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Microfluidic study of sustainable gold leaching using glycine solution
Hydrometallurgy 185 (2019) 186–193
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Microfluidic study of sustainable gold leaching using glycine solution ⁎
T
M.R. Azadi, A. Karrech , M. Elchalakani, M. Attar Department of Civil, Environmental and Mining Engineering, University of Western Australia, 35 Stirling Hwy, Crawley, WA 6009, Australia
A R T I C LE I N FO
A B S T R A C T
Keywords: Microchannel system Glycine Leaching Gold
The purpose of this paper is to investigate the in-situ leaching of gold using a microfluidic system with controlled flow, temperature and fluid composition. The microfluidic system is a microchannel embedding a nano-scale layer of gold over which the fluid can flow and etch (erode) gold at various temperatures and concentrations of lixiviant. The lixiviant in this case is an alkaline glycine system that has been identified as an environmentally friendly substance to dissolve precious metals, it is used to extract gold within the engineered micro-cavity. As the fluid flows in the microchannel, the thickness of the gold layer is monitored to control its reduction with respect to time and space. The experimental conditions were varied to evaluate their effects on the recovery rate. The results show that compared to traditional methods being used in laboratory testing, this technique can provide real time measurements with low consumption of chemicals and the results can be obtained in shorter periods of time.
1. Introduction
geological setting. However, a microchannel is an appropriate means to mimic the process within a single pore space of an established local fracture in a deposit. In hydrometallurgy, leaching is the main stage of mineral processing and often has significant effects on subsequent steps (Crundwell, 2011, 2013). Many studies have been conducted on metal dissolution in order to understand the behaviour of leachates and lixiviants, identify the kinetic parameters, and optimize the overall efficiency of hydrometallurgical operations (Crundwell, 2013). Recently, Kotova et al. (2017) conducted a pioneering experimental study on gold leaching by amonium thiosulfate using microchannels and monitored the etching rate of gold in real time under controlled conditions. In comparison with traditional leaching experiments, this new technique offers a visual and quantitative monitoring in a confined space under controlled laminar flow rates. Kotova et al. (2017) conducted their experiment using a high aspect ratio microchannel to mimic a crack in a mineralised sample. Yang and Priest (2018) introduced a microfluidic device to study thiosulfate leaching of gold by adding temperature screening to determine the activation energy. Their results show the difficulty of leaching above 270 °C due to possible diffusion of chromium adhesion between the gold film and substrate. They reveal that the leaching rate is affected by the stoichiometric ratio of thiosulfate, ammonia, and copper in the solution. In the current paper, we upgrade the design by including a temperature controlled system and we use alkaline glycine solutions instead of amonium thiosulfate. The flow rate can vary based on the joint aperture and the flow is assumed to be analogous and
Microelectronics technology has been developing over > 60 years and has produced valuable devices for a broad range of applications (Moore, 1965; Manz et al., 1990; Henry et al., 2000; Becker and Locascio, 2002; Temiz et al., 2015). In particular, microfluidic systems have been instrumental in biochemistry and microbiology and have helped characterising the chemical and physical mechanisms that occur at the micro-scale (Melin and Quake, 2007; Yeo et al., 2011; Sackmann et al., 2014; Halldorsson et al., 2015). As they can be used to manipulate small amounts of fluids in micro-scale channels (Whitesides, 2006), microfluidic systems have been used to extract some metals such as Ag (Nagai et al., 2009), Cu/Cr (Priest et al., 2011, 2012, 2013), Co (Minagawa et al., 2001), Pt (Kriel et al., 2015), Pd (Yin et al., 2013), and rare earth elements (Kolar et al., 2016; Chen et al., 2017). In recent years, in-situ leaching has emerged as a viable solution to extract various commodities including precious metals and started attracting a renewed interest Karrech et al. (2018). Understanding the process of in-situ leaching requires advanced tools of thermal-hydraulic-mechanical-chemical coupling within the principles of nonequilibrium thermodynamics (Poulet et al., 2010, 2012; Karrech et al., 2012; Karrech, 2013; Karrech et al., 2015) and mineralisation (Zhang et al., 2012, 2013). Such formulations require accurate description of leaching locally at the pore scale. To avoid ambiguity, it is important to note that the microfluidic system used in this paper is not to simulate the injection and extraction process per se as it happens in large scale
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Corresponding author. E-mail address: [email protected] (A. Karrech).
https://doi.org/10.1016/j.hydromet.2019.02.009 Received 18 September 2018; Received in revised form 20 December 2018; Accepted 15 February 2019 Available online 25 February 2019 0304-386X/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 2. Experimental set-up: alkaline glycine solution injected through a microchannel to leach gold. The thickness of the gold layer is monitored by a camera from the top while a white back LED backlight is used to illuminate the device.
Fig. 1. Schematic process of microchannel parapration steps. Oxygen plasma machine was used to etch the PMMA substrate to make the channel. Photoresist was used to protect the surface of PMMA rather than the channel area. Au (60 nm) was caoted on the channel using thermal evaporation techniques. The UV glue was applied to bond two substrates.
Fig. 3. Calibration curve of gold thickness versus luminosity.
pH and temperature. Table 1 The list of lixiviant samples used in the experiment at different temperature, pH, glycine and H2O2 concentrations. Lixiviant no.
Glycine (M)
H2O2 (%)
pH
Temperature ( ± 2 °C)
Flow rate (ml/h)
Sample Sample Sample Sample Sample Sample Sample Sample Sample Sample
1.0 1.5 1.0 0.5 1.0 1.0 1.2 0.1 0.5 0.5
0.56 0.56 0.56 0.56 0.56 0.40 0.40 0.40 0.40 0.40
11 11 12 10 9 12 12 12 12 12
60 60 60 60 60 75 75 75 75 70
7.0 7.0 7.0 7.0 7.0 7.0 7.0 7.0 7.0 7.0
1 2 3 4 5 6 7 8 9 10
2. Materials and methods 2.1. Microchannel preparation The microchannel preparation has been conducted at the Western Australian Centre for Semiconductor Optoelectronics and Microsystems (WACSOM) at The University of Western Australia. The WACSOM fabrication facility is a cleanroom environment that has a low level of environmental pollutants such as dust, airborne microbes, aerosol particles and chemical vapors. In this study, we used a transparent and colourless poly(methyl-methacrylate)(PMMA) material to make the substrate/chip. A groove was included in the chip to represent the microchannel and PMMA cover sheets were used to confine the flow. The PMMA sheets-purchased from Goodfellow (UK)-were cut to fit the chip size (150 × 150 × 2 mm). The nominal working temperature of this PMMA is between 50 and 90 °C, which is within the range of temperatures covered in this experiment. The PMMA has good abrasion and UV resistance and excellent optical clarity. It is inert to the chemicals used in this study and to the heat applied in the preparation process. PMMA characteristics have been examined and discussed in numerous studies (Henry et al., 1999; Henry, 2001; Aquino, 2005; Mathur et al., 2009; Prakash and Kumar, 2015). Inlet and outlet holes (1.6 mm) were drilled in the chip to circulate the fluid. The distance
laminar between two highly smooth parallel plates. However, as real surfaces are not necessarily smooth or parallel enough and they converge abruptly at their ends, turbulent flow may also occur (Gangi, 1978; Brown and Scholz, 1985). In this work, flow is laminar within the ranges of the velocity of fluid through the micro-channel, size of the aperture and physical properties of water-glycine. We present a visual and quantitative analysis of gold leaching by alkaline glycine in confined and high aspect ratio channels under controlled fluid flow rates, 187
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microchannel were checked by profilometer Dektak 150 and high resolution microscope ZYGO 6300. 2.2. Choice of lixiviant In recent years, there has been an increasing interest in studying insitu leaching (ISL) of gold using non-cyianidation system such as iodine (Karrech et al., 2018; Martens et al., 2012; Wu et al., 2012). Finding more environmently friendy chemicals as a replacement for cyanide can make ISL more viable. A considerable number of studies have been conducted on the amino acids (including glycine) and metals reactions (Monk, 1951; Martin et al., 1971; Farooq and Ahmad, 1974; Kiss et al., 1991; Mironov, 2007; Isaeva et al., 2008; Angkawijaya et al., 2012). The effect of pH and temperature on the solubility of gold in amino acids has been examined by Jingrong et al. (1996). It has been shown that the presence of hydrogen peroxide can enhancethe solubility of gold in amino acids (Brown et al., 1982). The effect of adding amino acids to thiosulphate for leaching gold has been evaluated by Feng and Van Deventer (2011). These authors found that gold recovery increases as amino acids are added to thiosulphate solutions. The following equations have been suggested by Aylmore (2005) as an example of gold dissolution reactions where the amino acid glycine is a ligand: Fig. 4. Images of gold layers captured after 0, 20 and 40 mins of the leaching. The points A, B and C show the distance from the injection point. The fluid is injected from left to right with a flow rate 7 ml/h: 1 M glycine, 0.4% H2O2, pH 12, 75 ± 2 °C.
4Au + 8NH2 CH2 COOH + 4NaOH + O2 = 4Na[Au(NH2CH2COO)2] + 6H2 O
(1)
2Au + 4NH2CH2 COOH + 2NaOH + H2 O2 = 2Na[Au(NH2 CH2 COO)2] + 4H2 O
between two holes has been adjusted with respect to the channel size (4 mm × 40 mm). To avoid scratching the surface after removing the plastic cover, a small tong was used to carry the substrate all the time. The substrate was washed with running water, dried by air and checked under microscope for any dirts. Fig. 1 shows a schematic representation of the microchannel preparation process. For the first step of fabrication, the substrate needed to be covered by photoresist to avoid etching outside the channel area. For this purpose, the photoresist AZ 4562 was selected. The substrate was placed in a spinning machine, covered with photoresist and allowed to spin for about 40 s at an angular speed of 2000 rpm. The substrate was then placed on a hot plate (90 °C) for 120 s to bake the layer of photoresist (8.8 μm). Using a mask, only the channel area was exposed to UV light for 1 min and washed up with the developer AZ 340. The substrate was then scanned under the microscope to make sure no photoresist was left in the channel area. To built a channel on the substrate, we used an oxygen plasma machine (Plasma100) and a practical recipe (60% O2 and 5% CF4) to etch the surface of the PMMA substrate (12 μm). Thermal evaporation was used to deposit a layer of 5 nm Cr (for adherence between substrate and gold) and a layer of 60 nm of gold on the channel. Thermal evaporation is a the physical vapor deposition (PVD) technique in which the metal is heated in a vacuum chamber until its surface atoms have sufficient energy to leave the surface. At this point they will traverse the vacuum chamber, at thermal energy (< 1 eV), and coat on the substrate positioned above the chamber. The rest of photoresist then was cleaned up from the substrate with the developer AZ 340. The final step was boding two PMMA substrates together. There are many different methods to bond two PMMA substrates including thermal bonding, chemical bonding using acetone/ ethanol, oxygen plasma activation, pressure bonding and UV assisted bonding (Leonard et al., 1985; Lei et al., 2004; Hsu and Chen, 2007; Harris et al., 2010; Kim et al., 2010; Tran et al., 2013; Yu et al., 2015; Wan et al., 2017). Among the above methods, we used UV assisted glue because it was easy to apply manually. In this method, a small amount of the glue was applied on the surface and carefully spread on the substrate while care was taken to keep the channel clean of glue. In the next step, two substrates were bonded by applying a load of 1 kg while curing by UV exposure. The thickness of the gold layer and the depth of
(2)
Recently, experimental studies have been carried out to investigate the application of glycine as an eco-friendly lixiviant for gold leaching in alkaline environment with the exposure to hydrogen peroxide (Oraby and Eksteen, 2015; Eksteen et al., 2017). The experimental data suggest that glycine is more effective at pH between 10 and 11 and temperature of 60 °C. In this study, we evaluate the leaching of gold using alkaline-glycine. For this purpose, deionized (DI) water and glycine (ReagentPlus, ≥99% HPLC) were used to prepare the solutions. Sodium hydroxide has been used to adjust the pH and hydrogen peroxide has been added to the solution right before exposure with gold. All chemicals have been purchased from Sigma-Altrich. In this experiment, nine different specimens of aqueous glycine (see Table 1) have been examined to evaluate the effect of pH, temperature, and glycine concentration on the leaching process. All sample were prepared using a beaker with a magnetic stirrer on a hot plate. The stirring process lasted for at least 1 h until a clear solution was obtained. Temperature and pH were regularly checked until stable conditions were reached. 2.3. Experiment set-up Fig. 2 is a schematic representation of the experiment in which the aqueous glycine is injected at a rate of 7 ml/h through the microchannel (4 mm × 50 mm × 12 μm) to leach gold from the 60 nm gold layer coated in the channel. To keep the temperature constant over the experiment duration, a heating pad was used and temperature was controlled by an infrared thermometer during the experiment. A white LED backlight was used to illuminate the device and the thickness of the gold layer was monitored using a microscope camera (Digitech) that captures the etching process in real time. Video files were recorded using the MicroCapture software through a 4 mm × 30 mm window. 2.4. Method of analysis The thickness of gold layer was monitored by a camera recording video files from the top of microchannel. The video files were analyzed using video Tracker 5.0.5 in which the brightness (luma) of a point can 188
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Fig. 5. Gold thickness versus time at different conditions of temperature and glycine concentration. The points A, B and C shows the distance from the injection point through the microchannel, respectively.
leaching. Fig. 5 shows the experimental results that were obtained under different leaching conditions. Samples 4, 5 and 8 were less affected after 12 h (no changes were observed on the gold thickness to be analyzed). Samples 6 and 7 had the most significant effects on the leaching process; the gold layer thicknesses decreased to zero after 30 mins. In the cases of samples 1, 3 and 9 the gold layer thicknesses reduced to zero after 12 h. As can be seen in Fig. 5, the leaching process was more effective at point A that is spatially closer to the injection point. This could be attributed to the decrease of flow velocity due to friction near the gold surface and towards the end of the channel. Since the regime of the flow is laminar (the channel is of small size), the speed profile in the confined channel is parabolic and the flow speed is maximum in the middle and minimum at the boundaries. Sample 2 also shows a low leaching rate, although glycine concentration is as high as 1.5 M. This may be explained by the passivated layers of gold
be tracked over a period of time. Samples with known gold layers thicknesses were prepared and used to calibrate the thickness-luminosity relationship as shown in Fig. 3. A thickness of 60 nm was selected because it represents the opacity threshold as suggested by Kotova et al. (2017). Image analysis was conducted on the three different points, A, B and C shown in Fig. 4 to see if the distance from the injection point has an effect on the leaching process. Additional images were processed to analyse the whole area of the micorchannel and examine the fingering patterns that are produced according to the fluid flow pathways. 3. Results and discussions The experiment was conducted with 10 samples (see Table 1) to check the effect of temperature, pH and glycine concentration on gold 189
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dissolution and/or polymerization of glycine which increases the viscosity of solution. Fig. 6a shows the white particles appeared as dark stains on the channel. Fig. 6b shows a microscopic (40×) images of possible polymerization of glycine extracted from the pregnant solution in sample 2. 3.1. Effect of pH Glycine complexes variably with metals as a function of pH (Kiss et al., 1991). In aqueous solutions containing amino acids such as glycine, increasing the pH increases the solubility of gold (Brown et al., 1982; Jingrong et al., 1996; Oraby and Eksteen, 2015). By comparing the results in samples 1, 3 and 5, it can be seen from Fig. 6 that increasing pH increases the leaching rate. Fig. 7 shows the graph of gold thickness versus time; the gold layer thickness decreases in less time as pH increases. It can be seen that pH beyond 11 can be more effective for alkaline glycine lixiviant. 3.2. Effect of temperature Fig. 6. (a) White particles in the microchannel in sample 2. (b) Possible polymerization of glycine within the pregnant solution.
As can be seen from Fig. 5, sample 6 and 7 show dramatic increases in the rate of gold dissolution at pH 12 and temperature 75 ± 2 °C. Some experimental studies on the solubility of gold in amino acid solutions at different temperatures have been conducted by Jingrong et al. (1996), which have shown the maximum solubility of gold is produced at 80 °C. The effect of temperature on gold leaching in alkaline glycine solutions have been experimentally studied by Oraby and Eksteen (2015) who conducted beaker-scale experiments over periods of time up to 48 h. Their results indicate that the gold dissolution increases dramatically as temperature increases. Comparing sample 3 and sample 9 (see Fig. 5), it can be seen that the recovery rates are close although the temperatures are from 60 to 75 °C, respectively while the glycine concentrations are 1 M and 0.5 M, respectively. Fig. 8 obtained by comparing the outputs of Samples 3, 6 and 10 shows a dramatical increase of gold recovery by increasing temperature from 60 ± 2 to 75 ± 2 °C. Following Darcy's law, the flow rate is inversely proportional to the viscosity of the liquid. As an example, it has been demonstrated that percolation rate into particles of aqueous leaching solution can be 5 times higher at at 80 °C than at 5 °C (Julian and Smart, 1907; Vizsolyi et al., 1963). It should be mentioned that although the injection rate is fixed at 7 ml/h, the flow rate may be affected by temperature and temporary evaporation.
Fig. 7. Effect of pH on gold dissolution in samples 1, 3 and 5. It shows that increasing pH increases the rate of gold dissolution as the thickness of gold layer is reduced.
3.3. Effect of glycine concentration Our experimental results indicate that increasing the concentration of glycine increases the gold dissolution rate, provided that the concentration remains below the polymerization level that depends on temperature and pH (Sakata et al., 2010). Excessively high glycine concentrations may result in adverse effects because if glycine polymerises, it hinders the flow. In addition, surface passivation may reduce the rate of etching/leaching (Fairley, 1998; Feng and Van Deventer, 2010; Kotova et al., 2017). Note that the solubility of glycine in water can be increased by increasing temperature Budavari et al. (1996). Fig. 9 shows that increasing the glycine concentration up to 1.2 M increases the gold dissolution in samples 6, 7, 8 and 9 at 75 °C (Table 1). However, Fig. 10 shows that increasing the glycine concentration from 1 to 1.5 M (in samples 1 and 2, respectively) reduces the rate of gold dissolution. As mentioned before, high concentration of glycine may cause passivation of the gold surface which can reduce the dissolution rate. 4. Monitoring gold leaching throughout the channel
Fig. 8. Effect of temperature on gold dissolution in samples 3 and 6.
In the previous sections, we show that the microchannel system is instrumental to evaluate the effects of temperature, pH and glycine 190
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Fig. 9. Effect of glycine concentration on gold dissolution in samples 6, 7, 8 and 9. Increasing glycine concentration at temperature 75 ± 2 °C increases the rate of gold dissolution.
Fig. 12. Contour plots of Au thickness across the microchannel at intervals of 5 min in sample 6.
concentration on gold leaching within a confined space that mimics micro-cracks. One of the main advantages of using the michrochannel is the ability to monitor leaching rates during the experiment and the possibility to observe the finger-like pattern that characterise the etching process in real time. Fig. 11 shows the three etch profiles across the microchannel in sample 6. As can be seen, the leaching rate is fully monitored with respect to space and time within the microchannel. The figure also shows that the leaching rate reduces close to the walls because the lowest flow velocity occurs in these areas. The leaching rate
Fig. 10. Effect of glycine concentration on gold dissolution in samples 1 and 2. Inceasing the glycine concentration at temperature 60 ± °C from 1 to 1.5 M decreases the rate of gold dissolution.
Fig. 11. Etch profile of Au thickness versus time in sample 6 at points A, B and C as shown in Fig. 4. 191
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also decreases from profiles A to C (see Fig. 4) because of the distance from injection point. To evaluate the effects of friction on the velocity of flow and consequently on the leaching rate, more profile lines were drawn across the channel for sample 6. By combining the profile lines and measuring the gold thickness at 300 points within the microchannel surface, contour plots of gold thickness were obtained at intervals of 5 min (Fig. 12). It can be seen in Fig. 12 that leaching started in the left area close to the injection points and moved towards the right side. In addition, the results show that leaching intensified in the middle of the microchannel and decreased at the areas close to the lateral sides. Video Tracker 5.0.5 and Surfer® 16 from Golden Software, LLC (www.goldensoftware.com) were employed to obtain the gold thickness across the channel and the contour graphs. Compared to other laboratory tests, this method mimics the in-situ gold etching process in micro-cavities. Yet, the system has certain limitations and can be improved in the future. For example selecting the right material to make the substrates and covering sheets can be critical as some materials can be opaque to the light, react with the lixiviants or sensitive to temperature. Controlling the temperature within a small range throughout the channel can be difficult with the current technology where a single thermocouple is used to measure temperature. In addition, preparing proper microchannels is challenging, expensive, and dependent on the type of metal or elements to be leached.
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5. Conclusion A microfluidic device has been developed to mimic the conditions of in-situ leaching of gold using an alkaline glycine system at various temperatures and lixiviant concentrations. The device builds on an existing device developed by Kotova et al. (2017) where amonium thiosulfate has been used to extract gold at room temperature. The device was instrumental to conduct the experiment and quantitatively monitor the leaching process under controlled conditions at full scale. It has been shown that increasing temperature around 75 °C dramatically increases the leaching rate of gold in alkaline glycine solutions. It has also been noticed that increasing pH to 11 and beyond can increase the leaching rate. Although increasing glycine concentration improves the rate of leaching to some extend, excessively high concentrations of glycine can reduce the recovery. This study shows that microchannel systems have a great potential for future mining and metallurgy studies where in-situ leaching under controlled conditions is relevant. Acknowledgements This project was supported by the Australian Research Council through the Discovery Project DP170104205. This support is highly appreciated. M.R. Azadi would like to acknowledge the financial support provided by Robert and Maude Gledden Postgraduate Scholarships. In addition, the authors thank Dr. Mariusz Martyniuk, Dr. Adrian Keating, Dr. Dhirendra Tripathi, Dr. Xiao Sun, Dr. Hemendra Kala and Dr. Martin Hill for valuable advices and great assistance in making the microchannels. References Angkawijaya, A.E., Fazary, A.E., Ismadji, S., Ju, Y.-H., 2012. Cu (ii), co (ii), and ni (ii)–antioxidative phenolate–glycine peptide systems: an insight into its equilibrium solution study. J. Chem. Eng. Data 57 (12), 3443–3451. Aquino, K.A.D.S., 2005. Estabilizacao radiolica do poli (metacrilato de metila) industrial. Aylmore, M.G., 2005. Alternative lixiviants to cyanide for leaching gold ores. In: Gold Ore Processing, Project Development and Operation, pp. 501–539. Becker, H., Locascio, L.E., 2002. Polymer microfluidic devices. Talanta 56 (2), 267–287. Brown, S.R., Scholz, C.H., 1985. Closure of random elastic surfaces in contact. J. Geophys. Res. 90 (B7), 5531–5545. Brown, D.H., Smith, W.E., Fox, P., Sturrock, R.D., 1982. The reactions of gold (0) with amino acids and the significance of these reactions in the biochemistry of gold. Inorg. Chim. Acta 67, 27–30.
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Microfluidic study of sustainable gold leaching using glycine solution ⁎
T
M.R. Azadi, A. Karrech , M. Elchalakani, M. Attar Department of Civil, Environmental and Mining Engineering, University of Western Australia, 35 Stirling Hwy, Crawley, WA 6009, Australia
A R T I C LE I N FO
A B S T R A C T
Keywords: Microchannel system Glycine Leaching Gold
The purpose of this paper is to investigate the in-situ leaching of gold using a microfluidic system with controlled flow, temperature and fluid composition. The microfluidic system is a microchannel embedding a nano-scale layer of gold over which the fluid can flow and etch (erode) gold at various temperatures and concentrations of lixiviant. The lixiviant in this case is an alkaline glycine system that has been identified as an environmentally friendly substance to dissolve precious metals, it is used to extract gold within the engineered micro-cavity. As the fluid flows in the microchannel, the thickness of the gold layer is monitored to control its reduction with respect to time and space. The experimental conditions were varied to evaluate their effects on the recovery rate. The results show that compared to traditional methods being used in laboratory testing, this technique can provide real time measurements with low consumption of chemicals and the results can be obtained in shorter periods of time.
1. Introduction
geological setting. However, a microchannel is an appropriate means to mimic the process within a single pore space of an established local fracture in a deposit. In hydrometallurgy, leaching is the main stage of mineral processing and often has significant effects on subsequent steps (Crundwell, 2011, 2013). Many studies have been conducted on metal dissolution in order to understand the behaviour of leachates and lixiviants, identify the kinetic parameters, and optimize the overall efficiency of hydrometallurgical operations (Crundwell, 2013). Recently, Kotova et al. (2017) conducted a pioneering experimental study on gold leaching by amonium thiosulfate using microchannels and monitored the etching rate of gold in real time under controlled conditions. In comparison with traditional leaching experiments, this new technique offers a visual and quantitative monitoring in a confined space under controlled laminar flow rates. Kotova et al. (2017) conducted their experiment using a high aspect ratio microchannel to mimic a crack in a mineralised sample. Yang and Priest (2018) introduced a microfluidic device to study thiosulfate leaching of gold by adding temperature screening to determine the activation energy. Their results show the difficulty of leaching above 270 °C due to possible diffusion of chromium adhesion between the gold film and substrate. They reveal that the leaching rate is affected by the stoichiometric ratio of thiosulfate, ammonia, and copper in the solution. In the current paper, we upgrade the design by including a temperature controlled system and we use alkaline glycine solutions instead of amonium thiosulfate. The flow rate can vary based on the joint aperture and the flow is assumed to be analogous and
Microelectronics technology has been developing over > 60 years and has produced valuable devices for a broad range of applications (Moore, 1965; Manz et al., 1990; Henry et al., 2000; Becker and Locascio, 2002; Temiz et al., 2015). In particular, microfluidic systems have been instrumental in biochemistry and microbiology and have helped characterising the chemical and physical mechanisms that occur at the micro-scale (Melin and Quake, 2007; Yeo et al., 2011; Sackmann et al., 2014; Halldorsson et al., 2015). As they can be used to manipulate small amounts of fluids in micro-scale channels (Whitesides, 2006), microfluidic systems have been used to extract some metals such as Ag (Nagai et al., 2009), Cu/Cr (Priest et al., 2011, 2012, 2013), Co (Minagawa et al., 2001), Pt (Kriel et al., 2015), Pd (Yin et al., 2013), and rare earth elements (Kolar et al., 2016; Chen et al., 2017). In recent years, in-situ leaching has emerged as a viable solution to extract various commodities including precious metals and started attracting a renewed interest Karrech et al. (2018). Understanding the process of in-situ leaching requires advanced tools of thermal-hydraulic-mechanical-chemical coupling within the principles of nonequilibrium thermodynamics (Poulet et al., 2010, 2012; Karrech et al., 2012; Karrech, 2013; Karrech et al., 2015) and mineralisation (Zhang et al., 2012, 2013). Such formulations require accurate description of leaching locally at the pore scale. To avoid ambiguity, it is important to note that the microfluidic system used in this paper is not to simulate the injection and extraction process per se as it happens in large scale
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Corresponding author. E-mail address: [email protected] (A. Karrech).
https://doi.org/10.1016/j.hydromet.2019.02.009 Received 18 September 2018; Received in revised form 20 December 2018; Accepted 15 February 2019 Available online 25 February 2019 0304-386X/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 2. Experimental set-up: alkaline glycine solution injected through a microchannel to leach gold. The thickness of the gold layer is monitored by a camera from the top while a white back LED backlight is used to illuminate the device.
Fig. 1. Schematic process of microchannel parapration steps. Oxygen plasma machine was used to etch the PMMA substrate to make the channel. Photoresist was used to protect the surface of PMMA rather than the channel area. Au (60 nm) was caoted on the channel using thermal evaporation techniques. The UV glue was applied to bond two substrates.
Fig. 3. Calibration curve of gold thickness versus luminosity.
pH and temperature. Table 1 The list of lixiviant samples used in the experiment at different temperature, pH, glycine and H2O2 concentrations. Lixiviant no.
Glycine (M)
H2O2 (%)
pH
Temperature ( ± 2 °C)
Flow rate (ml/h)
Sample Sample Sample Sample Sample Sample Sample Sample Sample Sample
1.0 1.5 1.0 0.5 1.0 1.0 1.2 0.1 0.5 0.5
0.56 0.56 0.56 0.56 0.56 0.40 0.40 0.40 0.40 0.40
11 11 12 10 9 12 12 12 12 12
60 60 60 60 60 75 75 75 75 70
7.0 7.0 7.0 7.0 7.0 7.0 7.0 7.0 7.0 7.0
1 2 3 4 5 6 7 8 9 10
2. Materials and methods 2.1. Microchannel preparation The microchannel preparation has been conducted at the Western Australian Centre for Semiconductor Optoelectronics and Microsystems (WACSOM) at The University of Western Australia. The WACSOM fabrication facility is a cleanroom environment that has a low level of environmental pollutants such as dust, airborne microbes, aerosol particles and chemical vapors. In this study, we used a transparent and colourless poly(methyl-methacrylate)(PMMA) material to make the substrate/chip. A groove was included in the chip to represent the microchannel and PMMA cover sheets were used to confine the flow. The PMMA sheets-purchased from Goodfellow (UK)-were cut to fit the chip size (150 × 150 × 2 mm). The nominal working temperature of this PMMA is between 50 and 90 °C, which is within the range of temperatures covered in this experiment. The PMMA has good abrasion and UV resistance and excellent optical clarity. It is inert to the chemicals used in this study and to the heat applied in the preparation process. PMMA characteristics have been examined and discussed in numerous studies (Henry et al., 1999; Henry, 2001; Aquino, 2005; Mathur et al., 2009; Prakash and Kumar, 2015). Inlet and outlet holes (1.6 mm) were drilled in the chip to circulate the fluid. The distance
laminar between two highly smooth parallel plates. However, as real surfaces are not necessarily smooth or parallel enough and they converge abruptly at their ends, turbulent flow may also occur (Gangi, 1978; Brown and Scholz, 1985). In this work, flow is laminar within the ranges of the velocity of fluid through the micro-channel, size of the aperture and physical properties of water-glycine. We present a visual and quantitative analysis of gold leaching by alkaline glycine in confined and high aspect ratio channels under controlled fluid flow rates, 187
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microchannel were checked by profilometer Dektak 150 and high resolution microscope ZYGO 6300. 2.2. Choice of lixiviant In recent years, there has been an increasing interest in studying insitu leaching (ISL) of gold using non-cyianidation system such as iodine (Karrech et al., 2018; Martens et al., 2012; Wu et al., 2012). Finding more environmently friendy chemicals as a replacement for cyanide can make ISL more viable. A considerable number of studies have been conducted on the amino acids (including glycine) and metals reactions (Monk, 1951; Martin et al., 1971; Farooq and Ahmad, 1974; Kiss et al., 1991; Mironov, 2007; Isaeva et al., 2008; Angkawijaya et al., 2012). The effect of pH and temperature on the solubility of gold in amino acids has been examined by Jingrong et al. (1996). It has been shown that the presence of hydrogen peroxide can enhancethe solubility of gold in amino acids (Brown et al., 1982). The effect of adding amino acids to thiosulphate for leaching gold has been evaluated by Feng and Van Deventer (2011). These authors found that gold recovery increases as amino acids are added to thiosulphate solutions. The following equations have been suggested by Aylmore (2005) as an example of gold dissolution reactions where the amino acid glycine is a ligand: Fig. 4. Images of gold layers captured after 0, 20 and 40 mins of the leaching. The points A, B and C show the distance from the injection point. The fluid is injected from left to right with a flow rate 7 ml/h: 1 M glycine, 0.4% H2O2, pH 12, 75 ± 2 °C.
4Au + 8NH2 CH2 COOH + 4NaOH + O2 = 4Na[Au(NH2CH2COO)2] + 6H2 O
(1)
2Au + 4NH2CH2 COOH + 2NaOH + H2 O2 = 2Na[Au(NH2 CH2 COO)2] + 4H2 O
between two holes has been adjusted with respect to the channel size (4 mm × 40 mm). To avoid scratching the surface after removing the plastic cover, a small tong was used to carry the substrate all the time. The substrate was washed with running water, dried by air and checked under microscope for any dirts. Fig. 1 shows a schematic representation of the microchannel preparation process. For the first step of fabrication, the substrate needed to be covered by photoresist to avoid etching outside the channel area. For this purpose, the photoresist AZ 4562 was selected. The substrate was placed in a spinning machine, covered with photoresist and allowed to spin for about 40 s at an angular speed of 2000 rpm. The substrate was then placed on a hot plate (90 °C) for 120 s to bake the layer of photoresist (8.8 μm). Using a mask, only the channel area was exposed to UV light for 1 min and washed up with the developer AZ 340. The substrate was then scanned under the microscope to make sure no photoresist was left in the channel area. To built a channel on the substrate, we used an oxygen plasma machine (Plasma100) and a practical recipe (60% O2 and 5% CF4) to etch the surface of the PMMA substrate (12 μm). Thermal evaporation was used to deposit a layer of 5 nm Cr (for adherence between substrate and gold) and a layer of 60 nm of gold on the channel. Thermal evaporation is a the physical vapor deposition (PVD) technique in which the metal is heated in a vacuum chamber until its surface atoms have sufficient energy to leave the surface. At this point they will traverse the vacuum chamber, at thermal energy (< 1 eV), and coat on the substrate positioned above the chamber. The rest of photoresist then was cleaned up from the substrate with the developer AZ 340. The final step was boding two PMMA substrates together. There are many different methods to bond two PMMA substrates including thermal bonding, chemical bonding using acetone/ ethanol, oxygen plasma activation, pressure bonding and UV assisted bonding (Leonard et al., 1985; Lei et al., 2004; Hsu and Chen, 2007; Harris et al., 2010; Kim et al., 2010; Tran et al., 2013; Yu et al., 2015; Wan et al., 2017). Among the above methods, we used UV assisted glue because it was easy to apply manually. In this method, a small amount of the glue was applied on the surface and carefully spread on the substrate while care was taken to keep the channel clean of glue. In the next step, two substrates were bonded by applying a load of 1 kg while curing by UV exposure. The thickness of the gold layer and the depth of
(2)
Recently, experimental studies have been carried out to investigate the application of glycine as an eco-friendly lixiviant for gold leaching in alkaline environment with the exposure to hydrogen peroxide (Oraby and Eksteen, 2015; Eksteen et al., 2017). The experimental data suggest that glycine is more effective at pH between 10 and 11 and temperature of 60 °C. In this study, we evaluate the leaching of gold using alkaline-glycine. For this purpose, deionized (DI) water and glycine (ReagentPlus, ≥99% HPLC) were used to prepare the solutions. Sodium hydroxide has been used to adjust the pH and hydrogen peroxide has been added to the solution right before exposure with gold. All chemicals have been purchased from Sigma-Altrich. In this experiment, nine different specimens of aqueous glycine (see Table 1) have been examined to evaluate the effect of pH, temperature, and glycine concentration on the leaching process. All sample were prepared using a beaker with a magnetic stirrer on a hot plate. The stirring process lasted for at least 1 h until a clear solution was obtained. Temperature and pH were regularly checked until stable conditions were reached. 2.3. Experiment set-up Fig. 2 is a schematic representation of the experiment in which the aqueous glycine is injected at a rate of 7 ml/h through the microchannel (4 mm × 50 mm × 12 μm) to leach gold from the 60 nm gold layer coated in the channel. To keep the temperature constant over the experiment duration, a heating pad was used and temperature was controlled by an infrared thermometer during the experiment. A white LED backlight was used to illuminate the device and the thickness of the gold layer was monitored using a microscope camera (Digitech) that captures the etching process in real time. Video files were recorded using the MicroCapture software through a 4 mm × 30 mm window. 2.4. Method of analysis The thickness of gold layer was monitored by a camera recording video files from the top of microchannel. The video files were analyzed using video Tracker 5.0.5 in which the brightness (luma) of a point can 188
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Fig. 5. Gold thickness versus time at different conditions of temperature and glycine concentration. The points A, B and C shows the distance from the injection point through the microchannel, respectively.
leaching. Fig. 5 shows the experimental results that were obtained under different leaching conditions. Samples 4, 5 and 8 were less affected after 12 h (no changes were observed on the gold thickness to be analyzed). Samples 6 and 7 had the most significant effects on the leaching process; the gold layer thicknesses decreased to zero after 30 mins. In the cases of samples 1, 3 and 9 the gold layer thicknesses reduced to zero after 12 h. As can be seen in Fig. 5, the leaching process was more effective at point A that is spatially closer to the injection point. This could be attributed to the decrease of flow velocity due to friction near the gold surface and towards the end of the channel. Since the regime of the flow is laminar (the channel is of small size), the speed profile in the confined channel is parabolic and the flow speed is maximum in the middle and minimum at the boundaries. Sample 2 also shows a low leaching rate, although glycine concentration is as high as 1.5 M. This may be explained by the passivated layers of gold
be tracked over a period of time. Samples with known gold layers thicknesses were prepared and used to calibrate the thickness-luminosity relationship as shown in Fig. 3. A thickness of 60 nm was selected because it represents the opacity threshold as suggested by Kotova et al. (2017). Image analysis was conducted on the three different points, A, B and C shown in Fig. 4 to see if the distance from the injection point has an effect on the leaching process. Additional images were processed to analyse the whole area of the micorchannel and examine the fingering patterns that are produced according to the fluid flow pathways. 3. Results and discussions The experiment was conducted with 10 samples (see Table 1) to check the effect of temperature, pH and glycine concentration on gold 189
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dissolution and/or polymerization of glycine which increases the viscosity of solution. Fig. 6a shows the white particles appeared as dark stains on the channel. Fig. 6b shows a microscopic (40×) images of possible polymerization of glycine extracted from the pregnant solution in sample 2. 3.1. Effect of pH Glycine complexes variably with metals as a function of pH (Kiss et al., 1991). In aqueous solutions containing amino acids such as glycine, increasing the pH increases the solubility of gold (Brown et al., 1982; Jingrong et al., 1996; Oraby and Eksteen, 2015). By comparing the results in samples 1, 3 and 5, it can be seen from Fig. 6 that increasing pH increases the leaching rate. Fig. 7 shows the graph of gold thickness versus time; the gold layer thickness decreases in less time as pH increases. It can be seen that pH beyond 11 can be more effective for alkaline glycine lixiviant. 3.2. Effect of temperature Fig. 6. (a) White particles in the microchannel in sample 2. (b) Possible polymerization of glycine within the pregnant solution.
As can be seen from Fig. 5, sample 6 and 7 show dramatic increases in the rate of gold dissolution at pH 12 and temperature 75 ± 2 °C. Some experimental studies on the solubility of gold in amino acid solutions at different temperatures have been conducted by Jingrong et al. (1996), which have shown the maximum solubility of gold is produced at 80 °C. The effect of temperature on gold leaching in alkaline glycine solutions have been experimentally studied by Oraby and Eksteen (2015) who conducted beaker-scale experiments over periods of time up to 48 h. Their results indicate that the gold dissolution increases dramatically as temperature increases. Comparing sample 3 and sample 9 (see Fig. 5), it can be seen that the recovery rates are close although the temperatures are from 60 to 75 °C, respectively while the glycine concentrations are 1 M and 0.5 M, respectively. Fig. 8 obtained by comparing the outputs of Samples 3, 6 and 10 shows a dramatical increase of gold recovery by increasing temperature from 60 ± 2 to 75 ± 2 °C. Following Darcy's law, the flow rate is inversely proportional to the viscosity of the liquid. As an example, it has been demonstrated that percolation rate into particles of aqueous leaching solution can be 5 times higher at at 80 °C than at 5 °C (Julian and Smart, 1907; Vizsolyi et al., 1963). It should be mentioned that although the injection rate is fixed at 7 ml/h, the flow rate may be affected by temperature and temporary evaporation.
Fig. 7. Effect of pH on gold dissolution in samples 1, 3 and 5. It shows that increasing pH increases the rate of gold dissolution as the thickness of gold layer is reduced.
3.3. Effect of glycine concentration Our experimental results indicate that increasing the concentration of glycine increases the gold dissolution rate, provided that the concentration remains below the polymerization level that depends on temperature and pH (Sakata et al., 2010). Excessively high glycine concentrations may result in adverse effects because if glycine polymerises, it hinders the flow. In addition, surface passivation may reduce the rate of etching/leaching (Fairley, 1998; Feng and Van Deventer, 2010; Kotova et al., 2017). Note that the solubility of glycine in water can be increased by increasing temperature Budavari et al. (1996). Fig. 9 shows that increasing the glycine concentration up to 1.2 M increases the gold dissolution in samples 6, 7, 8 and 9 at 75 °C (Table 1). However, Fig. 10 shows that increasing the glycine concentration from 1 to 1.5 M (in samples 1 and 2, respectively) reduces the rate of gold dissolution. As mentioned before, high concentration of glycine may cause passivation of the gold surface which can reduce the dissolution rate. 4. Monitoring gold leaching throughout the channel
Fig. 8. Effect of temperature on gold dissolution in samples 3 and 6.
In the previous sections, we show that the microchannel system is instrumental to evaluate the effects of temperature, pH and glycine 190
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Fig. 9. Effect of glycine concentration on gold dissolution in samples 6, 7, 8 and 9. Increasing glycine concentration at temperature 75 ± 2 °C increases the rate of gold dissolution.
Fig. 12. Contour plots of Au thickness across the microchannel at intervals of 5 min in sample 6.
concentration on gold leaching within a confined space that mimics micro-cracks. One of the main advantages of using the michrochannel is the ability to monitor leaching rates during the experiment and the possibility to observe the finger-like pattern that characterise the etching process in real time. Fig. 11 shows the three etch profiles across the microchannel in sample 6. As can be seen, the leaching rate is fully monitored with respect to space and time within the microchannel. The figure also shows that the leaching rate reduces close to the walls because the lowest flow velocity occurs in these areas. The leaching rate
Fig. 10. Effect of glycine concentration on gold dissolution in samples 1 and 2. Inceasing the glycine concentration at temperature 60 ± °C from 1 to 1.5 M decreases the rate of gold dissolution.
Fig. 11. Etch profile of Au thickness versus time in sample 6 at points A, B and C as shown in Fig. 4. 191
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also decreases from profiles A to C (see Fig. 4) because of the distance from injection point. To evaluate the effects of friction on the velocity of flow and consequently on the leaching rate, more profile lines were drawn across the channel for sample 6. By combining the profile lines and measuring the gold thickness at 300 points within the microchannel surface, contour plots of gold thickness were obtained at intervals of 5 min (Fig. 12). It can be seen in Fig. 12 that leaching started in the left area close to the injection points and moved towards the right side. In addition, the results show that leaching intensified in the middle of the microchannel and decreased at the areas close to the lateral sides. Video Tracker 5.0.5 and Surfer® 16 from Golden Software, LLC (www.goldensoftware.com) were employed to obtain the gold thickness across the channel and the contour graphs. Compared to other laboratory tests, this method mimics the in-situ gold etching process in micro-cavities. Yet, the system has certain limitations and can be improved in the future. For example selecting the right material to make the substrates and covering sheets can be critical as some materials can be opaque to the light, react with the lixiviants or sensitive to temperature. Controlling the temperature within a small range throughout the channel can be difficult with the current technology where a single thermocouple is used to measure temperature. In addition, preparing proper microchannels is challenging, expensive, and dependent on the type of metal or elements to be leached.
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5. Conclusion A microfluidic device has been developed to mimic the conditions of in-situ leaching of gold using an alkaline glycine system at various temperatures and lixiviant concentrations. The device builds on an existing device developed by Kotova et al. (2017) where amonium thiosulfate has been used to extract gold at room temperature. The device was instrumental to conduct the experiment and quantitatively monitor the leaching process under controlled conditions at full scale. It has been shown that increasing temperature around 75 °C dramatically increases the leaching rate of gold in alkaline glycine solutions. It has also been noticed that increasing pH to 11 and beyond can increase the leaching rate. Although increasing glycine concentration improves the rate of leaching to some extend, excessively high concentrations of glycine can reduce the recovery. This study shows that microchannel systems have a great potential for future mining and metallurgy studies where in-situ leaching under controlled conditions is relevant. Acknowledgements This project was supported by the Australian Research Council through the Discovery Project DP170104205. This support is highly appreciated. M.R. Azadi would like to acknowledge the financial support provided by Robert and Maude Gledden Postgraduate Scholarships. In addition, the authors thank Dr. Mariusz Martyniuk, Dr. Adrian Keating, Dr. Dhirendra Tripathi, Dr. Xiao Sun, Dr. Hemendra Kala and Dr. Martin Hill for valuable advices and great assistance in making the microchannels. References Angkawijaya, A.E., Fazary, A.E., Ismadji, S., Ju, Y.-H., 2012. Cu (ii), co (ii), and ni (ii)–antioxidative phenolate–glycine peptide systems: an insight into its equilibrium solution study. J. Chem. Eng. Data 57 (12), 3443–3451. Aquino, K.A.D.S., 2005. Estabilizacao radiolica do poli (metacrilato de metila) industrial. Aylmore, M.G., 2005. Alternative lixiviants to cyanide for leaching gold ores. In: Gold Ore Processing, Project Development and Operation, pp. 501–539. Becker, H., Locascio, L.E., 2002. Polymer microfluidic devices. Talanta 56 (2), 267–287. Brown, S.R., Scholz, C.H., 1985. Closure of random elastic surfaces in contact. J. Geophys. Res. 90 (B7), 5531–5545. Brown, D.H., Smith, W.E., Fox, P., Sturrock, R.D., 1982. The reactions of gold (0) with amino acids and the significance of these reactions in the biochemistry of gold. Inorg. Chim. Acta 67, 27–30.
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