- Email: [email protected]
Flotation behavior of enargite in the process of flotation using seawater
Minerals Engineering 142 (2019) 105897
Contents lists available at ScienceDirect
Minerals Engineering journal homepage: www.elsevier.com/locate/mineng
Flotation behavior of enargite in the process of flotation using seawater Rodrigo Yepsen a b c
a,b
a,⁎
, Leopoldo Gutierrez , Janusz Laskowski
T
c
Department of Metallurgical Engineering, Universidad de Concepcion, Chile Water Research Centre for Agriculture and Mining (CRHIAM), Universidad de Concepcion, Chile N.B. Keevil Institute of Mining Engineering, University of British Columbia, Canada
A R T I C LE I N FO
A B S T R A C T
Keywords: Flotation Enargite Seawater
The presence of enargite (Cu3AsS4) in porphyry copper deposits is an important matter as this mineral shows good flotability in the presence of the thiol collectors used in the flotation copper sulfides thus contaminating the final concentrate. Besides a relevant problem in Chilean mining is the scarcity of water resources so the use of seawater has become a feasible solution. The flotation behavior of copper sulfides and molybdenite in seawater was previously studied but there are no systematic studies related to the flotation of enargite using seawater. The objective of this work is to study the behavior of enargite in the process of flotation using seawater and to compare the flotation of enargite and chalcopyrite in this aqueous medium. The results indicate that the recovery of enargite is strongly depressed at pH > 9 in the process of flotation using seawater, behavior that can be explained in the first place by the adhesion of the hydrolysis products of magnesium on the surfaces of the enargite particles. The analyses of the Pourbaix diagrams for the As-Ca and As-Mg systems also suggest that the enargite depression in seawater observed at pH > 9 can not only be related to the adsorption of hydroxo complexes of magnesium and calcium, but also to the interaction of these metals with oxidized arsenic species such as arsenate and arsenite. As these compounds are hydrophilic in nature they reduce enargite recovery. Pretreatment of seawater before flotation to remove most of the calcium and magnesium present through the precipitation of hydrolyzed complexes of these cations is a suitable way to improve enargite recovery at pH > 9.
1. Introduction Chilean copper deposits have been extensively exploited during the last decades and the ores mined today also contain arsenic-bearing minerals. This makes difficult obtaining copper concentrates of suitable quality for commercialization. An example of these minerals is enargite (Cu3AsS4) which commonly appears in the porphyry copper deposits associated with copper sulfides. One of the complications associated with the presence of enargite in copper ores is that this sulfosalt shows a good flotability in the presence of the thiol collectors used in the flotation copper sulfides thus reporting to the final concentrate causing economical and environmental problems. The behavior of enargite in the flotation process and its relationship with its surface properties has been investigated under collectorless conditions (Kantar, 2002; Plackowski et al., 2012; Fullston et al., 1999a,b; Guo and Yen, 2008). It was found that enargite flotation strongly depends on pH and the pulp redox potential, and that it increases at low pH values and high Eh as a result of the presence of elemental sulfur on the enargite surface. In contrast, at more alkaline conditions a decrease of enargite flotation is observed which was
⁎
correlated with the presence of oxidized copper compounds on the surface. Fullston et al. (1999b) for example concluded from the zeta potential measurements of several sulfides that at oxidizing conditions a layer of copper hydroxide covers a metal-deficient sulfur-rich surface and that it increases at more oxidizing conditions. The following ranking of oxidation was proposed (Fullston et al., 1999b): chalcocite > tennantite > enargite > bornite > covellite > chalcopyrite. Although there are some studies that showed that enargite does not have an isoelectric point in the pH range between 2 and 12 (Castro and Honores, 2000; Castro and Baltierra, 2005), it was also reported that the sign of the enargite zeta potential changes when the ore is conditioned with a highly oxidizing agent such as hydrogen peroxide (Fulston et al., 1999a) which was explained by the process of progressive decomposition of the surface and release of copper, arsenic and sulfates (Pauporté and Schuhmann, 1996 ; Asbjornsson et al., 2004; Guo and Yen, 2008; Sasaki et al., 2010). In the presenece of an ethyl xanthate collector, Kantar (2002) reported that under alkaline conditions and Eh > 0.2 V a two layer of cuprous xanthate (CuX) and dixanthogen is formed on the surface of enargite which explains its hydrophobicity. In contrast, when enargite is floated
Corresponding author. E-mail address: [email protected] (L. Gutierrez).
https://doi.org/10.1016/j.mineng.2019.105897 Received 15 April 2019; Received in revised form 1 July 2019; Accepted 5 August 2019 0892-6875/ © 2019 Published by Elsevier Ltd.
Minerals Engineering 142 (2019) 105897
R. Yepsen, et al.
Germany in all the tests. Seawater was obtained from the coast of Concepcion. Salts of Mg (MgCl2·6H2O), Ca (CaCl2·2H2O), Na and K chlorides, used in the experiments, were obtained from Merk.
at lower Eh the recovery decreases due to the reduction of dixanthogen which reveals that the xanthate dimer is mainly responsible of enargite hydrophobicity (Guo and Yen, 2002). An important issue related to the presence of enargite in copper porphyry ores is its possible selective separation from chalcopyrite through flotation. Such a selective flotation involving the use of selective flotation reagents, selective oxidation and control of pulp potential, while conditioning with reagents such as sodium cyanide and sodium sulphide, has shown limited success (Plackowski et al., 2012). The use of MAA (magnesium-ammonium mixture) to depress enargite has shown varied results, with some studies reporting good selectivity (Castro et al., 2003) and others poor results (Pineda et al., 2015). However, methodological differences may explain these differences. Selective oxidation of the mineral surface to influence collector adsorption using a pre-conditioning step has been shown to be effective in separating enargite from non-arsenic copper sulphides (Guo and Yen, 2005). Typically, hydrogen peroxide or similar oxidizing reagents are used and can be combined with pulp aeration using air or oxygen gas (Kantar, 2002). However, further studies are needed to further develop a process more readily applicable by industry. The use of other potentially more selective flotation collectors different from traditional xanthates deserves investigation. A relevant problem in Chilean mining is the scarcity of water resources to carry out the processing of copper ores, so the use of alternative sources such as seawater has become a feasible solution. The flotation behavior of copper sulfides and molybdenite was studied (Castro and Laskowski, 2011; Castro et al., 2014). The main conclusions of these publications indicate that molybdenite flotation is negatively affected in seawater at pH > 9.5 when lime is used to depress pyrite which is explained by the action of the Mg2+ hydrolysis products, and that copper sulfides are not depressed. The use of dispersants to remove magnesium hydroxides from the molybdenite surface was later tested and positive results were reported by Rebolledo et al. (2017). However, there are no studies related to the flotation of enargite using seawater. The objective of this work is to study the behavior of enargite in the process of flotation using seawater. A comparison of the flotation of enargite and chalcopyrite using seawater is also carried out as the separation of these two minerals by flotation is a true technological challenge facing industry.
2.2. Microflotation tests The procedure of the micro-flotation tests is described in Fig. 2. These experiments were performed in a 150 cm3 Partridge Smith glass cell using nitrogen gas at a flow rate of 90 cm3/min. The flotation feed was wet ground in a laboratory carbon steel ball mill of 7 cm diameter and 12 cm length, charged with 270 g of carbon steel balls (12 balls of 13 mm diameter; 8 balls of 17 mm diameter). For the micro-flotation tests, 3 g of −2 + 1.41 mm enargite and chalcopyrite minerals samples were initially ground to a P80 of 74 µm at a solids content of 25% by weight. Then the pulp was split into 3 samples which were later conditioned during 3 min in 150 mL of solution (e.g., seawater, 0.01 M NaCl solution or treated seawater) to adjust pH to the required condition. After this, 25 ppm of PAX and 30 ppm of MIBC were added and conditioned for additional 3 min. The flotation tests were carried out for 2 min, scraping the froth off every 10 s. The pulp level in the microflotation cell was kept constant by adding a background solution prepared at the same composition and pH of the original solution. The concentrates and tailings were dried in an oven at 105 °C for 5 h and recovery was calculated dividing the mass of mineral in the concentrate by the mass of mineral in concentrate plus the tailings. 2.3. Zeta potential measurements The zeta potential of enargite at different conditions was studied using a Zetacompact Z9000 from CAD instrument. These measurements were performed using enargite particles 100%- 20 µm which were previously wet ground in the ball mill used in the micro-flotation tests. The effect of Mg and Ca ions on the zeta potential of enargite was studied using 0.005 M solutions of these ions dissolved in milli-Q water. 2.4. SEM-EDS Scanning Electron Microscopy with Energy Dispersive X-Ray (SEMEDS) was used to characterize the precipitates formed on the enargite surface. A JEOL JSM-6380 LV device coupled to the Digital MicrographTM software was used. The protocol used to prepare the sample was carried out according to the one used by Smart (1991) and consists of taking a wet ground sample of pulp and drying it with nitrogen into a sample tube for a period of 15 min. Then the sample tube is sealed under a nitrogen atmosphere with a silicone rubber sealant around the cap to prevent oxygen from entering into the tube. Then the sample is frozen for 30 mins and then the dry samples are mounted in a pin stub fixed with a carbon adhesive and covered with gold.
2. Materials and methods 2.1. Samples and reagents The enargite sample was obtained from the Quiruvilca mine in Perú. This sample was initially crushed to a −2 + 1.41 mm size fraction by hand grinding, and then concentrated by hand picking. The chalcopyrite sample used in the study, a high purity sample obtained from Ward‘s Natural Science Establishment, was initially treated following the same procedure. Purification of the crushed samples was carried out by using a magnetic separator to remove some magnetic impurities, and by desliming to remove ultra-fines. The mineral composition was analyzed by X-ray diffraction using Bruker® D4 Endeavor equipment, operated with Cu radiation and Ni Kβ radiation filter. XRD analyses indicate that enargite and chalcopyrite samples were 99.8% and 99% purity respectively with minor amounts of tennantite, and chalcopyrite in the case of the enargite sample (Fig. 1a), and low concentrations of pyrite and quartz (Fig. 1b) in the case of the chalcopyrite one. The chemical analysis of the sample gave 34.3% Cu, 30.4% Fe, and 35.2% S for the chalcopyrite sample and 18.7% As, 47.8% Cu, and 32.6% S for the enargite. Potassium amyl xanthate (PAX) obtained from Solvay previously purified with ether and acetone, and methyl isobutyl carbinol (MIBC) provided by Merk were used as collector and frother respectively. MilliQ water of 18.4 MΩ∙cm at 25 °C was used in all the experiments. pH was adjusted using sodium hydroxide of analytical grade from Merk,
3. Results 3.1. Microflotation tests Fig. 3 shows the recovery of enargite as a function of pH using a 0.01 M NaCl solution. The results show that the recovery of enargite remains constant at the whole studied pH range when the process is carried out in water with low ionic strength. Fig. 3 also shows that the increase of pH is associated with lower pulp electrochemical potentials (Eh) and dissolved oxygen values. Fig. 4 shows the recovery of enargite as a function of pH in seawater. It can be seen that at the whole pH range enargite flotation is lower than that observed in the tests carried out using a low ionic strength water. Additionally, at pH values between 8 and 10 the recovery of enargite remains relatively constant, but at pH > 10 a strong enargite depression is observed. Again the increase of pH lowers Eh and amount of dissolved oxygen. 2
Minerals Engineering 142 (2019) 105897
R. Yepsen, et al.
Fig. 1. XRD spectra of enargite (a) and chalcopyrite (b) samples.
Fig. 2. Micro-flotation procedure.
indicates that over the whole pH range chalcopyrite flotation is slightly lower than in the tests with 0.01 M NaCl. As it was observed in the experiments using enargite, the recovery of chalcopyrite also remains relatively constant below pH = 10 and strong depression is observed at more alkaline conditions. Again the increase of pH leads to lower Eh and dissolved oxygen values.
Sodium, potassium, calcium and magnesium ions are important constituents of seawater. To distinguish the influence of these ions on the recovery of enargite, additional micro-flotation tests were carried out using solutions of these elements at their characteristic concentrations in seawater (10,700 ppm sodium, 400 ppm potassium, 400 ppm calcium and 1,300 ppm magnesium). Fig. 5 shows that magnesium is the one that generates the strongest depressing effect on enargite, in particular at pH > 10. There is also a decrease in enargite recovery caused by calcium ions at pH > 10, but less significant than the one generated by magnesium. In both cases, it can be suggested that the hydrolysis products of these two divalent cations play a role in depressing enargite. An objective of this work is to compare the behavior of enargite and chalcopyrite under the conditions of flotation using seawater, and wet grinding. Fig. 6 presents the results of chalcopyrite flotation as a function of pH using a 0.01 M NaCl solution and indicate that chalcopyrite flotation is unaffected over the studied pH range. On the other hand, as observed in the tests with enargite, both the Eh and dissolved oxygen tend to decrease as the pH increases. Fig. 7 shows how pH affects chalcopyrite recovery in seawater. It
3.2. Zeta potential measurements Fig. 8 shows the zeta potential of enargite as a function of pH in Mili-Q water and solutions of 0.005 M of magnesium and calcium. The results obtained in Mili-Q water show that zeta potential values are negative throughout the pH range studied, which is in agreement with previous studies (Fullston et al., 1999a,b; Castro and Honores, 2000; Castro and Baltierra, 2005). The presence of magnesium and calcium in solution produces an important change in the enargite zeta potential and it is observed that at pH > 9 this mineral acquires a positive surface charge which can be associated with the adsorption of magnesium and calcium hydroxo complexes. 3
Minerals Engineering 142 (2019) 105897
R. Yepsen, et al.
Fig. 3. Enargite recovery as a function of pH in a 0.01 M NaCl solution using 25 ppm PAX and 30 ppm MIBC.
Fig. 4. Enargite recovery as a function of pH in seawater using 25 ppm PAX and 30 ppm MIBC.
3.3. SEM-EDS surface analysis SEM-EDS analyses were performed with the enargite which was conditioned in seawater at pH 9 and 11, and in the absence of PAX. The results obtained when the sample was prepared without a collector at pH 9 indicate similar concentrations of Cu, As, and S to those in untreated enargite. The analyzes of points 1 and 2 (see inserts in Fig. 9) measured at pH = 11 reveal the presence of oxygen, magnesium and calcium, which is explained by the presence of the hydrolyzed compounds of these metals on the enargite. This is directly related to the decrease in the recovery of enargite at pH values higher than 10 observed in Fig. 4. 4. Discussion Previous publications demonstrated that in the flotation of Cu-Mo ores in seawater, molybdenite flotation is strongly depressed by the hydrolysis products of Mg2+ at pH > 9.5, but no effect was reported on Cu recovery at the same pH range (Castro 2012; Castro et al., 2012). A recent publication by Ramirez et al. (2018) reported that in singlemineral flotation tests carried out in seawater chalcopyrite was
Fig. 5. Enargite recovery as a function of pH in solutions of 10,700 ppm sodium, 400 ppm potassium, 400 ppm claicum and 1300 ppm magnesium, and using 25 ppm PAX and 30 ppm MIBC.
4
Minerals Engineering 142 (2019) 105897
R. Yepsen, et al.
Fig. 6. Chalcopyrite recovery as a function of pH in a 0.01 M NaCl solution using 25 ppm PAX and 30 ppm MIBC.
Fig. 7. Chalcopyrite recovery as a function of pH in seawater using 25 ppm PAX and 30 ppm MIBC.
depressed at pH values above 10. The differences observed between both studies were attributed to the conditions at which minerals were ground and it was concluded that redox conditions affects the way in which hydroxo complexes of Mg depress chalcopyrite flotation. The results reported in Figs. 3 and 4 of this work indicate that the recovery of enargite is not affected at alkaline conditions (pH > 9) when the process is carried out in a 0.01 M NaCl solution. However, enargite flotation is strongly depressed at pH > 9 in seawater. The results reported in Fig. 5 show that, as previous publications revealed for molybdenite (Castro 2012; Castro et al., 2012) and for chalcopyrite (Ramirez et al., 2018), the hydroxo complexes of magnesium generated at pH > 9 explain such a depression, and that calcium hydrolysis products affect enargite flotation at pH > 10. These results suggest that coating of enargite by the hydrolysis products of these two divalent cations explain the observed depression. The change in the enargite zeta potential from negative to positive values observed at pH > 9 in Fig. 8 reveals the interactions of magnesium with enargite particles. In adition, SEM-EDS micrographics shown in Fig. 9 reveal the presence of magnesium and calcium on the enargite surface when the sample was prepared in seawater at pH = 11 which indicates that there is a
Fig. 8. Zeta potential of enargite as a function of pH in Mili-Q water and solutions 0.01 M of magnesium and calcium.
5
Minerals Engineering 142 (2019) 105897
R. Yepsen, et al.
Fig. 9. SEM micrographs of enargite conditioned in seawater at pH 9 and 11 without collector.
Fig. 10. Pourbaix diagram for (A) Ca-As system and (b) Mg-As system. Data obtained from HSC V6.00 software.
These authors indicate that species such as Ca4(OH)2(AsO4)2·4H2O and Ca5(OH)(AsO4)3 are likely to be formed at pH values above 10. Raposo et al (2004) proposed that in the case of the interaction of magnesium and calcium with arsenic, there is a distribution of the species that can be formed according to the following scheme: Ca3(AsO4)2·∙10H2O at pH 7–10; CaNaAsO4·∙7.5H2O pH 10–12; MgHAsO4·4H2O pH 6.5–7.1; and Mg3(AsO4)2·8H2O pH 7.2–9.0. Eq. (1) shows the general formula for the chemical reactions between Mg and Ca and AsO43- (Raposo et al., 2004).
precipitation of hydrolized species of calcium and magnesium which renders this mineral more hydrophilic causing a marked decrease in recovery as shown in Fig. 4. It has to be noted that Figs. 5 and 6 show that the behavior of chalcopyrite follow a similar trend to what is observed for enargite, which validates previous results presented by Ramirez et al. (2018) which indicated that in seawater chalcopyrite flotation is also depressed at pH > 9. The results presented in Figs. 3–7 indicate that the recovery of enargite and chalcopyrite at pH > 9 can be mainly explained by the coating by Mg and Ca hydroxo complexes of the mineral surfaces. This has been proposed by several authors to affect the surface chemistry of the flotation system (Fuerstenau et al., 1988; James and Healy, 1972; Laskowski and Castro, 2012). However, alkaline metals also strongly interact with inorganic arsenic. Bothe and Brown (1999) and Brown and Bothe (1999) proposed that different compounds can be formed resulting from the interaction between calcium and arsenic ions and that the extent of these chemical reactions strongly depends on pH.
pM 2 + + qH+ + rAsO43 − ↔ (M 2 +)p (H+)q (AsO43 −)r
(1)
Fig. 10 shows the Pourbaix diagrams for the As-Ca and As-Mg systems, and Fig. 11 shows the distribution of arsenate species in seawater conditions. The diagrams reveal that at the conditions of the microflotation experiments, that is at pH > 9 and Eh between −120 and −130 mV, the species that are likely to be formed on the surface of enargite are Ca(AsO2)2, Ca3(AsO4)2, Mg(AsO2)2 and Mg3(AsO4) 2, all of 6
Minerals Engineering 142 (2019) 105897
R. Yepsen, et al.
Fig. 11. Distribution of arsenate especies in seawater condition for (a) Ca2+ and (b) Mg2+ (Adapted from Raposo et al., 2004).
adsorption of hydroxo complexes of magnesium and calcium, but also to the interaction of these metals with oxidized arsenic species such as arsenate and arsenite. Since the compounds formed as a result of reaction between Mg and Ca and arsenates are in general more insoluble than the compounds formed with arsenites (Carpenter et al., 2014), it can be expected that an increase in Eh could generate the physicochemical conditions to promote the formation of hydrophilic compounds on the enargite surfaces, and therefore a decrease in recovery with respect to flotation performed at 0.01 M NaCl. To verify this idea, micro-flotation tests were carried out at different redox potential conditions in fresh and seawater. In these tests 1 g of dry ground enargite was initially conditioned for 20 min in a beaker with 100 cm3 of fresh or seawater, and at a given redox potential adjusted using hydrogen peroxide or sodium sulfite (Na2SO3). Once redox conditions reached equilibrium, pH was adjusted to 10.5 and PAX and MIBC were added to obtain concentrations of 25 and 30 ppm, respectively. The suspension was then conditioned for additional 3 min after which micro-flotation tests were performed as previously described. Fig. 12 shows the results of enargite recovery as a function of Eh. The first observation is that the natural Eh in these tests was around 350 mV/SHE which is higher than what is observed in Figs. 3 and 4; these were obtained in a more reducing environment (wet grinding in a mill using mild steel balls) while the results shown in Fig. 12 were obtained using enargite which was ground by dry grinding. A second important observation is that as the values of Eh rise from 350 mV to 500 mV, the depressing effect of seawater becomes stronger compared to a 0.01 M NaCl, which can be related to the formation of arsenate (oxidation of As3+ to As5+) and subsequent precipitation with the Mg2+ and Ca2+ ions. When the electrochemical potential rises to values above 500 mV/SHE, the recovery of enargite in both aqueous media decreases at the same rate, most likely due to excessive dissolution of Cu ions and precipitation of these with the collector in homogeneous phase. Since the presence of magnesium and calcium ions and the interactions of these elements with the surface of the enargite cause a depressing effect in this mineral, a possible option to avoid this negative effect is to pre-treat seawater before its use in flotation. Castro (2010) proposed a method that consists of eliminating most of the calcium and magnesium present in seawater through the precipitation of hydrolyzed
Fig. 12. Enargite recovery as a function of Eh using fresh and seawater at pH 9. 25 ppm PAX and 30 ppm MIBC.
Fig. 13. Recovery of enargite as a function of pH in raw and pre-treated seawater. 25 ppm PAX, 30 ppm MIBC.
them of hydrophilic nature. According to this analysis, the enargite depression observed at pH > 9 can not only be related to the 7
Minerals Engineering 142 (2019) 105897
R. Yepsen, et al.
hydrophilic they reduce enargite recovery. 3. Pre-treatment of seawater before flotation to remove most of the calcium and magnesium present through the precipitation of hydrolyzed complexes of these cations is a suitable way to improve enargite recovery at pH > 9.
complexes of these cations, bringing the pH to values of 11 with lime. Fig. 13 shows the recovery of enargite in raw seawater and the pretreated seawater according to the method described by Castro (2010). The results indicate that at pH values of 8 and 9, no differences in recovery are observed in both media, while at pH 11 the recovery of enargite is much greater in the pre-treated seawater. These results reinforce the idea that the hydrolyzed Mg compounds are adsorbed on the surface of enargite and cause a depressing effect. Other trends that are presented in Figs. 3–7 and that deserve to be discussed refer to the inverse relationship between the pH, and the values of Eh and dissolved oxygen. As was previously mentioned the type of grinding media used to reduce the size of particles before flotation strongly affects the redox conditions of the pulp, and as a result, the recovery of sulfides minerals (Huang and Grano, 2006; Peng and Grano, 2010). Previous studies showed that chalcopyrite flotation, for example, is depressed when carbon steel grinding media was used instead of stainless steel balls, which was attributed to the formation of iron hydroxides on the surface of chalcopyrite (Rao et al., 1976). These iron compounds are formed as a result of the anodic oxidation of carbon steel in aqueous media (Rao and Natarajan, 1988). In particular, the grinding of sulfide minerals with carbon steel balls in an aqueous medium causes the release of ferrous ions into solution according to Eq. (2) (Rao and Natarajan, 1989). As Eqs. (3) and (4) indicate ferrous iron under alkaline conditions oxidize to ferric iron and finally form a iron hydroxide. All these reactions release electrons which are captured by oxygen which explains the reduction of oxygen concentration, and reduction of Eh.
Fe 2 + + 2H2 O ↔ Fe (OH )2 + 2H+
Fe (OH )2 +
The authors acknowledge the financial support of CRHIAM sponsored by the CONICYT/FONDAP-15130015 project. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.mineng.2019.105897. References Asbjornsson, J., Kelsall, G.H., Pattrick, R.A.D., Vaughan, D.J., Wincott, P.L., Hope, G.A., 2004. Electrochemical and surface analytical studies of enargite in acid solution. J. Electrochem. Soc. 151 (7), E250–E256. Bothe, J.V., Brown, P.W., 1999. The stabilities of calcium arsenates at 23 °C. J. Hazard. Mater. B69, 197–207. Brown, P.W., Bothe, J.V., 1999. Arsenic inmobilization by calcium arsenate formation. Environ. Sci. Technol. 33, 3806–3811. Carpenter, J.S., Bai, C., Hwang, J.Y., Ikhmayies, S., Li, B., Monteiro, S.N., Peng, Z., 2014. Characterization of Minerals, Metals, and Materials 2014. John Wiley & Sons, New Yersey, USA, pp. 165–184. Castro, S., 2012. Challenges in flotation of Cu-Mo sulfide ores in sea water. In: Proc. 1st Water in Mineral Processing. Sociecty for Mining, Metallurgy and Exploration, Seattle Usa, pp. 29–40. Castro, S., Rioseco, P., Laskowski, J.S., 2012. Depression of molybdenite in sea water. In: Proc. 26th Internacional Mineral Processing Congress IMPC 2012, New Delhi, India, September 24-28, pp. 737–752. Castro, S., Uribe, L., Laskowski, J.S., 2014. Depression of inherently hydrophobic minerals by hydrolysable metal cations: molybdenite depression in seawater. In: 27th Int. Mineral Processing Congress IMPC 2014, Santiago, October 20-24, pp. 207–217. Castro, S., Laskowski, J.S., 2011. Froth flotation in saline water. Kona Powder Part. J. 29, 4–15. Castro, S., 2010. Proceso para pre-tratar agua de mar y otras aguas salinas para su utilización en procesos industriales. Universidad de Concepcion. Chilean Patent, No. 00475/INAPI (May 12, 2010). Castro, S.H., Baltierra, L., Muñoz, P., 2003. Depression of enargite by magnesium-ammonium mixtures. In: In: Proc. Proceedings of Copper 2003, Santiago, Chile, Volume III Mineral Processing, pp. 257–269. Castro, S.H., Baltierra, L., 2005. Study of the surface properties of enargite as a function of pH. Int. J. Miner. Process. 77 (2), 104–115. Castro, S.H., Honores, S., 2000. Surface properties and floatability of enargite. Proc. 21st International Mineral Processing Conference IMPC 2000. pp. B8b–47–B8b-53. Fuerstenau, M.C., Lopez-Valdivieso, A., Fuerstenau, D.W., 1988. Role of hydrolyzed cations in the natural hydrophobicity of talc. Int. J. Miner. Process. 23, 161–170. Fullston, D., Fornasiero, D., Ralston, J., 1999a. Oxidation of synthetic and natural samples of enargite and tennantite: 1. Dissolution and zeta potential study. Langmuir 15 (13), 4524–4529. Fullston, D., Fornasiero, D., Ralston, J., 1999b. Zeta potential study of the oxidation of copper sulfide minerals. Coll. Surf. A: Physicochem. Eng. Aspects 146 (1–3), 113–121. Guo, H., Yen, W.T., 2008. Electrochemical study of synthetic and natural enargites. In: In: Proc. 24th International Mineral Processing Conference IMPC 2008, pp. 1138–1145. Guo, H., Yen, W.T., 2002. Surface potential and wettability of enargite in potassium amyl xanthate solution. Miner. Eng. 15 (6), 405–414. Guo, H., Yen, W.T., 2005. Selective flotation of enargite from chalcopyrite by electrochemical control. Miner. Eng. 18 (6), 605–612. Huang, G., Grano, S., 2006. Galvanic interaction between grinding media and arsenopyrite and its effect on flotation: Part I. quantifying galvanic interaction during grinding. Int. J. Miner. Process. 78 (3), 182–197. James, R.O., Healy, T.W., 1972. Adsorption of hydrolyzable metal ions at the oxide—water interface. III. a thermodynamic model of adsorption. J. Colloid Interface Sci. 40 (1), 65–81. Kantar, C., 2002. Solution and flotation chemistry of enargite. Coll. Surf. A 210 (1), 23–31. Laskowski, J.S., Castro, S., 2012. Hydrolyzing ions in flotation circuits: seawater flotation. In: Proc. 13th International Mineral Processing Symposium, pp. 219–228. Pauporté, T., Schuhmann, D., 1996. An electrochemical study of natural enargite under conditions relating to those used in flotation of sulphide minerals. Coll. Surf.: A Phisicochem. Eng. Asp. A 111 (1–2), 1–19. Peng, Y., Grano, S., 2010. Effect of iron contamination from grinding media on the flotation of sulphide minerals of different particle size. Int. J. Miner. Process. 97 (1–4), 1–6. Pineda, D., Plackowski, C., Nguyen, A.V., 2015. Surface properties of enargite in MAA
(2)
Fe 0 → Fe 2 + + 2e−
OH−
Acknowledgements
↔ Fe (OH )3 +
(3)
e−
(4)
1 O2 + H2 O + 2e− ⇒ 2OH− 2
(5)
Besides, previous studies on the surface behavior of enargite and chalcopyrite describe the electrochemical reactions that take place under alkaline conditions (Eqs. (6) and (7)).
CuFeS2 + 3H2 O ⇒ CuS + Fe (OH )3 + S + 3H+ + 3e−
Cu3 AsS4 + 23H2 O ⇒ 3CuO +
HAsO42 −
+
4SO42 −
+
45H+
(6)
+
35e−
(7)
Eqs. (8) and (9) show the electrochemical potentials at 25 °C for chalcopyrite and enargite respectively and indicate an inverse relationship between the variation of the electrochemical potential and the pH.
Eh = 0.536 − 0.059pH
Eh = 0.490 + 1.685 ×
(8)
10−3log [HAsO42 −]
+ 6.743 ×
10−3log [AsO42 −]
− pH (9)
5. Conclusions 1. The recovery of enargite is strongly depressed at pH > 9 in the process of flotation using seawater, behavior that can be explained in the first place by the adhesion of the hydrolysis products of magnesium on the enargite surface. The change in the enargite zeta potential from negative to positive values observed at pH > 9 in solutions containing ions reveals the interactions of magnesium with enargite particles. 2. The analyses of the experimental data and the Pourbaix diagrams for the As-Ca and As-Mg systems also suggest that the enargite depression in seawater observed at pH > 9 can not only be related to the adsorption of hydroxo complexes of magnesium and calcium, but also to the interaction of these metals with oxidized arsenic species such as arsenate and arsenite. As these compounds are 8
Minerals Engineering 142 (2019) 105897
R. Yepsen, et al.
flotation of heavy metal sulphides. Flotation, A.M. Gaudin Memorial Volume. SMEAIME, New York, USA, pp 509-527. Raposo, J.C., Zuloaga, O., Olazabal, M.A., Madariaga, J.M., 2004. Study of the precipitation equilibria of arsenate anion with calcium and magnesium in sodium perchlorate at 25°C. Appl. Geochem. 19 (6), 855–862. Rebolledo, E., Laskowski, J.S., Gutierrez, L., Castro, S., 2017. Use of dispersants in flotation of molybdenite in seawater. Miner. Eng. 100, 71–74. Sasaki, K., Takatsugi, K., Ishikura, K., Hirajima, T., 2010. Spectroscopic study on oxidative dissolution of chalcopyrite, enargite and tennantite at different pH values. Hydrometallurgy 100 (3–4), 144–151. Smart, R.S.C., 1991. Surface layers in base metal sulphide flotation. Miner. Eng. 4 (7–11), 891–909.
depressant solutions. Miner. Eng. 71, 180–187. Plackowski, C., Nguyen, A.V., Bruckard, W.J., 2012. A critical review of surface properties and selective flotation of enargite in sulphide systems. Miner. Eng. 30, 1–11. Ramirez, A., Rojas, A., Gutierrez, L., Laskowski, J.S., 2018. Sodium hexametaphosphate and sodium silicate as dispersants to reduce the negative effect of kaolinite on the flotation of chalcopyrite in seawater. Miner. Eng. 125, 10–14. Rao, M.Y., Natarajan, K.A., 1988. Influence of galvanic interaction between chalcopyrite and some metallic materials on flotation. Miner. Eng. 1 (4), 281–294. Rao, M.Y., Natarajan, K.A., 1989. Electrochemical effects of mineral-mineral interactions on the flotation of chalcopyrite and sphalerite. Int. J. Min. Processing 27 (3–4), 279–293. Rao, S.R., Moon, K.S., Leja, J., 1976. Effect of grinding media on the surface reactions and
9
Contents lists available at ScienceDirect
Minerals Engineering journal homepage: www.elsevier.com/locate/mineng
Flotation behavior of enargite in the process of flotation using seawater Rodrigo Yepsen a b c
a,b
a,⁎
, Leopoldo Gutierrez , Janusz Laskowski
T
c
Department of Metallurgical Engineering, Universidad de Concepcion, Chile Water Research Centre for Agriculture and Mining (CRHIAM), Universidad de Concepcion, Chile N.B. Keevil Institute of Mining Engineering, University of British Columbia, Canada
A R T I C LE I N FO
A B S T R A C T
Keywords: Flotation Enargite Seawater
The presence of enargite (Cu3AsS4) in porphyry copper deposits is an important matter as this mineral shows good flotability in the presence of the thiol collectors used in the flotation copper sulfides thus contaminating the final concentrate. Besides a relevant problem in Chilean mining is the scarcity of water resources so the use of seawater has become a feasible solution. The flotation behavior of copper sulfides and molybdenite in seawater was previously studied but there are no systematic studies related to the flotation of enargite using seawater. The objective of this work is to study the behavior of enargite in the process of flotation using seawater and to compare the flotation of enargite and chalcopyrite in this aqueous medium. The results indicate that the recovery of enargite is strongly depressed at pH > 9 in the process of flotation using seawater, behavior that can be explained in the first place by the adhesion of the hydrolysis products of magnesium on the surfaces of the enargite particles. The analyses of the Pourbaix diagrams for the As-Ca and As-Mg systems also suggest that the enargite depression in seawater observed at pH > 9 can not only be related to the adsorption of hydroxo complexes of magnesium and calcium, but also to the interaction of these metals with oxidized arsenic species such as arsenate and arsenite. As these compounds are hydrophilic in nature they reduce enargite recovery. Pretreatment of seawater before flotation to remove most of the calcium and magnesium present through the precipitation of hydrolyzed complexes of these cations is a suitable way to improve enargite recovery at pH > 9.
1. Introduction Chilean copper deposits have been extensively exploited during the last decades and the ores mined today also contain arsenic-bearing minerals. This makes difficult obtaining copper concentrates of suitable quality for commercialization. An example of these minerals is enargite (Cu3AsS4) which commonly appears in the porphyry copper deposits associated with copper sulfides. One of the complications associated with the presence of enargite in copper ores is that this sulfosalt shows a good flotability in the presence of the thiol collectors used in the flotation copper sulfides thus reporting to the final concentrate causing economical and environmental problems. The behavior of enargite in the flotation process and its relationship with its surface properties has been investigated under collectorless conditions (Kantar, 2002; Plackowski et al., 2012; Fullston et al., 1999a,b; Guo and Yen, 2008). It was found that enargite flotation strongly depends on pH and the pulp redox potential, and that it increases at low pH values and high Eh as a result of the presence of elemental sulfur on the enargite surface. In contrast, at more alkaline conditions a decrease of enargite flotation is observed which was
⁎
correlated with the presence of oxidized copper compounds on the surface. Fullston et al. (1999b) for example concluded from the zeta potential measurements of several sulfides that at oxidizing conditions a layer of copper hydroxide covers a metal-deficient sulfur-rich surface and that it increases at more oxidizing conditions. The following ranking of oxidation was proposed (Fullston et al., 1999b): chalcocite > tennantite > enargite > bornite > covellite > chalcopyrite. Although there are some studies that showed that enargite does not have an isoelectric point in the pH range between 2 and 12 (Castro and Honores, 2000; Castro and Baltierra, 2005), it was also reported that the sign of the enargite zeta potential changes when the ore is conditioned with a highly oxidizing agent such as hydrogen peroxide (Fulston et al., 1999a) which was explained by the process of progressive decomposition of the surface and release of copper, arsenic and sulfates (Pauporté and Schuhmann, 1996 ; Asbjornsson et al., 2004; Guo and Yen, 2008; Sasaki et al., 2010). In the presenece of an ethyl xanthate collector, Kantar (2002) reported that under alkaline conditions and Eh > 0.2 V a two layer of cuprous xanthate (CuX) and dixanthogen is formed on the surface of enargite which explains its hydrophobicity. In contrast, when enargite is floated
Corresponding author. E-mail address: [email protected] (L. Gutierrez).
https://doi.org/10.1016/j.mineng.2019.105897 Received 15 April 2019; Received in revised form 1 July 2019; Accepted 5 August 2019 0892-6875/ © 2019 Published by Elsevier Ltd.
Minerals Engineering 142 (2019) 105897
R. Yepsen, et al.
Germany in all the tests. Seawater was obtained from the coast of Concepcion. Salts of Mg (MgCl2·6H2O), Ca (CaCl2·2H2O), Na and K chlorides, used in the experiments, were obtained from Merk.
at lower Eh the recovery decreases due to the reduction of dixanthogen which reveals that the xanthate dimer is mainly responsible of enargite hydrophobicity (Guo and Yen, 2002). An important issue related to the presence of enargite in copper porphyry ores is its possible selective separation from chalcopyrite through flotation. Such a selective flotation involving the use of selective flotation reagents, selective oxidation and control of pulp potential, while conditioning with reagents such as sodium cyanide and sodium sulphide, has shown limited success (Plackowski et al., 2012). The use of MAA (magnesium-ammonium mixture) to depress enargite has shown varied results, with some studies reporting good selectivity (Castro et al., 2003) and others poor results (Pineda et al., 2015). However, methodological differences may explain these differences. Selective oxidation of the mineral surface to influence collector adsorption using a pre-conditioning step has been shown to be effective in separating enargite from non-arsenic copper sulphides (Guo and Yen, 2005). Typically, hydrogen peroxide or similar oxidizing reagents are used and can be combined with pulp aeration using air or oxygen gas (Kantar, 2002). However, further studies are needed to further develop a process more readily applicable by industry. The use of other potentially more selective flotation collectors different from traditional xanthates deserves investigation. A relevant problem in Chilean mining is the scarcity of water resources to carry out the processing of copper ores, so the use of alternative sources such as seawater has become a feasible solution. The flotation behavior of copper sulfides and molybdenite was studied (Castro and Laskowski, 2011; Castro et al., 2014). The main conclusions of these publications indicate that molybdenite flotation is negatively affected in seawater at pH > 9.5 when lime is used to depress pyrite which is explained by the action of the Mg2+ hydrolysis products, and that copper sulfides are not depressed. The use of dispersants to remove magnesium hydroxides from the molybdenite surface was later tested and positive results were reported by Rebolledo et al. (2017). However, there are no studies related to the flotation of enargite using seawater. The objective of this work is to study the behavior of enargite in the process of flotation using seawater. A comparison of the flotation of enargite and chalcopyrite using seawater is also carried out as the separation of these two minerals by flotation is a true technological challenge facing industry.
2.2. Microflotation tests The procedure of the micro-flotation tests is described in Fig. 2. These experiments were performed in a 150 cm3 Partridge Smith glass cell using nitrogen gas at a flow rate of 90 cm3/min. The flotation feed was wet ground in a laboratory carbon steel ball mill of 7 cm diameter and 12 cm length, charged with 270 g of carbon steel balls (12 balls of 13 mm diameter; 8 balls of 17 mm diameter). For the micro-flotation tests, 3 g of −2 + 1.41 mm enargite and chalcopyrite minerals samples were initially ground to a P80 of 74 µm at a solids content of 25% by weight. Then the pulp was split into 3 samples which were later conditioned during 3 min in 150 mL of solution (e.g., seawater, 0.01 M NaCl solution or treated seawater) to adjust pH to the required condition. After this, 25 ppm of PAX and 30 ppm of MIBC were added and conditioned for additional 3 min. The flotation tests were carried out for 2 min, scraping the froth off every 10 s. The pulp level in the microflotation cell was kept constant by adding a background solution prepared at the same composition and pH of the original solution. The concentrates and tailings were dried in an oven at 105 °C for 5 h and recovery was calculated dividing the mass of mineral in the concentrate by the mass of mineral in concentrate plus the tailings. 2.3. Zeta potential measurements The zeta potential of enargite at different conditions was studied using a Zetacompact Z9000 from CAD instrument. These measurements were performed using enargite particles 100%- 20 µm which were previously wet ground in the ball mill used in the micro-flotation tests. The effect of Mg and Ca ions on the zeta potential of enargite was studied using 0.005 M solutions of these ions dissolved in milli-Q water. 2.4. SEM-EDS Scanning Electron Microscopy with Energy Dispersive X-Ray (SEMEDS) was used to characterize the precipitates formed on the enargite surface. A JEOL JSM-6380 LV device coupled to the Digital MicrographTM software was used. The protocol used to prepare the sample was carried out according to the one used by Smart (1991) and consists of taking a wet ground sample of pulp and drying it with nitrogen into a sample tube for a period of 15 min. Then the sample tube is sealed under a nitrogen atmosphere with a silicone rubber sealant around the cap to prevent oxygen from entering into the tube. Then the sample is frozen for 30 mins and then the dry samples are mounted in a pin stub fixed with a carbon adhesive and covered with gold.
2. Materials and methods 2.1. Samples and reagents The enargite sample was obtained from the Quiruvilca mine in Perú. This sample was initially crushed to a −2 + 1.41 mm size fraction by hand grinding, and then concentrated by hand picking. The chalcopyrite sample used in the study, a high purity sample obtained from Ward‘s Natural Science Establishment, was initially treated following the same procedure. Purification of the crushed samples was carried out by using a magnetic separator to remove some magnetic impurities, and by desliming to remove ultra-fines. The mineral composition was analyzed by X-ray diffraction using Bruker® D4 Endeavor equipment, operated with Cu radiation and Ni Kβ radiation filter. XRD analyses indicate that enargite and chalcopyrite samples were 99.8% and 99% purity respectively with minor amounts of tennantite, and chalcopyrite in the case of the enargite sample (Fig. 1a), and low concentrations of pyrite and quartz (Fig. 1b) in the case of the chalcopyrite one. The chemical analysis of the sample gave 34.3% Cu, 30.4% Fe, and 35.2% S for the chalcopyrite sample and 18.7% As, 47.8% Cu, and 32.6% S for the enargite. Potassium amyl xanthate (PAX) obtained from Solvay previously purified with ether and acetone, and methyl isobutyl carbinol (MIBC) provided by Merk were used as collector and frother respectively. MilliQ water of 18.4 MΩ∙cm at 25 °C was used in all the experiments. pH was adjusted using sodium hydroxide of analytical grade from Merk,
3. Results 3.1. Microflotation tests Fig. 3 shows the recovery of enargite as a function of pH using a 0.01 M NaCl solution. The results show that the recovery of enargite remains constant at the whole studied pH range when the process is carried out in water with low ionic strength. Fig. 3 also shows that the increase of pH is associated with lower pulp electrochemical potentials (Eh) and dissolved oxygen values. Fig. 4 shows the recovery of enargite as a function of pH in seawater. It can be seen that at the whole pH range enargite flotation is lower than that observed in the tests carried out using a low ionic strength water. Additionally, at pH values between 8 and 10 the recovery of enargite remains relatively constant, but at pH > 10 a strong enargite depression is observed. Again the increase of pH lowers Eh and amount of dissolved oxygen. 2
Minerals Engineering 142 (2019) 105897
R. Yepsen, et al.
Fig. 1. XRD spectra of enargite (a) and chalcopyrite (b) samples.
Fig. 2. Micro-flotation procedure.
indicates that over the whole pH range chalcopyrite flotation is slightly lower than in the tests with 0.01 M NaCl. As it was observed in the experiments using enargite, the recovery of chalcopyrite also remains relatively constant below pH = 10 and strong depression is observed at more alkaline conditions. Again the increase of pH leads to lower Eh and dissolved oxygen values.
Sodium, potassium, calcium and magnesium ions are important constituents of seawater. To distinguish the influence of these ions on the recovery of enargite, additional micro-flotation tests were carried out using solutions of these elements at their characteristic concentrations in seawater (10,700 ppm sodium, 400 ppm potassium, 400 ppm calcium and 1,300 ppm magnesium). Fig. 5 shows that magnesium is the one that generates the strongest depressing effect on enargite, in particular at pH > 10. There is also a decrease in enargite recovery caused by calcium ions at pH > 10, but less significant than the one generated by magnesium. In both cases, it can be suggested that the hydrolysis products of these two divalent cations play a role in depressing enargite. An objective of this work is to compare the behavior of enargite and chalcopyrite under the conditions of flotation using seawater, and wet grinding. Fig. 6 presents the results of chalcopyrite flotation as a function of pH using a 0.01 M NaCl solution and indicate that chalcopyrite flotation is unaffected over the studied pH range. On the other hand, as observed in the tests with enargite, both the Eh and dissolved oxygen tend to decrease as the pH increases. Fig. 7 shows how pH affects chalcopyrite recovery in seawater. It
3.2. Zeta potential measurements Fig. 8 shows the zeta potential of enargite as a function of pH in Mili-Q water and solutions of 0.005 M of magnesium and calcium. The results obtained in Mili-Q water show that zeta potential values are negative throughout the pH range studied, which is in agreement with previous studies (Fullston et al., 1999a,b; Castro and Honores, 2000; Castro and Baltierra, 2005). The presence of magnesium and calcium in solution produces an important change in the enargite zeta potential and it is observed that at pH > 9 this mineral acquires a positive surface charge which can be associated with the adsorption of magnesium and calcium hydroxo complexes. 3
Minerals Engineering 142 (2019) 105897
R. Yepsen, et al.
Fig. 3. Enargite recovery as a function of pH in a 0.01 M NaCl solution using 25 ppm PAX and 30 ppm MIBC.
Fig. 4. Enargite recovery as a function of pH in seawater using 25 ppm PAX and 30 ppm MIBC.
3.3. SEM-EDS surface analysis SEM-EDS analyses were performed with the enargite which was conditioned in seawater at pH 9 and 11, and in the absence of PAX. The results obtained when the sample was prepared without a collector at pH 9 indicate similar concentrations of Cu, As, and S to those in untreated enargite. The analyzes of points 1 and 2 (see inserts in Fig. 9) measured at pH = 11 reveal the presence of oxygen, magnesium and calcium, which is explained by the presence of the hydrolyzed compounds of these metals on the enargite. This is directly related to the decrease in the recovery of enargite at pH values higher than 10 observed in Fig. 4. 4. Discussion Previous publications demonstrated that in the flotation of Cu-Mo ores in seawater, molybdenite flotation is strongly depressed by the hydrolysis products of Mg2+ at pH > 9.5, but no effect was reported on Cu recovery at the same pH range (Castro 2012; Castro et al., 2012). A recent publication by Ramirez et al. (2018) reported that in singlemineral flotation tests carried out in seawater chalcopyrite was
Fig. 5. Enargite recovery as a function of pH in solutions of 10,700 ppm sodium, 400 ppm potassium, 400 ppm claicum and 1300 ppm magnesium, and using 25 ppm PAX and 30 ppm MIBC.
4
Minerals Engineering 142 (2019) 105897
R. Yepsen, et al.
Fig. 6. Chalcopyrite recovery as a function of pH in a 0.01 M NaCl solution using 25 ppm PAX and 30 ppm MIBC.
Fig. 7. Chalcopyrite recovery as a function of pH in seawater using 25 ppm PAX and 30 ppm MIBC.
depressed at pH values above 10. The differences observed between both studies were attributed to the conditions at which minerals were ground and it was concluded that redox conditions affects the way in which hydroxo complexes of Mg depress chalcopyrite flotation. The results reported in Figs. 3 and 4 of this work indicate that the recovery of enargite is not affected at alkaline conditions (pH > 9) when the process is carried out in a 0.01 M NaCl solution. However, enargite flotation is strongly depressed at pH > 9 in seawater. The results reported in Fig. 5 show that, as previous publications revealed for molybdenite (Castro 2012; Castro et al., 2012) and for chalcopyrite (Ramirez et al., 2018), the hydroxo complexes of magnesium generated at pH > 9 explain such a depression, and that calcium hydrolysis products affect enargite flotation at pH > 10. These results suggest that coating of enargite by the hydrolysis products of these two divalent cations explain the observed depression. The change in the enargite zeta potential from negative to positive values observed at pH > 9 in Fig. 8 reveals the interactions of magnesium with enargite particles. In adition, SEM-EDS micrographics shown in Fig. 9 reveal the presence of magnesium and calcium on the enargite surface when the sample was prepared in seawater at pH = 11 which indicates that there is a
Fig. 8. Zeta potential of enargite as a function of pH in Mili-Q water and solutions 0.01 M of magnesium and calcium.
5
Minerals Engineering 142 (2019) 105897
R. Yepsen, et al.
Fig. 9. SEM micrographs of enargite conditioned in seawater at pH 9 and 11 without collector.
Fig. 10. Pourbaix diagram for (A) Ca-As system and (b) Mg-As system. Data obtained from HSC V6.00 software.
These authors indicate that species such as Ca4(OH)2(AsO4)2·4H2O and Ca5(OH)(AsO4)3 are likely to be formed at pH values above 10. Raposo et al (2004) proposed that in the case of the interaction of magnesium and calcium with arsenic, there is a distribution of the species that can be formed according to the following scheme: Ca3(AsO4)2·∙10H2O at pH 7–10; CaNaAsO4·∙7.5H2O pH 10–12; MgHAsO4·4H2O pH 6.5–7.1; and Mg3(AsO4)2·8H2O pH 7.2–9.0. Eq. (1) shows the general formula for the chemical reactions between Mg and Ca and AsO43- (Raposo et al., 2004).
precipitation of hydrolized species of calcium and magnesium which renders this mineral more hydrophilic causing a marked decrease in recovery as shown in Fig. 4. It has to be noted that Figs. 5 and 6 show that the behavior of chalcopyrite follow a similar trend to what is observed for enargite, which validates previous results presented by Ramirez et al. (2018) which indicated that in seawater chalcopyrite flotation is also depressed at pH > 9. The results presented in Figs. 3–7 indicate that the recovery of enargite and chalcopyrite at pH > 9 can be mainly explained by the coating by Mg and Ca hydroxo complexes of the mineral surfaces. This has been proposed by several authors to affect the surface chemistry of the flotation system (Fuerstenau et al., 1988; James and Healy, 1972; Laskowski and Castro, 2012). However, alkaline metals also strongly interact with inorganic arsenic. Bothe and Brown (1999) and Brown and Bothe (1999) proposed that different compounds can be formed resulting from the interaction between calcium and arsenic ions and that the extent of these chemical reactions strongly depends on pH.
pM 2 + + qH+ + rAsO43 − ↔ (M 2 +)p (H+)q (AsO43 −)r
(1)
Fig. 10 shows the Pourbaix diagrams for the As-Ca and As-Mg systems, and Fig. 11 shows the distribution of arsenate species in seawater conditions. The diagrams reveal that at the conditions of the microflotation experiments, that is at pH > 9 and Eh between −120 and −130 mV, the species that are likely to be formed on the surface of enargite are Ca(AsO2)2, Ca3(AsO4)2, Mg(AsO2)2 and Mg3(AsO4) 2, all of 6
Minerals Engineering 142 (2019) 105897
R. Yepsen, et al.
Fig. 11. Distribution of arsenate especies in seawater condition for (a) Ca2+ and (b) Mg2+ (Adapted from Raposo et al., 2004).
adsorption of hydroxo complexes of magnesium and calcium, but also to the interaction of these metals with oxidized arsenic species such as arsenate and arsenite. Since the compounds formed as a result of reaction between Mg and Ca and arsenates are in general more insoluble than the compounds formed with arsenites (Carpenter et al., 2014), it can be expected that an increase in Eh could generate the physicochemical conditions to promote the formation of hydrophilic compounds on the enargite surfaces, and therefore a decrease in recovery with respect to flotation performed at 0.01 M NaCl. To verify this idea, micro-flotation tests were carried out at different redox potential conditions in fresh and seawater. In these tests 1 g of dry ground enargite was initially conditioned for 20 min in a beaker with 100 cm3 of fresh or seawater, and at a given redox potential adjusted using hydrogen peroxide or sodium sulfite (Na2SO3). Once redox conditions reached equilibrium, pH was adjusted to 10.5 and PAX and MIBC were added to obtain concentrations of 25 and 30 ppm, respectively. The suspension was then conditioned for additional 3 min after which micro-flotation tests were performed as previously described. Fig. 12 shows the results of enargite recovery as a function of Eh. The first observation is that the natural Eh in these tests was around 350 mV/SHE which is higher than what is observed in Figs. 3 and 4; these were obtained in a more reducing environment (wet grinding in a mill using mild steel balls) while the results shown in Fig. 12 were obtained using enargite which was ground by dry grinding. A second important observation is that as the values of Eh rise from 350 mV to 500 mV, the depressing effect of seawater becomes stronger compared to a 0.01 M NaCl, which can be related to the formation of arsenate (oxidation of As3+ to As5+) and subsequent precipitation with the Mg2+ and Ca2+ ions. When the electrochemical potential rises to values above 500 mV/SHE, the recovery of enargite in both aqueous media decreases at the same rate, most likely due to excessive dissolution of Cu ions and precipitation of these with the collector in homogeneous phase. Since the presence of magnesium and calcium ions and the interactions of these elements with the surface of the enargite cause a depressing effect in this mineral, a possible option to avoid this negative effect is to pre-treat seawater before its use in flotation. Castro (2010) proposed a method that consists of eliminating most of the calcium and magnesium present in seawater through the precipitation of hydrolyzed
Fig. 12. Enargite recovery as a function of Eh using fresh and seawater at pH 9. 25 ppm PAX and 30 ppm MIBC.
Fig. 13. Recovery of enargite as a function of pH in raw and pre-treated seawater. 25 ppm PAX, 30 ppm MIBC.
them of hydrophilic nature. According to this analysis, the enargite depression observed at pH > 9 can not only be related to the 7
Minerals Engineering 142 (2019) 105897
R. Yepsen, et al.
hydrophilic they reduce enargite recovery. 3. Pre-treatment of seawater before flotation to remove most of the calcium and magnesium present through the precipitation of hydrolyzed complexes of these cations is a suitable way to improve enargite recovery at pH > 9.
complexes of these cations, bringing the pH to values of 11 with lime. Fig. 13 shows the recovery of enargite in raw seawater and the pretreated seawater according to the method described by Castro (2010). The results indicate that at pH values of 8 and 9, no differences in recovery are observed in both media, while at pH 11 the recovery of enargite is much greater in the pre-treated seawater. These results reinforce the idea that the hydrolyzed Mg compounds are adsorbed on the surface of enargite and cause a depressing effect. Other trends that are presented in Figs. 3–7 and that deserve to be discussed refer to the inverse relationship between the pH, and the values of Eh and dissolved oxygen. As was previously mentioned the type of grinding media used to reduce the size of particles before flotation strongly affects the redox conditions of the pulp, and as a result, the recovery of sulfides minerals (Huang and Grano, 2006; Peng and Grano, 2010). Previous studies showed that chalcopyrite flotation, for example, is depressed when carbon steel grinding media was used instead of stainless steel balls, which was attributed to the formation of iron hydroxides on the surface of chalcopyrite (Rao et al., 1976). These iron compounds are formed as a result of the anodic oxidation of carbon steel in aqueous media (Rao and Natarajan, 1988). In particular, the grinding of sulfide minerals with carbon steel balls in an aqueous medium causes the release of ferrous ions into solution according to Eq. (2) (Rao and Natarajan, 1989). As Eqs. (3) and (4) indicate ferrous iron under alkaline conditions oxidize to ferric iron and finally form a iron hydroxide. All these reactions release electrons which are captured by oxygen which explains the reduction of oxygen concentration, and reduction of Eh.
Fe 2 + + 2H2 O ↔ Fe (OH )2 + 2H+
Fe (OH )2 +
The authors acknowledge the financial support of CRHIAM sponsored by the CONICYT/FONDAP-15130015 project. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.mineng.2019.105897. References Asbjornsson, J., Kelsall, G.H., Pattrick, R.A.D., Vaughan, D.J., Wincott, P.L., Hope, G.A., 2004. Electrochemical and surface analytical studies of enargite in acid solution. J. Electrochem. Soc. 151 (7), E250–E256. Bothe, J.V., Brown, P.W., 1999. The stabilities of calcium arsenates at 23 °C. J. Hazard. Mater. B69, 197–207. Brown, P.W., Bothe, J.V., 1999. Arsenic inmobilization by calcium arsenate formation. Environ. Sci. Technol. 33, 3806–3811. Carpenter, J.S., Bai, C., Hwang, J.Y., Ikhmayies, S., Li, B., Monteiro, S.N., Peng, Z., 2014. Characterization of Minerals, Metals, and Materials 2014. John Wiley & Sons, New Yersey, USA, pp. 165–184. Castro, S., 2012. Challenges in flotation of Cu-Mo sulfide ores in sea water. In: Proc. 1st Water in Mineral Processing. Sociecty for Mining, Metallurgy and Exploration, Seattle Usa, pp. 29–40. Castro, S., Rioseco, P., Laskowski, J.S., 2012. Depression of molybdenite in sea water. In: Proc. 26th Internacional Mineral Processing Congress IMPC 2012, New Delhi, India, September 24-28, pp. 737–752. Castro, S., Uribe, L., Laskowski, J.S., 2014. Depression of inherently hydrophobic minerals by hydrolysable metal cations: molybdenite depression in seawater. In: 27th Int. Mineral Processing Congress IMPC 2014, Santiago, October 20-24, pp. 207–217. Castro, S., Laskowski, J.S., 2011. Froth flotation in saline water. Kona Powder Part. J. 29, 4–15. Castro, S., 2010. Proceso para pre-tratar agua de mar y otras aguas salinas para su utilización en procesos industriales. Universidad de Concepcion. Chilean Patent, No. 00475/INAPI (May 12, 2010). Castro, S.H., Baltierra, L., Muñoz, P., 2003. Depression of enargite by magnesium-ammonium mixtures. In: In: Proc. Proceedings of Copper 2003, Santiago, Chile, Volume III Mineral Processing, pp. 257–269. Castro, S.H., Baltierra, L., 2005. Study of the surface properties of enargite as a function of pH. Int. J. Miner. Process. 77 (2), 104–115. Castro, S.H., Honores, S., 2000. Surface properties and floatability of enargite. Proc. 21st International Mineral Processing Conference IMPC 2000. pp. B8b–47–B8b-53. Fuerstenau, M.C., Lopez-Valdivieso, A., Fuerstenau, D.W., 1988. Role of hydrolyzed cations in the natural hydrophobicity of talc. Int. J. Miner. Process. 23, 161–170. Fullston, D., Fornasiero, D., Ralston, J., 1999a. Oxidation of synthetic and natural samples of enargite and tennantite: 1. Dissolution and zeta potential study. Langmuir 15 (13), 4524–4529. Fullston, D., Fornasiero, D., Ralston, J., 1999b. Zeta potential study of the oxidation of copper sulfide minerals. Coll. Surf. A: Physicochem. Eng. Aspects 146 (1–3), 113–121. Guo, H., Yen, W.T., 2008. Electrochemical study of synthetic and natural enargites. In: In: Proc. 24th International Mineral Processing Conference IMPC 2008, pp. 1138–1145. Guo, H., Yen, W.T., 2002. Surface potential and wettability of enargite in potassium amyl xanthate solution. Miner. Eng. 15 (6), 405–414. Guo, H., Yen, W.T., 2005. Selective flotation of enargite from chalcopyrite by electrochemical control. Miner. Eng. 18 (6), 605–612. Huang, G., Grano, S., 2006. Galvanic interaction between grinding media and arsenopyrite and its effect on flotation: Part I. quantifying galvanic interaction during grinding. Int. J. Miner. Process. 78 (3), 182–197. James, R.O., Healy, T.W., 1972. Adsorption of hydrolyzable metal ions at the oxide—water interface. III. a thermodynamic model of adsorption. J. Colloid Interface Sci. 40 (1), 65–81. Kantar, C., 2002. Solution and flotation chemistry of enargite. Coll. Surf. A 210 (1), 23–31. Laskowski, J.S., Castro, S., 2012. Hydrolyzing ions in flotation circuits: seawater flotation. In: Proc. 13th International Mineral Processing Symposium, pp. 219–228. Pauporté, T., Schuhmann, D., 1996. An electrochemical study of natural enargite under conditions relating to those used in flotation of sulphide minerals. Coll. Surf.: A Phisicochem. Eng. Asp. A 111 (1–2), 1–19. Peng, Y., Grano, S., 2010. Effect of iron contamination from grinding media on the flotation of sulphide minerals of different particle size. Int. J. Miner. Process. 97 (1–4), 1–6. Pineda, D., Plackowski, C., Nguyen, A.V., 2015. Surface properties of enargite in MAA
(2)
Fe 0 → Fe 2 + + 2e−
OH−
Acknowledgements
↔ Fe (OH )3 +
(3)
e−
(4)
1 O2 + H2 O + 2e− ⇒ 2OH− 2
(5)
Besides, previous studies on the surface behavior of enargite and chalcopyrite describe the electrochemical reactions that take place under alkaline conditions (Eqs. (6) and (7)).
CuFeS2 + 3H2 O ⇒ CuS + Fe (OH )3 + S + 3H+ + 3e−
Cu3 AsS4 + 23H2 O ⇒ 3CuO +
HAsO42 −
+
4SO42 −
+
45H+
(6)
+
35e−
(7)
Eqs. (8) and (9) show the electrochemical potentials at 25 °C for chalcopyrite and enargite respectively and indicate an inverse relationship between the variation of the electrochemical potential and the pH.
Eh = 0.536 − 0.059pH
Eh = 0.490 + 1.685 ×
(8)
10−3log [HAsO42 −]
+ 6.743 ×
10−3log [AsO42 −]
− pH (9)
5. Conclusions 1. The recovery of enargite is strongly depressed at pH > 9 in the process of flotation using seawater, behavior that can be explained in the first place by the adhesion of the hydrolysis products of magnesium on the enargite surface. The change in the enargite zeta potential from negative to positive values observed at pH > 9 in solutions containing ions reveals the interactions of magnesium with enargite particles. 2. The analyses of the experimental data and the Pourbaix diagrams for the As-Ca and As-Mg systems also suggest that the enargite depression in seawater observed at pH > 9 can not only be related to the adsorption of hydroxo complexes of magnesium and calcium, but also to the interaction of these metals with oxidized arsenic species such as arsenate and arsenite. As these compounds are 8
Minerals Engineering 142 (2019) 105897
R. Yepsen, et al.
flotation of heavy metal sulphides. Flotation, A.M. Gaudin Memorial Volume. SMEAIME, New York, USA, pp 509-527. Raposo, J.C., Zuloaga, O., Olazabal, M.A., Madariaga, J.M., 2004. Study of the precipitation equilibria of arsenate anion with calcium and magnesium in sodium perchlorate at 25°C. Appl. Geochem. 19 (6), 855–862. Rebolledo, E., Laskowski, J.S., Gutierrez, L., Castro, S., 2017. Use of dispersants in flotation of molybdenite in seawater. Miner. Eng. 100, 71–74. Sasaki, K., Takatsugi, K., Ishikura, K., Hirajima, T., 2010. Spectroscopic study on oxidative dissolution of chalcopyrite, enargite and tennantite at different pH values. Hydrometallurgy 100 (3–4), 144–151. Smart, R.S.C., 1991. Surface layers in base metal sulphide flotation. Miner. Eng. 4 (7–11), 891–909.
depressant solutions. Miner. Eng. 71, 180–187. Plackowski, C., Nguyen, A.V., Bruckard, W.J., 2012. A critical review of surface properties and selective flotation of enargite in sulphide systems. Miner. Eng. 30, 1–11. Ramirez, A., Rojas, A., Gutierrez, L., Laskowski, J.S., 2018. Sodium hexametaphosphate and sodium silicate as dispersants to reduce the negative effect of kaolinite on the flotation of chalcopyrite in seawater. Miner. Eng. 125, 10–14. Rao, M.Y., Natarajan, K.A., 1988. Influence of galvanic interaction between chalcopyrite and some metallic materials on flotation. Miner. Eng. 1 (4), 281–294. Rao, M.Y., Natarajan, K.A., 1989. Electrochemical effects of mineral-mineral interactions on the flotation of chalcopyrite and sphalerite. Int. J. Min. Processing 27 (3–4), 279–293. Rao, S.R., Moon, K.S., Leja, J., 1976. Effect of grinding media on the surface reactions and
9