Separation of Fine Mineral Particles by Selective Magnetic Coating

Journal of Colloid and Interface Science 256, 114–120 (2002) doi:10.1006/jcis.2001.7925

Separation of Fine Mineral Particles by Selective Magnetic Coating Georgios N. Anastassakis1 National Technical University of Athens (NTUA), Department of Mining and Metallurgical Engineering, 9 Heroon Polytechniou str., NTUA Campus, 157 80 Zographou, Greece Received April 26, 2001; accepted August 17, 2001; published online November 9, 2001

or artificial establishment of the magnetic susceptibility of minerals. The weakly magnetic properties of minerals are enhanced by pyrometallurgical treatment (roasting or reduction); artificial establishment is achieved by incorporating a strongly magnetic coating onto their surface. The artificial establishment of magnetic properties originated a long time ago and has been used to separate many minerals systems (9–17). More recently it has been used as a method of fine-particle selective separation (18–27). In the past, this process was also used for coarse particles (14). The magnetic coating methods differ only in their coating mechanisms. These methods can be summarized as follows (20):

This study deals with the separation of fine mineral particles using selective magnetic coating methods. The minerals used were quartz and magnesite, and the coating magnetic material was magnetite of heavy media grade ground to extremely fine particle size. The total interaction energies between each mineral and magnetite were calculated in the absence and the presence of dodecylamine as surfactant. Without surfactant, no magnetic coating was obtained on either of the single minerals, despite the favorable predictions. The magnetic coating results obtained on single quartz particles in the presence of surfactant were in full agreement with the predicted behavior based on the calculated energy barriers, while for magnesite the results were in partial agreement. The selective separation of quartz from magnesite was sufficiently confirmed when artificial mixtures of minerals were used. C 2002 Elsevier Science (USA) Key Words: magnetic separation; carrier methods; magnetic coating; ultrafine separation; quartz; magnesite.

1. Coating by selective decomposition of a gas on the particle surface (9, 10). 2. Coating based on the wettability of a particle’s surface by magnetite-laden oil (11–13). 3. Selective co-flocculation with magnetite (15, 16, 18). 4. Selective magnetic coating (19–27).

INTRODUCTION

During the processing of many mineral commodities, large quantities of fine and ultrafine particles are generated. The separation and recovery of particles of such size by conventional processing methods are difficult and inefficient even when the particles have magnetic properties (1, 2). This difficulty is attributed to problems posed by their very small mass, large specific area, and the high surface energy. The problem of processing fine particles is expected to worsen because of the depletion of high-grade deposits. Consequently, many efforts have been carried out in the laboratory to meet the need for fine-particle processing. Novel methods and techniques as well as combinations of known methods have been tested over the year (3). Among these methods selective aggregation, alone or in combination with flotation, has been given much attention, (3–8). The results were very encouraging in batch tests, but application on a larger scale seems less successful because the method is sensitive to many process-controlling parameters. Among methods developed to meet the problem of fineparticle processing, there is a group based on the enhancement 1 To whom correspondence should be addressed. Fax: +30 1 772 2119. E-mail: [email protected].

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The exact mechanism of magnetic coating is not yet clear because the system is complicated and the parameters determining particle interactions are not well known. The situation becomes more complicated when oily phases (soluble and/or insoluble in water) are added to promote selective coverage of the particles. It is generally believed that when an immiscible with water phase is added, oil droplets attach to hydrophobic particles and when the particles collide each other, the oil droplets merge and agglomerate. The effect of capillary forces in this process is very important as they influence both the preferential wetting of a solid surface with a liquid by displacing another one and the coalescence of the emulsion drops in the case of collision. As two emulsion drops approach, the pressure at the nearest surfaces increases, deforming the drops and enlarging the radius of curvature in the immediate area. That deformation causes the capillary pressure in the regions outside that area to decrease, suctioning continuous phase from between the drops and increasing the possibility of contact and film rupture or coalescence. The objective of this paper is to study the separation of quartz fine particles from magnesite fine particles by selective magnetic coating.

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SEPARATION OF FINE MINERAL PARTICLES

(b) Electrical (Coulombic) Interactions under Constant Charge, VE

THEORETICAL

The selective deposition of fine particles on coarser particles and the density of deposition are controlled by the total energy of interaction between the particles of the suspension. If a highenergy barrier exists, the magnetic and the mineral particles are prevented from close approach and, consequently, no coating occurs. On the other hand, if ideally no energy barrier exists, the magnetic material covers the particle surface. When surfactants are used, the total interaction energy includes the following components: (a) Van der Waals interactions, VA ; (b) electrical (Coulombic) interactions, VE ; and (c) hydrophobic interactions, VHPB : VT = VA + VE + VHPB .

[1]

(a) Van der Waals Interactions, V A For a system including mineral particles, these forces are attractive and are due to interactions between dipoles (Debye forces), dipoles/induced dipoles (Keesom forces), and nonpolar molecules or atoms (London forces). They may be modified by the presence of adsorbed layers. The magnitude of the interactions between two spheres, taking into account the effects of retardation, is calculated according to the following expression given by DLVO theory modified by Schenkel and Kitchener (28), VA = −

A R1 R2 f (P0 ) , 6(R1 + R2 )H

[2]

where, for P0 = 2π H/λ < 0.5,

f (P0 ) =

1 1 + 1.77P0

[3]

for 0.5 < P0 < ∞

  2.45 2.17 0.59 . f (P0 ) = 12 − + 60P0 180P02 420P03

[4]

The notation of the above expressions is given in the Nomenclature Appendix. In most applications the retardation factor f (P0 ) is ignored, resulting in overestimation of the energy due to Van der Waals forces, especially for long distances. The critical parameter determining the magnitude of the Van der Waals interactions is the Hamaker constant (A). It must be noted that different workers frequently report widely differing results even for the same system. For mineral particles in water, the value of the Hamaker constant ranges generally between 0.5 × 10−20 and 10 × 10−20 J. For quartz the literature values of the Hamaker constant range between 0.86 × 10−20 and 1.2 × 10−20 J, while for magnetite it is about 8.8 × 10−20 J.

These interactions arise from the surface charge of fine particles in suspension and the overlap of their electrical double layers. The result may be attraction or repulsion depending on the sign of their surface charge. The interactions between two spherical particles are considered under constant potential or constant charge, although in practice the actual interaction must lie between these two boundary conditions, depending on the rates of exchange of the potential determining ions (29–35). Despite some work following the constant potential boundary condition (23, 24, 36), many mineral systems seem to respond more closely to the constant charge (31–34). In this case, the interaction between two spherical particles is given by the expression (20)   π ε0 εr R1 R2 ψ12 + ψ22 2ψ1 ψ2  2  VE = (R1 + R2 ) ψ1 + ψ22

  1 + exp(−KH) × ln − ln[1 − exp(−2K H )] , [5] 1 − exp(−KH) 

where  K =

2e2 n 0 z 2 ε0 εr kT



1/2 =

2e2 NA cz 2 ε0 εr kT

1/2 .

[6]

(c) Hydrophobic Interactions, VHPB These interactions are due to attractive forces between hydrophobic particles. Although there are uncertainty and considerable debate regarding the origin and the mechanism of the hydrophobic interactions, they are believed to arise from the perturbation of the water structure as the particles approach each other (37). Many expressions have been proposed to describe the hydrophobic interactions between particles. Some researchers propose that the hydrophobic interactions consist of two components: one long-range component that is described by an exponential force law and another that arises from the association of the hydrophobic chains adsorbed on the particle surface and holds for very short particle distances L ≤ H ≤ 2L, where L is the length of the hydrocarbon chain of the surfactant (20, 23, 24). Direct force measurements with water-soluble surfactants suggest that the hydrophobic interactions are relatively short range, being discernible at separation distances in the range 0–15 nm. The interactions are usually described using an exponential force law (38). For two spherical particles, with adsorbed layer thicknesses δ1 and δ2 correspondingly, the hydrophobic energy of interactions is expressed in the form (39) VHPB = −

  CHPB (R1 + δ1 )(R2 + δ2 ) H − (δ1 + δ2 ) lHPB exp − . (R1 + δ1 ) + (R2 + δ2 ) lHPB [7]

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GEORGIOS N. ANASTASSAKIS

Published values for the hydrophobic interaction constant (CHPB ) are in the range 0.11–0.36 N/m while for decay length (lHPB ) 0.1–2.0 nm.

TABLE 1 Zeta-Potential of Quartz, Magnesite, and Magnetite in Water (W) and in 7.2 × 10−5 M Dodecylamine (DA) Solution as a Function of pH (Supporting Electrolyte: 5 × 10−4 M KCl)

MATERIALS AND METHODS

Pure natural samples of minerals (magnesite, quartz) were used in this study. The material used for magnetic coating was magnetite of heavy media grade. The mineral samples were hand-picked, subsequently crushed in a jaw and cone crusher, and ground in a porcelain ball mill by using porcelain grinding media. The size of the particles to be coated was −75 + 25 µm. Each sample of the minerals for zetapotential measurements was excessively ground for 6 h in the porcelain mill to a final size of −5 µm. Magnetite was ground for 4 h in a steel ball mill at a size of −5 µm for both zetapotential measurements and mineral coating. In all the cases the −5-µm size in the grinding product was 100%. The particle sizes of magnesite, quartz, and magnetite for zeta-potential measurements were defined in a Malvern Master Sizer SB.0D particle size analyzer. An electrophoretic mass transport analyzer (EMTA 1202, Micromeritics Instrument Corp.) was used for zeta-potential measurements. In all the electrokinetic experiments 5 × 10−4 M KCl was used as supporting electrolyte. The coating tests were carried out in a 400-cm3 beaker, and a stirrer with a plastic impeller was used to keep the particles in suspension. Initially for each test, 1 g of mineral was conditioned for 5 min with 100 ml of 7.2 × 10−5 M dodecylamine solution, kerosene (2.5 L/t), and pine oil (250 g/t) at the experimental pH value. The pH was regulated with HCl and NaOH solutions. In another beaker, a defined quantity (5 g) of finely ground magnetite was conditioned for 5 min with the same quantities of reagents as previously mentioned at the same experimental pH value. After this period, the suspensions were mixed by adding the magnetite suspension to that of the mineral. The mixture was further conditioned for 5 min. After this period, the magnetic particles were separated by high-intensity magnetic separation. The magnetic separator used was from Carpco Co., and the current setting was 1.0 A. The magnetic particles were held on steel balls of 8-mm diameter. The attached magnetite was removed from the mineral particle surface after treatment with ethanol. Then the mineral– magnetite mixture was dried and magnetite separation was followed using a hand magnet. The products were then weighed. For mineral mixtures, equal weights of 1 g for each mineral were used, and the coating procedure was the same as previously described. The results of the separation were determined on the basis of CO2 emission from magnesite during thermal gravimetric analysis (TGA) by using a Mettler Toledo Star System device. For quartz, the magnetic coating was also examined at pH 6 with kerosene and pine oil replaced by ethanol.

Zeta-potential (mV) Magnetite

Quartz

Magnesite

pH

W

DA

W

DA

W

DA

6 7 8 9 10 11

+25.5 −8.2 −6.9 −13.5 −29.7 −44.7

+25.9 +7.2 +18.3 +21.8 +17.7 +13.0

−54.2 −56.0 −59.4 −69.5 −81.5 −95.8

−43.4 −48.7 −52.2 −55.8 −49.0 −22.5

+31.0 +27.8 +14.9 −23.5 −33.1 −54.2

+31.6 +28.2 +15.5 −4.8 −7.5 −6.1

lamine solution 7.2 × 10−5 M are presented in Table 1 as well as in Figs. 1 and 2. From Fig. 1 it is observed that the surface of quartz is negatively charged in the whole pH range between 6 and 11 when only supporting electrolyte is present. Under the same experimental conditions, magnesite has a point of zero charge (pzc) at pH 8.3 and magnetite at pH 6.8. From Fig. 2 it is observed that in the presence of 7.2 × 10−5 M dodecylamine the surface charge of quartz remains negative in the pH range between 6 and 11, but it is more positive than previously. This shift is greater in the highly basic pH region. Similar observations can be made for magnesite at pH > 8.3, while the surface charge of magnetite is positive in the whole pH range examined. From the zeta-potential measurements before and after

RESULTS AND DISCUSSION

The results of zeta-potential measurements for magnesite, quartz, and magnetite in the absence and presence of dodecy-

FIG. 1. Zeta-potential of quartz, magnesite, and magnetite in water with 5 × 10−4 M KCl as supporting electrolyte.

SEPARATION OF FINE MINERAL PARTICLES

FIG. 2. Zeta-potential of quartz, magnesite, and magnetite in 7.2 × 10−5 M dodecylamine solution with 5 × 10−4 M KCl as supporting electrolyte.

the addition of surfactant, it seems that dodecylamine is adsorbed on the particle surface electrostatically both for minerals (quartz, magnesite) and for magnetite. The predicted energy barriers between minerals and magnetite as well as between magnetite particles are shown in Figs. 3 and 4.

FIG. 3. Energy barriers between minerals and magnetite in water as a function of pH.

117

FIG. 4. Energy barriers between minerals and magnetite in the presence of 7.2 × 10−5 M dodecylamine solution as a function of pH.

Magnetic coating is achieved when no barrier between mineral particles and magnetite exists. For the purposes of calculation the following set of values has been considered: Hamaker constants, quartz 1 × 10−20 J, magnetite 8.8 × 10−20 J, and magnesite 2 × 10−20 J; radius of mineral particles (quartz, magnesite), R1 = 50 µm; radius of magnetite particles, R2 = 1 µm; CHPB = 0.14 N/m; thickness of adsorbed surfactant layer on minerals and ˚ and lHPB = 1 nm. on magnetite, respectively, δ1 and δ2 = 18 A; The energy barriers were calculated according to Eq. [1] when surfactant was used. Only the Van der Waals and electrostatic interaction energies were taken into account when no surfactant was used. Figure 3 shows the calculated energy barriers when no surfactant is present. The approximate distance at which the theoretical maximum in the net repulsive force occurs is 1 nm for interactions between mineral (quartz, magnesite) and magnetite and about 2 nm for interactions between magnetite particles. From the profiles of the interaction energies between mineral and magnetite as a function of distance there was no secondary minimum obtained despite the relatively large size of the mineral particles. In some cases the net repulsive interaction energy is reduced below 17 kT for the interparticle distances of 54 nm. On the basis of the above-mentioned predictions, the pH favorable for selective magnetic coating seems to be restricted to pH 6 for quartz coating and at 7 < pH < 8 for magnesite. Magnetic coating tests carried out on single minerals at the predicted pH values did not confirm the predictions concerning the magnetic coating of either mineral. Corresponding microscopic examination of the minerals revealed that there was no seed of magnetite either on the quartz or on the magnesite particle surface. As it was observed, fine magnetite particles were attached on quartz

118

GEORGIOS N. ANASTASSAKIS

TABLE 2 Magnetic Separation of Single Minerals Treated with Reagents at Various pH Values

particles but so loosely that a soft hand shaking of the microscope plate was enough to detach the particles. This evidence indicates that the failure of coating could be attributed to the hydrophilicity of the minerals. The thin film of water that exists between the particles (quartz/magnetite and magnesite/magnetite) is not ruptured even when the particles are very close. Thus, the particles are prevented from tight aggregation even when the total interaction energy between particles is attractive. Consequently the particles are easily detached by stirring. According to Fig. 4, there seems to be no energy barrier for the magnetic coating of quartz in the pH range between 6 and 11 in the presence of dodecylamine, while energy barriers exist for magnesite when pH < 8.5. Consequently, the selective magnetic coating of quartz seems to be favored in this pH region. The approximate distance at which the theoretical maximum in the net repulsive force occurs is 9 nm for magnesite/magnetite interactions, while it is increased to about 11 nm for interactions between magnetite particles. From the profiles of the magnesite/magnetite interaction energies as a function of distance, no secondary minimum was obtained. Also in Fig. 5 is an example graph of the magnesite/magnetite interaction energies at pH 8 plotted as a function of separation distance. Magnetic coating tests on single minerals using only dodecylamine confirmed predictions regarding quartz, resulting in partial coverage of the particle surface that was enough to render them magnetic at relatively high field intensity. The aggregation between quartz and magnetite particles could be attributed to the hydrophobic coating of both kinds of particles. As the hydrophobic particles of magnetite and quartz approach each other, thinning and finally rupture of the liquid film between

them take place, possibly due to reduction in the interfacial viscosity. In contrast, scanty coating was observed on the surface of the magnesite particles, which did not respond positively in magnetic separation even at high intensity, despite the favorable predictions. When kerosene with pine oil was used along with dodecylamine, microscopic examination of the minerals revealed that quartz particles were strongly coated by magnetite in the range 6 < pH < 11. The observations were completely confirmed by magnetic separation tests that followed (Table 2 and Fig. 6). The role of kerosene seems to be very important, as oil droplets have the tendency to attach to hydrophobic particles and spread on their surface, making the hydrophobic film stronger. The magnetic coating was retained not only on single particles but also on aggregates of quartz particles. On the basis of the above-mentioned observations and results, the mechanism for the magnetic coating of quartz particles seems to be the

FIG. 5. Magnesite/magnetite interaction energies at pH 8 in the presence of dodecylamine.

FIG. 6. Recovery of quartz and magnesite after magnetic coating using 7.2 × 10−5 M dodecylamine solution, kerosene (2.5 L/t), and pine oil (250 g/t).

% Recovery pH

Quartz

Magnesite

6 7 8 9 10 11

97.6 97.8 98.0 98.7 99.3 99.6

0.0 0.0 0.0 4.7 10.2 17.5

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SEPARATION OF FINE MINERAL PARTICLES

TABLE 3 Magnetic Separation of Artificial Mixtures of Quartz–Magnesite at Various pH Values after Magnetic Coating

pH

Product

6 Magnetics Nonmagnetics Feed 8 Magnetics Nonmagnetics Feed 10 Magnetics Nonmagnetics Feed

Weight (%)

CO2 (%)

46.7 53.3 100.0 49.1 50.9 100.0 56.3 43.7 100.0

2.11 47.02 26.05 2.46 49.10 26.20 6.31 51.61 26.11

Grade (%)

Recovery (%)

Quartz Magnesite Quartz Magnesite 96.0 9.9 50.1 95.3 5.9 49.8 87.9 1.1 50.0

4.0 90.1 49.9 4.7 94.1 50.2 12.1 98.9 50.0

89.5 10.5 100.0 94.0 6.0 100.0 99.0 1.0 100.0

3.7 96.3 100.0 4.6 95.4 100.0 13.6 86.4 100.0

following. Initially the emulsified oil droplets are spread on the hydrophobic surfaces of quartz and magnetite, which were treated separately. Upon mixing of their suspensions and under the influence of stirring, the laden oil particles of quartz and magnetite collide and coalesce. Under the influence of a net attractive interaction force, fine magnetite particles are strongly attached to the quartz particle surface. Coating tests carried out on quartz at pH 6.3 using ethanol instead of kerosene and pine oil proved that the quartz particles retained no magnetic coating. This denotes that the driving force for the magnetic coating of quartz seems to be the emulsion coalescence. Similar observations for magnesite revealed that the particle surface retained no magnetic coating at pH < 9.5, while at higher values some seeds of magnetite were observed on the surface, with the extension of coating comparatively increasing with pH. Thus, it must be pointed out that the extension and strength of the magnetic coating of magnesite were much less than those of quartz. In Fig. 7 the results of magnetic coating at pH 9.2 are shown both for single minerals and for a mixture of them using dodecylamine, kerosene, and pine oil. In Table 3 the results of artificial mineral mixtures (1 g each) at selected pH values are presented. The results seem to be satisfactory compared to those with single minerals.

SUMMARY AND CONCLUSIONS

FIG. 7. Results of magnetic coating at pH 9.2 on quartz (A), magnesite (B), and artificial mixture (C).

The separation and recovery of fine and ultrafine particles are important problems that mineral processing must cope with. This study deals with the selective separation of quartz from magnesite fines by selective magnetic coating. The attachment of extremely fine magnetite on quartz seems possible in the presence of dodecylamine and kerosene in the pH range between 6 and 11. When kerosene with pine oil was replaced with ethanol, no surface magnetic coating was obtained. In contrast, the magnetic coating on the surface of magnesite particles was only partly confirmed despite the favorable predictions.

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GEORGIOS N. ANASTASSAKIS

The deposition of magnetite on mineral particles is achieved by controlling the physicochemical properties of the particles involved. In this case, emulsion coalescence seems to be the driving force for the selective coating of quartz. This method seems to have good potential application in the field of selective separation of mineral fine particles, which are inefficiently separated with conventional methods, with the prerequisite that the surfaces can be modified. APPENDIX: NOMENCLATURE

A CHPB c e f (P0 ) H K k lHPB NA n0 P0 R1 , R2 T VA VE VHPB VT z δ1 , δ2 ε0 εr λ ψ1 , ψ2

Hamaker constant (J) Hydrophobic interaction constant (N/m) Concentration (mol/m3 ) Electronic charge, 1.602 × 10−19 C Retardation coefficient Minimum separation distance between two spheres (m) Debye–H¨uckel parameter (m−1 ) Boltzmann’s constant, 1.3806 × 10−23 J/K Hydrophobic force decay length (lHPB = 1.0 nm) Avogadro’s number, 6.023 × 1023 mol−1 Bulk concentration of ionic species (m−3 ) Parameter in Eqs. [2–4], when calculating the retardation coefficient Radius of particles 1 and 2 (m) Temperature (K) Interaction energy due to Van der Waals forces (J) Interaction energy due to electrical double-layer effects (J) Interaction energy due to hydrophobic effects (J) Total interaction energy (J) Valence Adsorbed layer thickness on particles 1 and 2 (m) Permittivity of free space (ε0 = 8.854 × 10−12 F/m) Relative permittivity (for water εr = 81) Wavelength of intrinsic oscillations of atoms (m; λ = 10−7 m) Zeta- or Stern-potential of particles 1 and 2 (V) ACKNOWLEDGMENTS

The author acknowledges Dr. Pradip for fruitful discussion of the paper and his suggestions, as well as Dr. Th. Perraki (Assistant Professor NTUA) and Mrs. E. Mitsi (Technical staff NTUA) for their help with thermal gravimetric analysis.

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