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The effect of water chemistry on froth stability and surface chemistry of the flotation of a Cu–Zn sulfide ore
International Journal of Mineral Processing 102–103 (2012) 32–37
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The effect of water chemistry on froth stability and surface chemistry of the flotation of a Cu–Zn sulfide ore Özlem Bıçak, Zafir Ekmekçi ⁎, Metin Can, Yasemin Öztürk Hacettepe University, Mining Engineering Department, 06800, Beytepe, Ankara, Turkey
a r t i c l e
i n f o
Article history: Received 12 November 2010 Received in revised form 6 August 2011 Accepted 9 September 2011 Available online 24 September 2011 Keywords: Sulfide ores Water chemistry Froth stability True flotation Entrainment
a b s t r a c t Water shortages have a direct impact on the life of many mining and mineral processing operations. Therefore, a good understanding of the effects of water quality on flotation performance is essential. In this study, effects of dissolved ions (both anions and cations) were investigated on the flotation performance of a Cu-Zn complex sulfide ore from Çayeli Bakır İşletmeleri A.Ş. (CBI) (Turkey) by means of batch flotation tests. The results of the flotation tests revealed that accumulation of dissolved metal ions and sulfide ions, mainly in the form of SO42− and S2O32–, changed both the froth stability and surface chemistry of the sulfide minerals. The froth stability and hence the recovery by entrainment, increased in conjunction with the dissolved ion concentration in water. The presence of dissolved metal ions, such as Cu 2+ and Pb 2+, also increased the flotation rate and recovery of sphalerite. In the case of pyrite, the activation by dissolved metal ions was observed for moderately contaminated recycled water samples. High concentrations of sulfide ions however, counteracted the activation effect and reduced the recovery of pyrite by true flotation. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Managing water resources has become an increasingly important issue in the world because it is closely related to the quality of human life and environment. Therefore, water reuse is a growing practice in many regions of the world, even in those countries that have not typically been considered to have problems with water scarcity. A number of research works have focused on the treatment and utilization of recycled process water in mineral processing. In general, the recycled process water can be classified, treated and disposed selectively, and then returned to different sections of flotation operations according to its effect on mineral surface reactions. Such a method could enhance the efficiency, and utilization of wastewater, therefore greatly decreasing costs (Broman, 1980; Forssberg et al., 1985; Rao and Finch, 1989; Basilio et al., 1996; Leavay et al., 2001; Wei et al., 2006). In mineral processing plants, the process water is recycled from the tailings dams, thickener overflows, dewatering and filtration units. Typical contaminants in the recycle water are the colloid materials (silicates, clays, precipitated metal hydroxides, etc.), ions of base metals, thiosalts, sulfide, sulfite, sulfate, chloride, magnesium, calcium, sodium and potassium, as well as residual reagents such as frothers, collectors and depressants. The recycled water, particularly
⁎ Corresponding author. Tel.: + 90 312 2977660; fax: + 90 312 2992155. E-mail address: zafi[email protected] (Z. Ekmekçi). 0301-7516/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.minpro.2011.09.005
the water recycled from thickeners, dewatering and filtration units, have increased levels of total dissolved ions and solids (Leavay et al., 2001; Johnson, 2003; Slatter et al., 2009). In these streams, the dissolved ions, suspended solids and flotation reagents have insufficient time to decompose and precipitate. In this case, the concentration of the contaminants can adversely affect flotation performance. The effect of dissolved ions in recycled water on flotation performance has been investigated by a number of researchers for various ore types. Lui et al. (Lui et al., 1993) reported that the presence of calcium ions and thiosalts improved Cu flotation in a Cu/Zn ore by enhanced depression of pyrite. Kirjavainen et al., (Kirjavainen et al., 2002) showed that the addition of calcium and thiosulfate ions improved copper and nickel floatability after grinding in a steel mill, but resulted in depression of copper and nickel after grinding in a ceramic mill. Activation after steel milling was attributed to the galvanic interactions that occurred. Although, there is limited agreement on the electrochemical mechanisms, a number of researchers have shown the depressing effect of sulfite ions on galena, pyrite and sphalerite (Peres et al., 1981; Yamamoto, 1980; Wang and Forssberg, 1990). Haran et al. (Haran et al., 2008) investigated the effect of triple recycled water on the accumulation of dissolved ions in the recycle water on flotation of copper tailings from the Benambra Mine. An accumulation of organic and inorganic species was observed after using the triple recycled water. This had a detrimental effect on flotation performance. An accumulation of both organic and inorganic ions in the process water may also affect the froth stability in flotation. A poorly mineralized or over stabilized froth phase could form depending on the type
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Table 1 Characteristics of recycle process water in CBI Flotation Plant.
Cu recycle Process water Zn recycle Process water
Cu (ppm)
Zn (ppm)
Fe (ppm)
Ca (ppm)
Mg (ppm)
Cl− (ppm)
NO3– (ppm)
SO42– (ppm)
S2O32– (ppm)
Total
0.54
0.07
0.08
222.5
1.7
5.65
10.64
334.69
341.28
917
0.02
0.11
0.12
333.7
0.01
4.29
11.35
228.57
16.88
595
and concentration of dissolved species in the recycled water (Rao and Finch, 1989; Castro et al., 2010; Muzenda, 2010). At high electrolyte concentration, bubbles become stable and do not coalescence even in the absence of frother. It has been reported that bubble size measured in saturated brine solutions and seawater without frother and in distilled water at high frother concentrations were similar. Hence, both froth stability and flotation rate increases in solutions containing high concentrations of dissolved ions. However, Kurniawan et al. (Kurniawan et al., 2011) have shown that the enhancement in the flotation performance depends on the type and concentration of the salt. This was attributed to the differences in ion specificity (size and polarizability) of different salts at high concentrations. In this study, the effects of dissolved ions in process water were investigated on the flotation performance of a Cu-Zn ore from Çayeli Bakır İşletmeleri (CBI) (Turkey) by means of batch flotation tests using synthetic water samples. It is well known that the recycled water becomes progressively more contaminated over time and results in an accumulation of dissolved ions in process water due to the oxidation/dissolution of the sulfide minerals and repeated reagent addition (Laskowski and Castro, 2008; Rey and Raffinot, 1966). Therefore, synthetic water samples were prepared with increasing concentrations of cations (Cu 2+, Fe 2+, Zn 2+, Pb 2+) and anions (SO42− and S2O32−) to simulate the effects of water recycling. The combined effects of the dissolved ions both on froth stability and activation/ depression of the sulfide minerals were discussed.
In order to investigate effects of the chemistry of the recycle water on Cu flotation of the Cayeli Cu-Zn ore, four different synthetic recycle process water samples were prepared. Since the circulation of the process water results in a gradual build up of reagents and all dissolved products in the water, the synthetic recycle water samples were prepared accordingly to simulate the real conditions of the Cu and Zn recycle process water streams at the Çayeli Flotation Plant as determined by AAS and Ion Chromatography (equipped with an Ion Pack AS9-HC Model anion column, Table 1). The synthetic water samples were prepared using various salts containing Cu 2+, Fe 2+, Zn 2+, Pb 2+, Ca 2+, SO42− and S2O32− ions. The ionic concentration of the synthetic water samples is given in Table 2. For the four synthetic water samples, the concentration of the dissolved ions increased gradually from Water 1 to Water 4. Water 1 has a similar composition to Çayeli Cu recycle process water. Although, ferrous iron was also added to the water, e.g. 1 ppm in Water 1 and gradually increased to 8 ppm in Water 4, very little ferrous iron was detected in the recycle water because of the precipitation of ferric hydroxide. Similarly, most of the Pb and Ca ions added to the synthetic water precipitated out and very little remained in ionic form. The precipitates were then separated from the water used in the flotation tests. In spite of some precipitation of Fe, Pb and Ca, the characteristics of the synthetic recycle water samples were considered suitable to simulate the influence of the existing recycled water in the plant and also accumulation of ions in the water (Table 2).
2. Material and methods 2.1. Flotation tests A complex Cu-Zn sulfide ore from Çayeli Bakır İşletmeleri A.Ş. (CBI) in Turkey, was used for batch flotation experiments. The main copper mineral is chalcopyrite that is associated with sphalerite, pyrite and minor amounts of galena. The ore contains 3.63% Cu, 4.07% Zn, 0.39% Pb and 25.07% Fe. Approximately 30% of the ore is composed of the non-sulphide minerals, mainly quartz and barite (Ekmekçi et al., 2010). A representative ore sample was taken from the feed belt of the primary ball mill of the Çayeli Flotation Plant and crushed down to 2 mm using roll crushers. The sample was then split into 1.150 kg batch samples for the flotation experiments and stored in vacuum sealed bags to prevent oxidation of the sulfide minerals. Samples were milled to 80% finer than 38 μm in a ball mill at 60% w/w pulp density just prior to flotation. The flotation tests were then performed at 30% pulp density using a modified 3 L Leeds flotation cell. 30 g/t diisobutyl phosphinate (Cytec 3418A) and 15 g/t MIBC were used as the collector and the frother respectively. The impeller rotation speed and air flow rate were set at 1200 rpm and 3 lt/min respectively. The froth was scrapped manually at 10 second intervals and four concentrates were collected after 0.5, 1.5, 3.5 and 7.5 min of flotation. The mass and water recovery data was also recorded. The froth depth was kept constant at 2 cm by addition of synthetic water during the entire flotation test. The same flotation conditions were applied for all of the tests. Feed, concentrate and tails samples were then analyzed for Cu, Fe and Zn using AAS analysis.
2.2. Data analysis There are two main recovery mechanisms, namely recovery by true flotation and entrainment that determine the overall recovery and concentrate grade in flotation. The effect of various ions in process water may change both the surface state of the particles and froth phase. While the changes in the surface state, i.e. the degree of hydrophobicity, affect the recovery by true flotation, changes in froth phase will affect the amount of entrainment. Therefore, the contribution of true flotation and entrainment to the overall recovery was decoupled in this study in order to evaluate the effects of water chemistry on mineral activation-depression and froth stability. The degree of entrainment was calculated using Eq. (1). This was done using the non-sulfide minerals (mainly quartz and barite) as tracer minerals that were evaluated on an unsized basis. For the
Table 2 Ionic concentration of tap water and artificially prepared recycle water samples.
Tap water Water 1 Water 2 Water 3 Water 4
Cu ppm
Fe ppm
Pb (ppm)
Zn ppm
Ca ppm
SO42− ppm
S2O32− ppm
Total Ion (ppm)
0.01 0.92 1.72 3.46 6.90
0.03 n.a 0.01 0.05 0.33
0.02 0.13 0.26 0.71 2.06
1.36 1.55 2.83 4.70 9.55
82 308 576 1040 2037
256 281 329 432 461
n.a 400 796 1787 2748
340 991 1706 3269 5264
n.a.: below detection limit.
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Çayeli Cu-Zn ore, this is considered a reasonable choice because the average degree of liberation of the non-sulfide minerals is over 97% (Ekmekçi et al., 2010).
ENT ¼
mass free gangue per unit mass of water in concentrate mass free gangue per unit mass of water in pulp
ð1Þ
In Eq. (1), entrainment is considered to be independent of mineral composition or hydrophobicity. Hence, the degree of entrainment is the same for all minerals on an unsized basis. The recovery of different minerals by entrainment is calculated using Eq. (2) (Savassi et al., 1998):
Rent ¼
1−R :ENT:Rw 1−Rw
ð2Þ
where, Rw is the recovery of water in the cell (%) R is the overall recovery of mineral (%) Once the recovery by entrainment has been determined, the recovery by true flotation is calculated using Eq. (3). Rtrue ¼ Roverall –Rent
ð3Þ
The first order flotation rate equation was used to calculate the true flotation rate constant for Cu, Zn and pyrite. Repeat tests were performed on randomly selected conditions (Tap water three tests and Water 2 two tests) for statistical analysis of the reproducibility. The confidence intervals calculated at 95% confidence level are given as error bars in recovery graph for these tests for assessment of reproducibility. 3. Results and discussion
3.2. Froth stability Water recovery and mass recovery are generally considered to be good indicators of changes in the froth phase in batch flotation tests. Fig. 1 showed that the water recovery increased gradually with the higher ion concentration in the process water, indicating increased froth stability. The Cu and Zn recycle process water compositions of CBI are also shown to estimate the probable effects of process water in the plant on froth stability. Although, the actions of frothers and inorganic salts in water are different, both are able to form stable froth phase (Klassen and Plaksin, 1963; Laskowski, 1966; Pugh et al., 1997). Both batch scale flotation test and plant studies using seawater and process water with high salt content have shown that a voluminous and stable froth phase is obtained due to the frothing properties of dissolved ions (Yousef et al., 2003; Quinn et al., 2007). Castro et al. (Castro et al., 2010) reported that water with an increased concentration of electrolytes showed an important degree of frothing ability. It has been concluded that the rate of coalescence of air bubbles decreased either due to the frother molecules adsorbed on the water/air interface, or by the presence of a stable water layer on the surface of the bubbles. Presence of salt decreases the bubble size and increase froth stability. Both factors enhance flotation recovery. However, Kurniawan et al., (Kurniawan et al., 2011) have shown that enhancement of flotation performance depends on the type and concentration of the salt. The results illustrated in Fig. 1 correlate with those of previous works, indicating increased froth stability at high ion concentration. Comparison of the frothing effects of the recycle water in the CBI flotation plant can also be done with reference to the artificial water samples. The results show that the Zn recycle process water does not have a significant effect on froth stability. The Cu recycle water however, with a higher dissolved ion concentration can increase frothing in Cu flotation. The use of the Cu recycle water in the Cu flotation circuit at CBI could be one of the reasons for the stable but watery froth at CBI. This results in a high Cu recovery but low concentrate grade in the first cell of the Cu Rougher bank at the CBI flotation plant (Ekmekçi et al., 2010).
3.1. Water chemistry 3.3. Recovery by true flotation and entrainment The ionic composition of the synthetic water samples, as well as the composition of the water samples following grinding and batch flotation tests are given in Tables 2 and 3, respectively. Comparison of these results shows that significant amount of the base metal ions adsorbed on mineral surfaces after grinding through a precipitation/adsorption mechanism (Finkelstein, 1997; Peng and Grano, 2010). On the other hand, the concentration of Ca and the thiosalts increased due to the addition of lime to increase pulp pH to 11.5 and the oxidation/dissolution of sulfide ions from the sulfide minerals at this pH (Dinardo, 2000). It was noted that the majority of the adsorption and dissolution processes occurred during the grinding stage and only minor changes were observed in flotation.
In order to evaluate the influence of water quality on the activation and depression of the sulfide minerals, the contribution of the recovery by true flotation and entrainment needs to be decoupled (Fig. 2). The use of the recycled water increased both the Cu recovery by true flotation and entrainment. Cu flotation recovery increased gradually from ~ 80% in tap water up to ~ 90% in water 4 sample. The increase in Cu recovery could be attributed to the activation of slow floating, partially oxidized or tarnished chalcopyrite particles by the dissolved thiosalts (Ikumapayi et al., 2010) and coarse
Table 3 Ionic composition of the water samples after grinding and flotation.
Tap water Water 1 Water 2 Water 3 Water 4
After After After After After After After After After After
grinding flotation grinding flotation grinding flotation grinding flotation grinding flotation
n.a.: below detection limit.
Cu ppm
Fe ppm
Pb ppm
Zn ppm
Ca ppm
SO42−
S2O32−
n.a. n.a. n.a. n.a. n.a. 0.004 n.a. 0.049 0.046 0.225
n.a. n.a. n.a. n.a. n.a. n.a. n.a. 0.115 0.038 0.056
0.09 0.08 0.11 0.10 0.15 0.13 0.21 0.21 0.35 0.34
0.046 0.134 0.048 0.068 0.012 0.056 0.021 0.082 0.086 0.169
294 250 435 447 756 1154 1316 857
1009 829 868 1171 1681 1430 1673 1188 1382 1405
269 234 413 678 1236 1905 1874 1026 3057 2887
2382
Cu Recycle water Zn Recycle water
Fig. 1. Changes in water recovery and mass pull as a function of total dissolved ion concentration in the process water.
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Fig. 2. Recovery by true flotation and entrainment for Cu, Zn and pyrite (a, c, e: recovery by true flotation, b, d, f: recovery by entrainment).
composite particles (Fig. 2a). The increase in water recovery also resulted in an increase in the recovery by entrainment from ~ 2% in tap water to about ~ 5% in the recycled water (Fig. 2b). There was no significant difference in entrainment however, between the four synthetic water samples. For most of the synthetic water samples, the recovery of Cu by flotation was ~ 90%, with an addition 5% recovery due to entrainment, resulting in a total Cu recovery of ~ 95%. This is considered close to the maximum achievable copper recovery and higher water recoveries may not significantly affect the recovery by entrainment, as observed in Fig. 2b. Both the flotation rate and recovery of sphalerite by true flotation and entrainment increased with the use of recycled water. The dissolved copper and lead ions activated sphalerite (Gerson et al., 1999; Lascelles et al., 2001) and increased flotation recovery by ~ 6% (Fig. 2c). In addition to the activation by copper and lead ions, flotation can be enhanced by the smaller bubbles at high ion concentrations which increases bubble-particle collision efficiency (Kurniawan et al., 2011). The recovery by entrainment also increased from 3% to 9% with synthetic water 4 sample (Fig. 2d). This resulted in an overall increase in Zn recovery from 18% to ~30% when recycled water was used in the flotation tests. The results also showed that the use of recycled
water caused poorer selectivity between chalcopyrite and sphalerite, even when the concentrations of the Cu2+ and Pb2+ ions were low (Synthetic water 1). Although higher Zn recoveries were expected due to true flotation when the concentration of the Cu 2+ and Pb 2+ ions was increased to 6.90 ppm and 2.06 ppm in synthetic water 4, respectively, Zn recovery actually showed a slight decrease for the other synthetic water samples. (Fig. 2c). This small decrease in Zn recovery by true flotation is attributed to the balance between the activation effect of the Cu and Pb ions and the depression effect of the dissolved sulfide ions in the water. The thiosulfate and sulfate ion concentrations were increased in conjunction with the increase in Pb and Cu ions to simulate the accumulation of all dissolved ions in the actual recycled water samples (Table 2). Therefore, the small decrease in Zn recovery by true flotation could be attributed to the balance between activation effect of Cu and Pb ions and depression effect of dissolved sulfide ions in the water. The effects of recycled water on recovery of pyrite by true flotation, was similar to that of sphalerite (Fig. 2e). Pyrite was activated by the synthetic water 1 and 2 samples, due to the dissolved copper ions (Pecina et al., 2006). The flotation recovery of pyrite also increased from 4% up to 8%. For the synthetic water 3 and 4 samples
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that contained higher concentrations of the thiosulfate and sulfate ions however, the true flotation of pyrite decreased. This indicates that the activation of pyrite by the copper ions was counteracted by the depressive effect of the dissolved sulfur species in the pulp (Rao and Finch, 1989; Ikumapayi et al., 2010). The recovery of pyrite due to entrainment was similar to that of sphalerite (Fig. 2f). The recovery due to entrainment increased from ~ 3% to 6% when the tap water was replaced with synthetic water sample 1. The recovery due to entrainment then showed a further increase up to 8% as the dissolved ion concentration in the synthetic water was increased (synthetic water sample 4). The results showed that the use of recycled water with similar characteristics to synthetic water sample 1 would cause an increase in overall pyrite recovery from 7% to 13% due to both pyrite activation and entrainment. A further increase in sulfide ion concentration however (synthetic water sample 4) caused a decrease in the recovery by true flotation back to ~ 4%. This also coincided with the greatest degree of entrainment, overall resulting in a fairly unchanged pyrite final recovery of~13%. The effect of recycled water on the flotation rates are also of interest, and are illustrated in Fig. 3. The flotation rate of both chalcopyrite and sphalerite increased with higher dissolved ion concentration (Fig. 3a). This was due to the activation of the slow floating, tarnished chalcopyrite and sphalerite particles by Cu 2+ ions. The recovery of chalcopyrite increased gradually from ~77% to 87% (Fig. 3b). Sphalerite recovery also increased from 14% to 23% when the tap water was replaced with synthetic water sample 1. The sphalerite recovery was relatively unchanged for the other synthetic water samples. This is attributed to the balance between the activation effect of the dissolved copper ions and the depression effects of the sulfide species that prevented a further increase in Zn recovery. The flotation rate of pyrite was almost identical for all of the tests (Fig. 3a), whereas the true flotation recovery of pyrite, increased from ~ 4% to 8% when the tap water was replaced with the synthetic water (sample 2) due to the activation by the dissolved copper ions (Fig. 3b). The pyrite recovery however, decreased down to ~4% when synthetic water samples containing higher concentrations of copper and sulfide ions were used in flotation. This suggests that pyrite flotation is relatively unaffected by low concentrations of sulfide ions in the recycled water, and only when the concentration is increased do the sulfide ions have a significant depressing effect on pyrite flotation performance. 4. Conclusions A series of batch scale flotation tests were performed using the CBI Cu–Zn complex sulfide ore to investigate the influence of dissolved ions on flotation performance. The results of the flotation tests revealed that the dissolved metal ions and sulfide ions, mainly in the form of SO4− 2 and S2O3− 2, influenced both the froth stability and surface chemistry. Therefore, the effect of water chemistry was evaluated in terms of its influence on true flotation and entrainment. The froth stability increased in conjunction with the dissolved ion concentration, presumably due to the increased stability of the water layer between air bubbles. Consequently, the degree of entrainment, particularly for pyrite and sphalerite, increased with higher concentrations of dissolved ions in the recycled water. The presence of the dissolved metal ions, such as Cu 2+ and Pb 2+, increased both the rate and recovery of the copper minerals (mainly chalcopyrite) and sphalerite by true flotation. This was attributed to the activation of the slow floating, tarnished particles of chalcopyrite and also the sphalerite particles by the metal ions. The activation and depression effects of the metal ions were simultaneously observed on sphalerite and pyrite flotation performance. Both minerals were activated by the dissolved Cu and Pb ions with moderately contaminated water samples. The activation effect of metal ions, however was counteracted by the depressive effect of the sulfide ions with synthetic
a
Cu Recycle water Zn Recycle water
b
Fig. 3. Variations in true flotation rate (a) and recovery (b) of Cu, Zn and pyrite.
water samples containing high concentrations of thiosulfate and sulfate. The results have clearly shown the influence of process water contamination on the flotation performance for the Çayeli flotation plant. Further work should be performed considering the effect of residual organic reagents, such as collector and frother. The presence of these reagents may have a significant effect on the flotation performance by changing froth stability and activation, and should be considered in conjunction with the effect of the dissolved inorganic ions in future research. Acknowledgements The authors would like to acknowledge with great appreciation the contribution in the preparation of this work by Megan Backer from University of Cape Town, Center for Mineral Research. The technical and financial support from Çayeli Bakır İşletmeleri A.Ş. and the financial support of The Scientific and Technological Council of Turkey (TÜBİTAK) Project No: 107M275 are also acknowledged.
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Muzenda, E., 2010. An investigation into the effect of water quality on flotation performance. World Acad. Sci. Eng. Technol. 70, 237–241. Pecina, E.T., Uribe, A., Nava, F., Finch, J.A., 2006. The role of copper and lead in the activation of pyrite in xanthate and non-xanthate systems. Miner. Eng. 19, 172–179. Peng, Y., Grano, S., 2010. Effect of grinding media on the activation of pyrite flotation. Miner. Eng. 23, 600–605. Peres, A.E.C., 1981. The interaction between xanthate and sulfur dioxide in the flotation of nickel-copper sulfide ores, PhD Thesis, University of British Columbia. Pugh, R.J., Weissenborn, P., Paulson, O., 1997. Flotation of inorganic electrolytes; the relationship between recovery of hydrophobic particles, surface tension, bubble coalescence and gas solubility. Int. J. Miner. Process. 51, 125–138. Quinn, J.J., Kracht, W., Gomez, C.O., Gagnon, C., Finch, J.A., 2007. Comparing the effects of salts and frother (MIBC) on gas dispersion and froth properties. Miner. Eng. 20, 1296–1302. Rao, S.R., Finch, J.A., 1989. A review of water reuse in flotation. Miner. Eng. 2, 65–85. Rey, M., Raffinot, P., 1966. Flotation of ore in sea water: high frothing, soluble xanthate collecting. World Min. 18. Savassi, O.N., Alexander, D.J., Franzidis, J.P., Manlapig, E.V., 1998. Empirical model for entrainment in industrial flotation plants. Miner. Eng. 11 (3), 43–56. Slatter, K.A., Plint, N.D., Cole, M., Dilsook, V., De Vaux, D., Palm, N., Oostendorp, B., 2009. Water management in Anglo Platinum process operations: effects of water quality on process operations. Abstracts of International Mine Water Conference, Pretoria, South Africa, 19–23 October, pp. 46–55. Wang, X.H., Forssberg, K.S.E., 1990. Mechanisms of pyrite flotation with xanthates. Int. J. Miner. Process. 33 (1–4), 275–290. Wei, Y.H., Zhou, G.Y., Roelf, F.S., 2006. Effects of recycled water on flotation of a complex sulphide ore. Nonferrous Met. 58 (2), 82–85. Yamamoto, T., 1980. The mechanism of depression pyrite and sphalerite by sulphite. In: Jones, M.J. (Ed.), Complex Sulphide Ores. Institution of Mining and Metallurgy, London, pp. 71–78. Yousef, A.A., Arafa, M.A., Ibrahim, S.S., Abdel Khalek, M.A., 2003. Seawater usage in flotation for minerals beneficiation in arid regions (Arab Countries) (simulation and application). In: Lorenzen, L., Bradshaw, D.J. (Eds.), Proc. 22nd Mineral Processing Congress, South African Institute of Mining and Metallurgy, Cape Town, Vol. 2, pp. 1023–1031.
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International Journal of Mineral Processing journal homepage: www.elsevier.com/locate/ijminpro
The effect of water chemistry on froth stability and surface chemistry of the flotation of a Cu–Zn sulfide ore Özlem Bıçak, Zafir Ekmekçi ⁎, Metin Can, Yasemin Öztürk Hacettepe University, Mining Engineering Department, 06800, Beytepe, Ankara, Turkey
a r t i c l e
i n f o
Article history: Received 12 November 2010 Received in revised form 6 August 2011 Accepted 9 September 2011 Available online 24 September 2011 Keywords: Sulfide ores Water chemistry Froth stability True flotation Entrainment
a b s t r a c t Water shortages have a direct impact on the life of many mining and mineral processing operations. Therefore, a good understanding of the effects of water quality on flotation performance is essential. In this study, effects of dissolved ions (both anions and cations) were investigated on the flotation performance of a Cu-Zn complex sulfide ore from Çayeli Bakır İşletmeleri A.Ş. (CBI) (Turkey) by means of batch flotation tests. The results of the flotation tests revealed that accumulation of dissolved metal ions and sulfide ions, mainly in the form of SO42− and S2O32–, changed both the froth stability and surface chemistry of the sulfide minerals. The froth stability and hence the recovery by entrainment, increased in conjunction with the dissolved ion concentration in water. The presence of dissolved metal ions, such as Cu 2+ and Pb 2+, also increased the flotation rate and recovery of sphalerite. In the case of pyrite, the activation by dissolved metal ions was observed for moderately contaminated recycled water samples. High concentrations of sulfide ions however, counteracted the activation effect and reduced the recovery of pyrite by true flotation. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Managing water resources has become an increasingly important issue in the world because it is closely related to the quality of human life and environment. Therefore, water reuse is a growing practice in many regions of the world, even in those countries that have not typically been considered to have problems with water scarcity. A number of research works have focused on the treatment and utilization of recycled process water in mineral processing. In general, the recycled process water can be classified, treated and disposed selectively, and then returned to different sections of flotation operations according to its effect on mineral surface reactions. Such a method could enhance the efficiency, and utilization of wastewater, therefore greatly decreasing costs (Broman, 1980; Forssberg et al., 1985; Rao and Finch, 1989; Basilio et al., 1996; Leavay et al., 2001; Wei et al., 2006). In mineral processing plants, the process water is recycled from the tailings dams, thickener overflows, dewatering and filtration units. Typical contaminants in the recycle water are the colloid materials (silicates, clays, precipitated metal hydroxides, etc.), ions of base metals, thiosalts, sulfide, sulfite, sulfate, chloride, magnesium, calcium, sodium and potassium, as well as residual reagents such as frothers, collectors and depressants. The recycled water, particularly
⁎ Corresponding author. Tel.: + 90 312 2977660; fax: + 90 312 2992155. E-mail address: zafi[email protected] (Z. Ekmekçi). 0301-7516/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.minpro.2011.09.005
the water recycled from thickeners, dewatering and filtration units, have increased levels of total dissolved ions and solids (Leavay et al., 2001; Johnson, 2003; Slatter et al., 2009). In these streams, the dissolved ions, suspended solids and flotation reagents have insufficient time to decompose and precipitate. In this case, the concentration of the contaminants can adversely affect flotation performance. The effect of dissolved ions in recycled water on flotation performance has been investigated by a number of researchers for various ore types. Lui et al. (Lui et al., 1993) reported that the presence of calcium ions and thiosalts improved Cu flotation in a Cu/Zn ore by enhanced depression of pyrite. Kirjavainen et al., (Kirjavainen et al., 2002) showed that the addition of calcium and thiosulfate ions improved copper and nickel floatability after grinding in a steel mill, but resulted in depression of copper and nickel after grinding in a ceramic mill. Activation after steel milling was attributed to the galvanic interactions that occurred. Although, there is limited agreement on the electrochemical mechanisms, a number of researchers have shown the depressing effect of sulfite ions on galena, pyrite and sphalerite (Peres et al., 1981; Yamamoto, 1980; Wang and Forssberg, 1990). Haran et al. (Haran et al., 2008) investigated the effect of triple recycled water on the accumulation of dissolved ions in the recycle water on flotation of copper tailings from the Benambra Mine. An accumulation of organic and inorganic species was observed after using the triple recycled water. This had a detrimental effect on flotation performance. An accumulation of both organic and inorganic ions in the process water may also affect the froth stability in flotation. A poorly mineralized or over stabilized froth phase could form depending on the type
Ö. Bıçak et al. / International Journal of Mineral Processing 102–103 (2012) 32–37
33
Table 1 Characteristics of recycle process water in CBI Flotation Plant.
Cu recycle Process water Zn recycle Process water
Cu (ppm)
Zn (ppm)
Fe (ppm)
Ca (ppm)
Mg (ppm)
Cl− (ppm)
NO3– (ppm)
SO42– (ppm)
S2O32– (ppm)
Total
0.54
0.07
0.08
222.5
1.7
5.65
10.64
334.69
341.28
917
0.02
0.11
0.12
333.7
0.01
4.29
11.35
228.57
16.88
595
and concentration of dissolved species in the recycled water (Rao and Finch, 1989; Castro et al., 2010; Muzenda, 2010). At high electrolyte concentration, bubbles become stable and do not coalescence even in the absence of frother. It has been reported that bubble size measured in saturated brine solutions and seawater without frother and in distilled water at high frother concentrations were similar. Hence, both froth stability and flotation rate increases in solutions containing high concentrations of dissolved ions. However, Kurniawan et al. (Kurniawan et al., 2011) have shown that the enhancement in the flotation performance depends on the type and concentration of the salt. This was attributed to the differences in ion specificity (size and polarizability) of different salts at high concentrations. In this study, the effects of dissolved ions in process water were investigated on the flotation performance of a Cu-Zn ore from Çayeli Bakır İşletmeleri (CBI) (Turkey) by means of batch flotation tests using synthetic water samples. It is well known that the recycled water becomes progressively more contaminated over time and results in an accumulation of dissolved ions in process water due to the oxidation/dissolution of the sulfide minerals and repeated reagent addition (Laskowski and Castro, 2008; Rey and Raffinot, 1966). Therefore, synthetic water samples were prepared with increasing concentrations of cations (Cu 2+, Fe 2+, Zn 2+, Pb 2+) and anions (SO42− and S2O32−) to simulate the effects of water recycling. The combined effects of the dissolved ions both on froth stability and activation/ depression of the sulfide minerals were discussed.
In order to investigate effects of the chemistry of the recycle water on Cu flotation of the Cayeli Cu-Zn ore, four different synthetic recycle process water samples were prepared. Since the circulation of the process water results in a gradual build up of reagents and all dissolved products in the water, the synthetic recycle water samples were prepared accordingly to simulate the real conditions of the Cu and Zn recycle process water streams at the Çayeli Flotation Plant as determined by AAS and Ion Chromatography (equipped with an Ion Pack AS9-HC Model anion column, Table 1). The synthetic water samples were prepared using various salts containing Cu 2+, Fe 2+, Zn 2+, Pb 2+, Ca 2+, SO42− and S2O32− ions. The ionic concentration of the synthetic water samples is given in Table 2. For the four synthetic water samples, the concentration of the dissolved ions increased gradually from Water 1 to Water 4. Water 1 has a similar composition to Çayeli Cu recycle process water. Although, ferrous iron was also added to the water, e.g. 1 ppm in Water 1 and gradually increased to 8 ppm in Water 4, very little ferrous iron was detected in the recycle water because of the precipitation of ferric hydroxide. Similarly, most of the Pb and Ca ions added to the synthetic water precipitated out and very little remained in ionic form. The precipitates were then separated from the water used in the flotation tests. In spite of some precipitation of Fe, Pb and Ca, the characteristics of the synthetic recycle water samples were considered suitable to simulate the influence of the existing recycled water in the plant and also accumulation of ions in the water (Table 2).
2. Material and methods 2.1. Flotation tests A complex Cu-Zn sulfide ore from Çayeli Bakır İşletmeleri A.Ş. (CBI) in Turkey, was used for batch flotation experiments. The main copper mineral is chalcopyrite that is associated with sphalerite, pyrite and minor amounts of galena. The ore contains 3.63% Cu, 4.07% Zn, 0.39% Pb and 25.07% Fe. Approximately 30% of the ore is composed of the non-sulphide minerals, mainly quartz and barite (Ekmekçi et al., 2010). A representative ore sample was taken from the feed belt of the primary ball mill of the Çayeli Flotation Plant and crushed down to 2 mm using roll crushers. The sample was then split into 1.150 kg batch samples for the flotation experiments and stored in vacuum sealed bags to prevent oxidation of the sulfide minerals. Samples were milled to 80% finer than 38 μm in a ball mill at 60% w/w pulp density just prior to flotation. The flotation tests were then performed at 30% pulp density using a modified 3 L Leeds flotation cell. 30 g/t diisobutyl phosphinate (Cytec 3418A) and 15 g/t MIBC were used as the collector and the frother respectively. The impeller rotation speed and air flow rate were set at 1200 rpm and 3 lt/min respectively. The froth was scrapped manually at 10 second intervals and four concentrates were collected after 0.5, 1.5, 3.5 and 7.5 min of flotation. The mass and water recovery data was also recorded. The froth depth was kept constant at 2 cm by addition of synthetic water during the entire flotation test. The same flotation conditions were applied for all of the tests. Feed, concentrate and tails samples were then analyzed for Cu, Fe and Zn using AAS analysis.
2.2. Data analysis There are two main recovery mechanisms, namely recovery by true flotation and entrainment that determine the overall recovery and concentrate grade in flotation. The effect of various ions in process water may change both the surface state of the particles and froth phase. While the changes in the surface state, i.e. the degree of hydrophobicity, affect the recovery by true flotation, changes in froth phase will affect the amount of entrainment. Therefore, the contribution of true flotation and entrainment to the overall recovery was decoupled in this study in order to evaluate the effects of water chemistry on mineral activation-depression and froth stability. The degree of entrainment was calculated using Eq. (1). This was done using the non-sulfide minerals (mainly quartz and barite) as tracer minerals that were evaluated on an unsized basis. For the
Table 2 Ionic concentration of tap water and artificially prepared recycle water samples.
Tap water Water 1 Water 2 Water 3 Water 4
Cu ppm
Fe ppm
Pb (ppm)
Zn ppm
Ca ppm
SO42− ppm
S2O32− ppm
Total Ion (ppm)
0.01 0.92 1.72 3.46 6.90
0.03 n.a 0.01 0.05 0.33
0.02 0.13 0.26 0.71 2.06
1.36 1.55 2.83 4.70 9.55
82 308 576 1040 2037
256 281 329 432 461
n.a 400 796 1787 2748
340 991 1706 3269 5264
n.a.: below detection limit.
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Çayeli Cu-Zn ore, this is considered a reasonable choice because the average degree of liberation of the non-sulfide minerals is over 97% (Ekmekçi et al., 2010).
ENT ¼
mass free gangue per unit mass of water in concentrate mass free gangue per unit mass of water in pulp
ð1Þ
In Eq. (1), entrainment is considered to be independent of mineral composition or hydrophobicity. Hence, the degree of entrainment is the same for all minerals on an unsized basis. The recovery of different minerals by entrainment is calculated using Eq. (2) (Savassi et al., 1998):
Rent ¼
1−R :ENT:Rw 1−Rw
ð2Þ
where, Rw is the recovery of water in the cell (%) R is the overall recovery of mineral (%) Once the recovery by entrainment has been determined, the recovery by true flotation is calculated using Eq. (3). Rtrue ¼ Roverall –Rent
ð3Þ
The first order flotation rate equation was used to calculate the true flotation rate constant for Cu, Zn and pyrite. Repeat tests were performed on randomly selected conditions (Tap water three tests and Water 2 two tests) for statistical analysis of the reproducibility. The confidence intervals calculated at 95% confidence level are given as error bars in recovery graph for these tests for assessment of reproducibility. 3. Results and discussion
3.2. Froth stability Water recovery and mass recovery are generally considered to be good indicators of changes in the froth phase in batch flotation tests. Fig. 1 showed that the water recovery increased gradually with the higher ion concentration in the process water, indicating increased froth stability. The Cu and Zn recycle process water compositions of CBI are also shown to estimate the probable effects of process water in the plant on froth stability. Although, the actions of frothers and inorganic salts in water are different, both are able to form stable froth phase (Klassen and Plaksin, 1963; Laskowski, 1966; Pugh et al., 1997). Both batch scale flotation test and plant studies using seawater and process water with high salt content have shown that a voluminous and stable froth phase is obtained due to the frothing properties of dissolved ions (Yousef et al., 2003; Quinn et al., 2007). Castro et al. (Castro et al., 2010) reported that water with an increased concentration of electrolytes showed an important degree of frothing ability. It has been concluded that the rate of coalescence of air bubbles decreased either due to the frother molecules adsorbed on the water/air interface, or by the presence of a stable water layer on the surface of the bubbles. Presence of salt decreases the bubble size and increase froth stability. Both factors enhance flotation recovery. However, Kurniawan et al., (Kurniawan et al., 2011) have shown that enhancement of flotation performance depends on the type and concentration of the salt. The results illustrated in Fig. 1 correlate with those of previous works, indicating increased froth stability at high ion concentration. Comparison of the frothing effects of the recycle water in the CBI flotation plant can also be done with reference to the artificial water samples. The results show that the Zn recycle process water does not have a significant effect on froth stability. The Cu recycle water however, with a higher dissolved ion concentration can increase frothing in Cu flotation. The use of the Cu recycle water in the Cu flotation circuit at CBI could be one of the reasons for the stable but watery froth at CBI. This results in a high Cu recovery but low concentrate grade in the first cell of the Cu Rougher bank at the CBI flotation plant (Ekmekçi et al., 2010).
3.1. Water chemistry 3.3. Recovery by true flotation and entrainment The ionic composition of the synthetic water samples, as well as the composition of the water samples following grinding and batch flotation tests are given in Tables 2 and 3, respectively. Comparison of these results shows that significant amount of the base metal ions adsorbed on mineral surfaces after grinding through a precipitation/adsorption mechanism (Finkelstein, 1997; Peng and Grano, 2010). On the other hand, the concentration of Ca and the thiosalts increased due to the addition of lime to increase pulp pH to 11.5 and the oxidation/dissolution of sulfide ions from the sulfide minerals at this pH (Dinardo, 2000). It was noted that the majority of the adsorption and dissolution processes occurred during the grinding stage and only minor changes were observed in flotation.
In order to evaluate the influence of water quality on the activation and depression of the sulfide minerals, the contribution of the recovery by true flotation and entrainment needs to be decoupled (Fig. 2). The use of the recycled water increased both the Cu recovery by true flotation and entrainment. Cu flotation recovery increased gradually from ~ 80% in tap water up to ~ 90% in water 4 sample. The increase in Cu recovery could be attributed to the activation of slow floating, partially oxidized or tarnished chalcopyrite particles by the dissolved thiosalts (Ikumapayi et al., 2010) and coarse
Table 3 Ionic composition of the water samples after grinding and flotation.
Tap water Water 1 Water 2 Water 3 Water 4
After After After After After After After After After After
grinding flotation grinding flotation grinding flotation grinding flotation grinding flotation
n.a.: below detection limit.
Cu ppm
Fe ppm
Pb ppm
Zn ppm
Ca ppm
SO42−
S2O32−
n.a. n.a. n.a. n.a. n.a. 0.004 n.a. 0.049 0.046 0.225
n.a. n.a. n.a. n.a. n.a. n.a. n.a. 0.115 0.038 0.056
0.09 0.08 0.11 0.10 0.15 0.13 0.21 0.21 0.35 0.34
0.046 0.134 0.048 0.068 0.012 0.056 0.021 0.082 0.086 0.169
294 250 435 447 756 1154 1316 857
1009 829 868 1171 1681 1430 1673 1188 1382 1405
269 234 413 678 1236 1905 1874 1026 3057 2887
2382
Cu Recycle water Zn Recycle water
Fig. 1. Changes in water recovery and mass pull as a function of total dissolved ion concentration in the process water.
Ö. Bıçak et al. / International Journal of Mineral Processing 102–103 (2012) 32–37
35
Fig. 2. Recovery by true flotation and entrainment for Cu, Zn and pyrite (a, c, e: recovery by true flotation, b, d, f: recovery by entrainment).
composite particles (Fig. 2a). The increase in water recovery also resulted in an increase in the recovery by entrainment from ~ 2% in tap water to about ~ 5% in the recycled water (Fig. 2b). There was no significant difference in entrainment however, between the four synthetic water samples. For most of the synthetic water samples, the recovery of Cu by flotation was ~ 90%, with an addition 5% recovery due to entrainment, resulting in a total Cu recovery of ~ 95%. This is considered close to the maximum achievable copper recovery and higher water recoveries may not significantly affect the recovery by entrainment, as observed in Fig. 2b. Both the flotation rate and recovery of sphalerite by true flotation and entrainment increased with the use of recycled water. The dissolved copper and lead ions activated sphalerite (Gerson et al., 1999; Lascelles et al., 2001) and increased flotation recovery by ~ 6% (Fig. 2c). In addition to the activation by copper and lead ions, flotation can be enhanced by the smaller bubbles at high ion concentrations which increases bubble-particle collision efficiency (Kurniawan et al., 2011). The recovery by entrainment also increased from 3% to 9% with synthetic water 4 sample (Fig. 2d). This resulted in an overall increase in Zn recovery from 18% to ~30% when recycled water was used in the flotation tests. The results also showed that the use of recycled
water caused poorer selectivity between chalcopyrite and sphalerite, even when the concentrations of the Cu2+ and Pb2+ ions were low (Synthetic water 1). Although higher Zn recoveries were expected due to true flotation when the concentration of the Cu 2+ and Pb 2+ ions was increased to 6.90 ppm and 2.06 ppm in synthetic water 4, respectively, Zn recovery actually showed a slight decrease for the other synthetic water samples. (Fig. 2c). This small decrease in Zn recovery by true flotation is attributed to the balance between the activation effect of the Cu and Pb ions and the depression effect of the dissolved sulfide ions in the water. The thiosulfate and sulfate ion concentrations were increased in conjunction with the increase in Pb and Cu ions to simulate the accumulation of all dissolved ions in the actual recycled water samples (Table 2). Therefore, the small decrease in Zn recovery by true flotation could be attributed to the balance between activation effect of Cu and Pb ions and depression effect of dissolved sulfide ions in the water. The effects of recycled water on recovery of pyrite by true flotation, was similar to that of sphalerite (Fig. 2e). Pyrite was activated by the synthetic water 1 and 2 samples, due to the dissolved copper ions (Pecina et al., 2006). The flotation recovery of pyrite also increased from 4% up to 8%. For the synthetic water 3 and 4 samples
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that contained higher concentrations of the thiosulfate and sulfate ions however, the true flotation of pyrite decreased. This indicates that the activation of pyrite by the copper ions was counteracted by the depressive effect of the dissolved sulfur species in the pulp (Rao and Finch, 1989; Ikumapayi et al., 2010). The recovery of pyrite due to entrainment was similar to that of sphalerite (Fig. 2f). The recovery due to entrainment increased from ~ 3% to 6% when the tap water was replaced with synthetic water sample 1. The recovery due to entrainment then showed a further increase up to 8% as the dissolved ion concentration in the synthetic water was increased (synthetic water sample 4). The results showed that the use of recycled water with similar characteristics to synthetic water sample 1 would cause an increase in overall pyrite recovery from 7% to 13% due to both pyrite activation and entrainment. A further increase in sulfide ion concentration however (synthetic water sample 4) caused a decrease in the recovery by true flotation back to ~ 4%. This also coincided with the greatest degree of entrainment, overall resulting in a fairly unchanged pyrite final recovery of~13%. The effect of recycled water on the flotation rates are also of interest, and are illustrated in Fig. 3. The flotation rate of both chalcopyrite and sphalerite increased with higher dissolved ion concentration (Fig. 3a). This was due to the activation of the slow floating, tarnished chalcopyrite and sphalerite particles by Cu 2+ ions. The recovery of chalcopyrite increased gradually from ~77% to 87% (Fig. 3b). Sphalerite recovery also increased from 14% to 23% when the tap water was replaced with synthetic water sample 1. The sphalerite recovery was relatively unchanged for the other synthetic water samples. This is attributed to the balance between the activation effect of the dissolved copper ions and the depression effects of the sulfide species that prevented a further increase in Zn recovery. The flotation rate of pyrite was almost identical for all of the tests (Fig. 3a), whereas the true flotation recovery of pyrite, increased from ~ 4% to 8% when the tap water was replaced with the synthetic water (sample 2) due to the activation by the dissolved copper ions (Fig. 3b). The pyrite recovery however, decreased down to ~4% when synthetic water samples containing higher concentrations of copper and sulfide ions were used in flotation. This suggests that pyrite flotation is relatively unaffected by low concentrations of sulfide ions in the recycled water, and only when the concentration is increased do the sulfide ions have a significant depressing effect on pyrite flotation performance. 4. Conclusions A series of batch scale flotation tests were performed using the CBI Cu–Zn complex sulfide ore to investigate the influence of dissolved ions on flotation performance. The results of the flotation tests revealed that the dissolved metal ions and sulfide ions, mainly in the form of SO4− 2 and S2O3− 2, influenced both the froth stability and surface chemistry. Therefore, the effect of water chemistry was evaluated in terms of its influence on true flotation and entrainment. The froth stability increased in conjunction with the dissolved ion concentration, presumably due to the increased stability of the water layer between air bubbles. Consequently, the degree of entrainment, particularly for pyrite and sphalerite, increased with higher concentrations of dissolved ions in the recycled water. The presence of the dissolved metal ions, such as Cu 2+ and Pb 2+, increased both the rate and recovery of the copper minerals (mainly chalcopyrite) and sphalerite by true flotation. This was attributed to the activation of the slow floating, tarnished particles of chalcopyrite and also the sphalerite particles by the metal ions. The activation and depression effects of the metal ions were simultaneously observed on sphalerite and pyrite flotation performance. Both minerals were activated by the dissolved Cu and Pb ions with moderately contaminated water samples. The activation effect of metal ions, however was counteracted by the depressive effect of the sulfide ions with synthetic
a
Cu Recycle water Zn Recycle water
b
Fig. 3. Variations in true flotation rate (a) and recovery (b) of Cu, Zn and pyrite.
water samples containing high concentrations of thiosulfate and sulfate. The results have clearly shown the influence of process water contamination on the flotation performance for the Çayeli flotation plant. Further work should be performed considering the effect of residual organic reagents, such as collector and frother. The presence of these reagents may have a significant effect on the flotation performance by changing froth stability and activation, and should be considered in conjunction with the effect of the dissolved inorganic ions in future research. Acknowledgements The authors would like to acknowledge with great appreciation the contribution in the preparation of this work by Megan Backer from University of Cape Town, Center for Mineral Research. The technical and financial support from Çayeli Bakır İşletmeleri A.Ş. and the financial support of The Scientific and Technological Council of Turkey (TÜBİTAK) Project No: 107M275 are also acknowledged.
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