Particle size, surface coverage and flotation response

Colloids Elsevier

and Surfaces, 16 (1985) Science Publishers B.V.,

PARTICLE

P. BLAKE*

41-53 Amsterdam

SIZE, SURFACE

Printed

COVERAGE

41

in The Netherlands

AND FLOTATION

RESPONSE

and J. RALSTON**

School of Chemical Technology, Ingle Farm, S.A., 5098 (Australia) (Received

-

20 December

1984;

South

accepted

Australian in final form

institute

of Technology,

P.O. Box

1,

28 May 1985)

ABSTRACT The flotation behaviour of methylated quartz particles of known surface coverage has been studied as a function of surface coverage and particle size over the range from 13 to 47 pm in diameter. The effects of ionic strength and water structure modifiers, particularly urea, have also been investigated. Certain experiments have been performed with methylated glass ballotini. For a given particle size, a critical surface coverage is required before flotation will occur. Current theory cannot describe this phenomenon nor the recovery versus surface coverage behaviour above the critical value. Ionic strength effects are interpreted in terms of double layer theory. At concentrations above - 0.5 M, urea is shown to increase the rate of flotation of methylated glass ballotini, whilst similarly treated quartz particles remain unaffected.

INTRODUCTION

The froth flotation process for the separation of minerals generally relies on adding reagents to a slurry containing a mixture of minerals with the aim of rendering one mineral at a time selectively hydrophobic. The relationship between floatability, particle size and surface coverage or hydrophobicity has never been investigated satisfactorily, despite numerous attempts to do so [ 11. Hydrophobicity may be inherent (e.g. coal, molybdenite), or may be conferred by adsorbed collectors. It has recently been discovered that, by careful control of the chemistry of the flotation pulp, hydrophobicity may be induced on sulphide surfaces in the absence of conventional collectors [ 2-41. For example, elemental sulphur, which is naturally hydrophobic, is known to form on the surface of sphalerite and chalcopyrite, depending on Fh, the nature of the metal ions present, etc. [3, 41. The control of surface hydrophobicity is critically important, particularly when one realizes that serious losses of both fine and coarse particles occur in the majority of plants [ 1, 51. Contact angle measurements provide a measure of hydrophobicity and allow a link with surface coverage to be made. *Current address: Central Research Laboratories, Nicholas-Kiwi Drive, Boronia, Vie., 3155, Australia. **To whom all correspondence should be addressed. 0166.6622/85/$03.30

o 1985

Elsevier

Science

Publishers

Pty.

B.V.

Ltd.,

45 Wadhurst

42

Contact angle and hence hydrophobicity normally increases with surface coverage, although changes in adsorption behaviour, particularly at higher coverages, can alter this trend. In the absence of any firm link between floatability, particle size and surface coverage, improvements in the recovery of both fine and coarse particles will be inhibited. In order to develop this fundamental relationship, a new technique was developed in this laboratory [6, 71, enabling the surface of quartz particles to be tailored to various known surface coverages via a methylation process with trimethylchlorosilane. The flotation response of these chemically tailored quartz particles has been studied over the size range from 13 to 47 pm as a function of surface coverage. In certain experiments, the influence of water structure modifiers on flotation response was also investigated. EXPERIMENTAL

Analar grade or equivalent chemicals and conductivity water (conductivity < 0.8 X 1O-6 fit-l cm-‘, y = 72.8 mN m-l at 20°C) were used throughout this investigation. pH measurements were performed with an Orion Model 701A pH meter, calibrated at pH 4.00 and 7.00. Experiments were performed at 25°C. Preparation

of particles

Optical grade quartz (G. Bottley Pty. Ltd., London, U.K.) was crushed, sized and methylated with trimethylchlorosilane to varying, known surface coverages using procedures previously described [7]. Spherical glass ballotini (Potter Industries, Melbourne, Australia) were also used in selected experiments. The particle size and specific surface areas of six quartz size fractions are shown in Table 1. They have previously been characterized in detail [ 71. Particular attention was paid to surface cleanliness. TABLE

1

Particle

size and specific

surface

areas of six quartz size fractions

Particle size (weight average diameter, pm)

Specific surface areaa (m’ g-‘)

13* 3 17k 4 25i 6 36* 6 42+ 7 47 f 10

0.91 ?; 0.06 0.50 r 0.05 0.40 k 0.03 0.32 * 0.02 0.30 * 0.02 0.18 t 0.02

aSolution

adsorption:

Methylene

blue,

43

Flotation

experiments

Flotation experiments were carried out in a modified Hallimond Tube using standard procedures [8]. High purity Nz was used as the flotation through silica-water dispersions before passing gas and was “scrubbed” into the Hallimond Tube at a flow rate of 60 cm3 min-‘. A known mass of solid, usually 1.00 g unless otherwise stated, was conditioned for 5 min before the Nz was introduced. Flotation was then carried out for 2 min. The product was dried and weighed allowing the recovery to be determined. Note that blank experiments were also performed with unmethylated particles. Mechanical carry-over (“entrainment” in the absence of a frother) varied from 1.0 to 9.0% and increased systematically with decreasing particle size. Captive bubble tests performed on unmethylated particles, including the small amounts “carried over”, did not show any detectable hydrophobicity. These carry-over data were subtracted from the “total recovery” figures to yield “real flotation recovery”. Experiments were normally performed with 1O-3 M KN03 as the background supporting electrolyte in a total volume of 200 cm3. pH adjustments were performed with standard, aqueous solutions of KOH and HN03. In certain experiments involving glass ballotini and quartz, the concentration of urea, LiN03, KN03 and CsN03 was varied from 0 to 1.00 mol dmm3. Photographs of the swarm of bubbles in the Hallimond Tube were taken at regular intervals throughout the experimental programme. The bubble size distribution remained constant with an average bubble size of 1 mm. RESULTS

Flotation

as a function

of particle

size and surface

coverage

at pH 5.7

As noted in our previous paper [7], the I(CH3),Si (adsorption density) in mol g-’ plateau was taken as maximum surface coverage. The percentage surface coverage for any other I’(CH3)3Si is readily calculated as I’(CH3)3Si X 100

[r( CH313Wplateau It should be realized that this is an experimental quantity and assumes no detailed knowledge of either the surface area or the chemical characteristics of surface groups. Evidence suggests [7], however, that steric and other reasons dictate that 2.6 out of 4.6 total surface silanols react with trimethylchlorosilane at maximum surface coverage, giving a receding water contact angle of 72” for fully methylated quartz plates [ 151. Representative curves for 17 and 47 pm quartz particles are shown in

II/, 20

40

SURFACE

60

COVERAGE

89

100

(percentage)

Fig. 1. Flotation recovery as a function for 47 and 17 pm quartz particles at 10e3

of

surface coverage pH 5.7.

of trimethylsilyl

groups

M KNO,,

I

47 42 36 25

13 5 ,’

20-

20

40 SURFACE

60

COVE RAGE

Fig. 2. Flotation recovery as a function for various size fractions of quartz.

80

100

(percan+og*)

of

surface

coverage

of trimethylsilyl

groups

Fig. 1. Composite curves (deleting individual points for the sake of clarity) for the six particle sizes investigated are shown in Fig. 2. The salient features are that: (i) there is a critical surface coverage, for each particle size, below which

45

flotation does not occur. The “cut-off” values are found by extrapolation and verified by varying the surface coverage until a real flotation recovery is detected (i.e. above the carry-over value). In this way reliable data are obtained. For each particle size, a captive bubble pressed against a bed of particles with finite surface coverages below the “cut-off” value picks up particles. Bubble particle adhesion occurs under static conditions only in these circumstances. (ii) for the 13 pm quartz particles, flotation recovery increases only weakly at surface coverages above the critical value, and does not plateau, in marked contrast to the 47 I.trn quartz particles. Other particle sizes exhibit behaviour which is intermediate between these two extremes. (iii) the critical surface coverage required for flotation to commence for the 13 pm quartz particles is more than five times that required to induce the 47 I.trn quartz particles to float. When the mass of quartz available for flotation was varied from 0.2 to 5.0 g, there was no detectable influence on the critical surface coverage required for flotation to commence. Flotation

as a function

of ionic strength

at pH 5.7

The effect of increasing ionic strength on recovery (with KNO,) is shown in Table 2 for 47 pm quartz particles with a surface coverage of 10%. There is a small increase in flotation recovery up to 0.1 mol dme3, followed by a sharp increase which plateaus beyond - 0.25 mol dmw3. Similar behaviour occurs for other surface coverages and particle sizes. Influence

of water structure

modifiers

on flotation

at pH 5.7

Ions such as Li’ are classed as structure makers, whilst Cs’ is known as a structure breaker. Such a classification is dependent at least on the strength of the electric field of the ion concerned and is described in detail elseTABLE

2

Flotation response 10%) as a function

Flotation

[KNO, I

(mol dm-’

of tailored quartz particles (47 I.tm quartz of potassium nitrate concentration

x

lo+*)

0 (conductivity 0.1 1.0 10 25 50 100

water)

53 * 53 + 57 * 69+ 78 ? 81 + 81 r

2 3 2 2 2 3 2

recovery

(%)

particles,

surface

coverage

=

46

where [9]. Non-ionic additives can also modify the water structure - urea, for example is a structure breaker - and are known to have a marked effect on adsorption at interfaces and to influence contact angles on solid surfaces, e.g. Ref. [lo]. At low ionic strength, up to - 0.01 mol dme3, there was no detectable difference in the flotation behaviour of either quartz particles or glass ballotini in aqueous solutions of LiN03, KN03 or CsN03, nor was there any detectable change in the bubble size distribution in the Hallimond Tube. At higher salt concentrations, there was a marked increase in recovery with a concomitant decrease in bubble size and increase in bubble density (i.e. number per unit volume). Increasing the concentration of urea up to 1.0 mol dme3 enhanced the recovery of methylated glass ballotini (Fig. 3), but had no measurable effect on the recovery of methylated quartz particles. Varying the concentration of urea from 0 to 1.0 mol drn-j had no detectable effect on pH, bubble size distribution, surface tension of solutions in contact with methylated glass ballotini or on the recovery of unmethylated glass ballotini. The influence of urea is, therefore, a real effect and not an artifact. /

60/*

506-o/o

I

I

1

1.0

0.1

Fig. 3. constant

Flotation pH.

of methylated

glass ballotini

as a function

of urea

concentration

at

DISCUSSION

Recovery

data at fixed pH

For a batchwise flotation process it may be shown that (e.g. Refs [5, 6, 12, 141) the removal of particles as a function of time, t. is described by: dR = h(1 -R) dt

(1)

47

where R is the recovery, defined by [(C, - C)/C,)] where C is the particle concentration (mass or number per unit volume) and Co is the particle concentration at t = 0. h is a “rate constant” given by k=

3GE,E,E,h

(2)

2db Vr

where G is the gas flow rate, V, is the reference volume of height h through which bubbles rise, db is the appropriate bubble diameter and E,, E, and E, are the collision, attachment and stability efficiencies for bubble particle encounters. Through Eqn (1) and Figs 1 and 2 it may readily be shown, for example, that &=47 Mm > &=I3

pm

where the k values are experimental rate constants for the two particle sizes concerned. At present it is not possible to calculate either theoretical rate constants or recovery data from first principles with any degree of reliability. In Fig. 2, for each particle size examined, there is a distinct surface coverage below which particles do not float. Bubble-particle adhesion does occur, however, when a captive bubble is pressed against a bed of particles with finite surface coverages below the cut-off value. Film thinning and associated processes do not occur at a sufficiently rapid rate during the short time span, typically a few milliseconds, in which a bubble and a particle are in close proximity in the flotation cell. These “induction times” are clearly different for various particle size fractions. The critical surface coverage versus particle size data are shown in Fig. 4. At present there are no accurate techniques for measuring contact angles on particles. The most reliable data for receding contact angles of water on fully methylated quartz plates give ecornposite as 72” [ 151. The Cassie Equation, in conjunction with our previous study [7] of the methylation of quartz particles, enables the calculation of orWed@ for surfaces of varying surface coverage to be performed. Assuming that quartz plates and quartz particles show the same characteristics, oreceding angles for the various size fractions and critical surface coverages are given in Table 3. Theoretical studies of floatability limits attempt to describe the upper [ 16-181 or lower [ 161 limits of floatability in terms of particle diameter, d max or drnin, under varying conditions. These treatments of the upper limit have been reviewed in detail elsewhere [ 191 and predict a d,, as a function of oreceding trend which is quite the reverse of that exhibited by the data in Fig. 4 and Table 3. This does not imply that these treatments are invalid; they may simply relate to particle sizes greater than those examined in the present study. Trahar has performed a detailed and thorough examination of particle

L 0

I

1

Q-

I

10

20

SURFACE COVERAGE (percentag.)

Fig. 4. Particle size as a function (average measured particle size).

TABLE

of minimum

surface

coverage

required

for

flotation

3

Minimum contact tive particle size

angle

for flotation

Mean particle diameter (pm)

Surface coverage at zero recovery

13+3 17t 4 25* 6 36+ 6 42i 7 47 + 10

25 10 9 7 6.5 5.0

(%)

calculated

from

the Cassie

equation

for the respec-

Calculated minimum receding contact angle (0 ) 34 21 21 18 17 15

size data extracted from plant surveys and batch flotation tests [l] . He interprets his findings in terms of flotation probability, PF, following earlier work in this area. Now PF a: PC *PA *Ps, where PC, PA and Ps refer to the probabilities of collision, adhesion and stability of the bubble particle encounters. PC is of course directly related to d, the particle diameter, while

49

where f(4) increases with @ and f’(d) increases with d. q3 is taken to be a net measure of hydrophobicity. The interplay between PC, PA and Ps leads to a qualitative prediction of flotation recovery versus particle size curves. In the latter part of this paper, Trahar [l] remarks that: “There is a need for quantitative information on the collector requirements of individual size fractions of different minerals if the efficiency of selective flotation of minerals from increasingly refractory ores is to be improved. Experimental attempts to derive such relationships have produced results which have been so variable that little recent research in this topic has been undertaken.” In the presence of variable concentrations of water-soluble collectors, surface tension, bubble size, electrical double layer properties and rate of collector adsorption, to name but a few, parameters may vary so that it is not surprising that an experimental or theoretical link between floatability, particle size and surface coverage or hydrophobicity only has been so difficult to establish. The only theoretical treatment of d,h has been performed by Scheludko et al. [16]. The critical work of expansion of a three-phase contact (i.e. the work necessary to form a “hole” when a bubble just makes incipient contact with a particle) was equated with the kinetic energy of the particles, yielding a minimum particle diameter for flotation as d,i,=2

V2py(l-

1

l/3

3K2

cos e)

(3)

where K is the line tension, V is the bubble ascent velocity, p is the density difference between the particles and the liquid, y is the air-liquid surface tension and 8 is the contact angle. The value and significance of the line tension poses the greatest uncertainty. First suggested by Gibbs [20] and partially explained by Harkins [ 211, it has been analyzed by Lane [22], Pethica [23], White [24] and others (e.g. Ref. [28] ). Experimental data are scarce and often equivocal [251,so that calculations involving line tension are fraught with uncertainty. Using the extreme K values of 2.8 and 5.6 X lo-” N determined by Scheludko et al. [16], values of d,h were computed in the present study. V was determined to be 24 ? 5 X lo-’ m s-l by high speed photography, corresponding to an average bubble diameter of - 1.2 X 10m3 m [B]. y was measured as 72.0 mN m-’ and p was taken as 1.65 kg me3. Calculations were performed with V = 20 X 10d2 and 30 X 10m2 m s-l. The results of these calculations are shown in Fig. 5 as curves A to D. The experimental data from Table 3 are plotted in Fig. 5 as curve E. Clearly, there is very poor agreement between the experimental data

50

10

30

20 Receding Contact (degrees)

Fig. 5. Particle size (A), c&,~, K = 2.8 20 X 10.” m se’; 5.6 x lO-‘O N, V decrease in particle

40

Angle

versus minimum receding water contact angle required for flotation. x lo-” N, V = 30 x 10-l m s-l; (B), d,,, K = 2.8 X lo-” N, V = K = 5.6 x 10“’ N, V = 30 X 1O-2 m s-l; (D), c&h, K = (C), c&h, = 20 X 10m2 m s-‘; (E), results from this study - Table 3; (H), 50% size from curve E.

and those expected from Eqn (3), although the d versus oreceding data at least show the same trend, i.e. there is qualitative agreement with Scheludko’s theory. For a smaller particle with less kinetic energy, a larger contact angle is required for it to float, since a smaller amount of energy is required to nucleate a “hole” and allow the three-phase line of contact to expand. Even if the particle size were overestimated by 50%, curve E would be shifted only slightly (curve H). For the experimental data and theoretical values from Eqn (3) to be reconciled, the value of K would need to be an order of magnitude or so larger than those determined by Scheludko et al. [ 161. This appears to be physically unreasonable if theoretical estimates of K are considered [24], albeit in a system where only dispersion forces are considered. Furthermore, K should also be dependent on contact angle [24, 261. The evidence to date, therefore, suggests that the concept of line tension is inapplicable in this present flotation study and should be restricted only to very small droplets, bubbles and particles [23]. Influence

The

of ionic strength on floatability

effect

of increasing

ionic

strength

on floatability

is well known

51

[ 131. The aqueous film between a bubble and a particle must thin and rupture before flotation can occur; a reduction in electrical double layer repulsion will lead to a kinetically more efficient process. Rate constants will be increased and the recovery in a given period of time will be enhanced; this improved recovery is shown by the data in Table 2. There is clearly a direct correspondence with coagulation theory. At the higher salt concentrations (e.g. 0.5 M) bubble coalescence is retarded [ 111, leading to an increase in flotation recovery due to an increased number of bubbleparticle collisions per unit time, resulting from an increased number of bubbles per unit volume. Influence

of urea on floatability

The effect of water structure modifiers on mineral flotation has received very little attention, despite their well-known influence on water structure. Brill and Pratt [27] showed that the flotation rate of sphalerite was increased in the presence of high concentrations of the structure breaker urea and depressed in the presence of sucrose, a structure maker. Brill and Pratt suggested that urea increases the rate of bubble-particle attachment by disrupting water structure and enhancing the rate of thinning of the intervening liquid film. In this present study, low concentrations of water structure modifiers had no detectable influence on flotation response. At higher concentrations any effects on water structure resulting from the addition of simple salts were obscured by the effects on bubble size distribution alluded to in the previous section. The marked difference in behaviour between angular and smooth particles has been noted by Anfruns and Kitchener [ 131. They showed that strongly hydrophobic angular quartz particles were captured by bubbles with 100% efficiency whereas the capture efficiency of strongly hydrophobic glass ballotini was very much lower. The 100% capture efficiency of the angular particles was attributed to jagged projections or asperities on the particle surface leading to local thinning and rupture of the wetting film, i.e. the flotation rate was enhanced compared with the smooth surface of the glass ballotini. In this study it seems that any water disrupting effects due to urea are overridden in the case of the angular quartz particles - the asperities dominate the film-thinning phenomena. Urea at concentrations above ca. 0.5 M markedly enhances the flotation rate of smooth, methylated glass ballotini, presumably through its effect on short-range hydrodynamics resulting from a reduction in the local viscosity in the thin liquid film. The exact mechanism is unclear. These results are in conflict with the observations made by Brill and Pratt [ 271. It is just possible that their sphalerite particles possessed smooth faces so that, on the approach of an air bubble, the flotation behaviour parallels that of glass ballotini.

52 SUMMARY

The flotation response of methylated quartz particles has been studied as a function of particle size and surface coverage, ionic strength and in the presence of water structure modifiers. (1) For all particle sizes examined there is a critical surface coverage below which particles do not float. This is possibly an induction time effect and is not explained by present theory. In particular, the capillary theory advanced by Scheludko et al. [16] is shown to be in qualitative agreement only with the experimental findings. (2) Increasing ionic strength leads to an increased flotation rate. (3) Urea, at solution concentrations above about 0.5 M, enhances the flotation rate of hydrophobic glass ballotini, but not of hydrophobic, angular quartz particles. The effect is probably due to a reduction in the local viscosity in the thin liquid film between a particle and an approaching air bubble. ACKNOWLEDGEMENTS

Support for this project from the Australian Research Grants Scheme and from the Australian Mineral Industries Research Association is gratefully acknowledged. Fruitful discussions with Professor L.R. White, University of Melbourne, are gratefully acknowledged.

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W.J. Trahar, Int. J. Miner. Process., 8 (1981) 289. G.W. Heyes and W.J. Trahar, Int. J. Miner. Process., 4 (1977) 317. J. Ralston, P. Alabaster and T.W. Healy, Int. J. Miner. Process., 7 (1981) 279. J.R. Gardner and R. Woods, Int. J. Miner. Process., 6 (1981) 1. J. Ralston, Adv. Colloid Interface Sci., 19 (1983) 1. P. Blake, M. App. SC. Thesis, Swinburne Institute of Technology, Melbourne, 1983. P. Blake and J. Ralston, Colloids Surfaces, 15 (1985) 101. J. Leja, Surface Chemistry of Froth Flotation, Plenum, New York, NY, 1982. F. Franks, Water, Royal Society of Chemistry, London, 1983. M.J. Rosen, Surfactants and Interfacial Phenomena, Wiley, New York, NY, 1978. in R.J. Akers (Ed.), Foams, Academic, London, J.B. Melville and E. Matijevic, 1976. E.G. Kelly and D.J. Spottiswood, Introduction to Mineral Processing, Wiley, New York, NY, 1982. J.F. Anfruns and J.A. Kitchener, Trans. Inst. Min. Metall. Sec. C, 86 (1977) C9. G.L. Collins and G.J. Jameson, Chem. Eng. Sci., 32 (1977) 239. R.M. Lamb and D.N. Furlong, J. Chem. Sot. Faraday Trans. 1, 78 (1982) 61. A. Scheludko, B.V. Toshev and D. Bojadiev, J. Chem. Sot. Faraday Trans. 1, 72 (1976) 2815. H.J. Schulze, Int. J. Miner. Process., 4 (1977) 241. C. Huh and S.G. Mason, J. Colloid Interface Sci., 47 (1974) 271.

53 19 20 21 22 23 24 25 26 27 28

J.A. Finch and G.W. Smith, Miner. Sci. Eng., 11 (1979) 36. J.W. Gibbs, The Collected Works, Longmans, Green & Co., New York, NY, 1928. W.D. Harkins, J. Chem. Phys., 5 (1937) 135. J.E. Lane, J. Colloid Interface Sci., 52 (1975) 155. B.A. Pethica, J. Colloid Interface Sci., 62 (1977) 567. L.R. White, personal communication. J. Mingins and A. Scheludko, J. Chem. Sot. Faraday Trans. 1, 75 (1979) 1. H.J. Schulze, Physico-Chemical Elementary Processes in Froth Flotation, Elsevier, Amsterdam, 1984. G. Brill and J.M. Pratt, Int. J. Miner. Process., 6 (1979) 193. I.B. Ivanov, B.V. Toshev and B.P. Radoev, in J.F. Padday (Ed.), Wetting, Spreading and Adhesion, Academic, London, 1978.