Fine particle processing by magnetic carrier methods

Minerals Engineering, Vol. 7. No. 4, pp. 449-463, 1994 Copyright O 1994 Elsevier Science Ltd Printed in Great Britain. All fights reserved 0892-6875/94 $6.00+0.00

Pergalnon

0892-6875(93) E0031-R

FINE PARTICLE PROCESSING BY MAGNETIC CARRIER METHODS

QINGXIA LIU and F.J. FRlgDLAENDER School of Electrical Engineering, Purdue University, West Lafayette, IN 47907-1285, U.S.A. (Received 8 February 1993; accepted 12 October 1993)

ABSTRACT

Methods for selectively enhancing the magnetic properties offine particles to be recovered or removed by subsequent magnetic separation are reviewed. The comparison of magnetic carrier separation with carrier flotation is also discussed. The influences of physicochemical extent in the adsorption of fine magnetite onto particle surfaces are briefly described. It is concluded that the selectivity of magnetic reagent for thefine panicles and the hydrophobic interaction between the particles treated with chemicals and the carriers (e. g. magnetite laden oil or magnetic reagenO depends on factors similar to those in other mineral separation techniques such as hydrophobic agglomeration and shearflocculation. A large number of applications of the magnetic carrier methods in various areas are also introduced. Keywords Magnetic reagent, magnetite, magnetic separation, magnetic susceptibility, zeta-potential, oleate, and hydroxamate.

INTRODUCTION

With the depletion of high grade ores it becomes necessary to develop efficient methods for treating low grade ores or recovering a valuable component from waste material, which often involves handling and processing of free particles because extensive grinding of mineral ores is in most cases needed to liberate costly particles bound in the bulk ores. Since the specific surface area and the surface energy of particles become larger with a decrease of particle size, many problems such as a self- aggregation of fines and a lowered selectivity of the adsorption of a chemical reagent on the surface of fine target grains are encountered in the removal or recovery of free particles in micron and/or submicron size by means of surface control techniques. The entrapments of fine gangues in the concentrate or slime coating on the mineral surface to be recovered or removed, for example, have not been resolved in the traditional flotation cell, resulting in the loss of valuable fine particles in railings [1]. Thus, there is a great need to develop new methods to control the movement of individual particles and to upgrade a valuable component in the concentrate. In the last two decades, many attempts have been made and several results have been reported for improving the recovery or removal of fine grains using such approaches as column flotation, shear flocculation and carrier flotation, etc. [2-7]. Among these methods column flotation appears to be effective in handling some fine fractions and for this reason, much attention has been paid to the ME 7:4-B

449

450

QINGXIA LIU and F. J. FRIEDLAENDER

optimization, design, scale-up and development of various type.s of colunm flotation cells [8-9]. However, the contamination of fine slimes on the concentrate have not been overcome thoroughly in column flotation although washing water has been used to flush the concentrate and the retention time of particles in the column has been increased. Literature reviews as discussed below show that high gradient magnetic separation (HGMS) has made it possible to separate ferromagnetic particles of dimensions down to below 1 I.tm in size. A basic study on the magnetic separation of submieron particles by HGMS was reported by Takayasu, Gerber and Friedlaender [10]. The challenge in extending HGMS to the separation of weakly or non-magnetic fine particle, however, lies in the problem of enhancing the magnetic properties of weakly or non-magnetic particles to be recovered or removed selectively before or during magnetic separation. The objective of this paper is to review the magnetic methods for recovering weakly or non-magnetic free particles and to introduce recent developments in the application of magnetic carrier methods in different fields.

AN OVERVIEW OF METHODS FOR ENHANCING THE MAGNETIC SUSCEPTIBILITY OF WEAKLY MAGNETIC FINE PARTICLES It is well known that the bulk magnetic properties of certain minerals can be enhanced by roasting or reduction to chemically convert them to a more magnetic phase [11]. The basic principles of the roasting or reduction processes are based on the fact that the chemical or thermal reaction on the surface (or bulk) of certain minerals is different at reduction conditions so that the magnetic properties of target mineral grains can be enhanced selectively by converting them to a more magnetic composition. Alternatives of this process have been developed using microwave or x-ray as the external energy applied to the mineral system, instead of roasting, to selectively enhance the magnetic properties of target mineral grains to be recovered or removed. Kelland et al. [12] have studied the enhancement of magnetic properties of coal pyrite by high power density microwave irradiation. Their results indicate that the temperatures reached after heating are higher in the pyrite than in the coal and the coal pyrite can be selectively converted into ferromagnetic monoclinic pyrrhotite. Thus, the performance of high gradient magnetic separation (HGMS) in removing mineral pyrite from coal has been improved by increasing pyrite's magnetization. There is also a second method of increasing the magnetic response without chemically altering the minerals, and that is by incorporating a discrete magnetic phase onto the particles to be made more magnetic. The dramatic increase in magnetic response shown by a particle containing even small amounts (less than 1%) of a material such as magnetite arises because of the vastly greater magnetic susceptibility of ferromagnetic or ferrimagnetic materials compared with paramagnetie minerals such as siderite or most garnets. It has been calculated that only 0.01--0.1% by volume content of magnetite was required to render the non-magnetic materials sufficiently magnetic so that they could be recovered by a conventional highintensity magnetic separator [13]. A number of processes have been described differing in the mechanism by which selective attachment of the magnetic phase was brought about. These are described below and are summarized diagrammatically in Figure 1. These methods have been discussed in detail by Parsonage [14]. In our opinion, the selectivity in surface decomposition techniques may be higher than others due to chemical reaction on the surface. However, the system of surface decomposition may be quite difficult to control and operate, and the cost may be higher than others. Thus, the application of this technique may be limited only to remove the gangue or impurity from the suspension because the chemical reaction on the surface produces a new phase that results in the changes of properties of material to be made more magnetic.

Fine particle processing

451

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(D) Fig. 1 Methods for the selective enhancement of magnetic properties of mineral grains: (A) selective surface decomposition of iron pentacarbonyl (Magnex process); (B) selective wetting by magnetite laden oil ( Murex process); (C) selective co-flocculation with magnetite; (D) selective surface adsorption of fine magnetite. The selective adsorption of fine magnetite particles on the surface of material to be made more magnetic as shown in Figure 1 (D) is governed by the zeta-potential of the fine particles and the pH of the suspension. If the zeta-potential of the free particles to be removed or recovered has the opposite sign to that of magnetite at a certain pH, coagulation between them will take place due to electrostatic attraction so that the selective attachment of magnetite to the target mineral grains will be achieved. Unfortunately, the PZC (the point of zero charge) of mineral grains to be made more magnetic is often close to that of gangue grains (as shown in Figure 2), resulting in the contamination of the concentrate by impurities. Therefore, chemicals or surfactants will be needed, in many instances, to achieve highly selective agglomeration between the magnetite and target minerals. Our attention in this paper will focus on the magnetic carrier methods for enhancing the magnetic moments of weakly or non-magnetic materials by both the control of function groups of the magnetic reagents and the surface properties of free particles. The applications of various surface interactions between the fine particles are also considered.

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MAGNETIC CARRIER METHODS Magnetic carder methods employ the adsorption of magnetic reagent or the attachment of magnetite onto particles to be separated, to make them more magnetic. In the literature, many investigations have been described by many researchers [15-17] to attach ferromagnetic or ferdmagnetic material such as magnetite to the target mineral grains. Iwasaki [18], for example, added fine grained magnetite to a selectively flocculating suspension of hematite and silica. Under the conditions used, the hematite and the magnetite co-flocculated, whilst the silica remained dispersed. The resultant floes were strongly magnetic and were separated from the silica on a magnetic trough. Hwang et al. [19] have described the separation of gibbsite (AI(OH)3) from quartz in the 2-20 micron size range. A 5 wt % slurry was dispersed with Na2S or NaF and then fine magnetite was added. On the addition of a high molecular weight and high anionic polyacrylamide flocculant the co-flocculation of the gibbsite and magnetite occurred. The quartz remained dispersed. Recovery of the floes was achieved by passing the slurry through a magnetic separator with a steel wool matrix. A final magnetic product of 78.9% AI(OH)3 at a recovery of 87.1% was achieved from a feed grade of 39.6% AI(OH)3. More recently, to improve the selectivity and separation efficiency, two new methods have been developed to attach magnetic materials to the non-magnetic grains to be removed or recovered, namely magnetic reagents and hydrophobic magnetic agglomeration. A magnetic reagent, a composite of magnetic materials and surface active agent, was invented by J.Y. Hwang at Michigan Technological University [20,21]. The surface active agents contain functional groups that can react with magnetic and non-magnetic materials. Under appropriate conditions, the reagents can serve as a bridge to couple magnetic materials to other non-magnetic materials to be made more magnetic.

Fine particle processing

453

In many applications, a magnetic reagent was prepared by building two layers of surfactants on colloidal (nanometer size in most cases) magnetite, as shown in Figure 3. The inner layer surfactant has a functional group with an affinity for magnetite. Fatty acid, such as oleic acid, is an example. After covering the surface of magnetite with the inner layer surfactants, an outer layer surfactant can be built on top of the inner layer surfactant through hydrophobic interactions. The functional group of the outer layer will orient outward from the magnetite and provide the capability for coupling with non-magnetic materials. Since the functional group of the outer layer can be tailored, selectivity of the coupling can then be controlled. Magnetic reagents can also be classified into cationic, anionic, and nonionic types based on the head group of surfactant in the outer layer. Therefore, the adsorption of magnetic reagents on f'me particles is similar to the adsorption of surfactants on solid/water interface in many aspects. Common mechanisms for the adsorption of surfactants on particles may include all or most of the followings: chemical bonding, electrostatic force, Van der Waals force, and hydrogen bonding. The selective adsorption of magnetic reagent onto the particles can be controlled by both the conditions of slurries such as pH and the head group of surfactant. Hwang et al. [20] have studied the adsorption of anionic magnetic reagent with sodium oleate as outer layer on the calcite and quartz surface. Their results showed that calcite adsorbed anionic magnetic reagent consistently in the pH range studied. Magnetic susceptibility of calcite was increased from 3.94x10- 6 emu/g/Oe to 38x10-6 emu/g/Oe at the standard dosage (0.002 g magnetite per gram of mineral). On the other hand, quartz did not adsorb anionic magnetic reagent in the pH range from 5 to 9. Therefore, the separation of these two minerals was achieved by the control of pH using this anionic magnetic reagent. Recently, the adsorption of anionic magnetic reagent with sodium oleate as outer layer on the rutile and quartz has been studied in our laboratory as shown in Figure 4. In the pH range of study, ruffle adsorbed this anionic magnetic reagent consistently. The effective magnetic susceptibility of rutile was increased from 2.02 x 10-6 to 90.12 x 10-6 emu/g/Oe. The successful separation of futile from an artificial mixture of rutile and quartz was obtained using this anionic magnetic reagent and HGMS over a wide pH range as shown in Figure 5.

rant

group ~ineral

¢ end surl~tctant

with amnty eLlS

Fig.3 The structure of magnetic reagent. Another magnetic carrier method, i.e. hydrophobic magnetic agglomeration, is being developed in our laboratory based on the basic principles of hydrophobic agglomeration. From the point of view of the aggregation methods or carrier flotation methods described in the literature [23-25], it was considered that the common or critical requirements for hydrophobic magnetic agglomeration are:

454

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QINGXIA LXU and F. J. FRIEDLAENDER

highly selective surfactants or chemicals with special functional group's affinity with target grains in the mixture; hydrophobicity of the carrier (magnetite) and the particles to be attached; high agitation to form the hydrophobie agglomeration between hydrophobic particles.

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Fine particleprocessing

455

The whole process of hydrophobic magnetic agglomeration is described as follows:

a) b)

c) d)

conditioning slurry with dispersant and adjusting pH to the desired value; adding selective surfactants to slurry for rendering the surface of mineral grains to be recovered or removed hydrophobic; adding the magnetite-laden oil to the mineral slurry subsequent to high agitation; separation in a high gradient magnetic separator;

Potassium octylhydroxamate was the first surfactant used in our experiments for rendering the target mineral (rutile is an example here) hydrophobic. The hydrophobic magnetic carrier, magnetite-laden oil, is prepared by building up an inner layer on colloidal magnetite using sodium oleate and then covering with kerosene as an outer layer. The effects of hydroxamate concentration in suspension on the adsorption of magnetite laden oil onto futile are shown in Figure 6. It can be seen that the amount of adsorption of magnetite laden oil onto rutile increased at the lower hydroxamate concentration. However, at high hydroxamate concentration the amount of adsorption decreased. A likely explanation is that a bilayer adsorption of hydroxamate molecules was formed on the surface of futile to make the surface hydrophilic. Thus, the hydrophobic agglomeration between the rutile and the magnetite-laden oil was impaired. The adsorption of magnetite-laden oil on the futile and quartz treated with hydroxamate is shown in Figure 7. In the range of study, the amount of adsorption on the rutile was almost linearly proportional to the dosage of magnetite-laden oil. But quartz cannot adsorb hydroxamate as well as futile. The amount of adsorption (as shown in Figure 8) varies at different pH, depending on the stability constants of metal hydroxamate and the hydrolysis of cations in the solution. It is well known that the weakest complexes of hydroxamates are formed with alkaline earth metal cations (Ca2+, Ba2+, Sr2+) and rather strong complexes are formed with the highly charged multivalent rare earth cations and with the transition elements such as Nb, Ti, V, Mn, Zr, Hf, Ta [26]. It was also reported that differences in the stability constants of complexes formed with lattice cations of the minerals to be separated in a carbonatite ore (such as lanthanides from alkaline earth-containing gangue minerals) are much greater for hydroxamic acid-based collectors than for carboxylic acids (fatty acids). It was expected, therefore, that alkyl hydroxamates could be more selective than fatty acid in the beneficiation of rare metal-rich rutile from silicate oxide. The mechanism of adsorption of hydroxamate on the rutile and silicate oxide is being studied by us. |

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pH Fig.8 The adsorption of magnetite laden oil on the rutile at various pH. As discussed above, the aggregates were formed by hydrophobic interaction, once the hydrophobic particles collided with each other. The hydrophobic interaction plays a very important role in this process. Without the hydrophobic attraction, the electrical repulsion among the same charged particles would retain the mineral suspension dispersible. This can be verified by the following discussion and calculation.

Fine particle processing

457

INTERACTION BETWEEN PARTICLES IN SUSPENSIONS To determine whether fine magnetite-laden oil will tend to attach to the mineral particles, it is n~_,y~ary to consider the forces that exist between a droplet of magnetite-laden oil and the fine particle. It is possible to correlate the variation of potential energy with the separation of particles by calculating the total energy of the interaction. Both a net repulsive energy and a high energy barrier will prevent the mineral particles from agglomerating with magnetite-laden oil. Thus, the formation of the magnetic aggregation will be inhibited. On the other hand, a net attractive energy raised by hydrophobic interaction will enhance the agglomeration of magnetite-laden oil with the target mineral grains to be made more magnetic. The main interaction energy among particles adsorbed surfactants on the surface in the suspension includes at least the following: (a): Van der Waals interactions (VA); (b): electrical interaction (VR); (c): hydrophobic interaction (VH); If it is assumed that these energies are additive, the total interaction energy VT is given by: VT~- VA+VR+V H

(I)

If the particles are placed in a magnetic field, the magnetic interaction energy needs to be taken into aecount.

(1). Van der Waals interaction, V A. These are attractive forces due to dipoles, induced dipoles and London forces. They may be modified by the presence of adsorbed layers. Many expressions for the magnitude of the interaction have been given by Schenkel and Kitehener [27]. In order to simplify the calculation, the following expression is used;

v^ =

A-R,~ 6(Ri +R2)X

(2)

where g

complex Hamaker constant of particles (J); RI, R 2 : particle radii (m); X separation distance between particles assumed to be spherical (m); The critical parameter determining the magnitude of the Van der Waals interaction is the Hamaker constant. Values for various materials have been given by Gregory [28] and Visser [29]. A method for calculating the Hamaker constant from adsorption data has been described by Hough and White [30]. Fowkes [31,32] indicated that the Hamaker constant was determined by: Au = 6 x r 2 r d

(3)

in which Aii represents the Hamaker constants of the solid or medium; rii is the intermolecular distance within the interacting body of the substance and # refers to the dispersion component of the surface free energy of the substance in question. Fowkes suggested that 6~r~ approximates 1.44 x 10 -14 for most materials. The complex Hamaker constant of two spheres in a medium can be estimated by using [33]

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458

QINOXlA LIU and F. J. FRIEDLAENDER

(2). Electrical (Coulombic) interaction, V R. Electrical interactions arise from the charges of fine particles in suspension and the overlap of electrical double layers of particles. The stability of suspension can be controlled by adjusting the zeta-potential of fme particles using a surfactant or specific chemicals. The principal parameters for determining the magnitudes of electrical interaction are the relevant potential at the particle surface and the electrolyte concentration in the suspension. It is usually considered that the potential at the Stem plane is the important one in colloid stability, and this can, in many cases, be shown to be approximately equal to the potential measured by electrokinetic measurements. The interaction between spherical particles has been given by Hogg et al. [34]: for constant potential interaction,

Vit=

RI+R2

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:~, /j

(5)

and for constant charge interactions: 26102, ( l+e(-"o'~

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(6)

where

eo cr g

: : : :

zeta or Stem potential of particles (mV); permittivity of free space= 8.854 x 10"12 (F/m); relative permittivity; Debye-Huckel constant (cm-l);

e no z NA c k T

: : : : : : :

electronic charge--1.602 x 10"19 ((2); bulk concentration of ionic species, (m-3): valence of ions; Avogadro constantffi6.023 x 1023 (mole'l); concentration of ionic species (mole m-3); Boltzmann's constant--1.38 x 10.23 (J.K'I); temperature (K);

Traditionally, surface potential has been taken to be constant; it is equally plausible physically, however, that surface charge is constant, being fixed by the presence of charged sites on the surface. In the present work equation (5) was used to estimate an electrical interaction between particles. (3). Hydrophobic Interaction, Lilt" The hydrophobic interaction is produced by the collision of fme particles on which non-polar molecules are adsorbed. Usually, hydrophobic interactions consist of two components: One, an attractive, long-range force ( Ua~ ) decreasing exponentially with distance probably arises from the repulsion of the water molecule by hydrophobic particles. When R > > 8, equation:

U~ can be approximated from the following

Fine particle processing

R~R 2 u~ . ¢ - - X o e

Ri +~

<-±) '~

459

(7)

Published values for C (hydration constant) are in the range of 110 - 360 xl0 "7 J/cm2 and decay length x0 varies from 0.1 to 1.0 nm. A value C for the mica/CrAB monolayer system was determined to be 0.14 + 0.02 Nm "l with a decay length (x0) of the order of 1.0 nm [35-37]. Another attractive force,

U~, comes from the association of a hydrophobic chain adsorbed on the

particle surface in the process of collision.

U~ is given by: [38-39]

v ~ ffi U . A . V

(8)

where

U=X'$'~ ~b ffi 1.39kT and

X

: chain association factor (0.5
S

: degree of association (0 < S < 1);

A

: average density o f - C H 2- in the adsorbed layer (m-2). A =

4 R 2 " 0 .n

[(R+8)~-R 2] O

: the density of adsorption of surfactant on the particle surface(m-2);

nc

: the number of CH 2 in hydrocarbon chain;

8

: thickness of adsorbed layer on the surface (m);

V

: the overlapped volume of adsorbed layer on two particle surfaces (m3); V ,, f o d v

= -2x(~-X)2"(3R+28+X) 3 2 2

The electrical double-layer interaction between the mineral particles and a droplet of magnetite laden oil with various zeta-potentials was computed by us as shown in Figure 9. It can be seen that electrical repulsion forces between the particles having the same sign as the surface charges prevents the agglomeration of the free particles. However, for the hydrophobic surface, the energy of hydrophobic interaction computed by the above-mentioned equation as shown in Figure 10 shows a strong attractive force to overcome the electrical repulsion and to form hydrophobic agglomeration. Therefore, the removal or recovery of non-magnetic materials in the suspension can be achieved by the selective adsorption of specific surfactants on the particle surface to render them hydrophobic, followed by the addition of magnetite-laden oil to form a magnetic hydrophobic agglomeration.

460

QINGXIA LZU and F. J. FRIEDLAENDER

pm

~' "~ 'Q c UJ o

300

I

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...........

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:

-500 0

50

100

150

200

(A)

Distance

Fig.9 Electrical double-layer interaction energy between mineral particle and oil droplet (Rm=2pm;Xm=-45 mV; Ro=0.1pm; Xo=-45 mV; A=2.3x10 -13 erg; 1:1 electrolyte 2x10 -3 mole).

600 300

g 100 i _~

-

V r~ele©trlcal rop;,lelon V.~Van d.r W. |le attraction. vk~'hyaropnonio ~, mzraotion

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100 -300

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100

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150

m

2OO

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Fig. 10 Total energy of interaction curves between oil droplets and mineral particles (Rm=21.tm; Ro--0. lp.m; R= lp.m; Zm=-45mV; Xo=-45mV; C=218xl0"TJ/cm2; Xo--1.0nm; no:: 18; 8 : 15.24x10-8 cm'l; o-- 10"6mole/m2; K=50xl0"Scm-l).

Fine particle processing

461

APPLICATION OF MAGNETIC CARRIER METHODS Magnetic carrier methods have been used widely in many processes such as the separation of biological cells, the treatment of waste water, coal desulfurization and mineral processing. Owen [40] has presented three methods in his paper for magnetic separation of biological cells: permanently magnetized particulate labels, magnetic cell sorting, and T-cell removal. Magnetic labeling of specific cells and removal from a suspension is not limited in its chemistry to the use of antibodies alone. The avidin-biotin reaction was exploited in his study since it provides another specificity that is useful for indirect cell labeling. The work was then extended to the more interesting case of a particular subpopulation of white cells that could be labeled with a biotinated antibody against a cell surface determinant. The red cell experiments clearly show the feasibility and the convenience of using a colloidal magnetite reagent to label the surfaces of cells in suspension and then using HGMS magnetic filtration to immobilize them quickly on a filter matrix. When the approach was carried over to white cells, 99 % removal of the labeled cells was obtained in a rapid and simple process. An earlier application of the magnetic carrier approach in the removal of heavy metals from the waste water was studied by Anand et al. [41]. The effective removal (99.9%) of heavy metals such as cadmium, copper, nickel and zinc by adsorption onto ferric hydroxide flocs has been obtained at pH 10.511.0. Ferric sulfate was used as the source of ferric ions along with a small amount of magnetite that was added to make use of high gradient magnetic separation. The role of pH, flow rate through the filter, magnetic field strength and collection tube diameter was examined in this paper. It was confirmed in this paper that the exhausted ferric flocs can be regenerated either in the column or outside by adding sulfuric acid to a pH of 3.0-3.5. This would allow for the reuse of the ferric flocs and the metals released could be reused too because they were concentrated by a factor of 20 or more. Another application of the magnetic carrier approach in waste water treatment has recently been described by Krumm [42]. With his continuous technique waste water streams greater than 10 m3/h, containing phosphate, metals, heavy metals, or particles (pigments) were purified. The waste water treatment is based on precipitation/flocculation, attachment of the pollutants to a magnetic carrier material (magnetite), subsequent magnetic separation in a high gradient magnetic separator, and fmally, recovery of the magnetite from the sludge. The magnetic separation technique for waste water treatment is characterized by high elimination performance, compact process, small space requirements, low power input for magnet operation, no clogging because of open matrix structure. The application of the magnetic carrier approach in the coal desulfurization has been studied by Hwang et al. [43]. Good separation results were obtained at low to medium magnetic field strengths. Comparison with conventional flotation methods indicated that at about 1% total sulfur content, the magnetic reagent method had approximately 88% coal recovery, which is about 5% higher than the flotation method. The dosage of magnetic reagent is within a reasonable range. It is concluded that magnetic carrier technology is a promising method to serve the coal and mineral industries.

CONCLUSIONS The magnetic carrier approach is governed by many factors including both the selectivity of head group of surfactant and the zeta-potential of fine particles at various pH. Our experimental results and theoretical calculations on the interaction energy between the particles in the suspension indicate that hydrophobic interaction plays a more important role in the magnetic agglomeration. Magnetic carrier methods hold great promise for novel applications and, hence, are an important topic for future research.

ACKNOWLEDGMENT The authors wish to acknowledge the financial support provided by the US Bureau of Mines through the Mineral Institute of the University of Nevada (Grant No. 1115132).

462

QINGXlALIu and F. J.

FRIEDLAENDER

REFERENCES .

2.

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8. 9. 10. 11. 12.

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