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A study of the pyrrhotite depression mechanism by diethylenetriamine
Minerals Engineering, Vol. 8, No. 7, pp. 807-816, 1995 Elsevier Science Lid
Pergamon 0892-~7.5(9b')00041-0
Printed in Great Britain 0892--6875/95 $9.50+0.00
A STUDY OF THE PY RHOTITE DEPRESSION MECHANISM BY DIETHYLENETRIAMINE R...H. YOON§, C.I. BASILIO§, M.A. MARTICORENAt, A.N. KERR1"and R. STRATTON-CRAWLEY:~ § ('enter for Coal and Minerals Processing, ~irginia Polytechnic Institute and State University, Blacksburg, Virginia 24061-0258, U.S.A. t Central Mills, INCO Limited, Copper Cliff, Ontario P0M 1N0, Canada ~t J. Roy Gordon Research Laboratory, INCO Limited, Sheridan Park, Missisauga, Ontario L5K 1Z9, Canada (Received 25 January 1995; accepted 9 February 1995)
ABSTRACT Diethylenetriamine (DETA ) is a selective depressant for nickeliferous pyrrhotite during pentlandite flotation. Laboratory flotation tests conducted on ore and process samples showed that pyrrhotite rejection is greatly improved by small additions of DETA; however, the effectiveness of this depressant is most noticeable when the mineral sample is oxidized. LIMS and XPS analyses conducted on flotation products and pyrrhotite specimens showed that the dij~iculty in pyrrhotite rejection arises from inadvertent activation of the mineral by heavy metal ions, such as Ni2+, Cu2+ and Ag ÷, that are present in the process water. In the presence of DETA, however, the mineral is deactivated under oxidizing conditions. The deactivation mechanism may involve oxidation of the activation products, which are likely in t~e form of heavy metal sulfides, followed by solubilization by DETA. FTIR spectra of pyrrhotite electrodes contacted with DETA showed no trace of the reagent on the surface, substantiating the view that its role is one of complexing agent that enhances the dissolution of activation products under oxidizing conditions. FTIR spectra of pyrrhotite contacted with amyl xanthate solutions showed that both dixanthogen and iron xanthate are formed on the surface, the latter becoming more predominzmt at higher potentials. In the presence of DETA, however, only a small amount of xanthate is adsorbed on the mineral at potentials (Eh) approximately 200 mV higher than the case without DETA. Keyword.,~ Pyrrhotite rejection; depressants; diethylenetriamine; spectroscopy
INTRODUCTION
Since their discovery in the latter half of the 19th century, Sudbury Basin ores have been a major source of copper, nickel and precious metals. The minerals of economic interest include chalcopyrite (CuFeS2) and pentlandite ((NiFe)9Ss), which are associated with a greater abundance of pyrrhotite (FesS9) in a norite host rock. A more detailed discussion of the mineralogy of the Sudbury Basin deposits is given elsewhere [1].
807
808
R.-H. YOONet al.
In the smelting of the mill concentrates from this ore, pyrrhotite is the major source of sulfur dioxide emissions. Over the last 25 years, INCO has progressively reduced these emissions, culminating in 1994 with the completion of the SO 2 Abatement Project [2], This project has resulted in rejection or capture of >90% of the sulfur contained in the ore. The focus of these efforts in the context of mineral processing has been to maximize pyrrhotite rejection in milling. Sudbury area pyrrhotite is found in two different crystallographic forms, i,e., monoclinic and hexagonal. The monoclinic pyrrhotite, representing about 70% of the mineral i n the ore, is ferromagnetic, and can be rejected efficiently by magnetic separation. The hexagonal pyrrhotite is weakly (para-) magnetic and is rejected by flotation in what is essentially a nickel scavenging circuit. However, the feed to this circuit is of low-grade and oxidized, and consequently the separation is inefficient. As part of an intensive program aimed at identifying reagent(s) that would enhance penflandite-pyrrhotite separation in this circuit, a series of amino compounds were tested as pyrrhotite depressants. Diethylenetriamine (DETA) was identified as one of the more effective reagents tested and was subsequently introduced as a standard reagent in the Copper Cliff mill complex. The work carried out to determine the conditions under which DETA was effective has been described previously [3]. The present work deals with the investigation into the mechanism by which DETA depresses pyrrhotite. EXPERIMENTAL
Laboratory Flotation Tests Experiments were carried out in 2:2 L Denver flotation cell at 20 Hz impeller speed and 4 L/min air flow rate with a mechanical froth paddle rotating at a frequency of 0.2 sec-1. Slurry redox, pH and dissolved oxygen content were all monitored for the test duration. In tests involving ores, 1.25 kg Clarabelle rod mill feed was ground at 65% solids in saturated CaSO 4 water to a nominal product size of 12% retained on a 65-mesh screen. Magnetic pyrrhotite was removed prior to flotation using a 100-gauss hand magnet. In tests involving the feed to the Clarabelle pyrrhotite rejection circuit, samples of the plant stream were taken, settled, decanted and frozen before shipment to J.Roy Gordon Research Laboratory (JRGRL). In all cases, the % solids during flotation was approximately 37%. Potassium amyl xanthate (Aero 350, Cyanamid), Dowfroth 1263 and DETA were the reagents used. Materials Natural pyrrhotite specimens from the Sudbury ore, cut and polished, were used for contact angle and spectroscopy measurements. The DETA (99% purity) used was obtained from Aldrich Chemicals. The potassium amyl xanthate (KAX) was recrystallized three-times with acetone and ethyl ether before use. All measurements, unless otherwise noted, were conducted at pH 9.2 in 0.05 M Na2B407 buffer. In the experiments conducted under controlled-potential conditions, the buffer solution was deoxygenated by purging with low-oxygen (<0.05 ppm 02) nitrogen gas for at least one hour. Some XPS measurements were carried out using simulated Clarabelle Mill process water, which contained about 0.031 ppm Cu, 0.15 ppm Ni, 0.012 ppm Ag, 450 ppm Ca, 100 ppm Na, 30 ppm Mg and various sulfoxy ions (SxOy-2). The pH of this simulated plant water was adjusted to 9.5 using Ca(OH) 2.
Spectroscopy Measurements Fourier Transform Infrared (FFIR) spectroscopic measurements were conducted on pyrrhotite plates under controlled-potential conditions using external reflection technique. A total of 64 scans were recorded at four cm -1 resolution using a Bio-Rad FFS 60A FFIR spectrometer, The X-ray Photoelectron Spectroscopy (XPS) measurements were carried out using a Perkin-Elmer ESCA PHI 5400 spectrometer. Unmonoehromatized X-rays from a magnesium anode were used to excite the photoelectrons. Before each measurement, the electrode was wet-polished, washed in an ultrasonic bath of warm ethyl alcohol and rinsed in 18 Mf~ deionized water. The sample was then immediately immersed in ~the solution under investigation. A Laser Ionization Mass Spectrometer (LIMS 2a, Cambridge Mass Spectrometry) retrofitted with a Nd:YAG laser for post-ablation ionization was used in the LIMS analysis.
809
Pynhotitedepressionmechanism Contact Angle Memurcments
A specially-designed electrochemical cell was used to measure contact angles under controlled-potential conditions. In these measurements, the pyrrhotite electrode was first polarized at a cathodic potential. The potential was then increased to a desired value before depositing the nitrogen bubble on the electrode surface. The contact angle was measured using a Rame-Hart Model 100 Contact Angle Goniometer. The electrode potential was controlled with a PAR Model 371 Potentiostat/Galvanostat and a PAR Model 175 Universal ProgramrrLer. Although a Ag/AgC1 electrode was used as the reference electrode, the potentials reported here are expressed on the standard hydrogen electrode (SHE) scale.
RESULTS AND DISCUSSION Flotation Tests
Figure 1 shows typical laboratory flotation test results obtained with the feed to the Clarabelle pyrrhotite rejection circuit. In the absence of DETA, the flotation response of the sample did not change whether the sample was floated i) as-received, ii) after regrinding, or iii) after regrinding and aeration to oxidize the sample. This finding suggests that liberation was not a factor in determining the efficiency in pyrrhotite rejection. Use of DF,TA improved the pyrrhotite rejection; however, this improvement was observed only when the flotation was conducted i) as-received or ii) after regfinding followed by aeration. When the sample was reground and floated without subsequent aerative conditioning, the flotation response with DETA was similar to that observed without DETA. These results indicate that the improvement in pyrrhotite rejection is due to depression of the mineral by DETA, and that the reagent is effective under oxidizing condition.
NICKEL RECOVERY, % 100
80
• =
o
60
• 40
DO
o
O 20
II 0
' 0
i
20
,
I
40
,
,
I
60
,
i
80
100
PYRRHOTITE RECOVERY, % Fig. 1 Flotation restdts obtained with a feed to the Clarahelle pyrrhotite rejection circuit: i) as received with (m) and without (D) DETA, ii) r e g r o u n d with (o) and without (o) DETA, and iii) reground and aerated with (O) and without (x) DETA.
810
R.-H. YOON et al.
Figure 2 shows the flotation results obtained with the Clarabelle ore. When the as-received ore sample was allowed to oxidize, the flotation response in the absence of DETA deteriorated dramatically. However, conditioning both as-received and oxidized ore samples with DETA improved the pyrrhotite rejection. It is possible that the as-received ore was oxidized to some extent during collection and transportation to the laboratory. Therefore, both the as received and oxidized samples responded well to DETA. The results shown in Figure 2 clearly indicates that use of DETA shifted the recovery curves toward the left and upwards, demonstrating its effectiveness in depressing pyrrhotite and possibly enhancing the floatability of pentlandite.
NICKEL RECOVER% ~ 100
90
/
80
AS RECEIVED
70
50
0
20
40
60
80
PYRRHO~TR EECOVER~% Fig.2 Flotation results obtained with the Clarabelle ore. LIMS Study The pyrrhotite grains in the concentrates and tails obtained from the flotation tests were examined using LIMS. One of the more interesting observations was that both nickel (not shown) and specially copper (shown in Figure 3) are present on the pyrrhotite surface at levels generally higher than those on the substrate. Also, the amounts of nickel and copper found in the concentrates are greater than those found in the tails. These results suggest strongly that pyrrhotite is activated by these heavy metal ions, which may be the source of difficulty in pyrrhotite rejection. Figure 4 shows the calcium and carbon counts on the surface of pyrrhotite particles in the concentrates and tails obtained from the tests conducted using DETA. As shown, the tails show significantly higher calciumto-carbon ratios than the concentrates. When no DETA was used, the concentrates and tails show the same calcium-to-carbon ratios (not shown). FTIR Spectroscopy Studies Figure 5 shows the FTIR spectra of a pyrrhotite specimen conditioned for five minutes in a 10-3 M DETA solution under controlled-potential conditions. DETA has characteristic IR bands in the 1400-1600 cm -I region; however, no trace of DETA is shown in the spectra given in Figure 5. Only the oxidation of the mineral, as indicated by the broad bands in the 900-1200 cm -1 region, is shown. This finding suggests that the depressing action of DETA shown in flotation tests (Figures 1 and 2) may, not involve an adsorption mechanism, which is different from those observed for the more traditional depressants such as lime and cyanide.
Pyrrhotite depression mechanism
811
Gu(63) in Surface Layer (counts/total counts) 1E -1
1E -2
1E "3
o ° O
[]
1E4
[]
•0 'F1 1E -5 1E -5
1E -4
1E -3
1E -2
1E -1
Cu(65) in Surface Layer (count~total counts)
Fig.3 LIMS analysis of copper on the surface of pyrrhotite particles reporting to the flotation concentrate ( I ) and tails ([3); substrate(@). Ca(40) in Surface Layer (counts/total counts) 1E -1
1E -2 . . . . . . . . . . .
1E-3
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1E -5 1E -5
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C(12) in Surface Layer (counts/total counts)
Fig.4 LIMS analysis of calcium and carbon on the surface of pyrrhotite in flotation concentrate (11) and tails ([3); substrate ( 0 ) . Figure 6 shows the FTIR spectra of pyrrhotite contacted with a 10-3 M KAX solution under controlledpotential condition:;. It appears that both dixanthogen and iron xanthates were present. The presence of dixanthogen is indicated by the characteristic bands at: 1260 cm -1 (combination of S--C--S and C--O--C
812
R.-H. YOON et al.
asymmetric vibrations), 1136 cm -1 (symmetric C-O-C vibrations), and 1048 and 1015 c m -1 (C--S vibrations), while those for iron xanthates are at 1227 cm -I (S-C-S and C-O-C asymmetric vibrations), 1136, and 1015 cm -1. The spectra given in Figure 6 seem to show that iron xanthate formation is favored at higher potentials where the activity of iron hydroxide would be higher due to more severe oxidation. Pyrrhotite ~HI09-'~ M BETA
~D
0.4
0
0.3 v
V
< 0.2 V
-0.3 -1800
I -1700
I -1600
I -1500
I -1400
I -1300
I -1200
Wavenumber
I -1100
I -tO00
[ -900
V -800
( c m -1)
Fig.5 FTIR spectra of pyrrhotite conditioned in 10-3 M DETA solution at pH 9.2 and different potentials. No trace of DETA is shown. Pvrrhotite p[-I 9.2 IxlO -~ M
~
r
KAX,.~
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1200
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900
800
Wavenumber (cm -I) Fig.6 FTIR spectra of pyrrhotite conditioned in a 10-3 M KAX solution at Ph 9.2 and different potentials.
Pyrrhotite depression mechanism
813
One can see from Figure 6 that the ~ intensity increases with increasing potential. In order to obtain quantitative information on the effect of potential on the xanthate adsorption, the FI'IR intensities at 1260 cm -1 have been plotted in Figure 7. Also shown in Figure 7 are the water contact angles measured. Both the IR intensity and contact angle increased sharply at approximately 0 mV, which corresponds to the thermodynamic potential for dixanthogen formation. In the absence of KAX, the contact angle was zero over the entire Eh-range investigated. In the presence of 10-3 M DETA and KAX, however, the contact angle remained zero until the potential reached approximately 0.25 V. At higher potentials, only a small amount of dixanthogen was detected on pyrrhotite surface, with correspondingly reduced contact angles. 60
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Potential (V, SHE) Fig.7 Effect of potential on the adsorption of KAX, as measured by the IR intensity at 1260 cm -1, and on the contact angles on pyrrhotite at pH 9.2 in the presence and absence of DETA. The FTIR results obtained in the present work support the xanthate adsorption mechanism on pyrrhotite proposed by Hodgson and Agar [4]. Based on voltammetry studies, they suggested that pyrrhotite oxidizes as follows: FeS + H20 = Fe(C)H)[S]+ + H + + 2e
(1)
to form an iron (IH) hydroxy-polysulfide as the initial oxidation product. Xanthate ions adsorbs on the positively-charged oxidation product v/a coulombic attraction: Fe(OH)[S] + + X- --=Fe(On)[s]x,
(2)
which results in the formation of an iron hydroxy-polysulfide xanthate on the surface. The residual xanthate may be oxidized to dixanthogen, which subsequently adsorbs on the iron-hydroxy polysulfide xanthate possibly by hydrophobic interaction. The xanthate adsorption mechanism discussed above may provide an explanation for the prevention of xanthate adsorptiorL by DETA at potentials between 0 and 0.25 V (Figure 7). It is possible that DETA forms soluble complexes with the Fe 2+ ions formed on the surface of pyrrhotit¢ during oxidation. Since the Fe 2+ ions may be precursors for the formation of Fe(III)OH[S] + species, DETA may prevent its formation and,
814
R.-H. YOONet al.
hence, the xanthate adsorption. When the potential is above 0.25 V, however, small amounts of xanthate may still adsorb on pyrrhotite due to the large amount of iron hydroxides formed as a results of aggressive oxidation of the mineral. XPS Studies Table 1 shows the results of XPS analysis of pyrrhotite before and after conditioning the mineral for five minutes in a 10-4 M DETA solution at open circuit conditions. The atomic ratios of the different elements present on the surface were calculated from the intensity of the following signals: S(2p), Fe(2p), N(ls), Ni(2p), Cu(2p), and Ag(3d). The sensitivity factors used in the calculation were based on theoretical photoionization cross-sections and the calculated inelastic mean free paths of the electrons [5,6]. The results show no significant change in the N/S atomic ratio before and after DETA treatment. The XPS data also do not show any shifts in the binding energy of the N(ls) signal (not shown here). Since the presence of DETA is indicated by changes in the N(ls) signal, the results indicate that DETA does not adsorb on pyrrhotite, which agrees with the FTIR data.
TABLE 1 Atomic ratios of N, Fe, Ni, Cu, and Ag to S on pyrrhotite after various treatments.
Atomic Ratio Treatment
N/S
FedS
Ni/S
Cu/S
Ag/S
Wet-polished 10.4 M DETA
0.13
0.73
-
0.14
0.73
-
Process H20
0.24
0.99
0.11
0.01
0.015
Process H 2 0 , 10 "5 M D E T A
0.25
1.32
0.08
0.009
0.010
Process H 2 0 , 10 .4 M D E T A
0.27
1.34
0.05
0.008
0.009
Process H 2 0 , 5x10.4 M D E T A
0.21
1.25
0
0.002
0.009
Process H20, 10 .3 M D E T A
0.21
I.14
0
0.001
0.007
Table 1 also shows the results obtained for pyrrhotite conditioned for 15 minutes in a simulated Clarabelle Mill process water. Although the process water contained very small amounts (< one ppm) of Ni 2+, Cu 2+, and Ag + ions, considerable amount of these ions were detected on the pyrrhotite surface by XPS. Because these ions can form more insoluble sulfides than Fe 2÷ ions, pyrrhotite may be activated by these heavy metal ions. This is similar to the activation of sphalerite and pyrite. In order to study the effects of DETA, a pyrrhotite sample, which had been contacted with the simulated process water for 15 minutes, was immersed in different concentrations of DETA solutions for another 15 minutes. After immersing the sample in a 10-5 M DETA solution, the amounts of these heavy metals decreased considerably. It seems, however, that Cu is more difficult to remove than Ni and Ag as the change in the Cu/S ratio was significantly smaller than the Ni/S and Ag/S ratios. At higher DETA concentrations, a larger amount of Ni was removed. At DETA concentrations > 10-4 M, Ni was completely removed, while small amounts of Cu and Ag were still left on the pyrrhotite surface. This finding is not surprising because both CuS and AgS are thermodynamically more stable than NiS. Thus, the XPS analysis suggest that pyrrhotite can be activated by the heavy metal ions present in plant water. The activation mechanism may be similar to that of copper-activation of sphalerite. The driving force for the activation is that heavy metal sulfides are more stable (or less soluble) than pyrrhotite. Although thiol collectors can adsorb on pyrrhotite without activation, the amount of collectors required for the adsorption would be higher than those required for other sulfide minerals and the potentials at which collector adsorption occurs is higher, providing a basis for separation. When pyrrhotite is activated, however, thiol collectors can adsorb on the mineral at lower collector concentrations and lower potentials, making the separation more difficult.
Pyrrhotitedepressionmechanism
815
The XPS data given in Table 1 suggest that the role of DETA is to deactivate pyrrhotite. This is achieved by virtue of its ability to form aqueous complexes with certain heavy metal ions, such as Ni 2+, Cu 2+, and Ag + ions. However, sulfides of these heavy metals are so insoluble under reducing conditions that DETA is not effective as a deactivating agent. When the ore is subjected to oxidizing conditions, the activation product formed on pyrrhotite becomes an oxide that can more readily react with DETA and be removed from the surface. This mechanism may be similar to the ammoniacal leaching of copper oxides, which is supported by the flotation results (Figures 1 and 2) that show ,that DETA is an effective deactivator only when the ore is o~:idized. The proposed mechanism may also be supported indirectly by the chemical analysis of the tailings water after laboratory flotation tests. As shown in Table 2, the solubility of both Cu 2+ and Ni 2+ ions greatly increased when the flotation was conducted after aerative conditioning. TABLE 2 Analysis of water before and after batch flotation tests using DETA.
~tratica
(aW,/L)
:~nple
Cu
Hi
Fc
I~rocas H20
0.9.5
0.06
0.05
'Failings H20 TailinSs H20 (w/aeration)
0.21
5.1
0.04
3.6
9.4
0.04
The nickeliferous pyrrhotite in the Sudbury Basin ore contains Ni 2+ ions in the lattice. One might, therefore, suspect that the mineral may be self-activated; however, the amount of Ni 2+ ions found on the surface of nickeliferous pyrrhotite particles by LIMS is far in excess of what is originally present on the mineral. Therefore, the activation of pyrrhotite by Ni 2+ ions are induced by the ions present in the process water. It is well known that DETA is an excellent complexing agent for Cu 2+, Ni 2+, Co 2+, and Fe 2+ ions, but not for Fe 3+, which may be the basis for the excellent selectivity observed with this reagent as a pyrrhotite depressant. There are other complexing agents for Cu 2+ and Ni 2+ ions, but many form complexes with Fe 3+ ions as well, which are always present in mill water. A typical example is EDTA. However, such a reagent would not be effective as a pyrrhotite depressant probably because it is consumed in complexing Fe 3+ ions. It should also be noted here that DETA is most effective when the mill water is saturated with calcium ions. The LIMS data shown in Figure 4 suggest that in the presence of DETA, Ca 2+ ions become a more effective pyrrhotite depressant. However, it is not known at this point what the role of Ca 2+ ions in the deactivation mechanism discussed above. SUMMARY AND CONCLUSIONS DETA is a highly selective depressant for nickeliferous pyrrhotite during flotation of pentlandite. Laboratory flotation tests conducted with a feed to the Clarabelle pyrrhotite rejection circuit showed that the separation efficiency was greatly improved by using a small amount of DETA. However, the improvement was observed only when the feed was oxidized by aerative conditioning. The test results obtained with a Clarabelle ore also showed that DETA is effective only when the ore is oxidized. Examination of the flotation concentrates by LIMS showed that there was a significant increase in the amount of copper and nickel found on the surface of pyrrhotite, indicating that the mineral is activated by the heavy metal ions present in the plant water. The pyrrhotite activation by Cu 2+, Ni 2+ and Ag + ions was confirmed by XPS analysis of the mineral specimens immersed in simulated plant water. The XPS analysis showed also that when an activated pyrrhotite specimen was contacted with DETA solutions, the amount of heavy metal ion,,; found on the surface was reduced significantly, suggesting that the role of DETA is one of deactivator. At higher concentrations of DETA, the amount of Ni left on the surface was beyond the detection limit, while complete removal of Cu and Ag was more difficult. The activation products formed on pyrrhotite may be sulfides of Ni, Cu, Ag, etc., which are usually insoluble under reducing conditions. When the mineral is oxidized, however, the activation products will 14£ O:7-G
816
R.-H. YOONetal.
be converted to oxides, whose solubility may be increased in the presence of DETA. This may explain the observation that DETA works effectively as a pyrrbotite depressant only when the ore is oxidized. FFIR spectra of pyrrhotite contacted with DETA showed no trace of this reagent on the surface, supporting the role of DETA as a eomplexing agent. The role of DETA in pyrrhotite flotation was further investigated by monitoring collector adsorption and contact angles while controlling the electrochemical potential (Eh) of the mineral. The results showed that in the presence of DETA, xanthate adsorption is prevented below approximately 0.25 V, possibly due to the prevention of formation of the iron-hydroxy polysulfide. This species is regarded as the precursor to xanthate adsorption. At potentials above 0.25 V, however, small amounts of xanthate still adsorb on pyrrhotite.
ACKNOWLEDGEMENTS The authors acknowledge the personnel from AMTEL, particularly Greg Hill, for assisting in LIMS work, Germain Labonte and Kevin Stewart (JRGRL) for doing the flotation test work, Dong-Su Kim (CCMP) for assisting in the FI'IR and contact angle measurements, and INCO Ltd for permission to publish this work.
REFERENCES .
2. 3. 4.
.
6.
Hawley, J.E., The Sudbury ores, their mineralogy and origin. Can. Mineral., 7(1), 1 (1962). Sopko, M., Environmental programs at Sudbury. CIMM 96th Annual Gen. Meet., Toronto, 61 (1994). Martieorena, M,A., Hill, G., Kerr, A.N., Liechti, D. & Pelland, D.A. INCO develops new pyrrhotite depressant. Innovations in Mineral Proc., Sudbury, Ontario 15 (1994). Hodgson, M. & Agar, G.E., Electrochemical investigations into the flotation chemistry of pentlandite and pyrrhotite: Process water and xanthate interactions. Can. Met. Qtr., 28(3), 189 (1989). Scofield, J.H., Hartree-Slater subshell photoionization cross-sections at 1254 and 1487 eV. J. Electron Spectrosc., 8, 129 (1976). Wagner, C.D., Gale, L.H. & Raymond, R.H., Two-dimensional chemical plots: a standardized data set for use in identifying chemical states by XPS. Anal. Chem., $1, 466 (1979).
Pergamon 0892-~7.5(9b')00041-0
Printed in Great Britain 0892--6875/95 $9.50+0.00
A STUDY OF THE PY RHOTITE DEPRESSION MECHANISM BY DIETHYLENETRIAMINE R...H. YOON§, C.I. BASILIO§, M.A. MARTICORENAt, A.N. KERR1"and R. STRATTON-CRAWLEY:~ § ('enter for Coal and Minerals Processing, ~irginia Polytechnic Institute and State University, Blacksburg, Virginia 24061-0258, U.S.A. t Central Mills, INCO Limited, Copper Cliff, Ontario P0M 1N0, Canada ~t J. Roy Gordon Research Laboratory, INCO Limited, Sheridan Park, Missisauga, Ontario L5K 1Z9, Canada (Received 25 January 1995; accepted 9 February 1995)
ABSTRACT Diethylenetriamine (DETA ) is a selective depressant for nickeliferous pyrrhotite during pentlandite flotation. Laboratory flotation tests conducted on ore and process samples showed that pyrrhotite rejection is greatly improved by small additions of DETA; however, the effectiveness of this depressant is most noticeable when the mineral sample is oxidized. LIMS and XPS analyses conducted on flotation products and pyrrhotite specimens showed that the dij~iculty in pyrrhotite rejection arises from inadvertent activation of the mineral by heavy metal ions, such as Ni2+, Cu2+ and Ag ÷, that are present in the process water. In the presence of DETA, however, the mineral is deactivated under oxidizing conditions. The deactivation mechanism may involve oxidation of the activation products, which are likely in t~e form of heavy metal sulfides, followed by solubilization by DETA. FTIR spectra of pyrrhotite electrodes contacted with DETA showed no trace of the reagent on the surface, substantiating the view that its role is one of complexing agent that enhances the dissolution of activation products under oxidizing conditions. FTIR spectra of pyrrhotite contacted with amyl xanthate solutions showed that both dixanthogen and iron xanthate are formed on the surface, the latter becoming more predominzmt at higher potentials. In the presence of DETA, however, only a small amount of xanthate is adsorbed on the mineral at potentials (Eh) approximately 200 mV higher than the case without DETA. Keyword.,~ Pyrrhotite rejection; depressants; diethylenetriamine; spectroscopy
INTRODUCTION
Since their discovery in the latter half of the 19th century, Sudbury Basin ores have been a major source of copper, nickel and precious metals. The minerals of economic interest include chalcopyrite (CuFeS2) and pentlandite ((NiFe)9Ss), which are associated with a greater abundance of pyrrhotite (FesS9) in a norite host rock. A more detailed discussion of the mineralogy of the Sudbury Basin deposits is given elsewhere [1].
807
808
R.-H. YOONet al.
In the smelting of the mill concentrates from this ore, pyrrhotite is the major source of sulfur dioxide emissions. Over the last 25 years, INCO has progressively reduced these emissions, culminating in 1994 with the completion of the SO 2 Abatement Project [2], This project has resulted in rejection or capture of >90% of the sulfur contained in the ore. The focus of these efforts in the context of mineral processing has been to maximize pyrrhotite rejection in milling. Sudbury area pyrrhotite is found in two different crystallographic forms, i,e., monoclinic and hexagonal. The monoclinic pyrrhotite, representing about 70% of the mineral i n the ore, is ferromagnetic, and can be rejected efficiently by magnetic separation. The hexagonal pyrrhotite is weakly (para-) magnetic and is rejected by flotation in what is essentially a nickel scavenging circuit. However, the feed to this circuit is of low-grade and oxidized, and consequently the separation is inefficient. As part of an intensive program aimed at identifying reagent(s) that would enhance penflandite-pyrrhotite separation in this circuit, a series of amino compounds were tested as pyrrhotite depressants. Diethylenetriamine (DETA) was identified as one of the more effective reagents tested and was subsequently introduced as a standard reagent in the Copper Cliff mill complex. The work carried out to determine the conditions under which DETA was effective has been described previously [3]. The present work deals with the investigation into the mechanism by which DETA depresses pyrrhotite. EXPERIMENTAL
Laboratory Flotation Tests Experiments were carried out in 2:2 L Denver flotation cell at 20 Hz impeller speed and 4 L/min air flow rate with a mechanical froth paddle rotating at a frequency of 0.2 sec-1. Slurry redox, pH and dissolved oxygen content were all monitored for the test duration. In tests involving ores, 1.25 kg Clarabelle rod mill feed was ground at 65% solids in saturated CaSO 4 water to a nominal product size of 12% retained on a 65-mesh screen. Magnetic pyrrhotite was removed prior to flotation using a 100-gauss hand magnet. In tests involving the feed to the Clarabelle pyrrhotite rejection circuit, samples of the plant stream were taken, settled, decanted and frozen before shipment to J.Roy Gordon Research Laboratory (JRGRL). In all cases, the % solids during flotation was approximately 37%. Potassium amyl xanthate (Aero 350, Cyanamid), Dowfroth 1263 and DETA were the reagents used. Materials Natural pyrrhotite specimens from the Sudbury ore, cut and polished, were used for contact angle and spectroscopy measurements. The DETA (99% purity) used was obtained from Aldrich Chemicals. The potassium amyl xanthate (KAX) was recrystallized three-times with acetone and ethyl ether before use. All measurements, unless otherwise noted, were conducted at pH 9.2 in 0.05 M Na2B407 buffer. In the experiments conducted under controlled-potential conditions, the buffer solution was deoxygenated by purging with low-oxygen (<0.05 ppm 02) nitrogen gas for at least one hour. Some XPS measurements were carried out using simulated Clarabelle Mill process water, which contained about 0.031 ppm Cu, 0.15 ppm Ni, 0.012 ppm Ag, 450 ppm Ca, 100 ppm Na, 30 ppm Mg and various sulfoxy ions (SxOy-2). The pH of this simulated plant water was adjusted to 9.5 using Ca(OH) 2.
Spectroscopy Measurements Fourier Transform Infrared (FFIR) spectroscopic measurements were conducted on pyrrhotite plates under controlled-potential conditions using external reflection technique. A total of 64 scans were recorded at four cm -1 resolution using a Bio-Rad FFS 60A FFIR spectrometer, The X-ray Photoelectron Spectroscopy (XPS) measurements were carried out using a Perkin-Elmer ESCA PHI 5400 spectrometer. Unmonoehromatized X-rays from a magnesium anode were used to excite the photoelectrons. Before each measurement, the electrode was wet-polished, washed in an ultrasonic bath of warm ethyl alcohol and rinsed in 18 Mf~ deionized water. The sample was then immediately immersed in ~the solution under investigation. A Laser Ionization Mass Spectrometer (LIMS 2a, Cambridge Mass Spectrometry) retrofitted with a Nd:YAG laser for post-ablation ionization was used in the LIMS analysis.
809
Pynhotitedepressionmechanism Contact Angle Memurcments
A specially-designed electrochemical cell was used to measure contact angles under controlled-potential conditions. In these measurements, the pyrrhotite electrode was first polarized at a cathodic potential. The potential was then increased to a desired value before depositing the nitrogen bubble on the electrode surface. The contact angle was measured using a Rame-Hart Model 100 Contact Angle Goniometer. The electrode potential was controlled with a PAR Model 371 Potentiostat/Galvanostat and a PAR Model 175 Universal ProgramrrLer. Although a Ag/AgC1 electrode was used as the reference electrode, the potentials reported here are expressed on the standard hydrogen electrode (SHE) scale.
RESULTS AND DISCUSSION Flotation Tests
Figure 1 shows typical laboratory flotation test results obtained with the feed to the Clarabelle pyrrhotite rejection circuit. In the absence of DETA, the flotation response of the sample did not change whether the sample was floated i) as-received, ii) after regrinding, or iii) after regrinding and aeration to oxidize the sample. This finding suggests that liberation was not a factor in determining the efficiency in pyrrhotite rejection. Use of DF,TA improved the pyrrhotite rejection; however, this improvement was observed only when the flotation was conducted i) as-received or ii) after regfinding followed by aeration. When the sample was reground and floated without subsequent aerative conditioning, the flotation response with DETA was similar to that observed without DETA. These results indicate that the improvement in pyrrhotite rejection is due to depression of the mineral by DETA, and that the reagent is effective under oxidizing condition.
NICKEL RECOVERY, % 100
80
• =
o
60
• 40
DO
o
O 20
II 0
' 0
i
20
,
I
40
,
,
I
60
,
i
80
100
PYRRHOTITE RECOVERY, % Fig. 1 Flotation restdts obtained with a feed to the Clarahelle pyrrhotite rejection circuit: i) as received with (m) and without (D) DETA, ii) r e g r o u n d with (o) and without (o) DETA, and iii) reground and aerated with (O) and without (x) DETA.
810
R.-H. YOON et al.
Figure 2 shows the flotation results obtained with the Clarabelle ore. When the as-received ore sample was allowed to oxidize, the flotation response in the absence of DETA deteriorated dramatically. However, conditioning both as-received and oxidized ore samples with DETA improved the pyrrhotite rejection. It is possible that the as-received ore was oxidized to some extent during collection and transportation to the laboratory. Therefore, both the as received and oxidized samples responded well to DETA. The results shown in Figure 2 clearly indicates that use of DETA shifted the recovery curves toward the left and upwards, demonstrating its effectiveness in depressing pyrrhotite and possibly enhancing the floatability of pentlandite.
NICKEL RECOVER% ~ 100
90
/
80
AS RECEIVED
70
50
0
20
40
60
80
PYRRHO~TR EECOVER~% Fig.2 Flotation results obtained with the Clarabelle ore. LIMS Study The pyrrhotite grains in the concentrates and tails obtained from the flotation tests were examined using LIMS. One of the more interesting observations was that both nickel (not shown) and specially copper (shown in Figure 3) are present on the pyrrhotite surface at levels generally higher than those on the substrate. Also, the amounts of nickel and copper found in the concentrates are greater than those found in the tails. These results suggest strongly that pyrrhotite is activated by these heavy metal ions, which may be the source of difficulty in pyrrhotite rejection. Figure 4 shows the calcium and carbon counts on the surface of pyrrhotite particles in the concentrates and tails obtained from the tests conducted using DETA. As shown, the tails show significantly higher calciumto-carbon ratios than the concentrates. When no DETA was used, the concentrates and tails show the same calcium-to-carbon ratios (not shown). FTIR Spectroscopy Studies Figure 5 shows the FTIR spectra of a pyrrhotite specimen conditioned for five minutes in a 10-3 M DETA solution under controlled-potential conditions. DETA has characteristic IR bands in the 1400-1600 cm -I region; however, no trace of DETA is shown in the spectra given in Figure 5. Only the oxidation of the mineral, as indicated by the broad bands in the 900-1200 cm -1 region, is shown. This finding suggests that the depressing action of DETA shown in flotation tests (Figures 1 and 2) may, not involve an adsorption mechanism, which is different from those observed for the more traditional depressants such as lime and cyanide.
Pyrrhotite depression mechanism
811
Gu(63) in Surface Layer (counts/total counts) 1E -1
1E -2
1E "3
o ° O
[]
1E4
[]
•0 'F1 1E -5 1E -5
1E -4
1E -3
1E -2
1E -1
Cu(65) in Surface Layer (count~total counts)
Fig.3 LIMS analysis of copper on the surface of pyrrhotite particles reporting to the flotation concentrate ( I ) and tails ([3); substrate(@). Ca(40) in Surface Layer (counts/total counts) 1E -1
1E -2 . . . . . . . . . . .
1E-3
.-- -[13t . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . .
i
. . . . .
,. . . . . . . . . . .
:D h 1E-4
1E -5 1E -5
1E -4
1E -3
1E -2
1E -1
C(12) in Surface Layer (counts/total counts)
Fig.4 LIMS analysis of calcium and carbon on the surface of pyrrhotite in flotation concentrate (11) and tails ([3); substrate ( 0 ) . Figure 6 shows the FTIR spectra of pyrrhotite contacted with a 10-3 M KAX solution under controlledpotential condition:;. It appears that both dixanthogen and iron xanthates were present. The presence of dixanthogen is indicated by the characteristic bands at: 1260 cm -1 (combination of S--C--S and C--O--C
812
R.-H. YOON et al.
asymmetric vibrations), 1136 cm -1 (symmetric C-O-C vibrations), and 1048 and 1015 c m -1 (C--S vibrations), while those for iron xanthates are at 1227 cm -I (S-C-S and C-O-C asymmetric vibrations), 1136, and 1015 cm -1. The spectra given in Figure 6 seem to show that iron xanthate formation is favored at higher potentials where the activity of iron hydroxide would be higher due to more severe oxidation. Pyrrhotite ~HI09-'~ M BETA
~D
0.4
0
0.3 v
V
< 0.2 V
-0.3 -1800
I -1700
I -1600
I -1500
I -1400
I -1300
I -1200
Wavenumber
I -1100
I -tO00
[ -900
V -800
( c m -1)
Fig.5 FTIR spectra of pyrrhotite conditioned in 10-3 M DETA solution at pH 9.2 and different potentials. No trace of DETA is shown. Pvrrhotite p[-I 9.2 IxlO -~ M
~
r
KAX,.~
(9
i[0.
-~ R
0.4 V
O m ,.Q
0.2 V -0.3 [
1300
1200
I
I
1100 1000
V
I
900
800
Wavenumber (cm -I) Fig.6 FTIR spectra of pyrrhotite conditioned in a 10-3 M KAX solution at Ph 9.2 and different potentials.
Pyrrhotite depression mechanism
813
One can see from Figure 6 that the ~ intensity increases with increasing potential. In order to obtain quantitative information on the effect of potential on the xanthate adsorption, the FI'IR intensities at 1260 cm -1 have been plotted in Figure 7. Also shown in Figure 7 are the water contact angles measured. Both the IR intensity and contact angle increased sharply at approximately 0 mV, which corresponds to the thermodynamic potential for dixanthogen formation. In the absence of KAX, the contact angle was zero over the entire Eh-range investigated. In the presence of 10-3 M DETA and KAX, however, the contact angle remained zero until the potential reached approximately 0.25 V. At higher potentials, only a small amount of dixanthogen was detected on pyrrhotite surface, with correspondingly reduced contact angles. 60
i
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Pyrrhotite pH 9.2 50
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,
/
i
i
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/
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<
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0 -0.4 -~.3 ~-~'.2 ~-0'. i
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0.v4
0.5
Potential (V, SHE) Fig.7 Effect of potential on the adsorption of KAX, as measured by the IR intensity at 1260 cm -1, and on the contact angles on pyrrhotite at pH 9.2 in the presence and absence of DETA. The FTIR results obtained in the present work support the xanthate adsorption mechanism on pyrrhotite proposed by Hodgson and Agar [4]. Based on voltammetry studies, they suggested that pyrrhotite oxidizes as follows: FeS + H20 = Fe(C)H)[S]+ + H + + 2e
(1)
to form an iron (IH) hydroxy-polysulfide as the initial oxidation product. Xanthate ions adsorbs on the positively-charged oxidation product v/a coulombic attraction: Fe(OH)[S] + + X- --=Fe(On)[s]x,
(2)
which results in the formation of an iron hydroxy-polysulfide xanthate on the surface. The residual xanthate may be oxidized to dixanthogen, which subsequently adsorbs on the iron-hydroxy polysulfide xanthate possibly by hydrophobic interaction. The xanthate adsorption mechanism discussed above may provide an explanation for the prevention of xanthate adsorptiorL by DETA at potentials between 0 and 0.25 V (Figure 7). It is possible that DETA forms soluble complexes with the Fe 2+ ions formed on the surface of pyrrhotit¢ during oxidation. Since the Fe 2+ ions may be precursors for the formation of Fe(III)OH[S] + species, DETA may prevent its formation and,
814
R.-H. YOONet al.
hence, the xanthate adsorption. When the potential is above 0.25 V, however, small amounts of xanthate may still adsorb on pyrrhotite due to the large amount of iron hydroxides formed as a results of aggressive oxidation of the mineral. XPS Studies Table 1 shows the results of XPS analysis of pyrrhotite before and after conditioning the mineral for five minutes in a 10-4 M DETA solution at open circuit conditions. The atomic ratios of the different elements present on the surface were calculated from the intensity of the following signals: S(2p), Fe(2p), N(ls), Ni(2p), Cu(2p), and Ag(3d). The sensitivity factors used in the calculation were based on theoretical photoionization cross-sections and the calculated inelastic mean free paths of the electrons [5,6]. The results show no significant change in the N/S atomic ratio before and after DETA treatment. The XPS data also do not show any shifts in the binding energy of the N(ls) signal (not shown here). Since the presence of DETA is indicated by changes in the N(ls) signal, the results indicate that DETA does not adsorb on pyrrhotite, which agrees with the FTIR data.
TABLE 1 Atomic ratios of N, Fe, Ni, Cu, and Ag to S on pyrrhotite after various treatments.
Atomic Ratio Treatment
N/S
FedS
Ni/S
Cu/S
Ag/S
Wet-polished 10.4 M DETA
0.13
0.73
-
0.14
0.73
-
Process H20
0.24
0.99
0.11
0.01
0.015
Process H 2 0 , 10 "5 M D E T A
0.25
1.32
0.08
0.009
0.010
Process H 2 0 , 10 .4 M D E T A
0.27
1.34
0.05
0.008
0.009
Process H 2 0 , 5x10.4 M D E T A
0.21
1.25
0
0.002
0.009
Process H20, 10 .3 M D E T A
0.21
I.14
0
0.001
0.007
Table 1 also shows the results obtained for pyrrhotite conditioned for 15 minutes in a simulated Clarabelle Mill process water. Although the process water contained very small amounts (< one ppm) of Ni 2+, Cu 2+, and Ag + ions, considerable amount of these ions were detected on the pyrrhotite surface by XPS. Because these ions can form more insoluble sulfides than Fe 2÷ ions, pyrrhotite may be activated by these heavy metal ions. This is similar to the activation of sphalerite and pyrite. In order to study the effects of DETA, a pyrrhotite sample, which had been contacted with the simulated process water for 15 minutes, was immersed in different concentrations of DETA solutions for another 15 minutes. After immersing the sample in a 10-5 M DETA solution, the amounts of these heavy metals decreased considerably. It seems, however, that Cu is more difficult to remove than Ni and Ag as the change in the Cu/S ratio was significantly smaller than the Ni/S and Ag/S ratios. At higher DETA concentrations, a larger amount of Ni was removed. At DETA concentrations > 10-4 M, Ni was completely removed, while small amounts of Cu and Ag were still left on the pyrrhotite surface. This finding is not surprising because both CuS and AgS are thermodynamically more stable than NiS. Thus, the XPS analysis suggest that pyrrhotite can be activated by the heavy metal ions present in plant water. The activation mechanism may be similar to that of copper-activation of sphalerite. The driving force for the activation is that heavy metal sulfides are more stable (or less soluble) than pyrrhotite. Although thiol collectors can adsorb on pyrrhotite without activation, the amount of collectors required for the adsorption would be higher than those required for other sulfide minerals and the potentials at which collector adsorption occurs is higher, providing a basis for separation. When pyrrhotite is activated, however, thiol collectors can adsorb on the mineral at lower collector concentrations and lower potentials, making the separation more difficult.
Pyrrhotitedepressionmechanism
815
The XPS data given in Table 1 suggest that the role of DETA is to deactivate pyrrhotite. This is achieved by virtue of its ability to form aqueous complexes with certain heavy metal ions, such as Ni 2+, Cu 2+, and Ag + ions. However, sulfides of these heavy metals are so insoluble under reducing conditions that DETA is not effective as a deactivating agent. When the ore is subjected to oxidizing conditions, the activation product formed on pyrrhotite becomes an oxide that can more readily react with DETA and be removed from the surface. This mechanism may be similar to the ammoniacal leaching of copper oxides, which is supported by the flotation results (Figures 1 and 2) that show ,that DETA is an effective deactivator only when the ore is o~:idized. The proposed mechanism may also be supported indirectly by the chemical analysis of the tailings water after laboratory flotation tests. As shown in Table 2, the solubility of both Cu 2+ and Ni 2+ ions greatly increased when the flotation was conducted after aerative conditioning. TABLE 2 Analysis of water before and after batch flotation tests using DETA.
~tratica
(aW,/L)
:~nple
Cu
Hi
Fc
I~rocas H20
0.9.5
0.06
0.05
'Failings H20 TailinSs H20 (w/aeration)
0.21
5.1
0.04
3.6
9.4
0.04
The nickeliferous pyrrhotite in the Sudbury Basin ore contains Ni 2+ ions in the lattice. One might, therefore, suspect that the mineral may be self-activated; however, the amount of Ni 2+ ions found on the surface of nickeliferous pyrrhotite particles by LIMS is far in excess of what is originally present on the mineral. Therefore, the activation of pyrrhotite by Ni 2+ ions are induced by the ions present in the process water. It is well known that DETA is an excellent complexing agent for Cu 2+, Ni 2+, Co 2+, and Fe 2+ ions, but not for Fe 3+, which may be the basis for the excellent selectivity observed with this reagent as a pyrrhotite depressant. There are other complexing agents for Cu 2+ and Ni 2+ ions, but many form complexes with Fe 3+ ions as well, which are always present in mill water. A typical example is EDTA. However, such a reagent would not be effective as a pyrrhotite depressant probably because it is consumed in complexing Fe 3+ ions. It should also be noted here that DETA is most effective when the mill water is saturated with calcium ions. The LIMS data shown in Figure 4 suggest that in the presence of DETA, Ca 2+ ions become a more effective pyrrhotite depressant. However, it is not known at this point what the role of Ca 2+ ions in the deactivation mechanism discussed above. SUMMARY AND CONCLUSIONS DETA is a highly selective depressant for nickeliferous pyrrhotite during flotation of pentlandite. Laboratory flotation tests conducted with a feed to the Clarabelle pyrrhotite rejection circuit showed that the separation efficiency was greatly improved by using a small amount of DETA. However, the improvement was observed only when the feed was oxidized by aerative conditioning. The test results obtained with a Clarabelle ore also showed that DETA is effective only when the ore is oxidized. Examination of the flotation concentrates by LIMS showed that there was a significant increase in the amount of copper and nickel found on the surface of pyrrhotite, indicating that the mineral is activated by the heavy metal ions present in the plant water. The pyrrhotite activation by Cu 2+, Ni 2+ and Ag + ions was confirmed by XPS analysis of the mineral specimens immersed in simulated plant water. The XPS analysis showed also that when an activated pyrrhotite specimen was contacted with DETA solutions, the amount of heavy metal ion,,; found on the surface was reduced significantly, suggesting that the role of DETA is one of deactivator. At higher concentrations of DETA, the amount of Ni left on the surface was beyond the detection limit, while complete removal of Cu and Ag was more difficult. The activation products formed on pyrrhotite may be sulfides of Ni, Cu, Ag, etc., which are usually insoluble under reducing conditions. When the mineral is oxidized, however, the activation products will 14£ O:7-G
816
R.-H. YOONetal.
be converted to oxides, whose solubility may be increased in the presence of DETA. This may explain the observation that DETA works effectively as a pyrrbotite depressant only when the ore is oxidized. FFIR spectra of pyrrhotite contacted with DETA showed no trace of this reagent on the surface, supporting the role of DETA as a eomplexing agent. The role of DETA in pyrrhotite flotation was further investigated by monitoring collector adsorption and contact angles while controlling the electrochemical potential (Eh) of the mineral. The results showed that in the presence of DETA, xanthate adsorption is prevented below approximately 0.25 V, possibly due to the prevention of formation of the iron-hydroxy polysulfide. This species is regarded as the precursor to xanthate adsorption. At potentials above 0.25 V, however, small amounts of xanthate still adsorb on pyrrhotite.
ACKNOWLEDGEMENTS The authors acknowledge the personnel from AMTEL, particularly Greg Hill, for assisting in LIMS work, Germain Labonte and Kevin Stewart (JRGRL) for doing the flotation test work, Dong-Su Kim (CCMP) for assisting in the FI'IR and contact angle measurements, and INCO Ltd for permission to publish this work.
REFERENCES .
2. 3. 4.
.
6.
Hawley, J.E., The Sudbury ores, their mineralogy and origin. Can. Mineral., 7(1), 1 (1962). Sopko, M., Environmental programs at Sudbury. CIMM 96th Annual Gen. Meet., Toronto, 61 (1994). Martieorena, M,A., Hill, G., Kerr, A.N., Liechti, D. & Pelland, D.A. INCO develops new pyrrhotite depressant. Innovations in Mineral Proc., Sudbury, Ontario 15 (1994). Hodgson, M. & Agar, G.E., Electrochemical investigations into the flotation chemistry of pentlandite and pyrrhotite: Process water and xanthate interactions. Can. Met. Qtr., 28(3), 189 (1989). Scofield, J.H., Hartree-Slater subshell photoionization cross-sections at 1254 and 1487 eV. J. Electron Spectrosc., 8, 129 (1976). Wagner, C.D., Gale, L.H. & Raymond, R.H., Two-dimensional chemical plots: a standardized data set for use in identifying chemical states by XPS. Anal. Chem., $1, 466 (1979).