2019 年 60 巻 2 号 p. 287-296
The scope of this study was to improve the hydrometallurgical processes involving iodine-iodide leaching and precipitation for recovery of gold from waste printed circuit boards. Firstly, the influence of different precipitating agents, namely ascorbic acid, trisodium citrate and sodium hydroxide on the recovery of gold from gold-iodide leach liquor were investigated in order to define the most effective precipitating agent. The leach liquor was prepared by dissolving pure gold chips in 1:6 molar ratio of iodine-iodide solution at 40°C, 550 rpm for 12 h. The variables, which affect the efficiency of gold precipitation from the leach liquor, were the molar ratio of precipitating agents to gold, pH and redox potential of the solutions. The attained high gold precipitation efficiency from the leach liquor was more than 99% under the highly acidic (pH < 1.6) and alkaline conditions (pH > 13) induced by 0.1 M ascorbic acid and 0.1 M sodium hydroxide respectively, but 64.5% of gold at a weak alkaline condition (pH 8) with 0.1 M trisodium citrate. Secondly, physico-chemical properties of resultant colloidal solutions and prepared gold particles were examined. Finally, recycling of waste printed circuit boards (WPCBs) via iodine-iodide leaching followed by the ascorbic acid reduction was discussed. Results indicate that over 95% of gold extracted from WPCBs by two-step iodine-iodide leaching under the defined conditions, while the dissolution efficiencies of other precious metals (Ag, Pd) and metal impurities (Cu, Al, Fe, Ni, Pb and Zn) were less than 1% and 3%, respectively. The vast majority of Au (99.8%), Cu (95.6%) and Ag (76.8%) were precipitated from the pregnant leach solution by ascorbic acid reduction at ambient conditions.
Schematic diagram for the gold recovery from WPCBs.
In gold hydrometallurgy, cyanide leaching is a conventional approach to recover gold from its ores and concentrates and has been used for over 100 years in gold mines all over the world.1,2) Although this process has several advantages such as cost efficiency, technical effectiveness and suitable gold dissolution rate, its adverse impact on the environment and human health has led to the establishing economic and environmentally viable methods for recovery of gold.3–5) For this reason, a lot of efforts have been aimed to develop the cyanide leaching process6–9) and to replace cyanide with alternative lixiviants such as thiosulfate, thiourea, chlorine, bromine, iodine and ammonia leaching systems in the last three decades.10–12) Wilson (1974); Homick and Slaon (1976) were first proposed to leach gold in iodine and iodide solution.13,14) Taking into account the relatively faster reaction kinetics of gold dissolution in iodine-iodide solutions and stability of gold-iodide complexes in aqueous systems, many studies have been conducted on the dissolution of pure gold in iodine-iodide solutions, and focused to describe the reaction mechanism of gold leaching in the iodine-iodide system.13–18)
| \begin{equation} \text{I$_{\text{2(aq)}}$}+\text{I$^{-}$}\rightarrow\text{I$_{3}{}^{-}$} \end{equation} | (1) |
| \begin{equation} \text{2Au(s)}+\text{I$^{-}$}+\text{I$_{3}{}^{-}$}\rightarrow\text{2AuI$_{2}{}^{-}$ (overall)}\ \mathrm{E}^{0}=-0.042\,\text{V} \end{equation} | (2) |
Therefore, this work is firstly intended to investigate the effectiveness of different precipitating agents, namely ascorbic acid, sodium citrate and sodium hydroxide for the recovery of gold from gold-iodide leach liquor. The effect of the molar ratio of precipitating/reducing agents to gold, the pH and the oxidation-reduction potential (Eh) of the solution on gold precipitation, and characteristics of the resultant colloidal solutions as well as gold particles formed have investigated. Based on the experimental results, gold recovery from waste printed circuit boards (WPCBs) via the defined iodine-iodide leaching followed by a direct precipitation was examined. Influences of metal impurities exist in WPCBs on the iodine-iodide leaching and ascorbic acid precipitation were also discussed.
Commercially available pure gold chips (99.99%, 10 × 10 × 1 mm in size) purchased from the Kojunda Chemical Laboratory, Co. LTD, Japan, and a sample of WPCBs received from DOWA Metals & Mining Company, Japan were used in this study. The pure gold chips were flattened into thin sheets and then cut into small pieces for leaching experiments.
Reagent grade chemicals used to be: iodine, I (Nacalai Tesque, 99.8%), potassium iodide, KI (Nacalai Tesque, 99.5%), trisodium citrate, C6H5O7Na3 (Nacalai Tesque, 99.0%), L-ascorbic acid, C6H8O6 (Wako, 99.6%) and sodium hydroxide, NaOH (Nacalai Tesque, 97.0%). Distilled water purified by an automatic water distillation apparatus, AQUARIUS (Toyo Seisakusho Kaisha, Ltd, Japan) was used to prepare all solutions. 0.1 M ascorbic acid (L-AA) and 0.1 M sodium citrate (Na3Citrate) as weak reducing agents and 0.1 M NaOH as a strong base were prepared and used in a precipitation study.
2.2 Methods 2.2.1 Preparation of a leach liquorA leach liquor of gold-iodide was firstly prepared by dissolving the accurately weighed (0.1555 g) thin pure gold chips in a 0.6 L solution containing 2 g/L iodine and 12 g/L potassium iodide. A 1000 ml baffled volumetric flask containing the solution was immersed in a water bath with the temperature controller. The leaching experiment was conducted with the agitation speed of 550 rpm at 40°C for 12 h.21) The prepared leach liquor, brown in color, was kept in a brown glass bottle at room temperature for further study.
2.2.2 Gold precipitation from the leach liquor using various precipitating agentsThe precipitation experiments were carried out in a conical tube by mixing the leach liquor with solutions of 0.1 M L-AA, 0.1 M Na3Citrate, and 0.1 M NaOH as precipitating agents respectively, for 5 min at ambient temperature (25°C). After precipitation, the solution was centrifuged at 9200 rpm for 5 min using a flexifuge mini centrifuge (Cat. Number C0302, Argos Technologies, Inc) to separate solid and aqueous phases. The solid particles obtained were washed with 5 ml ethanol followed by 5 ml of distilled water and then dried at 70°C for 24 hours.
The pH and oxidation-reduction potential (ORP) of the aqueous solutions were controlled during the precipitation by a pH controller (Nissin, NPH-660) and ORP meter (Nissin, NOR-680D) with Pt counter electrode and Ag/AgCl reference electrode. The measured ORP (Emeasured, mV) values obtained using the commercially available electrode can be converted to standard electrode potential (Eh, mV) by providing the specific conversion factor (Ereference, mV), which is determined using Quinhydrone, MSDS (CAS: 106-34-3) solution:30,31)
| \begin{equation} \mathrm{Eh}=\mathrm{E}_{\text{measured}}+\mathrm{E}_{\text{reference}} \end{equation} | (3) |
The concentrations of gold and other metals in aqueous phases from leaching and precipitation were determined by inductively coupled plasma optical emission spectrometer (ICP-OES, SPS-5500, Seiko Instruments Inc.), and the efficiency (η) of metal precipitation was calculated by a mass balance equation (eq. (4)):
| \begin{equation} \eta=\frac{C_{0}-C_{\textit{Me}}}{C_{0}}\cdot 100\% \end{equation} | (4) |
The properties of gold particles and colloidal solutions after precipitation were measured by a particle size and zeta potential analyzer (Otsuka Electronics, ELSZ-1000ZS). Scanning electron microscopy (SEM) images, energy dispersive spectroscopy (EDS) spectra and electron backscatter diffraction patterns of gold particles were obtained via field emission-scanning electron microscope (FE-SEM/EDS), JSM-7800F.
2.2.3 Gold recovery from WPCBsGenerally, the study of gold recovery from WPCBs consists of 3 stages: 1). to remove metal impurities, the sample of WPCBs was treated with dilute sulphuric acid in an autoclave under high pressure oxidative conditions (1 M H2SO4, 750 rpm at 120°C for 30 min); 2). solid residues obtained was dissolved in an iodine-iodide solution under the conditions at which the leach liquor prepared; 3). gold and accompanying metals present in the 20 ml pregnant leach solution were reduced with various amounts of ascorbic acid ranging from 0.2 to 4 ml at the conditions defined from previous study (Section 2.2.2). The concentrations of elements in the WPCBs sample used to iodine-iodide leaching are shown in Table 1.
The leach liquor prepared by dissolving pure gold chips into an iodine-iodide solution contains 260 mg/L gold that indicates the complete dissolution of the gold chips in the mixture solution. The solution prepared has a pH of 4.4 and standard electrode potential of 0.63 V, respectively.
The Eh-pH diagrams of iodine species in the I−–I2–H2O system and gold species in the Au–I−–I2–H2O system at 25°C within the pH range from 0 to 14 were constructed with the thermodynamic data source using STABCAL software as shown in Figs. 1 and 2.32) The values of the standard free energy of AuI, AuI2−, AuI4−, and Au(OH)(a) species have imported from the literature data and dG Database (Helgeson Sup Crt, Stabcal) in the software components.32,33) The stability regions related to iodine and gold species at equilibrium state have defined by their boundaries of existence in the system and the stability limit of water marked by two dashed lines. Above the upper stability limit of water, iodine and gold-iodide species are oxidized to their oxidation states such as IO3−, IO4−, Au(OH)(a), Au(OH)2−, Au(OH)3 and (Au(OH)52−) with respect to the oxidation of water to oxygen. At the potential range from −0.5 to 0.58 V and broad pH range (pH 0–14) at 25°C, iodine is in the form of iodide ion, I−, whereas the potential range of 0.58–0.76 V and 0.76–1.2 V in the pH value below 8, tri-iodide ion (I3−) and aqueous iodine (I2(a)) appeared in the leaching system. The formation of the dominant iodine species in the iodine-iodide leaching system can be demonstrated by the following reactions:15–20,34,35)
| \begin{equation} \text{3I$^{-}$}\rightarrow\text{I$_{3}{}^{-}$}+\text{2e$^{-}$}\quad\mathrm{E}=0.535\,\text{V} \end{equation} | (5) |
| \begin{equation} \text{2I$_{3}{}^{-}$}\rightarrow\text{3I$_{\text{2(a)}}$}+\text{2e$^{-}$}\quad\mathrm{E}=0.784\,\text{V} \end{equation} | (6) |
| \begin{equation} \text{3I$_{3}{}^{-}$}+\text{6OH$^{-}$}\rightarrow\text{8I$^{-}$}+\text{IO$_{3}{}^{-}$}+\text{3H$_{2}$O}\quad\mathrm{pK}=-28.3 \end{equation} | (7) |
| \begin{equation} \text{IO$_{3}{}^{-}$}+\text{OH$^{-}$}\Leftrightarrow\text{IO$_{4}{}^{-}$}+\text{H$^{+}$}+\text{2e$^{-}$}\quad\mathrm{E}=1.6\,\text{V} \end{equation} | (8) |
| \begin{equation} \text{Au}+\text{2I$^{-}$}\rightarrow\text{AuI$_{2}{}^{-}$}+\text{e$^{-}$}\quad\mathrm{E}=0.578\,\text{V} \end{equation} | (9) |
| \begin{equation} \text{3Au}+\text{4I$_{3}{}^{-}$}\rightarrow\text{3AuI$_{4}{}^{-}$}+\text{e$^{-}$}\quad\mathrm{E}=0.757\,\text{V} \end{equation} | (10) |
| \begin{equation} \text{AuI$_{2}{}^{-}$}\leftrightarrow\text{AuI}+\text{I$^{-}$}\quad\mathrm{K}=6.33 \end{equation} | (11) |
Stability of the predominant species of iodine in the I−–I2–H2O system (STABCAL software). The conditions for the diagram construction were: 2 g/L iodine, 12 g/L potassium iodide at 25°C.
Stability of the predominant species of gold in the Au–I−–I2–H2O system (STABCAL software). The conditions for the diagram construction were: 260 mg/L gold, 2 g/L iodine, 12 g/L potassium iodide at 25°C.
In order to study the effectiveness of the precipitating agents, the stock solution containing 260 mg/L gold was treated with various amounts of 0.1 M L-AA, 0.1 M Na3Citrate, and 0.1 M NaOH, respectively under the same conditions. The results obtained are presented in the following sub-sections.
3.2.1 Gold precipitation under acidic mediumGold precipitation experiments were carried out by varying the molar ratio of L-AA and Au (L-AA:Au ratio) from 0.8 to 15 at 25°C with the agitation speed of 500 rpm for 5 min. The variations of pH value and the efficiency of gold precipitation from the leach liquor as a function of the L-AA:Au molar ratios are plotted in Fig. 3. It can be seen from Fig. 3 that the change in the L-AA:Au molar ratio from 0.8 to 4 led to a decrease in the pH value of the solution from 4.4 to 1.9, and an increase of the gold precipitation up to 20%. A further increase in the L-AA:Au molar ratio up to 8 forwards the gold precipitation to 93% at pH 1.7. The complete precipitation of gold from the leach liquor is accomplished at pH value of 1.6 when the L-AA:Au molar ratio is 15 (Fig. 3). The results showed that the efficiency of gold precipitation from the gold-iodide leach liquor is strongly dependent on changes in solution pH and L-AA:Au molar ratio. The reduction of the Au-I complexes by L-AA in connection with various L-AA:Au molar ratios perhaps attributed to the protonation of the L-AA/H2Asc under different pH ranges.36,37) The mechanism for precipitation of gold from gold-iodide leach liquor in the presence of L-AA/H2Asc is as follows:
| \begin{align} &\text{[AuI$_{2}$]$^{-}$}+\text{2H$_{2}$Asc}\\ &\quad \Rightarrow\text{Au$^{\circ}$}+\text{2HAsc}+\text{2HI}\quad(4.4>\text{pH}>1.7) \end{align} | (12) |
| \begin{align} &\text{[AuI$_{2}$]$^{-}$}+\text{2HAsc}\\ &\quad\Rightarrow\text{Au$^{\circ}$}+\text{2DHA}+\text{2HI}\quad(\text{pH}<1.7) \end{align} | (13) |
| \begin{align} &\text{2[AuI$_{2}$]$^{-}$}+\text{2H$_{2}$Asc}\\ &\quad\Rightarrow\text{2Au$^{\circ}$}+\text{2DHA}+\text{4HI}\quad (\text{overall reaction)} \end{align} | (14) |
The variation of pH value and gold precipitation efficiency as a function of L-AA:Au molar ratio in the leach liquor. Conditions: 260 mg/L gold, 2 g/L iodine, 12 g/L potassium iodide, 10 ml liquor solution, at 500 rpm, 25°C for 5 min.
It has noted that the ascorbic acid is a potential precipitating/reducing agent to produce elemental gold from gold-iodide solutions at strongly acidic conditions (pH ≤ 1.7). The L-AA:Au molar ratio is of vital importance in the complete precipitation of gold from gold-iodide leach solutions.
3.2.2 Gold precipitation under neutral/weak alkaline mediumPrecipitation experiments were conducted using different molar ratios of sodium citrate to Au (Na3Citrate:Au) ranging from 0.8 to 150 at the conditions as mentioned above and the results obtained are summarized in Fig. 4. It has shown that the pH value rose up very rapidly from a pH of 4.4 to 7.3 and then reached a pH of 8, gradually. The pH curve for varying Na3Citrate:Au molar ratios is the mirror image of the curve plotted from the different L-AA:Au molar ratios (Figs. 3 and 4). The efficiency of precipitation of gold from the leach liquor by citrate reduction increases successively with the increase of Na3Citrate:Au molar ratio up to 120 and then becomes nearly constant further. The maximum efficiency of gold precipitation reached 64.5% at pH value of 8 with Na3Citrate:Au molar ratio of 150. It was observed that molar ratio of Na3Citrate:Au ranging from 0.8–120 plays a vital role in the efficiency of gold recovery due to fast precipitation of gold, but that over 120 is a less obvious effect on gold precipitation (Fig. 4). It has appeared that the addition of Na3Citrate to the liquor results in the pH dependent hydroxylation of AuI2− and AuI4− via citrate protonation. The reduction of Au+ and Au3+ to Au0 by citrate reduction is considered as follows:
| \begin{align} \text{2AuI$_{\text{x}}{}^{-}$}+\text{3Na$_{3}$C$_{6}$H$_{5}$O$_{7}$}& =\text{2Au$^{\circ}$}+\text{3Na$_{2}$C$_{5}$H$_{4}$O$_{5}$}+\text{3CO$_{2}$}+{}\\ &\quad+\text{3Na$^{+}$}+\text{3H$^{+}$}+\text{2xI$^{-}$}\\ &\quad(\text{x = 2 & 4}) \end{align} | (15) |
| \begin{equation} \text{AuI$_{4}{}^{-}$}+\text{2I$^{-}$}=\text{AuI$_{2}{}^{-}$}+\text{I$_{3}{}^{-}$}\quad\mathrm{K}=17.7 \end{equation} | (16) |
The variation of pH value and gold precipitation efficiency as a function of Na3Citrate:Au molar ratio in the leach liquor. Conditions: 260 mg/L gold, 2 g/L iodine, 12 g/L potassium iodide, 10 ml liquor solution, at 500 rpm, 25°C for 5 min.
Gold precipitation reaction occurred in the pH range from 4.4 to 13 with the addition of a 0.1 M NaOH solution into the leach liquor under the conditions as stated above. As shown in Fig. 5, no obvious gold precipitation occurred in the pH range between 4.4 and 10 with the addition of NaOH, whereas gold precipitation drastically increased with increasing the pH (pH > 10) of the solution by adding NaOH further. It was observed that the vividness of a color of the liquor is changed from brown to pale yellow in the pH ranges between 4.4 and 10 but the color faded away with a further addition of 0.1 M NaOH solution. The alteration in the oxidation of Au-I complexes (Au+ and Au3+ ions) might be a reason for the change in the color with the addition of NaOH. It is worth mentioning that at pH 8, gold precipitation did not occurred with the addition of NaOH, whereas about 64.5% Au was precipitated in the presence of sodium citrate (Figs. 4 and 5). Therefore, it was possible to suggest that no obvious gold precipitation in the pH levels between 8 and 10 is attributable to formation of hydroxylated gold as shown in eq. (17). However, it can consider that above a pH of 10, NaOH promotes the formation of thermodynamically unstable Au(OH)2− compounds via hydrolysis, and further the compounds undergoing a subsequent hydrolysis to produce elemental gold. (Figs. 2 and 5). Consequently, the maximum gold precipitation efficiency of 99.2% had achieved at pH 13 in the presence of more sodium hydroxide (Fig. 5). The precipitation of gold from the Au-I leach liquor with NaOH can illustrate by the following reactions (eqs. (18)–(20)).33,43–46)
| \begin{equation} \text{[AuI$_{4}$]$^{-}$}+\text{OH$^{-}$}=\text{AuOH$_{\text{(a)}}$}+\text{4I$^{-}$}\quad(\text{pH}=8\text{-}10) \end{equation} | (17) |
| \begin{equation} \text{[AuI$_{4}$]$^{-}$}+\text{2OH$^{-}$}=\text{Au(OH)$_{2}{}^{-}$}+\text{4I$^{-}$}\quad(\text{pH}>10) \end{equation} | (18) |
| \begin{equation} \text{Au(OH)$_{2}{}^{-}$}+2\bar{\text{e}}=\text{Au$^{\circ}$}+\text{2OH$^{-}$}\quad\mathrm{E}^{\circ}=0.40\,\text{V} \end{equation} | (19) |
| \begin{equation} \text{Totally: [AuI$_{4}$]$^{-}$}+3\bar{\text{e}}=\text{Au$^{\circ}$}+\text{4I$^{-}$}\quad\mathrm{E}^{\circ}=+0.56\,\text{V} \end{equation} | (20) |
Effect of sodium hydroxide on gold precipitation from the leach liquor. Conditions: 260 mg/L gold, 2 g/L iodine, 12 g/L potassium iodide, 10 ml liquor solution, at 500 rpm, 25°C for 5 min.
The colloidal solutions obtained from the precipitation using different precipitating agents were characterized by pH meter and ORP meter, respectively. Figure 6 shows the effect of pH controlled by the addition of L-AA, Na3Citrate, and NaOH on the Eh of the resulting colloidal solutions. It can be seen that the value of Eh of the colloidal solutions resulted from precipitation is decreased from 0.63 V to 0.53 V and 0.45 V with the addition of L-AA and NaOH, respectively, whereas the addition of Na3Citrate solution resulted in a shift in value to more positive potentials (up to 0.67 V). The experimentally determined Eh (V) for the formation of elemental Au particles from AuI2−, AuI4− and Au(OH)2− species are in closer agreement with those of Bard et al., 1985 and Gadet et al., 1972 who determined the value for AuI2−, AuI4− and Au(OH)2− of 0.578, 0.56 and 0.40 V, respectively.33,46) However, the value for Au(OH)2− is noticeably lower than that values (0.88 V and 1.0 V) reported in references.45,47) The maximum gold precipitation from the leach liquor was achieved at pH 1.6, 8 and 13 in the presence of L-AA, Na3Citrate, and NaOH, respectively (Figs. 3–5). Furthermore, incomplete precipitation of gold from the solution at neutral and weak alkaline media may be associated with the formation of I3−, which is major ionic species of iodine in the solution to dissolve gold, under oxidation conditions [eqs. (10) and (21)].
| \begin{equation} \text{4H$^{+}$}+\text{3I$^{-}$}+\text{O$_{2}$}=\text{I$_{3}{}^{-}$}+\text{2H$_{2}$O}\quad\mathrm{E}^{\circ}=0.69\,\text{V} \end{equation} | (21) |
Effect of pH on standard electrode potentials of colloidal solutions (initial pH: 4.4, initial Eh: 0.63 V).
The properties of colloidal solutions and gold particles obtained were determined using a particle size and zeta-potential analyzer, and results are summarized in Fig. 7 and Table 2, respectively. Figure 7 shows about 60%, 18% and 11% of total gold particles come from 81 nm, 100 nm, and 6579 nm size particles that formed from the liquor in presence of L-AA at pH 1.6, while at pH 8, 58.6%, 19% and 13% are from the reduction by Na3Citrate. Whereas 33.6%, 24.6% and 24.3% of total gold particles come from 231 nm, 285 nm, and 6579 nm particles that formed with NaOH at pH 13. Cumulative frequency distribution (CFD) plots are represented by the distribution of the particle corresponds to any given percentile (Fig. 7). It was observed that the characteristics of the colloidal solutions with various pH levels are dependent on the precipitation behavior of gold from the gold-iodide leach liquor in the presence of different precipitating agents (Table 2). The pH and Eh are the core values of formation of gold particles with different properties and different morphologies. The Au particles reduced by L-AA, Na3Citrate, and NaOH are negatively charged due to the presence of Asc2− (di-anion of ascorbic acid), Cit3− (citrate) and OH− (hydroxide) ions on the surface of gold particles. Among them, the gold particles formed at pH 13 are more stable than these formed at pH 8 and pH 1.6, respectively due to the presence of more OH− ions. Incipient instability (from ±10 to ±30) of the Au particles induced by L-AA and Na3Citrate are attributed to the change in zeta potential and agglomeration of Au particles at pH of 1.6 and 8, respectively.48,49) As the decrease of the zeta potential (ζ), the electrophoretic mobility (U) decrease because of the directly proportional relationship between these two variables according to the following equation (eq. (22)):50,51)
| \begin{equation} \text{U}=\varepsilon_{r}\varepsilon_{o}\zeta/\eta\quad\text{(Smoluchowski equation)} \end{equation} | (22) |
Distributions of gold particles in the colloidal solutions at different pH conditions (pH: 1.6 (L-AA); pH: 8 (Na3Citrate); pH: 13 (NaOH)).
A decreasing number of the polydispersity index (PDI) from 2.98 to 0.58 indicates the increase in polydispersity distribution of gold particles formed under the studied conditions. The reduction of gold into the solution at pH 1.6 by L-AA results in the increase of PDI due to the agglomeration of gold particles as shown in Appendix figure 1 (a) (Fig. A1(a)). Thus, the size distribution of agglomerated gold particle at pH 1.6 is larger than that distributed in the solution with a pH of 8 and 13, respectively (Figs. A1(b) and A1(c)). It can be seen that the gold particles produced by the addition of NaOH become more monodisperse into the solution at pH 13 (Fig. A1(c)).
The SEM images of gold particles after precipitation. (a) the precipitate formed at pH 1.6 using L-AA; (b) the precipitate formed at pH 8 with Na3Citrate; (c) the precipitate formed at pH 13 in the presence of NaOH, respectively.
Energy dispersive spectroscopy (EDS) analysis identified the strong and weak characteristic peaks for AuMa, AuMb, AuMr, and AuMz, which confirm the formation of elemental gold with the precipitating agents under the conditions (Appendix Figs. A2(a)–(c)). Whereas other peaks detected from the particles after precipitation with Na3Citrate are attributed to Na, K and I which are products from coprecipitation with gold (Figs. A1(b) and A2(b)). The weak or strong peak of CKa detected in EDS spectrum was according to a carbon conductive adhesive tape on a FE-SEM specimen mount.
EDS spectrum for gold particles obtained from the Au-I leach liquor by precipitation. (a) the precipitate formed at pH 1.6 using L-AA; (b) the precipitate formed at pH 8 with Na3Citrate; (c) the precipitate formed at pH 13 in the presence of NaOH, respectively.
The preliminary results suggest that the gold would be efficiently and selectively recovered from raw materials by iodine-iodide leaching and direct precipitation by adding L-AA or NaOH. Therefore, the potential application of iodine-iodide leaching and ascorbic acid reduction for gold recovery from WPCBs is described in the sections below.
3.4.1 Dissolution of gold from WPCBs in an iodine-iodide solutionFigure 8 shows the dissolution efficiencies of metals from WPCBs in an iodine-iodide solution at various solid and liquid (S:L) phase ratios in the range of 1:20–1:5 under the conditions specified above. The dissolution efficiencies of Au, Ag and Pd decreased up to 62.0, 0.6 and 0.4% from 99.0, 3.7 and 1.8% with increasing the S:L phase ratio until 1:5. For metal impurities, their dissolution efficiency at the S:L phase ratio of 1:20 and 1:5 did not exceeded 5% and 2%, respectively. The loss in metals dissolution could be mainly due to the solubility of metals in high S:L phase ratios. To improve metals extraction, the residue obtained from the first-stage iodine-iodide leaching at the S:L phase ratio of 1:5 was dissolved in the iodide-iodide solution under same leaching conditions. Consequently, the efficiency of gold extraction was drastically increased and reached over 95% while the extraction of other metals was somewhat higher than those extracted from the first-stage leaching (Fig. 9). Results also indicate that iodine-iodide leaching is an effective process of recovering gold, because it dissolves gold more preferentially compared to other metals from WPCBs. On the other side, the metal impurities have no potential impact on the dissolution of gold from the WPCBs sample. The concentrations of main metals in the pregnant leach solution prepared by two-stage iodine-iodide leaching at the conditions are showed in Table 3. However the dissolution efficiencies of metal impurities are several ten times lower (less than 3%) than gold dissolution, the proper amounts of these metals exit in the PLS due to their high concentrations in the WPCBs. The pregnant leach solution (PLS) contains 112 mg/L Au, 1.3 mg/L Ag, <1 mg/L Pd and relatively high amounts of other metal impurities like Cu, Al, Fe, Ni, Pb and Zn. The PLS obtained had a pH of 3 and Eh of 0.58 V, and it was used in the subsequent study for separation of metals by ascorbic acid reduction.
The efficiencies of metals dissolution as a function of Solid:Liquid phase ratio (Conditions: 2 g/L iodine, 12 g/L potassium iodide, 1:20, 1:10, 1:5 S:L phase ratios, 500 rpm stirring speed at 40°C for 12 h).
Comparison of the metals dissolution in the iodine-iodide solution from the first and the second stage leaching under same leaching condition (Condition: 2 g/L iodine, 12 g/L potassium iodide, 1:5 S:L phase ratio, 500 rpm stirring speed at 40°C for 12 h).
The dissolved metals especially Au, Ag, Cu, Al, Fe, Ni, Pb and Zn in the PLS are considered in precipitation study, because the concentrations of other metals such as Pd and Co are less than 1 mg/L. The results after precipitation of the metals by adding the varying amounts of 0.1 M L-AA under the conditions explained above are plotted in Fig. 10. Results showed that the precipitation efficiencies of Au, Cu and Ag increased slightly first and then rose intensely with increasing of L-AA dosage from 0.01 to 0.05 and 0.05 to 0.1 ml/ml, respectively. Whereas a further increase in L-AA dosage up to 0.2 ml/ml results in a slight increase in the metals precipitation. The precipitation efficiency achieved with 0.2 ml/ml L-AA was 99.8% for Au, 95.6% for Cu and 76.8% for Ag, respectively. It can be seen that the metal impurities such as Al, Fe, Ni, Pb and Zn apart from Cu were increased continuously as the dose of L-AA increases, however their precipitation were not exceed 20%, respectively. It was estimated that the molar ratio for L-AA/Au was varied from 2 to 35 upon addition of the L-AA in the PLS (Fig. 10). Over 99% gold recovery was obtained from the PLS under the conditions where the L-AA/Au molar ratio was 17, precipitation efficiencies of Cu, Ag and metal impurities were 94%, 76% and <11%, respectively. A comparison of the results on gold precipitation from leach liquor and PLS showed a similar trend in the both solutions with the varying L-AA:Au molar ratios (Figs. 3 & 10). However, a small amount of extra L-AA is necessary for recovering gold completely from the PLS compare to its recovery from the leach liquor. This observation may be associated with the consumption of L-AA for the reduction of metal impurities in the PLS. The trend towards the change in pH of the solutions with L-AA has similar profile, whereas the change in Eh is opposite direction as shown in Figs. 6 & 11, respectively. The difference between the measured Eh values of the leach liquor and the PLS may derived from the reduction property of metal impurities with L-AA and oxidation of L-AA by metal-iodide complexes.52–54)
Precipitation of metals and variation of L-AA:Au molar ratio as a function of L-AA dosage (Conditions: 0.1 M L-AA, 500 rpm stirring speed at 25°C for 10 min).
Variations of pH and Eh values of colloidal solutions as a function of L-AA dosage.
Results suggest that the combination of iodine-iodide leaching and ascorbic acid reduction is an efficient method to achieve the best process outcomes regarding the selective leaching and effective recovery of gold from WPCBs.
The primary aim of this study was to find an efficient method for recovering of gold from WPCBs. To achieve the desired process outcomes, the specific purposes of this study were (1) to investigate the dissolution of gold in the iodine-iodide solution and recovery of gold from the gold-iodide leach liquor via direct precipitation with the addition of L-AA, Na3Citrate and NaOH, respectively, (2) to examine the characteristics and properties of colloidal solutions and gold particles from the precipitation, and (3) to evaluate the potential application of the iodine-iodide leaching and the L-AA reduction for recovery of gold from WPCBs. The main results are summarized as follows:
The authors gratefully acknowledge the financial support from the Japan Society for the Promotion of Science (JSPS) under the Program of Leading Graduate Schools, New Frontier Leader Program for Rare-Metals and Resources, and a research grant, KAKENHI (Grant Number 16H04182) at the Akita University.
