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Selective flotation of enargite from chalcopyrite by electrochemical control
Minerals Engineering 18 (2005) 605–612 This article is also available online at: www.elsevier.com/locate/mineng
Selective flotation of enargite from chalcopyrite by electrochemical control H. Guo *, W.-T. Yen Department of Mining Engineering, Queens University, Kingston, Ontario, Canada K7L 3N6 Received 6 May 2004; accepted 4 October 2004
Abstract Voltammetric studies, contact angle measurements, collector and collectorless microflotation tests were carried out in this study to investigate the oxidation properties and flotation characteristics of enargite as well as chalcopyrite. Selective flotation of enargite from chalcopyrite under varied pulp potentials was conducted to investigate the feasibility of enargite removal from a chalcopyrite concentrate. The test results indicate that chalcopyrite began to oxidize quickly at a much lower potential than enargite. Enargite could be floated well at a potential higher than +0.2 V vs. SCE while chalcopyrite was completely depressed at a potential higher than +0.2 V vs. SCE. Selective flotation revealed that enargite can be successfully removed from chalcopyrite through controlling the pulp potential higher than +0.2 V and lower than +0.55 V vs. SCE. 2004 Elsevier Ltd. All rights reserved. Keywords: Sulfide ores; Froth flotation; Redox reactions; Surface modification
1. Introduction Enargite (Cu3AsS4), which may associate with chalcopyrite, usually reports to the copper concentrate in conventional flotation due to their similar flotation properties. Arsenic, in the chalcopyrite concentrate, causes environmental problems and also problems in the subsequent pyrometallurgical process. Therefore, removal of arsenic from the copper concentrate is necessary. Industrially it has been shown that arsenic bearing minerals such as enargite, tennantite are difficult to remove from copper concentrate by conventional flotation techniques. Research on flotation separation of enargite and chalcopyrite by Fornasiero et al. (2001), *
Corresponding author. Address: CANMET MMSL, 555 Booth Street, Room 270, Ottawa, Ontario, Canada K1A 0G1. Tel.: +1 613 996 8303; fax: +1 613 996 9401. E-mail addresses: [email protected], [email protected] (H. Guo). 0892-6875/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2004.10.005
Yen and Tajadod (2000), and Menacho et al. (1993) indicated that pulp potential control flotation seemed to be a promising technique to eliminate arsenic from the chalcopyrite concentrates. Electrochemical depression of chalcopyrite has long been used in differential flotation separation of molybdenite and chalcopyrite. Both oxidants and reductants were used to depress chalcopyrite while molybdenite was selectively floated (Hu, 1982; Jiang et al., 1999). Electrochemical flotation has been studied intensively in the past decades. The use of pulp potential control to enhance flotation at commercial mineral process plants has been considered by a number of workers in Canada, China, USA, Russia and Finland (Woods, 2000). From a large number of investigations in this field, the oxidation of both mineral and collector were found to play an important role in the collection process. The hydrophilic behavior of most sulfide minerals is attributed to the presence of an oxide or a hydroxide layer; in the presence of collectors, a coating on the surface
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is produced that contributes to the hydrophobicity of the mineral (Kantar, 2002; Hu et al., 2000; Qin et al., 2000; Cordova et al., 1997; Pang and Chander, 1990; Luttrell and Yoon, 1984). In this paper, the oxidation characteristics of enargite and chalcopyrite, their wettability and electrochemical flotation behaviors under controlled potentials are investigated. Selective flotation of enargite from chalcopyrite was conducted. The selective flotation was conducted on synthetic enargite and natural chalcopyrite due to the availability of mineral samples.
Due to availability of the mineral samples, the single mineral flotation test was conducted on synthetic enargite and natural chalcopyrite. The synthetic composite ore used in selective flotation of enargite from chalcopyrite consisted of 50% synthetic enargite and 50% natural chalcopyrite. Buffer solutions used in this investigation consist of boric acid, potassium chloride, sodium hydroxide (to give a pH 10); sodium and potassium phosphate (to give a pH 7). All the buffer solutions were supplied by Fisher Scientific. Nitrogen gas (Ultra high purity, 99.9999% N2) was supplied by Praxair Products Inc. PAX (potassium amyl xanthate) used was supplied by Prospec Chemicals and was purified through acetone dissolution and ether re-precipitation. Sodium sulfide (Na2S) was supplied by Fisher Scientific with a purity of 99.8% Na2S Æ 9H2O (ACS grade). Sodium hypochlorite solution (10.8% NaOCl) was purchased through Canadian Tire Corporation Limited. Hexane was supplied by Fisher Scientific with ACS grade.
2. Experimental section 2.1. Samples and reagents Synthetic enargite (Cu3AsS4) was prepared from pure copper (99.99%), arsenic (99.99%) and sulfur (99.999%) powders in a stoichiometric ratio in a muffle furnace at 500 C for 17 days (Maske and Skinner, 1971) and the product has been confirmed by X-ray diffraction. The natural enargite sample supplied by Amethyst Galleries, Inc. was collected from the Maria Elana Mine, Chile. X-ray diffraction of the sample shows no impurity peaks. The chemical assay of natural enargite is presented in Table 1. The results in Table 1 indicate that the natural enargite sample contained trace amount of impurities such as Fe and Zn. The purity of the natural enargite sample was assumed over 98%. Synthetic chalcopyrite (CuFeS2) was prepared by reacting pure copper (99.99%), iron (99.9%) and sulfur powders (99.999%) at the stoichiometric ratio in a high vacuum pyrex-glass tube at 557 C for 17 days and the product was confirmed by X-ray diffraction. The natural chalcopyrite samples were supplied by Wards Natural Science Establishment, Inc. X-ray diffraction of the samples showed no impurity peaks. The chemical assay of the natural chalcopyrite sample is given in Table 2. The natural chalcopyrite contained trace amount of lead, zinc and nickel. The purity of the sample was assumed over 98%.
2.2. Procedures Mineral electrodes for electrochemical study were prepared according to the following procedure. Synthetic and natural mineral samples were cut and shaped into 1.4 · 1.4 · 1.0 cm blocks. Copper wires were connected to the shaped minerals with silver epoxy before they were encapsulated in epoxy resin. The electrode working areas varied from 2.0 cm2 to 2.5 cm2. A fresh surface was generated before each experiment by wet abrading with 600 grade sand paper, then polishing with 0.3 lm grit alumina and rinsing with deoxygenated distilled water. The whole fresh surface generating process was conducted under N2 atmosphere to avoid surface oxidation. For polarization tests, the electrode was put on the shaft immediately after a fresh surface was produced and connected with the corrosion test system, then the electrode was immersed into the test solution and a potential sweep on the electrode was initiated instantly and the current vs. potential curve was recorded automatically.
Table 1 Assay of natural enargite Element
Cu
As
S
Fe
Pb
Zn
Ni
Silica
Content (%)
47.4
18.94
32.3
0.093
0.029
0.055
0.01
0.13
Table 2 Assay of natural chalcopyrite sample Element
Cu
Fe
S
Pb
Zn
Ni
Silica
Content (%)
33.89
29.76
35
0.07
0.072
0.0045
0.05
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adsorption was 2 min. Then the mineral(s) was floated for 2 min. The flotation products were filtered with 6time washing, air dried, weighed and mixed. The copper and iron contents in the selective flotation products were determined by atomic adsorption spectrometry while arsenic content in the selective flotation products was assayed by a volumetric method. The method needed to digest the mineral sample in sulfuric acid and sodium sulfate. Then the As5+ in the solution was distilled under a reducing atmosphere to arsenious chloride. The As3+ was titrated to As5+ with a standard iodine solution.
3. Results 3.1. Electrochemical characteristics of enargite and chalcopyrite Fig. 1 shows the comparison of the cyclic polarization curve of synthetic, natural enargites, synthetic and natural chalcopyrites at pH 10. Fig. 2 shows the anodic polarization of synthetic enargite and natural chalcopyrite initiated from their open circuit potentials in the test solution. The scan rate used to obtain the polarization curves in Figs. 1 and 2 was 1 mV/s. The results in Fig. 1 indicate that natural chalcopyrite began to oxidize quickly at a potential around +0.3 V vs. SCE and the current density of its polarization curve increased dramatically when the potential applied on it was increased from +0.3 to +0.6 V vs. SCE. Synthetic chalcopyrite began to oxidize rapidly at a potential around +0.4 V vs. SCE and the current density of its polarization curve increased significantly when the potential applied on it was increased from +0.4 to +0.68 V vs. SCE. Both synthetic and natural enargite electrodes showed passivation capacity when the potential applied was lower than +0.75 V vs. SCE. The results in Fig. 2 illustrate that both synthetic enargite and natural chalcopyrite began to oxidize at
1.5E-03
NC
NC = Natural Chalcopyrite SC = Synthetic Chalcopyrite
1.0E-03
Current Density (A/cm 2 )
To obtain the contact angle of a mineral electrode under different potentials, the mineral electrode was first conditioned under a fixed potential in the PAX solution for a period of 10 min after the mineral electrode was immersed into the test solution. Then three air bubbles were produced on the mineral surface in sequence by a capillary tube. The contact angles on both sides of the air bubbles were measured and the average of the six angles was considered as the contact angle under the test potential. The average of the standard deviation of the contact angles under fixed potentials was 2.3. The microflotation samples were prepared by dry grinding after the samples were crushed to 1 mm. The 150 to +400 mesh fraction was used for flotation and was divided into 1 g samples after the fraction was mixed thoroughly. The flotation samples were stored in 20 ml glass vials. In order to wash away the oxidation products, especially the elemental sulfur on the particle surfaces of the flotation samples, the sample was washed with a 5% Na2S solution. The procedure for this process is as follows. First, 10 ml of 5% Na2S solution was added to the vial which contained the mineral sample. A small magnetic bar was put in the slurry and the mixture was agitated with a magnetic stirrer machine for 10 min at a moderate speed. Then the mineral particles were allowed to settle and the clear solution was drawn out and discarded. Another 10 ml of 5% Na2S solution was added into the vial and the mixture was agitated for 10 more minutes. After the agitation, the mineral particles in the slurry were allowed to settle for 1 min and then the unsettled fine particles were drawn out and discarded together with the Na2S solution. The washed sample was filtered under N2 atmosphere and was washed with 5 ml of 1% Na2S solution once and with double distilled deoxygenated water at least six times before it was transferred to the flotation tube (Guo, 2003). To carry out a chemical-controlled-pulp-potential microflotation, about 40 ml of the pH 10 buffer solution was transferred to the flotation chamber of the modified Hallimond tube (Guo, 2003) and about 10 ml to each of the other chambers. Then, the buffer solution in each chamber was injected with nitrogen through a gas disperser for 2 min to get rid of the oxygen in the solution. Next, the counter and reference electrodes were inserted into the smaller chambers and the Na2S solution washed mineral sample was transferred into the flotation chamber. A platinum electrode was inserted into the slurry to monitor the potential. The pulp potential was adjusted by the selected reagent (either NaOCl or Na2S) to the designated value and kept at this value for 10 min. The mineral was floated for 2 min after the 10 min conditioning in collectorless flotation. In flotation with collector, collector was added after the 10-min potential conditioning and the conditioning time for the collector
607
SC
NE = Natural Enargite
NE
SE = Synthetic Enargite 5.0E-04
SE
SE
0.0E+00
-5.0E-04
NC
SC NE SE
NE -1.0E-03 -0.9
-0.6
-0.3
0
0.3
0.6
0.9
Potential (V vs. SCE)
Fig. 1. Cyclic polarizations of enargite and chalcopyrite electrodes at pH 10.
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80
2.5E-04
) o
Contact Angle (
Current Density (A/cm 2 )
70 3.5E-04
Synthetic Enargite Natural Chalcopyrite
1.5E-04
60 50 40
Synthetic Enargite
30
Natural Enargite
20
0 -0.4 -5.0E-05 -0.3
Synthetic Chalcopyrite
10
5.0E-05
0
0.3
0.6
Natural Chalcopyrite -0.2
0
0.9
Potential (V vs. SCE)
0.2
0.4
0.6
0.8
Potential (V vs. SCE)
Fig. 3. Contact angle of enargite and chalcopyrite in a 7 · 10 PAX solution at pH 10.
4
M
Fig. 2. Anodic polarizations of synthetic enargite and natural chalcopyrite at pH 10 initiated from their open circuit potentials. 100
3.2. Wettability of enargite and chalcopyrite under controlled potentials The comparison of the contact angles on enargite and chalcopyrite surfaces obtained under externally applied potentials in a 7 · 10 4 M PAX solution at pH 10 and 7 are shown in Figs. 3 and 4. The contact angles on enargite and chalcopyrite surfaces obtained under externally applied potentials in a 7 · 10 5 M PAX solution at pH 10 are shown in Fig. 5. The results in Fig. 3 illustrate that both synthetic and natural enargites, as well as synthetic and natural chalcopyrites in a 7 · 10 4 M PAX solution at pH 10 became hydrophobic at the same potential, i.e. 0.2 V vs. SCE. The contact angle on the natural enargite surface remained unchanged in the potential range from 0.1 V
90
Contact Angle ( o )
80 70 60 50 40 30
Synthetic Ena rgite
20
N atural Enarg ite Synthetic Cha lcopyrite
10 0 -0.4
N atural Chalcopyrite -0.2
0
0.2
0.4
0.6
0.8
Potential (V vs. SCE)
Fig. 4. Contact angle of enargite and chalcopyrite in a 7 · 10 PAX solution at pH 7.
4
M
80 70
o
)
60
Contact Angle (
0.0 V vs. SCE. The current density of the natural chalcopyrite polarization curve began to increase rapidly from +0.3 V. Synthetic enargite showed passivation ability at potentials lower than +0.72 V vs. SCE. The main difference between the oxidation characteristics of enargite and chalcopyrite was that chalcopyrite could be oxidized at a lower potential than enargite. The difference might be attributed to the electrical conductivity difference between the two minerals. Chalcopyrite had a much better conductivity than enargite for electricity. The electrical resistance of a 1 · 1 · 1 cm synthetic chalcopyrite cube was about 2–3 X. The electrical resistance of a 1 · 1 · 1 cm synthetic enargite cube was about 1000 X. A better conductivity would facilitate the charge transfer for the oxidation of the mineral. As can be seen from the contact angle measurement and flotation results presented later in this paper, the oxidation characteristics of the two minerals had significant impacts on their floatabilities. The ease of quick oxidation of chalcopyrite at a lower potential indicate not only that chalcopyrite was easy to oxidize but also that the passivation film at the chalcopyrite surface was easy to destroy.
Synthetic Enargite
50
Natural Enargite 40
Synthetic Chalcopyrite Natural Chalcopyrite
30 20 10 0 -0.2
0
0.2
0.4
0.6
0.8
Potential (V vs. SCE)
Fig. 5. Contact angle of enargite and chalcopyrite in a 7 · 10 PAX solution at pH 10.
5
M
to +0.84 V vs. SCE. The contact angle on the synthetic enargite increased slightly in the potential range from 0.1 V to +0.2 V and then remained unchanged in the potential range from +0.2 V to +0.85 V. In contrast, the synthetic and natural chalcopyrites only demonstrated good hydrophobicity in a narrow potential range from 0.1 V to 0.0 V vs. SCE. Contact
H. Guo, W.-T. Yen / Minerals Engineering 18 (2005) 605–612 100 90 80 70
Recovery (%)
angles on the synthetic and natural chalcopyrite electrodes began to decrease when the potential was higher than +0.1 V and reached its minimum at a potential of +0.2 V. Although the contact angle on the synthetic and natural chalcopyrites restored when the applied potential was increased to +0.4 V, the results in Fig. 3 indicate that the oxidation product film on the chalcopyrite electrode surface was more hydrophilic than the film on the enargite electrode surface at a potential higher than 0.2 V. As indicated by the polarization study of chalcopyrite and enargite (Guo, 2003), the oxidation products on the chalcopyrite surface, such as S0, which contributed to the hydrophobicity of chalcopyrite at a moderate potential ( 0.1 to 0.1 V vs. SCE) were further oxidized to hydrophilic species, such as SO23 or SO24 , at a potential higher than 0.2 V. The hydrophobic entities on the enargite surface produced from a moderate oxidation of the mineral was not destroyed at a potential lower than 0.75 V. The restoration of contact angle on chalcopyrite surface at potentials higher than 0.4 V was due to the high formation rate of dixanthogen at the mineral surface in a quiescent solution (Guo and Yen, 2003). Fig. 4 shows similar results as Fig. 3 except the mineral electrodes became hydrophobic at lower potentials, i.e. 0.28 to 0.25 V vs. SCE. The minimum contact angle on chalcopyrite electrode surfaces occurred in the potential range of +0.3 to +0.4 V. The effect of applied potential on the wettability of enargite and chalcopyrite was enhanced in a diluted PAX solution, i.e. 7 · 10 5 M at pH 10 (Fig. 5). The results in Fig. 5 illustrate that the contact angle at the chalcopyrite electrode surfaces was much smaller than the contact angle at enargite electrode surfaces. The difference between the hydrophobicity of enargite and chalcopyrite surfaces was more significant when the potential was higher than +0.2 V vs. SCE. This indicates that there is a great chance to separate enargite from chalcopyrite by electrochemical flotation at a potential higher than +0.2 V vs. SCE in a 7 · 10 5 M PAX solution at pH 10. Thus, the selective flotation tests of this investigation were conducted in 7 · 10 5 M PAX and collectorless solutions.
60 50 40 30 20
Chalcopyrite
10
Enargite
0 -0.6
-0.4
-0.2
0
0.2
0.4
0.6
Potential (V vs. SCE)
Fig. 6. Effect of pulp potential on floatabilities of enargite and chalcopyrite in the 7 · 10 5 M PAX solution at pH 10.
tential was increased from +0.2 V to +0.3 V. Chalcopyrite did not float when the pulp potential was higher than +0.3 V vs. SCE and enargite exhibited good floatability even when the pulp potential was up to +0.45 V vs. SCE. However, enargite flotation was reduced at a potential higher than +0.5 V vs. SCE. The low floatability of chalcopyrite at potentials higher than 0.3 V was due to the ease of oxidation of the mineral. The oxidation of S0 to SO24 reduced the hydrophobicity of the mineral surface. Further more, the increase of the hydrophobic species such as Fe(OH)3, Cu(OH)2 on the chalcopyrite surface also reduced the affinity between dixanthogen and the mineral surface and gave no chance for chalcopyrite to float at a potential higher than 0.3 V vs. SCE. 3.3.2. Floatability of enargite and chalcopyrite in collectorless solutions The flotation results of enargite and chalcopyrite in a collectorless pH 10 solution are shown in Fig. 7. Fig. 7 indicates that chalcopyrite floated well in the potential range from 0.09 V to +0.2 V vs. SCE in a collectorless solution at pH 10. Enargite did not float at a
100 90
Chalcopyrite Enargite
80
Recovery (%)
3.3. Flotation characteristics of enargite and chalcopyrite 3.3.1. Floatabilities of enargite and chalcopyrite in PAX solutions Single mineral flotation of synthetic enargite and natural chalcopyrite was conducted in the 7 · 10 5 M PAX solution at pH 10. Fig. 6 demonstrates that the floatabilities of enargite and chalcopyrite were almost the same in the pulp potential range from 0.4 V to +0.2 V. The floatability of chalcopyrite decreased dramatically while the floatability of enargite remained unchanged when the pulp po-
609
70 60 50 40 30 20 10 0 -0.6
-0.4
-0.2
0
0.2
0.4
0.6
Potential (V vs. SCE)
Fig. 7. Effect of pulp potential on floatabilities of enargite and chalcopyrite in the collectorless solution at pH 10.
610
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potential lower than 0.15 V, and its flotation recovery increased from 10.7% to 33.1% when the pulp potential increased from 0.15 V to 0.07 V. The recovery of enargite increased gradually from 33.1% to 87.6% as the pulp potential was increased from 0.07 V to +0.47 V and decreased to 57.5% when the pulp potential was further increased to +0.55 V vs. SCE. The low floatability of enargite in the collectorless solution at a potential lower than 0.3 V was due to the lack of S0 at enargite surface. Cyclic polarization study of the synthetic enargite confirmed that S0 was not produced on the enargite surface at a potential lower 0.3 V (Guo, 2003). The floatability of enargite in the potential range from 0.07 V to 0.3 V was due to a metal deficient surface. S0 was responsible for the good floatability of chalcopyrite in the potential range from 0.1 V to 0.2 V (Pang and Chander, 1990; Gardner and Woods, 1979). Both the collector and collectorless flotation results indicate that there was a potential to separate enargite from chalcopyrite by controlling the pulp potential.
Recovery or Selectivity (%)
100.00 90.00 80.00 70.00 60.00 50.00
Enargite Recovery
40.00
Chalcopyrite Recovery
30.00
Selectivity
20.00 10.00 0.00 -0.10
0.00
0.10
0.20
0.30
0.40
Fig. 8. Results of collectorless selective flotation at pH 10.
ing effect at a high pulp potential on chalcopyrite was much more severe than on enargite. At a potential of +0.3 V, 80.3% enargite was removed from chalcopyrite at a cost of 13.3% chalcopyrite. Under these conditions, a chalcopyrite product with a grade of 81.8% was obtained in the tailings and the flotation concentrate contained 85.7% enargite. At a potential of +0.4 V, 76.0% enargite was separated from chalcopyrite. Under these conditions, a chalcopyrite product with a grade of 80.6% was obtained in the tailings and the flotation concentrate contained 92.0% enargite. At a pulp potential of +0.5 V vs. SCE, the floatability of enargite was greatly reduced and only 50.5% enargite was floated. The selectivity curve in Fig. 8 indicates that as the pulp potential increased from 0.08 V to 0.4 V, the selectivity of the separation increased and best separation was obtained at 0.4 V. The selectivity decreased significantly as the pulp potential was further increased to 0.5 V.
3.4. Selective flotation of enargite and chalcopyrite 3.4.1. Selective flotation of enargite and chalcopyrite in a collectorless solution In order to investigate the possibility of enargite removal from a copper concentrate, selective flotation of enargite and chalcopyrite was conducted. A synthetic composite ore consisting of 50% synthetic enargite and 50% natural chalcopyrite was used in the tests. The flotation results of the synthetic composite ore in a collectorless solution at pH 10 are shown in Table 3. The recovery of enargite and chalcopyrite in the flotation concentrate and the selectivity criteria, which is calculated by subtracting the chalcopyrite recovery in the concentrate from the enargite recovery in the concentrate, are shown in Fig. 8. The results in Table 3 and Fig. 8 indicate that only 38.7% enargite and 26.1% chalcopyrite were floated at a pulp potential of 0.08 V vs. SCE. As the pulp potential was increased to +0.1 V, the floatability of both minerals was increased significantly to 90.6% enargite and 69.4% chalcopyrite respectively. When the pulp potential was +0.2 V or higher, the flotation of both enargite and chalcopyrite was depressed. However, the depress-
3.4.2. Selective flotation of enargite and chalcopyrite in a diluted PAX solution The electrochemical flotation results of the synthetic composite ore sample in a 7 · 10 5 M PAX solution at pH 10 are tabulated in Table 4 and illustrated in Fig. 9. The results in Table 4 and Fig. 9 illustrate that a better selective flotation of enargite and chalcopyrite was achieved in the PAX solution. At a potential of +0.2 V in a 7 · 10 5 M PAX solution, 97.6% enargite was recovered in the concentrate with 20.6% chalcopyrite. The chalcopyrite product in the tailings had a grade of 97.1%
Table 3 Selective flotation results in collectorless solution Potential (V) Enargite recovery in concentrate (%) Enargite recovery in tailing (%) Enargite grade in concentrate (%) Chalcopyrite recovery in concentrate (%) Chalcopyrite recovery in tailing (%) Chalcopyrite grade in tailing (%)
0.08 38.7 61.3 59.8 26.1 73.9 54.6
0.50
Pulp Potential (V vs. SCE)
0.10
0.20
0.30
0.40
0.50
90.6 9.4 56.2 69.4 30.5 76.9
75.6 24.5 80.4 18.2 81.8 77.2
80.3 20.0 85.7 13.3 86.7 81.8
76.0 24.0 92.1 6.2 93.8 80.6
50.5 49.6 88.7 6.4 93.6 65.4
H. Guo, W.-T. Yen / Minerals Engineering 18 (2005) 605–612 Table 4 Selective flotation results in 7 · 10
5
PAX solution
Potential (V)
0.20
0.30
0.40
0.50
Enargite recovery in concentrate (%) Enargite recovery in tailing (%) Enargite grade in concentrate (%) Chalcopyrite recovery in concentrate (%) Chalcopyrite recovery in tailing (%) Chalcopyrite grade in tailing (%)
97.6 2.4 82.6 20.6 79.4 97.1
95.4 4.6 83.8 19.0 81.0 94.4
95.7 4.3 84.4 18.1 81.9 94.9
93.4 6.6 92.8 7.4 92.6 93.2
Recovery or Selectivity (%)
100.00 90.00 80.00 70.00 60.00 Enargite Recovery
50.00
Chalcopyrite Recovery
40.00
Selectivity
30.00 20.00 10.00 0.00 0.20
0.25
0.30
0.35
0.40
0.45
0.50
Pulp Potential (V vs. SCE)
Fig. 9. Results of selective flotation in a 7 · 10 pH 10.
5
M PAX solution at
chalcopyrite. As the pulp potential was further increased, the recovery of enargite in the flotation concentrate was slightly reduced from 97.6% at +0.2 V to 93.3% at +0.5 V while the recovery of chalcopyrite in the flotation concentrate was reduced from 20.6% at +0.2 V to 7.4% at +0.5 V vs. SCE. At a pulp potential of +0.5 V, 93.4% enargite was removed at a cost of 7.4% chalcopyrite. The flotation concentrate contained 92.8% enargite. 92.6% chalcopyrite was recovered in the sink (flotation tailings) with a grade of 93.2% chalcopyrite. The selectivity curve indicates that best separation was obtained at 0.5 V vs. SCE.
4. Discussions The reasons for the restoration of the contact angle at the chalcopyrite surface and the low flotation recovery of chalcopyrite obtained at a potential higher than +0.2 V vs. SCE were explained in details in a previous publication (Guo and Yen, 2003). The contact angle restoration was due to the high formation rate of dixanthogen at the chalcopyrite surface under high potentials. The fact that the actual pulp potential controlled flotation system could not maintain the xanthate concentration at high potentials explains why chalcopyrite did not float at a potential higher than +0.2 V vs. SCE. The flotation results of the synthetic composite ore sample concluded that enargite could be separated from chalcopyrite by electrochemical flotation. The results
611
also indicate that single mineral electrochemical flotation results are useful in predicting the electrochemical flotation behaviors of the mineral in a composite mineral sample. On the other hand, mixing of enargite and chalcopyrite had a considerable effect on their floatabilities. Under all the pulp potentials tested, enargite in the synthetic composite ore was always more floatable than chalcopyrite in both collector and collectorless flotations. The single mineral flotation conducted in a collectorless solution showed that chalcopyrite floated better than enargite in the pulp potential range from 0.1 V to +0.2 V vs. SCE. The phenomenon that enargite floated better in the synthetic composite ore might be due to the galvanic reactions between enargite and chalcopyrite and should be further investigated. It has been proved that galvanic reactions between sulfides had remarkable effects on flotation response of the sulfides in the pulp system (Trahar et al., 1994; Rao and Finch, 1988). The flotation response of enargite and chalcopyrite could be explained by their oxidation properties under different potentials. The polarization studies on enargite and chalcopyrite indicate that chalcopyrite was much easier to oxidize at high potentials than enargite and this accounted for the floatability difference of enargite and chalcopyrite at high potentials. Another phenomenon observed in the single mineral flotation and selective flotation of enargite and chalcopyrite was that enargite floated much faster in the collector solution than in the collectorless solution. This phenomenon might be due to the fact that the hydrophobic entity at the enargite surface in the collectorless solution was S0 and the hydrophobic entity at enargite surface in the collector solution was S0 + X2 (dixanthogen). The enargite surface with S0 and dixanthogen was more hydrophobic than the surface with only S0. This means the enargite in collectorless solution will need a longer time to float. Both collector and collectorless selective flotations can be used to separate enargite from a chalcopyrite concentrate. The better method is the collector selective flotation since it guarantees a better selectivity and a faster flotation rate. The arsenic content in a copper concentrate has to be lower than 0.1% to avoid any penalties. Although the actual penalty could be different from one contract to another, the penalty for every 0.1% of arsenic exceeding the 0.1% limit equals to the value of 0.2–0.3% copper in the concentrate. It will be economically feasible if an enargite flotation product with an As/Cu ratio greater than one could be generated.
5. Conclusions 1. Chalcopyrite began to oxidize at a much lower potential than enargite.
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2. The contact angles measured on the chalcopyrite electrode decreased dramatically, especially in a solution with low PAX concentrations, when the applied potential was higher than +0.2 V vs. SCE. The contact angles measured on enargite remained unchanged in the potential range from 0.0 V to +0.8 V vs. SCE. 3. Single mineral electrochemical flotation results indicate that enargite showed good floatability at potentials higher than +0.2 V vs. SCE while chalcopyrite was completely depressed at potentials higher than +0.2 V vs. SCE. 4. Synthetic composite ore sample flotation results illustrate that enargite can be successfully separated by electrochemical flotation from chalcopyrite. At a pulp potential of 0.5 V vs. SCE, 93.4% enargite was floated into the concentrate that had a grade of 92.8% enargite and 92.6% chalcopyrite was recovered in the tailings that contained 93.2% chalcopyrite.
Acknowledgments The authors wish to thank the Natural Sciences and Engineering Research Council of Canada for financial support. Mr. Mike Fulton is acknowledged for editorial advice.
References Cordova, R., Gomez, H., Real, S.G., Schrebler, R., Vilche, J.R., 1997. Characterization of natural enargite aqueous solution systems by electrochemical techniques. Journal of the Electrochemical Society 144 (8), 2628–2636. Fornasiero, D., Fullston, D., Li, C., Ralston, J., 2001. Separation of enargite and tennantite from non-arsenic copper sulfide minerals by selective oxidation or dissolution. International Journal of Mineral Processing 61, 109–119. Gardner, J.R., Woods, R., 1979. An electrochemical investigation of the natural flotability of chalcopyrite. International Journal of Mineral Processing 6 (1), 1–16. Guo, H., 2003, Electrochemistry and flotation of enargite and chalcopyrite. Ph.D. Thesis, Queens University at Kingston, Kingston, Ontario, Canada.
Guo, H., Yen, W.T., 2003. Pulp potential and floatability of chalcopyrite. Minerals Engineering 16 (3), 247–256. Hu, W.B., 1982. Flotation. Metallurgical Industry Publishing House, Beijing China, pp. 249–259. Hu, Y.H., Qiu, G.H., Sun, S.Y., Wang, D.Z., 2000. Recent development in researches of electrochemistry of sulfide flotation at Central South University of Technology. In: Qiu, G.Z., Hu, Y.H., Qin, W.Q. (Eds.), Proceedings of an International Workshop on Electrochemistry of Flotation of Sulfide Minerals, Changsha, China. Transaction of Metal Society of China 10, 1–7 (Special Issue). Jiang, Y.R., Zhou, L.H., Xue, Y.L., Zhang, Q.X., 1999, Electrochemical flotation separation of chalcopyrite from molybdenite by CM Eh control. In: Proceedings of an International Workshop on Electrochemistry of Flotation of Sulfide Minerals, Changsha, China, pp. 110–113. Kantar, C., 2002. Solution and flotation chemistry of enargite. Colloids and Surfaces A: Physicochemical and Engineering Aspects 210, 23–33. Luttrell, G.H., Yoon, R.-H., 1984. The Collectorless flotation of chalcopyrite ores using sodium sulfide. International Journal of Mineral Processing 13, 271–283. Maske, S., Skinner, B.J., 1971. Studies of the sulfosalts of copper, I. phase and phase relation in the system Cu–As–S. Economic Geology 66, 901–918. Menacho, J.M., Aliaga, W., Valenuela, R., Ramos, V., Olivares, I., 1993. Selective flotation of enargite and chalcopyrite. Minerals 48 (203), 33–39. Pang, J., Chander, S., 1990. Oxidation and wetting behavior of chalcopyrite in the absence and presence of xanthates. Minerals & Metallurgical Processing 7 (3), 149–155. Qin, W.Q., Qiu, G.Z., Xu, J., 2000. Electrodeposition of dixanthogen on surface of pyrrhotite electrode. In: Qiu, G.Z., Hu, Y.H., Qin, W.Q. (Eds.), Proceedings of an International Workshop on Electrochemistry of Flotation of Sulfide Minerals, Changsha, China. Transaction of Metal Society of China 10, 61–63 (Special issue). Rao, S.R., Finch, J.A., 1988. Galvanic interactions studies on sulfide minerals. Canadian Metallurgy Quarterly 27 (4), 253– 259. Trahar, W.J., Senior, G.D., Shannon, L.K., 1994. Interactions between sulfide minerals—the collectorless flotation of pyrite. International Journal of Mineral Processing 40, 287– 321. Woods, R., 2000. Recent advances in electrochemistry of sulfide mineral flotation. In: Qiu, G.Z., Hu, Y.H., Qin, W.Q. (Eds.), Proceedings of an International Workshop on Electrochemistry of Flotation of Sulfide Minerals, Changsha, China. Transaction of Metal Society of China 10, 26–29 (Special issue). Yen, W.T., Tajadod, J., 2000. Selective flotation of enargite and chalcopyrite. Proceeding of the XX IMPC B 8A, 49– 55.
Selective flotation of enargite from chalcopyrite by electrochemical control H. Guo *, W.-T. Yen Department of Mining Engineering, Queens University, Kingston, Ontario, Canada K7L 3N6 Received 6 May 2004; accepted 4 October 2004
Abstract Voltammetric studies, contact angle measurements, collector and collectorless microflotation tests were carried out in this study to investigate the oxidation properties and flotation characteristics of enargite as well as chalcopyrite. Selective flotation of enargite from chalcopyrite under varied pulp potentials was conducted to investigate the feasibility of enargite removal from a chalcopyrite concentrate. The test results indicate that chalcopyrite began to oxidize quickly at a much lower potential than enargite. Enargite could be floated well at a potential higher than +0.2 V vs. SCE while chalcopyrite was completely depressed at a potential higher than +0.2 V vs. SCE. Selective flotation revealed that enargite can be successfully removed from chalcopyrite through controlling the pulp potential higher than +0.2 V and lower than +0.55 V vs. SCE. 2004 Elsevier Ltd. All rights reserved. Keywords: Sulfide ores; Froth flotation; Redox reactions; Surface modification
1. Introduction Enargite (Cu3AsS4), which may associate with chalcopyrite, usually reports to the copper concentrate in conventional flotation due to their similar flotation properties. Arsenic, in the chalcopyrite concentrate, causes environmental problems and also problems in the subsequent pyrometallurgical process. Therefore, removal of arsenic from the copper concentrate is necessary. Industrially it has been shown that arsenic bearing minerals such as enargite, tennantite are difficult to remove from copper concentrate by conventional flotation techniques. Research on flotation separation of enargite and chalcopyrite by Fornasiero et al. (2001), *
Corresponding author. Address: CANMET MMSL, 555 Booth Street, Room 270, Ottawa, Ontario, Canada K1A 0G1. Tel.: +1 613 996 8303; fax: +1 613 996 9401. E-mail addresses: [email protected], [email protected] (H. Guo). 0892-6875/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2004.10.005
Yen and Tajadod (2000), and Menacho et al. (1993) indicated that pulp potential control flotation seemed to be a promising technique to eliminate arsenic from the chalcopyrite concentrates. Electrochemical depression of chalcopyrite has long been used in differential flotation separation of molybdenite and chalcopyrite. Both oxidants and reductants were used to depress chalcopyrite while molybdenite was selectively floated (Hu, 1982; Jiang et al., 1999). Electrochemical flotation has been studied intensively in the past decades. The use of pulp potential control to enhance flotation at commercial mineral process plants has been considered by a number of workers in Canada, China, USA, Russia and Finland (Woods, 2000). From a large number of investigations in this field, the oxidation of both mineral and collector were found to play an important role in the collection process. The hydrophilic behavior of most sulfide minerals is attributed to the presence of an oxide or a hydroxide layer; in the presence of collectors, a coating on the surface
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is produced that contributes to the hydrophobicity of the mineral (Kantar, 2002; Hu et al., 2000; Qin et al., 2000; Cordova et al., 1997; Pang and Chander, 1990; Luttrell and Yoon, 1984). In this paper, the oxidation characteristics of enargite and chalcopyrite, their wettability and electrochemical flotation behaviors under controlled potentials are investigated. Selective flotation of enargite from chalcopyrite was conducted. The selective flotation was conducted on synthetic enargite and natural chalcopyrite due to the availability of mineral samples.
Due to availability of the mineral samples, the single mineral flotation test was conducted on synthetic enargite and natural chalcopyrite. The synthetic composite ore used in selective flotation of enargite from chalcopyrite consisted of 50% synthetic enargite and 50% natural chalcopyrite. Buffer solutions used in this investigation consist of boric acid, potassium chloride, sodium hydroxide (to give a pH 10); sodium and potassium phosphate (to give a pH 7). All the buffer solutions were supplied by Fisher Scientific. Nitrogen gas (Ultra high purity, 99.9999% N2) was supplied by Praxair Products Inc. PAX (potassium amyl xanthate) used was supplied by Prospec Chemicals and was purified through acetone dissolution and ether re-precipitation. Sodium sulfide (Na2S) was supplied by Fisher Scientific with a purity of 99.8% Na2S Æ 9H2O (ACS grade). Sodium hypochlorite solution (10.8% NaOCl) was purchased through Canadian Tire Corporation Limited. Hexane was supplied by Fisher Scientific with ACS grade.
2. Experimental section 2.1. Samples and reagents Synthetic enargite (Cu3AsS4) was prepared from pure copper (99.99%), arsenic (99.99%) and sulfur (99.999%) powders in a stoichiometric ratio in a muffle furnace at 500 C for 17 days (Maske and Skinner, 1971) and the product has been confirmed by X-ray diffraction. The natural enargite sample supplied by Amethyst Galleries, Inc. was collected from the Maria Elana Mine, Chile. X-ray diffraction of the sample shows no impurity peaks. The chemical assay of natural enargite is presented in Table 1. The results in Table 1 indicate that the natural enargite sample contained trace amount of impurities such as Fe and Zn. The purity of the natural enargite sample was assumed over 98%. Synthetic chalcopyrite (CuFeS2) was prepared by reacting pure copper (99.99%), iron (99.9%) and sulfur powders (99.999%) at the stoichiometric ratio in a high vacuum pyrex-glass tube at 557 C for 17 days and the product was confirmed by X-ray diffraction. The natural chalcopyrite samples were supplied by Wards Natural Science Establishment, Inc. X-ray diffraction of the samples showed no impurity peaks. The chemical assay of the natural chalcopyrite sample is given in Table 2. The natural chalcopyrite contained trace amount of lead, zinc and nickel. The purity of the sample was assumed over 98%.
2.2. Procedures Mineral electrodes for electrochemical study were prepared according to the following procedure. Synthetic and natural mineral samples were cut and shaped into 1.4 · 1.4 · 1.0 cm blocks. Copper wires were connected to the shaped minerals with silver epoxy before they were encapsulated in epoxy resin. The electrode working areas varied from 2.0 cm2 to 2.5 cm2. A fresh surface was generated before each experiment by wet abrading with 600 grade sand paper, then polishing with 0.3 lm grit alumina and rinsing with deoxygenated distilled water. The whole fresh surface generating process was conducted under N2 atmosphere to avoid surface oxidation. For polarization tests, the electrode was put on the shaft immediately after a fresh surface was produced and connected with the corrosion test system, then the electrode was immersed into the test solution and a potential sweep on the electrode was initiated instantly and the current vs. potential curve was recorded automatically.
Table 1 Assay of natural enargite Element
Cu
As
S
Fe
Pb
Zn
Ni
Silica
Content (%)
47.4
18.94
32.3
0.093
0.029
0.055
0.01
0.13
Table 2 Assay of natural chalcopyrite sample Element
Cu
Fe
S
Pb
Zn
Ni
Silica
Content (%)
33.89
29.76
35
0.07
0.072
0.0045
0.05
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adsorption was 2 min. Then the mineral(s) was floated for 2 min. The flotation products were filtered with 6time washing, air dried, weighed and mixed. The copper and iron contents in the selective flotation products were determined by atomic adsorption spectrometry while arsenic content in the selective flotation products was assayed by a volumetric method. The method needed to digest the mineral sample in sulfuric acid and sodium sulfate. Then the As5+ in the solution was distilled under a reducing atmosphere to arsenious chloride. The As3+ was titrated to As5+ with a standard iodine solution.
3. Results 3.1. Electrochemical characteristics of enargite and chalcopyrite Fig. 1 shows the comparison of the cyclic polarization curve of synthetic, natural enargites, synthetic and natural chalcopyrites at pH 10. Fig. 2 shows the anodic polarization of synthetic enargite and natural chalcopyrite initiated from their open circuit potentials in the test solution. The scan rate used to obtain the polarization curves in Figs. 1 and 2 was 1 mV/s. The results in Fig. 1 indicate that natural chalcopyrite began to oxidize quickly at a potential around +0.3 V vs. SCE and the current density of its polarization curve increased dramatically when the potential applied on it was increased from +0.3 to +0.6 V vs. SCE. Synthetic chalcopyrite began to oxidize rapidly at a potential around +0.4 V vs. SCE and the current density of its polarization curve increased significantly when the potential applied on it was increased from +0.4 to +0.68 V vs. SCE. Both synthetic and natural enargite electrodes showed passivation capacity when the potential applied was lower than +0.75 V vs. SCE. The results in Fig. 2 illustrate that both synthetic enargite and natural chalcopyrite began to oxidize at
1.5E-03
NC
NC = Natural Chalcopyrite SC = Synthetic Chalcopyrite
1.0E-03
Current Density (A/cm 2 )
To obtain the contact angle of a mineral electrode under different potentials, the mineral electrode was first conditioned under a fixed potential in the PAX solution for a period of 10 min after the mineral electrode was immersed into the test solution. Then three air bubbles were produced on the mineral surface in sequence by a capillary tube. The contact angles on both sides of the air bubbles were measured and the average of the six angles was considered as the contact angle under the test potential. The average of the standard deviation of the contact angles under fixed potentials was 2.3. The microflotation samples were prepared by dry grinding after the samples were crushed to 1 mm. The 150 to +400 mesh fraction was used for flotation and was divided into 1 g samples after the fraction was mixed thoroughly. The flotation samples were stored in 20 ml glass vials. In order to wash away the oxidation products, especially the elemental sulfur on the particle surfaces of the flotation samples, the sample was washed with a 5% Na2S solution. The procedure for this process is as follows. First, 10 ml of 5% Na2S solution was added to the vial which contained the mineral sample. A small magnetic bar was put in the slurry and the mixture was agitated with a magnetic stirrer machine for 10 min at a moderate speed. Then the mineral particles were allowed to settle and the clear solution was drawn out and discarded. Another 10 ml of 5% Na2S solution was added into the vial and the mixture was agitated for 10 more minutes. After the agitation, the mineral particles in the slurry were allowed to settle for 1 min and then the unsettled fine particles were drawn out and discarded together with the Na2S solution. The washed sample was filtered under N2 atmosphere and was washed with 5 ml of 1% Na2S solution once and with double distilled deoxygenated water at least six times before it was transferred to the flotation tube (Guo, 2003). To carry out a chemical-controlled-pulp-potential microflotation, about 40 ml of the pH 10 buffer solution was transferred to the flotation chamber of the modified Hallimond tube (Guo, 2003) and about 10 ml to each of the other chambers. Then, the buffer solution in each chamber was injected with nitrogen through a gas disperser for 2 min to get rid of the oxygen in the solution. Next, the counter and reference electrodes were inserted into the smaller chambers and the Na2S solution washed mineral sample was transferred into the flotation chamber. A platinum electrode was inserted into the slurry to monitor the potential. The pulp potential was adjusted by the selected reagent (either NaOCl or Na2S) to the designated value and kept at this value for 10 min. The mineral was floated for 2 min after the 10 min conditioning in collectorless flotation. In flotation with collector, collector was added after the 10-min potential conditioning and the conditioning time for the collector
607
SC
NE = Natural Enargite
NE
SE = Synthetic Enargite 5.0E-04
SE
SE
0.0E+00
-5.0E-04
NC
SC NE SE
NE -1.0E-03 -0.9
-0.6
-0.3
0
0.3
0.6
0.9
Potential (V vs. SCE)
Fig. 1. Cyclic polarizations of enargite and chalcopyrite electrodes at pH 10.
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80
2.5E-04
) o
Contact Angle (
Current Density (A/cm 2 )
70 3.5E-04
Synthetic Enargite Natural Chalcopyrite
1.5E-04
60 50 40
Synthetic Enargite
30
Natural Enargite
20
0 -0.4 -5.0E-05 -0.3
Synthetic Chalcopyrite
10
5.0E-05
0
0.3
0.6
Natural Chalcopyrite -0.2
0
0.9
Potential (V vs. SCE)
0.2
0.4
0.6
0.8
Potential (V vs. SCE)
Fig. 3. Contact angle of enargite and chalcopyrite in a 7 · 10 PAX solution at pH 10.
4
M
Fig. 2. Anodic polarizations of synthetic enargite and natural chalcopyrite at pH 10 initiated from their open circuit potentials. 100
3.2. Wettability of enargite and chalcopyrite under controlled potentials The comparison of the contact angles on enargite and chalcopyrite surfaces obtained under externally applied potentials in a 7 · 10 4 M PAX solution at pH 10 and 7 are shown in Figs. 3 and 4. The contact angles on enargite and chalcopyrite surfaces obtained under externally applied potentials in a 7 · 10 5 M PAX solution at pH 10 are shown in Fig. 5. The results in Fig. 3 illustrate that both synthetic and natural enargites, as well as synthetic and natural chalcopyrites in a 7 · 10 4 M PAX solution at pH 10 became hydrophobic at the same potential, i.e. 0.2 V vs. SCE. The contact angle on the natural enargite surface remained unchanged in the potential range from 0.1 V
90
Contact Angle ( o )
80 70 60 50 40 30
Synthetic Ena rgite
20
N atural Enarg ite Synthetic Cha lcopyrite
10 0 -0.4
N atural Chalcopyrite -0.2
0
0.2
0.4
0.6
0.8
Potential (V vs. SCE)
Fig. 4. Contact angle of enargite and chalcopyrite in a 7 · 10 PAX solution at pH 7.
4
M
80 70
o
)
60
Contact Angle (
0.0 V vs. SCE. The current density of the natural chalcopyrite polarization curve began to increase rapidly from +0.3 V. Synthetic enargite showed passivation ability at potentials lower than +0.72 V vs. SCE. The main difference between the oxidation characteristics of enargite and chalcopyrite was that chalcopyrite could be oxidized at a lower potential than enargite. The difference might be attributed to the electrical conductivity difference between the two minerals. Chalcopyrite had a much better conductivity than enargite for electricity. The electrical resistance of a 1 · 1 · 1 cm synthetic chalcopyrite cube was about 2–3 X. The electrical resistance of a 1 · 1 · 1 cm synthetic enargite cube was about 1000 X. A better conductivity would facilitate the charge transfer for the oxidation of the mineral. As can be seen from the contact angle measurement and flotation results presented later in this paper, the oxidation characteristics of the two minerals had significant impacts on their floatabilities. The ease of quick oxidation of chalcopyrite at a lower potential indicate not only that chalcopyrite was easy to oxidize but also that the passivation film at the chalcopyrite surface was easy to destroy.
Synthetic Enargite
50
Natural Enargite 40
Synthetic Chalcopyrite Natural Chalcopyrite
30 20 10 0 -0.2
0
0.2
0.4
0.6
0.8
Potential (V vs. SCE)
Fig. 5. Contact angle of enargite and chalcopyrite in a 7 · 10 PAX solution at pH 10.
5
M
to +0.84 V vs. SCE. The contact angle on the synthetic enargite increased slightly in the potential range from 0.1 V to +0.2 V and then remained unchanged in the potential range from +0.2 V to +0.85 V. In contrast, the synthetic and natural chalcopyrites only demonstrated good hydrophobicity in a narrow potential range from 0.1 V to 0.0 V vs. SCE. Contact
H. Guo, W.-T. Yen / Minerals Engineering 18 (2005) 605–612 100 90 80 70
Recovery (%)
angles on the synthetic and natural chalcopyrite electrodes began to decrease when the potential was higher than +0.1 V and reached its minimum at a potential of +0.2 V. Although the contact angle on the synthetic and natural chalcopyrites restored when the applied potential was increased to +0.4 V, the results in Fig. 3 indicate that the oxidation product film on the chalcopyrite electrode surface was more hydrophilic than the film on the enargite electrode surface at a potential higher than 0.2 V. As indicated by the polarization study of chalcopyrite and enargite (Guo, 2003), the oxidation products on the chalcopyrite surface, such as S0, which contributed to the hydrophobicity of chalcopyrite at a moderate potential ( 0.1 to 0.1 V vs. SCE) were further oxidized to hydrophilic species, such as SO23 or SO24 , at a potential higher than 0.2 V. The hydrophobic entities on the enargite surface produced from a moderate oxidation of the mineral was not destroyed at a potential lower than 0.75 V. The restoration of contact angle on chalcopyrite surface at potentials higher than 0.4 V was due to the high formation rate of dixanthogen at the mineral surface in a quiescent solution (Guo and Yen, 2003). Fig. 4 shows similar results as Fig. 3 except the mineral electrodes became hydrophobic at lower potentials, i.e. 0.28 to 0.25 V vs. SCE. The minimum contact angle on chalcopyrite electrode surfaces occurred in the potential range of +0.3 to +0.4 V. The effect of applied potential on the wettability of enargite and chalcopyrite was enhanced in a diluted PAX solution, i.e. 7 · 10 5 M at pH 10 (Fig. 5). The results in Fig. 5 illustrate that the contact angle at the chalcopyrite electrode surfaces was much smaller than the contact angle at enargite electrode surfaces. The difference between the hydrophobicity of enargite and chalcopyrite surfaces was more significant when the potential was higher than +0.2 V vs. SCE. This indicates that there is a great chance to separate enargite from chalcopyrite by electrochemical flotation at a potential higher than +0.2 V vs. SCE in a 7 · 10 5 M PAX solution at pH 10. Thus, the selective flotation tests of this investigation were conducted in 7 · 10 5 M PAX and collectorless solutions.
60 50 40 30 20
Chalcopyrite
10
Enargite
0 -0.6
-0.4
-0.2
0
0.2
0.4
0.6
Potential (V vs. SCE)
Fig. 6. Effect of pulp potential on floatabilities of enargite and chalcopyrite in the 7 · 10 5 M PAX solution at pH 10.
tential was increased from +0.2 V to +0.3 V. Chalcopyrite did not float when the pulp potential was higher than +0.3 V vs. SCE and enargite exhibited good floatability even when the pulp potential was up to +0.45 V vs. SCE. However, enargite flotation was reduced at a potential higher than +0.5 V vs. SCE. The low floatability of chalcopyrite at potentials higher than 0.3 V was due to the ease of oxidation of the mineral. The oxidation of S0 to SO24 reduced the hydrophobicity of the mineral surface. Further more, the increase of the hydrophobic species such as Fe(OH)3, Cu(OH)2 on the chalcopyrite surface also reduced the affinity between dixanthogen and the mineral surface and gave no chance for chalcopyrite to float at a potential higher than 0.3 V vs. SCE. 3.3.2. Floatability of enargite and chalcopyrite in collectorless solutions The flotation results of enargite and chalcopyrite in a collectorless pH 10 solution are shown in Fig. 7. Fig. 7 indicates that chalcopyrite floated well in the potential range from 0.09 V to +0.2 V vs. SCE in a collectorless solution at pH 10. Enargite did not float at a
100 90
Chalcopyrite Enargite
80
Recovery (%)
3.3. Flotation characteristics of enargite and chalcopyrite 3.3.1. Floatabilities of enargite and chalcopyrite in PAX solutions Single mineral flotation of synthetic enargite and natural chalcopyrite was conducted in the 7 · 10 5 M PAX solution at pH 10. Fig. 6 demonstrates that the floatabilities of enargite and chalcopyrite were almost the same in the pulp potential range from 0.4 V to +0.2 V. The floatability of chalcopyrite decreased dramatically while the floatability of enargite remained unchanged when the pulp po-
609
70 60 50 40 30 20 10 0 -0.6
-0.4
-0.2
0
0.2
0.4
0.6
Potential (V vs. SCE)
Fig. 7. Effect of pulp potential on floatabilities of enargite and chalcopyrite in the collectorless solution at pH 10.
610
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potential lower than 0.15 V, and its flotation recovery increased from 10.7% to 33.1% when the pulp potential increased from 0.15 V to 0.07 V. The recovery of enargite increased gradually from 33.1% to 87.6% as the pulp potential was increased from 0.07 V to +0.47 V and decreased to 57.5% when the pulp potential was further increased to +0.55 V vs. SCE. The low floatability of enargite in the collectorless solution at a potential lower than 0.3 V was due to the lack of S0 at enargite surface. Cyclic polarization study of the synthetic enargite confirmed that S0 was not produced on the enargite surface at a potential lower 0.3 V (Guo, 2003). The floatability of enargite in the potential range from 0.07 V to 0.3 V was due to a metal deficient surface. S0 was responsible for the good floatability of chalcopyrite in the potential range from 0.1 V to 0.2 V (Pang and Chander, 1990; Gardner and Woods, 1979). Both the collector and collectorless flotation results indicate that there was a potential to separate enargite from chalcopyrite by controlling the pulp potential.
Recovery or Selectivity (%)
100.00 90.00 80.00 70.00 60.00 50.00
Enargite Recovery
40.00
Chalcopyrite Recovery
30.00
Selectivity
20.00 10.00 0.00 -0.10
0.00
0.10
0.20
0.30
0.40
Fig. 8. Results of collectorless selective flotation at pH 10.
ing effect at a high pulp potential on chalcopyrite was much more severe than on enargite. At a potential of +0.3 V, 80.3% enargite was removed from chalcopyrite at a cost of 13.3% chalcopyrite. Under these conditions, a chalcopyrite product with a grade of 81.8% was obtained in the tailings and the flotation concentrate contained 85.7% enargite. At a potential of +0.4 V, 76.0% enargite was separated from chalcopyrite. Under these conditions, a chalcopyrite product with a grade of 80.6% was obtained in the tailings and the flotation concentrate contained 92.0% enargite. At a pulp potential of +0.5 V vs. SCE, the floatability of enargite was greatly reduced and only 50.5% enargite was floated. The selectivity curve in Fig. 8 indicates that as the pulp potential increased from 0.08 V to 0.4 V, the selectivity of the separation increased and best separation was obtained at 0.4 V. The selectivity decreased significantly as the pulp potential was further increased to 0.5 V.
3.4. Selective flotation of enargite and chalcopyrite 3.4.1. Selective flotation of enargite and chalcopyrite in a collectorless solution In order to investigate the possibility of enargite removal from a copper concentrate, selective flotation of enargite and chalcopyrite was conducted. A synthetic composite ore consisting of 50% synthetic enargite and 50% natural chalcopyrite was used in the tests. The flotation results of the synthetic composite ore in a collectorless solution at pH 10 are shown in Table 3. The recovery of enargite and chalcopyrite in the flotation concentrate and the selectivity criteria, which is calculated by subtracting the chalcopyrite recovery in the concentrate from the enargite recovery in the concentrate, are shown in Fig. 8. The results in Table 3 and Fig. 8 indicate that only 38.7% enargite and 26.1% chalcopyrite were floated at a pulp potential of 0.08 V vs. SCE. As the pulp potential was increased to +0.1 V, the floatability of both minerals was increased significantly to 90.6% enargite and 69.4% chalcopyrite respectively. When the pulp potential was +0.2 V or higher, the flotation of both enargite and chalcopyrite was depressed. However, the depress-
3.4.2. Selective flotation of enargite and chalcopyrite in a diluted PAX solution The electrochemical flotation results of the synthetic composite ore sample in a 7 · 10 5 M PAX solution at pH 10 are tabulated in Table 4 and illustrated in Fig. 9. The results in Table 4 and Fig. 9 illustrate that a better selective flotation of enargite and chalcopyrite was achieved in the PAX solution. At a potential of +0.2 V in a 7 · 10 5 M PAX solution, 97.6% enargite was recovered in the concentrate with 20.6% chalcopyrite. The chalcopyrite product in the tailings had a grade of 97.1%
Table 3 Selective flotation results in collectorless solution Potential (V) Enargite recovery in concentrate (%) Enargite recovery in tailing (%) Enargite grade in concentrate (%) Chalcopyrite recovery in concentrate (%) Chalcopyrite recovery in tailing (%) Chalcopyrite grade in tailing (%)
0.08 38.7 61.3 59.8 26.1 73.9 54.6
0.50
Pulp Potential (V vs. SCE)
0.10
0.20
0.30
0.40
0.50
90.6 9.4 56.2 69.4 30.5 76.9
75.6 24.5 80.4 18.2 81.8 77.2
80.3 20.0 85.7 13.3 86.7 81.8
76.0 24.0 92.1 6.2 93.8 80.6
50.5 49.6 88.7 6.4 93.6 65.4
H. Guo, W.-T. Yen / Minerals Engineering 18 (2005) 605–612 Table 4 Selective flotation results in 7 · 10
5
PAX solution
Potential (V)
0.20
0.30
0.40
0.50
Enargite recovery in concentrate (%) Enargite recovery in tailing (%) Enargite grade in concentrate (%) Chalcopyrite recovery in concentrate (%) Chalcopyrite recovery in tailing (%) Chalcopyrite grade in tailing (%)
97.6 2.4 82.6 20.6 79.4 97.1
95.4 4.6 83.8 19.0 81.0 94.4
95.7 4.3 84.4 18.1 81.9 94.9
93.4 6.6 92.8 7.4 92.6 93.2
Recovery or Selectivity (%)
100.00 90.00 80.00 70.00 60.00 Enargite Recovery
50.00
Chalcopyrite Recovery
40.00
Selectivity
30.00 20.00 10.00 0.00 0.20
0.25
0.30
0.35
0.40
0.45
0.50
Pulp Potential (V vs. SCE)
Fig. 9. Results of selective flotation in a 7 · 10 pH 10.
5
M PAX solution at
chalcopyrite. As the pulp potential was further increased, the recovery of enargite in the flotation concentrate was slightly reduced from 97.6% at +0.2 V to 93.3% at +0.5 V while the recovery of chalcopyrite in the flotation concentrate was reduced from 20.6% at +0.2 V to 7.4% at +0.5 V vs. SCE. At a pulp potential of +0.5 V, 93.4% enargite was removed at a cost of 7.4% chalcopyrite. The flotation concentrate contained 92.8% enargite. 92.6% chalcopyrite was recovered in the sink (flotation tailings) with a grade of 93.2% chalcopyrite. The selectivity curve indicates that best separation was obtained at 0.5 V vs. SCE.
4. Discussions The reasons for the restoration of the contact angle at the chalcopyrite surface and the low flotation recovery of chalcopyrite obtained at a potential higher than +0.2 V vs. SCE were explained in details in a previous publication (Guo and Yen, 2003). The contact angle restoration was due to the high formation rate of dixanthogen at the chalcopyrite surface under high potentials. The fact that the actual pulp potential controlled flotation system could not maintain the xanthate concentration at high potentials explains why chalcopyrite did not float at a potential higher than +0.2 V vs. SCE. The flotation results of the synthetic composite ore sample concluded that enargite could be separated from chalcopyrite by electrochemical flotation. The results
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also indicate that single mineral electrochemical flotation results are useful in predicting the electrochemical flotation behaviors of the mineral in a composite mineral sample. On the other hand, mixing of enargite and chalcopyrite had a considerable effect on their floatabilities. Under all the pulp potentials tested, enargite in the synthetic composite ore was always more floatable than chalcopyrite in both collector and collectorless flotations. The single mineral flotation conducted in a collectorless solution showed that chalcopyrite floated better than enargite in the pulp potential range from 0.1 V to +0.2 V vs. SCE. The phenomenon that enargite floated better in the synthetic composite ore might be due to the galvanic reactions between enargite and chalcopyrite and should be further investigated. It has been proved that galvanic reactions between sulfides had remarkable effects on flotation response of the sulfides in the pulp system (Trahar et al., 1994; Rao and Finch, 1988). The flotation response of enargite and chalcopyrite could be explained by their oxidation properties under different potentials. The polarization studies on enargite and chalcopyrite indicate that chalcopyrite was much easier to oxidize at high potentials than enargite and this accounted for the floatability difference of enargite and chalcopyrite at high potentials. Another phenomenon observed in the single mineral flotation and selective flotation of enargite and chalcopyrite was that enargite floated much faster in the collector solution than in the collectorless solution. This phenomenon might be due to the fact that the hydrophobic entity at the enargite surface in the collectorless solution was S0 and the hydrophobic entity at enargite surface in the collector solution was S0 + X2 (dixanthogen). The enargite surface with S0 and dixanthogen was more hydrophobic than the surface with only S0. This means the enargite in collectorless solution will need a longer time to float. Both collector and collectorless selective flotations can be used to separate enargite from a chalcopyrite concentrate. The better method is the collector selective flotation since it guarantees a better selectivity and a faster flotation rate. The arsenic content in a copper concentrate has to be lower than 0.1% to avoid any penalties. Although the actual penalty could be different from one contract to another, the penalty for every 0.1% of arsenic exceeding the 0.1% limit equals to the value of 0.2–0.3% copper in the concentrate. It will be economically feasible if an enargite flotation product with an As/Cu ratio greater than one could be generated.
5. Conclusions 1. Chalcopyrite began to oxidize at a much lower potential than enargite.
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2. The contact angles measured on the chalcopyrite electrode decreased dramatically, especially in a solution with low PAX concentrations, when the applied potential was higher than +0.2 V vs. SCE. The contact angles measured on enargite remained unchanged in the potential range from 0.0 V to +0.8 V vs. SCE. 3. Single mineral electrochemical flotation results indicate that enargite showed good floatability at potentials higher than +0.2 V vs. SCE while chalcopyrite was completely depressed at potentials higher than +0.2 V vs. SCE. 4. Synthetic composite ore sample flotation results illustrate that enargite can be successfully separated by electrochemical flotation from chalcopyrite. At a pulp potential of 0.5 V vs. SCE, 93.4% enargite was floated into the concentrate that had a grade of 92.8% enargite and 92.6% chalcopyrite was recovered in the tailings that contained 93.2% chalcopyrite.
Acknowledgments The authors wish to thank the Natural Sciences and Engineering Research Council of Canada for financial support. Mr. Mike Fulton is acknowledged for editorial advice.
References Cordova, R., Gomez, H., Real, S.G., Schrebler, R., Vilche, J.R., 1997. Characterization of natural enargite aqueous solution systems by electrochemical techniques. Journal of the Electrochemical Society 144 (8), 2628–2636. Fornasiero, D., Fullston, D., Li, C., Ralston, J., 2001. Separation of enargite and tennantite from non-arsenic copper sulfide minerals by selective oxidation or dissolution. International Journal of Mineral Processing 61, 109–119. Gardner, J.R., Woods, R., 1979. An electrochemical investigation of the natural flotability of chalcopyrite. International Journal of Mineral Processing 6 (1), 1–16. Guo, H., 2003, Electrochemistry and flotation of enargite and chalcopyrite. Ph.D. Thesis, Queens University at Kingston, Kingston, Ontario, Canada.
Guo, H., Yen, W.T., 2003. Pulp potential and floatability of chalcopyrite. Minerals Engineering 16 (3), 247–256. Hu, W.B., 1982. Flotation. Metallurgical Industry Publishing House, Beijing China, pp. 249–259. Hu, Y.H., Qiu, G.H., Sun, S.Y., Wang, D.Z., 2000. Recent development in researches of electrochemistry of sulfide flotation at Central South University of Technology. In: Qiu, G.Z., Hu, Y.H., Qin, W.Q. (Eds.), Proceedings of an International Workshop on Electrochemistry of Flotation of Sulfide Minerals, Changsha, China. Transaction of Metal Society of China 10, 1–7 (Special Issue). Jiang, Y.R., Zhou, L.H., Xue, Y.L., Zhang, Q.X., 1999, Electrochemical flotation separation of chalcopyrite from molybdenite by CM Eh control. In: Proceedings of an International Workshop on Electrochemistry of Flotation of Sulfide Minerals, Changsha, China, pp. 110–113. Kantar, C., 2002. Solution and flotation chemistry of enargite. Colloids and Surfaces A: Physicochemical and Engineering Aspects 210, 23–33. Luttrell, G.H., Yoon, R.-H., 1984. The Collectorless flotation of chalcopyrite ores using sodium sulfide. International Journal of Mineral Processing 13, 271–283. Maske, S., Skinner, B.J., 1971. Studies of the sulfosalts of copper, I. phase and phase relation in the system Cu–As–S. Economic Geology 66, 901–918. Menacho, J.M., Aliaga, W., Valenuela, R., Ramos, V., Olivares, I., 1993. Selective flotation of enargite and chalcopyrite. Minerals 48 (203), 33–39. Pang, J., Chander, S., 1990. Oxidation and wetting behavior of chalcopyrite in the absence and presence of xanthates. Minerals & Metallurgical Processing 7 (3), 149–155. Qin, W.Q., Qiu, G.Z., Xu, J., 2000. Electrodeposition of dixanthogen on surface of pyrrhotite electrode. In: Qiu, G.Z., Hu, Y.H., Qin, W.Q. (Eds.), Proceedings of an International Workshop on Electrochemistry of Flotation of Sulfide Minerals, Changsha, China. Transaction of Metal Society of China 10, 61–63 (Special issue). Rao, S.R., Finch, J.A., 1988. Galvanic interactions studies on sulfide minerals. Canadian Metallurgy Quarterly 27 (4), 253– 259. Trahar, W.J., Senior, G.D., Shannon, L.K., 1994. Interactions between sulfide minerals—the collectorless flotation of pyrite. International Journal of Mineral Processing 40, 287– 321. Woods, R., 2000. Recent advances in electrochemistry of sulfide mineral flotation. In: Qiu, G.Z., Hu, Y.H., Qin, W.Q. (Eds.), Proceedings of an International Workshop on Electrochemistry of Flotation of Sulfide Minerals, Changsha, China. Transaction of Metal Society of China 10, 26–29 (Special issue). Yen, W.T., Tajadod, J., 2000. Selective flotation of enargite and chalcopyrite. Proceeding of the XX IMPC B 8A, 49– 55.