~ ) Pergamon Minerals Engineering, Vol. 7, No. 9, pp. 1085-1098, 1994

Elsevier Science Ltd Printed in Great Britain

0892-6875194 $7.00+0.00

0892-6875(94)00054-9 T H E S U R F A C E P R O P E R T I E S AND F L O T A T I O N

B E H A V I O U R O F X E N O T I M E

T A - W U I C H E N G , A . C . P A R T R I D G E , T A M T R A N a n d P . L . M . W O N G

Centre for Minerals Engineering, University of New South Wales, P.O. Box 1, Kensington, 2033, NSW, Australia

(Received 18 Januasy 1994; accepted 1 March 1994)

ABSTRACT Smface properties and flotation behaviour of xenotime using sodium oleate as a collector have been investigated. Adsorption isotherm attd flotation studies in combination with chemical modelling were conducted to investigaie the surface properties of xenothne and mechanisms of the interactions of the smface with the collector and the collector-treated stnface with air bubbles. It was foutwl that the adsot.'ption process on a xenotime sulface is very much pH dependent. The amount of active sites on the sulface available for reacting with the collector is related to the distribution of hydroxyl species. Front both experimental observations and chemical modelling, sulface precipitation is believed to play an important role its governing the mechanisms of flotation its acidic and alkaline media. Surface precipitation may render the mineral sulface more hydrophilic. From the adsotption isotherm at 25°C, the orientation of oleate molecules was found to be vertical, having a cross-sectional area of 21,~ 2. On the other hand, at a higher temperature of 70°C, the cross-sectional area changed to a higher vahse of 42e~ 2, indicating a combination of vertical and flat orientations.

Keywords Flotation; xenotime; adsorption; sodium oleate

INTRODUCTION

With the development of advanced technological applications of rare-earth elements and new uses based on their unique chemical and physical properties, the demand for rare-earth elements has increased rapidly. Xenotime, a major source for heavy rare-earth dements, is recovered as a by-product from the concentration of heavy minerals such as monazite, zircon, ruffle, ilmenite and garnet. With some heavy mineral deposits, the valuable constituents are in a fine size range, for which standard physical processing methods are less efficient and heavy losses occur. Therefore, for technical and economic reasons, flotation is an attractive method and offers a better alternative for xenotime recovery. The separation of rare earth minerals from each other is difficult due to their similar atomic structures and surface characteristics. The effectiveness of flotation is primarily controlled by the surface properties of the minerals and is significantly related to the collector adsorption at the mineral-water interface. The most extensively used collectors in rare-earth mineral separation are the long-chain fatty acids and their alkaline salts, especially oleic acid and sodium oleate. It is therefore necessary to systematically investigate the surface chemistry of xenotime in oleate solution. This paper reports results of adsorption isotherms, flotation and physico-chemical modelling studies, and discusses the mechanisms of xenotime flotation in the presence of sodium oleate.

1085

1086 TA-WuI CHENG et al.

A useful measure of the affinity between bubble and particle surfaces is the attachment time, the time required for thinning and rupture of the fluid film on contact. Cross and Miller [1] measured the bubble attachment time for monazite and xenotime with various sodium fluorite additions at different temperatures in sodium oleate solutions. By increasing the oleate concentration from 10 -6 M to 10 -5 M, a slight decrease in the bubble attachment time was found. However, on increasing the concentration to 10 -4 M, the bubble attachment time dramatically decreased. The effects of oleate concentration and temperature on the bubble attachment time for different phosphate minerals have also been reported and the zeta potential [2,3] and microflotation [3] of xenotime have been investigated. However, there seems to be limited information available on the details of adsorption isotherms or the flotation behaviour of xenotime.

EXPERIMENTAL

After screening into different size fractions, pure samples were selected under a microscope for use in flotation tests and adsorption studies. Flotation tests were performed in a modified Hallimond Tube.

Each flotation test used 1.2 g of xenotime and the air flow rate was constant at 60 cm 3 per minute. Flotation tests were carried out for 3 minutes. The particle size of the xenotime particles used for flotation was in the range of +90-106 Izm.

Adsorption isotherms were determined by equilibrating 0.5 g of finely ground xenotime particles in 100 ml of oleate solution at predetermined concentrations. The surface area of the xenotime was determined by the BET adsorption method using nitrogen gas and was tbund to be 1.15 m 2 g-l. The amount of oleate adsorbed was determined by the difference of oleate concentration in the solution before and after the addition of the solids. After conditioning for 60 minutes, the solution was centrifuged at 3000 rpm for 10 minutes. The concentration of oleate was determined by spectrophotometer using a triethylenetetramine copper(II) complex method [4].

CHEMICAL MODELLING

Computer programs DIASTAB [5] and THERMOCHEMISTRY [6] were used to generate the solubility and species distribution diagrams, respectively. Free energy of formation data were obtained from National Bureau of Standards (NBS) data [7] in the THERMOCHEMISTRY database, and from work by Firsching and Brune [8] and Turner et al. [9].

EXPERIMENTAL RESULTS

Flotation Tests

Figure 1 shows the relationship between collector concentration and flotation recovery of xenotime at pH 7. The recovery increases continuously with increasing sodium oleate concentration till it reaches complete flotation recovery at approximately 5 x 10 -5 M. The effect of pH was also investigated Ibr various sodium oleate concentrations (Figure 2) and for various flotation times (Figure 3). With increasing concentration of sodium oleate, the recovery increases with increasing pH values till a maximum is reached and then decreases. Maximum flotation recovery is tbund to be around pH 7.5. Figure 4 shows that flotation recovery of xenotime decreases with increasing temperature.

Adsorption Studies

A preliminary study of the kinetics of adsorption showed that the adsorption of oleate on xenotime reaches near completion within 60 minutes. The most important factor that affects the degree of adsorption of collector is the pH of the system. Variation of pH affects the type of predominant species in the bulk solution and on the mineral's surt'ace, causing different type of reactions at the aqueous-solid interface.

Surface properties and flotation of xenotime 1087

Figure 5 shows the adsorption density of oleate on xenotime as a function of pH for 5 x 10 °06 M and 1 x 10 -4 M initial oleate concentrations. This result is replotted in terms of the percentage of adsorption and is shown in Figure 6. It can be seen that adsorption occurs at pH values up to 8, with a sharp decrease at pH 7.5. Adsorption troughs occur at around pH 5 and 6 for lx l0 "4 M and 5x10 "6 M initial concentrations, respectively.

Fig. 1

20

Recovery (%)

0 ~" I I I t I I I I I I I ' I I I i i I

lO .6 10 .5 10 .4

S o d i u m O l e a t e C o n c e a t r a f i o n (M)

Flotation recovery of xenotime as a function of sodium oleate concentration

Fig.2

HE 7 :9 -B

Recovery (%) 100 -- - "~ - ,.., n

90 lO-4, M

--'/" . \+ + 5 . ,o-0, , ao ,o / \ * ,o-0 ..

6 0

5 0

4O

BO

2 0 I

t0! 0 t i t ,

0 1 g 8 4 5 6 7 8 9 10 11 12 13 14.

pH Xenotime flotation recovery as a function of pit with various collector concentrations

(flotation time 3 minutes)

1088 TA-WuI CHENG et al.

1oo

9o

0o

70

60

flO

40

3O

2O

10

0 0

Recovery %

a m b i .

Q l m l n .

0 10 s e o .

I I I I I I I I I I I I I

1 2 3 4 5 e 7 8 9 10 11 12 13

p H

14

Fig.3 Effect of pH on recovery of xenotime with 5 x 10 -6 M

R e c o v e r y % 100

90-

80

70

0 0 -

5 0 -

4 0 -

80

2O

10

0 1o

I I I I I I I

20 30 40 50 60 70 80 80

T e m p e r a t u r e (*C)

Fig.4 Flotation recovery as a function of temperature (3 x 10 -6 M sodium oleate, pH=7.0 :t: 0.2, flotation time=3 min.)

Adsorption Isotherms

Isotherms of adsorption of oleate on the surface of xenotime in an aqueous solution at various equilibrium oleate concentrations were obtained at pH 7.5. This pH was selected since it corresponds to the pH at which maximum adsorption occurs. The isotherms at 25°C and 70°C are presented in Figure 7. The plateau indicates monolayer coverage. The monolayer density at 25°C is found to be 8 x 10 -6 mole.m -2. Assuming vertical orientation, the oleate molecule cross-sectional area is calculated from these results to be 21 A 2. This calculated cross-sectional area is very close to that reported in previous investigations [I0], which was 20.5 ~,2.

10 - 4

10 "5

10-6

lO-7

10"8

pH

A(k~, ~ ( ~ )

Surface properties and flotation of xenotime 1089

1 • IO'4M

5 • IO'6M

I I I I I I I I I I I i I

0 1 2 8 4 8 6 7 8 0 10 11 12 18 14

AdmrptJo. (%) . , i n

Fig.5 Effect of pH on the adsorption of sodium oleate on xenotime at initial concentration of 1 x 10 -4 M and 5 x 10 -6 M.

100

O0

80

70

60

50

40

30

20

10

0 0

1 • 10 .4 M

Sx 10"6M

I I I I I I I I I I I I I

1 2 3 4 5 8 7 8 9 10 11 12 18 14

pH

Fig.6 The percentage of the uptake of sodium oleate on xenotime.

Isotherms exhibit three different regions. The first region is where adsorption increases with equilibrium oleate concentration until a plateau, the second region, is reached where a monolayer exists. As equilibrium oleate concentration is further increased beyond the monolayer equilibrium concentrations, adsorption again increases, entering into the third region.

An increase in temperature has an obvious effect on the adsorption isotherms. It can be seen that the isotherms shift downward at 70°C. The shifting of the isotherm plateaus to a lower adsorption density indicates horizontal monolayering. The horizontal monolayer density for xenotime is found to be 4 x 10 -6

1o90 TA-WuI CHENG et al.

mol.m -2 which produces a horizontal cross-sectional area of 55 ~2. This value is about 50% of that reported earlier by Yehia and his co-workers [10], of 92 ~2. It is noted that the adsorbed oleate molecules may have a combination of vertical and flat orientations, instead of lying totally flat on the surface. This may explain the variation of the cross-sectional area for horizontal oleate molecules on xenotime surfaces.

10_4 Adsorption Denlity (mole/m2)

, o - 5 . . . . . . . . . . . . . . . .

-7 10

250C 70 ° C

I I I I I I I I I I I I I I I I I I I I I I I I I I

10-6 10-5 10-4 10-3

Equilibrium Concentration (mole/litre)

Fig.7 Adsorption isotherms for xenotime-oleate system at pH 7.5 + 0.2

DISCUSSION

To study the flotation of xenotime, surfactant adsorption and flotation behaviour must both be evaluated, due to their significant effects on the efficiency of flotation. When minerals are conditioned with collectors under suitable conditions, a hydrophobic surface is formed by either physical or chemical adsorption, sometimes a combination of the two adsorption processes.

The adsorption of oleate ions on xenotime is believed to be chemisorption. This is shown by the fact that adsorption still occurs above the point of zero charge of the xenotime (pH = 3), despite the fact that xenotime surface and oleate ions are both negatively charged [3]. The standard free energy of adsorption for oleate on xenotime up to monolayer coverage can be determined by the equation [11]:

0

1-0 55.55

where 0 is the fraction of surface sites occupied, C is the equilibrium concentration of surfactant.

In this study, the standard free energy of adsorption at 25°C and 70°C (Figure 7) are computed to be -38.9 kJ/mol and -39.9 kJ/mol respectively, indicating ehemisorption [12].

According to the solubility diagram of sodium oleate [13], oleate acid will be precipitated from aqueous solution at lower pHs. Hence the increasing adsorption of oleate on xenotime at lower pHs (Figures 5 and 6) may be due to the aqueous precipitation or surface precipitation. Surface precipitation would be the

Surface properties and flotation of xenotime 1091

likely process since mineral surfaces are present. The surface precipitate may be in the form of admicelles (surface aggregates). As the pH is decreased gradually, the proportion of chemisorption to surface precipitation decreases. The transition between ehemisorptio.n and surface precipitation may have caused the adsorption troughs at pH 5 and 6 in Figure 6.

Similar adsorption isotherm results were also obtained by Parkins and Shergold [14] with an ilmenite-oleate system. It was found that the adsorption process at lower oleate concentration is an irreversible process, implying chemisorption. At higher concentrations, second layer starts to form and the process becomes reversible, which shows that the bonding in the second layer is weaker than the first layer. This second layer is probably due to surface precipitation as mentioned before. This can be seen from the solubility diagram of sodium oleate system [13], that as the equilibrium concentration of oleate increases beyond 1 x 10 -5 M at pH 7.5, aqueous precipitation occurs. With the xenotime-oleate system (Figure 7), second layer formation starts at around 1 x 10- 4 M and pH 7.5. Therefore, the increasing adsorption on the xenotime surfaces at higher concentrations could be due to a combination of chemisorption and surface precipitation of oleate.

The effect of temperature on the floatability of xenotime in the presence of sodium oleate is strongly related to the solubility of oleic acid at different temperatures. Oleic acid is a weak acid with limited solubility (6 x 10 -7 M at 25°C). The nature of the oleate species in aqueous solution depends on the extent of hydrolysis and on the pH of the solution [15]. Read and Manser [16] pointed out that for a weak electrolyte surfactant adsorbed at the mineral-water interface, the degree of ionisation of the surfactant will decrease due to the decreased hydration of the polar group. The degree of hydrolysis of sodium oleate for natural pH values with different temperatures has been determined by Powney and Jordan [17]. With increasing temperature, the degree of hydrolysis of sodium oleate increases. According to Dixit and Biswas' [18] theory, the concentration of O1" and HOI(1 ) at each pH value at different temperatures can be computed. The computed results are shown in Figure 8. It is clear that, with increasing temperature, the oleic acid species, HOI(I ), becomes more predominant at higher pH. This indicates that at any nominated total concentration, the solubility of oleic acid decreases with increasing temperature. This confirms Read and Manser's [16] hypothesis. Therefore, the concentration of monomer species, O1-, which can react with the mineral surface, would decrease and the adsorption density decreases at higher temperature. This could be the reason for the decreasing flotation recovery of xenotime (Figure 4) at a higher temperature.

0

1

2

3

4

5

6

7

8

9

10 0

- l t ~ C ~ t r . t i ~ 04)

1 - - 25°(7

2--seOc

3 - - 80°(2

HO!

I I I I I I I I

1 2 8 4 1 0 I I 1 2 1 3 1 4

./" //Y'//::" /:" ... /

1 2 3

. . . .

/" / / /

/:'" / ... / / / " . . / / ' . /" - .:.'/

/ ./ .:/' .." i m .... .."i i I

5 8 ? 8 9

p i t

Fig.8 Solubility diagram of sodium oleate (dotted lines indicate extrapolated values).

1092 T A - W m C H E N G et al.

The adsorption density at any particular concentration usually decreases with increasing temperature [ 19]. The same phenomenon applies to the adsorption of oleate on xenotime surfaces, especially at lower equilibrium concentrations. In the ease of adsorption at a lower concentration, the adsorption density decreases with increasing temperature, probably due to the reduced solubility of oleic acid. But at higher concentrations the adsorption density increases with increasing temperature. This is due to the physical adsorption of oleic acid on mineral surfaces in addition to the purely chemical adsorption experienced at lower concentrations. Similar result was also found in a monazite-oleate system [13] and ilmenite-oleate system [14].

In the xenotime-solution system, cations transfer from the mineral surfaces into solution forming hydroxylated complex ions. From the ionic species distribution diagram for xenotime (YPO4) (Figure 9), and solubility diagram of yttrium phosphate (Figure 10), the predominate aqueous species below pH 10 are y3+, Y(OH)2+ and Y(OH)2 +.

-3

-5

-7

-9

-11

-13

-15 0

Log Concentration (M)

YPO 4 (s) Y(OH) 3 (s) 1

1 2 3 4 5 6 7 8 9 10 11 12 13 14

pH

Fig.9 Ionic species distribution diagram for xenotime (YP04) and associated species at 104 M total concentration

0

!

3

3

4

S

6

7

8

9

10

i i

12

JlY+++]I]'04--] l I t I I I I I I | ! ! I

y l q D 4 (8)

J

, , , , , , , , i , J , ' , , ,

0 I Z $ 4 S i 7 I • I0 I I 12 I$ 14

pH

Fig. 10 Solubility diagram of Yttrium-Water System

Surface prol~rties and flotation of xenotime 1093

Figure 11 illustrates the effects of concentration on adsorption, flotation recovery and probability of detachment (obtained from Cheng and co-workers[3]) in the xenotime-oleate systems. It can be seen that by increasing the sodium oleate concentration (in the ease ot ~ adsorption the concentration corresponds to the equilibrium concentration), the adsorption density and flotation recovery increased and the probability of detachment decreased. Figure 12 illustrates the observed trends in adsorption and flotation response for xenotime as a function of pH. It is clear that the distribution of the first hydroxy species Y(OH) 2+ shows a good correlation with the adsorption and flotation results and a sharp peak was found at about pH 7.5.

Ad~rptiee Demlty (,-de/m2) Prelmbilli~ of Detschmm* Recovery %

10 "$ 0.7 70

0.6 80

0.5 50

0.4 4 0 I0 4

L3 50

0~2 20

0.1 10

10"7 0 0

10 4 10 -5 10 -4 10 -3

Concentrttion (mole/Utre)

Fig. 11 The effect of collector concentration on adsorption, flotation recovery and probability of detachment of xenotime-oleate system. (Adsorption studies were carried out at pH 7.5 +0.2, whereas the

flotation and detachment studies were done at pH 7.0+0.2)

Recovery %

120 _ -II

/ J ~ \ oo

- - *, I~ 80 l O 0 Itl~"o~7 % =.,.._

/= \ v -12 70

V(Ol~ "M" / 60

8 0 5 0

4 0

- -13

8 0 iv,. ~ ~ 20

D ~ 10

4 0 i i i i t i i i t i i i t -14 0

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

pli

Fig. 12 The effect of pH on distribution of RE(OH) 2+ species, adsorption, flotation recovery and adhesion strength in xenotime-oleate system (5 x 10 -6 M of sodium oleate).

1 0 9 4 T A - W U i C H E N G et al.

The flotation results have good correlation with the distribution of the rare earth first hydroxyl species, adsorption, and adhesion strength (obtained from Cheng and co-workers [3]). The relationship between each other is presented in Figure 13 and is suggested as follows:

.

2.

.

.

Hydroxyl species are formed on the mineral surface when it is conditioned in the solution (Figure 13 A). These hydroxyl species, especially the first hydroxyl species [RE(OH)2+], are very dependent on pH, and may bind with oleate ions to form an adsorption layer on the mineral surface (Figure 13 B). This adsorption layer, in the form of hemicelles, makes the mineral surface hydrophobic and assists the rupture of the thin film when the mineral particle encounters a bubble. Thereafter, a three-phase contact is formed allowing flotation (Figure 13 C and D). The adsorption of collector directly affects the hydrophobicity of the mineral surface and three-phase contact angle formation, and consequently affects the adhesion strength in the bubble-particle interface and flotation.

(A)

Surface Hydroxyl Groups

f ~ 4 ~ / H 0 0 0 0 /

i °,

(B)

(c)

(D)

R R Adsorption Layer [ I (Hydrophobic Surface)

Jl m /~\ / / \

0 0 0 0 ~ .W..W.III

Induction time < contact tlme Thin film rupture

.................. 3

Three Phase Contact

Fig.13 The mechanism of particle-bubble attachment process: (A) Formation of the surface hydroxyl groups on the mineral surface. (B) Adsorption of collector molecules on mineral surthce. (C) Collision

of a particle with a bubble, followed by the rupture of the thin film. (D) Formation of a three phase contact.

Surface properties and flotation of xenotime 1095

In the alkaline region, a sharp decrease in flotation recovery was found. This is considered to be due to the decreased concentration of the first hydroxyl species, RE(OH) 2+, and the formation of rare earth hydroxide, RE(OH) 3 (s), on the surface of minerals (see Figures 9 and 10). Though oleate ions are abundant in the aqueous solution, the number of reaction sites is much less than before (see Figure 14 A). This leads to the lower adsorption density of oleate on the mineral surfaces, and hence reduced hydrophobicity. This also implies that the thin film remains stable (Figure 14 B), leading to a decrease in flotation recovery.

(A) species decrease ion decreues ,ce bydrophobicity decreases

(n) thin film rtmaial stabl~

~ / induction time > contact time , d r

I ~ probability of detaehn~lt increases

Fig.14 The mechanism of xenotime flotation system in the alkaline region: (A) Particle surface hydrophobicity decreases due to the decreased concentration of the first hydroxyl species. (B) Thin film

remains stable, and the probability of detachment increases.

In the acidic range, with decreasing pH, the flotation recovery also decreases. The two contributing factors are:

(1) (2)

the gradual decrease of the first hydroxyl species, and the aqueous or surface precipitation of oleic acid.

The increase in adsorption density in the lower pH region is mainly due to surface precipitation, instead of direct adsorption of the oleate ions onto the minerals surface. Oleic acid would initially precipitate in the form of micelles in the aqueous solution, and the hydrophilic head of the micelles are simultaneously chemisorbed by the minerals. The adsorption of micelles in this manner makes the mineral surface more hydrophilic, as the hydrophilic heads of the adsorbed oleate are directed towards the aqueous solution (see Figure 15 A). Therefore, the adhesion strength between particle and air bubble decreases. This multiple action causes the aggregation of mineral particles (Figure 15 B). The mass of the aggregates compared to single particles could be several times larger, causing poor recovery (Figure 15 C). In addition, very few oleate ions are attached to the air bubbles, due to the very low concentration of oleate ions in the low pH solution. Therefore, the bubble transfer mechanism [3], in which the bubble plays an active role in transferring surfactant from the solution to the mineral surface, will not occur.

1096 TA-WuI CrlENO et al.

(A)

~"

( l i )

~ " ...................... IP' nurt'n~

( B u b b l e )l littlle mono~rs ut~cln to bubble \ / / r,.n.,-.,,.bl.

induction time :~ contact time

(c)

Fig. 15 The mechanisms of xenotime flotation system in the acidic region: (A) Particle surface becomes more hydrophilic due to the formation of admicelles on the surface. (B) Formation of particle aggregates.

(C) Thin film remains stable, the probability of detachment increases, leading to poor recovery.

In the low pH oleate solution, when a mineral particle is brought into contact with an air bubble, the hydrophilic heads have to flip over and let the hydrophobic tails of the adsorbed micelles diffuse into the bubble, The diffusion time is therefore longer than that at higher pHs, requiring a longer induction time for the particle to attach to an air bubble. Consequently, the thin film remains stable, the probability of detachment increases, and the flotation recovery decreases. This may explain the lower flotation recovery in the lower pH region (pH < 6).

CONCLUSION

Microflotation tests using a modified Hallimond tube showed that the recovery of xenotime is a function of pH, flotation time, sodium oleate concentration and temperature. Maximum flotation recovery was found at pH 7 to 8.

If the mineral is slightly soluble, the dissolved metallic species will be hydrolysed and readsorbed on the mineral surface, subsequently activating the mineral surface 'for flotation. Computer-assisted calculations of thermodynamic equilibria were successfully carried out to simulate the solution chemistry of the mineral systems in this study. From the chemical modelling of the speciation of xenotime, it was found that the distribution of the first hydroxyl rare earth species, RE(OH) 2+, shows a good correlation with the adsorption, bubble-particle adhesion strength, and flotation results. It is, therefore, suggested that the distribution of the first hydroxyl species is the key factor in the flotation of xenotime with sodium oleate as a collector.

From a physico-chemical point of view, it is clear that the pH has two important roles in adsorption and flotation processes:

Surface properties and flotation of xenotime 1097

(1)

(2)

In the acidic region, the solubility of oleic acid decreases with decreasing pH, causing aqueous or surface precipitation and hence poor flotation. In the alkaline region, oleate remains soluble and hence the role of the hydroxyl ions and the precipitation of rare earth hydroxides becomes the predominant processes in deciding the extent of the adsorption of oleate species and hence flotation.

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4.

5.

6.

7.

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9.

10.

11.

12.

13.

14.

15.

16.

REFERENCES

Cross, W.M. & Miller, J.D., Bubble attachment time measurements for selected rare-earth phosphate minerals in oleate solutions, Rare Earth, Extraction, Preparation attd Applications, R.G. Bautista and M.M. Wong Editors, The Minerals, Metals and Materials Society, 45-55 (1988). Harada, T., Owada, S., Takiuchi, T. & Kurita, M., A flotation study for effective separation of the heavy mineral sands, XVIII InternationalMineral Processing Congress, Sydney, 4, 1017-1024 (1993). Cheng, T.W., Holtham, P.N. & Tran, T., Froth Flotation of Monazite and Xenotime, Minerals Engineering, 6, No 4, 341-351 (1993). Gregory, G.R.E.C., The determination of residual anionic surface-active reagents in mineral flotation liquors, Analyst, 91(4), 251-257 (1966). Robins, R.G. & Stillman, R.W., DIASTAB - A computer program for generating stability diagrams, Version 4.02, (1989). Turnbull, A.G., A general computer program for the calculation of chemical equilibria and heat balances, CALPHAD, 7, 137-147 (1983). Wagman, D.D., Evans, W.H., Parker, V.B., Schumm, R.H., Halow, I., Bailey, S.M., Churney, K.L. & Nuttal, R.L., The NBS tables of chemical thermodynamic properties Selected values for inorganic and C1 and C2 organic substances in SI units, J. Physical attd Chemical Reference Data, 11, (1982). Firsching, F.H. & Brune, S.N., Solubility products of the trivalent rare-earth phosphates, J. Chem. Eng. Data, 36, 93-95 (1991). Turner, D.R., Whitfield, M. & Dickson, A.G., The equilibrium speciation of dissolved components in freshwater and seawater at 25°C and 1 atm pressure, Geochimica et Cosmochimica Acta, 45, 855-881 (1981). Yehia, A., Miller, J.D. & Ateya, B.G., Analysis of the adsorption behaviour of oleate on some synthetic apatites, Minerals Engineering, 6, No. 1, 79-86 (1993). Hough, D.B. & Rendall, H.M., Adsorption of ionic surfactants, In: AdsorptionjS"om solution at the solid~liquid interface, eds. G.D. Parfitt and C.H. Rochester, Academic Press, New York, 247-314 (1983). Fuerstenau, D.W., An alternate reagent scheme for the flotation of Mountain Pass rare-earth ore, In Preprints - XIV btternational Mineral Processing Congre.ss, Toronto, session IV, pap.6, pp 12 (1982). Tran, T., Cheng, Ta-Wui, Partridge, A.C. & Won=-, P.L.M., The adsorption of oleate and its effect on the flotation of monazite, Rare Earths '93 Processing & Utilisation Workshop, Australian Nuclear Science & Technology Organisation (ANSTO), Lucas Heights, October, (1993). Parkins, E.J. & Shergold, H.L., The effect of temperature on the conditioning and flotation of an ilmenite ore, In: Flotation - A.M. Gaudin memorial volume, ed., M.C. Fuerstenau, AIME, New York, chapter 20, (1976). Ananthapadmanabhan, K., Somasundaran, P. & Healy, T.W., Chemistry of Oleate and Amine Solutions in Relation to Flotation, Trans. Society of Mining Engineers of AIME, 266, 2003-2009 (1979). Read, A.D. & Manser, R.M., Surface polarizability and flotation: study of the effect of cation type on the oleate flotation of three orthosilicates, Trans. btst. Mitt. Metall., 81, C69-C78 (1972).

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17.

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Powney, J. & Jordan, D.O., The hydrolysis of Soap as Determined by Glass Electrode pH Measurements, Transactions of the Fara. Soc., 34, 366-371 (1938). Dixit, S.G. & Biswas, A.K., pH - Dependence of the Flotation and Adsorption Properties of Some Beach Sand Minerals, Trans., AIME, 244, 173-178 (1969). Parfitt, G.D, & Rochester, C.H., Adsorption of small molecules, In: Adsorption fi'om solution at the solid~liquid intelface, G.D. Parfitt and C.H. Rochester eds., Academic Press, New York, 3-44 (1983).