al. [H30] showed that HGMS

provides comparable e!ciency at substantially higher ﬂow rates, lower space

requirements and considerably lower running costs.

A detailed investigation of the performance and cost eectiveness of the

HGMS as applied to the ﬁltration of steel mill waste water was carried out by

Yano and Eguchi [Y9]. They found that while the installation costs of HGMS

and a sand ﬁlter are almost the same, HGMS surpasses the sand ﬁlter with

regard to operating costs and space requirements. Comparison between HGMS

6.3. WASTE WATER TREATMENT 515

Table 6.21: Comparison between HGMS and sand ﬁlter for waste water treat-

ment from the scubber for an oxygen converter [Y9].

Parameter HGMS Sand ﬁlter

Inlet concentration of solids 150 mg/c 150 mg/c

Target solids in e"uent ? 15 mg/c ? 15 mg/c

Throughput 20 m

/h 20 m

Flow rate 6·10

2

m/s 8·10

3

m/s

Feed time 20 min 8 hours

Flush time 1min 15 min

Back-ﬂush water volume 16 m

/d 30 m

Space requirement 4.5 m

Relative operating cost 0.7 1

Relative installation cost 1.1 1

Magnet input power 5kW -

and sand ﬁlter as applied to the ﬁltration of waste water from an oxygen con-

verter scrubber is shown in Table 6.21.

The ﬁrst full-scale HGMS device for steel mill water treatment was installed

in 1977 at the Chiba Plant of the Kawasaki Steel Corporation (Japan) to treat

scrubber waste water from the vacuum degassing process. The operating con-

ditions and the results of magnetic ﬁltration are summarized in Table 6.22.

In subsequent years several HGMS machines were installed in Japan and

other countries, to remove suspended solids from waste water from hot rolling

mills, continuous casting plants and others. Figure 6.41 illustrates the installa-

tion of a Metso Minerals high-gradient magnetic ﬁlter at a steel plant.

More recently a high-gradient magnetic ﬁlter has been installed at the Chiba

Plant to remove ﬁne iron particles from a coolant used in a cold rolling plant

[T10]. The coolant that is used in cold rolling processing of steel sheets usually

contains 1% to 10% of emulsiﬁed oil. With increasing concentration of ﬁne solid

particles in this recycled coolant, surface defects develop on the steel sheets.

By removing iron particles from the coolant by HGMS, viscous oil is captured,

together with iron particles, on the matrix. In order to reduce the loss of oil

and to eliminate matrix blockage, the viscosity of oil is reduced with steam and

the matrix is then ﬂushed with hot water and compressed air. [T10]. With this

procedure, the matrix can be kept clean for two to six months. The ﬁltration

e!ciency is a function of the solids concentration, as is shown in Fig. 6.42.

In spite of considerable advantages of HGMS, most steel plants are still

using conventional ﬁltration techniques such as sand ﬁlters. One of the reasons

is relatively high energy consumption of the magnet. In order to overcome

this problem, Franzreb et al. [F23] developed an HGMS based on rare-earth

permanent magnets. The magnetic circuit is designed in such a way that the

magnetic ﬁeld in the separation chamber can be switched on and o by rotating

the permanent magnet system in and out of the iron yoke which magnetizes

the matrix. Pilot-plant tests at several German steel mills are reported to show

516 CHAPTER 6. INDUSTRIAL APPLICATIONS

Table 6.22: Magnetic ﬁltration results and operating conditions of HGMS as

applied to gas scrubber waste water treatment at Chiba Works [T9].

Parameter Speciﬁcation

Solid constituents FeandMnoxides

Particle size ? 100 m, 85% - 20 + 5 m

Concentration of solids 100 mg/c

HGMS e"uent ? 20 mg/c

Back ﬂushed sludge 1000 to 3000 mg/c

Filtration e!ciency A 80%

Magnetic induction 0.3 T

Flow velocity max. 0.08 m/s

Matrix Steel wool, 100 m

Matrix height 150 mm

Canister diameter 2.1 m

Throughput 900 m

Input power 20 kW

Feed time 40 to 90 min

Flush time 1min

Mass 40 t

Figure 6.41: Metso Minerals high-gradient magnetic ﬁlter installed at a steel

plant (courtesy of Metso Minerals).

6.3. WASTE WATER TREATMENT 517

Figure 6.42: The iron removal from a cold rolling mill coolant, as a function of

inlet iron concentration. B = 0.3 T, y = 400 m/h (adapted from

[T10]).

performance comparable to that of electromagnet-based HGMS. The e!ciency

of solids removal from a rolling mill e"uent, as a function of the feed rate,

obtained using the Franzreb’s separator, is shown in Fig. 5.78.

6.3.2 Thermal power plants

Corrosion products (crud), in the primary coolant of thermal power plants, are

deposited on the surfaces of circuits, where they interfere with ﬂuid ﬂow and heat

transfer. Corrosion products can also become radioactive in nuclear reactors,

thereby leading to the irradiation of personnel and the restriction of access.

HGMS was found to be eective for the removal of these ﬁne, predominantly

ferromagnetic particulates.

Heitman [H31] described the operational experience with magnetic ﬁlters in

conventional thermal power plants. The e!ciency of removal of iron oxide from

clean turbine condensate ranged between 90% and 99%. Table 6.23 describes

the ﬁltration e!ciency and selected technical data of these magnetic ﬁlters.

In nuclear power plants, crud composition varies between pressurized wa-

ter reactors (PWR) and boiling water reactors (BWR). The principal dose-

associated radionuclides are Co

,Co

,Fe

and Mn

. Under normal PWR

operating conditions of 300

C, the only stable oxides are magnetite and other

substituted spinel ferrites of Co, Ni and Mn, which are ferromagnetic.

In BWRs, system conditions vary from approximately 300

Catthecoreto

C to 100

C at the feed water puriﬁcation system. Under these conditions,

about 30% of the oxides are hematite, the balance being formed by Fe

spinels

518 CHAPTER 6. INDUSTRIAL APPLICATIONS

Table 6.23: Water ﬁltration by HGMS in conventional power plants [H31, M39].

Parameter Speciﬁcation

Filtration e!ciency 90% (total iron)

Inﬂuent Fe concentration ? 1 mg/kg

E"uent Fe concentration ? 0.02 mg/kg

Magnetic induction 0.2 T

Flow velocity 0.3 m/s

Feed time 3to7days

Matrix 6 mm balls

Matrix height 1000 mm

Throughput 100 to 1000 m

Input power 35 kW at 700 m

Diameter of magnet 1.5 m at 700 m

as found in the PWR [E10].

In the USA, the application of magnetic ﬁltration to the primary and sec-

ondary sides of PWRs was studied jointly by EPRI and Westinghouse [T11].

The results indicate that the treatment of 10% of the steam generator secondary

ﬂow can reduce sludge accumulation by 70% to 80%, and that treatment of 0.5%

of the primary coolant has the potential to reduce radioactivity by a factor of 2

to 5.

Heitmann et al. [H32] studied the performance of various types of magnetic

ﬁlters in the puriﬁcation of feed water in PWR with Fe solids concentration

of the order of 10 to 20 ppb. It was observed that, depending on the type of

matrix, the e!ciency of ﬁltration ranged from 85% to 99%, while the maximum

time of operation amounted to 650 days with a combination of steel ball and

steel wool matrix.

Filtration of the cooling water from the multiple force circulation circuit

(MFC) and of a condensate of the steam separator of the St. Petersburg (Rus-

sia) nuclear power plant was investigated by Alikhanov et al. [A39]. The ﬁl-

tration e!ciency of a permanent magnet-based magnetic ﬁlter generating 0.2

T in the expanded metal matrix ranged from 67% to 73% with the solids con-

centration in the inﬂuent ranging from 9 to 29 g/c. The dose rate based on

the measurement of -radiation at various sections of the matrix, and shown in

Fig. 6.43, illustrates that the large majority of radionuclides were retained by

the upper part of the matrix. The measurement of the speciﬁc activity of the

coolant also revealed that the highest retention, as high as 99%, was obtained

with the radionuclides present at high concentrations, such as Zr

and Nb

6.4. MAGNETIC CARRIER TECHNIQUES 519

Figure 6.43: The -radiation dose rate from the matrix of a magnetic ﬁlter, as

a function of the duration of ﬁltration (adapted from [A39]).

6.4 Magnetic carrier techniques

6.4.1 General principles

Magnetic separation can be extended to materials that are not naturally mag-

netic by introducing a material (a carrier) which can selectively enhance the

magnetic properties of a non-magnetic material (a target) that needs to be sep-

arated. Magnetic carriers are of particular interest for the separation of ﬁne

particles, colloids and organics, which are slow or di!cult to separate by con-

ventional methods. The magnetic carrier technique has been applied in ﬁelds as

diverse as mineral separation, e"uent treatment, biological cell separation and

drug delivery.

The action of magnetic carrier particles is to enhance the magnetic properties

of a non-magnetic target material so that its magnetic properties are su!cient

to make the target material separable in conventional magnetic separators. In

order to achieve that, only a small amount of a strongly magnetic material is

necessary. The initial mass magnetic susceptibility of ferrosilicon or magnetite

is approximately 3×10

4

/kg, while weakly magnetic materials, with suscep-

tibility of the order of 2×10

7

/kg can be fairly easily separated in a variety

of high-intensity magnetic separators. Magnetic polarization of such a weakly

magnetic material, of density 5000 kg/m

,placedinanexternalmagneticﬁeld

of 0.2 T, is 0.2×10

3

T. Since the saturation polarization of the industrial-grade

magnetite is 0.45 T (see Table 5.16), only 0.05% of magnetite is thus required

to cause a magnetic polarization in the target material su!cient for its recovery