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Lead activation of sphalerite during galena flotation
Pergamon
LEAD ACTIVATION
MineraLs b~gineering, Vol. 9, No. 8, pp. 869-879, 1996 Copyright © 1996 Published by Elsevier Science Ltd Printed in Great Britain. All rights reserved P I I : S0892--6875(96)00078-7 0892-6875/96 $15.00+0.00
OF SPHALERITE
DURING
GALENA
FLOTATION
C.I. BASILIO§, I.J. KARTIO? and R.-H. YOON§ § (.'enter for Coal and Minerals Processing, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061-0258, USA I" Laboratory of Materials Science, University of Turku, Itiiinen Pitkiikatu 1, FIN-20520 Turku, Finland (Received 8 April 1996; accepted 6 May 1996)
ABSTRACT
Kinetic studies on the flotation of a fine-grained complex lead-zinc ore showed that sphalerite exhibits considerable floatability during the later stages of galena flotation, causing a loss of zinc to lead concentrate. The lead concentrates obtained toward the end of the second lead rougher flotation were examined by X-ray photoelectron spectroscopy (XPS). The results showed that the lead-to-zinc atomic ratios on the surface are significantly higher than in the bulk, suggesting that the flotation of sphalerite is caused by the activation of the mineral by the lead ions present in the flotation pulp. Further evidence of the activation mechanism was given by examining the surfaces of the monominerallic sphalerite specimens that had been immersed in the flotation pulp and other solutions containing lead ions; XPS analysis of the specimens showed significant amounts of lead on: the surface. When the specimen was contacted by ore pulp (or simulated plant water) and then with a xanthate solution, the Fourier Transform Infrared (FTIR) spectrum of the specimen showed the presence of lead xanthate and dixanthogen. The proposed activation mechanism is discussed in view of the thermodynamic data available in the literature. Copyright ©1996 Published by Elsevier Science Ltd
Keywords Sulphide ores; flotation activators; flotation kinetics; ion exchange; surface modification
INTRODUCTION Most of the lead-zinc ores are processed by differential flotation, in which galena is floated first before floating sphalerite. The separation is possible because thiol collectors can directly adsorb on galena via an electrochemical mechanism, while sphalerite requires activation. When the ore is oxidized or contains oxidized minerals, however, a considerable amount of sphalerite can float in the lead flotation circuit, causing a loss of zinc recovery [1,2]. Rey and Formaneck [3] conducted a series of Hallimond tube flotation tests using a pure zinc sulfide (ZnS) sample precipitated from a zinc sulfate solution. When using amyl xanthate as coUector at pH 9, the precipitated ZnS showed no flotation without activation. In the presence of 10% galena, however, the zinc recovery was 3.5%, while it increased to 14.1 and 21.9% in the presence of 10 % cerussite and anglesite, respectively. The 3.5 % recovery obtained with 10 % galena may seem small, but the ZnS sample used by Re), and Formaneck floated only 21% after activation with 1 g/t of copper sulfate, most probably due to the small particle size of the precipitate. These results are consistent with a more recent work reported by Houot et al. [4], who showed that an artificial addition of Pb 2+ ions to a flotation pulp increases the recovery of sphalerite. Thus, sphalerite can be significantly activated by the lead ions derived from galena and oxidized lead minerals such as cerussite and anglesite. 869
870
c. 1. Basilioet al.
The lead activation of sphalerite in the presence of galena may become more serious when a lead-zinc ore is finely ground with a long retention time, followed by a long flotation time with a large circulating load. In such cases, a control of the inadvertent activation of sphalerite may be important for improving selectivity and maximizing recovery. Finkelstein and Allison [5] suggested that sphalerite can be activated by the heavy metal ions present in the mineral lattice, which was confirmed by Mielczarski [6]. He showed that lead ion concentration is higher on the surface than in the bulk of sphalerite, as shown by X-ray photoelectron spectroscopy (XPS), and that lead xanthate is formed on the surface when the mineral is exposed to a xanthate solution, as determined by Fourier transform infrared spectroscopy (FTIR). Fuerstenau et al. [7] and Marouf et al. [8] showed that sphalerite can be floated without activation at pH below 6, where the mineral becomes soluble enough to form zinc xanthate (ZnX2) on the surface. Leroux et al. [9] suggested, on the other hand, that sphalerite can be activated by ferric hydroxy species in acidic media. Girczys and Laskowski [10] proposed a different mechanism for the flotation of sphalerite in acidic solutions. They suggested that the Fe 3 + ions derived from sphalerite (actually marmatite) oxidize xanthate to dixanthogen. However, Mukherjee and Sen [11] and Fuerstenau et al. [7] found no correlation between the flotation recovery and iron content. In the present work, batch flotation tests have been conducted on a complex lead-zinc ore. In the plant operation, the ore is finely ground to achieve liberation, and the plant water is recycled. A major problem in this operation is the low zinc recovery, which may be attributed to incomplete liberation and/or the heavy metal ions present in the recycled plant water. It has been the objective of the present investigation to study the possibility that the heavy metal ions present in the plant water inadvertently activate sphalerite in the lead circuit, causing poor selectivity and, hence, the loss of zinc to lead concentrates. To identity the species responsible for the inadvertent activation, flotation products were subjected to XPS and FTIR spectroscopic analyses.
EXPERIMENTAL Flotation Tests
Batch flotation tests were conducted on a complex lead-zinc sulfide ore assaying 6.5% Pb, 22.8% Zn, 4.2% Fe, 48.2% SiO2 and 0.65% Ba. The flotation tests were conducted using the conditions similar to those employed at the plant where the ore was being processed. The laboratory tests were limited to rougher flotation since the objective of this investigation was to identify the causes for sphalerite loss in the lead circuit. A 1000 g ore sample ( - 1.7 mm) was ground in a ball mill at 60% solids using iron balls as grinding media; 75 % of the mill product passed a 44/~m screen. Both the grinding and flotation tests were conducted using simulated plant water, whose composition is given in Table 1. The ground ore pulp was transferred to a 2.8 liter Denver laboratory flotation cell, and then conditioned for two minutes after adding 15 g/t of sodium cyanide (NaCN) and 100 g/t of potassium amyl xanthate (KAX). The first lead rougher flotation was carried out for eight minutes using an appropriate amount of methyl isobutyl carbinol (MIBC) as frother. After the first lead rougher flotation, 60 g/t of KAX was added to the first lead rougher tail and conditioned for two minutes before conducting the second lead rougher flotation for eight minutes. For zinc rougher flotation, 600 g/t of copper sulfate (CuSO4) and 250 g/t of KAX were added to the second lead rougher tall and conditioned for two minutes. After the conditioning, additional KAX (150 g/t) was added to the pulp before commencing the zinc rougher flotation for 20 minutes using MIBC as frother. In all of the flotation tests, timed-cut froth products were collected to obtain kinetic information. Various flotation products were filtered, dried, and assayed for Pb, Zn, and Fe using a Philips Energy Dispersive X-ray Fluorescence (XRF) spectrometer (Model PV9550).
Lead activation of sphalerite
871
TABLE 1 Composition of Simulated Plant Water
Elements
Concentration (ppm)
Ca
500
Fe
20
Pb
3
K
16
Mg
45
Mn
15
Na
23
Zn
400
CI 5 *All cations are in sulfates except for Na, which is chloride.
Spectroscopic Analysis Sample Preparation: Some of the flotation products obtained in the present work were subjected to XPS analysis. These included: i) the froth products obtained at the eighth minute mark of the first and the second lead rougher flotation tests, and ii) the zinc rougher concentrate obtained during the first minute of flotation. The samples were dried in a heated vacuum desiccator prior to the XPS measurements. A mono-mineraUic sphalerite specimen from Santander, Spain, was cut and polished to a rectangular plate with approximate dimensions of 20x8xl mm and used as a probe to monitor the reactions occurring on the mineral surface during flotation. The sphalerite specimen contained 0.035 % Fe and 0.012% Cu by weight. The probe was wet-polished with silicon carbide paper (600 grit), rinsed with double-distilled deionized water, and then inmaediately immersed into the solution under investigation. After 5 to 15 minutes of immersion time, the probe was removed from the solution, rinsed, and air-dried before spectroscopic analysis. For XPS analysis, the probe was immersed in the following solutions: i) distilled water, ii) a slurry containing the ore ground with simulated plant water, iii) plant water containing three ppm Pb 2+ ions, iv) simulated plant water, and v) 10 -4 M PbSO 4 solution (natural pH). For FTIR spectroscopic analysis, the sphalerite probe was immersed in one of the following solutions: i) 10 -4 M PbSO 4 solution for one minute, ii) simulated plant water for 30 minutes, and iii) actual plant water for 30 minutes before immersing it in 10 -4 M KAX solution for 15 minutes at natural pH. XPS Analysis: The; XPS measurements of the flotation concentrates were carried out using a Kratos XSAM 800 spectrometer, 'while a Perkin-Elmer ESCA PHI 5400 spectrometer was used for the sphalerite probes. In both instruments, unmonochromatized X-rays from a magnesium anode were used to excite the photoelectrons at a vacuum pressure of approximately 10 -7 Pa. The full width at half maximum (FWHM) of the Ag(3d)3/2 line was 1.1 and 1.0 eV for the Kratos XSAM 800 and Perkin-Elmer ESCA PHI 5400 spectrometers, respectively. The energy scales of both instruments were calibrated by using the Au(4f)7/2 (BE = 84.0 eV) and Cu(2P)3/2 (BE = 932.6 eV) lines. The sensitivity factors used for quantification of the spectra obtained with the Kratos spectrometer were based on the theoretical cross-section and calculated mean free paths of electrons [12]. In the case of the Perkin-Elmer spectrometer, the experimental sensitivity factors used were provided by the manufacturer for this particular instrument.
872
c. 1. Basilioet
al.
FI'IR Analysis: FTIR spectra were recorded using a Bio-Rad FTS 60A Rapid Scan FTIR spectrometer equipped with a liquid nitrogen-cooled mercury cadmium telluride (MCT) wide-band detector. In each measurement, a total of 100 spectra were obtained with a spectral resolution of 4 cm -1 and signalaveraged. The transmission spectra of the sphalerite probe were recorded using a Bio-Rad UMA-300A infrared microscope sampling accessory.
RESULTS AND DISCUSSION Batch Flotation Studies Figure 1 shows the kinetics test results obtained during the first lead rougher flotation. The galena flotation was fast for the first three minutes and tapered off afterwards. After the first 3 or 4 minutes of flotation time, the rate of galena flotation was sufficiently reduced that it became comparable to the rate of sphalerite flotation. As a result, significant amounts of sphalerite floated especially toward the end of the 8 minute flotation time. After the 8 minutes of flotation time, the zinc-to-lead loss amounted to 11%, while the lead recovery was 84.3 %. Although part of the zinc loss to the lead concentrate must be due to the flotation of locked particles (of galena and sphalerite) and entrainment, significant portion of zinc loss may be due to the flotation of free sphalerite.
v
.I3 n
a. @ m
C C
143
138
Binding Energy (eV) Fig. 1 Cumulative grade and recovery of Pb and Zn to the first lead rougher as a function of time. Figure 2 shows the kinetics results obtained during the second lead rougher flotation. Surprisingly, the lead recovery rate was low, while the zinc recovery rate was sharply increased due to the addition of 60 g/t of KAX. The low lead recovery may be attributed to the likelihood that most of the floatable galena particles had already been floated in the first rougher. What was surprising was that the zinc recovery rate increased sharply without the addition of the traditional activator, i.e., copper sulfate. It is possible that some of the heavy metal ions present in the pulp activated the sphalerite and caused the xanthate collector to adsorb on the mineral surface, thereby rendering the mineral hydrophobic. After the eight minutes of flotation time, 42.5% of the sphalerite floated at the second lead rougher with the froth product assaying more than 50%
Lead activation of sphalerite
873
Zn. Although a large portion of the sphalerite floated in the second lead rougher is rejected at the cleaner circuit and sent to the zinc rougher in the plant operation, the sphalerite flotation in the second lead rougher constitutes a significant source of zinc loss. 100
80
100
Zn o • a •
Rouflher Pb Kecovery Pb Grade Zn Recovery Zn Grade
80 q)
> 0 O
"I0 60
60
0 k.
C9
G)
n-
G) > 40
40
.~--
_Q
0 20
0 0
05
- = _ _ _ , _ ~ _ ^ ^ ^ ~, . - R - . . . . . . . . ~ 5 ' ' - =10 15
200
Time (min) Fig.2 Cumulative grade and recovery of Pb and Zn to the second lead rougher as a function of time. The kinetics test results obtained during the zinc rougher flotation are shown in Figure 3. The lead recovery to the zinc rougher was negligible since most of the galena had already been recovered during the two stages of lead rougher flotation. The rate of sphalerite flotation was high, most of it being recovered in the first five minutes..As a result, the zinc grade was maximum in the first minute and decreased afterwards, indicating that slow-floating particles were recovered during the later stages of flotation. There was no problem in floating sphalerite at the zinc rougher as the zinc loss to the final tall was only 0.7% of the feed. Nevertheless, the overall zinc recovery was less than 50% because a total of 52.5 % of zinc was lost during the first and second lead rougher flotation. The flotation resullls show that the high flotation rate of sphalerite particularly in the second lead rougher was responsible for the low zinc-to-zinc recovery. Since the ore was finely ground (75 % finer than 44#m), the sphalerite flotation during the later stages of galena flotation may be attributed to surface chemistry problems rather than to incomplete liberation. Surface Analysis of Flotation Concentrates
To identify the reason(s) for the high floatability causing poor selectivity in the lead rougher circuit, the flotation concentrates were analyzed for both lead and zinc using XPS and X-ray fluorescence (XRF) spectroscopy. Since XRF spectrometry has a sampling depth of at least one micron, the analysis gives information on bulk composition. On the other hand, XPS is a surface-sensitive technique with a sampling depth of only about a few nanometers. In the XPS measurements, the atomic concentration of lead was calculated from the intensity (spectral area) of the Pb(4f) doublet. Since the Zn(3s) peak is overlapping with the Pb(4f) doublet, as shown in Figure 4, the spectrum has been resolved by curve-fitting to subtract the area due to Zn(3s) peak. The atomic concentration of zinc was obtained from the intensity of the Zn(3p) doublet (not shown here).
C.i. Basilioet al.
874 100
100
Rougher 2 P• o Pb b Recovery Pb Grode a Zn Recovery • Zn Grode
80 "~"
.= o
6o
60
,,=__=._.4--=----=
III/
=---4--
0
~
20 L
.
.
.
p
40
~1~
20
8
.
d 0
_
0
0
1
2
3
4
5
6
7
8
Time (rain) Fig.3 Cumulative grade and recovery of Pb and Zn to the zinc rougher as a function of time. 100
80
100
Rougher I Pob Pb Recovery • Pb Grade
0, Zn Recovery
80 ID
"10 0
IIJ
> o 60 o
60
._> (1~ 40
40
._> o
0
E
E --! 2o 0
2O ~ /
.il
o 6 ~
I
~
rl
n
,
2
,
3
Time
,
4
,
5
,
6
,
7
8
0
(rain)
Fig.4 XPS spectrum (Pb(4f) and Zn(3s) region) of the first lead rougher concentrate. Table 2 shows the lead-to-zinc (Pb/Zn) atomic ratios of the different flotation concentrates as determined by XRF spectrometry and XPS. The first lead rougher concentrate (Sth minute product) has a surface Pb/Zn atomic ratio, as measured by XPS, much higher than the bulk Pb/Zn ratio measured by XRF. The same can be said of the second lead rougher and zinc rougher concentrates. The excess lead found on the surface suggests that sphalerite has been activated by the Pb2+ ions present in the plant water. The activation of sphaierite by Pb2+ ions may be represented by the following exchange reaction,
Lead activation of sphalerite
+ Pb 2+" PbS + Zn 2+
875
(1)
pK = -3.64
whose equilibrium constant K being given by:
(2)
K=
in which
g~ s
is the solubility product of the following reaction:
ZnS ~
Zn 2 + + S2 - ,
while
/t, PbS "*sp
(3)
is the same of the reaction:
P b S - * P b 2+ + S2-.
(4)
TABLE 2 Bulk and Surface Pb/Zn Atomic Ratios of Different Flotation Products Sample
Bulk Pb/Zn (XRF)
Surface Pb/Zn (XPS)
First lead rougher concentrate ~
0.14
0.40
Second lead rougher concentrate 2
0.007
0.08
Zinc rougher concentrate 3
0.003
0.05
8th min sample (31.4% Zn, 13.5 Pb); z 8th rain sample (54.2% Zn, 1.2% Pb); 3 1st min sample (55.3% Zn, 0.5% Pb).
Table 3 shows a survey of
K szns
and
/fPbS "~se
values reported in the literature. Substituting these values
into Eq. (2), one can obtain the values of K for reaction (1) in the range of 32.3 and 3.3x106. Since these values are large, the reaction should proceed to the right irreversibly. After Pb 2 + ions have replaced Zn 2 + ions from the sphalerite surface, stable lead xanthate and/or dixanthogen can be formed to render the mineral hydrophobic.
TABLE 3 Thermodynamic Data for Zinc and Lead Sulfides
Latimer t~
References Leckie and James ~'~
ZnS Sphalerite
7 . 2 x 1 0 -26
7.1 x 10 -26
2.2x10 -27
1.9x10 -26
Wurzite
4 . 6 x 1 0 -23
1.6 × 10 -23
1.6x10 -23
2 . 2 × 1 0 -24
precipitated
8 . 6 x 10 -23
8.9 x 10 -23
8.7x10 -23
PbS
6 . 8 x 1 0 -29
6.3 x 10 -29
6.6x10 -29
Sulfides
Bard et al. 15
Helgeson m
2 . 7 x 1 0 -29
Table 1 shows that the plant water contained 400 ppm of Zn 2+ ions and 1 ppm of Pb 2+ ions. If one uses
876
C. 1. Basilio et al.
the lowest value of K (=32.3) for reaction (1), ZnS can be activated only when P b 2 + ion concentration exceeds 12.4 ppm. On the other hand, if one uses the highest value of K (=3.3x106), the activation should occur when Pb 2+ ion concentration is larger than 1.2x10 -4 ppm, which is an exceedingly small concentration. Table 1 shows that the plant water contained 3 ppm of Pb 2+ ions; therefore, it is difficult to conclude whether the thermodynamic calculation provides a definitive answer to the question concerning lead activation of sphalerite. It should be noted, however, that the value of K=32.3 was obtained using the value of
KZpns
= 2.2x10 -27, which is the only one of the
KZpns
values given in Table 3 that do
not support the proposed activation mechanism. All other thermodynamic data suggest that at 400 ppm of Zn 2+ ions in solution, ZnS should be activated at Pb 2+ ion concentrations of approximately 1 ppm. For example, the Latimer's data for sphalerite (
KZpns
=
Itr P b S 7.2x10 -26) and lead sulfide ( "'~sP
=
6.8x10-29) give K = 1. lxl03 in accordance to (Eq. 2), in which case the lead activation should occur when Pb 2+ ion concentration exceeds 0.34 ppm at 400 ppm Zn 2+ ions. The Pb 2 + ions present in the flotation pulp may have been derived from the oxidized minerals present in the ore (i.e., anglesite). The lead-activation allows xanthate adsorption to take place resulting in the flotation of sphalerite in the lead rougher circuit. It is also possible that the inadvertent activation of sphalerite may have been caused by the Pb2+ ions produced from the oxidation of galena as suggested by Houot and Ravenau [4]. This would be particularly the case when an ore is ground for a long time, and the finely-ground ore requires a long retention time with large circulating loads. A solution to this problem may be an electrochemical potential control to minimize the galena oxidation.
Surface Analysis of Sphalerite Probe Further evidence that zinc loss to the lead concentrate may be caused by the lead-activation of sphalerite was obtained by analyzing the surface of the mono-minerallic sphalerite probe immersed in different solutions. Table 4 shows the results of the XPS analysis. After conditioning for 5 minutes in distilled water, no lead was detected on the surface. However, conditioning for 5 minutes in an ore slurry resulted in a Pb/Zn atomic ratio of 0.068 on the surface, which is close to that obtained with a sphalerite probe contacted with a 10 -4 M PbSO 4 solution for one minute. These results established that sphalerite was activated by Pb 2 + ions under the conditions employed for processing the ore.
TABLE 4 Surface Pb-to-Zn Atomic Ratios of Sphalerite Probes Contacted with Various Solutions Immersion Condition 5 rain. in distilled water 1 min. in 10 -4 M PbSO4 5 min. in ore slurry 5 rain. in plant water 15 rain, in plant water 5 rain. in simulated plant water
] Pb/Zn 0.000 0.085 0.068 0.043 0.056 0.023
For a sphalerite probe conditioned for 5 minutes in the plant water, the Pb/Zn atomic ratio on the surface was 0.043. This ratio was smaller than that obtained by immersing the probe in the ore slurry for five minutes, suggesting that the ground ore pulp provided additional Pb 2+ ions. It is possible that the oxidized minerals present in the ore serve as continual sources of Pb 2+ ions. After 15 minutes of conditioning in the plant water, the Pb/Zn ratio increased to 0.056, indicating that the lead-activation continued to increase
Lead activationof sphalerite
877
with conditioning time but at a slower rate. Conditioning the mono-mineralic sphalerite probe for five minutes in the simulated plant water gave a Pb/Zn ratio of 0.02',3, which was almost half of that observed after conditioning the probe in the plant water for the same period of time. Considering that the plant and simulated waters contain about the same amount of Pb 2 + ions, spha~lerite is more easily activated in the former, the reason for which is unknown at present. The results shown in Table 4 clearly establish that sphalerite is activated by Pb 2+ ions in solution via reaction (1), despite the fact that the amount of the Pb 2+ ions in the pulp is relatively small while the Zn 2+ ion concentration is high. Ralston et al. [17] showed that the kinetics of lead-activation is relatively slow, as compared to that of copper-activation, which may be one of the reason that sphalerite begins to float at the later stages of the lead rougher flotation. FTIR measurements were also carried out on the mono-mineraUic sphalerite probe treated under conditions similar to those us~xl for the XPS measurements, except that the samples used for the FTIR measurements were contacted with KAX solutions afterwards. Figure 5 shows the FTIR spectra obtained after immersing the probe in solutions containing different amounts of Pb 2+ ions and then contacting with 10 -4 M KAX solutions. The probe conditioned in a 10 -4 M PbSO 4 solution for one minute and then in KAX solution for 15 minutes showed the presence of lead xanthate (PbX2) and dixanthogen (X2) as shown in Figure 5a. The presence of PbX 2 is indicated by its characteristic absorption bands at 1204, 1164 and 1029 cm-1 while the bands at 1260, 1236, 1109 and 1029 cm -1 show the presence of X 2 [18,19]. No evidence for zinc xanthate formation was observed, which agrees with the work of Finkelstein and Allison [5].
a) @ 0 r-
U
c) L__
0 .Q
<
Wavenumber (cm") Fig.5 FT[R spectra of sphalerite probe immersed initially in the following: a) 10 - 4 M PbSO 4 solution for one minute, b) simulated plant water for 30 minutes, and c) plant water for 30 minutes; and then conditioned in 10 - 4 M KAX solution for 15 minutes.
878
c.I. Basilioet al.
Figures 5b and 5c show the FTIR spectra for the mono-minerallic sphalerite probe immersed for 30 minutes in simulated and actual plant water, respectively, and then contacted with a 10-4 M KAX solution for 15 minutes. The presence of PbX2 and X2 on the probe surface is also indicated on both spectra. These results show that sphalerite is inadvertently activated by the Pb2+ ions present in plant water causing a significant zinc loss to the lead circuit.
SUMMARY AND CONCLUSIONS An ore sample containing galena and sphalerite was subjected to flotation kinetics studies using a simulated plant water containing various heavy metal ions. The results showed that significant amounts of sphalerite float during the later stages of galena flotation, causing a loss of zinc to lead concentrates. Spectroscopic analysis of the lead rougher concentrates showed that lead-to-zinc atomic ratios are substantially higher on the surface than in the bulk of the minerals floated, suggesting that sphalerite is activated by the lead ions present in the flotation pulp. Evidence of the lead activation of sphalerite was also given by the XPS analysis of the mono-minerallic sphalerite probes that had been contacted with different solution; the probes contacted with the ore pulp, simulated plant water, and lead sulfate solutions show lead-to-zinc ratios substantially higher than that of the sphalerite specimen contacted with distilled water. When the sphalerite probe was contacted with a lead sulfate solution and then with an amyl xanthate solution, the FTIR spectra showed the presence of lead amyl xanthate and dixanthogen. Similar results were obtained when the monominerallic sphalerite probe was contacted first with simulated plant water (or actual plant water) and then with amyl xanthate solutions. These findings provided further evidence that sphalerite is inadvertently activated by the lead ions present in the plant water, react with xanthate collector, and acquire a significant floatability in the lead rougher flotation circuit, causing a significant loss of zinc to lead concentrate.
ACKNOWLEDGMENTS The authors wish to acknowledge Cominco Metals and Cominco Alaska for the financial support. They would also like to thank Prof. Eero Suoninen for reading the manuscript.
REFERENCES .
2. .
.
5.
.
7. 8.
9.
10.
Gaudin, A.M., Flotation. McGraw-Hill, London, 254 (1957). Bogdanov, O.S., Podnek, A.K. & Semenova, E.A., Issledovanie flotacii raznovidnostiei sfalerita (Flotation study of different sphalerite forms). Trudy Inst. Mechanobr, Vypusk, 135 (1965). Rey, M. & Formanek, V., Some Factors Affecting Selectivity in the Differential Flotation of Lead-Zinc Ores, Particularly in the Presence of Oxidized Lead Minerals. Paper No. 18, p. 343-353, Proceedings of the International Mineral Processing Congress, IMM, London (1960). Houot, R. & Ravenau, P., Activation of sphalerite flotation in the presence of lead ions. Int. J. Miner. Process., 35, 253 (1992). Finkelstein, N.P. & Allison, S.A., The chemistry of activation, deactivation, and depression in the flotation of zinc sulfide: a review. In: M.C. Fuerstenau (Editor), Flotation, A.M. Gaudin Memorial Volume. SME/AIME, New York, 414 (1976). Mielczarski, J., The role of impurities of sphalerite in the adsorption of ethyl xanthate and its flotation. Int. J. Miner. Process., 16, 179 (1986). Fuerstenau, M.C., Clifford, K.L. & Kuhn, M.C., The role of zinc-xanthate precipitation in sphalerite flotation. Int. J. Miner. Process., 1, 307 (1974). Marouf, B., Bessiere, J., Houot, R. & Blazy, P., Flotation of sphalerite without prior activation by metallic ions. Trans. Inst. Min. Metall., 95, C1, (1986). Leroux, M., Rao, S.R. & Finch, J.A., Selective flotation of sphalerite from Pb-Zn ore without copper activation. CIM Bull., 80(902), 41 (1987). Girczys, J. & Laskowski, J., Mechanism of flotation of unactivated sphalerite with xanthates. Trans. Inst. Min. Metall., 81, Cl18, (1972).
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18. 19.
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Mukherjee, A.D. & Sen, P.K., Floatability of sphalerite in relation to its iron content. J. Mines Metals Fuels, 327 (1976). Scofield, J.H., Hartree-Slater subshell photoionization cross-sections at 1254 and 1487 eV. J. Electron Spectrosc., 8, 129 (1976). Latimer, W.M., The Oxidation States of the Elements and their Potentials in Aqueous Solutions. Prentice-Hall, Inc., New Jersey, pp. 72, 152, 169 (1952). Leckie, J.O. & James, R.O., Aqueous-Environmental Chemistry of Metals. Alan J. Rubin, Editor, Arm Arbors Science Pub., Inc., Ann Arbor, Michigan, 1-76 (1974). Bard, A.J., Parsons, R. & Jordan J., Standard Potentials in Aqueous Solutions. Marcel Dekker, Inc., New York (1985). Helgeson, N.C., Thermodynamics of Hydrothermal Systems at Elevated Temperatures and Pressures. Am. J. Sci., 267, 729-804 (1969). Ralston, J., Alabaster, P. & Healy, T.W., Activation of zinc sulphide with CulI, Cd II, and Pb n, III. The mass spectrometric determination of elemental sulphur. Int. J. Miner. Process., 7, 279 (1981). de Donato, P., Cases, J.M., Kongolo, M. & Bumeau, A., Infrared investigation of amylxanthate adsorption of galena: Influence of oxidation, pH and grinding. Colloids Surfaces, 44, 207 (1990). Kongolo, M., Cases, J.M., Burneau, A. & Predali, J.J., Spectroscopic study of potassium amylxanthate adsorption on finely ground galena: relation with flotation. In: M.J. Jones and R. Oblat (Editors), Reagents in the Mineral Industry, 79 (1985).
LEAD ACTIVATION
MineraLs b~gineering, Vol. 9, No. 8, pp. 869-879, 1996 Copyright © 1996 Published by Elsevier Science Ltd Printed in Great Britain. All rights reserved P I I : S0892--6875(96)00078-7 0892-6875/96 $15.00+0.00
OF SPHALERITE
DURING
GALENA
FLOTATION
C.I. BASILIO§, I.J. KARTIO? and R.-H. YOON§ § (.'enter for Coal and Minerals Processing, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061-0258, USA I" Laboratory of Materials Science, University of Turku, Itiiinen Pitkiikatu 1, FIN-20520 Turku, Finland (Received 8 April 1996; accepted 6 May 1996)
ABSTRACT
Kinetic studies on the flotation of a fine-grained complex lead-zinc ore showed that sphalerite exhibits considerable floatability during the later stages of galena flotation, causing a loss of zinc to lead concentrate. The lead concentrates obtained toward the end of the second lead rougher flotation were examined by X-ray photoelectron spectroscopy (XPS). The results showed that the lead-to-zinc atomic ratios on the surface are significantly higher than in the bulk, suggesting that the flotation of sphalerite is caused by the activation of the mineral by the lead ions present in the flotation pulp. Further evidence of the activation mechanism was given by examining the surfaces of the monominerallic sphalerite specimens that had been immersed in the flotation pulp and other solutions containing lead ions; XPS analysis of the specimens showed significant amounts of lead on: the surface. When the specimen was contacted by ore pulp (or simulated plant water) and then with a xanthate solution, the Fourier Transform Infrared (FTIR) spectrum of the specimen showed the presence of lead xanthate and dixanthogen. The proposed activation mechanism is discussed in view of the thermodynamic data available in the literature. Copyright ©1996 Published by Elsevier Science Ltd
Keywords Sulphide ores; flotation activators; flotation kinetics; ion exchange; surface modification
INTRODUCTION Most of the lead-zinc ores are processed by differential flotation, in which galena is floated first before floating sphalerite. The separation is possible because thiol collectors can directly adsorb on galena via an electrochemical mechanism, while sphalerite requires activation. When the ore is oxidized or contains oxidized minerals, however, a considerable amount of sphalerite can float in the lead flotation circuit, causing a loss of zinc recovery [1,2]. Rey and Formaneck [3] conducted a series of Hallimond tube flotation tests using a pure zinc sulfide (ZnS) sample precipitated from a zinc sulfate solution. When using amyl xanthate as coUector at pH 9, the precipitated ZnS showed no flotation without activation. In the presence of 10% galena, however, the zinc recovery was 3.5%, while it increased to 14.1 and 21.9% in the presence of 10 % cerussite and anglesite, respectively. The 3.5 % recovery obtained with 10 % galena may seem small, but the ZnS sample used by Re), and Formaneck floated only 21% after activation with 1 g/t of copper sulfate, most probably due to the small particle size of the precipitate. These results are consistent with a more recent work reported by Houot et al. [4], who showed that an artificial addition of Pb 2+ ions to a flotation pulp increases the recovery of sphalerite. Thus, sphalerite can be significantly activated by the lead ions derived from galena and oxidized lead minerals such as cerussite and anglesite. 869
870
c. 1. Basilioet al.
The lead activation of sphalerite in the presence of galena may become more serious when a lead-zinc ore is finely ground with a long retention time, followed by a long flotation time with a large circulating load. In such cases, a control of the inadvertent activation of sphalerite may be important for improving selectivity and maximizing recovery. Finkelstein and Allison [5] suggested that sphalerite can be activated by the heavy metal ions present in the mineral lattice, which was confirmed by Mielczarski [6]. He showed that lead ion concentration is higher on the surface than in the bulk of sphalerite, as shown by X-ray photoelectron spectroscopy (XPS), and that lead xanthate is formed on the surface when the mineral is exposed to a xanthate solution, as determined by Fourier transform infrared spectroscopy (FTIR). Fuerstenau et al. [7] and Marouf et al. [8] showed that sphalerite can be floated without activation at pH below 6, where the mineral becomes soluble enough to form zinc xanthate (ZnX2) on the surface. Leroux et al. [9] suggested, on the other hand, that sphalerite can be activated by ferric hydroxy species in acidic media. Girczys and Laskowski [10] proposed a different mechanism for the flotation of sphalerite in acidic solutions. They suggested that the Fe 3 + ions derived from sphalerite (actually marmatite) oxidize xanthate to dixanthogen. However, Mukherjee and Sen [11] and Fuerstenau et al. [7] found no correlation between the flotation recovery and iron content. In the present work, batch flotation tests have been conducted on a complex lead-zinc ore. In the plant operation, the ore is finely ground to achieve liberation, and the plant water is recycled. A major problem in this operation is the low zinc recovery, which may be attributed to incomplete liberation and/or the heavy metal ions present in the recycled plant water. It has been the objective of the present investigation to study the possibility that the heavy metal ions present in the plant water inadvertently activate sphalerite in the lead circuit, causing poor selectivity and, hence, the loss of zinc to lead concentrates. To identity the species responsible for the inadvertent activation, flotation products were subjected to XPS and FTIR spectroscopic analyses.
EXPERIMENTAL Flotation Tests
Batch flotation tests were conducted on a complex lead-zinc sulfide ore assaying 6.5% Pb, 22.8% Zn, 4.2% Fe, 48.2% SiO2 and 0.65% Ba. The flotation tests were conducted using the conditions similar to those employed at the plant where the ore was being processed. The laboratory tests were limited to rougher flotation since the objective of this investigation was to identify the causes for sphalerite loss in the lead circuit. A 1000 g ore sample ( - 1.7 mm) was ground in a ball mill at 60% solids using iron balls as grinding media; 75 % of the mill product passed a 44/~m screen. Both the grinding and flotation tests were conducted using simulated plant water, whose composition is given in Table 1. The ground ore pulp was transferred to a 2.8 liter Denver laboratory flotation cell, and then conditioned for two minutes after adding 15 g/t of sodium cyanide (NaCN) and 100 g/t of potassium amyl xanthate (KAX). The first lead rougher flotation was carried out for eight minutes using an appropriate amount of methyl isobutyl carbinol (MIBC) as frother. After the first lead rougher flotation, 60 g/t of KAX was added to the first lead rougher tail and conditioned for two minutes before conducting the second lead rougher flotation for eight minutes. For zinc rougher flotation, 600 g/t of copper sulfate (CuSO4) and 250 g/t of KAX were added to the second lead rougher tall and conditioned for two minutes. After the conditioning, additional KAX (150 g/t) was added to the pulp before commencing the zinc rougher flotation for 20 minutes using MIBC as frother. In all of the flotation tests, timed-cut froth products were collected to obtain kinetic information. Various flotation products were filtered, dried, and assayed for Pb, Zn, and Fe using a Philips Energy Dispersive X-ray Fluorescence (XRF) spectrometer (Model PV9550).
Lead activation of sphalerite
871
TABLE 1 Composition of Simulated Plant Water
Elements
Concentration (ppm)
Ca
500
Fe
20
Pb
3
K
16
Mg
45
Mn
15
Na
23
Zn
400
CI 5 *All cations are in sulfates except for Na, which is chloride.
Spectroscopic Analysis Sample Preparation: Some of the flotation products obtained in the present work were subjected to XPS analysis. These included: i) the froth products obtained at the eighth minute mark of the first and the second lead rougher flotation tests, and ii) the zinc rougher concentrate obtained during the first minute of flotation. The samples were dried in a heated vacuum desiccator prior to the XPS measurements. A mono-mineraUic sphalerite specimen from Santander, Spain, was cut and polished to a rectangular plate with approximate dimensions of 20x8xl mm and used as a probe to monitor the reactions occurring on the mineral surface during flotation. The sphalerite specimen contained 0.035 % Fe and 0.012% Cu by weight. The probe was wet-polished with silicon carbide paper (600 grit), rinsed with double-distilled deionized water, and then inmaediately immersed into the solution under investigation. After 5 to 15 minutes of immersion time, the probe was removed from the solution, rinsed, and air-dried before spectroscopic analysis. For XPS analysis, the probe was immersed in the following solutions: i) distilled water, ii) a slurry containing the ore ground with simulated plant water, iii) plant water containing three ppm Pb 2+ ions, iv) simulated plant water, and v) 10 -4 M PbSO 4 solution (natural pH). For FTIR spectroscopic analysis, the sphalerite probe was immersed in one of the following solutions: i) 10 -4 M PbSO 4 solution for one minute, ii) simulated plant water for 30 minutes, and iii) actual plant water for 30 minutes before immersing it in 10 -4 M KAX solution for 15 minutes at natural pH. XPS Analysis: The; XPS measurements of the flotation concentrates were carried out using a Kratos XSAM 800 spectrometer, 'while a Perkin-Elmer ESCA PHI 5400 spectrometer was used for the sphalerite probes. In both instruments, unmonochromatized X-rays from a magnesium anode were used to excite the photoelectrons at a vacuum pressure of approximately 10 -7 Pa. The full width at half maximum (FWHM) of the Ag(3d)3/2 line was 1.1 and 1.0 eV for the Kratos XSAM 800 and Perkin-Elmer ESCA PHI 5400 spectrometers, respectively. The energy scales of both instruments were calibrated by using the Au(4f)7/2 (BE = 84.0 eV) and Cu(2P)3/2 (BE = 932.6 eV) lines. The sensitivity factors used for quantification of the spectra obtained with the Kratos spectrometer were based on the theoretical cross-section and calculated mean free paths of electrons [12]. In the case of the Perkin-Elmer spectrometer, the experimental sensitivity factors used were provided by the manufacturer for this particular instrument.
872
c. 1. Basilioet
al.
FI'IR Analysis: FTIR spectra were recorded using a Bio-Rad FTS 60A Rapid Scan FTIR spectrometer equipped with a liquid nitrogen-cooled mercury cadmium telluride (MCT) wide-band detector. In each measurement, a total of 100 spectra were obtained with a spectral resolution of 4 cm -1 and signalaveraged. The transmission spectra of the sphalerite probe were recorded using a Bio-Rad UMA-300A infrared microscope sampling accessory.
RESULTS AND DISCUSSION Batch Flotation Studies Figure 1 shows the kinetics test results obtained during the first lead rougher flotation. The galena flotation was fast for the first three minutes and tapered off afterwards. After the first 3 or 4 minutes of flotation time, the rate of galena flotation was sufficiently reduced that it became comparable to the rate of sphalerite flotation. As a result, significant amounts of sphalerite floated especially toward the end of the 8 minute flotation time. After the 8 minutes of flotation time, the zinc-to-lead loss amounted to 11%, while the lead recovery was 84.3 %. Although part of the zinc loss to the lead concentrate must be due to the flotation of locked particles (of galena and sphalerite) and entrainment, significant portion of zinc loss may be due to the flotation of free sphalerite.
v
.I3 n
a. @ m
C C
143
138
Binding Energy (eV) Fig. 1 Cumulative grade and recovery of Pb and Zn to the first lead rougher as a function of time. Figure 2 shows the kinetics results obtained during the second lead rougher flotation. Surprisingly, the lead recovery rate was low, while the zinc recovery rate was sharply increased due to the addition of 60 g/t of KAX. The low lead recovery may be attributed to the likelihood that most of the floatable galena particles had already been floated in the first rougher. What was surprising was that the zinc recovery rate increased sharply without the addition of the traditional activator, i.e., copper sulfate. It is possible that some of the heavy metal ions present in the pulp activated the sphalerite and caused the xanthate collector to adsorb on the mineral surface, thereby rendering the mineral hydrophobic. After the eight minutes of flotation time, 42.5% of the sphalerite floated at the second lead rougher with the froth product assaying more than 50%
Lead activation of sphalerite
873
Zn. Although a large portion of the sphalerite floated in the second lead rougher is rejected at the cleaner circuit and sent to the zinc rougher in the plant operation, the sphalerite flotation in the second lead rougher constitutes a significant source of zinc loss. 100
80
100
Zn o • a •
Rouflher Pb Kecovery Pb Grade Zn Recovery Zn Grade
80 q)
> 0 O
"I0 60
60
0 k.
C9
G)
n-
G) > 40
40
.~--
_Q
0 20
0 0
05
- = _ _ _ , _ ~ _ ^ ^ ^ ~, . - R - . . . . . . . . ~ 5 ' ' - =10 15
200
Time (min) Fig.2 Cumulative grade and recovery of Pb and Zn to the second lead rougher as a function of time. The kinetics test results obtained during the zinc rougher flotation are shown in Figure 3. The lead recovery to the zinc rougher was negligible since most of the galena had already been recovered during the two stages of lead rougher flotation. The rate of sphalerite flotation was high, most of it being recovered in the first five minutes..As a result, the zinc grade was maximum in the first minute and decreased afterwards, indicating that slow-floating particles were recovered during the later stages of flotation. There was no problem in floating sphalerite at the zinc rougher as the zinc loss to the final tall was only 0.7% of the feed. Nevertheless, the overall zinc recovery was less than 50% because a total of 52.5 % of zinc was lost during the first and second lead rougher flotation. The flotation resullls show that the high flotation rate of sphalerite particularly in the second lead rougher was responsible for the low zinc-to-zinc recovery. Since the ore was finely ground (75 % finer than 44#m), the sphalerite flotation during the later stages of galena flotation may be attributed to surface chemistry problems rather than to incomplete liberation. Surface Analysis of Flotation Concentrates
To identify the reason(s) for the high floatability causing poor selectivity in the lead rougher circuit, the flotation concentrates were analyzed for both lead and zinc using XPS and X-ray fluorescence (XRF) spectroscopy. Since XRF spectrometry has a sampling depth of at least one micron, the analysis gives information on bulk composition. On the other hand, XPS is a surface-sensitive technique with a sampling depth of only about a few nanometers. In the XPS measurements, the atomic concentration of lead was calculated from the intensity (spectral area) of the Pb(4f) doublet. Since the Zn(3s) peak is overlapping with the Pb(4f) doublet, as shown in Figure 4, the spectrum has been resolved by curve-fitting to subtract the area due to Zn(3s) peak. The atomic concentration of zinc was obtained from the intensity of the Zn(3p) doublet (not shown here).
C.i. Basilioet al.
874 100
100
Rougher 2 P• o Pb b Recovery Pb Grode a Zn Recovery • Zn Grode
80 "~"
.= o
6o
60
,,=__=._.4--=----=
III/
=---4--
0
~
20 L
.
.
.
p
40
~1~
20
8
.
d 0
_
0
0
1
2
3
4
5
6
7
8
Time (rain) Fig.3 Cumulative grade and recovery of Pb and Zn to the zinc rougher as a function of time. 100
80
100
Rougher I Pob Pb Recovery • Pb Grade
0, Zn Recovery
80 ID
"10 0
IIJ
> o 60 o
60
._> (1~ 40
40
._> o
0
E
E --! 2o 0
2O ~ /
.il
o 6 ~
I
~
rl
n
,
2
,
3
Time
,
4
,
5
,
6
,
7
8
0
(rain)
Fig.4 XPS spectrum (Pb(4f) and Zn(3s) region) of the first lead rougher concentrate. Table 2 shows the lead-to-zinc (Pb/Zn) atomic ratios of the different flotation concentrates as determined by XRF spectrometry and XPS. The first lead rougher concentrate (Sth minute product) has a surface Pb/Zn atomic ratio, as measured by XPS, much higher than the bulk Pb/Zn ratio measured by XRF. The same can be said of the second lead rougher and zinc rougher concentrates. The excess lead found on the surface suggests that sphalerite has been activated by the Pb2+ ions present in the plant water. The activation of sphaierite by Pb2+ ions may be represented by the following exchange reaction,
Lead activation of sphalerite
+ Pb 2+" PbS + Zn 2+
875
(1)
pK = -3.64
whose equilibrium constant K being given by:
(2)
K=
in which
g~ s
is the solubility product of the following reaction:
ZnS ~
Zn 2 + + S2 - ,
while
/t, PbS "*sp
(3)
is the same of the reaction:
P b S - * P b 2+ + S2-.
(4)
TABLE 2 Bulk and Surface Pb/Zn Atomic Ratios of Different Flotation Products Sample
Bulk Pb/Zn (XRF)
Surface Pb/Zn (XPS)
First lead rougher concentrate ~
0.14
0.40
Second lead rougher concentrate 2
0.007
0.08
Zinc rougher concentrate 3
0.003
0.05
8th min sample (31.4% Zn, 13.5 Pb); z 8th rain sample (54.2% Zn, 1.2% Pb); 3 1st min sample (55.3% Zn, 0.5% Pb).
Table 3 shows a survey of
K szns
and
/fPbS "~se
values reported in the literature. Substituting these values
into Eq. (2), one can obtain the values of K for reaction (1) in the range of 32.3 and 3.3x106. Since these values are large, the reaction should proceed to the right irreversibly. After Pb 2 + ions have replaced Zn 2 + ions from the sphalerite surface, stable lead xanthate and/or dixanthogen can be formed to render the mineral hydrophobic.
TABLE 3 Thermodynamic Data for Zinc and Lead Sulfides
Latimer t~
References Leckie and James ~'~
ZnS Sphalerite
7 . 2 x 1 0 -26
7.1 x 10 -26
2.2x10 -27
1.9x10 -26
Wurzite
4 . 6 x 1 0 -23
1.6 × 10 -23
1.6x10 -23
2 . 2 × 1 0 -24
precipitated
8 . 6 x 10 -23
8.9 x 10 -23
8.7x10 -23
PbS
6 . 8 x 1 0 -29
6.3 x 10 -29
6.6x10 -29
Sulfides
Bard et al. 15
Helgeson m
2 . 7 x 1 0 -29
Table 1 shows that the plant water contained 400 ppm of Zn 2+ ions and 1 ppm of Pb 2+ ions. If one uses
876
C. 1. Basilio et al.
the lowest value of K (=32.3) for reaction (1), ZnS can be activated only when P b 2 + ion concentration exceeds 12.4 ppm. On the other hand, if one uses the highest value of K (=3.3x106), the activation should occur when Pb 2+ ion concentration is larger than 1.2x10 -4 ppm, which is an exceedingly small concentration. Table 1 shows that the plant water contained 3 ppm of Pb 2+ ions; therefore, it is difficult to conclude whether the thermodynamic calculation provides a definitive answer to the question concerning lead activation of sphalerite. It should be noted, however, that the value of K=32.3 was obtained using the value of
KZpns
= 2.2x10 -27, which is the only one of the
KZpns
values given in Table 3 that do
not support the proposed activation mechanism. All other thermodynamic data suggest that at 400 ppm of Zn 2+ ions in solution, ZnS should be activated at Pb 2+ ion concentrations of approximately 1 ppm. For example, the Latimer's data for sphalerite (
KZpns
=
Itr P b S 7.2x10 -26) and lead sulfide ( "'~sP
=
6.8x10-29) give K = 1. lxl03 in accordance to (Eq. 2), in which case the lead activation should occur when Pb 2+ ion concentration exceeds 0.34 ppm at 400 ppm Zn 2+ ions. The Pb 2 + ions present in the flotation pulp may have been derived from the oxidized minerals present in the ore (i.e., anglesite). The lead-activation allows xanthate adsorption to take place resulting in the flotation of sphalerite in the lead rougher circuit. It is also possible that the inadvertent activation of sphalerite may have been caused by the Pb2+ ions produced from the oxidation of galena as suggested by Houot and Ravenau [4]. This would be particularly the case when an ore is ground for a long time, and the finely-ground ore requires a long retention time with large circulating loads. A solution to this problem may be an electrochemical potential control to minimize the galena oxidation.
Surface Analysis of Sphalerite Probe Further evidence that zinc loss to the lead concentrate may be caused by the lead-activation of sphalerite was obtained by analyzing the surface of the mono-minerallic sphalerite probe immersed in different solutions. Table 4 shows the results of the XPS analysis. After conditioning for 5 minutes in distilled water, no lead was detected on the surface. However, conditioning for 5 minutes in an ore slurry resulted in a Pb/Zn atomic ratio of 0.068 on the surface, which is close to that obtained with a sphalerite probe contacted with a 10 -4 M PbSO 4 solution for one minute. These results established that sphalerite was activated by Pb 2 + ions under the conditions employed for processing the ore.
TABLE 4 Surface Pb-to-Zn Atomic Ratios of Sphalerite Probes Contacted with Various Solutions Immersion Condition 5 rain. in distilled water 1 min. in 10 -4 M PbSO4 5 min. in ore slurry 5 rain. in plant water 15 rain, in plant water 5 rain. in simulated plant water
] Pb/Zn 0.000 0.085 0.068 0.043 0.056 0.023
For a sphalerite probe conditioned for 5 minutes in the plant water, the Pb/Zn atomic ratio on the surface was 0.043. This ratio was smaller than that obtained by immersing the probe in the ore slurry for five minutes, suggesting that the ground ore pulp provided additional Pb 2+ ions. It is possible that the oxidized minerals present in the ore serve as continual sources of Pb 2+ ions. After 15 minutes of conditioning in the plant water, the Pb/Zn ratio increased to 0.056, indicating that the lead-activation continued to increase
Lead activationof sphalerite
877
with conditioning time but at a slower rate. Conditioning the mono-mineralic sphalerite probe for five minutes in the simulated plant water gave a Pb/Zn ratio of 0.02',3, which was almost half of that observed after conditioning the probe in the plant water for the same period of time. Considering that the plant and simulated waters contain about the same amount of Pb 2 + ions, spha~lerite is more easily activated in the former, the reason for which is unknown at present. The results shown in Table 4 clearly establish that sphalerite is activated by Pb 2+ ions in solution via reaction (1), despite the fact that the amount of the Pb 2+ ions in the pulp is relatively small while the Zn 2+ ion concentration is high. Ralston et al. [17] showed that the kinetics of lead-activation is relatively slow, as compared to that of copper-activation, which may be one of the reason that sphalerite begins to float at the later stages of the lead rougher flotation. FTIR measurements were also carried out on the mono-mineraUic sphalerite probe treated under conditions similar to those us~xl for the XPS measurements, except that the samples used for the FTIR measurements were contacted with KAX solutions afterwards. Figure 5 shows the FTIR spectra obtained after immersing the probe in solutions containing different amounts of Pb 2+ ions and then contacting with 10 -4 M KAX solutions. The probe conditioned in a 10 -4 M PbSO 4 solution for one minute and then in KAX solution for 15 minutes showed the presence of lead xanthate (PbX2) and dixanthogen (X2) as shown in Figure 5a. The presence of PbX 2 is indicated by its characteristic absorption bands at 1204, 1164 and 1029 cm-1 while the bands at 1260, 1236, 1109 and 1029 cm -1 show the presence of X 2 [18,19]. No evidence for zinc xanthate formation was observed, which agrees with the work of Finkelstein and Allison [5].
a) @ 0 r-
U
c) L__
0 .Q
<
Wavenumber (cm") Fig.5 FT[R spectra of sphalerite probe immersed initially in the following: a) 10 - 4 M PbSO 4 solution for one minute, b) simulated plant water for 30 minutes, and c) plant water for 30 minutes; and then conditioned in 10 - 4 M KAX solution for 15 minutes.
878
c.I. Basilioet al.
Figures 5b and 5c show the FTIR spectra for the mono-minerallic sphalerite probe immersed for 30 minutes in simulated and actual plant water, respectively, and then contacted with a 10-4 M KAX solution for 15 minutes. The presence of PbX2 and X2 on the probe surface is also indicated on both spectra. These results show that sphalerite is inadvertently activated by the Pb2+ ions present in plant water causing a significant zinc loss to the lead circuit.
SUMMARY AND CONCLUSIONS An ore sample containing galena and sphalerite was subjected to flotation kinetics studies using a simulated plant water containing various heavy metal ions. The results showed that significant amounts of sphalerite float during the later stages of galena flotation, causing a loss of zinc to lead concentrates. Spectroscopic analysis of the lead rougher concentrates showed that lead-to-zinc atomic ratios are substantially higher on the surface than in the bulk of the minerals floated, suggesting that sphalerite is activated by the lead ions present in the flotation pulp. Evidence of the lead activation of sphalerite was also given by the XPS analysis of the mono-minerallic sphalerite probes that had been contacted with different solution; the probes contacted with the ore pulp, simulated plant water, and lead sulfate solutions show lead-to-zinc ratios substantially higher than that of the sphalerite specimen contacted with distilled water. When the sphalerite probe was contacted with a lead sulfate solution and then with an amyl xanthate solution, the FTIR spectra showed the presence of lead amyl xanthate and dixanthogen. Similar results were obtained when the monominerallic sphalerite probe was contacted first with simulated plant water (or actual plant water) and then with amyl xanthate solutions. These findings provided further evidence that sphalerite is inadvertently activated by the lead ions present in the plant water, react with xanthate collector, and acquire a significant floatability in the lead rougher flotation circuit, causing a significant loss of zinc to lead concentrate.
ACKNOWLEDGMENTS The authors wish to acknowledge Cominco Metals and Cominco Alaska for the financial support. They would also like to thank Prof. Eero Suoninen for reading the manuscript.
REFERENCES .
2. .
.
5.
.
7. 8.
9.
10.
Gaudin, A.M., Flotation. McGraw-Hill, London, 254 (1957). Bogdanov, O.S., Podnek, A.K. & Semenova, E.A., Issledovanie flotacii raznovidnostiei sfalerita (Flotation study of different sphalerite forms). Trudy Inst. Mechanobr, Vypusk, 135 (1965). Rey, M. & Formanek, V., Some Factors Affecting Selectivity in the Differential Flotation of Lead-Zinc Ores, Particularly in the Presence of Oxidized Lead Minerals. Paper No. 18, p. 343-353, Proceedings of the International Mineral Processing Congress, IMM, London (1960). Houot, R. & Ravenau, P., Activation of sphalerite flotation in the presence of lead ions. Int. J. Miner. Process., 35, 253 (1992). Finkelstein, N.P. & Allison, S.A., The chemistry of activation, deactivation, and depression in the flotation of zinc sulfide: a review. In: M.C. Fuerstenau (Editor), Flotation, A.M. Gaudin Memorial Volume. SME/AIME, New York, 414 (1976). Mielczarski, J., The role of impurities of sphalerite in the adsorption of ethyl xanthate and its flotation. Int. J. Miner. Process., 16, 179 (1986). Fuerstenau, M.C., Clifford, K.L. & Kuhn, M.C., The role of zinc-xanthate precipitation in sphalerite flotation. Int. J. Miner. Process., 1, 307 (1974). Marouf, B., Bessiere, J., Houot, R. & Blazy, P., Flotation of sphalerite without prior activation by metallic ions. Trans. Inst. Min. Metall., 95, C1, (1986). Leroux, M., Rao, S.R. & Finch, J.A., Selective flotation of sphalerite from Pb-Zn ore without copper activation. CIM Bull., 80(902), 41 (1987). Girczys, J. & Laskowski, J., Mechanism of flotation of unactivated sphalerite with xanthates. Trans. Inst. Min. Metall., 81, Cl18, (1972).
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18. 19.
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Mukherjee, A.D. & Sen, P.K., Floatability of sphalerite in relation to its iron content. J. Mines Metals Fuels, 327 (1976). Scofield, J.H., Hartree-Slater subshell photoionization cross-sections at 1254 and 1487 eV. J. Electron Spectrosc., 8, 129 (1976). Latimer, W.M., The Oxidation States of the Elements and their Potentials in Aqueous Solutions. Prentice-Hall, Inc., New Jersey, pp. 72, 152, 169 (1952). Leckie, J.O. & James, R.O., Aqueous-Environmental Chemistry of Metals. Alan J. Rubin, Editor, Arm Arbors Science Pub., Inc., Ann Arbor, Michigan, 1-76 (1974). Bard, A.J., Parsons, R. & Jordan J., Standard Potentials in Aqueous Solutions. Marcel Dekker, Inc., New York (1985). Helgeson, N.C., Thermodynamics of Hydrothermal Systems at Elevated Temperatures and Pressures. Am. J. Sci., 267, 729-804 (1969). Ralston, J., Alabaster, P. & Healy, T.W., Activation of zinc sulphide with CulI, Cd II, and Pb n, III. The mass spectrometric determination of elemental sulphur. Int. J. Miner. Process., 7, 279 (1981). de Donato, P., Cases, J.M., Kongolo, M. & Bumeau, A., Infrared investigation of amylxanthate adsorption of galena: Influence of oxidation, pH and grinding. Colloids Surfaces, 44, 207 (1990). Kongolo, M., Cases, J.M., Burneau, A. & Predali, J.J., Spectroscopic study of potassium amylxanthate adsorption on finely ground galena: relation with flotation. In: M.J. Jones and R. Oblat (Editors), Reagents in the Mineral Industry, 79 (1985).