Interfacial Chemistry of Mineral Processing Separations

1. Laskowski

1. Introduction

4

Modern industry depends on a continuous supply of raw materials for the production of metals, fuels, ceramics, fertilizers, etc. This growing need for materials and energy is accompanied by the decreasing availability of mineral raw materials. Ore deposits, that is heterogeneous mixtures of solidified phases with some valuable minerals occurring in high amounts, are now having to be processed at lower grades. Thus, we are faced with limitations of the sources and supplies of mineral raw materials, and continued research is necessary if in the future we are to rely on low-grade raw materials. (1)

Traditionally, mineral processing has been defined as the art of treating raw materials by physical processes to yield marketable products, that is, to extract the useful constituents from available deposits in the form of concentrates without changing the identity of the minerals. Low-grade ores, however, usually contain minerals of economic interest disseminated through a mass of rock; to liberate them, fine crushing and grinding are essential before separation. The efficiency of conventional mineral beneficiation techniques is low at feed sizes below 20 ILm. This can be illustrated by the following example.

A mixture of quartz and hematite particles can be separated by gravity concentration techniques based on the differences in density of these minerals. However, if the mineral particles are very fine, say, below llLm,

I. Laskowski • Institute of Inorganic Chemistry and Metallurgy of Rare Elements, Wroclow Technical University, Wroc1ow, Poland. Present address: Department of Materials Science and Mineral Engineering, University of California-Berkeley, Berkeley, California.

315

E. Matijevi (ed.), Surface and Colloid Science© Plenum Press, New York 1982

316 1. Laskowski

then the properties of such suspensions are no longer controlled by the bulk properties of the solid phase but are determined by the surface properties of the particles. Such particles in suspension collide with each other as a consequence of Brownian motion, and if the potential barrier that arises on approach is high enough, a suspension can be stable. By chemical control of the surface potentials of these two different species, it is possible to achieve a wide range of their coagulation rates. (2) For example around pH 7.5 particles of hematite can be coagulated selectively with one another but not with quartz particles. Under these conditions quartz particles do not coagulate appreciably with each other.(3) Separation then can be obtained.

Consider a sphere: if its diameter decreases, then the ratio of the sphere surface to a sphere volume equaling 6 d-1 increases, and at a given value of this ratio, the surface effects predominate over bulk effects. In such a case, to separate a mineral mixture, the differences in the surface properties of the minerals must be exploited. In other words, for particles of the order of 1 J.tm or less, the body forces of gravity and inertia can be neglected, compared with the surface forces; and the collisions between the particles resulting from Brownian motion can lead to perikinetict coagulation. Thus, the properties of such suspensions depend on the surface properties of the dispersed solid phase. For larger, but still fine particles, the collisions resulting from mixing or from differences in speed of sedimentation can lead to orthokinetic§ coagulation that can also influence the behavior of such suspensions to a large extent. Such surface effects can, however, be neglec­ted completely if the behavior of coarse particles in liquid is considered.

It is, therefore, obvious that the effectiveness of conventional mineral beneficiation techniques becomes unacceptably low for the separation of values from the fines obtained by extensive comminution of ores. Hence, if the increasing mineral demand is to be met, techniques that enable recovery of valuable minerals from finely comminuted ores are urgently needed. New methods are based on colloid chemistry, and this new mineral-processing area is, in fact, applied colloid chemistry.

Concentration of slime fractions can be achieved either by modifying the established processes, so that they can cope with a wider size distribution in the feed, or by developing new processes. That is why many new methods have been recently proposed. Some of them have so far received little publicity. Furthermore, a confusion has arisen in the literature as to the naming of these methods. To place these processes in their proper perspec­tive, it seems important to classify them according to the principles common to the methods.

t The process is called perikinetic aggregation if the collisions are caused by Brownian motion. § Orthokinetic aggregation describes the process with collisions caused by hydrodynamic

motions.

Interfacial Chemistry of Mineral Processing Separations

Figure 1. Schematic classification of the separation methods in mineral processing.

2. Classification of Separation Methods in the Field of Mineral Processing

317

The grounds for separation of particles are differences in the properties of the various mineral species such as density, magnetic permeability, electrical conductivity, and surface properties.

Generalization of the basic phenomena involved in mineral separation techniques leads to classification as shown in Figure 1. (4)

Most numerous are the gravity concentration methods. These are very effective in dealing with coarse particles (with sizes that fall in the range 103_105 JLm). The size of particles that can be separated with magnetic and electrodynamic methods fall in the range of 102-103 JLm. With flotation methods the size of mineral grains separated are usually in the range of

1 2 10 -10 JLm. For a very long period of time, flotation has been the only physico­

chemical separation process widely applied in the mineral processing area, but now flotation has become only one of several physicochemical separ­ation methods.

3. Physicochemical Methods of Separation

3.1. Classification of Various Methods

If physicochemical separation methods given in Figure 1 are put in the place of flotation in the chart, then the next diagram (Figure 2) showing the total nomenclature scheme can be constructed. (5)

The main subdivision categorizes all physicochemical separation methods as methods of separation of grains and methods of separation of ions and molecules. Generally, the separation of ions is carried out after

318 1. Laskowski

Physicochemical J Separation Methods

I Separation of Grains

I Separation of I Ions and Molecules

I I I r.---4-------~------------~ I

Colloid I I Ionic and Adsorbing I Extraction : Flotation Precipitate Colloid: to Organic

Methods : Flotation Flotation I Phase

I '-- 1 - - - - - - _J

t Fractionation Flotation Methods

by Surface Coagulation

I I I Electrophoresis II Selecti~e Coagulation

I Selective I I Selective ,I Flocculation Shear-Flocculation

I Carrier Colloid Spherical

Separation Methods Agglomeration

Figure 2. Schematic classification of the physicochemical separation methods.

leaching in hydrometallurgical processing. These techniques are usually outside of the scope of the mineral-processing field.

The next subdivision that categorizes the physicochemical methods of separation of mineral grains according to the basic principles governing the mechanism of separation divides the field into colloid and flotation methods. Both are based on differences in the surface properties of the minerals separated, but in the case of flotation, the dispersed air is blown in and separation is carried out by means of air bubbles. In the colloid methods, separation can be achieved, for example, by the selective aggregation of particles of one of the species into flocs while leaving the other species in a dispersed state. The flocs can then be separated from the dispersed material by a sedimentation technique.

It is accepted that at a given concentration of potential-determining ions, a positive or negative charge is created at a solid/solution interface. For oxides and silicates, H+ and OH- ions have been shown to be potential determining. Different electrical charges that are created on the surface of various minerals at a given pH can be exploited in the electrophoresis for separating fine mineral mixtures.

The possibility of selective coagulation in colloidal mineral suspension was investigated by Pugh and Kitchener.(2) They computed stability curves

Interfacial Chemistry of Mineral Processing Separations 319

and indicated that a very wide range of coagulation rates was possible. Consequently, separation should be possible by chemical control of the surface potentials of minerals.(3.6)

According to the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory of colloid stability,(7-9) in a lyophobic colloid that is stable, the repulsive forces arising from the presence of electrical double layers keep the particles apart. Aggregation can be achieved either by the neutralization of charge that manifests in bringing to light the van der Waals attraction forces or by adding a polyelectrolyte that aggregates the particles by a "bridging" mechanism. The latter process is known as flocculation after La Mer.(lO)

Studies on selective flocculation have come mostly from Kitchener's laboratory.01-13) The essentials of the process are visualized graphically in the paper by Collins and Read. (14) The industrial application of the selective flocculation of hematite from quartz was described by Villar and Dawe. OS)

Warren(16.17) has recently reported a new aggregation process. He has found that a suspension of highly charged but hydrophobic particles, pre­viously treated with flotation collectors and stable on standing, can be aggregated by generating a sufficiently high shear rate in the suspension. The process requires minimum mechanical energy to overcome an energy barrier arising from the high electrical charge of the particles. While coagulation and flocculation produce aggregates that are broken up by intense agitation, in shear flocculation the extent of aggregation and the size of aggregates increase with the speed of stirring of the suspension.

Finely divided solids in a liquid can be agglomerated by the addition of a small amount of a second liquid that preferentially wets the solid and is immiscible with the first liquid. With a critical amount of bridging liquid and suitable agitation, the solids can be separated (as shown by Puddington et al. (18-20» as highly spherical bodies. The process has become known as "spherical agglomeration."

Since the bridging liquid must wet the solid that is to be separated, the problem is based on wetting phenomena as in flotation. Hence, it is desirable to apply agents like flotation collectors in the usual way. The oil wets hydrophobic particles, and the water retains those solids that are hydro­philic. The oil-coated particles adhere to each other on collision, and then agglomerates form during agitation.

Carrier colloid separation methods should also be classed in this group, for example, the carrier agglomeration that has already been described in the literature.(21) However, carrier coagulation, that is, the coagulation of fine particles on coarse particles of a carrier, is also quite possible.

In Figure 2 fractionation by surface coagulation methods has been placed between the colloid and the flotation methods. This is a very new technique, and only one short note has so far been published on it. (22) The

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Interfacial Chemistry of Mineral Processing Separations 321

method utilizes the surface coagulation process, that is, a coagulation taking place at a gas-liquid interface when gas is bubbled through the suspension. Heller et al. have shown(23-27) that some stable colloidal systems under proper conditions can undergo surface coagulation. After 3 h of bubbling of nitrogen through the suspension of Ti02 and CuO, he demonstrated that 90% of the Ti02 was coagulated and could be removed by settling, while 85% of the CuO remained in suspension.

A group of flotation methods will be discussed in detail in Section 3.3.

3.2. Effect of Particle Size

Beneficiation is preceded by the liberation process, that is, by the crushing and grinding of an ore. In the liberation step, it is intended to reduce the size of particles until the valuable minerals are in the form of pure grains, i.e., have been freed from adjoining phases. The experimental methods that have recently become available make it possible to follow the process of size reduction (Figure 3).(28)

3.2.1. Effect of Crushing on the Surface Reactivity of Minerals

Mention should be made of the fact that the stress to which particles are subjected during grinding must be very large so that the physical process, the decreasing of the size of material (or an increase in its surface area), can be accompanied by a change in the surface reactivity of the solid.

There is evidence in the literature that during grinding the lattice of the solid is disturbed and numerous disclocations produced in the surface layers are known to enhance activity.(29-33) Carbonates such as those of calcium or magnesium begin to decompose at a lower temperature after grinding. The grinding of quartz, for instance, gives an amorphous surface layer that is more soluble in sodium hydroxide. The "broken bonds" formed on dry grinding of graphite cause an increase in chemisorption of oxygen that yields an increase in the hydrophilic nature of the graphite. The lattice disturbances may bring about phase transformations: calcite can be trans­formed to aragonite; vaterite transforms rapidly to calcite and then to a calcite-aragonite mixture. (34) Many hydrated salts have been found to decompose during grinding. Grinding of galena for a long time leads to the formation of anglesite. (30) This has also been proved by electrokinetic measurements. (31)

The differences in adsorption of surfactants on crystalline and ground calcite have also been reported. (35,36) The rupture of bonds caused by grinding results in a surface with unsatisfied valencies. This favors reactions between the solid and the surrounding medium, especially if it is an aqueous solution, leading to disappearance of broken bonds through chemisorption.

322 1. Laskowski

The effect of the grinding of minerals on their surface properties has even been noted in the flotation process when grinding was carried out under dry conditions. (30,37)

Such an influence becomes quite clear if Eadington's paper(38) is analyzed. In order to study the surface compQsition of minerals, he used Auger-electron spectroscopy. The results for hematite showed that the concentrations of some elements at fracture surfaces can differ from their bulk concentrations by several orders of magnitude and that the composi­tion of the surface is very much dependent on the nature of the fracture and on the subsequent teatment of the mineral.

The effect of mineral heterogeneity on the interfacial properties was studied by Kulkarini and Somasundaran. (39) They investigated the proper­ties of hematite particles that were found to be richer in silica and clay in the surface region than in the bulk, the properties of minerals in the surface region being dependent upon the pretreatment that the particles received. They are of the opinion that the behavior of natural ore particles can be explained on the basis of the mineral heterogeneity of the surface and the concentration of impurities in the surface region.

Such effects must be taken into account since mineral beneficiation depends to a large extent on the exploitation of surface phenomena.

3.2.2. Effect of Grain Size on Coagulation

According to Smoluchowski,(40) the collision frequency between parti­cles of radius ri and rj present at concentrations ni and nj is given by the relation

(1)

where

Diffusion coefficients of individual particles are given by the Stokes­Einstein relation

~= kT 617"71r

and thus ~ is inversely proportional to the radius of the particle. Smoluchowski introduced the approximation

(2)

@ijRij = (@i + @j)(ri + rj) = @lr1(.!. + .!.) (ri + rj) = 4~lr1 (3) ri rj

where ~1 and r1 refer in this context to a particle equal in size to the average in the dispersion.

Interfacial Chemistry of Mineral Processing Separations 323

Assuming that the interaction radius R cannot be very different from 2'1: equation (3) can be written as follows:

(4)

The rate of disappearance of primary particles of the total concentra-tion n equal to the collision frequency between particles is then given by

dn 4kT 2 -=--n dt 311

(5)

The rate of coagulation, as seen from equation (5) does not depend on the radius of the particles.

Miiller(41) has extended Smoluchowski's expressions to polydisperse systems giving the equation

1/2 1/2 2 9)jjR jj = 9)1'1 {4 + [ (~) - (;;) ]} (6)

The correction introduced 9) jjR jj > 49)1r1 leads to the conclusion that col­lision between particles differing in size is more likely. (42) Cooper(43) has shown that for dispersions having some stability, a Gaussian distribution of radii or surface potential about a mean value results in a less-stable dispersion than in the corresponding monodisperse system.

Smoluchowski's expression for collision frequency among particles in a laminar shear field is

(7)

where D is the shear rate with dimensions of reciprocal time. For a system of particles of uniform size (rj = rj = r), the rate of change

of the concentration is given by

dn = -'¥r3n2D dt

(8)

As can be seen, the rate of coagulation by shear flow depends strongly on particle size.

The overall rate of decrease in the concentration of particles of any size . . b (44-46) IS given y

_ dn = 4kT n2 + 3lr3n2D dt 311

(9)

Comparison of collision frequency in a laminar shear field [equation (8)] with that for Brownian conditions [equation (5)] gives

4 ,2 f3 = "3 7rqg D (10)

324 1. Laskowski

and using gjJ = 2.13 X 10-13 r-1 cm s -1 for r = 10-4 cm

(3=D

This means that even at low shear values, shear coagulation, the orthokinetic coagulation, dominates over perikinetic conditions for particles of r ~ 10-4 cm.

As calculated by Stumm and Morgan, (46) the second term in equation (9) becomes negligible for particles with a diameter of d < 1 I'm, whereas for representative velocity gradients of D > 5 S-1 only the second term is significant for particles of size d > 1 I'm.

The collision frequency under turbulent conditions has been given by Levich. (47,48)

Assuming that the coefficient of diffusion under such conditions is given by

(11)

where gjJl is the coefficient of turbulent diffusion, he has found that for the coagulation radius R larger than A, the characteristic microscale of tur­bulence, the collision frequency of particles is determined by ~I' For R < A, even under turbulent conditions, Brownian diffusion is a dominant factor in the transport of particles. Levich estimated that in water at room temperature and under mean conditioning intensity, for particles of radius r, r > 10-5_10-6 cm, the turbulent collision frequency reaches higher values than the Brownian collision frequency.

Mineral suspensions usually contain particles that are characterized by r> 10-4 cm. Typically, the suspensions are polydisperse. If the separation of mineral species forming the suspension is to be carried out by the selective coagulation technique, then it is clear that such a process can be carried out under orthokinetic conditions. If the suspension is polydisperse, high­frequency collisions are also possible as a result of differences in the velocity of sedimentation of various grains.

Manley and Mason(49) working with glass spheres differing in size (spheres up to 190 I'm in size) showed that when 1 < rt/r2 < 2, collisions by velocity gradients are similar to those for spheres of equal size and can be described by equation (7). It can be supposed that for particles very different in size, the hydrodynamic conditions at the surface of these particles must also be taken into account.

The collision frequency of large particles with radius r1 that sediment with a velocity of vsp(rh 8) through a suspension of small particles of concentration n is given by

b = 7T3R 2vsp(rt, 8)n (12)

where 3 is the factor accounting for the hydrodynamic flow pattern around

Interfacial Chemistry of Mineral Processing Separations

Figure 4. Collisions between fine and coarse sedimentating particles leading to orthokinetik coagulation.

325

a big particle. Fuchs(SO) and Levich(48) gave in a graphical form the values for the function a = f(Stk), where

The Stokes number characterizes the inertial forces acting, in this case, on the small particles in the streamlines near the large particle. Particles that are not very small deviate from the streamlines because of inertia. Accord­ing to the data given in the references by Fuchs(SO) and Levich,(48) 3 = 1 for Stk ~ 50 and for creeping flow 3 ~ 0 where Stk ~ 0.5. The situation is illustrated in Figure 4. As seen, the distance from the limiting trajectory of particles to the axis is h. In other words, the factor a introduced into equation (12) means that the number of collisions of small particles with a larger one should be proportional to IIh2 and not to IIR2.

The situation shown in Figure 4 is for Stokesian flow of liquid around a sphere. Levich(48) concluded that because of the high values of turbulent diffusion coefficients for the small solid particles colliding with large ones, the distortion of trajectories of small particles does not have any significant effect on the collision frequency when the radius of the larger particle is r .;:; 10-3 cm. This conclusion should be very important for mineral particles that are not spheres but are angular.

In the case of the energy barriers between the particles, Fuchs(Sl) has shown that the rate of coagulation is decreased by the collision efficiency factor, W, which denotes the ratio of rapid to slow coagulation:

(13)

326 1. Laskowski

According to the classical theory for VI = 0, for all H, W is unity,(52) and then the rate equations are identical to that of Smoluchowski for rapid coagulation. Under conditions of V; =F 0, the probability of collision between two particles of types i and j is reduced by a factor ~P3)

foo (Vii) ds ~i = 2f 2f exp k; S2

(14) _ rl + r2 r=--'

2 '

This means that the stability factor WI [by which the terms on the right-hand side in equation (9) must be divided] for dispersion summed over all combinations of particles of types i and j is given by

(15)

According to the well-known DLVO theory,(7-9) the total potential energy of the interaction VI between two particles is defined as

(16)

where VA and VR represent the attractive and repulsive potential energies, respectively.

Hogg et al. (53) give the following expression for the energy of interaction of two different spheres of radii ri and rj and with surface potentials !/Ii and !/Ii:

VI = Ci :i r) [ -6~0 + ~ f(!/Ii,·!/IrK, Ho)] (17)

Then, V; is approximately proportional to the particle radius, and it can be expected that the coagulation of large particles is slowed down more rapidly than that of small particles. However, this is not confirmed by experiment. (54-59)

Mention should be made of the fact that the interaction between two spherical particles under constant surface charge density conditions depends on the radius of interacting particles in a way very similar to the conditions under constant surface potential. (60-62)

In studies(63-64) it has been shown that for colloidal particles, Smoluchowski's value 4kT/3T/ in equation (5) is diminished by the hydro­dynamic interaction factor of about 0.4 to 0.6. However, the factor proposed does not depend on particle size.

The effect of electrolytes on suspensions is similar to its effects on classical colloids, but the calculated total potential energy curves show that the particles of suspension should be subject to a very large repulsion with the energetic barrier much larger than the kinetic energy of the particles. To explain this apparent discrepancy between experiment and theory,

Interfacial Chemistry of Mineral Processing Separations 327

Schenkel and Kitchener(54) showed the possibility of coagulation into the secondary minimum in the potential energy curve at large distances. This follows from the fact that V A decreases according to a low power while V R

decreases exponentially. With true colloids the minimum is shallow com­pared with kT but becomes important for coarse suspensions.

Parfitt and Picton(56) found rapid coagulation of 0.25-#£m-diam carbon black particles, while the dispersion of 0.025-#£m-diam particles were stable under similar conditions. They then used the concept of secondary minima coagulation to explain the experimental findings.

According to Wiese and Healy, (57) the secondary minima become more important for systems in which interaction approaches the sphere-plate case (e.g., rdr2> 10). Efremov and Us'yarov(S8) showed that the interaction energy of small particles can be altered to a large extent if they are in the vicinity of a large particle.

Mager and Laskowski(65) used the hindered settling technique that consists in measuring the rate of fall of the slurry-supernatant liquid interface of concentrated suspension to investigate the coagulation of quartz suspensions. Three quartz size fractions-2-4, 6-10, and 12-18 #£m-were used. It was found that mean apparent Stokes diameter determined by a hindered settling method depended on the pH of quartz suspension as shown in Figure 5. The coagulation of the 2-4-#£m fraction was observed at pH below 3.2, but the coagulation range extended up to pH 5.8 for the 12-18-#£m fraction. Calculations show that for the 12-18-#£m size fraction a primary coagulation takes place at pH < 3 and a secondary coagulation at pH> 3. The demarcation line between the secondary coagulation domain and the stability zone is placed at about - 50 kT. The experiment revealed that

30.-----,.---------------------------.

20

10

o~~--~~--~~--~~--~--~~~ 2 3 4 5 6 7 8 9 10

pH

Figure 5. The relationship between measured apparent mean diameter d and pH of sus­pension for three different size fractions of quartz sand: (1) 2-4-".m fraction. (2) 6-10-".m fraction, (3) 12-18-".m fraction.

328 1. Laskowski

whereas the primary coagulation is mainly responsible for the behavior of the quartz suspension prepared with the 2-4-~m size fraction, the 12-18-~m fraction can coagulate into the primary and secondary minima.

A mathematical model for the formation and breakdown of aggregates in a mineral suspension under turbulent conditions was proposed by Derjaguin et al. (66) They showed the rate of coagulation of small onto big particles to be 400-500 times higher than coagulation among small particles. Samygin et al. (67) observed an increase of 103-104 times in the sticking of small mineral particles (below 10-20 ~m) to large carrier mineral particles (60 ~m) as compared to the rate of sticking of small particles among themselves.

Because of possible coagulation into the secondary minimum, the energetic hindrance for coagulation of coarse particles does not seem to be higher than that for small particles. However, the aggregates that form under such conditions cannot be as stable as the ones formed if coagulation into the primary minimum takes place. Since Brownian motion in sus­pensions is not sufficient for high collision frequency, it is necessary in such systems to apply stirring. For polydisperse suspensions, the collision frequency caused by gravity sedimentation can provide effective coagula­tion. (68-70) Coarse particles in polydisperse suspensions cannot be, however, too large. The separation of minerals in the selective coagulation process occurs by sedimentation, and individual particles settle all the time since Brownian motion is insufficient to counteract gravity sedimentation in the suspension. Only for small particles does sedimentation depend on coagula­tion; if the particles are too large, coagulation has no significant effect on the velocity of their sedimentation. For such a system, selective coagulation cannot be suggested as a means of separation of the mineral species.

Consider, for instance, a quartz-hematite suspension. Pugh, (3) working with suspensions that contained over 80% of particles in the size range of 0.05-0.2 ~m, has shown that the region of selective coagulation of the mixture was anticipated at pH 7-7.5. In this region the quartz remained relatively stable, while the hematite was observed to undergo coagulation.

It is very likely that the quartz-hematite suspension containing quartz particles with average particle radius of about 1 ~m and, for instance, hematite particles with size distribution in the range 1-50 ~m, can be separated by selective coagulation at pH 7-7.5. If the coagulation of hematite particles takes place under conditions of shearing caused by sedimentation, it can be supposed that very fine hematite particles will be captured by larger ones and disappear from the system. Separation of the relatively large hematite aggregates and small quartz particles will then be possible. However, look at Figure 6 showing the sedimentation velocity of the quartz and the hematite particles vs. their radius. If, as the next example, a suspension containing -50 +1-~m hematite particles and -10 +1-~m

Interfacial Chemistry of Mineral Processing Separations 329

III -5 -1 10

~

Z 0 i= ~ Z -2 LU 10 ~ (5 LU l/)

U. 0 >- -3 I- 10 U 9 LU >

10-' 10 20 30 i.0 50

PARTICLE RADIUS, fm Figure 6. Velocity of sedimentation of quartz and hematite particles. The velocity has been calculated from the Stokes equation for spheres.

quartz particles is considered, then the situation is not so clear. Selective separation could be obtained only if during the formation of the hematite aggregates, all of the hematite particles finer than 10 I'm disappeared. Differences between the sedimentation velocity of individual quartz par­ticles and the hematite agglomerates formed could then be enough for the separation of such species by, say, elutration. Thus, the creation of the proper hydrodynamic conditions in coagulation permitting the capture of all of the fine particles by larger ones seems to be the decisive stage in such a process.

3.2.3. Effect of Grain Size on Flotation

In the flotation process, rising bubbles capture the hydrophobic par­ticles and transfer the particle-laden bubbles from the pulp to the froth.

The mechanism of particle capture is usually divided into three strages(71.72): (i) bubble-particle collision, (ii) thinning and rupture of the disjoining film, and (iii) formation of the stable particle-bubble aggregate capable of withstanding a considerable disruptive turbulence in a flotation cell.

330 1. Laskowski

All stages depend on the sizes of the particle and the bubble. By analogy with coagulation, the efficiency of the overall process can be given in terms of collision frequency and collision efficiency (attachment). The collision frequency depends on the hydrodynamics of the system, while the attach­ment efficiency depends on the physicochemical conditions (most of all on the hydrophobicity of the solid).

The classical condition for bubble contact to be possible is given by

AG = 'YIV(COS (J - 1) < 0 (18)

where AG represents the change in Gibbs' free energy of the system, 'Ylv

stands for interfacial tension at the liquid/vapor interface and (J denotes the contact angle. This equation can be called the thermodynamic criterion of flotation. It is often interpreted as follows(73); "The more negative the AG value, the greater is the probability of flotation." Such a thermodynamic treatment, however, clearly describes only the change of free energy in the process of collection of particle by bubble without taking into account the "activation energy." Sheludko(74) concluded that the angle of contact could directly characterize flotation if there were no kinetic resistances to the attachment or if the kinetic resistance depended on the same parameters as the angle of contact. In other words, the problem cannot be solved by examining the forces acting after the formation of the contact angle, i.e., those forces acting after all three stages of approach of the particle to the bubble have been completed. (71,75) The contact angle can characterize the third stage, namely, the probability of the formation of the stable particle­bubble aggregate and from these results the upper limit for floatability of large particles. Schulze and Espig(76) point out that the detachment energy of particles of 100-200 /Lm in radius is comparable to the kinetic energy of these particles under turbulent conditions.

The second stage-the thinning and rupture of the disjoining film-is influenced by capillary effects of the second kind to a large extent. (77-81) This results from the fact that the surface free energy, 1', is not independent of the form of the liquid if one or more of the dimensions become comparable with the range of action of the surface forces. Capillary effects of the second kind manifest themselves in a deviation from surface energy arising from an additional pressure, the so called disjoining pressure, II, acting on the film at a certain thickness, h. Thus, the specific surface free energy of the liquid laminae is ps (h) = 21' + h J: II dh, where I' stands for the ordinary surface free energy.

Frumkin and Gorodetzkaya(78) point out that the problem of the stability of the liquid layer between the solid surface and bubble can be compared with the stability of the layer between two solids. Even in the absence of a surface charge on the bubble, a double layer on the solid surface prevents the squeezing out of the film of water by an air bubble since ions

Interfacial Chemistry of Mineral Processing Separations 331

of the double layer are repelled from the air/water interface. Adopting the treatment used in the theory of negative adsorption of ions at an air/water interface, they concluded that the interaction between the ions of a double layer and the free surface of water will be exactly the same as if there were a mirror image of the double layer on the other side of the surface. It follows from their discussion that the forces acting in this system are equal to the forces between two solid surfaces charged to the same potential and immersed in the same solution at a distance equal to twice the thickness of the layer.

It is known that a layer of liquid, after thinning to a given critical value, becomes unstable and that rupture is followed by the formation of a contact angle. (74) The study of the thinning process of the wetting layer on the surface of a mineral seems to be essential for the development of the flotation theory.(7S) Because of these kinetic interferences, flotation is possible only when the induction time defined as the time required for the disjoining film to thin to such a thickness that rupture can take place, is shorter than the time of contact. Only when this kinetic condition is fulfilled does the thermodynamic condition given by equation (18) become significant. It leads to the conclusion that hydrophobicity defined by the contact angle is not sufficient to describe the flotation process. The thinning and rupture of the wetting film, related to the stability of this film, depends not only on hydrophobicity of a solid but also on other parameters such as viscosity of the film.

Derjaguin et al.(82) predict that the shorter the time of contact between a particle and a bubble the less favorable the hydrodynamic interaction becomes for collection.

There are direct flotation experiments showing examples when there is no correlation between the contact angles and flotation. (83) For instance, it was shown that the contact angle of quartz that has been methylated by reaction with trimethylchlorosilane did not depend on the concentration of KCl solution, but flotation and induction times were markedly influenced by KCI.

Laskowski and Kitchener(84) have found that clean hydrophilic silica and silicas that have been rendered hydrophobic by treatment with trimethylchlorosilane have practically the same zeta potential values. This was later substantiated by Iskra(85) and Harding.(86) These measurements, as do some previous ones, (87) show that the presence of electrical double layers does not exclude hydrophobicity. Taking into account the fact that the stability of a liquid film is mainly because of the electrical contribution to the disjoining pressure, it becomes clear that for a given range of thicknesses, the stabilities of the wetting films on these two surfaces should be similar. This means a total hydrophilicity for the silica/water interface (curve A, Figure 7) and an energetic barrier for the silica that had been

332

FILM THICKNESS -

1. Laskowski

Figure 7. Diagram of specific surface energy of a polar solid with a liquid film of variable thickness on it. A, Stable wet­ting film. B, Film becomes unstable below a critical thickness.

rendered hydrophobic (curve B, Figure 7). Blake and Kitchener(88) suc­ceeded in measuring the thickness of such metastable films on hydrophobic surfaces. Working with special care, they were able to establish the existence of equilibrium films for surfaces with contact angles of 95° (advancing) and 90° (recedin~) after rupture. This, as well as further work by Anfruns and Kitchener,(89 explains the ditlerent induction times for solids with similar contact angles.

Anfruns(89.90) has shown that hydrophobic angular particles are collec­ted in flotation at their maximum efficiency, i.e., at the efficiency at which particles collide to bubbles. Such particles did not show any significant change in efficiency when KCI was added. However, hydrophobic spherical particles were collected at a much lower efficiency. The reason for this seems to be a less favorable configuration for the thinning and rupture of the wetting film. KCI was found to increase the efficiency of collection of hydrophobic spheres to a large extent. This confirms that the main factor hindering the collection of spherical particles by bubbles is the thinning and rupture of the disjoining film.

The papers quoted clearly show the influence of the particle size and shape curvature on the second stage of particle capture, and this is in line with earlier findings by Eigeles and Volova.(91)

The collision frequency between the bubble and the mineral particles in the situation shown in Figure 8 (71.92.93) is very similar to that given in Figure 4. In Figure 4 the particles are of the same density and move in the same directions. In the flotation case shown in Figure 8, the bubble and particle move in opposite directions.

Interfacial Chemistry of Mineral Processing Separations 333

Bubbles in the flotation range of diameters (up to about 2 mm) and under flotation conditions remain spherical. (94 .9S) The size of bubbles as compared to mineral particles is very large. Thus, to calculate the collision frequency of the particles with bubbles that rise through the pulp, the distortion of the trajectories of the particles around the bubbles must be taken into account.

An equation of the motion of solid particles relative to a rising spherical bubble is usually given in the form(92}

dv* Stk-=-v* -v*+u* dt sp

* vsp VSP --' - Ub'

u u*=-

Ub

(19)

where v is the particle velocity relative to bubble; Vsp is the particle settling velocity; u is the fluid velocity relative to bubble; Ub is the bubble rising velocity.

Flint and Howarth(92} calculated collision efficiencies for Stokes and potential flow around the bubble for values of Stk down to 0.001 and Vsp

up to 0.3. For coarse particles, i.e., those characterized by Stk greater than about 1.0, collision efficiency depends strongly upon inertial forces. For fine particles characterized by Stk less than about 0.1, collision efficiency is virtually independent on Vsp, i.e., for fine particles the inertial effects are

Figure 8. Capture of mineral particles by bubble. The dashed line shows the limiting trajectory of particles colliding to bubble.

h

334 1. Laskowski

very small in comparison with viscous effects. The collision efficiency of a particular sized particle is increased by a reduction in the bubble size:

When Stk -+ 0 equation (19) gives

v* = -v~p + u* (20)

and it is seen from this relation that particles would follow the streamlines of the fluid if their settling velocity can be ignored.

Reay and Ratcliff(93) showed that for very fine particles, collision is mainly by diffusion, and flotation rate is inversely proportional to particle radius. When particles are coarse and are unaffected by Brownian motion (that means that particles contact the bubble only if their trajectories come within one particle radius of the bubble surface), the flotation rate increases with the square of the particle diameter. Since in the diffusion regime collision efficiency decreases with increasing particle radius and in the collision regime the collision efficiency increases with increasing particle radius, there should be an intermediate region where the collision efficiency is a minimum. With submicron particles coagulation should be beneficial for flotation only if the agglomerate size is well into the collision regime.

In line with the calculations are the early findings by Gaudin et al. (96)

They showed that the floatability of galena particles above a particle diameter of about 4 #Lm increased with particle size; below 4 #Lm all particles behaved similarly. To explain these observations, they put forward the hypothesis that particles finer than critical form agglomerates and then become attached to air bubbles, while particles coarser than critical become attached to air bubbles individually. According to their view, the critical size range may be regarded as the range in which the surface forces causing agglomeration are approximately equal to the disruptive mechanical forces produced by agitation and, hence, are determined by hydrodynamic condi­tions in the flotation cell.

According to Derjaguin et al. (97) there is an increase in the collection efficiency as the diameter of bubbles is reduced; however, for the motion of bubbles under the conditions of Re« 1 E oc r~/ r~ and for very high Reynolds number (Re » 1) E oc rp/rb'

It is worth noting here the results of de Vivo and Karger. (98) They found the optimum flotation in a system employing bubbles of 1- to 2-mm average diam to be for dispersed kaolin particles, while for 0.2-mm diam bubbles coagulation was beneficial.

Experimental data on the rate of flotation of various researchers give different results. For example, Morris(99) found the flotation rate constant k oc In dpo while Tomlinson and Fleming(lOO) give k oc d~ for easily floated mineral and k oc dp for poorly floated minerals. The range in particle size for the optimum floatability limited on one side by the lifting power of surface forces and on the other by the collision and attachment efficiencies

Interfacial Chemistry of Mineral Processing Separations 335

b f d · h (101 102) A' . . f h can e oun 10 many monograp s.· n 1Oterest1Og review 0 t e subject has been published by Trahar and Warren.(103)

Recently, Collins and Jameson (104,105) have presented an analysis of the collection of small particles by bubbles that seems to clarify several points. They correctly assumed that in the flotation of small particles electrical double layers should be an important factor and they investigated the flotation of polystyrene particles of size 4-20 p,m under conditions in which the effect of particle diameter on the rate of flotation could be analyzed. In their experiments electrokinetic potential of particles and bubbles in the flotation process was measured. According to them k ex: d!·s. The exponent in this equation was substantially independent of the zeta potential, whereas the flotation rate was observed to increase by an order of magnitude when zeta potential of particles was reduced from 60 to 30 mY.

3.3. Flotation Methods

Use of collectors is characteristic of flotation. These reagents are employed to increase the hydrophobicity of a mineral surface.

The classical boundary condition for the hydrophilic-hydrophobic transition is equality of W A, the work of adhesion of liquid to solid, and We,

the work of cohesion of the liquid:

W A = y.v + ylv _ y.1

2 Iv We = Y

(21)

(22)

where superscripts indicate solid/vapor, liquid/vapor, and solid/liquid interfaces.

By introducing Young's equation, equation (21) can be converted to

then

which gives

WA = ylV(l + cos 8) (23)

WA cos 8 = 2--1

We

(24)

(25)

The condition for hydrophobicity follows from equation (25); 0 ¢ 0° for WA < We.

The work of adhesion of water to a solid can be divided into three terms:(106)

(26)

336 J. Laskowski

where w! is the contribution from dispersion forces, w~ is the contribution from the hydrogen bonding of water to hydroxyl groups, and w~ is the contribution from the electrical charge at the interface.

According to Fowkes(107)

d 2( 1 • )1/2 WA= "Yd'"Yd (27)

where "y~20 ... 22 X 10-3 Jm-2•

A list of values of "Yd is given in Fowkes' papers. w! calculated for solids and water for various solid/water systems fall in the range from 47 x 10-3 to 132 X 10-3 Jm-2• For an ideal nonpolar solid like graphite, w~ ... 0, w~ ... 0, and WA'" w!. Since for water We ... 146 X 10-3 Jm-2,

equation (25) for graphite gives (J > 0°. It follows then that all minerals would be hydrophobic if they did not carry polar or ionic groups. As a matter of fact, only a few minerals are known to be hydrophobic. They show so-called natural floatability. Other minerals must be converted into a

-2

0.2 / .!!!I E E ~

/ -1 ::t>

/ ~ /

...J

0 iii

/ 0 ~

0.4

/ ()

/1 ~ w a: 0 J: Q..

0

/ 2 a: lJ

/ w ...J

/ w

3 /

0.6

0.8

/ / l,

1)-11 10-10 10-9

ADSORPTION, mole/cm2

Figure 9. Effect of adsorption of sodium dodecyl sulfonate on wettability and electrophoretic mobility of alumina. Experimental conditions: pH 7.2, ionic strength 2 x 10-3 M of NaCI. (Adapted from T. Wakamatsu and D. W. Fuerstenau.(109l)

Interfacial Chemistry of Mineral Processing Separations 337

hydrophobic state to be floatable as condition (J > 0° must be fulfilled for flotation to be possible. This is why collectors are used. The effect of their action can be explained on the basis of equations (25) and (26).

The influence of the adsorption of sodium dodecyl sulfonate on the contact angle and zeta potential of alumina at pH 7.2 is shown in Figure 9.(108) The experimental data are taken from papers by Wakamatsu and Fuerstenau.(109,1l0) As seen, adsorption of the anionic collector on the solid that is positively charged causes a decrease in the positive zeta-potential value of alumina.

Equation (26) predicts a variation of the work of adhesion of water to a solid with adsorption of a collector. A decrease of the surface charge of alumina caused by the adsorption of the anionic collector at pH 7 .2 [the point of zero charge (pzc) of alumina lies at pH 9.1] lowers w~ in equation (26). Formation of the film with long-chained molecules at surface of the solid also leads to a decrease of the w ~ term. According to equation (25), a decrease in w A leads to an increase in the contact angle. In agreement with this mechanism, the contact angle of alumina increases with adsorption of dodecyl sulfonate. The zeta potential reaches zero, but further adsorp­tion of the collector makes the zeta potential more negative. It may be predicted that now w ~ again increases, but w A, as given by equation (26), is approximately constant because an increase in w~ is compensated by a continuous decrease of w~ with the adsorption of a collector. This shows that a change in the hydrophobicity of a solid caused by a collector can be related to the w ~ and w ~ terms in the work of adhesion of water to a solid. The effect of a collector on the wi term can be neglected in such considerations.

I Flotation Processes

I I I

Froth I Separation

Froth I Flotation

I Nonfoaming I Separation

I 1 I I

Ore Precipitate Microflotation Adsorbing Ion

Flotation Flotation Colloid Flotation Flotation

I I 1

Flotation with Emulsion Agglomerate Carrier Water Soluble Collectors Flotation Flotation Flotation

Figure 10. Schematic classification of flotation separation techniques.(lOS)

338 J. Laskowski

A schematic classification of flotation separation techniques is shown in Figure 10. (lOS) The classification is based on that by Karger et al. (111) and Lemlich. (112) The changes introduced follow mainly from the fact that the classification system has been adapted to mineral flotation processes without taking into account all ion and precipitate flotation modifications. These are techniques that are of much more interest for chemical and hydrometallur­gical rather than for mineral processing purposes.

In the classification of the bubble separation techniques of Karger et al.(111) the first division is into two main groups: foam separation and nonfoaming adsorptive bubble separation. The former require the gener­ation of a froth to carry off material; the latter does not. The foam separation group is then subdivided into foam fractionation and froth flotation. In the classification shown in Figure 10 the froth separation method is introduced in the place of foam fractionation.

Froth separation is a method of ore separation that has been recently developed. (113-116) In this process a solution containing various hydrophobic and hydrophilic particles as well as surface-active agents is fed onto the layer of froth. Hydrophobic particles are concentrated in the upper layers of the froth, while the hydrophilic particles together with the solution are filtered through the layer of froth. The size of the mineral grains concentrated on the froth separators can be much increased compared with the particle size in froth flotation. Upper size of the particles is claimed to be 3-4 mm for sylvanite, 2 mm for sulfides, and even 10 mm for coal.(llS) Decrease of the angle of contact from 90 to l O is followed by a rather slow decrease in critical diameter from 1 to 0.13 units. (116)

The flotation process with an organic phase described by Shergold and Mellgren(117.11S) can be placed in the group of nonfoaming separation methods. In such a process, sufficiently large quantities of oil are used, and a separate oil phase is formed. Fine particles can then be extracted from an aqueous suspension into an oil phase if their surfaces are sufficiently hydrophobic. Zambrana et al. (119) described the extraction method that is used for the processing of -10-#£m cassiterite size fractions. Raghavan and Fuerstenau(120) have shown the feasibility of such an oil flotation process for the treatment of submicron particles.

Froth flotation methods applied in the field of mineral processing will be discussed further. One general method is conventional froth flotation that utilizes collectors that are soluble in water and adsorb selectively from the pulp. The second is emulsion flotation that utilizes oils that are practi­cally insoluble in water. The former process has been investigated exten-. I hi' f h d' (73 121122) slve y; t e atter reqUlres urt er stu les. . .

In the latter process, nonpolar collectors are introduced into the flotation system in the form of an emulsion. Investigations usually concern two major problems in emulsion flotation:

Interfacial Chemistry of Mineral Processing Separations 339

1. conditions necessary for the adhesion of oil drops to the surface of mineral particles; and

2. the mechanism in which a nonpolar agent increases the force of adhesion of a particle to a bubble.

It is known that the adhesion of oil drops to the surface of a solid immersed in water is possible only for solids that show some degree of hydrophobicity. Mackenzie(123,124) demonstrated that collectors that increase the hydrophobicity of a solid facilitate the adhesion of oil drops to that solid. However, the best conditions for adhesion are not produced at the greatest hydrophobicity but, in agreement with coagulation theory, at the minimum energetic barrier.

A R · , . hi' , d (121 125) 1 11 s USSJan sCientists ave ong mamtame, ' a nonpo ar co ector is accumulated at the contact of the solid/solution/air interfaces as shown in Figure 11. This explains why such a nonpolar collector shows better flotation properties when it is less soluble in water.(126)

The introduction of the emulsion of a nonpolar collector into the froth flotation system increases the size of floated particles. This means that such an agent increases the force of adhesion between a particle and a bubble. However, to increase the size of floated particles 1.5 times, the force of adhesion of the particle to the bubble must be increased by 3 to 8 times. The mechanism that makes such a situation possible was proposed by Melik_GaykazianY27,128) In his analyses he also considered the diameter of the particle-bubble contact.

Figure 12 gives some results extracted from the paper by Melik­azian. (127) The situation considered is shown in the upper part of Figure 12. The bubble contacts with the particle; the diameter of the contact is constant and equals 1 mm. The following forces were taken into account: Fl, the force of adhesion of bubble to particle; F 2, the force of detachment that results from the Law of Archimedes; and F 3, the additional detachment force resulting from the difference in capillary pressure in the bubble and hydrostatic pressure at the level of contact between bubble and particle.

Figure 11. Schematic illustration showing the adhesion of bubble to solid particles in presence of nonpolar collector. {After V. I. Klassen. (125»

340 J. Laskowski

20

~ F3 (1yne

16

12

e

F3

o - 0.1

At equilibrium

Figure 12. Adhesion forces Fl and detachment forces F2 and F3 acting between a solid surface of 0.1 cm in diameter and a bubble. (After V. I. Melik-Gaykazian.(\27»

(28)

Dimensionless parameter {3 describes the form of the bubble. The forces acting between a given particle and various bubbles are then analyzed in Figure 12. The curves give the forces for various surface tension values of the solution. As seen, force F3 can be much greater than F2. With an increase in bubble size at constant surface tension, F2 increases and F3 decreases in such a way that Fi passes through a minimum. When changing bubble size and surface tension (see dashed lines in Figure 12), F2 can be constant whereas Fi and F3 decrease.

Melik-Gaykazian claims that under dynamic conditions turbulent dis­ruptive forces of short duration suddenly change the shape of the bubble. In the presence of a nonpolar agent (as shown in Figure 11), expansion of the surface of the bubble causes the outflow of the agent from the bubble­particle contact increasing the surface tensionY29) Change of the shape of the bubble also influences the contact angle. This leads to a sudden increase in the adhesion forces between the bubble and the particle at the very

Interfacial Chemistry of Mineral Processing Separations 341

moment of action of the disruptive forces. These phenomena could explain the influence of a nonpolar collector on the size of floated particles.

Emulsion flotation is usually applied in the processing of minerals that show native hydrophobicity, such as sulfur and coals. Molybdenite also can be floated under such conditions. (130) Flotation of other hydrophilic minerals requires the use of conventional collectors.(131)

Froth flotation can be extended to lower size ranges by the use of techniques that permit agglomeration between fines. In principle agglomer­ate flotation and carrier flotation (also called ultraflotation) are based on the same phenomena.

As already pointed out, it was shown in Gaudin's early studies(96) that very fine particles can be floated if they have been previously flocculated. Note should be taken of the fact that now, after La Mer,(1O) the term flocculation is used for the aggregation process caused by polymeric sub­stances (flocculants) leading to the formation of a loose structure called a floc. Aggregation of hydrophobic particles that takes place under flotation conditions will be called hydrophobic flocculation.

Gaudin and Malozemoff(132) showed that near-colloidal mineral par­ticles are best floated if they are first selectively aggregated by means of hydrophobic flocculation leaving the gangue particles in a dispersed state. Accordingly, Derjaguin et al.(133) point out that aggregates formed under turbulent flotation conditions mainly contain two primary particles. For very fine primary particles, such a secondary particle is too small to be floated effectively. They conclude, therefore, that fines can be floated effectively only after aggregation onto course particles.

Hydrophobic flocculation is considered an indication of flotability.(134) Figure 13 gives the relation between wettability, flotation, and hydro­

phobic flocculation of galena particles upon addition of potassium ethyl xanthateY34) A similar clear correlation is seen in Figure 14 from Polkin and BergerY3S)

Hydrophobic flocculation is obviously at a maximum when particles are sufficiently hydrophobic and these conditions correspond to optimum flota­tion.(36) Recently, Warren's(16,17) findings suggest that so-called shear flocculation can also be responsible for the hydrophobic flocculation pro­cesses taking place under flotation conditions.

As discussed in Section 3.2.3 near-colloidal particles float by a diffusion regime, and their flotation can be much improved by aggregation if the size of aggregates is well into the collision regime. Then, aggregation should obviously be beneficial, especially for flotation of very fine particles.

In agglomeration flotation, the conditioning of pulp is usually carried out at 50-70% solids with an ionic collector-fuel oil combination prior to flotation. (137,141) The oil drops cause the agglomeration of hydrophobic ionic collector coated mineral particles.

342

...... U 0..

100 11--

90 0.9 r a:l 80 Q8

Ul

'-_ J 0

70U O.7

If< 60 0.6 ....

~ ../ 50 0.5 V r . .1

.. ~o o.~ .. 30 03 ~ 20 0.2

.\ 10 0.1

\ ~O

~O 21i 30

~ -0.1 6

-0.2 A

, ..

..

2

40 50 60 70

3 6. t--

1. Laskowski

- -II

- ~ ..

200 gil

A

50

45

40 -+-'

K 35

30 c .Q

25 ] :J U

20 g u:

15

10

5

Figure 13. Effect of potassium ethyl xanthate on flotation (curve 1), hydrophobic flocculation (curve 2), and wettability (curve 3) of galena. Flocculation curve shows content of floes assaying five and more primary particles. (After P. A. Rehbinder.(134»)

Lapidot and Mellgren (140) showed that when the ore was conditioned with reagents at a high concentration of solid, first flocculation and then de flocculation takes place. This led to variations in the viscosity of the pulp and corresponding variations in power consumption.

In the flocculation period, a gradual nonselective increase in recoveries of ilmenite and gangue minerals reaches a maximum at the flocculation peak. During the deflocculation period, the ilmenite recovery increases slightly, while the recovery of gangue decreases. At the end of this period, maximum selectivity is observed. In the dispersion period, recoveries of valuable and gangue minerals decrease gradually. This suggests that after the distribution of reagents onto mineral particles in the first step, desorp­tion of reagents from the gangue particles, intensified by attrition, takes place improving the selectivity of flotation. This implies that the condition­ing process proceeds from a state of all the particles being nonfloatable, through successive stages of all the particles being floatable, followed by only the valuable mineral being floatable. The final state of the process led

Interfacial Chemistry of Mineral Processing Separations 343

100 -r--------::------....., 160

.,..; BO u a. III

>- W c:: 60 ~ w f= > 8

[; 150 Z

W a c:: LO f= u ::> c:: t-I/)

20 w 0

1LO

0 2.5 5 10 15

CONCENTRATION, mg/l

Figure 14. Effect of concentration of laurylamine on flotation (curve 1) and flocculation (curve 2) of +O.1-0.1S-mm quartz particles. Flocculation was characterized by the time of destruction of flocculated sediment method of Waksmundzk et al,036) (After S. I. Polkin and G. S. Berger. (135»

to all particles being poorly floatable. (140) A similar mechanism was pre­viously postulated for the flotation of hematite ore. (142)

Agglomeration flotation with conditioning of the pulp with high solid content utilizes slightly different phenomena from those already described. Attrition and redistribution of reagents leading to the selective agglomer­ation seem to be essential in the process. High power consumption is also needed.

Carrier flotation, the process that is based on the same phenomena as agglomeration flotation, is so far only used to remove fine anatase particles from kaolin slurries. (143)

Another noteworthy carrier flotation method listed in Figure 2 is an adsorbing colloid flotation process. (144-146) In this process the ferric hydroxide or other particulates enriched with trace metal ions are floated by use of the appropriate surfactant. Since the charge of the carrier is pH dependent, the pH has marked effect on the recovery; for example, for separation of uranium with ferric hydroxide from sea water with the use of dodecyl sodium suHate the maximum recovery was obtained between pH 5.5 and 6.0Y4S)

The possible relevance of the phenomenon of shear flocculation to carrier flotation was analyzed by Warren. (17) He pointed out that the critical requirements for carrier flotation were (1) that the carrier particles be less

344 1. Laskowski

than 50 j..Lm, (2) that the carrier, as well as the anatase, be made hydro­phobic, and (3) that conditioning be carried out at higher energies than normal. Thus, Warren concluded that the shear flocculation is evidently essential to the carrier flotation.

3.4. Colloidal Separation Methods

3.4.1. Selective Coagulation

Examples of selective coagulation can be found in the papers by Pugh and Kitchener(6) and by Pugh. (3)

-90

-80

-70

-60 • -50

> -40 ~/ E

r~ -30 -.J <! -20 t-Z UJ - 10 t-O a.. 0

~ +10 UJ N

+20

+ 30

+40

+SO

+ 60

2 3 4 5 6 7 8 9 10 11 pH

Figure 15. Zeta potential of the quartz, the rutile, and the hematite vs. pH. (After R. J. Pugh.(3))

Interfacial Chemistry of Mineral Processing Separations 345

The zeta potentials of quartz, rutile, and hematite are shown in Figure 15.(3)

Coagulation of the quartz-hematite suspension with an initial solid content of 2.2 wt%,1.1 % of each mineral, is shown in Figure 16. Over 80% of the particles of each mineral had a size range of between 0.05 and 0.2 p. m radius with a mean of about 0.1 p.m. As seen, a region of selective coagula­tion of the mixture was anticipated at pH 7-7.5 since in this region the quartz remained relatively stable, while the hematite with a small negative zeta potential undergoes coagulation.

3

Jl :l

.!;; 2 E a M

L. 1 C1I --o c .Q 0 III C

~ III :l III

.S; 3

.§ 'g 2 ~

~ o

-2 3

Hematite unstable Quartz stable

, , , , 1

- -

01 1 1° I L-_---J

I. 5 6 7 B

(AI

9 10 11 12 13 11. pH

IB) unstable stable - - -

selective coagulat ion zone

2 3 456 7 8 9 10 11 12 13 1~ pH

Figure 16. Stability regions for the quartz suspension and the hematite suspension. e, Quartz; 0, hematite. (B.) Stability regions for the hematite-quartz mixed suspension. Stability was measured by wt% solid remaining in suspension after 30 min. (After R. J. Pugh.(3)

346 1. Laskowski

A similar region of selective coagulation was found while working at a pH of 9 but varying the concentration of NaCI.

Figure 17 gives the coagulation of the quartz/rutile system. (6) A region of selective coagulation was found at about pH 5.6. At this point negatively charged quartz particles remained stable, while the rutile coagulated.

Figure 18 gives data extracted from the paper by Wiese and Healy.(147) As seen from the values of W, selective coagulation of Ti02 and Alz03

should be possible round pH 9.5. However, for this system the solubility of Alz03 can cause some problems. At a concentration of AI(N03h of 2.1 x 10-5 M the zeta potential of Ti02 reaches the same value as that of Alz03• (148) Such secondary effects of solubility can make selective coagula­tion impossible.

(AI

tfl UNSTAaE STABLE UNSTABLE ~2 rn-----~--~~~~~~\

~ g1

Z 03 Vi Z W B;2 ~ Z1

~ 2 :J o Vl ~1

I-3 0

2 3 4 5 6 7 8 9 '0 11 12 13 14 pH

(B) STABLE.

UNSTABLE UNSTABLE

1 2 3 4 5 6 7 8 9 '0 11 12 13 14 pH

UNSTABLE STABLE (e)

2 3 4 5 6 7 8 9 10 11 12 13 14 pH

Figure 17. Stability regions for the quartz (A), the rutile (B), and the quartz-rutile mixed suspension (C). Stability was measured by wt% solid remaining in suspension after 30 min. (After R. J. Pugh and J. A. Kitchener. (6»

3: 0 i= <{ a: >-I-..J

iii <{ I-CI)

Interfacial Chemistry of Mineral Processing Separations

20

10

5

2

TiC. ,r--

347

80

60 > E

40

20

..J' <{

I­Z

~~~~~==~~-==-~~==-~~~~~=-~~~~~~o ~ 9.0~9.5 10.0 10.511.0 0 4.0

20 Q"

<t: 40 Iii

pH

60

80

4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9:0 9.5 10.0 10.5 11.0

Figure 18. The zeta potentials and the stability factors W for Al20 3 and Ti02 as a function of pH in 10-3 M KN03 aqueous solution. (After G. R. Wiese and T. W. Healy.(13S»)

The industrial process of selective coagulation that is used for the purification of kaolin by removing coagulated colored minerals was described by Maynard et al. (149)

3.4.2. Selective Flocculation

The aggregation process brought about by polymers soluble in water is called flocculation. Some polyelectrolytes can be also used as coagulants, but in that case in contrast to flocculation the molecular weight of polymer has little effect. Principles of the action of the polymeric flocculants can be found in the papers by Kitchener et al.(1l-13,1S0,151)

Flocculants are typical hydrophilic colloids with a molecular weight in the range 105_107 • These substances have been applied in industry for a long time in solid-liquid separations when the solid is in the form of finely suspended material. The reason for their industrial application was their ability to produce open network floes larger than those obtained by coagula­tion with inorganic electrolytes. The "bridging" mechanism of their action was suggested by Ruehrwein and Ward(152) and by Michaels and Morelos. (153)

The adsorption of a flocculant is an irreversible process; it cannot be easily washed off with water. Flocculants with high molecular weight are

N

348 J. Laskowski

usually nonselective, and flocs are formed from various mineral grains present in the suspension. This is desirable in the clarification of water but not in the beneficiation processes.

K· h I (11 12 150 154) h h h t b h . h . Itc ener et a. . " ave sown t a y c oosmg t e appropn-ate flocculant, pH, and other conditions, selective adsorption of a flocculant can be obtained that gives selectivity of flocculation. In that case it is not desirable to choose a flocculant of very high molecular weight giving very large and cohesive flocs as that type of flocculation would enhance the entrapment problem setting limits to the selectivity.

It is important that the pulp initially be in a dispersed condition that usually requires the use of a dispersant. This and other modifying agents change the surface properties of the minerals altering the mechanism of adsorption of the polymer. It has been shown(lS4,13) that it is possible to synthetize polymers containing chelating or coordinating functional groups that specifically chemisorb on minerals.

The separation of the galena-quartz, calcite-galena, (11) calcite-rutile, aIumina-quartz, (150) and synthetic cassiterite-quartz(lSS) systems by selec­tive flocculation has been demonstrated. Read and HoIlick(156) have given more examples in a review published in 1976. A commercial selective floculation process was reported by Villar and Dawe. (15)

The selective flocculation process requires special hydrodynamic condi­tions. A flocculant should be distributed uniformly through the pulp, and at this stage, conditioning should be at a low rate of shear since too intense stirring can irreversibly break flocs. Gentle stirring provides orthokinetic conditions for the formation of large flocs. However, even under favorable surface-chemical conditions, the entrapment of nonftocculated mineral particles in the flocs inevitably lowers the grade of the flocculated fraction. One of the ways that allows the avoidance of such a complication is the use of a lower pulp density; another consists in the cleaning stages.

3.4.3. Spherical Agglomeration

As shown in this paper, there are many processes that involve an oil, and they differ according to the oil-to-water ratio and the presence (or absence) of air bubbles in the system.

In Puddington's spherical agglomeration process, fine particles in sus­pension are treated with a "bridging liquid," which preferentially wets the solid and is immiscible with the first liquid. Since the hydrophobicity of solid surfaces is a necessary condition for this process, it involves basically the same stage operations as in flotation.

The type of agglomeration produced on agitation of a suspension with an oil is critically dependent on the amount of the oil, the degree and type of agitation, particle size, size distribution, and surface properties of the

Interfacial Chemistry of Mineral Processing Separations 349

solid components. (157) With an increase in the amount of bridging liquid, the agglomerates change from unconsolidated flocs to larger, densified spheres. The bridging liquid is in the pendular state when up to 20% of the space between primary particles in agglomerate is occupied. This gives a flocculated product with an open structure that is easily broken down. When the space between primary particles in agglomerates is completely filled with bridging liquid, the capillary state is reached and agglomerates reach a peak of strength and sphericity.

If the mineral particles are placed in the presence of two immiscible liquids, they can pass preferentially into one of these liquids. For 8 > 90° the particle will tend to be drawn into the oil phase. Thus, it is quite clear that beneficiation by this method requires some of the mineral constituents of the ore to be made hydrophobic prior to agglomeration. The selectivity of the overall process depends on this particular stage.

Specific examples and a list of references are given in the review by Puddington and Sparks. (157)

3.4.4. Carrier Colloid Separation Methods

Clearly, in dealing with even two-component polydisperse suspensions, the selective coagulation technique cannot provide the suitable conditions for the gravitational separation of one-mineral aggregates from the primary particles of the second mineral if these primary particles are not very fine, say, below several microns in diameter. It should be noted that the hydrodynamic conditions for orthokinetic collisions between fine and coarse particles are extremely favorable. The only reasonable solution, then, seems to be carrier selective coagulation or carrier selective agglomeration. An ideal situation would be if the valuable fines could be selectively captured by the coarse particles of the carrier, especially if the carrier possessed properties permitting its easy extraction from the system.

Fine magnetite particles selectively aggregate without the addition of any reagent. Using magnetite as carrier, its ferromagnetic properties can be exploited for the extraction of the carrier loaded by valuable fines. Schinkorienko and Bodnaraschek(21) studied this idea further. Since surface properties of magnetite, hematite, and martite are very similar, they were able to agglomorate hematite and martite with magnetite using fatty acids and nonpolar hydrocarbons. Hydrophobic flocculation occurred selectively and could be followed by conventional magnetic separation. Two low-grade hematite-martite ores assaying approximately 42 % Fe were finely ground (content of a -50-lJ.m size fraction amounted to 75%) and were agglomer­ated with the magnetite (content of a-50 IJ.m was 90%). The concentrates that were obtained contained above 60% Fe at recovery of 90%. The amounts of reagents were, however, very high: 2 kg/t of tall oil and 10 kg/t of diesel oil.

350 1. Laskowski

It was shown that results were better at high pulp density, at 65% solids; the time for conditioning was 30 min.

Since the agglomerates formed under such conditions are not very strong, it should be possible to redisperse the concentrate obtained by intense stirring with subsequent reuse of the carrier.

Agglomeration can also be because of the physical magnetizing process and applies to fine ferromagnetic particles. Such particles, after exposure to a magnetic field strong enough to cause reorientation of the particles, tend strongly to retain this induced magnetism. Bartnik and Giermak(158) called this process magnetic flocculation and showed that nonmagnetic sus­pensions seeded with ferromagnetic particles can also be agglomerated in such a process. An example that seems to be an industrial application of this idea can be found in the paper by Nesterova et al. (159)

The potentialities of magnetic flocculation as one of the colloid separa­tion methods is obvious. Another way may be the application of the carrier in the shear-flocculation process.

4. Physicochemical Beneficiation Processes

In mineral processing the raw materials are treated without changing the identity of the minerals. Physicochemical beneficiation processes may offer another approach.

The term "physicochemical beneficiation processes" will be applied to the beneficiation processes in which previously described physicochemical separation methods are applied or to processes with pyro- or hydrometallur­gical pretreatments that facilitate the application of conventional mineral separation techniques. Thus, physicochemical beneficiation processes rely on a combination of chemical processes and mineral separation techniques. It is evident that there is need for research on this important and new approach. (160)

Figure 19 indicates some of the principal operations involved in physicochemical beneficiation processes. An obvious division is in terms of the pretreatment operations that precede the concentration stage.

The best known and most conventional preoperation is the comminu­tion of ore and treatment with chemical agents. After liberation, effected by crushing and grinding, which can be carried out in the presence of the specific reagents, (161) a concentration process is employed. In some instances selective grinding (or selective dispersion) can lead directly, by sizing of the selectively ground ore, to the separation of the grains of various minerals.

Physicochemical separation methods must be placed in this group of processes covering a wide variety of methods.

As seen from Figure 19, electrodynamic separation can also be classed in this group of processes. This process is based on differences in the surface

Interfacial Chemistry of Mineral Processing Separations

Comminution and Treating with

Reagents Roasting

Magnetic Roasting, Segregation, etc.

Leaching

Figure 19. Schematic classification of the physicochemical beneficiation processes.

351

conductivity of minerals. Adsorption of water vagor on the surface of grains decreases the difference in the conductivities 162.163). Humidity can be eliminated by "hydrophobic coating," This treatment may be necessary in the case of high humidity(164) and usually involves conditioning with a collector aqueous solution, but treatment with reagents applied in the form of aerosol has also been described. (165)

Pretreatment operation by roasting is applied when difficult-to­beneficiate ore can be converted into artificial minerals that are easily concentratable.

This process is widely applied for the beneficiation of nonmagnetic iron ores. Iron oxides roasted in a weakly reducing atmosphere at about 700°C can be converted into magnetite.

Oxidized copper or nickel ores can be beneficiated by a "segregation process." Here the ore is heated together with small amounts of salts NaCI or CaCh and coal. HCl produced in the first stage transforms copper (or nickel) oxides into chlorides that vaporize and are reduced to metal, mostly on the surface of a solid reductant. The segregated metal is recovered by flotation or by magnetic separation. (166-168)

Leaching is a conventional operation in hydrometallurgy. However, it can also be used as a pretreatment operation in mineral processing. Examples can be found in work by Krukiewicz and Laskowski. (169) In the cited paper the so-called magnetizing alkali leaching process was described, and its application to the processing of low-grade siderite ores was demon­strated. It was shown that this process proceeds in two steps: decomposition of FeC03 in alkaline solutions with the formation of Fe(OHh and oxidation

352 1. Laskowski

to a-, 8-, 'Y-FeZ03, and Fe304. Under optimum conditions magnetic coat­ings can contain mainly 'Y-FeZ03 and Fe304. Low-tension magnetic separators can then be applied to the beneficiation of the ore.

Symbols

A

P'(h)

G H Ho

h

k K

Greek

Iv sv 'Y ,'Y , 'Y.I

I • 'Yd, 'Yd

E

Van der Waals-Hamaker's con­stant collision frequency diameter of sphere diameter of particle shear rate diffusion coefficient coefficient of turbulent diffusion force of adhesion of a bubble to a particle force of detachment that results from the Law of Archimedes additional detachment force resulting from the difference in capillary pressure in the bubble and hydrostatic pressure at the line of contact between bubble and particle total specific surface free energy of a thin plane parallel lamella Gibbs free energy separation of particle centers minimum separation of the par­ticle surfaces distance of a grazing trajectory from its axis at infinity Boltzmann constant Debye-Hiickel reciprocal double-layer thickness concentration probability of collision between i and j particles

liquid-vapor, solid-vapor, and solid-liquid interfacial tensions dispersion force contribution to the surface free energy of liquid and of solid, respectively specific density dielectric constant

, R Stk t T

We W v

VIP

V.

radius of particle interaction radius R jj = 'j + 'j Stokes number time absolute temperature fluid velocity relative to bubble bubble rising velocity work of adhesion contribution from dispersion forces to the work of adhesion contribution from hydrogen bonding to the work of adhesion contribution of electric charge at the interface to the work of adhesion work of cohesion the stability rath;) factor particle velocity relative to bubble particle settling velocity total potential energy of interac­tion between two particles energy of interaction (attraction) between the particles because of London-van der Waals disper­sion forces energy of interaction (repulsion) between the electrical double layers associated with the par­ticles

viscosity contact angle surface potential disjoining pressure coefficient accounting for the hydrodynamic flow pattern around a big particle (or bubble)

Interfacial Chemistry of Mineral Processing Separations 353

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