Investigation of Chalcopyrite Leaching Using an Ore-on-a-Chip

Dec 10, 2018 - This paper reports on an ore-on-a-chip that enables efficient investigations of mineral leaching using real ore samples. Here, chalcopy...
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Investigation of Chalcopyrite Leaching using an Ore-on-a-Chip Die Yang, Melissa Kirke, Rong Fan, and Craig Priest Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b04802 • Publication Date (Web): 10 Dec 2018 Downloaded from http://pubs.acs.org on December 16, 2018

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Analytical Chemistry

Investigation of Chalcopyrite Leaching using an Ore-on-a-Chip Die Yang a, Melissa Kirke a,b, Rong Fan a,c and Craig Priest a,d* a Future

Industries Institute, University of South Australia, Mawson Lakes, SA 5095, Australia of Chemical Engineering, University College London, Torrington Place, London WC1E 7 JE, United Kingdom. c Natural and Built Environments Research Centre, School of Natural and Built Environments, University of South Australia, Mawson Lakes, SA 5095, Australia. d School of Engineering, University of South Australia, Mawson Lakes, SA 5095, Australia. b Department

ABSTRACT: This paper reports an ore-on-a-chip that enables efficient investigations of mineral leaching using real ore samples. Here, a chalcopyrite (CuFeS2) ore samples are cut, polished flat, and sealed against a polydimethylsiloxane microchannel. The leach solution is collected for analysis and the ore sample is then recovered for surface analysis. Compared to conventional bulkscale leach tests, the ore-on-a-chip allows for faster, more efficient screening of leach parameters using real ore samples obtained from mine sites. Insight and optimization of leach conditions is demonstrated here for chalcopyrite, which has been extensively studied yet leach performance is still strongly dependent on the origin of the ore. Two grades of chalcopyrite were chosen for this study (moderate and high purity) and the effect of ferric ion concentration and pH was studied on moderate and high purity chalcopyrite ores, respectively. The leach rate of Cu was faster in the presence of ore impurities (moderate grade) compared to the higher purity ore under the same conditions. The results also suggest that Fe is preferentially leached in the early stages to form an iron-deficient sulfide, according to x-ray photoelectron spectroscopy. Longer leach studies (48 h) reported no measurable surface passivation for the conditions studied. The ore-on-a-chip offers a new approach of case specific leach studies, which will enable rapid and tailored optimization of leach strategies for mineral processing.

Chalcopyrite (CuFeS2) is the most abundant source of Cu in the world, accounting for 70% of the Earth’s copper.1 Recovery of Cu is often achieved by pyrometallurgy; however, the continuous depletion of high-grade Cu-bearing mineral ores and potential environmental and economic benefits have increased interest in hydrometallurgical extraction.2-4 Chalcopyrite leaching has not been widely adopted by industry due to the extremely slow leach rate that stems from surface passivation during the leach.1,2,5 Despite intense study, the composition of chalcopyrite passivation layers and the mechanism by which they form remains unclear. The chemical species formed on leached chalcopyrite surface have been reviewed, suggesting that sulfur (𝑆0), metal deficient sulfide (𝑆2𝑛 ― ) and jarosite (KFe3(SO4)2(OH)6) are among passivation candidates.1,6-7 In the active redox region, Fe was selectively leached out prior to Cu, forming a Fedeficient Cu polysulfide layer. Some of the studies reported that copper rich polysulfide 𝐶𝑢𝑆𝑛 (n>2) was the main component in the surface layer resulting from leaching process. The formation of surface passivating layers and consequently decreased leach rate is affected by a number of related factors including pH, temperature, oxidant, redox potential 𝐸ℎ, and external ions in the lixiviant.4,8-9 Specifically, mineral impurities such as pyrite are found to significantly enhance the leach rate of chalcopyrite in some cases.10-11 It is therefore important to understand chalcopyrite dissolution under different reaction conditions as well as the specific surface chemistry associated with chalcopyrite passivation.

This information is prerequisite to elucidating the mechanism of passivation and test strategies to minimize it during chalcopyrite leaching. Unfortunately, chalcopyrite ore varies greatly from site-to-site, within sites, and even within batches of sample, making the challenge even more difficult. It follows that successful leach optimization requires a fast, efficient, and insightful method to screen chalcopyrite leach conditions using real ore samples obtained from mine sites. In addition to their chemical complexity, mined ores are inherently structured, porous, or fractured to create micro/nano-scale environments that are chemically reactive and critical to the effectiveness of leach strategies. During leaching, these small-scale environments may affect flow of the leach solution, enhance surface-to-volume ratios and consequently affect leaching behavior.12 Microscale environments can be created on small ore samples obtained from mine sites to investigate these leach phenomena. Such miniaturized systems can yield valuable insights into the physicochemical process involved in the leach process and will be particularly useful in chalcopyrite leaching, where complex oxidation-reduction (redox) reactions occur. The new approach avoids the pitfalls of sample-tosample variability (less reproducible) or bulk averaging (less insightful) and enables high throughput screening on a simple platform. To date, a few microfluidic systems have been developed for investigating the dissolution (leaching) of metals or displacement of liquid in solid matrix. For example, Song et al.13 visualized calcite dissolution for CO2 capture/storage using a microchannel, showing effects of flow

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(‘wormholing’) and crystal orientation on the dissolution profile. Later, Ciceri and Allanore14 studied leaching kinetics of K+ from a K-feldspar-bearing sample (syenite) in a microchannel to mimic the environments relevant to agronomy. More recently, Gerold et al.15 created a rock-on-achip device that incorporates naturally occurring reservoir rock ore samples for studying oil recovery through direct visualization of the process, based on previous studies on modeled oil reservoirs, i.e. synthetic microfluidic devices16-19. These studies demonstrated several important applications of the rock-on-a-chip approach on the investigation of the dissolution/leaching or liquid displacement taking place in solid matrix at micro- or nano-scale. Our studies have considered leaching of metallic gold in microfluidic channels to mimic the microscale environment found in natural samples.12, 20 Rapid screening of parameters for leach optimization and studying mechanisms and kinetics using minimal reagent and time was demonstrated.12 Here, for the first time, we use an ore-on-a-chip method to the study of chalcopyrite leaching and demonstrate the potential benefit to mineral processing, where leaching is of fundamental importance. In many cases, mineral leaching involves complex redox chemistry that is both poorly understood and extremely sensitive to the specific source of the ore sample. This study (1) elucidates potential benefits of an ore-on-a-chip platform for use in mining, including fast, safe, and sample-specific optimization of leaching conditions and (2) investigates leach phenomena that are difficult to test using bulk analysis, including the role of leach chemistry, ore grade, passivation, and surface chemistry from a very complex ore sample (chalcopyrite). In this study, leaching of chalcopyrite ore takes place in a microfluidic environment (125 µm x 500 µm x 8 mm) created by a polydimethylsiloxane channel. Acidic ferric sulfate is used as the leach solution, for several pHs and Fe3+ concentrations at 70-80 ℃. A demountable chip assembly is demonstrated for rapid process optimization (pH, Fe3+ concentration, leach time, and sample grade) of ore-specific samples and direct correlation between leaching behavior and surface chemistry. The reported experiments showed: (i) preferential leaching of Fe (compared with Cu) from chalcopyrite in both short (4 h) and long (48 h) experiments; (ii) formation of Fe-deficient surface structure; (iii) no evidence of surface passivation; (iv) beneficial ore heterogeneity for leaching, with evidence that Fe and Cu are leached faster from heterogeneous chalcopyrite, for the conditions and samples studied.

EXPERIMENTAL Materials. The ore samples were obtained from Durango, Mexico. A mixed chalcopyrite ore (moderate grade) and a high purity chalcopyrite ore (high grade) were selected for short-time screening tests and long-time experiments, respectively. Due to the irregular shape of the original ores, they were cut into 24 mm x 58 mm x 2 mm and 62 mm x 24 mm x 6 mm blocks respectively and polished on both sides (Adelaide Petrographic Laboratories). EDS analysis showed that the spatial distribution of the mineral grains at randomly selected areas among the different ore blocks were very different. Figure 1 shows typical SEM/EDS images of the moderate-grade (Sample A) and high-grade (Sample B) chalcopyrite, which confirms the presence of chalcopyrite

(CuFeS2), quartz (SiO2), and iron (oxides) as the main mineral constituents in Sample A. Sample B (high purity) is dominated by chalcopyrite, with small amounts of quartz found at selected areas. Both samples are sporadically spotted with zinc sulfide and

Figure 1. SEM/EDS images of the two chalcopyrite samples. Sample A is moderate grade. Sample B is high grade.

Figure 2. Set-up for the ore-on-a-chip experiment, showing chip assembly using mechanical pressure (black clamps) and temperature control (heated sand bed).

contain chalcopyrite as the only copper mineral present according to EDS analysis. Sulfuric acid (98%, AR) and Fe2(SO4)3 (AR) were purchased from Chem-Supply, Australia. Milli-Q water (18 MΩ.cm resistivity) was used to prepare all solutions. Fabrication of Microfluidic Chips. The microfluidic device for the investigation of minerals leaching is assembled by sealing a thin layer of polydimethylsiloxane (PDMS) layer on the polished chalcopyrite block, with channels embedded in the PDMS. The fabrication of the PDMS microchannels was carried out according to a procedure described elsewhere.20 A mass ratio of 10:1 of Sylgard 184 silicone elastomer base and curing agent was mixed thoroughly and poured onto a

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Analytical Chemistry hydrophobized hexamethydisilazane (HMDS) master, i.e. a patterned SU8 photoresist on a silicon wafer. The PDMS was cured at 60 ̊C for 4 h, then peeled off from the silicon master. After which, inlet/outlet ports were cored using a 1.5 mm biopsy punch. The HMDS-coated master with microchannels of 125 µm thickness was prepared according to a procedure described elsewhere.21 The microfluidic device was then constructed by creating a watertight seal between the PDMS layer (containing microchannels) and the flat and polished ore sample via mechanical pressure applied to the ore and plastic top cover (green), see Figure 2. The top cover was structured to allow optical inspection of the channels and connection of the TYGON® tubing. A flow test was conducted with pure water prior to each experiment to check for leakage before introducing the acidic leach solution. Leach Experiment. A syringe pump (KD Scientific) was used to pump the leach solution through capillary tubing (0.5 mm inner and 1.58 mm outer diameter) and the microchannel at 0.5 mL/h. The leach solutions were collected at the outlet in a glass vial over a period of 1 h for each sample. A schematic of the overall experimental setup is given in Figure 2. For each side of polished ore sample, six experiments were possible (in two batches of three, due to the top cover design). This approach ensured that experiments on one type of ore sample could be compared. Screening experiments on Sample A were carried out in triplicate at 70-80 ̊C, i.e. 12 individual experiments. The samples collected at the outlet were analyzed by inductively-coupled plasma mass spectrometry (ICP-MS) (Agilent 8800). The average leach rate during the collection period (1 h for each measurement) was determined from the metal ion concentration in the collected leach solution. Surface Analysis. After leaching, the PDMS layer was easily detached from the ore sample and then rinsed with water, ethanol and isopropanol to remove any remaining leach solution and surface debris. Surface profile analysis was conducted using optical profilometry (Olympus LEXT OLS 500 and Wyko NT9100) and DektakXT stylus profilometry to measure surface roughness, height profiles, and for initial qualitative mineral identification. ZEISS SEM Crossbeam 540 was used to take high resolution images of the ore surfaces, including leached and unleached regions. EDS was used to confirm the identification of the minerals present in SEM imaging. The chemical composition of the leached and unleached ore surfaces were determined by XPS. Spectra were collected using a Kratos AXIS Ultra DLD spectrometer. The x-ray was a monochromatic aluminium x-ray running at 225 W with a characteristic energy of 1486.6 eV. The area of analysis (Iris aperture) was a 0.3 mm x 0.7 mm slot; the analysis depth was approximately 15 nm into the surface of the sample.

dichromate27 with Fe3+ being the most commonly applied2,8,28Here, a pH 1.0 H2SO4 leach solution was prepared with ferric ion concentrations from 2 to 20 mM. Chalcopyrite Sample A was used to examine the effect of Fe3+ concentration on the hourly leach rate over the first 4 h of contact in flow at 70-80 ℃ (Figure 3). ICP-MS revealed that the collected leach solution contained 30.

RESULTS AND DISCUSSION Two parameters are known to play central roles in determining leach rate and surface passivation under acidic leach conditions: pH and Fe3+ concentration. According to the Pourbaix diagram for the CuFeS2-H2O system, a pH lower than 4 and an oxidizing redox potential higher than + 0.4 V is required to dissolve copper from chalcopyrite.2 To achieve these reaction conditions, various oxidants have been adopted as aids for chalcopyrite oxidation, such as, hydrogen peroxide22, 23, oxygen24,25, sodium nitrate26, and potassium

Figure 3. Effect of Fe3+ concentration on chalcopyrite leaching of (A) copper; (B) nickel; (C) zinc and (D) iron, at pH =1 and temperature of 70-80 ̊C on Sample A.

Cu, Ni, Zn, and Fe, accounting for 98% of metal ions measured in the leach solution.

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Figure 3 clearly shows that Fe differs in both the magnitude and time-dependence of leach rate, compared to Cu, Zn, and Ni. Note that the feed Fe3+ was subtracted from the measured concentration in the collected samples to ensure that the reported rates represent leached Fe only. The high concentration of Fe in the leached solution may be due to a combination of effects. Previous studies31-33 suggest that during the early stages of chalcopyrite leaching, Fe is preferentially released to form an intermediate Fe-deficient (and Cu-rich) sulfide structure. This may account for the faster initial leach rates for Fe but does not explain the relatively constant leach rate observed throughout the experiment, compared with the increasing rates for Cu, Ni, and Zn. Alternatively, rapid leaching from other Fe-containing minerals present in Sample A may be responsible. For example, EDS revealed a considerate amount of iron oxides in Sample A. Increasing the concentration of Fe3+ in initial leach solution increased the dissolution rate of Fe from the ore sample as high redox potential favor the dissolution of chalcopyrite as indicated by Eq. (1). For Cu, Ni, and Zn, the leach rate increases significantly with time, indicating a transient surface chemistry with increasing reactivity. As the focus of this paper is chalcopyrite, the following discussion will focus on Cu and Fe leaching. Effect of Fe3+ Concentration. Increasing Fe3+ concentration from 2 to 10 mM significantly enhanced the leach rate of Cu in 4 h (Figure 3A). The maximum Cu leach rate measured at 4 h ranged between 1.9 and 10.6 µmol.m-2.s-1 for 5 – 20 mM Fe3+. The observed Cu leach rate is in the same order of that previously reported for chalcopyrite under similar reaction conditions.16 The differences observed over the 5 – 20 mM Fe3+ range are not significant for Cu, so we must conclude that a threshold concentration of Fe3+ is required for efficient Cu leaching. Nonetheless, the higher Fe leach rate observed for 20 mM Fe3+ does appear to be associated with lower leach rates for Cu, Ni, and Zn, which might have indicated competition between Fe and the other elements present. Effect of pH. pH affects chalcopyrite dissolution under acidic aqueous conditions34 and is more complicated with the addition of Fe3+ to the solution1,2. Briefly, pH can induce hydrolysis and precipitation of ferric salts (precipitation of Fe3+ as jarosite is possible even at pH 0.9)2. A low pH (< 0.5) competition between H+ and Fe3+ at the chalcopyrite surface can cause rapid surface passivation34. To investigate potential passivation over long time-scales, Sample B was subjected to 48 h leaching and samples were collected for ICP-MS analysis. Figure 4A shows that, at a fixed concentration of Fe3+ (5 mM), pH 0.4 gives a much higher leach rate of copper (≈ 0.6 µmol.m-2.s-1) compared to pH 1 (≈ 0.09 µmol.m-2.s-1). This may be due to pH 1 favoring Fe3+ hydrolysis and precipitation, resulting in decreased leach rate of Cu. Similar results were obtained for Zn on Sample B (Figure 4C); however Al leach rate was unaffected by pH (Figure 4B). For pH 0.4 and 1.0, Fe is preferentially leached in Sample B (high grade), consistent with the results for Sample A (moderate grade). However, EDS analysis of Sample B showed that there were no other Fe-containing minerals present (the sample is a high grade and relatively homogeneous chalcopyrite ore). Thus, the results in Figure 4 reflect the specific leach chemistry of chalcopyrite ore. Based on this, it is clear that a significant change in Fe/Cu ratio on the surface of the ore occurs. This was confirmed by XPS

analysis on leached and unleached areas of Sample B (Table 1). Compared with the unleached area, there is a significant change in the ratio of Fe/Cu/S. In particular, less Fe was present than expected for the stoichiometric ratio of chalcopyrite, suggesting the formation of Fe-deficient surface structure after the leaching. The small amount of Zn was completely removed from the surface, consistent with the ICPMS results presented earlier. No Al was detected in the region of interest, despite evidence for Al in the ICP-MS results. This is likely due to some mineral heterogeneity of the ore surface. The XPS results are discussed in the Surface Analysis section below.

Figure 4. Effect of pH on long-time leaching of (A) Cu; (B) Al; (C) Zn and (D) Fe from chalcopyrite Sample B at pH 1 and 0.4 in 5 mM Fe3+ solution at 70-80 ̊C.

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Analytical Chemistry The formation of iron-deficient phases on the mineral surface at the early stage of chalcopyrite leach has been regarded as one of the candidates for the formation of the surface passivating layer;32,35-36 however, both the short leach experiments on Sample A (4 h, Figure 3) and the long leach experiments on Sample B (48 h, Figure 4) do not indicate significant surface passivation. This further validated the conjecture that the formation of Fe-deficient and Cu-rich sulfide intermediates on chalcopyrite surface allow the diffusive transport of reactive species and has no surface passivation effect, as proposed by Nava and González33. Figure 4 shows clearly that pH 1.0 favors leaching of Fe compared to Cu, and vice versa for pH 0.4. Table 1. Near-surface (2) formed in the surface layer for the conditions and samples studied, which was proposed as the main component in the surface layer resulting from the leaching process in previous studies.2 Figure 5D shows typical height profiles at the boundary of the leached and unleached (channel) regions on Sample B for the pH 0.4 and 1.0 experiments. The ore sample is inherently rough despite polishing and, therefore, scans are presented for regions that showed an obvious height step at the

Figure 5. (A) Optical image of the channel; (B) SEM image of selected area in Figure 5A; (C) XPS measurement of the leached and unleached area; and (D) Typical profile image of the channel depth, leached at pH 0.4 on Sample B.

chalcopyrite enhances leaching of Cu from chalcopyrite, despite less available mineral surface area on the target ore.

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Similar results have been reported on pyrite/chalcopyrite mixed ores, where pyrite can act as a catalyst for increasing both the Cu leach rate and yield.10-11 This important role of coexisting minerals illustrates the importance of optimizing leach conditions for a specific ore sample, rather than conducting generic studies for a single mineral type (and is not limited to chalcopyrite). Industry may benefit greatly from testing specific ore samples obtained from mines using this ore-on-a-chip analytical method, with outcomes including fast recovery at higher efficiency and selectivity.

CONCLUSION An ore-on-a-chip microfluidic leaching method is presented as an alternative approach to leach optimization in mineral processing. Chalcopyrite was chosen as an industrially important mineral, with complex leach behavior and surface passivation. The key advantage of this method is the ability to screen parameters on a single ore sample, which avoids the pitfalls of sample-to-sample variability (less reproducible) or bulk averaging (less insightful). For chalcopyrite, where the leach chemistry and rate are not fully understood, the ore-ona-chip approach offers easy correlation between solution (e.g. ICP-MS) and surface (e.g. XPS, EDS) analysis. Critical leach parameters (pH, Fe3+ concentration, leach time, and sample grade) were explored. Preferential leaching of Fe (compared with Cu) from chalcopyrite was observed in both short (4 h) and long (48 h) experiments. However, no clear evidence of surface passivation was observed for the conditions and samples studied. Interestingly, the Cu leach rate was faster on a moderate-grade chalcopyrite ore, when compared to that on a high-grade ore under the same conditions (pH 1; Fe3+ concentration; 5 mM; 70-80 ̊C). This illustrates how sensitive leaching is to the presence of other minerals and impurities, and highlights the potential for the ore-on-a-chip approach to impact industrial process optimization through reduced costs, better efficiencies and recovery, and greater insight into complex leach behavior.

AUTHOR INFORMATION Corresponding Author *Corresponding author. E-mail address: [email protected]

Author Contributions MK, DY and CP designed the experiments. MK and DY conducted the experiments. All authors interpreted the results, prepared the manuscript, and gave approval to the final version of the manuscript.

ACKNOWLEDGMENT The authors thank Dr. Eric Charrault for help with stylus profilometry, Dr. Gujie Qian for helpful discussion on chalcopyrite leaching in acidic ferric sulfate and Chris Bassell for XPS analysis. Support for MK from University of South Australia and University College London is acknowledged. This work was performed in part at the South Australian node of the Australian National Fabrication Facility (ANFF-SA) under the National

Collaborative Research Infrastructure Strategy to provide nanoand microfabrication facilities for Australia’s researchers.

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