International Journal of Mineral Processing, 23 (1988) 33-53 33 Elsevier Science Publishers B.V., Amsterdam m Printed in The Netherlands

Froth Stabi l i ty, Part ic le Entra inment and Dra inage in Flotation m A Rev iew

T.V. SUBRAHMANYAM .1 and ERIC FORSSBERG

Division of Mineral Processing, Lule~t University of Technology, S-951 87 Lule~t (Sweden)

(Received September 11, 1986; accepted after revision March 27, 1987)

ABSTRACT

Subrahmanyam, T.V. and Forssberg, E., 1988. Froth stability, particle entrainment and drainage in flotation -- a review. Int. J. Miner. Process., 23: 33-53.

The froth and its stability, the entrainment and the drainage of particles in flotation were long before recognised as important factors which affect recovery and grade. A too stable froth is dif- ficult to handle but, on the other hand, an unstable froth is least desirable. Therefore, a froth of correct stability is of utmost importance. However, the question is whether this phenomenon is related to the frothing properties of a frother or to the physical, chemical and geometrical condi- tions of a system! The entrainment and drainage of particles in flotation are concerned with the pulp/froth phases. The present paper deals with the current aspects on froth stability, particle entrainment and drainage, based on a detailed literature review.

INTRODUCTION

The flotation process is known to be governed by a multitude of interacting variables and it becomes necessary to have a knowledge of those that contrib- ute to the final yield. In a three-phase system like froth flotation involving solid-liquid-gas, the efficiency of separating the values from the gangue is con- sidered to be a function of the adherence of an air bubble to the required min- eral particle. The role of the gas phase in the form of dispersed bubbles in a pulp is to carry the hydrophobic mineral particles, obtained as concentrate along with the froth from the flotation cell. From the view point of handling, the froth thus collected should die down immediately. The froth phase is im- portant since it can affect the flotation recovery and grade. It is considered that a small bubbled closely knit froth is favourable for high recoveries and a loosely knit froth of large bubbles for good grades. This needs to be examined in terms of particle entrainment and drainage since all these phenomena are

*1On leave from the departamento de Geologia, Laboratory of Mineral Processing, UFRN, 59.000 Natal-RN, Brazil.

0301-7516/88/$03.50 1988 Elsevier Science Publishers B.V.

;]4

AIR CONCENTRAfF (-measured rate of ~ecovery

ceLl

bulk T draLnage transport ent ralnment and by colLapse bubbles

A~R I FEED I TA/MNG

Fig. 1. Transport paths of materials in flotation (Flint, 1973).

J Finepo~tic,es J

I sin" . . . . I 1 H gh I

Fig. 2. Schematic diagram showing the relationship between the physical and chemical properties of fine particles and their behaviour in flotation. (G) and (R) refer to whether the phenomenon affects grade and/or recovery (Fuerstenau et al., 1973).

interrelated and influence the recovery and grade, i.e. low grade at high recov- ery and vice versa. Figs. I and 2 illustrate the material transport paths and the adverse effects of the presence of fine particles in flotation, respectively.

A flotation plant operator is often baffled with the problem of choosing a frother for a given ore-collector system because of lack of sufficient data on frother performance. The selection of a frother is mostly based on a trial-and- error basis. A survey carried out in different flotation plants reveals that the most commonly used frothers are pine oil, cresylic acid, methytisobutylcarbi- nol and triethoxybutane (Booth and Freyberger, 1962 ) either individually or in combination with other frothers. Table I gives some important properties of these frothers and some collectors with frothing properties.

A considerable number of papers have appeared in the literature on flotation

TABLEI

Generalfro~ingproportiesof~mecommon~usedfr~he~andcoll~tors

35

Type Name General properties of froth Solubility at 25C g/1000 g of soln.

Produced from

R-OH Pine oil Small bubbles, closely knit 2.5 (cyclic alcohol type) structure, breaks down

readily

Cresylic acid Similar to pine oil but (aromatic alcohol type) larger bubble size

1.6 (at 20 )

Methylisobutylcarbinol (aliphatic alcohol )

R'O(RO)xH Polypropyleneglycols ( polyglycol type)

(R' O ) xR Triethoxybutane (alcoxy type )

Larger in bubble size, less compact structure; re- quires large quantities to form a close structured froth

Compact, lasting froth complete/ structure; breaks down partial readily

Resemble those of pine oil 8 froths (at 20 )

Closely textured stable bubble aggregates, difficult to disintegrate even by water jets and sprays

RCOOH Fatty acids

RSO3Na Sulfonates

RNH2 Amine collectors

Turpentine or pine stump by distillation or solvent extraction. Is a mixture of terpene alcohols, ketones, ethers and hydrocarbons. The alcohol components and B terpineols, fenchyl alcohol and borneol and camphor (a ketone) are the chief frothing components.

Byproduct of coking of coal and thermal cracking of petroleum. The petroleum cresylics are more widely used than those obtained from coal.

Polypropyleneglycols of low mol.wt. Prepared by reacting propylene oxide with propylene glycol.

Synthetic non-ionic frother shows limited solu- bility in water, readily dispersible.

Petroleum Refining of white oils and sulfonation of other hydro- carbon fractions.

frothers (Wrobel, 1952; Gaudin, 1957; Booth and Freyberger, 1962; Klassen and Mokrousov, 1963; Harris, 1982; and Lovell, 1982 ) and any attempt to deal with the fundamental aspects of frothers will merely be a repetition. The role of particle size in flotation has been discussed by Gaudin et al. (1931) and in detail reviewed by Trahar (1981). Hausen (1974) discussed the types of froths and related problems in flotation circuits and metallurgical plants.

36

The scope of the present paper is restricted to the related aspects of froth stability, particle entrainment and drainage in flotation and the topics are dis- cussed as shown below; Froth stability -Froth stability measurement -Influence of frother -Frother interaction with collectors -Classification of frothers -Effect of interaction products on froth -Effect of particles Particle entrainment -Particle size - - flotation and entrainment -Degree of entrainment -Liquid lamella thickness - - entrainment Drainage Summary and conclusions

FROTH STABILITY

Froth stability measurement

The stability of a froth is the time of its persistence. Harris (1982) referred to two types of froths - - unstable and metastable. Unstable froths are those which continuously break due to draining of liquid from between the bubbles and metastable froths can persist for quite longer times in the absence of dis- turbances. The quantitative properties of a froth depend upon a number of variables. Sun (1952) investigated the frothing characteristics of pine oils and explained that the frothability is governed by rate and time of aeration, height of liquid column, chemical composition of the frother, solution pH, tempera- ture and frother concentration in solution. Two methods, i.e. compressed air frothing and sucked air frothing, were used to measure the froth stability in a froth meter. Livshits and Dudenkov (1965) described mechanical and pneu- matic methods. In the former a known volume of aqueous frother solution (generally 25-100 ml) is subjected to shaking in a glass cylinder for a given time. The time of persistence of the froth after shaking is the degree of froth stability. In the pneumatic method, air was blown into the aqueous frother solution through a porous glass filter at a constant rate and pressure. The degree of dynamic stability is marked by the maximum volume and the char- acteristic of static stability by its rate of destruction.

For three-phase froths the stability is governed by a number of factors in- cluding particle size, hydrophobicity, etc. The stability of a three-phase froth was measured by Dippenar (1982) in a glass cylinder containing solids and

37

aqueous frother solution. The froth was produced by agitation and the imme- diate froth volume was noted. The method is similar to the one described by Livshits and Dudenkov (1965).

The effect of solids on froth stabilization is measured in terms of froth height or volume with respect to time.

Influence of frother

A bubble produced in water is unstable. One of the prerequisites for a suc- cessful flotation operation is the stability of the bubble-particle aggregate. A stable bubble is produced by using a frother, the function of which is to de- crease the surface tension of the air-liquid interface. The frother molecules impart stability to a bubble by migrating from a region of low surface tension to a higher value due to the surface tension gradient. Wrobel (1951) measured the rate of adsorption of commonly used frothers at the air-solution interface and found appreciable variation in surface tension with time, indicating a low rate of migration of molecules with aqueous solutions of pine oil and triethox- ybutane. With solutions of isoamyl and methyl alcohols equilibrium was achieved in a few seconds. Brown et al. (1953) and De Vries (1957) considered the overall stability of froth to be a function of the bubble size.

The influence of frother concentration on bubble size was investigated by Grunder et al. (1956) and Benett et al. (1958). The bubble diameters observed by them varied between 0.2 and 0.7 mm for pine oil concentrations of 10-20 mg/1 which corresponds to concentrations generally used in practice. The ef- fect of bubble size on the rate of flotation of fine particles was investigated by Ahmed and Jameson (1985). The mean bubble size varied between 0.075 and 0.655 mm with particles less than 50 mm and they conclude that fine bubbles were superior for the flotation of fine particles. This is due to the fact that fine bubbles behave like solid spheres and take up more load than larger bubbles. Recently Szatkowski and Freyberger {1985) observed that fine bubbles (10-100 /~m) with their load of quartz particles were resistent to coalescence and the production of a stable froth required mineralization. Laplante et al. (1983) observed a lower flotation rate in a system having no frother and attributed this to an increase in the air flow rate which caused an increase in bubble diameter. It was found that even a ppb concentration of CsH13OH prolonged the time of coalescence of bubbles ( Sagert et al., 1976).

Frother interaction with collectors

The frother-collector interaction in flotation was first postulated by J.H. Christman in 1930 (see Leja and Schulman, 1954). It received both contra- dicting (Wark, 1938) and supporting (Taggart and Hassialis, 1946) views. The theory gained ground only after the work of Leja and Schulman (1954) who explained the effectiveness of a frother on interaction with a collector.

38

Air

! t s

Fig. 3. Mechanism of bubble attachment: bubble approaching a collector coated solid surface: diffused monolayers of associated and unassociated molecules at interfaces and in solution (Leja and Schulman, 1954).

Air

Fig. 4. Mechanism of bubble attachment: adherence of an air bubble established through the penetration of the monolayer at the solid/liquid interface by the monolayer at the air/liquid in- terface (Leja and Schulman, 1954).

Wrobel (1951-52) discussed linear and lamellar theories for bubble-particle adhesion. The linear theory states that the conditioned particles maintain con- tact with the gaseous phase by penetration through the air bubble film while the lamellar (or film) theory holds that the attachment takes place without penetration. Leja and Schulman (1954) state: "...as soon as the air bubble contacts the solid surface [Figs. 3 and 4] the collector-frother molecules of the air/water interface can now penetrate the diffuse monolayer at the solid, and adsorb strongly on the solid surface, greatly increasing local surface con- centration and local hydrophobic character of the surface."

The mechanism of particle-bubble adhesion suggested by Leja and Schul- man could be considered an extension of the linear theory discussed by Wrobel (1951-52). Several workers (Fuerstenau and Yamada, 1962; Lekki and Las- kowski, 1971; Bansal and Biswas, 1974; Malysa et al., 1981a, b) with different objectives on different mineral-collector-frother systems in their investiga- tions confirmed frother-collector interactions and consequently the effects in

39

flotation, for example, in bubble-particle collision, attachment, stabilization, etc. Despite the importance of such interactions in flotation processes little information is available on the choice or combination of a frother for a given collector (or mineral-collector) system. Certainly investigations of this type need quantitative data like frother and collector concentrations at the air-liquid, air-solid and solid-liquid interfaces, which means very low concentrations. Although Bansal and Biswas (1974) suggested the use of C 14 radioactive la- belled compounds for investigations of this nature, problems such as their ready availability, long half-life periods, high costs, environmental restrictions, etc., make the investigation impractical.

Among the alcohols (ethyl to butyl) studied as frothers for the flotation of galena with ethyl xanthate as collector, Mukai et al. (1972) obtained a better recovery with amyl alcohol. The phenomenon was considered to be due to the mutual co-adsorption of the collector-frother molecules. It was explained on the basis of the shift of the wave number of the C-H stretching vibration of alcohols and xanthates. The shift in the asymmetric stretching vibration of the CH3 group around 2960 cm- 1 was found to be a function of the number of carbon atoms in the alkyl group (Figs. 5 and 6). Certainly data of this type would be useful in the selection of a frother for a given mineral-collector sys- tem. However, further work along these lines on other systems is necessary for a better understanding of the interactions and consequently their effects on the yield parameters.

Classification of frothers

A frother has a number of functions in flotation - - first, it reduces the sur- face tension of the air-liquid interface in order that a stable bubble is produced in the system; secondly, it influences the kinetics of bubble-particle adhesion; thirdly, it thins the liquid layer by interacting with collector molecules and finally, it stabilizes the bubble-particle aggregate (Schulman and Leja, 1954; Leja, 1956-57). Flotation froths are three-phase with solids in the froth. In addition to the frother properties like surface activity, surface viscosity of the medium, etc., the nature of solids, i.e. the particle size and the hydrophobicity, play a dominant role in imparting stability to the froth. Lekki and Laskowski (1975) classified the frothers into surface-active and surface-inactive based on their mechanism of action, i.e. the ability to adsorb or co-adsorb on the mineral surface with a collector (Table II and Fig. 7). They observed froth stabilization in the presence of solids with surface-inactive compounds such as diacetone alcohol and ethyl acetal which do not possess froth-forming properties.

Effect of interaction products on froth

The stabilization or destabilization of froth is influenced by several factors and no single theory could explain the mechanism by which these factors affect

40

3000

2990

E u 2980

m

g z 2970

<

2960

2950

/ I I I I

I I XANTHAT(

o ALCOHOL

'~~o~. . . . o I

I I I I I I I 2 3 4 5 6

NUMBEROF CARBON ATOMS IN THE ALKYL CHAIN OF XANTHATE OR ALCOHOL

Fig. 5. Relation between the wave number of CHa stretching vibration and the number of carbon atoms in the alkyl chain of xanthate or alcohol (Mukai et al., 1972).

2990

- - 2980

E u

m 2970

< 2960

2950 I I [ I I 1 2 3 4 5

NUMBER OF CARBON ATOMS IN THE ALKYL CHAIN

100

80

60 v~

< l.O o

d u_

Fig. 6. Comparison of the floatability of galena with the wave number of alcohol (Mukai et al., 1972).

the stability of froth. In general, a reduction in the volume of froth or desta- bilization is known to take place due to bubble coalescence, and the various factors which promote or lead to coalescence should be considered on the basis of experimental conditions. Wark (1938) observed absence of froth with sul- phide minerals on the addition of xanthate while Moeller (1955) and Leja and Schulman (1954) noticed an increase in froth stability on the addition of xan- thate in the absence of minerals. The nature of solid particles -- the size, hy- drophobicity, concentration, state of aggregation, interaction products formed in the pulp -- exercise influence on flotation froths. Livshits and Dudenkov

TABLE II

Classification of flotation frothers (Lekki and Laskowski, 1975)

41

Groups Characteristic of Interactions at the aqueous solution liquid/gas interface

at flotation range of concn.

Froth

surface- colloidal solutions cause a large two- and three- active (fatty acids, amines ) change in surface phase

tension

molecular solutions lower the surface two- and three- (alcohols) tension of solutions phase

surface- molecular solutions do not alter the three-phase inactive (diacetone alcohol surface tension froth only

and ethyl acetal)

inorganic raise the surface poor two- electrolytes tension of solutions phase froth and

good three- phase froth with hydrophobic minerals

(1965) thought that it was impossible to destroy the froth even at a high xan- thate consumpt ion in the absence of surface oxidation products or heavy metal ions present in the suspension. A reduct ion in the dynamic and static stabil ity of f roth was observed when xanthate was added to frother solution contain ing

Q. b.

surface active frothef ) surface inactive frother

Fig. 7. Mechanism of action of: a. surface-active frnther; b. surface-inactive frother (Lekki and Laskowski, 1975).

42

soluble salts of copper or lead (50 mg/1) ; with zinc salts no such effect was found. Similarly with calcium or barium in the frother solution a sharp de- crease in the stability of froth was observed on the addition of sodium oleate. The presence of mercury xanthate in the pulp was found to weaken the froth (Ray and Brewers, 1941). The soluble salts form hydrophobic precipitates after interaction with the collector. Then the effect of these precipitates on froth should be the same as that of the hydrophobic solid particles. The size of the heavy metal precipitate particles formed by reaction with xanthate was found to be 0.2-0.5 ttm (Glembotsky and Kolchemanova, 1958). Livshits and Dudenkov (1965) interpreted from the work of Leja and Schulman (1954) that the precipitate particles adsorb the frother molecules thus leading to a low frother concentration in the pulp and consequently causing a reduction in the stability of froth.

Effects of particles on froth

The influence of solids on froth stabilization is already well known. The phenomenon was attributed to be due to an increase in viscosity of surface films. Klassen and Mokrousov (1963) observed that the stronger the hydro- phobicity of the particle the greater was the effect in stabilizing the froth. The fact that hydrophobic solids can stabilize or destabilize the froth is dependent on the particle size and their concentration. Livshits and Dudenkov (1965) believe that the destruction of froth by hydrophobic particles is size-dependent and further that the coarser hydrophobic particles might even prevent bubble coalescence by acting as buffers between bubbles. The fine particles too do not affect the coalescence rate. There seems to be an optimum size range for par- ticles to stabilize or destabilize the froth.

Lovell (1976) investigated the frothing characteristics of a phosphate ore containing essentially apatite and calcite. Both minerals at low solid concen- trations destabilized the froth in the presence of tall oil fatty acid, and at higher concentrations the froth obtained was more than the two-phase froth, thus indicating froth stabilization. With tall oil fatty acid and other modifying agents apatite destabilized the froth irrespective of the solid concentration studied while calcite still had a stabilizing effect. This behaviour was examined in terms of hydrophobicity (contact angles) and particle size. The froth destabilization at lower solids concentration was attributed to hydrophobicity and increase in froth stabilization at higher concentration of solids to be due to an increase in particle size by agglomeration. A similar explanation that flocculation of par- ticles leads to increased froth stability was presented by Dudenkov (1967). Dippenar (1982) measured the effects of particle size of hydrophobic quartz and galena on froth volume. The effect of a mass of 0.16 mg of 4-6 ffm hydro- phobic quartz particles on reducing the immediate froth volume was found to be the same as that of 18.8 mg of 500-589/tin size. With hydrophobic galena the masses required to reduce the froth volume were 1.7 mg and 140 mg for

43

- - 12

s - - tO

6 ,V

4

2

Manual o d ispers ion

Ultrasonic x d ispers ion

x x

x x

10 100 Solids concentration mg t-l(Liquid)

1000

16

14

- - 12

~1o 2 i8 ~6

4

2

0

i

100 Solid concentration gl-~(pulp)

1000

Fig. 8. Low futile concentration effect on froth stability (Hemmings, 1981 ).

Fig. 9. High rutile concentration effect on froth stability (Hemmings, 1981 ).

8-10/~m and 90-106 #m sizes, respectively. A similar investigation by Hem- mings (1981) on -8/Lm rutile particles in a solution containing 25 ppm oleate with 0.2 ppm Fe +3 at a low solid concentration showed a destabilizing effect and at higher solid concentration the froth was found to be stable (Figs. 8 and 9).

Recent investigations by Dippenar (1982) on the effects of particles of dif- ferent shapes and degrees of hydrophobicity on film rupture with an artificially thinned film of water by high speed cinematography revealed some interesting features (Table III). The equilibrium contact angle (without rupturing the film) was established in the case of spherical glass beads with a contact angle of 74 and the film was found to rupture only on attaining its natural rupture thickness. The particle with a contact angle of 102 ruptured the film within milliseconds of contact with the lower interface; rough particles of quartz and sulphur with a contact angle of > 90 were oriented with more than half the body in the air phase and with < 90 towards the liquid phase. Livshits and Dudenkov (1960, 1965) and Dudenkov (1967) observed that very small hy- drophobic particles with contact angles > 90 ruptured the thin films between bubbles. This occurred when two bubbles came into contact with hydrophobic particles, and each interphase while trying to establish its equilibrium on the particle thinned the film in between the bubbles finally resulting in a rupture.

PARTICLE ENTRAINMENT

The two important mechanisms by which particle collection takes place in a flotation operation are adhesion and entrainment. Not much information exists on the other possible mechanisms like entrapment (Gaudin, 1957) and carrier flotation (Greene and Duke, 1962).

44

TABLE III

Effects of particles of different shapes and degrees of hydrophobicity on film rupture (data taken from Dippenar, 1982)

Mineral Surface Size Treatment Contact Induction Total nature (#m) angle time .2 interaction

(degr.) (ms) time *:~ (ms)

glass beads Spherical

Spherical Quartz Rough Galena Sharp

rectangular/ smooth

Sulphur Smooth Rough

250 DDMS *l 74 30 -- 102 12 106

160 DDMS 7 10 160 98 5.5 9

160

K Hexyl 80 _+ 8 2 10 xanthate

70 8 -- In presence of emulsified paraffin 95 _+ 0.5 5 6

tDichlorodimethylsilane. 2The interval between apparent contact of the solid with the meniscus and formation of a three phase boundary. 3The time taken to rupture the film (values shown in the table include the induction time).

Particle size - - f lotation and entra inment

The effects of particle and bubble sizes on flotation rate recovery were in- vestigated by several workers; experimental studies and theoretical analysis have led to varying results. These aspects were dealt with in earlier works (Trahar, 1976, 1981; Jameson et al., 1977; Szatkowski and Freyberger, 1985), therefore only the results reported by some workers are given in Table IV. The influence of particle size on the rate of recovery of minerals was investigated by Gaudin et al. (1931), Anthony et al. (1975) and Trahar (1976). In indus- trial concentrators the recovery for copper, lead and zinc minerals was shown to be maximum in the size range of 10-100 ~tm (Gaudin et al., 1931 ). While the slow recovery rate of fine particles was due to decreased particle-bubble collisions, that of coarse particles was attributed to the disruption of bub- ble-particle aggregate in turbulent zones ( Morris, 1950; Schultze, 1977). One of the reasons for the low flotation rate of coarse particles was that with in- creasing particle size the density of the bubble-particle aggregate approaches that of the pulp density and thereby the aggregate becomes less buoyant (Jameson et al., 1977 ). T l~ argument put forth by Jowett (1980) for the poor recovery is in agreement with the observations made by Dippenar (1982). With

45

TABLE IV

Relationship between the flotation rate constant k and the relative diameters of particles (dp) and of air bubbles (rib) where indicated

(a)

(b) (c) (d) (e) (f)

(g)

k independent of particle size for dp = 1-4 #m (galena) h~:dp for dp=4-28#m (galena) h ~: dp/ db 2 kocln dp k independent of dp koc dp 2 (for apatite, hematite and galena)

kocd,; d,=4-20/lm (quartz)

(h) k ~c dp2 / db 3 (i) k~c 1/db ~

Gaudin et al. (1942) Gaudin et al. (1942) Sutherland (1948) Morris (1952) Bushell (1962) Tomlinson and Fleming (1963) Tomlinson and Fleming (1963) Reay and Ratcliff {1973) Jameson et al. (1977)

increasing particle size the induction time increases and hence poor floatabil- ity. In addition to the particle size and surface chemical characteristics, the shape factor also seems to influence the induction time (see Table III).

For bubble-particle adhesion it is necessary that the bubble should collide with the particle and consequently film rupture will follow by the establish- ment of a three-phase contact. Whereas the entrainment is a characteristic feature of fine particles and is non-selective, there being no distinction be- tween hydrophilic or hydrophobic particles. An arbitrary classification of par- ticles into fine (5-10 ~m), intermediate (10-70/lm) and coarse ( > 70 #m) on the basis of their floatabilities was discussed by Trahar (1981). It is, of course, difficult to generalize since the floatabilities vary from mineral to mineral, as can be observed from Table V. However, it is known that the adhesion of par- ticles > 10 #m in size occurs due to collision with air bubbles. For particles < 10 #m the collision efficiencies are low and the mechanism of collection takes place by entrainment, i.e. the particles are carried in the liquid septa forming the bubbles.

In general the high recoveries obtained in flotation either at plant level or in batch tests are not separately accounted for in terms of recovery by entrain- ment and by true flotation. The contribution to the final yield by entrainment is significant especially with fine particles present in the system and it is im- portant to estimate this factor for a better evaluation of the process performance.

Degree of ent ra inment

The entrainment of particles in flotation is closely related to the recovery of water. Several workers investigated the effects of operating variables like froth

46

TABLE V

Observed size ranges of flotation recovery for different minerals as reported by different workers (from Trahar and Warren, 1976)

Mineral Size range Conditions Reference (~m)

Barytes 10- 30 laboratory, Clement and Klossel (1963) batch

Cassiterite 3- 20 industrial Kelsall et al. (1974)

Fluorite 40-110 laboratory, Klassen and Mokrousov batch {1963, p. 390)

10- 90 industrial Klassen and Mokrousov (1963, p. 390)

50-150 industrial Lay and Bell {1962)

Galena 37-295 laboratory, batch Gaudin et al. (1931)

170-240 laboratory, Klassen and Mokrousov batch (1963, p. 391)

7- 70 industrial Cameron et al. (1971) 6- 70 industrial Kelsall et al. (1974)

13- 75 industrial Klassen and Mokrousov (1963, p. 391 )

20-100 industrial Lynch and Thorne (1974)

Pyrite 50-100 laboratory, Imaizumi and Inoue {1965) continuous

Pyrite- 20-- 70 laboratory, pyrrhotite batch Morris (1952)

Quartz 10- 40 laboratory, De Bruyn and Modi (1956) continuous

9- 50 laboratory, Robinson (1959) batch

Sphalerite 15-100 industrial Cameron et al. (1971) 8- 70 laboratory,

batch Anthony et al. (1975)

Wolframite 20- 50 laboratory, batch Clement et al. (1966)

depth, frother concentration and air addition which influence the water recov- ery. Mitrofanov et al. (1985) pointed out that the factors responsible for en- trainment are the ascending and descending streams as a result of cell hydrodynamics, bubble population and size, amount of slimes and the concen- trations of the reagents.

Wrobel (1953), in an investigation on flotation frothers, observed a direct relationship between water content in the froth and the concentrate grade. The

TABLE VI

Degree of entrainment for different minerals

Mineral Sp.gr. Particle size Degree of Reference (/~m) entrainment

Quartz 2.65 3.5 0.72 > 40.0 0.10

Silica < 12.0 0.99

Cassiterite 6.80- 7.10 < 5.0 0.85

Coal 1.00- 1.80 < 38.0 1.00

Ultraf'me gangue 0.87 Warren (1985)

Fine gangue < 40.0 0.78 Subrahmanyam and Forssberg (1986)

47

Trahar (1981) Engelbrecht and Woodburn (1975)

Engelbrecht and Woodburn (1975)

Goodman and Trahar (1977)

Lynch et al. (1981)

varying behaviour of a mineral (flotation and entrainment) with different size fractions present in the pulp is evident from the work of Engelbrecht and Woodburn (1975) and Subrahmanyam and Forssberg (1986). Engelbrecht and Woodburn (1975) observed a linear relationship with water recovery in the case of hydrophobic pyrite < 7.7/~m and silica < 12/~m. The recovery of coarse pyrite 20-27/ira was found to be dependent on hydrophobicity. A similar ob- servation was made by Subrahmanyam and Forssberg (1986) in an investi- gation on the performance of different flotation frothers with a copper ore

~containing graphite and silica in major proportion. For hydrophobic particles the recovery by entrainment was measured by differences in the relative con- tributions of true flotation in the presence of collector and by entrainment in the absence of the collector, i.e. with frother only present at the same water recoveries (Trahar, 1981 ). The relationship between the recovery of fine gan- gue by entrainment and the recovery of water is represented by the equation:

Rg----eg Rwater (1)

where Rg is the recovery of fine gangue of a given size in a given time, eg a constant for a given particle size and specific gravity and Rwat~r is the recovery of water for the same time. The slope of the line eg in the plot of the recovery of solids versus the recovery of water is termed the degree of entrainment and the values reported by several workers are given in Table VI.

For hydrophobic particles, Warren (1985) proposed:

Rm=Fm+em Wwat~r (2)

45

TABLE VII

Liquid lameUa thickness (data taken from Hemmings, 1981 )

Circuit Lamella thickness in different cells (/zm)

Coal 280- 830 Copper 70- 340 Lead 70- 290 Zinc 80- 210 Tin 50- 130 Scheelite 60-1080

where Fm is the intercept of the extrapolated line on the mineral recovery axis independent of water recovery and em is the entrainment factor for floatable ( hydrophobic ) mineral.

Liquid lamella thickness-entrainment

Hemmings (1980) concluded that the thickness of the aqueous lametla would be the principal factor for the concentration of ultrafines ( - 10/an). In a later investigation Hemmings (1981) measured the froth liquid lamella thickness with a specially developed conductivity probe in different beneficiation plants ( Table VII). He found that the froth liquid lamella thickness was far too high for the concentration of ultrafine particles which would otherwise be possible if the liquid lamella were thinner than 10-20 pro. This observation holds for the liquid that is carried by the bubbles (or the liquid septa forming the bub- bles). The thicker the liquid lamella the higher is the water content and hence greater is the possibility of fine particles being recovered by entrainment rather than by true flotation.

DRAINAGE

The particles are carried into the froth by rising bubbles through the pulp phase. These bubbles with their load of particles accumulate at the pulp/froth interface. A variable amount of the material carried into the froth returns to the pulp by drainage. The re-entry of particles into the pulp occurs because of the continuous drainage of liquid and bubble coalescence. Although several factors described above contribute to bubble coalescence which can lead to drainage of hydrophobic particles also, but little evidence on this aspect elim- inates this possibility. The drainage of gangue particles is desirable from the view point of enriching the grade. It was suggested that water spraying on the

49

froth layer might improve both grade and recovery (Klassen and Mokrousov, 1963). Obviously this accelerates the liquid drainage and hence the drainage of gangue material.

Recent investigations of Cutting and Devenish (1975), Moys (1978, 1984), Cutting et al. (1981, 1982, 1986), Kuzkin et al. (1983) and Mitrofanov et al. (1985) have given an insight into the possible mechanisms operating in the flotation froths.

Investigations carried out by Moys (1978) deal with the mineral concentra- tion gradients, froth properties and the effect of froth removal on froth grades with respect to froth height. A steady increase in the grades of Cu, Zn and Fe sulphides and percentage of solids in the froth and a decrease in the gangue grade with increasing froth height were observed. Above a certain froth height the concentration of the gangue was found to be constant.

The displacement of less hydrophobic (Fe) by more hydrophobic (Cu, Zn) particles in the froth was attributed to the lower film surface area as a result of drainage and bubble coalescence. Cutting et al. (1981) explained this in terms of the degree of hydrophobicity of the mineral particles and the degree of competition; while the former is an effect of the chemical environment the latter is a consequence of the machine operating variable. And finally the froth structures determine the appearance of the mineral species depending on the degrees of hydrophobicity.

A general conclusion from the investigations of Cutting et al. (1986), Moys (1984) and Kuzkin et al. (1983) is that the conventional flotation cells in which the froth is removed by mechanical means will cause froth collapse re- sulting in recovery losses. In order to allow maximum drainage, the removal of the upper froth layers only is recommended. Bisshop and White (1976) have shown that the residence time is the most important factor in the particle recovery.

SUMMARY AND CONCLUSIONS

The froth forms an important phase in flotation but there has been relatively little work on the several factors that govern its stability and the mechanisms within the froths. Froth behaviour is different under different experimental conditions and is influenced by the system variables - - physical, chemical and geometrical and their interactions. As many of us have observed, even in the same flotation circuit mineralized froths behave differently, i.e. variation of bubble sizes and stability in rougher, scavenger and cleaner cells. This could be explained to a certain extent by online measurements such as reagent con- centrations, interaction products formed, particle sizes and concentrations, air flow, etc.

Although there is general agreement on the formation of interaction prod- ucts in flotation, their influence on froth is controversial. The frother-collector

50

combinations, their interact ions with minerals and consequently their effects on process need further investigations.

The ent ra inment is unavoidable with fine particles present in the system and the recovery of hydrophil ic gangue is closely related to the recovery of water. By manipulat ing the variables that affect the recovery of water, better grades and recoveries can be obtained. Both the residence t ime of the froth and froth removal mechanisms in f lotation cells influence the recoveries and grades. For example, in the absence of froth removal higher grades of f loatable min- erals and lower gangue mineral concentrat ions were observed. With longer froth residence t imes the entra ined hydrophil ic particles drain back to the pulp thus yielding a high grade concentrate, whereas high recoveries and low grades would result from shorter residence times.

Recent investigations on drainage mechanisms have provided an insight into the operating mechanisms of the froth phase. It has been shown that size clas- sif ication occurs on the basis of hydrophobic propert ies (degree of hydropho- bicity). Depending on this the particles appear in the froth layer which in turn is dictated by the froth mobil ity structures. More knowledge is necessary on the drainage aspects.

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