Interactions in the sphaleriteCaSO4CO3 systems

COLLOIDS AND Colloids and Surfaces

A

SURFACES

A: Physicochemicaland EngineeringAspects 137 (1998) 69-77

ELSEVIER

Interactions in the sphalerite-Ca-SO4-CO3 systems C. Sui a, F. Rashchi a, Z. Xu b, j. Kim c, j. E. Nesset c, j. A. Finch a,. a Dept. Mining and Metallurgical Engineering, McGill University, Montreal, Que., Canada b Dept. Material and Chemical Engineering, University of Alberta, Edmonton, Aha., Canada ¢ Mineral Processing Laboratory, Noranda Technology Centre, Pointe Claire, Que., Canada

Received 29 January 1997; accepted 12 September 1997

Abstract

The interaction of sphalerite with calcium, sulphate and/or carbonate was studied using zeta potential measurements, scanning electron microscopy, and X-ray photoelectron spectroscopy. The interaction in the Ca and Ca-SO4 systems appeared to be the same, namely adsorption of ionic Ca species in the double layer. Neither Ca n o r CaSO 4 was detected by surface analysis. In the Ca-CO3 system, some CaCO3 precipitates were found on the surface. Addition of increasing CO3 to the Ca-SO4 system progressively converted CaSO4 to CaCO3 precipitates and the system eventually became identical to Ca-CO3 alone. The role of CO3 addition in flotation appears to be to remove ionic calcium species, whether SO4 is present or not. © 1998 Elsevier Science B.V. Keywords: Precipitates; Ca ion adsorption; Calcium sulphate; Calcium carbonate; Sphalerite flotation

1. Introduction In sulphide flotation plants, in particular those practising water recycling, high levels of Ca are frequently experienced. In some process waters Ca concentrations exceed 2.5 x 1 0 - 2 M (1000 ppm) [1,2]. Calcium ions result mainly from dissolution of Ca-containing minerals and the addition of lime as p H modifier and pyrite depressant. In the case of sphalerite, the presence of Ca ions may reduce the exchange rate with activating ions (primarily Cu) [3], resulting in a decrease in recovery, A high concentration of sulphate also exists in the process waters of many sulphide flotation plants owing to oxidation of sulphur species. If

* Corresponding author, 0927-7757/98/$19.00© 1998 ElsevierScienceB.V. All rights reserved. PII S0927-7757 (97) 00336-1

both Ca and SO4 levels are sufficiently high, precipitation of C a S O 4 o c c u r s . It has long been suspected that C a S O 4 precipitates coat sulphide minerals indiscriminately, leading to reduced selectivity and recovery. A recent example is the study of galena flotation at the Hilton Concentrator ( M o u n t Isa Mines Limited, Australia) [4]. An effective method to combat the detrimental effect of both Ca and CaSO4 on flotation is to add carbonate (e.g. soda ash) to precipitate Ca as CaCO 3 or convert C a S O 4 t o C a C O 3. When soda ash was added to the grinding mills at the Hilton Concentrator, galena flotation recovery was significantly improved. Surface analysis showed a decrease in both Ca and SO4 on galena [4]. In the present study, the interaction of sphalerite with Ca and precipitates of calcium-sulphate/ calcium-carbonate was investigated.

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2. Experimental 2.1. Mineral and reagents 2.1.1. Minerals Sphalerite, obtained from Ward's Natural Science Establishment, originated from the Kidd Creek Mine, Timmins, Ontario, and was selected because its high iron content (10.45%) resembles that of many base metal ores of Eastern Canada. It was crushed, and hand picked specimens were ground and screened to obtain a 38-74 ~m size fraction. The sample was purified by repeated processing on a shaking table to remove silica, followed by washing, first with dilute HC1 (pH 2) to remove products of oxidation, then three times with distilled water. The sample was stored under acetone until required. The chemical analysis is shown in Table 1. 2.1.2. Reagents The reagents used were Ca(NO3) 2 •4H20 (analytical grade from J. T. Baker Chemical Co.), Na2SO 4 (analytical grade from BDH Inc.), Na2CO 3 (Reagent grade from Fisher), and KNO3, HNO3, NaOH (analytical grade from Anachemical ). Analytical grade powder C a S O 4 • 2H20 (A and C American Chemicals) and CaCO3 (J. T. Baker Chemical Co.) were used for reference zeta potential measurements, 2.2. Methodology 2.2.1. Zeta potential measurement A Laser-Zee meter (model 501, Penkem Inc., USA) was used. The sphalerite sample (38-74 ~tm), 0.5 g was further ground with a ceramic mortar and pestle, and transferred to a 500 ml beaker containing 0.001 M KNO 3 electro-

Chemical composition of sphalerite used in experiments

Sphalerite

2.2.2. Scanning electronmicroscopy (SEM) Sample preparation was the same as for the zeta potential measurements except that the suspension was left for 20h (the beakers were sealed by paraffin film). After filtering, the solids were washed twice with distilled water and dried in air. A small portion of solids was mounted on double sided carbon tape or conductive silver paint. Finally, a thin gold coating was applied using a sputter technique. Particles and any precipitates were analyzed by energy dispersive spectroscopy, and micrographs of back scattered electron images were taken. 2.2.3. X-ray photoelectron spectroscopy (XPS) A portion of the conditioned samples (prepared for SEM) was used for XPS measurements.

Table 1 Mineral

lyte solution. The measurements were started from natural pH (5-6) and then adjusted toward the low pH direction using HNO3 or the high pH direction using NaOH. Following pH adjustment, the suspension was stirred for 3 min, and the zeta potential was measured. For each pH, the tests were repeated twice, and the average of the three results is reported (standard deviation was about 5 mV). In the cases where ions were added, a given concentration was introduced and the suspension mixed for 3 min. Then the pH was set, followed by stirring for a further 3 min. The sequence of adding chemicals was Ca, followed by SO4, and/or CO 3. Zeta potential measurement of C a S O 4 and CaCO3 compounds was similar to that of the sphalerite except in the amount used. To ensure a sufficient quantity of CaSO4 or CaCO 3 particles remained in suspension, 0.75 and 0.5 g C a S O 4 and CaCO 3 were added to 500ml distilled water, respectively. After pH was adjusted, the suspension was conditioned for 6 min, and zeta potential was measured.

Size range

% Wt. of element

(~n)

Zn

Fe

Pb

Cu

38 74

53.54

10.45

0.1

0.01

Spectra were obtained on an ESCALAB 220i-XL spectrometer with an A1Kat anode (hv = 1486.6 eV) at a normal take off angle. The X-ray was monochromated and the instrument was calibrated against the Ag3a5/2 band-pass energy of 20 eV

C Sui et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 137 (1998) 69-77

2o

corresponding to an energy resolution of 0.6 eV. The sample was prepared by pressing the powders onto a foil of adhesive-backed copper tape and surrounding with a stainless steel washer (for charge removal). The samples were maintained under the instrumental background pressure (10 -9 torr) for about 30 min in the sample chamber before spectra acquisition.

i - . - - -

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~-~0 -20 -30 -40

0

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2

4

6

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71

12

3. Results and discussion

pit

Fig. I. The zeta potential of sphalerite with and without Ca ions.

20 l0 ~

t

~

'

~

c.s~ ~ --[] .'-" ,_

'

-. iq0 -20

"%./

p+Ca+S04(ppt)

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2

. .6 . . 8 . . 10 #

12

14

Fig. 2. The zeta potential of sphalerite in the presence of Ca with CaSO 4 compound.

and SO4 ions; comparison

3.1. In presence o f Ca ions

The zeta potential of sphalerite alone became more negative as pH increased (Fig. 1). The iep was about pH 1.9. The iep of sphalerite ranges approximately from pH 2 to 8, depending on the degree of oxidation, conditioning time, pH, and solid/liquid ratio [5]. The low pHiep suggests that dominant species on the surface were sulphide or elemental sulphur, i.e. sphalerite was not significantly oxidized. The increase in the zeta potential at approximately pH 8.5 is tentatively attributed to the presence of ferrous ions (the sample contained 10% iron) by analogy with the observation of Zhang et al. [6]. In the presence of 1.25 x 10 - 2 M (500 ppm) Ca ions, the zeta potential increased significantly. Three ieps were observed, pH 3, 8, and 10. The increase in zeta potential was attributed to adsorption of Ca ions.

20 I

. 10

{ o i-10 ~.

-20

3.2. In thepresence o f Ca and S O 4 '

als~ca

., :: , -~, 2 \~,¢~ ~ .

J

, ~

.- ~ ~

~ "I3 ~ / ~ b )

~30 ~

~ ~

-40 2

' 4

°

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Sp+C~a(pp +St)O,

s

.....1'/~ . . . . . ~

i 8

i l0

i 12

14

pn Fig. 3. The zeta potential of sphalerite in the presence of: (a) 1.25 × 1 0 - 2 M Ca(NO3)2; (b) saturated CaSO4 with precipitates; and (c) supernatant of saturated CaSO4. Note: [Ca] was approximately 1.25 × 10 -~ M in both (b) and (c).

The zeta potential of sphalerite in the presenceof Ca (3 x 10 - 2 M) and S O 4 ions (5 x 10 -2 M ) is given in Fig. 2. The concentration of Ca and SO 4 used exceeded the solubility product of CaSO 4 (1 × 10 -5, [7]), i.e. the zeta potential of sphalerite was measured in the presence of CaSO 4 precipitates. Of significance is the fact that this zeta potential c u r v e i s s i m i l a r to t h a t i n t h e p r e s e n c e o f Ca ions alone shown in Fig. 1, but quite different from CaSO4 (included in Fig. 2). These observations imply that the effect on zeta potential is due t o C a ions To test

and n o t t o t h e precipitates of CaSO4. this, an experiment was designed t o

72

C Sui et al. / Colloids Surfaces A. Physicochem. Eng. Aspects 137 (1998) 69-77

Ca2p

ICd+ I~ 3xlO 2 M, IS02, 1=SzlO~M

i So + Ca÷S04

i 360

356

352

(a)

,

i , 344

i

,

i , 340

i

,

i 336

Binding energy ( m V )

S2p

(a)

i 348

led+ l= 3xlO 2 M, ISO~,l=5xl o 2 M

'~

,E

168 (b)

[

!

166

164

162

Binding

160 energy

158

156

154

152

(mV)

Fig. 5. XPS spectra of sphalerite in the presence of 3x 10-2 M Ca and 5 × 10-4M SO4 at pH9.5: (a) Ca2p and (b) S2p.

maYexplored.related be to SO24- ions and should be further

(b) Fig. 4. SEM micrograph: (a) sphalerite alone and (b) in the presence of 3 x 10-2 M Ca and 5 x 10-2 M SO4 ions (pH 9.5). measure the zeta potential in the supernatant of saturated CaSO4, i.e. in the absence of precipitates, The Ca concentration was the same as in Fig. 1 (500 ppm). The zeta potential proved to be similar to that in the presence of precipitate. Fig. 3 summarizes these observations. One difference between the result for S p + C a and the other two is that at approximately pH 3.5 there is a sharp dip which

The SEM images support the conclusions from zeta potential measurements. Calcium sulphate precipitates were not detected on the surface of sphalerite. Instead, the needle-shaped CaSO4 precipitates remained separate (Fig. 4). A possibility considered was that CaSO 4 colloids were present on the surface of sphalerite which may not be detected by SEM analysis. Sphalerite, after exposure to Ca (5 × 10 -3 M ) and SO4 (5 x 10 -z M) at pH 9.5 was analyzed by XPS (Fig. 5). No Ca or sulphur in sulphate (168 eV) peaks were observed (Fig. 5). The doublet at 161.3 and 162.5 eV from clean sphalerite (Fig. 5b) is assigned to sulphide, and the peak at 163 eV in the presence of Ca-SO4 is assigned to polysulphide

C. Sui et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 137 (1998) 69-77

73

10

0

~ - I0

*. . . . . .

~ -20

I

*--...CaCO3

......~.--

-3o u -40

a+CO3 Sp

~

7

8

9

10

I1

12

i

13

oH

Fig. 6. The zeta potential of sphalerite in the presence of Ca and C03; comparison with CaCO 3 compound. owing to superficial oxidation of the sphalerite. The results from all three analytical techniques suggest that Ca ions but not C a S O 4 precipitates interact with the sphalerite.

.:~ ,it~ ~ ......

j~ (a)

3.3. In the presence of Ca and COs The zeta potential of sphalerite after addition of Ca and CO3 (3 x 10 - 2 M ) was similar to that of CaCO 3 (Fig. 6). The SEM micrograph showed that some CaCO s precipitates (cubic shaped) were present on the surface (Fig. 7).

3.4. Effect of adding C03 to Ca and S04 Since CaCO3 is less soluble than CaSO4 (Ksp ( C a C O 3 ) ~--- 8.7 X 10- 9 and Ksp (caso4) = 2.45 x 10 -5, [7]), it is expected that addition of CO a will decrease the concentration of Ca ions in solution and convert CaSO 4 to CaCOs. This is the case: When 3 x l 0 - a M N a 2 C O 3 w a s added to the S p - C a - S O 4 system, the zeta potential curve moved towards that of S p - C a - C O 3 (Fig. 8). Further increasing [CO3] to 3 × 10 -2 M and 0.1 M, however, did not further decrease the zeta potential, indicating the surface charge was insensitive to an excess of COs ions. Samples prepared at pH 9.5 were analyzed by SEM (Fig. 9). The images showed that the addition of 3 x 10 -3 M COs resulted in nodule shaped precipitates which attached to the edges of sphalerite particles and to CaSO 4 precipitates (Fig. 9a).

(b) Fig. 7. SEM micrograph of sphalerite in the presence of

3 x 10-2 M Ca and CO3. Magnification:(a) 400 and (b) 2000. The EDS spectrum of the nodule showed that it was composed of mainly Ca and SO4, and a small amount of CO3 (not shown). When [COs] was increased to 3 x 10 -2 M and 1 x 10 -1 M, the same cubic shaped precipitates as shown in Fig. 7 were observed (Figs. 9b and c), which consisted of Ca and COs. No needle shaped CaSO4 precipitates were observed at [CO3] above 3 × 10 - 2 M.

C Sui et al. / Colloids Surfaces A." Physicochem. Eng. Aspects 137 (1998) 69-77

74 10

a) 0 M -J0

.

b) 0.003M

.

.

.

-

-

4. Significance to sulphide flotation

~ d)O.J M

~-20 -30

~ .~ e) 0.03 M

•- - 2 . . . .

~

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•

'

.................. Sp+Ca+C03

" So

-40 -50

6

L

~

~

7

8

9

i

10

tJ

~2

~3

pn Fig. 8. The zeta potential of sphalerite in the presence of Ca (3 x 1 0 - 2 M ) , SO4 (5 x 1 0 - 2 M ) , and CO3: (a) 0; (b) 3 x 10 .3 M; (c) 3 x 10 .2 M; and (d) 1 x 10 -~ M.

Table2 Percentage distribution of Ca upon addition of CO3 for initial [Ca] =3 x 10 -2 M and [SO4] =5 x 10 -2 M Form of Ca

As free metal ions Precipitated with CO2Precipitated with SO ] -

Bound with SO2Bound with NO3

Total

Concentration of COl-(M) 3×10 -3

3 x 1 0 -2

27.7 9.8 41.5 18.2 2.8

1.6 96.7 0 1.5 0.2

100.0

100.0

l x l 0 -1

0 99.8 0 0 0

99.8 a

aComputer program lists mole percentages only if they are at

least 0.1% in value,

Thermodynamic calculation (by SOILCHEM, a computer program for calculation of solution ionic balance) showed that 9.8, 97, 100% of Ca ions precipitate a s CaCO 3 at [CO3] of 3 x 10 - 3 , 3 x 10 -2, and 1 x 10 -1 M (Table 2). XPS analysis supported the SEM images: Ca was found on sphalerite only when the CO3 concentration was greater than 3 x 1 0 - 2 M where CaCO 3 precipitates are formed (Fig. 10a). Carbon in carbonate was detected when a large amount of CaCO 3 precipitate was produced ( S p - f a - C O 3 and S p - C a - S O 4 - C O 3 with [ C O 3 ] > 3 × 10 - 2 M ) (Fig. 10b). Sulphur peaks varied from sample to sample, but no sulphur peak from sulphate was detected in any of the cases (Fig. 10c).

The motivation for this work was to understand the interactions among sphalerite, calcium, sulphate, and carbonate. This is relevant to flotation practice where such ions often occur in significant concentrations. For example, the concentration of Ca and SO4 in the grinding circuit at the Brunswick Mining and Smelter Concentrator (Canada) is as high as 1000 ppm and 4000 ppm, respectively [2]. Reduced sphalerite recovery and selectivity is sometimes attributed to adsorption of calcium [3] o r the interference from calcium sulphate precipit a t e s [4]. O n e r e s p o n s e is t o a d d c a r b o n a t e ions to form calcium carbonate to remove both Ca and CaSO4 precipitates. In the presence of Ca only, the results indicate a significant effect of calcium on one surface property, the zeta potential, but XPS failed to detect any calcium species on sphalerite. An artifact of the sample preparation for XPS analysis (i.e. the Ca is somehow 'lost' from the surface) was considered, but various procedures (e.g. washing with water or not) gave the same result. A separate study on silica-Ca also gave the same result: an effect on the zeta potential, but no surface Ca species were detected [8]. In apparent contrast, Moignard et al. [3] do report Ca adsorption on sphalerite. Their samples were chemically prepared (by sulphide precipitation) and probably had a much higher specific surface area than our samples, which may have contributed to the different response. The observations in the presence of Ca and SO4 ions strongly suggested that only Ca ions played a significant role. The zeta potential data did not suggest calcium sulphate was present on the sphalerite surface, nor were any precipitates detected by SEM or XPS. This seems to contradict evidence from plant-derived samples [4]. One possibility is that because the sphalerite samples in this study were first rigorously cleaned (note the low pHiep), the various surface oxidation products, including sulphoxy species, which are probably present on plant-derived samples, were absent. Such species may act as sites for calcium uptake. This hypothesis is subject to experimental scrutiny. Another possibility is that the Ca signals (detected

C Sui et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 137 (1998) 69-77

(a)

75

(c)

(b) Fig. 9. SEM micrograph of sphalerite in the presence of Ca (3 x 10 -2 M), SO4 (5 x 10 -2 M ) and CO3: (a) 3 x 10 -3 M; (b) 3 x 10-2; and (c) 0.1 M.

by XPS [4]) derived from Ca species in the sample, but not necessarily precipitates associated directly with the sulphide minerals. For example, Kim et al. [9] reported that Ca species on the surface of plant-derived samples, detected by SEM, most likely originated from Ca-containing minerals such

as gypsum (CaSO4), apatite (CaPO4), and calcite (CaCO3). The addition of carbonate is traditionally considered to remove or disperse the calcium. The present results partially uphold this view. Most of the calcium is precipitated as dispersed fine cubic

C. Sui et al. / Colloids Surfaces A." Physicochem. Eng. Aspects 137 (1998) 69-77

76

C,2p iC2+|=3,IO-2M,ISO41*I~,IO'IM

localised areas thus cleaning it from the rest of the surface. If calcium interferes with the uptake of activating ions, this problem would consequently be eliminated from most of the sphalerite surface. This result is similar to that reported in the silica-Ca-CO 3 and silica-Ca-SO4-CO3 systems [8], where again most of the calcium was dispersed as fine carbonate precipitates, but some was still detected on the silica surface.

2)~'-~a*S04

-~ ,~s~.~-~o~,0-'~ ,

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.

.

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.

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348

,'f' 340

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Binding energy ( m V )

(a)

5. C o n c l u s i o n s

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•" 2,s..... ~

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292

290

288

286

284

282

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1~,+l°3~,o.,~.tsot~,o_~ ~

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~

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166

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The increase in the zeta potential of sphalerite in the presence of Ca or Ca-SO4 was attributed to adsorption of Ca 2+ or Ca-hydroxy species. Needle shaped CaSO 4 precipitates formed at a high concentration of Ca and S04, but were not present on the surface of sphalerite. The zeta potential in the presence of Ca-CO3 was similar to that of CaCO3. Some cubic shaped CaCO 3 precipitates were found on the surface of sphalerite, while the majority were dispersed. Calcium sulphate precipitates were converted t o CaCO 3 upon addition of sufficient CO 3. The properties of sphalerite then became indistinguishable from the Ca-CO 3 system. The apparent benefit of adding CO3 to either Sp-Ca or Sp-Ca-SO 4 systerns is to reduce the Ca concentration and thus eliminate Ca from interfering with desirable surface reactions such as those involving Cu activating ions and collectors.

~

164

162

160

,

158

~)_~L~

i

I

J

156

154

152

Bindingene~y(mV)

Fig. 10. XPS spectra of sphalerite in the presence of Ca (3 x 10 -z M), SO 4 (5 × 10 -2 M) and CO3: (a) Ca2p; (b) Cls;

and(c) S2p. calcium carbonate but some remains firmly attached to the sphalerite. This may still result in a net benefit by collecting most of the calcium in

Acknowledgment

Funding from INCO, Comico, Falconbridge, Hudson Bay Mining and Smelting, Noranda, and Teck, coordinated by CAMIRO and the National Science and Engineering Research Council of Canada under the Collaborative Research and Development program is acknowledged. Additional support from the Noranda Technology Centre and Brunswick Mining and Smelting is also gratefully acknowledged.

C. Sui et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 137 (1998) 69-77

77

References

[5] T.W. Healy, M.S. Moignard, A review of electrokinetic

[1 ] J.T. Woodcock, M.H. Jones, Oxygen concentrations, redox potentials, xanthate residuals, and other parameters in flotation plant pulps, in: M.J. Jones (Eds.), Mineral Processing and Extractive Metallurgy, Institution of Mining and

studies of metal sulphides, in: I M.C. Fuerstenau (Ed.), Flotation, Gaudin Memorial Volume, vol. 1, chapter 9, AIME, New York, 1976, pp. 275-297. [6] Q. Zhang, Z. Xu, S. Brienne, I. Butler, J. Finch, The effect of iron ions on the flotation of sphalerite and pyrite, International Zinc Conference, 22-24 May, Sendai, Japan, The Mining and Materials Processing Institute of Japan, 1995, pp. 167-176.

Metallurgy, London, 1970, pp. 439-468. [2] M. Cooper, 1995, Internal Report of Noranda, unpublished. [3] M.S. Moignard, R.O. James, T.W. Healy, Adsorption of calcium at the zinc sulphide-water interface, Aust. J. Chem. 33 (1977) 733-740. [4] S.R. Grano, P.L.M. Wong, W. Skinner, N.W. Johnson, J. Ralston, Detection and control of calcium sulphate precipitation in the lead circuit of the Hilton concentrator of Mount Isa Mines Limited, Australia, XIX Inter. Miner. Process. Congress, vol. 3, SME, Colorado, USA, 1995, pp. 171-179.

[7] R.C. Weast (Ed. in Chief), CRC Handbook of Chemistry and Physics, 66th edition, CRC Press, Florida, 1985-1986. [8] F. Rashchi, Z. Xu, J.A. Finch, Adsorption on silica in Pband Ca-SO4-CO 3 systems, Colloids and Surfaces, in press. [9] J. Kim, 1996, Internal Report of Noranda Technology Centre, unpublished.