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Fine particles processing: shear-flocculation and carrier flotation — a review
International Journal of Mineral Processing, 30 ( 1 9 9 0 ) 2 6 5 - 2 8 6
265
Elsevier Science Publishers B.V., Amsterdam
Fine particles processing: shear-flocculation and carrier flotation - a review T.V. Subrahmanyama and K.S.Eric Forssberg b aDepartamento de Geologia, CCE/UFRN, Campus Universitario, 59. 072 Natal-RN, Brazil bDivision of Mineral Processing, Luled University of Technology, S-951 87 Lule& Sweden (Received October 9, 1989; accepted after revision July 2, 1990)
ABSTRACT
Subrahmanyam, T.V. and Forssberg, K.S.E., 1990. Fine particles processing: shear-flocculation and carrier flotation - a review. Int. J. Miner. Process., 30: 265-288. Shear-flocculation/flotation and carrier flotation are methods by which fine mineral values are aggregated and then recovered by flotation. The aggregates formed have better floatabilities. Without prior treatment ahead of flotation, significant quantities of fines are lost in tailings. An overview of the problems associated with fine particles is presented. Like froth flotation, the mechanisms ofshearflocculation and carrier flotation are governed by physical, chemical and geometrical variables. The present paper deals with the effects of these variables on shear-flocculation and carrier flotation, based on a detailed literature review.
INTRODUCTION
Froth flotation is one of the most important methods for the separation of minerals. With the depletion of high grade ores it becomes necessary to develop efficient methods for treating low grade ores, which often involves handling and processing of fine particles. It was estimated that 1/ 3 of phosphate (Tyler and Waggman, 1953 ), 1/6 of copper (Goldberger, 1973 ), 1/ 10 of iron explored in the U.S., 1/2 of the tin produced in Bolivia and 1/5 of the tungsten mined in the world (Somasundaran, 1976 ) and millions of tons of other minerals are lost in the form of fines. The gravity of the problem of fine particles and the emergent necessity for suitable processing methods to recover the valuable components from fines has resulted in a large number of publications and symposia. Fuerstenau (1988) rightly concludes: "To meet the economic challenges facing the mineral industry worldwide, it is important that industry applies novel concepts to increase efficiency and reduce costs of mineral producing operations. Research and development in flotation science and engineering have a definite role in the efforts for production improvement. Enhancement in grade and recovery and reduction of costs in 0301-7516/90/$03.50
© 1990 - - Elsevier Science Publishers B.V.
266
r . v . SUBRAHMANYAM AND K.S.E. FORSSBERG
flotation plants can be brought about by research on comminution, fine particle recovery, flotation reagent development, novel developments in flotation machines, the design of new flotation circuits and the development of automatic control technology..." Several papers (Fuerstenau et al., 1978; Sennett and Young, 1978; Somasundaran, 1978; Sastry, 1978; Yang, 1978; Stratton-Crawley, 1978 ) deal with fine particles processing. Among the modified flotation techniques are: high temperature flotation, column flotation, oil flotation, liquid-liquid extraction, agglomerate flotation/floto-flocculation, electroflotation, vaccum or pressure release flotation, precipitate flotation, selective flocculation, flotation with controlled dispersion, spherical agglomeration, shear-flocculation/ flotation and carrier flotation. The scope of the present paper is restricted to the recovery of ultrafine and fine particles ,-. 5-10/~m by shear-flocculation/ flotation and carrier flotation. In these methods the principal objective is to enhance the particle size by uniting the particles so that the aggregates formed may have better floatabilities. Depending on the mechanism involved earlier workers referred to ultra- or piggyback flotation (Green and Duke, 1962; Deryagin et al., 1964; Samygin et al., 1968), floc-flotation (Gaudin et al., 1942; Clement et al., 1970); or to autogenous carrier- or ramification carrier flotation (Hu et al., 1987, 1988). FINE PARTICLES IN FLOTATION CIRCUITS AND PROBLEMS: AN OVERVIEW
The influence of particle size on the rate of recovery of minerals was investigated by several workers (Gaudin et al., 1931; Anthony et al., 1975; Trahar, 1976 ). In general, fine particles < 10#m in size have low collision efficiencies with gas bubbles and are accessible to mechanical entrainment. The entrainment mechanism does not distinguish between hydrophilic or hydrophobic particles and is closely related to water recovery. Earlier workers (Engelbrecht and Woodburn, 1975; Goodman and Trahar, 1977; Lynch et al., 1981; Warren, 1985; Subrahmanyam and Forssberg, 1988a,b,c) observed a linear relationship between the water and mineral recoveries. Table I gives the degree of entrainment for different minerals. It can be observed that with decreasing particle size the entrainment becomes the dominant mechanism of particle collection. Figure 1 illustrates the adverse effects of the presence of fine particles in flotation circuits. Due to the small mass and large surface area of fines, problems like high reagent consumption, non-selectivity of the collector, excess froth stability, mechanical entrainment of fines, etc., affect the recovery and grade of the concentrate product. Several papers (Trahar and Warren, 1976; Fuerstenau, 1980; Jameson, 1984; Warren, 1984; Subrahmanyam and Forssberg, 1988a) deal with the problems associated with fine particles. In the flotation size range particles have better floatabilities due to colli-
SHEAR-FLOCCULATIONAND CARRIERFLOTATION
267
TABLE I Degree of entrainment for different minerals (from Subrahmanyam and Forssberg, 1988b ) Mineral
Spec. gray.
Particle size (/tin)
Degree of entrainment
Reference
Quartz
2.65
3.5 >40.0
0.72 0.10
Trahar ( 1981 ) Engelbrecht and Woodburn (1975)
< 12.0
0.99
Engelbrecht and Woodburn ( 1975 )
< 5.0
0.85
Goodman and Trahar ( 1977 )
< 38.0
1.00
Lynch et al. ( 1981 )
0.87
Warren ( 1985 )
0.78
Subrahmanyam and Forssberg ( 1988c )
Silica Cassiterite
6.80-7.10
Coal
1.00
Ultrafine gangue Fine gangue
< 40.0
sions with gas bubbles. The size range for floatability may vary with the mineral and is also dependent upon the magnitude of operation - batch, laboratory or industrial. Table II shows the size ranges of flotation recovery for different minerals. Below the minimum (fines) and above the maximum (coarse) sizes the flotation recoveries start to decrease.
FTTC. REAGENTI I I HIGHSURFACE] ,
ENERGY
~ --
_
HIGH
/
I~
-->lO SSOLUTION (G~R)I
\
[LbWMOMENTUM/ IFINEPARTICLE L
~
EIENTRAINMENTIG) r'-
SMALL MASS
~h"~HIC-H~ ~/
FROTH /
\\FTAO'-'Z~NC~RII ,
/
1
l STAa"-,TY Ix
~"\.~_J C O A G U L A O T IN I~-----~/ L O J l WC O L S L O IINL l
I
I RATEIR)
J
LPROBABILITY F
Fig. 1. Relationship between the physical and chemical properties of fine particles and their behaviour in flotation. ( G ) and ( R ) refer to whether the phenomena affect grade a n d / o r recovery. The arrows indicate the various factors contributing to a particular phenomenon observed in flotation of fine particles (from Fuerstenau, 1980).
268
T.V. SUBRAHMANYAMANDK.S.E.FORSSBERG
TABLE II Observed size ranges of flotation recovery for different minerals as reported by different workers (from Trahar and Warren, 1976) Mineral
Size range (~m)
Barytes
10- 30
Cassiterite
3- 20
Conditions
Reference
laboratory, batch
Clement and Klossel ( 1963 )
industrial
Kelsall et al. (1974)
Fluorite
40-110 10- 90 50-150
laboratory, batch industrial industrial
Klassen and Mokrousov ( 1963, p. 390) Klassen and Mokrousov ( 1963, p. 391 ) Lay and Bell (1962)
Galena
37-295 170-240 7- 70 6- 70 13- 75 20-100
laboratory, batch laboratory, batch industrial industrial industrial industrial
Gaudin et al. ( 1931 ) Klassen and Mokrousov ( 1963, p. 391 ) Cameron et al. (1974) Kelsall et al. (1974) Klassen and Mokrousov ( 1963, p. 391 ) Lynch and Thorne (1974)
Pyrite
50-100
laboratory, continuous
Imaizumi and Inoue (1965)
Pyrite-pyrrhotite
20- 70
laboratory, batch
Morris (1952)
Quartz
10- 40 9- 50
laboratory, continuous laboratory, batch
De Bruyn and Modi (1956) Robinson (1950)
Sphalerite
15-100 8- 70
industrial, laboratory batch
Cameron et al. ( 1971 ) Anthonyetal. (1975)
Wolframite
20- 50
laboratory, batch
Clement et al. (1966)
References: see original publication. In view of the poor floatabilities of fines their treatment prior to flotation becomes essential for improving the recoveries. AGGREGATION OF PARTICLES W h e n i n d i v i d u a l p a r t i c l e s are b o u n d t o g e t h e r o r u n i t e d t h e r e s u l t i n g m a s s o r b o d y is a n a g g r e g a t e . A d e s c r i p t i o n o f t h e t e r m s is g i v e n a l o n g w i t h s o m e d e t a i l s in t h e p r e s e n t s e c t i o n in o r d e r t o d i s t i n g u i s h t h e m e c h a n i s m s o f p a r t i cle a g g r e g a t i o n . F o r g e n e r a l i n f o r m a t i o n T a b l e I I I g i v e s t h e h y d r o p h o b i c aggregation separation methods.
Coagulation and flocculation In coagulation simple inorganic electrolytes are used to aggregate the par-
rhodocrosite ( < 10/tm )
Combined aggregation separation
References can be found in original publication.
oxides of iron and titanium
ilmenite ( < 10#m) hematite ( < I/zm)
Magnetic adhesion separation
baryte (mostly < 7/~m)
Sphere agglomerationseparation Two-liquid extraction
martite
Hospaton-18 Hospaton-21 Medialan- 16
manganese ore
Emulsion flotation
Magnetic seed separation
sodium oleate
anatase (90-94% < 2/tin )
Carrier flotation
sodium oleate
fatty acid
sodium oleate
tall oil, petroleum sulphonate
tall oil
sodium oleate
scheelite (90% 0.75-1.4 /lm )
Shear flocculationflotation
Collector/reagent
Materials treated
Process
kerosene
non-polar oil
non-polar oil
fuel oil
kerosene
petroleum
diesel oil
Non-polar oil
Hydrophobic Aggregation Separation Methods (from Shouci and Zongfu, 1988)
TABLE 111
2050 rpm
> 780 rpm
> 100 Kw-h/ short ton
1600-2000 rpm
850 rpm 1700 rpm
intensity
Agitation
screening
flotation
flotation
flotation
10-15
desliming in magnetic field
magnetic separation
magnetic separation
phase Fayed (1965) separation
30
90 20
time ( min )
Separation method
Lu et al. (1988)
Tihonov and Babushkina (1979)
Xin and Xu ( 1983 )
Takamori et al. (1980)
Gate (1957)
Green and Duke ( 1962 )
Warren ( 1975 )
Reference
"~ "~
t" O
~
¢3
7
O
.~
270
T.V. SUBRAHMANYAM
AND K.S.E. FORSSBERG
ticles. Flocculation is generally effected by polymer reagents which bind the particles by a bridging mechanism. Oliver ( 1961 ) discussed the types of flocculants and applications in various industries dealing with solid-liquid separations. In spontaneous flocculation processes the energy barrier between particles is of the order of kT and flocculation occurs without any energy input. Princen and De Vena-Peplinski (1964) observed the mutual flocculation of two oppositely charged hydrophobic colloids, ZnO and TiO2, suspended in water, to be essentially dependent upon the particle size and not on the pH and time. Further, mutual flocculation was found to occur even when the sign of charge was the same.
Floc flotation In floc flotation the formation of floes was found to be a pre-requisite for the flotation of fines from a mixture ofhaematite and quartz in a tall oil emulsion. The floes were formed in oleic acid but not in sodium oleate and it was attributed to the molecular form of oleic acid (Clement et al., 1970).
Slime coatings In flotation systems where particles are oppositely charged, the problem of slime coatings may affect the flotation behaviour of the valuable mineral for example, the flotation of galena in the presence of alumina slimes (Fig. 2 ). Below pH 10, alumina is positively charged and forms slime coatings on negatively charged galena (Gaudin et al., 1960). In this example, when the Ioo o c
>: cr
60 t.) w
,-,z 40
GALENA Z5 mgll. KEX
o_
I-0 -J ~-
ALUMINA SLIME o 0 g/I. • 0.1
20
o
i 0
6
0.2
• r'l
0.3 0.5
I
I
2
I 4
~
I
1
6
8
i
I I0
,
I 12
I 14
pH
Fig. 2. Flotation recovery vs. pH for galena floated in the presence of various amounts of alumina slimes (from Gaudin et al., 1960).
SHEAR-FLOCCULATION AND CARRIER FLOTATION
271
repulsive force due to like charges was greater than the attractive van der Waal's force, slime coatings did not form.
Shear-flocculation In shear-flocculation the energy barrier resulting in repulsion of similarly charged hydrophobic particles is overcome by intense stirring. The energy barrier in this case is much greater than kT. A direct contact due to collision between particles is favoured by an energy of hydrophobic association (Warren, 1975a). For the formation of hydrophobic aggregates the hydrophobic interaction energy is a significant factor since it is a few hundred times higher than the energy of molecular or electrostatic repulsive energy. From the potential energy values calculated as per the DLVO theory and the hydrophobic interaction theory, it has been shown by Shouci and Zongfu ( 1988 ) that for hydrophobic rhodocrosite the potential energy begins to decrease for interparticle distances of 8 nm, assuming negative values for distances less than 6-7 nm. In the context of shear-flocculation, Warren (1975a) refers to slime coatings which involved the adhesion of fines to coarse particles, both similarly charged and collector-coated. This is a different mechanism of slime coatings from the one referred to by Gaudin et al. (1960) (see Fig. 2 ).
Carrier flotation and other similar methods Among early investigations, Green and Duke (1962) were the first to recognise the beneficial effect of the use of a coarse ( ~ 325 mesh) reagentized carrier mineral, in the present context calcite, for the separation of anatase impurity from kaolin clay. Other carrier minerals tested were ground silica, fluorspar, kyanite, anatase and sulphur. The mechanism of calcite-anatase aggregation was explained to be due to the preferential adsorption and stronger binding of oleate to calcite and anatase and not to kaolinite (Chia and Somasundaran, 1983 ). In autogenous- or ramification carrier flotation (Hu et al., 1987, 1988 ) the same pre-conditioned coarse mineral particle serves as a carrier (Fig. 3 ). Among the aggregation methods described, 'the common or critical requirements for shear-flocculation, carrier flotation or autogenous carrier flotation are: ( 1 ) presence of coarse particles; (2) use of higher energy agitation; and ( 3 ) hydrophobicity of the carrier (coarse) and the carried (fine) particles. MECHANISMSOF SHEAR-FLOCCULATIONAND CARRIER FLOTATION A pulp containing ultrafine to fine ( ,-, 5/~m) and coarse ( ~ 40/~m) particles when agitated at high speeds, forms three types of aggregates, viz., fine-
272
T.V. SUBRAHMANYAM AND K.S.E. FORSSBERG
fine, fine-coarse and coarse-coarse. The first two types of aggregates decrease the turbidity of the solution caused by the fines in suspension. The rate of decrease of the number of single particles in the absence (eq. 1 ) and in the presence (eq. 2) of carrier mineral is given by (Samygin et al., 1968): dn dt = -2k,
n o2
( 1)
and
drt
--=
dt
_ 2k, nXo_ 2 k 3 n o n b
UNEVENLY
(a)
(2)
DISSEMINATED
ORE
1
CLASSIFICATION
SLIME
SAND FLOTATION
CONCENTRATE
TAILI NG
AUTOGENOUS CARRIER FLOTATION
COMPLEX, OXIDIZED, SULPHIDE
ORE
(b) OXIDE
PART
DESLIMING
SAN D
OX I DE FLOTATION
SULPHIDE
SULPHIDE
S L IME l
PART
FLOTATION
CONCENT RATE CARRIER
AUTOGE NOUS CARRIER FLOTATION
Fig. 3. Autogenous carrier flotation (from Hu et al., 1988).
TA I L I N G
SHEAR-FLOCCULATION ANDCARRIERFLOTATION COMPLEX Fe (C)
273
ORE
1 MAGNETIC
SEPARATION
1 MAGNETtTE
WEAK MAGNETIC
NI
PART HEMATITE
CONCE RATE USED AS CARRIER
I
AUTOGENOUS ILCARRIER FLOTATION
REFRACTORY, OXIDIZED, SULPHIDE AMORPHOUS Cu ORE
(d)
1 SULPHIDE FLOTATION
SULPHIDE CONCiNTRATE
OXIDE 1
PART USED AS CARRIER
LEACHING l CuS PRECIPITATION
1
It-
AUTOGENOUS CARRIER FLOTATION
Fig. 3. Continued.
where no and nb are the concentrations of fines and the carrier mineral, respectively; k~ and k3 refer to fine and large particles, respectively, and are the rate constants for mutual cohesion of the fine particles and for adhesion of fines to larger particles. Two types of collision mechanisms are predominant depending on the particle size, i.e. diffusion, when the particle size is less than
274
T.V. SUBRAHMANYAM
AND
K.S.E.
FORSSBERG
the internal turbulence scale and inertia, in the case of larger particles in polydisperse suspensions. The number of particle collisions increases with the particle radii and the relative velocity in the liquid. The relationship between the scale of turbulence (A) and particle size is given by, (3) where, k, and k, are the total number of collisions between the particles in unit volume per unit time by the inertia and diffusion mechanisms respectively; d is the average particle diameter; p and p. are the densities of the mineral particles and the liquid. A=
1
(4)
where, 1 is the diameter of the chamber; R is the radius of the impeller; N is the impeller rotation speed and v is the kinematic viscosity. For the conditions maintained and considering ki/kd = 1, i.e. for equal probabilities of inertia and diffusion mechanisms, the value of A was calculated to be 33.5 pm (Samygin et al., 1968). However, since the particle size of the coarse mineral used in their experiments was 60 pm, inertia was stated to be the predominant collision mechanism between fine and coarse particles. Because of their greater mass the larger particles move relative to the liquid with tine particles suspended in it and collide with them. The relative significance of the two mechanisms under given hydrodynamic conditions is shown by: k, =a
vRe3i2r3 l2
(5)
where, cy and ~1, are the fractions of adhered particles out of total numbers involved in collisions; Re is the Reynolds number; a and r are the average radii of the carrier mineral and of the fine particles, respectively. From eqs. 5 and 6, assuming cr , = a: k3
a, npRe-3’4(a+f)2(a2+f2) cup, Ir 3 k,=
SHEAR-FLOCCULATION AND CARRIER FLOTATION
275
From this relation the adhesion rate of fines (f is 2.5-5.5/tm ) to coarse (a is 3 0 / t m ) particles (p is 2-3.2) was found to be 103-104 as high as the rate of cohesion between the fines. Such effect was attributed to different mechanisms at work, i.e. diffusion for collision between fines and inertia for collision between fine and coarse particles. For particles < 20/tm under normal agitation in water the collision mechanism being diffusion, the collision rate is given by (Levich, 1962): N, urb = 127rflR3n:o(Eo/v) 1/2
(8)
where N, urb=collision rate per unit volume by turbulent diffusion mechanism, fl= a constant, R = radius of colliding particles, no = initial concentration of particles, Eo= energy loss occurring in the flow per second per unit volume and v = kinematic viscosity. For collisions between particles of different size, R in eq. 8 was substituted by (RI + R 2 ) / 2 and no2 by nl and n:. For different suspensions stirred at the same speed: Nturb ~ (RI +R2)3nl n2
(9)
From this relation the weights of ultrafine and coarse particles required to equalize the collision rates were calculated (Warren, 1975a). By the method of Princen and De Vena-Peplinski (1964) the degree of slime coating may be defined as the ratio of the actual number n of ultrafines (radius R l) on each large particle (radius R2) to the m a x i m u m possible number M that could be fitted on to the surface (surface coverage), where: M=3.63{ (RI + R 2 ) / R 2 } 2
(10)
From the observed turbidity change Warren (1975a) calculated the average number of ultrafines adhering to each large particle. The degree of slime coating and the proportion of slime removed were found to increase with increasing size of the carrier. However, the actual situation deviates from this and aggregation is opposed by a process of re-dispersion, i.e. attrition. The effect of carrier size on the recovery of - 5 / ~ m wolframite was investigated by Hu et al., (1988). With increasing carrier size the fraction of - 5 /~m particles adhering to coarse particles increased but when the carrier size was greater than 2, the microscale of turbulence, the adhesion of fines was found to decrease. From eq. 4 2 was calculated to be 25.2/~m for wolframite and the experimentally observed value was 2 5 - 3 6 / l m . Similarly, the values of 2 with respect to conditioning speed and the carrier size, assuming laminar flow for Reynolds number R e = 1: /
\1/4
(ll)
276
T.V. SUBRAHMANYAM AND K.S.E. FORSSBERG
where e = energy consumption per unit volume within unit time, p = density of fluid, v = kinematic viscosity of the fluid. Since the collision of particles in turbulent motion is influenced by the input energy, the energy can be expressed as fluid velocity (Vr) and the maximum scale of turbulence L; r is the radius of the particle and C is the particle consistency in slurry. The collision rate per unit volume of particles NT, is then:
r3CV3/2
(12)
i.e., there is an increase in collision rate with fluid velocity (Hu et al., 1987 ).
Measurement of particle aggregation The techniques used in general for the measurement of particle aggregation are: ( 1 ) the turbidity of the solution; (2) sedimentation; and (3) the size distribution of the primary particles and the aggregates formed. In addition to these scanning electron micrographs can provide supporting evidence. Sresty and Venkateswar (1980) reviewed the methods of particle size measurement and analysis.
Turbidity The method is based on the principle of transmission or dispersion of light by suspended solid particles in solution. For colourless solutions a normal turbidity meter can be used. In the case of coloured solutions (due to suspended mineral particles) a ratio turbidity meter is used to compensate for the colour. Sufficient care must be taken while measuring the turbidities of solutions containing particles of higher densities, such as scheelite, wolframite, galena, where settling of even individual particles might be mistaken for flocs.
Sedimentation The terminal velocity of a particle settling under gravity, and according to Stokes law is given by: V,_
2 (Ps -Pr)gr 2 9/~
(13)
where: Vt=terminal settling velocity; ps, & = t h e densities of solid and fluid, respectively; g = acceleration due to gravity; r = Stokes radius of the particle; a n d / t = viscosity of the medium.
277
SHEAR-FLOCCULATION AND CARRIER FLOTATION 35 HEMATITE 30
5X'10 -3 30 -4 pH
25
M
M
FEED
I-I 4 0 0
SDS
A
3
rDm
1200
rpm
O 2000
rpm
-
// /
z
i,i
O
~
-NaCI
20
W I..(.9
\
"' .
1o-
"--
/
/I /
o
\
" - . x ZO---.~ ....~ ' l~\
,.
l
1
I
1
1
5
10
20
50
100
PARTICLE
150
SIZE,microns
Fig. 4. Size distribution of primary hematite particles and aggregates produced at different agitation speeds (from Fuerstenau et al., 1988).
Size distribution of the aggregates Figure 4 shows the size distribution of the primary particles and the aggregates formed at different agitation speeds. Similarly the effect of different variables on aggregate formation can be investigated. A simple binocular microscope can be used to determine the cross-sectional size and the shape of the aggregates. FACTORS INFLUENCING SHEAR-FLOCCULATION A N D CARRIER FLOTATION
Among the important variables which influence shear-flocculation and carrier flotation are: the intensity and time of agitation, pulp density, the size of the carrier, proportion of coarse to fine particles, pH, collector concentration, degree of hydrophobicity, surface charge and the cell geometry. These variables may be grouped into physical, chemical and geometrical ones.
Physical variables Table IV shows the energies of particles in suspension due to various interactions. Particle size influences the collision mechanism, the effectiveness and
278
T.V SUBRAHMANYAM AND K.S.E. FORSSBERG
TABLE IV Energies o f particles in suspension due to various interactions, in units o f kT (data o f Warren taken from Healy, 1978 ) Interaction
Particle size, izm 0.1
Van der Waals attraction Electrostatic repulsion Brownian motion Kinetic energy o f sedimentation Kinetic energy o f stirring
~ 10 kT 0-100 1 10-13 ~ 1
1.0 ~ 100 kT 0-1000 1 10- 6 1000
10.0 ~ 1000 kT 0-10 000 1 10 10 6
the number of particle collisions, the last increasing with the particle radii and the relative velocity in the liquid. The effectiveness of collisions also depends on the surface properties of the solids (Samygin et al., 1968 ). At higher agitations the interparticle forces due to electrostatic repulsion, van der Waal's forces and specific chemical interactions leading to an energy barrier between particles are overcome. Once the particles collide the adhesion takes place via hydrophobic association, i.e. due to the overlapping of hydrocarbon chains. Subrahmanyam et al. (1990a,b) observed stable aggregates between coarse ( - 38 + 20/~m) and fine ( - 5 / l m ) galena particles in xanthate solution at 1500 rpm. However, when synthetic PbS (particle size << 5 ¢tm; median size ~ 2/zm) was used in place of - 5 ¢tm galena, the solution turbidities were found to increase and decrease with time, even for prolonged periods of stirring, i.e. both formation and disruption of the aggregates. Such fluctuations in solution turbidities were explained by the variation in the proportion of fines to coarse. Though the weights of - 5/~m galena and synthetic PbS ( ~ 2 /~m) used in the tests were the same, the number of particles in the case of synthetic PbS would be more due to a much smaller size. Either an increase in the proportion of fines to coarse or vice versa, decrease the adhesion of fines to coarse. In the case of the former the coarse particle surface available to fines is reduced; whereas an increase in the proportion of coarse to fines may result in higher collision rates between coarse particles alone, thus resuiting in the detachment of already adhered fines. When the carrier size exceeds the microscale of turbulence, the aggregates break due to attrition and shear forces generated in the liquid (Dianzuo et al., 1988 ). Further, the number of collisions between coarse and fine particles may also decrease if the difference in size is large, i.e. fines may begin to flow past the coarse instead of colliding (Samygin et al., 1968 ). The agitation speeds generally applied are 800-3000 rpm and vary from system to system, for example Samygin et al. ( 1968 ) used impeller speeds of 225 rpm in their flocculation studies. However, factors like particle hydro-
SHEAR-FLOCCULATIONAND CARRIERFLOTATION
279
TABLE V Effect of particle size and stirring speed on aggregation of particles in shear flocculation of scheelite (data taken from Warren, 1975a,b) Particle size (/zm)
Stirring speed (rpm)
Observation
1/tm (0.75-1.4 )
1000 850
7.6-l 1.1 16.6-23.6 23.6-31.2 38.0-44.0 1.0-12.0
850 850 850 850 1700
Aggregation Negligible aggregation after 90 min stirring in 10-4M Na oleate Particles readily aggregated Slight aggregation Negligible aggregation No aggregation Aggregation
Single size fractions
Mixtures of particles 0.75- 1.4 and 16.6-23.6
850
0.75- 1.4 and 7.6-11.1
1700
Ultrafines did not aggregate themselves but adhered to coarse particles ("slime coating" ) Three types of aggregates, i.e. fine-fine, coarsefine and coarse-coarse
phobicity and charge influence the aggregation. At high shear rates, Fuerstenau et al. ( 1988 ) observed breakage of larger flocs. Flocculation was maximum at 1200 rpm and for a low shear rate of 400 rpm there was no aggregate formation (Fig. 4 ). Table V shows the effect of particle size and stirring speed on the aggregation of scheelite particles. Chemical variables The double layer repulsion of two particles depends on the Stern potential which is a function of the electrolyte concentration, and the zeta potential is a good approximation to the Stern potential. The question that remains is with respect to the magnitude of charge and consequently the effect on particle aggregation, since particles are highly negatively charged at high pH's and collector concentrations. In the carrier flotation of wolframite the optimum recovery obtained at pH 6-7 was explained to be due to less electrostatic repulsion and favourable reagent adsorption since wolframite possesses its lowest potential at the pH 6-7 (Hu et al., 1982, 1988). Shouci et al. (1988) investigated the aggregation behaviour of rhodocrosite in sodium oleate with non-polar oil, kerosene. In the range pH 5-11 studied, the maximum aggregation and hydrophobicity of the particles (measured in terms of contact angles) were observed at pH 7-8, where the zeta potentials were higher in comparison to the potential at pH 5. The absence of aggregates of galena particles at a high xanthate concentration (6.94 X 10-aM) w a s explained to be due to
280
T.V. SUBRAHMANYAM AND K.S.E. FORSSBERG
a high negative potential resulting in repulsion. However, the aggregates of galena observed in the absence of xanthate were attributed to hydrophobic products that may have formed as a result of the surface oxidation of the sulphide mineral (Subrahmanyam and Forssberg, 1989, 1990a,b). The degree of sulfur enrichment and the concentration of surface oxides are involved in determining the hydrophobicity (Buckley and Walker, 1988) and the flotation of sulphide minerals, viz. galena, chalcopyrite and pyrrhotite, under oxidizing conditions (Trahar, 1984). For non-sulphide minerals, viz. scheelite, quartz, the aggregates were found to form only in the presence of a collector without which there was no aggregation even under turbulent stirring conditions (Warren, 1975a; Bhaskar Raju et al., 1990). From the foregoing discussion it is evident that the degree of hydrophobicity, surface charge and the hydrodynamic conditions govern the phenomenon of aggregation. Hydrophobicity is a critical factor and the shear-flocculated aggregates can directly be subjected to flotation. The question remains to be answered if the concentration of the collector that is required for shear flocculation is higher than that needed for flotation. Warren (1975a) observed that the minimum collector concentration for slime coatings was higher than that necessary for good flotation of scheelite. The slime coatings were sparse for oleate concentrations below 2 × 10- 5M and substantial above 6 × 10- 5M.
BOTTOM SIDE
(b) LIQUID
BOTTOM SIDE
Fig. 5. Unbaffled tanks with minimum shear rate (from Oldshue, 1978).
SHEAR-FLOCCULATIONAND CARRIERFLOTATION
281
tal
~ .-_... ~,
~BAFFLES 1.,
~ J / ~---~' SIDEVIEW
BOTTOMVIEW
(b)
~ - -
'
p
[
I:
BAFFLES
~/\,1.
I
.\i° ~ i I
tl ~
I
SIDEVIEW Fig. 6. Baffled tanks with m a x i m u m shear rate - top to b o t t o m turnover (from Oldshue, 1978).
In general, with fine particles in the system the collector consumption can be expected to be higher due to the large surface area of fines. Depending upon the mineral-collector system the adsorption of a surfactant on a mineral surface can be physical or chemical. Physical adsorption involves attraction between opposite charges, often leading to neutralization of surface charge. Under such conditions the aggregation can be m a x i m u m at the PZC even for a low stirring speed. The chemical adsorption occurs due to bonding between the surfactant and the surface metal sites in which case a negatively charged surface becomes more negative due to collector adsorption - for example, xanthate adsorption on galena. Higher negative charges between two particles lead to a stronger repulsion and to overcome this energy barrier it becomes necessary to apply higher agitations. At the same time the chemical bonds are comparatively stronger to resist the high energy input without desorption of the surfactant from the mineral surface.
282
T.V. SUBRAHMANYAMAND K.S.E. FORSSBERG
Geometrical variables While collisions between the solid particles and gas bubbles are of primary importance for flotation, in shear-flocculation and carrier flotation, collisions between the fine and coarse particles are of utmost importance for aggregation. The conditions to effect aggregation in shear-flocculation or carrier flotation are not much different from those maintained in a flotation system apart from agitation speed, pulp density, cell geometry. At this stage an important question remains to be answered, i.e. does particle aggregation take place in the conventional flotation process? Subrahmanyam et al. (1990b) tested cells with and without baffles for similar experimental conditions. Particle collisions and consequently their adhesion are effective in baffled cells though very sparse aggregates were also observed in unbaffled cells. The geometry of the mixers with m i n i m u m and m a x i m u m shear rates was described by Perry and Chilton ( 1974 ) and a discussion of the role of mixing in mineral processing operations was given by Oldshue (1978). Figure 5 shows unbaffled cells where swirl and vortex formation are c o m m o n in low viscosity fluids. Baffled cells with radial or axial flow impellers produce top to bottom turnover with m a x i m u m shear rate (Fig. 6). For laboratory investigations a baffled cell with a paddle-type stirrer was used by Warren (1975a). The cell dimensions were 80 m m high and 63 m m in diameter with six baffle plates (each 50 m m × 7 m m X 1.5 m m ) and a single bladed paddle stirrer 14 m m X 25 m m X 3 mm. FLOATABILITY OF AGGREGATES
While the slow recovery rate of fines was due to decreased bubble-particle collisions, at the other extreme that of coarse particles was attributed to the disruption of bubble-particle aggregates in turbulent zones (Morris, 1950; Schultze, 1977 ). One of the reasons for the low flotation rate of coarse particles was that with increasing size the density of the bubble-particle aggregate approaches that of the pulp and thereby the aggregate becomes less buoyant (Jameson et al., 1977). Yet another reason for poor floatability is that with increasing particle size the induction time also increases. When particle size is increased by aggregation the efficiency of floatability of large aggregates, like coarse particles is debatable. In the shear-flocculation of haematite the m a x i m u m aggregate size measured was 125/lm for an agitation speed of 1200 rpm (Fuerstenau et al., 1988 ). With scheelite particles ( 7.6-11.1 p m ) aggregates of 30-50 p m were measured for 90 min stirring at 1700 rpm (Warren, 1975b). The aggregates produced are well within the flotation size range and it was observed by Warren (1975a) that the floatability of coarse particles was unaffected by an attached layer of ultrafine scheelite particles. Further, larger aggregates, even if formed, are unstable and may be
283
SHEAR-FLOCCULATION AND CARRIER FLOTATION
TABLE Vl Comparison between conventional and autogenous carrier flotation (data from Hu et al., 1987, 1988 ) Ore
Grade of Conventional flotation Grade of Autogenous carrier riot. feed (%) feed (%) conc. grade conc. rec. conc. grade conc. rec.
(%)
(%)
0.30
21.84
44.18
Complex oxidized Pb- Pb Zn sulphide ore slime Zn
3.24 5.55
5.20 10.10
Malachite
2.94
Wolframite
Wo3
(%)
(%)
0.31
28.13
59.10
85.40 96.71
3.24 5.53
10.33 20.95
94.20 98.15
7.60
62.95
2.94
20.76
91.43
Cassiterite slime
Sn
0.42
0.59
63.98
0.42
2.76
57.45
Complex low-grade fine-grained Fe ore (hematite)
Fe
34.50
61.23
82.30
34.50
65.60
87.93
Slimes-Fe
Fe
18.70
20.75
84.60
18.70
22.77
86.51
Antimony (flotation of Sb stibnite and depression As of arsenopyrite
6.22 0.58
57.48 2.74
91.20 -
6.53 0.52
56.63 0.35
94.50 -
Refractory, oxidized sulphide amorphous copper ore
2.30
15.00
35.00
2.03
24.94
81.21
Cu
b r o k e n u p d u e to a t t r i t i o n a n d s h e a r forces g e n e r a t e d in the liquid. H o w e v e r , the h y d r o d y n a m i c c o n d i t i o n s in the case o f c o a r s e p a r t i c l e f l o t a t i o n a n d in the f l o t a t i o n o f aggregates f o r m e d b y s h e a r - f l o c c u l a t i o n a n d c a r r i e r f l o t a t i o n are different. T a b l e V I gives t h e r e c o v e r i e s o f s o m e m i n e r a l s b y c o n v e n t i o n a l and by carrier flotation. SUMMARY AND CONCLUSIONS S h e a r - f l o c c u l a t i o n / f l o t a t i o n a n d c a r r i e r f l o t a t i o n are m e t h o d s b y w h i c h p a r t i c l e size is e n h a n c e d b y a g g r e g a t i o n o f fines w i t h c o a r s e p a r t i c l e s p r i o r to f l o t a t i o n to i m p r o v e t h e f l o a t a b i l i t y o f fines. T h e aggregates t h u s f o r m e d are c o n s i d e r e d to h a v e b e t t e r floatabilities. D e s p i t e the b e n e f i c i a l effect, i.e. i m p r o v e m e n t in the f l o t a t i o n r e c o v e r y o f .fines a n d the g r a d e o f t h e c o n c e n t r a t e b y s h e a r - f l o c c u l a t i o n / f l o t a t i o n or b y c a r r i e r f l o t a t i o n , the w o r k o n t h e s e m e c h a n i s m s r e p o r t e d in the l i t e r a t u r e is m e a g r e . Since h y d r o p h o b i c i t y is a critical f a c t o r for p a r t i c l e a g g r e g a t i o n it is o f p a r t i c u l a r i n t e r e s t to e x a m i n e t h e b e h a v i o u r o f s u l p h i d e m i n e r a l s w h i c h e x h i b i t h y d r o p h o b i c i t y in the ab-
284
T.V. SUBRAHMANYAM AND K.S.E. FORSSBERG
sence of collectors. Like the flotation process the mechanisms of shear-flocculation and carrier flotation are governed by physical, chemical and geometrical variables.
REFERENCES Anthony, R.M., Kelsall, D.F. and Trahar, W.J., 1975. The effect of particle size on the activation and flotation ofsphalerite. Proc. Aust. Inst. Min. Metall., 254: 47-58. Bhaskar Raju, G., Subrahmanyam, T.V., Sun, Z. and Forsling, W., 1990. Shear flocculation of quartz. Submitted to Int. J. Miner. Process. Buckley, A.N. and Walker, G.W., 1988. Sulfur enrichment at sulfide mineral surfaces. In: K.S. Eric Forssberg (Editor), Proc. XVI Int. Miner. Process. Congr., Part A: 589-599. Chia, Y.H. and Somasundaran, P., 1983. Carrier flotation ofanatase from clay and its physicochemical mechanisms. In: Malaghan G. Subhas (Editor), Ultrafine Grinding and Separation of Industrial Minerals. SME AIME, 12:117-131. Clement, M., Harms, H. and Trondle, H.M., 1970. Ober das Flotations verhalten verschiedener Mineralanen unter besonderer Beriicksichtigung der Kornfeinheit. Proc. Int. Miner. Process. Congr., 9th, Prague, 1: 179-187. Deryagin, B,V., Samygin, V.D. and Livshits, A.K., 1964. Study of the flocculation of mineral particles under turbulent conditions, 1. Flocculation mechanism. Colloid J., 26:149-154. Dianzuo, W., Guanzhou, Q. and Weibei, H., 1988. The effect of carrier-promoting aggregation of coarse particles. In: A.J. Plumpton (Editor), Proc. Int. Syrup. Production and Processing of Fine Particles, 17:309-316. Engelbrecht, J.A. and Woodburn, E.T., 1975. The effects of froth height, aeration rate and gas precipitation on flotation. J. S. Afr. Inst. Min. Metall., 125-132. Fuerstenau, D.W., 1980. Fine particle flotation. In: P. Somasundaran (Editor), Fine Panicles Processing. Proc. Int. Symp. Fine Panicles Processing, Las Vegas, Vol. 1: 669-705. Fuerstenau, D.W., 1988. Flotation science and engineering: advances and challenges. In: K.S.Eric Forssberg (Editor), Proc. XVI Int. Miner. Process. Congr., Elsevier, Amsterdam, Part A: 63-80. Fuerstenau, D.W., Chander, S. and Abouzeid, A.M., 1978. The recovery of fine particles by physical separation methods. In: P. Somasundaran and N. Arbiter (Editors), Beneficiation of Mineral Fines: Problems and Research Needs. Nat. Sci. Found. Workshop Rep., pp. 359. Fuerstenau, D.W., Li, C. and Hanson, J.S., 1988. Shear flocculation and carrier flotation of fine hematite. In: A.J. Plumpton (Editor), Proc. Int. Symp. Production and Processing of Fine Particles, pp. 329-335. Gaudin, A.M., Groh, J.O. and Henderson, H.B., 1931. Effect of particle size on flotation. Am. Inst. Min. Metall. Eng., Tech. Publ., 414: 3-23. Gaudin, A.M., Schuhmann, R., Jr. and Schlechten, A.W., 1942. Flotation kinetics, II. The effect of size on the behaviour of galena panicles. J. Phys. Chem., 46: 902-910. Gaudin, A.M., Fuerstenau, D.W. and Miaw, H.L., 1960. Slime coatings in galena flotation. Trans. Can. Inst. Min. Metall., 63: 668-671. Goldberger, W.M., 1973. An analysis of technology for improved beneficiation of ultrafine mineral particle systems. Paper presented at Annual General Meeting of Can. Inst. Mining, Vancouver. Goodman, R.H. and Trahar, W.J., 1977. Flotation of cassiterite at the Renison Tin Mine, Renison Bell, Tasmania, Int. Tin Symp., La Paz.
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Greene, E.W. and Duke, J.B., 1962. Selective froth flotation of ultrafine minerals or slimes. Min. Eng., 14: 51-55. Healy, T.W., 1978. Principles of dispersion and aggregation of mineral fines. In: P. Somasundaran and N. Arbiter (Editors), Beneficiation of Mineral Fines: Problems and Research Needs. Nat. Sci. Found. Workshop Rep., pp. 183-196. H u. W., Wang, D. and Huaai, J., 1982. Flotation of wolframite slime particle and technological innovation. In: Proc. XIV Int. Min. Proc. Congress., IV. 10. l-IV. 10.14. Hu, W., Wang, D.Z. and Qu, G., 1987. Principle and application of carrier flotation. J. Cent. South. Inst. Min. metall., 4: 408-414. Hu, W., Wang, D.Z. and Qu, C.Z., 1988. Autogenous carrier flotation. In: K.S. Eric Forssberg (Editor), Proc. XVI Int. Miner. Px~ocess.Congr., Elsevier, Amsterdam, Part A: 445-452. Jameson, G.J., 1984. Physical aspects of fine particle flotation. In: M.H. Jones and J.T. Woodcock (Editors), Principles of Mineral Flotation. The Wark Symp., Ser. 40, pp. 215-232. Jameson, G.J.+ Nam, S. and Young, M.M., 1977. Physical factors affecting recovery rates in flotation. Miner. Sci. Eng., 9(3): 103-118. Levich, V.G., 1962. Physico-Chemical Hydrodynamics, 2nd ed. Prentice-Hall, Englewood Cliffs, N.J., p. 213. Lynch, A.J., Johnson, N.W., Manlapig, E.V. and Thorne, C.G., 1981. Mineral and Coal Flotation Circuits-Their Simulation and Control. Elsevier, Amsterdam. Morris, T.M., 1950. Measurement of equilibrium forces between an air bubble and an attached solid in water. Trans. AIME, 187: 91-95. Oldshue, J.Y., 1978. The role of mixing in beneficiation of mineral fines. In: P. Somasundaran and N. Arbiter (Editors), Beneficiation of Mineral Fines: Problems and Research Needs. Nat. Sci. Found. Workshop Rep., pp. 201-223. Oliver, R.H., 1961. Flocculation - key to more economical solid-liquid separation. Trans. Am. Inst. Min. Eng., 220: 434-443. Perry, R.H. and Chilton, C.H., 1974. Chemical Engineer's Hand Book, 5th ed., 19-6. Princen, L.H. and De Vena-Peplinski, M., 1964. Effect of particle size on the mutual flocculation between zinc oxide and titanium dioxide. J. Colloid Sci., 19: 786-797. Samygin, V.D., Barskii, A.A. and Angelova, S.M., 1968. Mechanism of mutual flocculation of particles differing in size. Colloid J., 30: 435-439. Sastry, K.V.S., 1978. Flotation of mineral fines. In: P. Somasundaran and N. Arbiter (Editors), Beneficiation of Mineral Fines: Problems and Research Needs. Nat. Sci. Found. Workshop Rep., pp. 283-290. Schultze+ H.J., 1977. New theoretical and experimental investigations on stability of bubble/ particle aggregates in flotation: a theory on the upper particle size of floatability. Int. J. Miner. process., 4: 241-259. Sennett, P. and Young, R.H., 1978. Current problems in beneficiation of kaolin clay. In: P. Somasundaran and N. Arbiter (Editors), Beneficiation of Mineral Fines: Problems and Research Needs. Nat. Sci. Found. Workshop Rep., pp. 115-133. Shouci, L. and Zongfu, D., 1988. Separation of ultrafine mineral particles by hydrophobic aggregation methods. In: A.J. Plumpton (Editor), Proc. Int. Symp. Production and Processing of Fine Particles, 17: 317-327. Shouci, L., Shaoxian, S. and Zongfu, D., 1988. The hydropbobic and magnetic combined aggregation of paramagnetic minerals. In: K.S. Eric Forssberg (Editor), Proc. XVI Int. Miner. Process. Congr. Elsevier, Amsterdam, Part A: 999-1009. Somasundaran, P., 1976. Fine panicles treatment. In: P. Somasundaran and D.W. Fuerstenau (Editors), Research Needs in Mineral Processing. Nat. Sci. Found. Workshop Rep. Somasundaran, P., 1978. Principles of selective aggregation. In: P. Somasundaran and N. Arbiter (Editors), Beneficiation of Mineral Fines: Problems and Research Needs. Nat. Sci. Found. Workshop Rep., pp. 183-196.
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Sresty, G.C. and Venkateswar, R., 1980. Particle size analysis - a review. In: P. Somasundaran (Editor), Fine Particles Processing. Proc. Int. Symp. Fine Particles processing, Las Vegas, Vol. 1: 329-365. Stratton-Crawley, R., 1978. Oil flotation: two liquid flotation techniques. In: P. Somasundaran and N. Arbiter (Editors), Beneficiation of Mineral Fines: Problems and Research Needs. Nat. Sci. Found. Workshop Rep., pp. 317-329. Subrahmanyam, T.V. and Forssberg, K.S.E., 1988a. Froth stability, particle entrainment and drainage in flotation - a review. Int. J. Miner. Process., 23: 33-53. Subrahmanyam, T.V. and Forssberg, K.S.E., 1988b. A study of particle entrainment in flotation with different frothers - The case of copper ore. In: K.S. Eric Forssberg (Editor), Proc. XVI Int. Miner. Process. Congr., Elsevier, Amsterdam, Part A: 785-796. Subrahmanyam. T.V. and Forssberg, K.S.E., 1988c. Frother performance in flotation of copper and lead-zinc ores. Trans. Ins. Min. Metall., Sect. C, Miner. Process. Extr. Metall., 97:C134142. Subrahmanyam, T.V. and Forssberg, K.S.E., 1988d. Froth characteristics and grade-recovery relationships in the flotation of lead-zinc and copper ores. Miner. Eng., 1( 1 ): 41-52. Subrahmanyam, T.V. and Forssberg, K.S.E., 1989. Carrier flotation of galena. In: Proceedings of Konferens i Mineralteknik, Hogskolan i Lulea, Lulea (Sweden), 61-73. Subrahmanyam, T.V., Luz, G.R. and Muzzi, R.M.M., 1987. Arraste na Flotaq~o. Brasil Miner., 48:83-88 (in Portuguese). Subrahmanyam. T.V., Sun, Z., Forssberg, K.S.E. and Forsling, W., 1990a. Shear flocculation and flotation of galena and synthetic PbS. Int. Symp. Advances in Fine Particles processing. Fine particles Society, Boston, Mass., 22-26 August, 1989. In: J. Hanna and Y.A. Airia (Editors), Advances in Fine Particle Processing. Elsevier, Amsterdam, pp. 269-278. Subrahmanyam, T.V., Sun, Z., Forssberg, K.S.E. and Forsling, W., 1990b. Variables in the shear flocculation of galena. In: Sulphide Mineral Deposits: Their Origin and Processing. Int. Min. Metall., London. Trahar, W.J., 1976. The selective flotation of galena from sphalerite with special reference to the effect of particle size. Int. J. Miner. Process., 3:151-166. Trahar, W.J., 1981. A rational interpretation of the role of particle size in flotation. Int. J. Miner. Process., 8: 289-327. Trahar, W.J., 1984. The influence of pulp potential in sulphide flotation. In: M.H. Jones and J.T. Woodcock (Editors), Principles of Mineral Flotation. The Wark Symposium, Aust. Inst. Min. Metall., Melbourne, pp. 117-135. Trahar, W.J. and Warren, L.J., 1976. The floatability of very fine particles - a review. Int. J. Miner. Process,, 3:103-131. Tyler, P.M. and Waggman, W.H., 1953. Report on Possible Utilization of Phosphate Rock Slimes. National Academy of Science, MMAB Report, 45-C. Warren, L.J., 1975a. Slime coating and shear flocculation in the scheelite-sodium oleate system. Trans. Inst. Min. Metall., Sect. C, Min. Proc. Ext. Metall., 84: C99-104. Warren, L.J., 1975b. Shear flocculation of ultrafine scheelite in sodium oleate solutions. J. Colloid Interface Sci., 50 (2): 307-318. Warren, L.J., 1984. Ultrafine particles in flotation. In: M.H. Jones and J.T. Woodcock (EdiIors), Principles of Mineral Flotation. The Wark Symposium, Ser. 40, pp. 185-213. Warren, L.J., 1985. Determination of the contributions of true flotation and entrainment in batch flotation tests. Int. J. Miner. Process., 14: 33-44. Yang, C.D., 1978. Flotation in systems with controlled dispersion-carrier flotation etc. In: P. Somasundaran and N. Arbiter (Editors), Beneficiation of Mineral Fines: Problems and Research Needs. Nat. Sci. Found. Workshop Rep., pp. 295-308.
265
Elsevier Science Publishers B.V., Amsterdam
Fine particles processing: shear-flocculation and carrier flotation - a review T.V. Subrahmanyama and K.S.Eric Forssberg b aDepartamento de Geologia, CCE/UFRN, Campus Universitario, 59. 072 Natal-RN, Brazil bDivision of Mineral Processing, Luled University of Technology, S-951 87 Lule& Sweden (Received October 9, 1989; accepted after revision July 2, 1990)
ABSTRACT
Subrahmanyam, T.V. and Forssberg, K.S.E., 1990. Fine particles processing: shear-flocculation and carrier flotation - a review. Int. J. Miner. Process., 30: 265-288. Shear-flocculation/flotation and carrier flotation are methods by which fine mineral values are aggregated and then recovered by flotation. The aggregates formed have better floatabilities. Without prior treatment ahead of flotation, significant quantities of fines are lost in tailings. An overview of the problems associated with fine particles is presented. Like froth flotation, the mechanisms ofshearflocculation and carrier flotation are governed by physical, chemical and geometrical variables. The present paper deals with the effects of these variables on shear-flocculation and carrier flotation, based on a detailed literature review.
INTRODUCTION
Froth flotation is one of the most important methods for the separation of minerals. With the depletion of high grade ores it becomes necessary to develop efficient methods for treating low grade ores, which often involves handling and processing of fine particles. It was estimated that 1/ 3 of phosphate (Tyler and Waggman, 1953 ), 1/6 of copper (Goldberger, 1973 ), 1/ 10 of iron explored in the U.S., 1/2 of the tin produced in Bolivia and 1/5 of the tungsten mined in the world (Somasundaran, 1976 ) and millions of tons of other minerals are lost in the form of fines. The gravity of the problem of fine particles and the emergent necessity for suitable processing methods to recover the valuable components from fines has resulted in a large number of publications and symposia. Fuerstenau (1988) rightly concludes: "To meet the economic challenges facing the mineral industry worldwide, it is important that industry applies novel concepts to increase efficiency and reduce costs of mineral producing operations. Research and development in flotation science and engineering have a definite role in the efforts for production improvement. Enhancement in grade and recovery and reduction of costs in 0301-7516/90/$03.50
© 1990 - - Elsevier Science Publishers B.V.
266
r . v . SUBRAHMANYAM AND K.S.E. FORSSBERG
flotation plants can be brought about by research on comminution, fine particle recovery, flotation reagent development, novel developments in flotation machines, the design of new flotation circuits and the development of automatic control technology..." Several papers (Fuerstenau et al., 1978; Sennett and Young, 1978; Somasundaran, 1978; Sastry, 1978; Yang, 1978; Stratton-Crawley, 1978 ) deal with fine particles processing. Among the modified flotation techniques are: high temperature flotation, column flotation, oil flotation, liquid-liquid extraction, agglomerate flotation/floto-flocculation, electroflotation, vaccum or pressure release flotation, precipitate flotation, selective flocculation, flotation with controlled dispersion, spherical agglomeration, shear-flocculation/ flotation and carrier flotation. The scope of the present paper is restricted to the recovery of ultrafine and fine particles ,-. 5-10/~m by shear-flocculation/ flotation and carrier flotation. In these methods the principal objective is to enhance the particle size by uniting the particles so that the aggregates formed may have better floatabilities. Depending on the mechanism involved earlier workers referred to ultra- or piggyback flotation (Green and Duke, 1962; Deryagin et al., 1964; Samygin et al., 1968), floc-flotation (Gaudin et al., 1942; Clement et al., 1970); or to autogenous carrier- or ramification carrier flotation (Hu et al., 1987, 1988). FINE PARTICLES IN FLOTATION CIRCUITS AND PROBLEMS: AN OVERVIEW
The influence of particle size on the rate of recovery of minerals was investigated by several workers (Gaudin et al., 1931; Anthony et al., 1975; Trahar, 1976 ). In general, fine particles < 10#m in size have low collision efficiencies with gas bubbles and are accessible to mechanical entrainment. The entrainment mechanism does not distinguish between hydrophilic or hydrophobic particles and is closely related to water recovery. Earlier workers (Engelbrecht and Woodburn, 1975; Goodman and Trahar, 1977; Lynch et al., 1981; Warren, 1985; Subrahmanyam and Forssberg, 1988a,b,c) observed a linear relationship between the water and mineral recoveries. Table I gives the degree of entrainment for different minerals. It can be observed that with decreasing particle size the entrainment becomes the dominant mechanism of particle collection. Figure 1 illustrates the adverse effects of the presence of fine particles in flotation circuits. Due to the small mass and large surface area of fines, problems like high reagent consumption, non-selectivity of the collector, excess froth stability, mechanical entrainment of fines, etc., affect the recovery and grade of the concentrate product. Several papers (Trahar and Warren, 1976; Fuerstenau, 1980; Jameson, 1984; Warren, 1984; Subrahmanyam and Forssberg, 1988a) deal with the problems associated with fine particles. In the flotation size range particles have better floatabilities due to colli-
SHEAR-FLOCCULATIONAND CARRIERFLOTATION
267
TABLE I Degree of entrainment for different minerals (from Subrahmanyam and Forssberg, 1988b ) Mineral
Spec. gray.
Particle size (/tin)
Degree of entrainment
Reference
Quartz
2.65
3.5 >40.0
0.72 0.10
Trahar ( 1981 ) Engelbrecht and Woodburn (1975)
< 12.0
0.99
Engelbrecht and Woodburn ( 1975 )
< 5.0
0.85
Goodman and Trahar ( 1977 )
< 38.0
1.00
Lynch et al. ( 1981 )
0.87
Warren ( 1985 )
0.78
Subrahmanyam and Forssberg ( 1988c )
Silica Cassiterite
6.80-7.10
Coal
1.00
Ultrafine gangue Fine gangue
< 40.0
sions with gas bubbles. The size range for floatability may vary with the mineral and is also dependent upon the magnitude of operation - batch, laboratory or industrial. Table II shows the size ranges of flotation recovery for different minerals. Below the minimum (fines) and above the maximum (coarse) sizes the flotation recoveries start to decrease.
FTTC. REAGENTI I I HIGHSURFACE] ,
ENERGY
~ --
_
HIGH
/
I~
-->lO SSOLUTION (G~R)I
\
[LbWMOMENTUM/ IFINEPARTICLE L
~
EIENTRAINMENTIG) r'-
SMALL MASS
~h"~HIC-H~ ~/
FROTH /
\\FTAO'-'Z~NC~RII ,
/
1
l STAa"-,TY Ix
~"\.~_J C O A G U L A O T IN I~-----~/ L O J l WC O L S L O IINL l
I
I RATEIR)
J
LPROBABILITY F
Fig. 1. Relationship between the physical and chemical properties of fine particles and their behaviour in flotation. ( G ) and ( R ) refer to whether the phenomena affect grade a n d / o r recovery. The arrows indicate the various factors contributing to a particular phenomenon observed in flotation of fine particles (from Fuerstenau, 1980).
268
T.V. SUBRAHMANYAMANDK.S.E.FORSSBERG
TABLE II Observed size ranges of flotation recovery for different minerals as reported by different workers (from Trahar and Warren, 1976) Mineral
Size range (~m)
Barytes
10- 30
Cassiterite
3- 20
Conditions
Reference
laboratory, batch
Clement and Klossel ( 1963 )
industrial
Kelsall et al. (1974)
Fluorite
40-110 10- 90 50-150
laboratory, batch industrial industrial
Klassen and Mokrousov ( 1963, p. 390) Klassen and Mokrousov ( 1963, p. 391 ) Lay and Bell (1962)
Galena
37-295 170-240 7- 70 6- 70 13- 75 20-100
laboratory, batch laboratory, batch industrial industrial industrial industrial
Gaudin et al. ( 1931 ) Klassen and Mokrousov ( 1963, p. 391 ) Cameron et al. (1974) Kelsall et al. (1974) Klassen and Mokrousov ( 1963, p. 391 ) Lynch and Thorne (1974)
Pyrite
50-100
laboratory, continuous
Imaizumi and Inoue (1965)
Pyrite-pyrrhotite
20- 70
laboratory, batch
Morris (1952)
Quartz
10- 40 9- 50
laboratory, continuous laboratory, batch
De Bruyn and Modi (1956) Robinson (1950)
Sphalerite
15-100 8- 70
industrial, laboratory batch
Cameron et al. ( 1971 ) Anthonyetal. (1975)
Wolframite
20- 50
laboratory, batch
Clement et al. (1966)
References: see original publication. In view of the poor floatabilities of fines their treatment prior to flotation becomes essential for improving the recoveries. AGGREGATION OF PARTICLES W h e n i n d i v i d u a l p a r t i c l e s are b o u n d t o g e t h e r o r u n i t e d t h e r e s u l t i n g m a s s o r b o d y is a n a g g r e g a t e . A d e s c r i p t i o n o f t h e t e r m s is g i v e n a l o n g w i t h s o m e d e t a i l s in t h e p r e s e n t s e c t i o n in o r d e r t o d i s t i n g u i s h t h e m e c h a n i s m s o f p a r t i cle a g g r e g a t i o n . F o r g e n e r a l i n f o r m a t i o n T a b l e I I I g i v e s t h e h y d r o p h o b i c aggregation separation methods.
Coagulation and flocculation In coagulation simple inorganic electrolytes are used to aggregate the par-
rhodocrosite ( < 10/tm )
Combined aggregation separation
References can be found in original publication.
oxides of iron and titanium
ilmenite ( < 10#m) hematite ( < I/zm)
Magnetic adhesion separation
baryte (mostly < 7/~m)
Sphere agglomerationseparation Two-liquid extraction
martite
Hospaton-18 Hospaton-21 Medialan- 16
manganese ore
Emulsion flotation
Magnetic seed separation
sodium oleate
anatase (90-94% < 2/tin )
Carrier flotation
sodium oleate
fatty acid
sodium oleate
tall oil, petroleum sulphonate
tall oil
sodium oleate
scheelite (90% 0.75-1.4 /lm )
Shear flocculationflotation
Collector/reagent
Materials treated
Process
kerosene
non-polar oil
non-polar oil
fuel oil
kerosene
petroleum
diesel oil
Non-polar oil
Hydrophobic Aggregation Separation Methods (from Shouci and Zongfu, 1988)
TABLE 111
2050 rpm
> 780 rpm
> 100 Kw-h/ short ton
1600-2000 rpm
850 rpm 1700 rpm
intensity
Agitation
screening
flotation
flotation
flotation
10-15
desliming in magnetic field
magnetic separation
magnetic separation
phase Fayed (1965) separation
30
90 20
time ( min )
Separation method
Lu et al. (1988)
Tihonov and Babushkina (1979)
Xin and Xu ( 1983 )
Takamori et al. (1980)
Gate (1957)
Green and Duke ( 1962 )
Warren ( 1975 )
Reference
"~ "~
t" O
~
¢3
7
O
.~
270
T.V. SUBRAHMANYAM
AND K.S.E. FORSSBERG
ticles. Flocculation is generally effected by polymer reagents which bind the particles by a bridging mechanism. Oliver ( 1961 ) discussed the types of flocculants and applications in various industries dealing with solid-liquid separations. In spontaneous flocculation processes the energy barrier between particles is of the order of kT and flocculation occurs without any energy input. Princen and De Vena-Peplinski (1964) observed the mutual flocculation of two oppositely charged hydrophobic colloids, ZnO and TiO2, suspended in water, to be essentially dependent upon the particle size and not on the pH and time. Further, mutual flocculation was found to occur even when the sign of charge was the same.
Floc flotation In floc flotation the formation of floes was found to be a pre-requisite for the flotation of fines from a mixture ofhaematite and quartz in a tall oil emulsion. The floes were formed in oleic acid but not in sodium oleate and it was attributed to the molecular form of oleic acid (Clement et al., 1970).
Slime coatings In flotation systems where particles are oppositely charged, the problem of slime coatings may affect the flotation behaviour of the valuable mineral for example, the flotation of galena in the presence of alumina slimes (Fig. 2 ). Below pH 10, alumina is positively charged and forms slime coatings on negatively charged galena (Gaudin et al., 1960). In this example, when the Ioo o c
>: cr
60 t.) w
,-,z 40
GALENA Z5 mgll. KEX
o_
I-0 -J ~-
ALUMINA SLIME o 0 g/I. • 0.1
20
o
i 0
6
0.2
• r'l
0.3 0.5
I
I
2
I 4
~
I
1
6
8
i
I I0
,
I 12
I 14
pH
Fig. 2. Flotation recovery vs. pH for galena floated in the presence of various amounts of alumina slimes (from Gaudin et al., 1960).
SHEAR-FLOCCULATION AND CARRIER FLOTATION
271
repulsive force due to like charges was greater than the attractive van der Waal's force, slime coatings did not form.
Shear-flocculation In shear-flocculation the energy barrier resulting in repulsion of similarly charged hydrophobic particles is overcome by intense stirring. The energy barrier in this case is much greater than kT. A direct contact due to collision between particles is favoured by an energy of hydrophobic association (Warren, 1975a). For the formation of hydrophobic aggregates the hydrophobic interaction energy is a significant factor since it is a few hundred times higher than the energy of molecular or electrostatic repulsive energy. From the potential energy values calculated as per the DLVO theory and the hydrophobic interaction theory, it has been shown by Shouci and Zongfu ( 1988 ) that for hydrophobic rhodocrosite the potential energy begins to decrease for interparticle distances of 8 nm, assuming negative values for distances less than 6-7 nm. In the context of shear-flocculation, Warren (1975a) refers to slime coatings which involved the adhesion of fines to coarse particles, both similarly charged and collector-coated. This is a different mechanism of slime coatings from the one referred to by Gaudin et al. (1960) (see Fig. 2 ).
Carrier flotation and other similar methods Among early investigations, Green and Duke (1962) were the first to recognise the beneficial effect of the use of a coarse ( ~ 325 mesh) reagentized carrier mineral, in the present context calcite, for the separation of anatase impurity from kaolin clay. Other carrier minerals tested were ground silica, fluorspar, kyanite, anatase and sulphur. The mechanism of calcite-anatase aggregation was explained to be due to the preferential adsorption and stronger binding of oleate to calcite and anatase and not to kaolinite (Chia and Somasundaran, 1983 ). In autogenous- or ramification carrier flotation (Hu et al., 1987, 1988 ) the same pre-conditioned coarse mineral particle serves as a carrier (Fig. 3 ). Among the aggregation methods described, 'the common or critical requirements for shear-flocculation, carrier flotation or autogenous carrier flotation are: ( 1 ) presence of coarse particles; (2) use of higher energy agitation; and ( 3 ) hydrophobicity of the carrier (coarse) and the carried (fine) particles. MECHANISMSOF SHEAR-FLOCCULATIONAND CARRIER FLOTATION A pulp containing ultrafine to fine ( ,-, 5/~m) and coarse ( ~ 40/~m) particles when agitated at high speeds, forms three types of aggregates, viz., fine-
272
T.V. SUBRAHMANYAM AND K.S.E. FORSSBERG
fine, fine-coarse and coarse-coarse. The first two types of aggregates decrease the turbidity of the solution caused by the fines in suspension. The rate of decrease of the number of single particles in the absence (eq. 1 ) and in the presence (eq. 2) of carrier mineral is given by (Samygin et al., 1968): dn dt = -2k,
n o2
( 1)
and
drt
--=
dt
_ 2k, nXo_ 2 k 3 n o n b
UNEVENLY
(a)
(2)
DISSEMINATED
ORE
1
CLASSIFICATION
SLIME
SAND FLOTATION
CONCENTRATE
TAILI NG
AUTOGENOUS CARRIER FLOTATION
COMPLEX, OXIDIZED, SULPHIDE
ORE
(b) OXIDE
PART
DESLIMING
SAN D
OX I DE FLOTATION
SULPHIDE
SULPHIDE
S L IME l
PART
FLOTATION
CONCENT RATE CARRIER
AUTOGE NOUS CARRIER FLOTATION
Fig. 3. Autogenous carrier flotation (from Hu et al., 1988).
TA I L I N G
SHEAR-FLOCCULATION ANDCARRIERFLOTATION COMPLEX Fe (C)
273
ORE
1 MAGNETIC
SEPARATION
1 MAGNETtTE
WEAK MAGNETIC
NI
PART HEMATITE
CONCE RATE USED AS CARRIER
I
AUTOGENOUS ILCARRIER FLOTATION
REFRACTORY, OXIDIZED, SULPHIDE AMORPHOUS Cu ORE
(d)
1 SULPHIDE FLOTATION
SULPHIDE CONCiNTRATE
OXIDE 1
PART USED AS CARRIER
LEACHING l CuS PRECIPITATION
1
It-
AUTOGENOUS CARRIER FLOTATION
Fig. 3. Continued.
where no and nb are the concentrations of fines and the carrier mineral, respectively; k~ and k3 refer to fine and large particles, respectively, and are the rate constants for mutual cohesion of the fine particles and for adhesion of fines to larger particles. Two types of collision mechanisms are predominant depending on the particle size, i.e. diffusion, when the particle size is less than
274
T.V. SUBRAHMANYAM
AND
K.S.E.
FORSSBERG
the internal turbulence scale and inertia, in the case of larger particles in polydisperse suspensions. The number of particle collisions increases with the particle radii and the relative velocity in the liquid. The relationship between the scale of turbulence (A) and particle size is given by, (3) where, k, and k, are the total number of collisions between the particles in unit volume per unit time by the inertia and diffusion mechanisms respectively; d is the average particle diameter; p and p. are the densities of the mineral particles and the liquid. A=
1
(4)
where, 1 is the diameter of the chamber; R is the radius of the impeller; N is the impeller rotation speed and v is the kinematic viscosity. For the conditions maintained and considering ki/kd = 1, i.e. for equal probabilities of inertia and diffusion mechanisms, the value of A was calculated to be 33.5 pm (Samygin et al., 1968). However, since the particle size of the coarse mineral used in their experiments was 60 pm, inertia was stated to be the predominant collision mechanism between fine and coarse particles. Because of their greater mass the larger particles move relative to the liquid with tine particles suspended in it and collide with them. The relative significance of the two mechanisms under given hydrodynamic conditions is shown by: k, =a
vRe3i2r3 l2
(5)
where, cy and ~1, are the fractions of adhered particles out of total numbers involved in collisions; Re is the Reynolds number; a and r are the average radii of the carrier mineral and of the fine particles, respectively. From eqs. 5 and 6, assuming cr , = a: k3
a, npRe-3’4(a+f)2(a2+f2) cup, Ir 3 k,=
SHEAR-FLOCCULATION AND CARRIER FLOTATION
275
From this relation the adhesion rate of fines (f is 2.5-5.5/tm ) to coarse (a is 3 0 / t m ) particles (p is 2-3.2) was found to be 103-104 as high as the rate of cohesion between the fines. Such effect was attributed to different mechanisms at work, i.e. diffusion for collision between fines and inertia for collision between fine and coarse particles. For particles < 20/tm under normal agitation in water the collision mechanism being diffusion, the collision rate is given by (Levich, 1962): N, urb = 127rflR3n:o(Eo/v) 1/2
(8)
where N, urb=collision rate per unit volume by turbulent diffusion mechanism, fl= a constant, R = radius of colliding particles, no = initial concentration of particles, Eo= energy loss occurring in the flow per second per unit volume and v = kinematic viscosity. For collisions between particles of different size, R in eq. 8 was substituted by (RI + R 2 ) / 2 and no2 by nl and n:. For different suspensions stirred at the same speed: Nturb ~ (RI +R2)3nl n2
(9)
From this relation the weights of ultrafine and coarse particles required to equalize the collision rates were calculated (Warren, 1975a). By the method of Princen and De Vena-Peplinski (1964) the degree of slime coating may be defined as the ratio of the actual number n of ultrafines (radius R l) on each large particle (radius R2) to the m a x i m u m possible number M that could be fitted on to the surface (surface coverage), where: M=3.63{ (RI + R 2 ) / R 2 } 2
(10)
From the observed turbidity change Warren (1975a) calculated the average number of ultrafines adhering to each large particle. The degree of slime coating and the proportion of slime removed were found to increase with increasing size of the carrier. However, the actual situation deviates from this and aggregation is opposed by a process of re-dispersion, i.e. attrition. The effect of carrier size on the recovery of - 5 / ~ m wolframite was investigated by Hu et al., (1988). With increasing carrier size the fraction of - 5 /~m particles adhering to coarse particles increased but when the carrier size was greater than 2, the microscale of turbulence, the adhesion of fines was found to decrease. From eq. 4 2 was calculated to be 25.2/~m for wolframite and the experimentally observed value was 2 5 - 3 6 / l m . Similarly, the values of 2 with respect to conditioning speed and the carrier size, assuming laminar flow for Reynolds number R e = 1: /
\1/4
(ll)
276
T.V. SUBRAHMANYAM AND K.S.E. FORSSBERG
where e = energy consumption per unit volume within unit time, p = density of fluid, v = kinematic viscosity of the fluid. Since the collision of particles in turbulent motion is influenced by the input energy, the energy can be expressed as fluid velocity (Vr) and the maximum scale of turbulence L; r is the radius of the particle and C is the particle consistency in slurry. The collision rate per unit volume of particles NT, is then:
r3CV3/2
(12)
i.e., there is an increase in collision rate with fluid velocity (Hu et al., 1987 ).
Measurement of particle aggregation The techniques used in general for the measurement of particle aggregation are: ( 1 ) the turbidity of the solution; (2) sedimentation; and (3) the size distribution of the primary particles and the aggregates formed. In addition to these scanning electron micrographs can provide supporting evidence. Sresty and Venkateswar (1980) reviewed the methods of particle size measurement and analysis.
Turbidity The method is based on the principle of transmission or dispersion of light by suspended solid particles in solution. For colourless solutions a normal turbidity meter can be used. In the case of coloured solutions (due to suspended mineral particles) a ratio turbidity meter is used to compensate for the colour. Sufficient care must be taken while measuring the turbidities of solutions containing particles of higher densities, such as scheelite, wolframite, galena, where settling of even individual particles might be mistaken for flocs.
Sedimentation The terminal velocity of a particle settling under gravity, and according to Stokes law is given by: V,_
2 (Ps -Pr)gr 2 9/~
(13)
where: Vt=terminal settling velocity; ps, & = t h e densities of solid and fluid, respectively; g = acceleration due to gravity; r = Stokes radius of the particle; a n d / t = viscosity of the medium.
277
SHEAR-FLOCCULATION AND CARRIER FLOTATION 35 HEMATITE 30
5X'10 -3 30 -4 pH
25
M
M
FEED
I-I 4 0 0
SDS
A
3
rDm
1200
rpm
O 2000
rpm
-
// /
z
i,i
O
~
-NaCI
20
W I..(.9
\
"' .
1o-
"--
/
/I /
o
\
" - . x ZO---.~ ....~ ' l~\
,.
l
1
I
1
1
5
10
20
50
100
PARTICLE
150
SIZE,microns
Fig. 4. Size distribution of primary hematite particles and aggregates produced at different agitation speeds (from Fuerstenau et al., 1988).
Size distribution of the aggregates Figure 4 shows the size distribution of the primary particles and the aggregates formed at different agitation speeds. Similarly the effect of different variables on aggregate formation can be investigated. A simple binocular microscope can be used to determine the cross-sectional size and the shape of the aggregates. FACTORS INFLUENCING SHEAR-FLOCCULATION A N D CARRIER FLOTATION
Among the important variables which influence shear-flocculation and carrier flotation are: the intensity and time of agitation, pulp density, the size of the carrier, proportion of coarse to fine particles, pH, collector concentration, degree of hydrophobicity, surface charge and the cell geometry. These variables may be grouped into physical, chemical and geometrical ones.
Physical variables Table IV shows the energies of particles in suspension due to various interactions. Particle size influences the collision mechanism, the effectiveness and
278
T.V SUBRAHMANYAM AND K.S.E. FORSSBERG
TABLE IV Energies o f particles in suspension due to various interactions, in units o f kT (data o f Warren taken from Healy, 1978 ) Interaction
Particle size, izm 0.1
Van der Waals attraction Electrostatic repulsion Brownian motion Kinetic energy o f sedimentation Kinetic energy o f stirring
~ 10 kT 0-100 1 10-13 ~ 1
1.0 ~ 100 kT 0-1000 1 10- 6 1000
10.0 ~ 1000 kT 0-10 000 1 10 10 6
the number of particle collisions, the last increasing with the particle radii and the relative velocity in the liquid. The effectiveness of collisions also depends on the surface properties of the solids (Samygin et al., 1968 ). At higher agitations the interparticle forces due to electrostatic repulsion, van der Waal's forces and specific chemical interactions leading to an energy barrier between particles are overcome. Once the particles collide the adhesion takes place via hydrophobic association, i.e. due to the overlapping of hydrocarbon chains. Subrahmanyam et al. (1990a,b) observed stable aggregates between coarse ( - 38 + 20/~m) and fine ( - 5 / l m ) galena particles in xanthate solution at 1500 rpm. However, when synthetic PbS (particle size << 5 ¢tm; median size ~ 2/zm) was used in place of - 5 ¢tm galena, the solution turbidities were found to increase and decrease with time, even for prolonged periods of stirring, i.e. both formation and disruption of the aggregates. Such fluctuations in solution turbidities were explained by the variation in the proportion of fines to coarse. Though the weights of - 5/~m galena and synthetic PbS ( ~ 2 /~m) used in the tests were the same, the number of particles in the case of synthetic PbS would be more due to a much smaller size. Either an increase in the proportion of fines to coarse or vice versa, decrease the adhesion of fines to coarse. In the case of the former the coarse particle surface available to fines is reduced; whereas an increase in the proportion of coarse to fines may result in higher collision rates between coarse particles alone, thus resuiting in the detachment of already adhered fines. When the carrier size exceeds the microscale of turbulence, the aggregates break due to attrition and shear forces generated in the liquid (Dianzuo et al., 1988 ). Further, the number of collisions between coarse and fine particles may also decrease if the difference in size is large, i.e. fines may begin to flow past the coarse instead of colliding (Samygin et al., 1968 ). The agitation speeds generally applied are 800-3000 rpm and vary from system to system, for example Samygin et al. ( 1968 ) used impeller speeds of 225 rpm in their flocculation studies. However, factors like particle hydro-
SHEAR-FLOCCULATIONAND CARRIERFLOTATION
279
TABLE V Effect of particle size and stirring speed on aggregation of particles in shear flocculation of scheelite (data taken from Warren, 1975a,b) Particle size (/zm)
Stirring speed (rpm)
Observation
1/tm (0.75-1.4 )
1000 850
7.6-l 1.1 16.6-23.6 23.6-31.2 38.0-44.0 1.0-12.0
850 850 850 850 1700
Aggregation Negligible aggregation after 90 min stirring in 10-4M Na oleate Particles readily aggregated Slight aggregation Negligible aggregation No aggregation Aggregation
Single size fractions
Mixtures of particles 0.75- 1.4 and 16.6-23.6
850
0.75- 1.4 and 7.6-11.1
1700
Ultrafines did not aggregate themselves but adhered to coarse particles ("slime coating" ) Three types of aggregates, i.e. fine-fine, coarsefine and coarse-coarse
phobicity and charge influence the aggregation. At high shear rates, Fuerstenau et al. ( 1988 ) observed breakage of larger flocs. Flocculation was maximum at 1200 rpm and for a low shear rate of 400 rpm there was no aggregate formation (Fig. 4 ). Table V shows the effect of particle size and stirring speed on the aggregation of scheelite particles. Chemical variables The double layer repulsion of two particles depends on the Stern potential which is a function of the electrolyte concentration, and the zeta potential is a good approximation to the Stern potential. The question that remains is with respect to the magnitude of charge and consequently the effect on particle aggregation, since particles are highly negatively charged at high pH's and collector concentrations. In the carrier flotation of wolframite the optimum recovery obtained at pH 6-7 was explained to be due to less electrostatic repulsion and favourable reagent adsorption since wolframite possesses its lowest potential at the pH 6-7 (Hu et al., 1982, 1988). Shouci et al. (1988) investigated the aggregation behaviour of rhodocrosite in sodium oleate with non-polar oil, kerosene. In the range pH 5-11 studied, the maximum aggregation and hydrophobicity of the particles (measured in terms of contact angles) were observed at pH 7-8, where the zeta potentials were higher in comparison to the potential at pH 5. The absence of aggregates of galena particles at a high xanthate concentration (6.94 X 10-aM) w a s explained to be due to
280
T.V. SUBRAHMANYAM AND K.S.E. FORSSBERG
a high negative potential resulting in repulsion. However, the aggregates of galena observed in the absence of xanthate were attributed to hydrophobic products that may have formed as a result of the surface oxidation of the sulphide mineral (Subrahmanyam and Forssberg, 1989, 1990a,b). The degree of sulfur enrichment and the concentration of surface oxides are involved in determining the hydrophobicity (Buckley and Walker, 1988) and the flotation of sulphide minerals, viz. galena, chalcopyrite and pyrrhotite, under oxidizing conditions (Trahar, 1984). For non-sulphide minerals, viz. scheelite, quartz, the aggregates were found to form only in the presence of a collector without which there was no aggregation even under turbulent stirring conditions (Warren, 1975a; Bhaskar Raju et al., 1990). From the foregoing discussion it is evident that the degree of hydrophobicity, surface charge and the hydrodynamic conditions govern the phenomenon of aggregation. Hydrophobicity is a critical factor and the shear-flocculated aggregates can directly be subjected to flotation. The question remains to be answered if the concentration of the collector that is required for shear flocculation is higher than that needed for flotation. Warren (1975a) observed that the minimum collector concentration for slime coatings was higher than that necessary for good flotation of scheelite. The slime coatings were sparse for oleate concentrations below 2 × 10- 5M and substantial above 6 × 10- 5M.
BOTTOM SIDE
(b) LIQUID
BOTTOM SIDE
Fig. 5. Unbaffled tanks with minimum shear rate (from Oldshue, 1978).
SHEAR-FLOCCULATIONAND CARRIERFLOTATION
281
tal
~ .-_... ~,
~BAFFLES 1.,
~ J / ~---~' SIDEVIEW
BOTTOMVIEW
(b)
~ - -
'
p
[
I:
BAFFLES
~/\,1.
I
.\i° ~ i I
tl ~
I
SIDEVIEW Fig. 6. Baffled tanks with m a x i m u m shear rate - top to b o t t o m turnover (from Oldshue, 1978).
In general, with fine particles in the system the collector consumption can be expected to be higher due to the large surface area of fines. Depending upon the mineral-collector system the adsorption of a surfactant on a mineral surface can be physical or chemical. Physical adsorption involves attraction between opposite charges, often leading to neutralization of surface charge. Under such conditions the aggregation can be m a x i m u m at the PZC even for a low stirring speed. The chemical adsorption occurs due to bonding between the surfactant and the surface metal sites in which case a negatively charged surface becomes more negative due to collector adsorption - for example, xanthate adsorption on galena. Higher negative charges between two particles lead to a stronger repulsion and to overcome this energy barrier it becomes necessary to apply higher agitations. At the same time the chemical bonds are comparatively stronger to resist the high energy input without desorption of the surfactant from the mineral surface.
282
T.V. SUBRAHMANYAMAND K.S.E. FORSSBERG
Geometrical variables While collisions between the solid particles and gas bubbles are of primary importance for flotation, in shear-flocculation and carrier flotation, collisions between the fine and coarse particles are of utmost importance for aggregation. The conditions to effect aggregation in shear-flocculation or carrier flotation are not much different from those maintained in a flotation system apart from agitation speed, pulp density, cell geometry. At this stage an important question remains to be answered, i.e. does particle aggregation take place in the conventional flotation process? Subrahmanyam et al. (1990b) tested cells with and without baffles for similar experimental conditions. Particle collisions and consequently their adhesion are effective in baffled cells though very sparse aggregates were also observed in unbaffled cells. The geometry of the mixers with m i n i m u m and m a x i m u m shear rates was described by Perry and Chilton ( 1974 ) and a discussion of the role of mixing in mineral processing operations was given by Oldshue (1978). Figure 5 shows unbaffled cells where swirl and vortex formation are c o m m o n in low viscosity fluids. Baffled cells with radial or axial flow impellers produce top to bottom turnover with m a x i m u m shear rate (Fig. 6). For laboratory investigations a baffled cell with a paddle-type stirrer was used by Warren (1975a). The cell dimensions were 80 m m high and 63 m m in diameter with six baffle plates (each 50 m m × 7 m m X 1.5 m m ) and a single bladed paddle stirrer 14 m m X 25 m m X 3 mm. FLOATABILITY OF AGGREGATES
While the slow recovery rate of fines was due to decreased bubble-particle collisions, at the other extreme that of coarse particles was attributed to the disruption of bubble-particle aggregates in turbulent zones (Morris, 1950; Schultze, 1977 ). One of the reasons for the low flotation rate of coarse particles was that with increasing size the density of the bubble-particle aggregate approaches that of the pulp and thereby the aggregate becomes less buoyant (Jameson et al., 1977). Yet another reason for poor floatability is that with increasing particle size the induction time also increases. When particle size is increased by aggregation the efficiency of floatability of large aggregates, like coarse particles is debatable. In the shear-flocculation of haematite the m a x i m u m aggregate size measured was 125/lm for an agitation speed of 1200 rpm (Fuerstenau et al., 1988 ). With scheelite particles ( 7.6-11.1 p m ) aggregates of 30-50 p m were measured for 90 min stirring at 1700 rpm (Warren, 1975b). The aggregates produced are well within the flotation size range and it was observed by Warren (1975a) that the floatability of coarse particles was unaffected by an attached layer of ultrafine scheelite particles. Further, larger aggregates, even if formed, are unstable and may be
283
SHEAR-FLOCCULATION AND CARRIER FLOTATION
TABLE Vl Comparison between conventional and autogenous carrier flotation (data from Hu et al., 1987, 1988 ) Ore
Grade of Conventional flotation Grade of Autogenous carrier riot. feed (%) feed (%) conc. grade conc. rec. conc. grade conc. rec.
(%)
(%)
0.30
21.84
44.18
Complex oxidized Pb- Pb Zn sulphide ore slime Zn
3.24 5.55
5.20 10.10
Malachite
2.94
Wolframite
Wo3
(%)
(%)
0.31
28.13
59.10
85.40 96.71
3.24 5.53
10.33 20.95
94.20 98.15
7.60
62.95
2.94
20.76
91.43
Cassiterite slime
Sn
0.42
0.59
63.98
0.42
2.76
57.45
Complex low-grade fine-grained Fe ore (hematite)
Fe
34.50
61.23
82.30
34.50
65.60
87.93
Slimes-Fe
Fe
18.70
20.75
84.60
18.70
22.77
86.51
Antimony (flotation of Sb stibnite and depression As of arsenopyrite
6.22 0.58
57.48 2.74
91.20 -
6.53 0.52
56.63 0.35
94.50 -
Refractory, oxidized sulphide amorphous copper ore
2.30
15.00
35.00
2.03
24.94
81.21
Cu
b r o k e n u p d u e to a t t r i t i o n a n d s h e a r forces g e n e r a t e d in the liquid. H o w e v e r , the h y d r o d y n a m i c c o n d i t i o n s in the case o f c o a r s e p a r t i c l e f l o t a t i o n a n d in the f l o t a t i o n o f aggregates f o r m e d b y s h e a r - f l o c c u l a t i o n a n d c a r r i e r f l o t a t i o n are different. T a b l e V I gives t h e r e c o v e r i e s o f s o m e m i n e r a l s b y c o n v e n t i o n a l and by carrier flotation. SUMMARY AND CONCLUSIONS S h e a r - f l o c c u l a t i o n / f l o t a t i o n a n d c a r r i e r f l o t a t i o n are m e t h o d s b y w h i c h p a r t i c l e size is e n h a n c e d b y a g g r e g a t i o n o f fines w i t h c o a r s e p a r t i c l e s p r i o r to f l o t a t i o n to i m p r o v e t h e f l o a t a b i l i t y o f fines. T h e aggregates t h u s f o r m e d are c o n s i d e r e d to h a v e b e t t e r floatabilities. D e s p i t e the b e n e f i c i a l effect, i.e. i m p r o v e m e n t in the f l o t a t i o n r e c o v e r y o f .fines a n d the g r a d e o f t h e c o n c e n t r a t e b y s h e a r - f l o c c u l a t i o n / f l o t a t i o n or b y c a r r i e r f l o t a t i o n , the w o r k o n t h e s e m e c h a n i s m s r e p o r t e d in the l i t e r a t u r e is m e a g r e . Since h y d r o p h o b i c i t y is a critical f a c t o r for p a r t i c l e a g g r e g a t i o n it is o f p a r t i c u l a r i n t e r e s t to e x a m i n e t h e b e h a v i o u r o f s u l p h i d e m i n e r a l s w h i c h e x h i b i t h y d r o p h o b i c i t y in the ab-
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sence of collectors. Like the flotation process the mechanisms of shear-flocculation and carrier flotation are governed by physical, chemical and geometrical variables.
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