Principles of mineral separation by selective magnetic coating

International Journal of Mineral Processing, 24 (1988) 269-293

269

Elsevier Science Publishers B.V., AmsterdAm - - Printed in The Netherlands

Principles of Mineral Separation by Selective Magnetic Coating P. PARSONAGE

Warren Spring Laboratory, Stevenage, Hertfordshire SG1 2BX (U.K.) (Received November 23, 1987; accepted after revision April 29, 1988)

ABSTRACT Parsonage, P., 1988. Principles of mineral separation by selectivemagnetic coating. Int.J. Miner. Process., 24: 269-293. Methods for selectivelyenhancing the magnetic properties of minerals as a basis for subsequent magnetic separation are reviewed. The physico-chemical parameters influencing the adsorption of fine magnetite onto particlesurfaces is discussed. Effects due to Born repulsion,van der Waals forces,electricaldouble-layer interactions,hydration, stericand hydrophobic effects,hydrocarbon chain association and magnetic interactions are considered. Equations to describe these interactions are given. The influence of hydrodynamic factorsin the adsorption stage,and aspects of the magnetic recovery stage are brieflydiscussed. It is concluded that the selectivityof the magnetic coating process is dependent on factorsdifferentfrom those exploited in other mineral separation techniques, and that it can be used to fractionate mixtures which are difficultto treat using conventional processes.

INTRODUCTION

In the search for new methods to separate difficult to treat ores, a number of processes have been described which involve increasing the magnetic response of selected phases in the mixture. In general these involve the incorporation of a strongly magnetic phase into a particle of an otherwise nonmagnetic or weakly magnetic material. A brief review of some of these techniques is included in this paper. The underlying physico-chemical principles of one of these techniques, which has been termed selective magnetic coating, are dealt with in more detail. A REVIEW OF PHYSICAL METHODS OF SELECTIVE MAGNETISATION

It is well-known that it is possible to alter the bulk magnetic properties of

© Crown copyright, 1988

270

certain minerals by roasting or reduction to chemically convert them to a more magnetic phase. 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 magnetised. 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 permeability of ferromagnetic materials compared with paramagnetic minerals such as siderite or garnet. It has been calculated that only 0.01-0.1% by volume content of magnetite should be required to impart sufficiently strong magnetic properties to a particle to enable it to be recovered by conventional high-intensity separation (Parsonage, 1984 ). A number of processes have been described differing in the mechanism by which selective attachment of the magnetic phase is brought about. These are described below and are summarized diagrammatically in Fig. 1.

OO Fe(CO)5{GAS~

Fe~COATING~+ 5(CO)~GASt (A)

a ~

Ill/Ill/l/

//////////

Ill/I/I///

(B~

QQ

oa~

-

,," "~'

"

~ 0 6

(C)

vv

+ -...

.

(D) Fig. 1. Methods of selective magnetisation: (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.

271

Coating by selective decomposition of a gas at the particle surface Hazen Research have developed a method, known as the Magnex process, in which the decomposition of iron pentacarbonyl takes place selectively onto pyrite and ash surfaces in the presence of coal (Kindig and Turner, 1976; Porter and Goens, 1977). The decomposition products, proposed to be Fe on the ash and FeS on the pyrite, produced magnetisation of these phases and allowed their removal by magnetic means. Meech and Paterson (1980) extended the application to magnetise chrysocolla in the presence of quartz. Magnetic separation was by high-intensity induced rolls at a flux density of 0.29 T. The basis of the selectivity is the high specific surface area of chrysocolla relative to quartz, and also the lower activation energy of the decomposition reaction on chrysocolla.

Coating based on differences in wettability of the particles If hydrophobic (or oleophilic ) particles are contacted with a magnetite laden oil, then the magnetic oil will tend to spread over the particles' surfaces. This can form the basis for selective magnetisation, and hence separation, of mixtures of hydrophobic and hydrophilic phases. Taggart (1945) describes several examples of this type which is known as the Murex Process. At Mawchi Mines Ltd, Burma, coarse sulphides were removed from gravity tin and tungsten concentrates and at Clausthal, galena was separated from barite and silica. In the treatment of a partially oxidized lead-silver ore at Darwin Lead and Silver Mines, reagent consumption was given as: oil, 6.8 kg t-1; oleic acid, 0.36 kg t-1; and magnetite, 7.7 kg t-1. A similar process has been described by Shubert (1980). He used a kerosenebased magnetic fluid to selectively wet hydrophobic particles. The coated grains were extracted using either a hand magnet or other low intensity device. Separations included chalcocite from silica, sphalerite from gangue, and coal from ash. Reeson (1983) has described the separation of coal and ash using a suspension of magnetite in gas oil. The magnetite/oil apparently agglomerated the coal selectively. High shear agitation (using a Silverson mixer at 8000 rpm) was necessary to achieve selectivity and high recovery of coal to the magnetics. In the treatment of a sample of coal with a particle size of 7 microns it was found that the recovery of magnetics decreased from 61.7% to 42.9% with increasing particle size of the magnetite from 2.5 microns to 9 microns. Magnetic separations were carried out using a laboratory-scale high-intensity separator.

Selective co-flocculation with magnetite If a suspension is flocculated using a high molecular weight polymer in the presence of fine magnetite, then it is highly likely that some of the magnetite

272

will become incorporated in the flocs. This can be due either to adsorption of the f]occulant onto the magnetite or to simple mechanical entrainment of the magnetite within the flocs. In either case the flocs are then amenable to recovery by magnetic means. Use is made of this effect in the water industry for the bulk removal of suspended solids or bacteria (de Latour, 1973: de Latour and Kolm, 1976; Kurinobu and Uchiyama, 1982). It is also known that by controlling the chemical conditions in a mixed pulp of fine particles that selective flocculation of one mineral phase may occur (Yarar and Kitchener, 1970; Read, 1971; Attia and Fuerstenau, 1976). The introduction of fine magnetite to such a system can result in the formation of magnetite-bearing flocs of one of the mineral phases. Such flocs may then be recovered from the dispersed phase by magnetic means. Iwasaki (1981), 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 flocs were strongly magnetic and could be separated from the silica on a magnetic trough. Hwang et al. (1982) have described the separation of gibbsite (AI(OH)3) from quartz in the 2-20 micron size range. A 5% slurry was dispersed with Na,,S or NaF and then fine magnetite was added. On the addition of a high molecular weight, highly anionic polyacrylamide flocculant, co-flocculation of the gibbsite and magnetite occurred. The quartz remained dispersed. Recovery of the flocs was achieved by passing the slurry through a magnetic separator with a steel wool matrix. A final magnetic product analysing 78.9% A1(OH)3 at a recovery of 87.1% was achieved from a feed grade of 39.6% AI(OH):,. In a more recent paper the same authors report the separation of 2-20 micron alunite (KAI:~( SO4 ) ( OH ) 6) and quartz ( Hwang et al., 1985 ). The pulp was dispersed using sodium carbonate and sodium tripolyphosphate. A 1% addition of 5 micron magnetite was used. Co-flocculation of quartz and magnetite was achieved using a non-ionic polyacrylamide. Following magnetic separation an alunite grade of 89.1% at a recovery of 81.7 % was obtained in the non-magnetic product. Co-flocculation with magnetite followed by magnetic separation has several technical advantages over the conventional process of selective flocculation followed by settling. It is less important to produce large, fast settling flocs and may therefore allow the use of higher shear rates in the conditioning stage. High shear mixing may help to reduce entrainment of gangue in the flocs and so improve grade. The problem of contamination of the flocculated concentrate by coarse gangue particles settling with the flocs wilt also be avoided. A further advantage could be in the removal of clays from soluble salt minerals where high extraction of the liquor is required; in these cases conventional desliming by hydrocyclone is not applicable as it results in unacceptable losses of brine (Parsonage et al., 1988).

273

Surface adsorption of fine magnetite (selective magnetic coating) In this process, it is the surface adsorption of individual magnetite particles onto selected grains which forms the mechanism of magnetisation. It is not necessarily dependent on the wettability of the constituent minerals nor on their flocculation state. It is the discussion of the parameters controlling the selectivity of the coating in this process which is the main purpose of this paper. The earliest work located in the literature on a process which involves this principle was reported by Frangiskos and Gambopolous (1977). An ore containing magnesite, serpentines, pegmatite veins and calcite was conditioned with ARQUAD-2C ( a dialkyl quaternary ammonium chloride ) ( 0.5-0.8 kg t - 1), diesel oil (2.8-3.2 1 t -1) and Flotol B (0.4-0.5 l t-~). On addition of heavy media grade magnetite or ferrosilicon, the silica and calcite were coated whereas the magnesite was not. Magnetic separation was achieved using a wet belt-type separator. The process was found to be effective over the size range 0.5-8 mm. Significantly, the reagents used were those which had been found to be applicable for selective flotation of the silica and calcite but much lower additions were needed for the separation by magnetic coating. The removal of the titanium-bearing minerals anatase and rutile from kaolin using magnetite seeding of the slurry has been described by Nott and Price (1978a,b). The particle size of the kaolin was 90% below 2 microns. Both chemically precipitated and ground mineral magnetite were found to be effective. The conditioned slurry was separated using a wire wool matrix separator operating at 1.5 T. The titaniferous impurities were removed with the magnetite as a magnetic product. Cook (1981) extended this work and proposed the use of fatty acids such as dodecanoic acid as hydrophobizing agents. Flotation of the pulp prior to magnetic separation removed a proportion of the magnetite and reduced the load on the subsequent magnetic separation stage. Hydrophobic flocculation of the titanium minerals with the magnetite was proposed as a possible mechanism of aggregation. Separation of calcite and dolomite from phosphate minerals by selective coating of the carbonates with magnetite has been described (Parsonage, 1984, 1985b, 1986). Lower additions of magnetite were required if it was used in conjunction with a surfactant such as sodium oleate. It was shown that selective coating could be achieved even in conditions under which the different particles had similar wettabilities. Other mixtures separated were quartz/ fluorite and metallic lead/copper. Results were interpreted by means of theories of colloid stability. Separations at both laboratory and pilot scale have been described using both synthetic and ground natural magnetite (Parsonage, 1986). The importance of controlling the particle interactions through changes in zeta potential and surfactant adsorption was emphasized. In the treatment of a phosphatic chalk reagent additions of 1.1-2.8 kg t - 1 surfactant and 20-50

274

kg t ~ magnetite were used to selectively coat the carbonates (Parsonage, 1985b). A large proportion of the added magnetite was found to remain free and is amenable to recovery for recycling by low intensity magnetic treatment ahead of the main high intensity stage. TABLE I Examples of separations using selective magnetic coating (1) Cassiterite/quartz, - 2 0 microns, 5.10-4 M Na oleate, pH 4.5, 2-10 -:* M NaC104 Wt.% Magnetics Non-magnetics Feed

Cassiterite (%)

Recovery (%)

3.7 96.3

74.9 1.1

71.4 28.6

100.0

3.8

100.0

(2) Beryl/glass/quartz, - 150 + 75 microns, 0.1% Aeropromoter 825, pH 2.5 Wt.% Magnetics Non-magnetics Feed

Beryl (%)

Recovery ( % )

27.4 72.6

93 2

95 5

100.0

27

100

(3) Barite/calcite, - 150 + 75 microns, 5. l0 -~ M Na oleate, 10-3 M Na2SO4, pH 9, 2" 10-3 M NaC104 Wt.%

Magnetics Non-magnetics Feed

Barite (%)

Calcite (%)

Barite recovery

Calcite recovery

39.2 60.3

13.9 94.9

86.1 5.1

8.6 91.4

91.6 8.4

100.0

63.1

36.9

100.0

100.0

(%)

(%)

(4) Scheelite/calcite, - 150 + 75 microns, 10-4 M Na oleate, pH 9, 2.10 -:3 M NaCIO4 Wt.% Magnetics Non-magnetics Feed

Scheelite (%)

Recovery ( % )

59.1 40.9

0.39 97.2

0.6 99.4

100.0

40.0

60.0

275 Other previously unpublished results of tests carried out at Warren Spring Laboratory are shown in Table I. Separation of sulphides, e.g. pyrite from chalcopyrite in flotation concentrates, has also been shown to be possible. The basic steps in the process of magnetic coating are as follows. (1) The pulp is dispersed by mechanical or chemical methods to prevent the heterocoagulation of different mineral species. Some selective coagulation or flocculation at this stage may be beneficial. (2) The second stage involves the introduction of fine magnetite (or other ferromagnetic phase). Conditions of the particle surfaces in the pulp should be such that the magnetite selectively adsorbs on the target mineral. (In cases involving the use of flocculants it is often more effective to apply these reagents after the magnetite, to produce more intimately mixed fiocs. ) (3) Following a conditioning period during which collision and adhesion of the magnetite takes place, the pulp is passed through a low-intensity magnetic separator. This stage removes the bulk of the magnetite which remains unattached to other minerals and reduces the load on the subsequent high intensity separation. (4) The final stage is the treatment of the slurry by high intensity magnetic separation to produce a magnetic concentrate consisting of those minerals which have been coated with magnetite. If required the coated product may be treated further to remove the adhering magnetite for recycling. The most important stage of the process is in obtaining selectivity of adsorption of the fine magnetite onto the required mineral. The parameters influencing the magnitude and selectivity of the adsorption are related to the forces of interaction between the magnetite particles and the minerals and are described in more detail in the next section. INTERACTIONS BETWEEN PARTICLESIN SUSPENSION To understand whether or not fine magnetite will tend to coat a mineral particle, it is necessary to consider the forces which act between the magnetite and the surfaces of the particle. Such forces arise from a number of sources which are described below. By adding together the energy contributions it is possible to predict the variation in potential energy with particle separation. A net repulsive energy or a high energy barrier will tend to prevent contact of the mineral and magnetite particle and will inhibit the formation of a magnetite coating. On the other hand, a net attractive energy of interaction will favour contact of the magnetite and mineral and will tend to produce a coating of the magnetite on the mineral surface. The conditions for coating are similar to those which lead to slime coatings in flotation (Fuerstenau et al., 1958; Parsonage et al., 1982; Parsonage, 1985a). The phenomenon of selective coating is also made use of in carrier ("piggy-back") flotation (Green and Duke, 1961; Chia and Somasundaran, 1983).

276

The important energies of interaction between particles comprise those due to: Born repulsion, VB; van der Waals interactions, VA; electrical (Coulombic) interactions, VR; steric interactions, VST; hydrocarbon chain association, Va~8oc; hydration forces, VHDN;hydrophobic effects, VHPB;magnetic interactions, VM; and bridging forces, VBR. If it is assumed that these energies are additive, the total interaction energy, VT, is given by: Vv = V~ + VA + VH + V.s~v+ V...... + VHDN + VHpB + VM + VBa

(1)

Most of these interactions are amenable to modification by the use of reagents, such as surfactants and polymers, and it is by the judicious use of these that control over the selectivity of the coating process can be achieved. Expressions which may be used in calculating the various energies are described below. The notation used is given in the Appendix. More extensive discussion of these equations has been presented elsewhere (Parsonage, 1987 ). ( 1 ) Born repulsion, VB. This is a close range repulsion caused by overlapping electron clouds as two atoms or particles approach each other. Feke et al. (1984) assumed a pairwise additivity of interaction potentials between all the molecules in the system and derived the following expression for the Born repulsion between particles: n--6

lib =

(n--2)! R

× [ - R 2- ( n - 5 ) (R2/R1 - 1 ) R - ( n - 6) [ (R2/Ra)2- ( n - 5 ) ( R J R I ) + 1 ] ( R - I + R2/R1) n-5

+ - R 2 + ( n - 5 ) (RJR~ - 1 ) R - ( n - 6 ) [ (R2/R~)e

( n - 5 ) (R2/R~) + 1]

(R+ I - R 2 / R 1 ) n-s

R2 + ( n - 5 ) (R2/RI + I )R + ( n - 6 ) [Rz/R~)2 + ( n - 5 ) (R2/R~) + I ] (R + I + R2/R1) n-s) R 2 - ( n - 5 ) ( R J R I + I )R + ( n - 6 ) [ (R2/R1)2 + ( n - 5 ) (R2/R1) + I ] ] (R - 1 - Re/R1 ) "-s

(2)

where R = (Rl+Rz+h)/Rv Eq. 2 is valid for all values of n except n = 7, 6, 5, 4, 3 or 2; n represents the degree of hardness in the repulsion. In the absence of any information to the contrary, values of n = 12, and a = 5-10- lO m may be used as standard. For the case of identical spheres and n = 12, eq. 2 reduces to:

277 I 2 ( 3 \ R"~. 4! oo] VB =-R-4A(a~6

~TaR2) +R2+14R+54(R+2) 7 +R2-14R+541~J

(3)

(2) Van der Waals interactions, VA. These are attractive forces due to dipoles, induced dipoles and London forces. They may be modified by the presence of adsorbed layers. The following expression for the magnitude of the interaction, taking into account the effects of retardation has been given by Schenkel and Kitchener (1960). For Po < 0.5: VA=

A RxR2 ( 1 ) 6(RI+R2)h 1 + 1.77Po

(4)

For Po=0.5 to ~ and h<< R1 or R2:

A RIR2 [ 2.45 2.17 6(RI+R2)h 12 -60---~o+ lS0Po 2 where Po = 2rth/2. VA =

0.59 ] 420Po 3

(5)

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 (1969) and Visser (1972). A method for calculating the Hamaker constant from absorption data has been described by Hough and White (1980). Further forms of the retarded van der Waals interaction in the presence of adsorbed layers have been described by Vincent (1973). (3) Electrical (Coulombic) interactions, Vs. Electrical interactions arise due to the charges on the particles and the overlap of the ionic double layers. They have a particular importance in the control of suspension stability because they are amenable to control by the use of reagents. The principal parameters determining their magnitude are the relevant potential at the particle surface and the electrolyte concentration. It is usually considered that the potential at the Stern 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, namely the zeta potential. Pure magnetite is positively charged at pH values below about 6.5 and negatively charged above this value. The interaction between spherical particles has been given by Hogg et al. (1966) as: for constant potential interaction,

VR=

~,o,,R1R2(~tlZ+~2 2) ~" 2~,~z lnFl+exp(-tch)] (R~+R2) ~ (~q 2+~u22) [_1 - exp ~ - ~ J + ln[1-exp(-2~ch)] t

(6)

278 and ibr constant charge interactions, Y R _-

7re°erRIR2(~12+~'22) t (R1 A-R2)

2~1~2 (t//l 2-4- ~12 2)

ln[l+exp( -s:h)~ [1-exp( ~)_] - |n [1 - e x p ( - 2a:h) ]

)

(7)

where: 2 2 ~1/2 (2e~noz~ (2e2NAcz2~ 1/2 (2.103e2NACmZ2~1/2 K = \ eoerkT/ = \ eoGkT ] = \ eo~,kT ]

(8)

The expressions in eqs. 6 and 7 hold exactly for ~1 or ~2 of less than 25 mV and are good approximations for ~1 and ~2 less than 50-60 inV. The particle radius should be greater than 10/K. (4) Steric effects, VST. These are usually repulsive and arise due to interaction between adsorbed species on opposing surfaces as they approach. Two mechanisms are identified, the volume restriction effect and the interpenetration or osmotic pressure effect. The volume restriction effect arises due to the mechanical difficulty of compressing the adsorbed layers as the particles approach. The osmotic pressure effect arises due to mixing of polymer chains as the particles approach. For the conditions h > 25 steric interactions are absent. When the particles approach such that h < 25 then either compression of the layers or mixing of the chains takes place. Compression is probably not appreciable until h < g. For steric interactions due to compressive forces Jaeckel (1964) has given the following expression for identical spheres:

VST~ 1.325 Ey (252--h)5/e .... (Ro +5o) 1/2

(9)

where Ey is the elastic modulus of the adsorption layer, which for cross-linked gelatin gels is in the order of 105 N m -2. For the interpenetration model, again for the condition that h < 2g, Ottewill (1977) has given the following expression:

-

3Vlp~

(~]ent-Z1) 8 -

3Ro+25+

(10)

Reviews of the effects of polymers on the stabilisation of suspensions have been published by Napper (1977), Vincent and Whittington (1982) and Vincent (1984). (5) Hydrocarbon chain association, V~o¢. This is, in effect, a variety of steric interaction but specifically describes the attraction due to association of hy-

279 drocarbon chains on opposing surfaces as the particles approach each other. An expression for the magnitude of this interaction has been derived (Parsonage, 1987). For identical spheres: Vassoc = 2[CH21]

(11)

"VoC)Kf

where [CH21] =

3Ro 2Cs,rs nc

[ (Ro + ~)3-Ro ~]

2~

approx. 0to1 (6) Hydration effects, VHDN. These effects arise through changes in the water structure and are induced by hydrated surfaces or by hydrated ions in the vicinity of the surface. Most authors express the hydration force as decaying exponentially with distance, i.e.: for equal spheres, =

- 0.8RT

K~ =

7~RoI~DNKHDN exp ( h/IHDN ) for unequal spheres with layers of reagent of thickness $1 and $2,

( 12 )

VHDN = 2~z(R1+c~1)(R2 +$2)I~DNKHDNeXp[--(h--($I+$2))/IHDN] (R1 +$1) + (R2 +$2)

(13)

VHDN =

-

Published values of the hydration constant, KHDN, a r e in the range 8.105107 N m -2 for silicate minerals, with decay lengths/HDN, of the order of 1 nm (Israelachvili and Adams, 1978; Churaev and Derjaguin, 1985). A number of papers concerned with the hydration effects between mica surfaces have been published by Pashley (1981a, b, 1982 ). (7) Hydrophobic effects, VHPB-Whereas hydrated surfaces give rise to repulsive forces and aid dispersion, hydrophobic surfaces can produce attractive forces which can lead to destabilization. Ottewill and Rastogi (1960a, b) and Ottewill et al. (1960) for example showed that in silver iodide-amine systems, maximum instability was not at the point of zero electrical charge but at the point of maximum oleophilicity. Also an additional long range attractive force was present. Hydrophobic flocculation is often considered to be an indication of flotability. A number of examples have been reviewed by Laskowski (1982). Israelachvili and Pashley (1982, 1984) proposed that the hydrophobic force was of long range and decayed exponentially with distance. The equation they proposed may be expressed in the form:

280 CHPB (R 1 -}-61) ( R 2 "}-6,2) ~/-HPB

=

--

(R1 + ~ 1 ) + (R2 + ~ )

/HPB exp [ - ( h - 61 + 62 )//HPB ]

( 14 )

for two spheres bearing adsorbed layers of thickness 51 and 52. A value of 0.14 +_0.02 N m - 1 was determined for CHPB, with a decay length, /HPB, of 1.0 nm for the mica/CTAB monolayer system. For the case of one hydrophobic surface approaching a hydrophilic surface, the interaction is intermediate to those between two hydrophobic surfaces and between two hydrophilic surfaces (Claesson et al., 1987). This suggests that hydrophobization of either the magnetite or the target mineral should lead to greater forces of attachment compared with the case when both are hydrophilic. (8) Magnetic interactions, VM. Watson ( 1976 ) has given the following equation for the magnetic interaction between two particles: YM =

4n2Ro 6Xv2So 2 (2Ro + h ) 3 g~)

(15)

This represents the maximum interaction, i.e. when the direction of the collision is in the direction of the magnetic field. (9) Polymer bridging forces, VBR. Unlike the other interactions considered here, the interactions due to bridging by long chain polymers are not of the equilibrium type and are dependent on factors such as time and mixing regime. These influence the conformation of the polymer at the particle surface and make any straightforward analytical solution impossible. Also, in cases where flocculant is present, the effects of its adsorption on the behaviour of the suspension is likely to dominate all other interactions and the most fruitful approach to predicting the behaviour of such systems would be by predicting the conditions for polymer adsorption. EXAMPLES OF POTENTIAL ENERGY INTERACTION CURVES

In order to give an idea of how variations in the components of the interaction energy affect the overall interaction between two particles, some examples of interaction curves are given in this section. The curves (Figs. 2-7 ) are plotted showing the variation in potential energy, VT, with separation distance, h. This energy is plotted in units of thermal energy, kT. The curves have been calculated using the equations presented in the previous section. The majority of the curves show a similar form. At large separations the interaction energy approaches zero; the particles are not influenced by each other. As the particles approach a slight attractive force may be present causing VT to become negative. This is not always present but can be seen for example on curves (a) and (b) of Fig. 5. This trough in the potential energy curve is known as the "secondary minimum". As the particles approach each

281

other more closely, VT may become either more positive, if strong repulsive forces are dominant, or more negative if attractive forces dominate. If VT becomes more positive with decreasing separation, then this acts as an energy barrier to further close approach of the particles; particles will only be able to approach more closely if they have sufficient energy. Kinetic energy of the particles, for example, may be sufficient to enable closer approach. Further close approach generally leads to a steep decrease in VT; for most of the examples in Figs. 2-7 this occurs at separations below 1-2 nm. The potential energy well at close separations is known as the "primary minimum". This is the region of the equilibrium position of coagulated particles. The slope of the curve in the primary minimum gives a measure of the force required to break up the particle pair. Strong short range forces due to Born repulsion prevent actual touching of the particles. The curves have been calculated assuming that the two particles have radii of 0.5/~m and 50/lm to approximate the situation where a particle of fine magnetite approaches the surface of a larger particle. Fig. 2 shows the effect of varying the zeta potential of one of the particles. At high potentials (curve a) high positive values of VT are developed which result in an energy barrier preventing the close approach of particles. At lower zeta potentials the energy barrier may be absent (curve d); under such conditions the two particles should aggregate on collision. Increasing electrolyte concentration (Fig. 3 ) leads to a reduction in the electrical repulsive forces. At some critical concentration the energy barrier disappears leading to aggregation of particles. For the conditions shown in Fig. 3 the critical concentration for aggregation is between 0.01 M and 0.1 M. Differences in effective Hamaker constant, A, affect the magnitude of the van der Waals attractive forces (Fig. 4). With increasing Hamaker constant the attractive forces increase resulting in lower energy barriers and deeper primary minima. The prevention of aggregation by hydration forces is shown in Fig. 5. In the absence of hydration forces the interaction is purely attractive down to below 0.5 nm for the conditions shown (curve c). With increasing hydration forces (curve b) greater repulsion occurs at close separations. Strong hydration forces can eliminate the primary minimum altogether; the conditions shown for curve (a) are approaching this point. Fig. 5 illustrates how hydration forces can account for lack of aggregation of particles which would be predicted to aggregate if only van der Waals and electrical interactions were considered. Hydrophobic forces are attractive, they decrease the value of VT and lead to stronger aggregation (Fig. 6). The much deeper primary minimum developed will make such aggregates more difficult to disrupt than for non-hydrophobic aggregates. Association between adsorbed hydrocarbon chains on opposing particles leads to stronger aggregation provided that the particles have approached close

28'2 4000~

3000

2000

1000 ~

a

h/nm 5

10

15

20

d -1000

-2000

-3000

-4000 L

Fig. 2. Effect of zeta potential on interaction energy. ~1 = 45 mV, A = 5.10 -~° J, R1 = 0.5 #m, R2 = 50 pm, 2 . 1 0 - 3 M 1:1 electrolyte, constant potential. (a)~2 = 45 mV; (b) ~2 = 30 mV; (c) (~ = 15 mV; (d) (2 = 5 mV. 4000

3000

2000

1000

h/nm d

i0

15

20

- 1000

-2000

-3000

-400C

Fig. 3. Effect of electrolyte concentration on interaction energy. ~1 = ~2 = 45 mV, A = 5.10 -20 J, R1 = 0.5 #m, R~ = 50/~m, constant potential. Concentration of 1:1 electrolyte: (a) I0 -4 M; (b) 10 a M ; (c) 1 0 - 2 M ; (d) 1 0 - 1 M .

283

2000

1000

0

~ 5

10

h/nm 15

20

C -1000

-2000

-3000

-4000

-5000

-6000

Fig. 4. Effect of Hamaker constant on interaction energy. ~, = 45 mV, ~2 = 15 mV, R 1 = 0.5 #m, R2 = 50 ~m, 2" 10 -3 M 1:1 electrolyte, constant potential. (a) A = 10 -2o J; (b) A = 5 - 1 0 - 2 ° J ; ( c ) A = 10-19J. 4000

3000

2000

1000

~

0~

h/nm 5

10

15

20

-1000

-2000

-3000

-4000 L

Fig. 5. Effect of hydration force on interaction energy. ~1 = 45 mV, ~2 -- 5 mV, A = 5.10 .20 J, R, = 0.5 #m, R2 = 50 pxa, 2" 10 -3 M 1:1 electrolyte, constant potential. (a) K H D N = 1 0 7 N m -2, /HDN = 1 nm; (b) KHDN ---- 5"106 N m-2,/HDN ---- 1 rim; (c) KHDN = 0.

284 5000

O

~

h

/

n

r

5

n

10

15

20

-5000

-10000

-15000 Ul

Fig. 6. Effect of hydrophobic effect on interaction energy. ~1 = 45 mV, ~2 = 15 mV, A = 5 . 1 0 -2o J, R1 = 0 . 5 / a n , R2 = 50/~m, 2 . 1 0 -3 M 1:1 electrolyte, constant potential. (a) no hydrophobic effect, CHp B = 0; (b) hydrophobic effect, CHpB = 0.14 N m -1,/HPB = 1 nm. 5000 I

0

.

.

.

.

.

i

. . . .

I

'

'

,

'

I h/nm

_5ooo

-10000

-15000

Fig. 7. Effect of adsorbed hydrocarbon chains on interaction energy. ~I "~" 45 mV, ~2 = 15 mV, A = 5" 10 -2° J, R1 = 0.5 #m, R2 = 50 ~ , 2-10 -3 M 1:1 electrolyte, constant potential. (a) no adsorbed hydrocarbon; (b) 10 - s tool m -2 adsorbed hydrocarbon, adsorbed layer thickness J = 1.57 nm, chain association factor Kf = 1; (c) as (b), but assuming hydrophobic force originating at outer surface of adsorbed layers, CHPB = 0.14 N m -1,/HPB = 1 nm.

285 enough so that the adsorbed layers overlap (Fig. 7, curve b). If the outer surface of the adsorbed layer behaves as a hydrophobic surface then the longer range attraction due to the hydrophobic interaction will manifest itself producing even stronger aggregation (Fig. 7, curve c). It should be borne in mind that in reality the adsorption of surfactant with hydrocarbon chain will usually alter the zeta potential of the minerals and will therefore affect the magnitude of the electrical interaction; this may result in either a greater or lesser potential energy barrier. These curves explain the observation that adhesion between surfactantcoated particles is generally more tenacious than between bare particles, and also the improvement in the efficiency of the magnetic coating process with the use of surfactants (Parsonage, 1984, 1985). CONDITIONS FOR SELECTIVECOATING To summarise, the interaction between the magnetite and the mineral to be coated should ideally be of the type with no potential energy barrier; and with a deep primary minimum to ensure strong adhesion. For prevention of coating formation, there should be a high potential energy barrier to prevent close approach of the magnetite. This should be coupled with a very shallow, or nonexistent primary minimum to minimize the force required to remove any magnetite particles which do happen to reach the particle surface. If fine particles (below about 20 microns) are being treated, conditions should be such to minimize heterocoagulation of the different ore minerals in the pulp. HYDRODYNAMICEFFECTS

Effects of agitation on particle coating In real systems where a pulp is being treated, agitation in conditioning tanks will occur. The energy imparted to the components of the pulp will affect the aggregation properties of the particles. In some circumstances for example, it may be great enough for the colliding particles to overcome any potential energy barriers which may be present. This needs to be taken into account when predicting the extent of coating. It is known that increasing the shear rate in a suspension can result in one of two opposing effects with regard to particle aggregation. In most cases, increasing shear rate results in greater dispersion of an aggregated suspension as the energy of agitation disrupts flocs and aggregates. Hunter (1982) has considered this problem in relation to aggregated suspensions and has related parameters such as the energy of particle doublet separation and the shear rate to physical suspension properties such as floc size and density, relative viscosity and yield stress.

286 In other cases, in which the particles are initially dispersed, it is sometimes observed that increasing the shear rate can lead to increased particle aggregation. It was fbund, for example, that a stable suspension of fine scheelite in the presence of sodium oleate may be aggregated by the use of high shear (Warren, 1975a, b, 1982). This effect, which is observed when particles possess adsorbed layers of surfactant, has been termed shear flocculation. It has been proposed to arise by imparting sufficient kinetic energy to the colliding particles to allow them to surmount the potential energy barrier, which under less agitated conditions prevents their close approach. Once contact has been achieved, stability of the doublet will be favoured due to the adhesion brought about by the association of the hydrocarbon chains on the opposing surfaces. Van de Ven and Mason (1977) have performed theoretical calculations to predict the formation of particle doublets of equal-sized spheres under the influence of shear forces taking into account Coulombic and van der Waals interactions between the particles. They predict that at low shear rates particles bearing high surface potentials may aggregate in the secondary minimum whilst at high shear rates aggregation in the primary minimum can occur. At intermediate shear rates, however, the capture efficiency can be zero with no aggregation occurring. It is evident from the above that hydrodynamic conditions may be of critical importance where the selective aggregation of magnetite onto target minerals is to be achieved as is the case in the magnetic coating process. It is proposed that the aggregation of particles in a collision is principally controlled by three parameters, namely the potential function of the particle pair (which may be predicted using the equations given in the earlier section), their diameters, and the local shear rate. These parameters control the time for which the particles are in contact, and the thinning time required for the particles to approach to such a degree that they are stable to subsequent disruption by the shear field. If the contact time is less than the time needed for the thinning of the interparticle water film then the particles will separate after contact and aggregation will not occur. Increasing shear rate increases the relative velocity of approach of the particle pair. This will result in a decreased time of contact. The increased pressure resulting from the greater approach velocities will however result in a greater rate of thinning. Conversely, it will also increase the hydrodynamic stress leading to break up of the particle doublet. It is proposed that it is the relative magnitudes of these parameters which determine the effectiveness of the collisions in an ore-magnetite pulp. As yet, however, insufficient experimental data exist to quantify these effects and there is need for further research into this area. MAGNETIC SEPARATIONOF MAGNETITECOATEDPARTICLES Both low and high intensity magnetic separation will be applicable for treating the conditioned slurry.

287 For most separations it will be advisable to pass the treated material through a low intensity magnetic separator, such as a drum type, to reduce the load on the subsequent high intensity separator. The low intensity stage should aim to recover that part of the magnetite which remains unattached to the mineral surfaces and which may therefore be suitable for direct recycling. Although conventional high intensity magnetic separation is normally used for the recovery ofparamagnetic materials such as beach sand minerals or iron ores (Jones, 1960; Oberteuffer, 1974), previous work has shown that separators of this type (using a grooved plate matrix ) should be suitable for fractionating slurries which have been coated with a ferromagnetic phase (Parsonage, 1986). It should be borne in mind, however, that a paramagnetic particle will have a different dependence on the control parameters of the magnetic separator than will a non-magnetic particle coated with magnetite. The magnetic force on a non-magnetic particle coated with magnetite may be given by (Hopstock, 1985): F M ---- }2o

YviMsati (dH/dZ)

(16)

This applies for fields sufficient to magnetically saturate the magnetite. The saturation magnetisation of magnetite, Msati, is about 4.7.105 A m-1, and will be exceeded under normal operating conditions of high intensity separation. Also at high intensities, the matrix of the magnetic separator will become magnetically saturated. Increasing the current to the coils beyond this point will not increase the field gradient, dH/dZ, which remains constant. Hence the magnetic force on the particle cannot be further increased. This means that provided the current to the magnet is sufficient to produce saturation of both magnetite and the matrix then its precise magnitude will not influence the magnetic force on a coated particle. This magnetic force will be dependent only on the total volume of magnetite coating the particle and the magnetic field gradient. The latter parameter depends on the physical dimensions of the elements forming the matrix. Provided the elements are saturated the gradient will be constant for a given geometry. The above considerations show that the most important factor determining the magnetic force experienced by the coated particle will be its volume content of magnetite. It has been estimated (Parsonage, 1984) that 0.01-0.1 vol.% magnetite per particle should be sufficient for non-magnetic particles to display a magnetic response enabling them to be captured on a high intensity magnetic separator. The competing forces tending to prevent capture are those due to fluid drag and gravity. These forces will be a function of particle size and density and fluid flow rate and viscosity. Experimental confirmation of the coating level necessary for recovery to the magnetite product is currently under investigation.

2S8 DISCUSSION

The use of magnetic particles to selectively increase the magnetic response of minerals offers the possibility of achieving separation of materials on the basis of properties not exploited in other separation processes. The main condition required for recovery is that the target mineral particles should acquire a resilient coating of fine magnetite of sufficient density such that they can be recovered by conventional magnetic separation. As described in the present paper, this is achieved by controlling the surface properties of the minerals and the magnetite in the pulp, and the conditions of mixing. As yet there are insufficient reported experimental data with which to test the predictions which have been made here. Commercial exploitation of the process is dependent on economic factors. Since the cost of magnetite is expected to comprise a substantial fraction of the processing costs, methods for producing fine sized products of this material should be investigated. The exact specifications for the magnetite (or alternative ferromagnetic phase) are as yet unknown but it is expected that a product with a particle size of below about 10 microns will be required. Problems involved with recovering and recycling the magnetite will also need study. It is possible that metallurgical fume or precipitates from leach liquors may be a low-cost alternative to ground natural magnetite. Research is also necessary to define the particle size range for which the magnetic coating process is applicable. However, since successful separations have been reported on particles from below 20 microns (Nott and Price, 1978a; Hwang et al., 1982; present paper) to above 1 mm (Frangiskos and Gambopolous, 1977), it would appear that the process has the capability of treating a wide particle size range. It is believed that the full theoretical analysis of the process will be considerably aided by the large body of literature which exists on processes such as froth flotation, flocculation and adhesion. CONCLUSIONS

The controlled deposition of a fine ferromagnetic phase, such as magnetite, onto the surfaces of minerals in a pulp is brought about principally by the regulation of the surface properties of the particles. Equations for quantifying the particle interaction energies involved have been reported in the literature. Hydrodynamic factors play a part in the kinetics of the coating process, but at present insufficient data are available to make firm predictions regarding the influence of the mixing regime. The selectivity of the magnetic coating process is dependent on different factors in froth flotation and can be used to fractionate mixtures of similar wettabilities.

289

Conventional high intensity magnetic separators can be used to recover the magnetite coated particles. APPENDIX List of symbols used A Bo [CH21],[CH22]

CHPB Csurfl,esurf2 C Cm Cg

dH/dZ Ey e h

K~ KHDN k ~HDN /HPB Msati

N~ n no nc

Po R R

R1,R2 Ro T V

V~R V~ V~ VR VHDN YsT Yahoo VHpB

VM

VT V1 Vi /)o

effective Hamaker constant, J magnetic flux density, T concentration of CH2 groups in adsorbed layers on particles I and 2, where [CH21] ~< [CH22], mol m -3 hydrophobic interaction constant, N m surface concentration of surfactant on particles 1 and 2, mol m -2 concentration, mol m-3 concentration, tool 1-1 concentration, kg m -3 magnetic field gradient, A m -2 elastic modulus of adsorption layer, N m -2 electronic charge -- 1.602-10 - i s C minimum separation distance between two spheres, m fraction of CH2 groups which associate on overlap hydration force constant, N m-2 Boltzmann's constant = 1.3806.10 -23 J K hydration force decay length, m hydrophobic force decay length, m saturation magnetisation, A m Avogadro constant -- 6.022-10 -23 m o l parameter in Born repulsion equation bulk concentration of ionic species, m -3 number of CH2 groups in hydrocarbon chain distance parameter in retardation equations {4,5) gas content = 8.314 J K -1 tool -~ (eqs. 10, 11) parameter in Born repulsion eqs. 2 and 3, m radii of particles 1 and 2, m particle radius when both the same, m temperature, K volume of particle, m 3 interaction energy due to polymer bridging, J interaction energy due to Born repulsion, J interaction energy due to van der Waals forces, J interaction energy due to electricaldouble layer effects,J interaction energy due to hydration effects,J interaction energy due to steric effects,J interaction energy due to association of hydrocarbon chains, J interaction energy due to hydrophobic effects,J interaction energy due to magnetic forces, J total interaction energy, J molar volume of solvent molecules, m 3 fractional volume of magnetite overlap volume of adsorbed layers, m a

290 Z

,~j,&

(o (r K

P~ 0"

valency adsorbed layer thickness on particles 1 and 2, m adsorbed layer thickness when both the same, m adsorbed layer thickness in uncompressed state, m permittivity of free space = 8.854" 10 ~~F m relative permittivity Debye-Huckel parameter, in wavelength of intrinsic oscillations of atoms, m (taken as 10- 7 m 1 permeability of free space = 4r~10 7 H m - 1 normal usage (3.14159 ...) density of adsorbed material kg m :~ interatomie separation at which E, the atom pair interaction potential = 0, n l

¢ Xz

free energy change for removal of CH2 groups from aqueous solution, J tool Flory interaction parameter characterising interaction of polymer with solvent volume magnetic susceptibility (SI units ) zeta or Stern potential of particles 1 and 2, V entropy parameter, ideally 0.5

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