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Certain cyanide leaching tailings in Guizhou contain 1.66 g/t of gold, primarily existing in the forms of exposed gold and sulfide-encapsulated gold. The tailings have a high fineness, with particles of 0.038 mm accounting for over 80%. Given the high slime content of the cyanidation tailings, a sand-slime separation process was employed. Cyclones are used for grading and flotation is performed for the recovery of the underflow and overflow separately. A closed-circuit test with roughing once, scavenging twice, and cleaning three times was conducted, with process parameters and reagent regimes tailored to the characteristics of the underflow and overflow. The sand-slime separation process achieved a concentrate with a gold grade of 12.42 g/t and a gold recovery rate of 50.13%, while the ungraded flotation process of the cyanidation tailings produced a gold concentrate with a grade of 9.49 g/t and a recovery rate of 38. 10%. The results indicate that the sand-slime separation process improved the gold grade by approximately 2.9 g/t and the gold recovery rate by about 12 percentage points, effectively recovering gold from the cyanidation tailings. [ABSTRACT FROM AUTHOR]

Thiosulfate leaching has been regarded as a promising alternative to cyanidation, but it still faces the challenge of the recovery of low content of gold from high concentrations of thiosulfate solutions. Liquid–liquid extraction is a method to address this issue but is still limited by the use of volatile and toxic organic solvents. To overcome this limitation, this work synthesized some functionalized dicationic ionic liquids (DILs) to serve as extraction solvents for the recovery of the gold–thiosulfate complex, [Au(S2O3)2]3−, from thiosulfate solutions. Experimental results indicated that the DILs showed higher extraction rates toward [Au(S2O3)2]3− compared with their monocationic-based counterparts, likely due to the stronger electrostatic interaction between the dications of the ILs and [Au(S2O3)2]3−. The transfer of [Au(S2O3)2]3− from the water phase to the IL phase was identified as an anion exchange and endothermic process. The rate of extraction was limited by the anion exchange process occurring at the IL–water interface. The extraction ability of ILs highly depended on the type of anion; specifically, the ILs with anions that had strong hydrogen-bonding ability exhibited high extraction ability toward [Au(S2O3)2]3−. Finally, DILs proved effective in the recovery of [Au(S2O3)2]3− from an actual gold leaching solution and exhibited high selectivity toward coexisting ions, indicating their potential as environmentally friendly solvents for gold recovery. [ABSTRACT FROM AUTHOR]

Hydrometallurgical gold recovery processes play a pivotal role in the gold mining industry, contributing to more than 90% of global gold production. Among the array of techniques available, the Merrill–Crowe process, adsorption, ion exchange, and solvent extraction are central in extracting gold from leach solutions. While the Merrill–Crowe process and gold complex adsorption onto activated carbon represent historical cornerstones, their inherent limitations have prompted the emergence of more recent innovations in ion exchange and solvent extraction, offering enhanced selectivity, control, and sustainability. The evolution of modern organic chemistry has significantly influenced the progress of ion exchange technology, mainly through the introduction of advanced polymer matrix synthetic resins. At the same time, novel solvents tailored to gold complex interactions have revitalized ion exchange and solvent extraction. Introducing ionic liquids and deep eutectic solvents has also added a new dimension to efforts to improve gold extraction metallurgy. This paper reviews these cutting-edge developments and their potential to revolutionize the hydrometallurgical gold recovery process, addressing the pressing need for improved efficiency and environmental responsibility. [ABSTRACT FROM AUTHOR]

Low- and middle-income countries suffer from excessive accumulation of electronic waste (e-waste), partly owing to a lack of recycling infrastructure and relevant policy frameworks, which can lead to significant toxic exposure of the surrounding populations and the environment. Using Ghana as an example, the complexity of the network of actors involved in the e-waste management value chain is discussed from this perspective. More specifically, this study aims to characterize the structure of the e-waste sector in Accra, Ghana, and investigate approaches to render a more feasible circular economy. An alternative framework for gold recycling is proposed. Emerging environmentally friendly gold recycling technologies are discussed to support the feasibility of this approach. Overall, this would enable circular management of e-waste and improve the health of the informal sector. Based on this analysis, the recovery of gold could ideally generate 450 million USD/year, while significantly reducing the environmental footprint associated with the life cycle of gold by extending its lifetime. Examining the global warming potential (GWP) linked to the production of key materials in e-waste shows that gold exhibits the most significant environmental impact, accounting for 86% of the total GWP of the studied e-waste sample. [ABSTRACT FROM AUTHOR]

Electronic waste grows globally at a rate of 5% annually, which makes electronic waste recycling (WEEE recycling) an urgent task aimed at achieving (i) environmental protection and (ii) the preservation of mineral resources through the re-introduction of strategic metals into the market. As it turns out, WEEE recycling produces further waste, called WEEE secondary waste, which still contains valuable metals such as gold, silver, and copper. This study assessed the economic viability of recovering these metals and identified the most promising targets and approaches. WEEE secondary waste produced at a plant in southern Europe was sampled and examined for this purpose. The study methods included an ANOVA (analysis of variance) and an OCCP (orthogonal central composition plan). Over 90% gold, silver, and copper extraction was achieved with hydrochloric acid leaching in sodium hypochlorite with sodium chlorite as an oxidizing agent at 60 °C. The significance of the variation in the response for each factor, calculated using the Yates algorithm, demonstrates that by excluding sodium chloride and optimizing the doses of hydrochloric acid and sodium hypochlorite, gold can be selectively recovered through the leaching process. The scenario of HCl (2.5M)/NaClO (5%) leaching results in the maximum extraction effect of Au (91.6%) at the lowest concentrations of Ag and Cu (37–44%). [ABSTRACT FROM AUTHOR]

This work investigates the potential of 70% ethanolic leaf extracts of Rubus idaeus L., Cistus ladanifer L. and Erica andevalensis in the metal separation from synthetic unimetallic solutions of different metals and a leachate obtained from the leaching of PCBs. Results from the experiments with unimetallic solutions revealed R. idaeus and E. andevalensis extracts induced separation of more than 95% of the initial Au(III), while C. ladanifer separated ~78% of this metal. Thereafter, application of three plant extracts to real Au bearing leachate obtained from PCBs leaching, revealed about 96, 95 and 90% Au recovery with R. idaeus, C. ladanifer and E. andevalensis extracts, respectively with 15–60% co-removal of Pb and less than 15% of other metals. The reduction of Au(III) ions into Au(0) nanoparticles by R. idaeus extract was confirmed by molecular UV–Visible, and FT-IR analysis showed the involvement of plant secondary metabolites in Au bio-reduction and bio-stabilization. Particles obtained from the application of R. idaeus extract to the leachate were initially analyzed with XRD and results confirmed the presence of Au(0) with contamination of PbSO4, which was completely removed by washing with 1 M HCl. Thereafter, results from STEM-EDS analysis showed the presence of Au particles conjugated with organic material and other metals. Consequently, particles were subjected to another washing step with acetone. Afterwards, STEM-EDS showed pure Au microparticle clusters (~0.8 μm) with flower-shaped or apparently cubic morphologies, and HRSTEM showed the tiny nanoparticles (~20 nm), which form the clusters. [ABSTRACT FROM AUTHOR]

In response to the issue of resource waste caused by the inability to recover and utilize microfine low-grade gold in the sulfur separation operation process of cyanide tailings in a gold production enterprise, a systematic experimental study was conducted. Based on the existing sulfur separation system, cyanide tailings and sulfur concentrate were used as experimental objects to test the gold recovery conditions of the technical scheme, forming a technical scheme for producing high-grade gold and sulfur concentrate and low-grade gold and sulfur concentrate through the enrichment of cyanide tailings by the existing sulfur separation system and further classification with a hydrocyclone. The scheme has the advantages of low cost and easy implementation for industrialization. After on-site transformation, two products were produced: high-grade gold and sulfur concentrate with a gold grade greater than 3.0 g/t and a sulfur grade greater than 46.5 %,and low-grade gold and sulfur concentrate with a sulfur grade greater than 46.5 %,which maximized the economic benefits. [ABSTRACT FROM AUTHOR]

The recovery of gold by adsorption using activated carbon from sodium cyanide and thiourea leached solutions are reported in this study. The leached solutions were obtained under real operating conditions from the beneficiation plant “Paz Borja”, Machala-Ecuador. Calgon Carbon DG-11 6X12 type, widely used in the local metallurgical industry was used as adsorbent material. The operational parameters varied during the adsorption process experiments included the concentration of leaching agent, agitation speed, dose of activated carbon and initial concentration of gold. The control parameters included density, percentage of solid, pH, temperature, and solution potential. The obtained results were adjusted to mass transfer model by diffusion through the interface and the Freundlich model for the equilibrium isotherms. The analysis of the results indicates a higher adsorption rate of the gold di-cyanide complex on activated carbon compared to gold-thiourea complexes.

For over two hundred years, cyanide has served as the primary reagent for gold extraction. However, due to its high toxicity, the use of cyanide poses significant risks. Traditional low-toxicity leaching reagents have limitations that restrict their widespread industrial application, leading to the necessity for the development of new, efficient, and low-toxic gold leaching reagents to support sustainable gold production. In this study, a novel, efficient, and low-toxicity gold extraction reagent was synthesized at high temperatures by combining urea, sodium carbonate, and a specific iron salt. The research delved into the leaching ability of the reagent under different synthesis conditions and examined the generation of free cyanide content as a by-product. Findings indicated that reagents synthesized with either potassium ferrocyanide or potassium ferricyanide displayed comparable leaching capabilities. Reagents synthesized at 800 °C exhibited lower levels of free cyanide ions and reduced toxicity. Additionally, this reagent demonstrated exceptional selectivity for gold, while in minimal dissolution of copper, iron, nickel, lead, and iron from computer central processing unit (CPU) pins. Under optimal conditions, the efficiency of gold extraction from CPU pins reached 94.65%. Hence, this reagent holds significant potential for the low-toxicity extraction of gold from electronic waste or auriferous concentrates. [ABSTRACT FROM AUTHOR]

With the fast generation of electronic waste (e-waste) and the increasing depletion of metal resources, "urban mining" that can selectively recover gold from secondary resources has attracted great interest. Construction of materials with high extraction capacity and satisfying selectivity in complex aqueous-based matrices still remains challenging. Here, a novel metal-organic framework/polymer composite (Fe-BTC/poly(thioctic acid), denoted as Fe-BTC/pTA) has been newly synthesized and applied for selective gold recovery in different matrices (river water, seawater, and leaching solution of e-waste). Benefiting from the high specific surface area and suitable pore sizes as well as the rational design of active sites, the composite exhibits high adsorption capacity (920 mg/g), high removal efficiency (> 99%), fast kinetics (below 0.1 ppb within 10 min), and good applicability in complex matrices, which are better than those of most reported sulfur-containing adsorbents. Solid-state metallic gold with high purity can be effectively enriched due to the high recyclability and long-term stability of the composite. The material after adsorption can be further applied as a heterogeneous catalyst for water remediation due to the in situ generated gold nanoparticles by the redox reaction between Au(III) ions and the S-containing groups in the composites. [ABSTRACT FROM AUTHOR]

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Microbially assisted bio-oxidation of sulfide concentrates in stirred-tank reactors (stirred-tank reactor bio-oxidation (STRB)) and acid pressure oxidation (POX) are widely used to pretreat refractory sulfide concentrates and increase gold extraction via cyanidation. Continuous STRB requires a comparatively long residence time; however, in some cases, it cannot effectively oxidize some sulfide minerals. POX enables oxidation in a short residence time. At the same time, if a processed concentrate contains a large amount of sulfur, it decreases the ratio of the solid mineral phase to liquid (pulp density) during POX and limits its economic attractiveness. In the present work, experiments were performed to investigate the problems associated with both processing methods for refractory sulfide concentrates. The experiments combined both treatments (STRB and POX) based on the example of a pyrite–arsenopyrite gold-bearing concentrate. The gold recovery from the untreated concentrate via cyanidation reached 58%. Continuous STRB for 2, 4, and 6 days oxidized 43, 74, and 79% of the sulfide sulfur (Ss), respectively. The gold recovery rates from the bio-oxidation residues were 68, 82, and 88%, respectively. The pressure oxidation of both the concentrate and STRB residues increased Ss oxidation by 97–99% and gold recovery by 96–97%. For 2 days, STRB decreased the Ss content and increased the possible liquid-to-solid ratio for POX. The combined processes result in a new promising direction because the POX stage allows high gold recovery, whereas combining STRB and POX provides products for further POX in terms of Ss content and increases POX productivity. [ABSTRACT FROM AUTHOR]

Hydrometallurgical gold recovery processes play a pivotal role in the gold mining industry, contributing to more than 90% of global gold production. Among the array of techniques available, the Merrill–Crowe process, adsorption, ion exchange, and solvent extraction are central in extracting gold from leach solutions. While the Merrill–Crowe process and gold complex adsorption onto activated carbon represent historical cornerstones, their inherent limitations have prompted the emergence of more recent innovations in ion exchange and solvent extraction, offering enhanced selectivity, control, and sustainability. The evolution of modern organic chemistry has significantly influenced the progress of ion exchange technology, mainly through the introduction of advanced polymer matrix synthetic resins. At the same time, novel solvents tailored to gold complex interactions have revitalized ion exchange and solvent extraction. Introducing ionic liquids and deep eutectic solvents has also added a new dimension to efforts to improve gold extraction metallurgy. This paper reviews these cutting-edge developments and their potential to revolutionize the hydrometallurgical gold recovery process, addressing the pressing need for improved efficiency and environmental responsibility.

Thiosulfate leaching has been regarded as a promising alternative to cyanidation, but it still faces the challenge of the recovery of low content of gold from high concentrations of thiosulfate solutions. Liquid–liquid extraction is a method to address this issue but is still limited by the use of volatile and toxic organic solvents. To overcome this limitation, this work synthesized some functionalized dicationic ionic liquids (DILs) to serve as extraction solvents for the recovery of the gold–thiosulfate complex, [Au(S2O3)2]3−, from thiosulfate solutions. Experimental results indicated that the DILs showed higher extraction rates toward [Au(S2O3)2]3− compared with their monocationic-based counterparts, likely due to the stronger electrostatic interaction between the dications of the ILs and [Au(S2O3)2]3−. The transfer of [Au(S2O3)2]3− from the water phase to the IL phase was identified as an anion exchange and endothermic process. The rate of extraction was limited by the anion exchange process occurring at the IL–water interface. The extraction ability of ILs highly depended on the type of anion; specifically, the ILs with anions that had strong hydrogen-bonding ability exhibited high extraction ability toward [Au(S2O3)2]3−. Finally, DILs proved effective in the recovery of [Au(S2O3)2]3− from an actual gold leaching solution and exhibited high selectivity toward coexisting ions, indicating their potential as environmentally friendly solvents for gold recovery.

Electronic waste grows globally at a rate of 5% annually, which makes electronic waste recycling (WEEE recycling) an urgent task aimed at achieving (i) environmental protection and (ii) the preservation of mineral resources through the re-introduction of strategic metals into the market. As it turns out, WEEE recycling produces further waste, called WEEE secondary waste, which still contains valuable metals such as gold, silver, and copper. This study assessed the economic viability of recovering these metals and identified the most promising targets and approaches. WEEE secondary waste produced at a plant in southern Europe was sampled and examined for this purpose. The study methods included an ANOVA (analysis of variance) and an OCCP (orthogonal central composition plan). Over 90% gold, silver, and copper extraction was achieved with hydrochloric acid leaching in sodium hypochlorite with sodium chlorite as an oxidizing agent at 60 °C. The significance of the variation in the response for each factor, calculated using the Yates algorithm, demonstrates that by excluding sodium chloride and optimizing the doses of hydrochloric acid and sodium hypochlorite, gold can be selectively recovered through the leaching process. The scenario of HCl (2.5M)/NaClO (5%) leaching results in the maximum extraction effect of Au (91.6%) at the lowest concentrations of Ag and Cu (37–44%).

The adhesive properties of microorganisms on the surface of minerals play an important role in the biooxidation efficiency of sulfidic refractory gold ores. In this research, the simultaneous effects of monosaccharides, ore content, pyrite content, and time on the activity and growth rate of Ferroplasma acidiphilum-from native Acid Mine Drainage (AMD)- was investigated during biooxidization alongside finding the best machine learning approach for the prediction of process efficiency using the independent variables. The results revealed that the optimum condition for reaching the highest pyrite dissolution (∼75 %) is 15 days of operating time, pyrite content of 7.2 wt%, and ore content of 5 wt%, pH of 1.47, and D-+-sucrose, D-+-galactose, and D-+-fructose concentrations of 0.52, 0.09, and 0.12 wt%, respectively. The results of the model comparison indicated that the Artificial Neural Network (ANN) model was able to predict the experimental results of this study with acceptable accuracy and better than Genetic Programming (GP) and Polynomial Regression informed by Response Surface Methodology (PR-RSM) from experimental data. Finally, the results showed that the change in D-+-fructose and D-+-galactose concentration has no significant effect on ferric ions concentration and pyrite dissolution content, while the influence of alteration in D-+-sucrose concentration is significantly high.

Regardless of the effectiveness of the industrial process design for gold recovery encompassing acidic bio-oxidation and cyanidation, the substantial neutralization of effluents required after bio-oxidation results in a considerable generation of waste streams. Conducting a bio-oxidation at circumneutral pH with in-situ neutralization is a novel technological opportunity for strengthening environmental protection and safety. For the first time, this research reports the capacity of Phanerochaete chrysosporium to oxidize a low-grade refractory gold ore at circumneutral pH. The fungal treatment was evaluated in five culture media regarding pH variation, enzyme activity, and sulfide oxidation. Thereafter, optimal conditions were investigated and then scaled up in a bioreactor. Results indicated that pH > 5.0 decreased the enzyme concentration released by P. chrysosporium , reducing sulfide oxidation. An initial pH of 5.8 led to a 24.4 ± 2.2% sulfide oxidation after 14 days, comparatively better to reported studies in the same pH range. A higher efficiency was attained by replenishing the fungal culture for three cycles, reaching 47.6 ± 2.3% sulfide oxidation and 49.5 ± 2.4% gold recovery. The results were confirmed by UV–visible, C/S detector, SEM and gold fire assay. Overall, this study opens a possibility to explore a bio-oxidation at higher pH, contributing to the vision of a net-zero target. [Display omitted] [ABSTRACT FROM AUTHOR]

The resource utilization of cyanide tailings has significant environmental and economic benefits. The efficient recovery of gold from low-grade cyanide tailings containing 1.71 g/t Au was performed by a chlorination roasting–flotation process. The effects of roasting temperature, calcium chloride, internal coke, external coke, copper sulfide concentrate, and kaolin on the recovery rate of concentrate, gold grade, and sorting efficiency were investigated. The optimized process parameters were as follows: 16 wt% calcium chloride dihydrate, 6 wt% internal carbon, 1 wt% external carbon, 9 wt% copper sulfide concentrate, 2 wt% kaolin, and roasting temperature of 730 °C. The sorting rate, gold grade, and recovery rate of gold concentrate can reach 88.48%, 33.46 g/t, and 76.7%, respectively, and the gold grade of tailings was as low as 0.17 g/t. In the matte phase, gold can be enriched in the form of gold grains. Therefore, through chlorination roasting, the trapped gold can be released and deposited on the surface of the matte phase due to the transformation from hematite to magnetite. The gold-deposited metal sulfide can be effectively recycled through flotation. These results have potential guiding significance for the efficient recovery of gold from cyanide tailings. [ABSTRACT FROM AUTHOR]

The efficient and selective recovery of gold from secondary sources is key to sustainable development. However, the complexity of the recovery environment can significantly complicate the compositions of utilized sorbents. Here, we report a straw-derived mesoporous carbon as an inexpensive support material. This mesoporous carbon is modified by anions (sulfur modulation, C-S-180) to improve its electron-transfer efficiency and tune the electronic structure of its skeleton toward enhanced gold reduction. The high surface area of C-S-180 (989.4 m2/g), as well as the presence of abundant C–S in the porous structure of the adsorbent, resulted in an outstanding Au3+-uptake capacity (3422.75 mg/g), excellent resistance to interference, and favorable Au3+ selectivity. Dissimilar to most existing carbon-based adsorbents, electrochemistry-based studies on the electron-transfer efficiencies of adsorbents reveal that sulfur modulation is crucial to optimizing their adsorption performances. Furthermore, the density functional theory reveals that the optimization mechanism is attributable to the adjustment of the electronic structure of the carbon skeleton by C–S, which optimizes the band-gap energy for enhanced Au3+ reduction. These findings offer a strategy for constructing green and efficient adsorbents, as well as a basis for extending the applications of inexpensive carbon materials in gold recovery from complex environments.

Gold is a valuable commodity with numerous industrial and technological applications. Traditional adsorbents for gold adsorption using activated carbon or ion exchange adsorption resin are limited by low capacity and selectivity. To overcome these limitations, this study introduces a core–shell structured magnetic adsorbent, Fe3O4@CuS, which can capture and recover gold through in situ reduction. The resulting gold-loaded adsorbent can be easily separated using an external magnet, offering significant advantages such as high efficiency, ease of operation, low cost, low materials consumption, and low waste generation. Results show that this adsorbent has a gold loading capacity of 407 mg/g within 4 h, and the maximum capacity can reach up to 558.7 mg/g. Moreover, this magnetic adsorbent exhibited good selectivity for gold in the presence of other base metals. This research shows the feasibility of using Fe3O4@CuS for efficient gold recovery and provides insights into its adsorption behavior and mechanism. [ABSTRACT FROM AUTHOR]

Designing adsorption materials with high adsorption capacities and selectivities is highly desirable for precious metal recovery. Desorption performance is also particularly crucial for subsequent precious metal recovery and adsorbent regeneration. Herein, a metal–organic framework (MOF) material (NH2‐UiO‐66) with an asymmetric electronic structure of the central zirconium oxygen cluster has an exceptional gold extraction capacity of 2.04 g g−1 under light irradiation. The selectivity of NH2‐UiO‐66 for gold ions is up to 98.8 % in the presence of interfering ions. Interestingly, the gold ions adsorbed on the surface of NH2‐UiO‐66 spontaneously reduce in situ, undergo nucleation and growth and finally achieve the phase separation of high‐purity gold particles from NH2‐UiO‐66. The desorption and separation efficiency of gold particles from the adsorbent surface reaches 89 %. Theoretical calculations indicate that ‐NH2 functions as a dual donor of electrons and protons, and the asymmetric structure of NH2‐UiO‐66 leads to energetically advantageous multinuclear gold capture and desorption. This adsorption material can greatly facilitate the recovery of gold from wastewater and can easily realize the recycling of the adsorbent. [ABSTRACT FROM AUTHOR]

Utilizing weak interactions to effectively recover and separate precious metals in solution is of great importance but the practice remains a challenge. Herein, we report a novel strategy to achieve precise recognition and separation of gold by regulating the hydrogen‐bond (H‐bond) nanotrap within the pore of covalent organic frameworks (COFs). It is found that both COF‐HNU25 and COF‐HNU26 can efficiently capture AuIII with fast kinetics, high selectivity, and uptake capacity. In particular, the COF‐HNU25 with the high density of H‐bond nanotraps exhibits an excellent gold uptake capacity of 1725 mg g−1, which is significantly higher than that (219 mg g−1) of its isostructural COF (COF‐42) without H‐bond nanostrap in the pores. Importantly, the uptake capacity is strongly correlated to the number of H‐bonds between phenolic OH in the COF and [AuCl4]− in water, and multiple H‐bond interactions are the key driving force for the excellent gold recovery and reusability of the adsorbent. [ABSTRACT FROM AUTHOR]

Adsorption recovery of precious metals on a variety of solid substrates has steadily gained increased attention in recent years. Special attention was paid to the studies on the characterization of the adsorptive properties of materials with a high affinity for gold depending on the nature of the pendant groups present in the structure of the material. The aim of the present work was to synthesize and characterize a new material by using the sol-gel synthesis method (designated as BCb/CB). In this case, synthesis involved the following precursors: bismuth carbonate (III), carbon black, and IGEPAL surfactant (octylphenoxypolyethoxyethanol). Immobilization of the heterojunction as bismuth oxide over a flexible support such as carbon black (CB) can prevent their elution in solution and make it versatile for its use in a system. In this work, a new adsorbent material based on bismuth carbonate supported over carbon black (BCb/CB) was developed and used further for gold recovery from aqueous solutions. The required material was characterized physically/chemically by scanning electron microscopy (SEM); energy dispersive X-ray spectrometry (EDX); X-ray diffraction (XRD); thermal analysis (DTG/DTA); atomic force microscopy (AFM). The Brunauer–Emmett–Teller (BET) method was used to determine the specific surface area indicating a value of approximately 40 m2/g, higher than the surface of CB precursor (36 m2/g). The adsorptive properties and the adsorption mechanism of the materials were highlighted in order to recover Au(III). For this, static adsorption studies were carried out. The parameters that influence the adsorption process were studied, namely: the pH, the contact time, the temperature, and the initial concentration of the gold ions in the used solution. In order to establish the mechanism of the adsorption process, kinetic, thermodynamic, and equilibrium studies were carried out. Experimental data proved that the gold recovery can be conducted with maximum performance at pH 3, at room temperature. Thermodynamic studies proved that the gold adsorption on BCb/CB material is a spontaneous and endothermal process. The results indicate a total adsorption capacity of 13.1 mg Au(III)/g material. By using this material in real solutions, a recovery efficiency of 90.5% was obtained, concomitant with a higher selectivity (around 95%). [ABSTRACT FROM AUTHOR]

Ionic liquids (ILs) have received considerable attention as a promising green solvent for extracting metal ions from aqueous solutions. However, the recycling of ILs remains difficult and challenging because of the leaching of ILs, which is caused by the ion exchange extraction mechanism and hydrolysis of ILs in acidic aqueous conditions. In this study, a series of imidazolium-based ILs were confined in a metal–organic framework (MOF) material (UiO-66) to overcome the limitations when used in solvent extraction. The effect of the various anions and cations of the ILs on the adsorption ability of AuCl4− was studied, and 1-hexyl-3-methylimidazole tetrafluoroborate ([HMIm]+[BF4]−@UiO-66) was used for the construction of a stable composite. The adsorption properties and mechanism of [HMIm]+[BF4]−@UiO-66 for Au(III) adsorption were also studied. The concentrations of tetrafluoroborate ([BF4]−) in the aqueous phase after Au(III) adsorption by [HMIm]+[BF4]−@UiO-66 and liquid–liquid extraction by [HMIm]+[BF4]− IL were 0.122 mg/L and 18040 mg/L, respectively. The results reveal that Au(III) coordinated with the N-containing functional groups, while [BF4]− was effectively confined in UiO-66, instead of undergoing anion exchange in liquid–liquid extraction. Electrostatic interactions and the reduction of Au(III) to Au(0) were also important factors determining the adsorption ability of Au(III). [HMIm]+[BF4]−@UiO-66 could be easily regenerated and reused for three cycles without any significant drop in the adsorption capacity. [ABSTRACT FROM AUTHOR]

We conducted a series of column experiments on tailing heap samples from the Picacho mine in California to determine the ability of the native Gram-negative bacteria, Alcaligenes faecalis, to leach gold. To mimic heap leaching using our technique, unprocessed and unsorted tailings of mixed grain sizes were placed into columns and leached for three weeks with four treatments: (1) deionized water, (2) bacteria, (3) NaCN by bacteria and (4) NaCN. In all of the experimental runs, the total Au (mg) recovered from the columns treated with A. faecalis and NaCN followed by A. faecalis yielded gold amounts that were higher than those from the deionized water control, and lower than the columns treated with cyanide. However, the total yields were variable across runs, which we attribute to the inherent heterogeneity of gold distribution in the samples. Statistical tests show that the yields from the treatments employing bacteria and/or cyanide were different from those that employed deionized water alone. Our results support previous studies that showed that exudates of A faecalis promoted reduction of Au3+, catalyzing extracellular Au0 particles under alkaline conditions. We propose that A. faecalis is a possible novel alternative to cyanide treatment for recovering Au from mine tailings, and recommend optimization of the method. [ABSTRACT FROM AUTHOR]

The metallurgical testings to treat the Kenyan artisanal gold mining samples were performed with several processes such as gravity concentration (i.e., Knelson Concentrator and panning), flotation, and cyanidation or leaching. These tests were conducted to find the best processing stages to improve the Kenyan artisanal mining recovery. From the three categories of samples treated, sample A, which was processed through gravity concentration and flotation, produced 95.64% of gold recovery. Meanwhile, sample B could produce 98.74% of gold recovery with the cyanidation test. The results from sample A and sample B confirmed that the combination of the Knelson concentrator and flotation, which the Processing Center should handle, was the perfect combination to reduce the use of cyanide during the leaching process. On the other hand, the study also showed that the tailing sample (sample C) could still be recovered through flotation. With the results obtained, the best scenario was proposed. The government played a critical role in facilitating both the artisanal miners and the Processing Center for both cases, in Kenya or Aceh.

Gold recovery from secondary resources has been an interesting topic due to its high market prices and vast industrial applications. Reductive adsorption is a new promising technique that provides a cost-effective, efficient and eco-friendly way for gold recovery. In this study, we prepared a gallic acid-functionalised Mg/Al hydrotalcite (Mg/Al HT-GA) as well as first and second refunctionalised calcined Mg/Al hydrotalcite (Mg/Al CHT-GA_1 and Mg/Al CHT-GA_2) for reductive adsorption of AuCl4− in aqueous solution. The as-prepared adsorbents were characterised using a Fourier transform infrared (FTIR) spectrometer, an X-ray diffractometer (XRD) and a scanning electron microscope (SEM). Adsorption test showed that Mg/Al HT-GA, Mg/Al CHT-GA_1 and Mg/Al CHT-GA_2 were effective in adsorbing AuCl4– at pH 3 with adsorption capacities (b) according to Langmuir isotherm model were 175.43, 112.36 and 86.20 mg g–1, respectively. Kinetics study showed that all adsorbents followed pseudo-second order with rate constant (kp2) of 9.81 × 10–4, 33.17 × 10–4 and 9.80 × 10–4 g/(mg.min) for Mg/Al HT-GA, Mg/Al CHT-GA_1 and Mg/Al CHT-GA_2, respectively. The characterisation of the spent adsorbent by using an XRD showed that Au(0) was formed. It was revealed that phenolic – OH groups of gallic acid played a key role for the reduction mechanism of Au(III). [ABSTRACT FROM AUTHOR]

In recent years, the recovery of gold from electronic waste, especially waste printed circuit boards (PCBs), has grown in importance. PCBs consist of organic epoxy resins overlaid with valuable metals. The first step in recovering metals from waste PCBs is the removal of these resins. In this paper, autoclave-assisted alkaline pretreatment employing NaOH was used to remove resins from the PCBs. The process is shown to remove epoxy efficiently when conducted at 350 °C for 18 h using 0.8 mol·L−1 NaOH, 18 bar, and a liquid-to-solid (L/S) ratio of 18 mL·g−1. After autoclaving, the solid-phase sample (autoclaved powder) was treated with a mixture of FeCl 3 and HCl solutions – a process known as "FeH extraction" – to remove industrial metals. Under the optimal conditions (0.8 mol·L−1 FeCl 3 , 0.6 mol·L−1 HCl, L/S 24 mL·g−1, 50 °C, 100 min, 300 rpm), this process proved highly efficient in removing industrial metals. Following this treatment, a KClO 3 -HCl-CaCl 2 extraction system was employed to extract gold from the FeH-extracted sample. With this system, a 99.5% gold extraction efficiency was achieved under the optimal conditions (0.55 mol·L−1 KClO 3 , 1.2 mol·L−1 HCl, 1.5 mol·L−1 CaCl 2 , 60 °C, 80 min, L/S 12 mL·g−1, 350 rpm). Gold ions (AuCl 4 −) in the extracted solution were subsequently recovered by using hydroquinone as a reducing agent to form gold nanoparticles (AuNPs) with the assistance of NaOH. Finally, the AuNPs were stabilized using benzoquinone (an oxidized product of hydroquinone). [Display omitted] [ABSTRACT FROM AUTHOR]

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Two-step post-synthetic modification of bipydine-based sp 2 c -COF for stepwise gold recovery. [Display omitted] • A two-step PSM strategy was developed in sp2c -COF for gold recovery; • Ionization and oximation PSMs demonstrated high selectivity and efficiency • A gold adsorption capacity of 1389 mg · g−1 was achieved via stepwise adsorption; • Experimental and theoretical studies confirmed the adsorption and anchoring process; • isp2c -COF ox -NH 2 shows excellent stability, recyclability, and practical potential. Recovery of gold element from electronic waste is a promising approach for mitigating the rising gold demand in semiconductor industry. However, the complex and harsh environment in gold recovery restricted the selectivity and stability of absorbents. Herein, taking advantage of the robust sp 2-conjugated covalent organic frameworks (COFs), we proposed a two-step post-synthetic modification (PSM) strategy to prepare the isp 2 c -COF ox -NH 2 for efficient Au extraction with high selectivity and recyclability. The stepwise electrostatic and chelation interaction of the modified quaternary ammonium and amidoxime groups resulted in an efficient adsorption capacity of 1389 mg/g. Experimental and theoretical studies show that the quaternary ammonium groups ionize the sp 2 c -COFs and significantly enhance the selectivity of gold adsorption (Ni2+, Co2+ and Cu2+ for negligible influence), and the amidoxime groups further increase the adsorption capacity of the COFs via chemical reduction. Furthermore, the isp 2 c -COF ox -NH 2 retained 95.1 % of its adsorption efficiency even after 10 adsorption–desorption cycles. It is worth mentioning that isp 2 c -COF ox -NH 2 displays good adsorption performance for trace amounts of gold in complex water samples and practical electronic waste, demonstrating excellent potential for practical applications. This study not only provides novel inspirations for improving the efficiency of gold resource recovery but also paves the way for the widespread application of sp 2c-COF materials. [ABSTRACT FROM AUTHOR]

A sulfhydryl-functionalized vinylene-linked covalent organic framework (viCOF-mtd) has been successfully synthesized and possesses excellent performance for gold recovery with light-assistance. [Display omitted] • A vinylene-linked viCOF-mtd with sulfhydryl groups shows excellent performance for gold recovery. • The vinyl groups endow viCOF-mtd with a highly conjugated structure and good photoelectrical properties. • viCOF-mtd has a twofold higher adsorption capacity for gold with light assistance. • The gold adsorption of viCOF-mtd is accomplished by electrostatic interaction, S–Au coordination, and redox effect. Gold, as one of the precious metals, has a wide range of applications and a high market demand. Recovering gold from secondary resources not only safeguards the natural environment but also optimizes resource utilization. Extensive efforts have been devoted to developing materials that can efficiently and selectively recover gold from solutions via adsorption in a rapid manner. In this study, we designed and prepared a vinylene-linked covalent organic framework (viCOF-mtd) through post-synthesis modification by sulfhydryl groups for efficient recovery of gold in aqueous solution. viCOF-mtd exhibits good porosity and structural stability, and possesses an attractive gold adsorption capacity up to 1249 mg g−1. By introducing light stimulation, the gold adsorption capacity of viCOF-mtd can be further enhanced to 3386 mg g−1 along with improved adsorption kinetics. The results of various structural and property characterization tests and density-functional theory (DFT) calculations confirm the presence of electrostatic interaction, S–Au coordination, and redox effects during the gold adsorption process. Notably, light stimulation can effectively increase the photogenerated electrons in viCOF-mtd, which enhance the redox effect on gold ions. Furthermore, viCOF-mtd exhibits remarkable adsorption selectivity towards gold even in the solutions with multiple competing ions and in the complex seawater system. [ABSTRACT FROM AUTHOR]

The defatted microalgae Haematococcus pluvialis (HP), obtained from various production lots, were subjected to two different treatments: drying without chemical treatment (DH1 , DH2 , and DH3) and treatment with concentrated sulfuric acid (TH1 , TH2 , and TH3). These treated materials were employed as efficient and eco-friendly adsorbents to investigate the adsorption behavior of Au(III) in acidic chloride media. The treated adsorbents demonstrated exceptional performance, achieving 100 % adsorption of Au(III) across a broad range of hydrochloric acid (HCl) concentrations from 0.01 to 5.00 M, whereas the dry adsorbents exhibited up to 90 % adsorption only at low HCl concentrations. Both dry and treated adsorbents exhibited superior selectivity for Au(III) in the presence of a mixture of precious metal ions. The adsorption isotherms of Au(III) followed the Langmuir type of adsorption mechanism, and the maximum adsorption capacities for Au(III) were estimated as 1.57, 0.88, and 1.52 mol kg–1 for DH1 , DH2 , and DH3 , and 6.17, 4.80, and 5.37 mol kg–1 for TH1 , TH2 , and TH3 , respectively. The adsorbed Au(III) ions were reduced to the elemental state, as confirmed by X-ray diffraction patterns, scanning electron microscopy/energy-dispersive X-ray analyses, and digital micrographs of the adsorbents after Au(III) adsorption. Successful desorption of the adsorbed Au(III) was achieved using acidic thiourea solution. [Display omitted] • Defatted microalgal adsorbents were prepared with or without treating with concentrated H 2 SO 4. • Adsorbents treated using concentrated H 2 SO 4 exhibited better Au(III) adsorption capacity and selectivity. • Au(III) was reduced to metallic form during the adsorption. • Loaded and reduced Au was successfully desorbed using HCl and thiourea mixed solution. • Component of microalgae would affect the adsorption behavior. [ABSTRACT FROM AUTHOR]

The recovery of gold (Au) from electronic waste (e-waste) has gained significant attention due to its high Au content and economic feasibility compared to natural ores. This study presents a facile, single-step approach to prepare the chitosan-thioglycolic acid composite crosslinked with glutaraldehyde (CS-TGA-GA) and demonstrates its unique capability for precious metal management, which is a less investigated application area for thiolated chitosan materials. The novel cost-effective biosorbent CS-TGA-GA demonstrated a very high adsorption capacity of 1351.9 ± 96 mg/g and selectivity for Au(III) from an acidic e-waste solution at pH 1 and 298 K. The high adsorption capacity and selectivity of the sorbent can be attributed to the abundance of –NH 2 , –OH, and –SH groups present on its surface. Various characterizations, such as scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffractometry, Fourier-transform infrared spectroscopy, and X-ray photoelectron spectroscopy, as well as sorption experiments, including pH, kinetic, and isotherm studies, were performed. The kinetic data align with a pseudo-second-order model and the isotherm data can be well expressed by the Freundlich model. The CS-TGA-GA composite effectively facilitated the conversion of Au(III) to Au(0), leading to the formation of Au nanoparticles that aggregated in the reaction vessel over time. Subsequently, the Au-loaded CS-TGA-GA underwent an incineration procedure, yielding recovered Au with a purity of 99.6%, as measured by X-ray fluorescence. In addition to its large uptake capacity, acid stability, and recyclability, the prepared sorbent showed a highly selective uptake of Au(III) ions in a solution containing various metal ions leached from waste printed circuit boards. These results highlight the potential of CS-TGA-GA as an adsorbent for the recovery of Au from e-waste leachate, thereby contributing to sustainable resource management. [Display omitted] • A novel biosorbent CS-TGA-GA was developed for selective recovery of Au(III) from e-waste leachate. • A one-pot, single-step, low-cost sorbent synthesis strategy was developed. • CS-TGA-GA demonstrated rapid kinetics, high uptake capacity and selectivity for Au(III). • CS-TGA-GA exhibits an "adsorption-reduction-crystallization" mechanism. • No toxic reductants were used to collect Au(III) in its elemental form. [ABSTRACT FROM AUTHOR]

[Display omitted] • A Ti-Al-C MAX phase (Ti 3 AlC 2) is used to recover gold from acidic solutions. • High gold extraction capacity (>6000 mg/g) is achieved at pH < 1. • High selectivity for gold is observed in the presence of interfering metal ions. • Gold recovery is likely to proceed via a reduction reaction similar to cementation. • High purity gold (92 mass%) is recovered without any posttreatment. Given the scarcity of natural gold sources and associated mining challenges, the recovery of gold from secondary sources, such as electronic equipment waste (e-waste), becomes imperative. E-waste is typically treated with aqua regia to obtain highly acidic gold-containing solutions, which are subsequently processed by various methods to recover gold. Gold recovery materials offer numerous benefits but often exhibit insufficient selectivity, efficiency, and cost-effectiveness. To address this problem, we herein used a nonmodified pristine Ti-Al-C MAX phase (Ti 3 AlC 2) to realize the one-pot room-temperature recovery of gold from highly acidic solutions (pH < 1) in the form of easily collectable bulky ball-shaped Au0 aggregates. High gold selectivity (K d = 2.03 × 106 mL/g) and extraction efficiency (∼99 %) were observed even in the presence of interfering metal ions (Pd2+, Pt4+, Co2+, Ni2+, Cu2+, Zn2+), and the maximum gold extraction capacity (>6000 mg/g) surpassed that of previously reported gold recovery materials. The recovered gold was highly pure (>90 mass%) and contained minimal amounts of the original MAX phase, which obviated the need for intensive postpurification. The results of instrumental analyses suggested that gold recovery relied on electron transfer, similar to cementation, and primarily involved the reduction of solution-phase Au3+ to Au0 via electron transfer from the Ti and Al of Ti 3 AlC 2. Thus, our work demonstrates the significant potential of pristine Ti 3 AlC 2 for the energy-efficient recovery of gold from highly acidic solutions and paves the way for the efficient valorization of e-waste. [ABSTRACT FROM AUTHOR]

As an important precious metal, the recovery of gold can not only relieve the shortage of resources, but also has a high economic value. In this work, a novel silsesquioxane-based fluorescent porous hybrid polymer (PTSC) was prepared successfully by Heck coupling reaction of octavinylsilsesquioxane with a dibromated distyry-bithiophenes containing cyano groups. The structure of PTSC was fully investigated, which featured a high surface area of 455 m2 g−1, wide light adsorption band, narrow optical band gap (1.73 eV) and excellent semiconductor performance. As Au(III) adsorbent, the adsorption capacity of PTSC reached 3080 mg g−1 under light irradiation for 4 h at pH = 5, and the purity of the recovered gold in e-waste could reach 23.92 K. The adsorption mechanism showed that photoelectrons produced by PTSC could reduce all Au(III) adsorbed to Au(I) and Au(0). Moreover, a hybrid aerogel device based on PTSC and chitosan was fabricated to realize the enrichment and recovery of gold ions in wastewater. Besides, PTSC could also act as a probe to detect Au(III) with high selectivity and specificity. This study provided a new strategy for efficient recovery and detection of Au(III). [Display omitted] • A novel silsesquioxane-based hybrid semiconductor polymer can adsorb and detect Au(III) simultaneously. • PTSC can reduce Au(III) to Au(I) and Au(0) under light irradiation, and the maximum adsorption capacity is 3080 mg g−1. • A new device containing aerogel is fabricated to recover gold ions in e-wastes. [ABSTRACT FROM AUTHOR]

Thiosulfate has been considered as a more environmentally-friendly alternative to cyanide salts for the extraction of gold from gold ores and the development of affordable, green and efficient adsorbents for the isolation of gold-thiosulfate complex (Au(S 2 O 3) 2 3−) from the leaching solution remains a significant challenge. To address this issue, chitosan, a natural macromolecule, was selected as a carrier and chemically modified with ionic liquids. The ionic liquids modified chitosan showed greater adsorption capacity towards Au(S 2 O 3) 2 3− compared with pristine chitosan. The adsorption of Au(S 2 O 3) 2 3− on ionic liquid modified chitosan followed Freundlich isotherm and pseudo-second order kinetic models, involving an anion-exchange mechanism with liquid film diffusion as the rate-limiting step. The chitosan modified with butylimidazolium-based ionic liquid had an adsorption capacity of 5.0 mg g−1 for gold (10 mg L−1 of gold, pH 6, 2 g L−1 of adsorbent dosage), outperforming other reported adsorbents. The ionic liquid modified chitosan showed a high adsorption efficiency of up to 96.7 % for Au(S 2 O 3) 2 3− in an actual thiosulfate leaching solution with a desorption efficiency of 98.4 %, suggesting that the ionic liquid modified chitosan has the potential to be a eco-friendly, biocompatible and effective adsorbent for the recovery of Au(S 2 O 3) 2 3−. • Ionic liquids functionalized chitosan (ILs-CS) was synthesized. • ILs-CS exhibited high adsorption ability for gold-thiosulfate complex, Au(S 2 O 3) 2 3−. • Adsorption of Au(S 2 O 3) 2 3− on ILs-CS followed Freundlich isotherm model. • Adsorption of Au(S 2 O 3) 2 3− on ILs-CS was an anion-exchange process. • Liquid film diffusion was the rate-limiting step in the adsorption process. [ABSTRACT FROM AUTHOR]

Cyanidation, a conventional process to extract gold from gold ores, has been used for over 130 years in industrial mining because of the high efficiency and rate of formation of Au(CN) 2 − and the high recovery efficiency by adsorption of Au(CN) 2 − on activated carbon. However, carbonaceous refractory gold ores are not targeted because Au(CN) 2 − is easily adsorbed on carbonaceous matter in the ores, resulting in high recovery loss. In this study, the flotation concentrates of a carbonaceous refractory gold ores was subjected to biooxidation at 45 °C using a mixed culture containing iron-oxidizing and sulfur-oxidizing bacteria, followed by gold extraction using thiourea under strongly acidic conditions. The gold extraction efficiency reached ∼100% in 12 h without re-adsorption. Finally, the quantitative recovery of the Au(CS(NH 2) 2) 2 + complex was confirmed by adsorption on strongly cationic exchange resin. Biooxidation reduced the amount of Fe-containing metal sulfides, which minimized the decomposition of thiourea, and the Au(CS(NH 2) 2) 2 + complex had a low affinity toward carbonaceous matter, different to Au(CN) 2 −. Since the process described in this study does not require roasting to remove carbonaceous materials in pretreatment and does not use cyanide for gold extraction, it is environmentally friendly and should be considered for practical applications in carbonaceous gold ore-producing mines. [Display omitted] • Complete extraction of Au from double refractory gold ores (DRGOs) after BIOX was achieved in 12 h by thiourea. • Carbonaceous matter in DRGOs is needless to degrade prior to thiourea leaching. • Complete recovery of Au(CS(NH 2) 2) 2 + was confirmed on cation exchange resin. • Simple procedure was established for extraction and recovery of Au from DRGOs. [ABSTRACT FROM AUTHOR]

The resource utilization of cyanide tailings has significant environmental and economic benefits. The efficient recovery of gold from low-grade cyanide tailings containing 1.71 g/t Au was performed by a chlorination roasting–flotation process. The effects of roasting temperature, calcium chloride, internal coke, external coke, copper sulfide concentrate, and kaolin on the recovery rate of concentrate, gold grade, and sorting efficiency were investigated. The optimized process parameters were as follows: 16 wt% calcium chloride dihydrate, 6 wt% internal carbon, 1 wt% external carbon, 9 wt% copper sulfide concentrate, 2 wt% kaolin, and roasting temperature of 730 °C. The sorting rate, gold grade, and recovery rate of gold concentrate can reach 88.48%, 33.46 g/t, and 76.7%, respectively, and the gold grade of tailings was as low as 0.17 g/t. In the matte phase, gold can be enriched in the form of gold grains. Therefore, through chlorination roasting, the trapped gold can be released and deposited on the surface of the matte phase due to the transformation from hematite to magnetite. The gold-deposited metal sulfide can be effectively recycled through flotation. These results have potential guiding significance for the efficient recovery of gold from cyanide tailings.

Abstract Recycling of waste printed circuit boards (PCBs) has attracted increasing attention because of its high annually produced amount and high content of gold. In this study, gold recovery from waste PCBs was carried out by using the processes including microwave pyrolysis, acid leaching, solvent extraction and oxidative precipitation. The leaching efficiency of copper was approximately 95% when using a lixiviant composed of sulfuric acid and hydrogen peroxide, and the leaching efficiencies of gold were approximately 59, 95 and 95% by using thiourea, thiosulfate and aqua regia, respectively. The gold ions contained in the leachate previously produced by the leaching processes were not satisfactorily extracted by using organic solvents including di-(2-ethylhexyl)phosphoric acid, tributyl phosphate, dibutyl carbitol and trioctylamine, so the leachate was decided to bypass solvent extraction and directly apply to the oxidative precipitation process. By using the oxidants of hydrogen peroxide and perchloric acid, the precipitation efficiencies of gold were approximately 95 and 99%, and the final recovery rates were approximately 90 and 93%, respectively. The high recovery rates of gold can be attributable to the use of microwave pyrolysis that prevents the loss of gold caused by shredding and grinding processes. In addition, perchloric acid can provide higher selectivity for gold recovery than hydrogen peroxide. The maximum processing capacity of microwave pyrolysis of waste PCBs would be approximately 1.23 kg. The gold recovered from 1 t of waste PCBs can be sold for approximately USD 10,000, and thus the return on investment can be as high as approximately 1400%.

This work presents the synthesis of Au nanoparticles from gold coatings recovered from processor pins with minimal waste generation. The process consisted of four main steps: (1) physical recovery of pins, (2) recovery of gold coatings by acid digestion, (3) synthesis of HAuCl4 under mild conditions and, (4) synthesis of Au nanoparticles by the Turkevich method. The small dimensions of Au coatings allowed the synthesis of HAuCl4 with lower amounts of HClconc and HNO3conc than those used with aqua regia. This method has significant advantages, such as lower NO2(g) emission, easy post-treatment and purification, low synthesis cost and high yields. Gold nanoparticles synthesized from HAuCl4 were characterized by transmission electron microscopy (TEM) and UV-Vis spectroscopy. Size distribution analysis showed particles 14.23 nm in length and 12.05 nm in width, while absorption spectra showed a surface plasmon located at 523 nm; these characteristics were very similar to those observed with Au nanoparticles obtained with Aldrich's reagent. It is suggested that recycling procedures can be improved by taking into account the size and shape of the metals to be recovered, thus introducing a new field of research known as hydronanometallurgy. [ABSTRACT FROM AUTHOR]

Environmental desulfurization is gaining attention as an efficient approach to manage sulfidic mine tailings by separating sulfide minerals from tailings. While extensive research has been done to reuse desulfurized tailings in mine site reclamation, the responsible management or potential of valorization of desulfurization concentrates did not receive as much attention. The objective of this study was to evaluate the potential to recover Au from desulfurization concentrates originating from active gold mine sites. Desulfurization concentrates were produced by froth flotation of tailings and characterized. Cyanidation tests and gravity separation experiments were performed on the concentrates. Desulfurization concentrates, mostly composed of fine particles, contained high amounts of S (between 23.0 and 34.5% S) and variable contents of Au (between 0.7 and 1.9 g/t). Mineralogical characterization by SEM showed that 99% of Au-bearing particles were locked, mainly in pyrite (94%). Cyanidation allowed the recovery of 13 to 78% of Au, depending on the desulfurization concentrate. Low Au-bearing particles liberation, and possibly the presence of Cu, were identified as parameters negatively affecting the Au recovery. Gravity separation was poorly efficient (<50%) in recovering Au, which is quite consistent with physical (small particle size) and mineralogical characterizations. A desulfurization concentrate reprocessing flowsheet is proposed. [ABSTRACT FROM AUTHOR]

The feasibility of hydrometallurgy for metal extraction from electronic wastes was investigated in the current work. Samples were ground and preliminary extracted by nitric acid and aqua regia. The gold concentrations in nitric acid and aqua regia were measured as 1.82 mg L 1 and 29.45 mg L 1, respectively. In addition, extraction performances using Cl 2 (oxidizer) were studied. Experiments were conducted by connecting an extraction reactor to an oxidizing electro-generator. It was found that copper and gold were mostly extracted by adjusting different extraction conditions, such as extraction time, HCl concentration, extraction temperature, current density, particle size range, sample mass (solution density), and stirring speed. Two specific extraction steps were employed to selectively extract copper and gold. It was detected that 98.1 % of copper was extracted using the first extraction step twice, whereas, in the final step, 95.1 % of gold was extracted from the residue sample of the first step. The gold recovery performances of five different resins were investigated, and IRA402 Cl and HPR9700 resins manifested the highest adsorption ability and selectivity. The adsorption mechanism was found to be a monolayer adsorption process, and the adsorption capacities of IRA402 Cl and HPR9700 resins reached 303.8 mg g 1 and 295.4 mg g 1, respectively. Finally, gold desorption from these resins was successfully performed using an acetone-HCl mixture as the eluent. Reusability tests revealed that these two resins could be reused at least 6–7 times with an adsorption capacity loss of less than 10 %. [Display omitted] [ABSTRACT FROM AUTHOR]

The redox reaction mechanism of gold−cyanide [Au(CN)2−] during hydrocyclone electrowinning was investigated in this work by conducting Hull cell test, potentiodynamic polarization, and computational fluid dynamics (CFD) simulations. A leaching solution of waste printed circuit boards (WPCBs) was prepared as an electrolyte, and the effects of flow rate and applied voltage on the recovery of gold and destruction of cyanide through hydrocyclone electrowinning were investigated. A numerical simulation of the flow field in the cell was performed, and the results were visualized by conducting a finite element analysis in FLUENT. The CFD simulations and Hull cell test showed that the cylindrical cathode region had a flow velocity of ~11.05 m·s−1, which was higher than that at the center of the cylindrical part (<6.45 m·s−1). The optimum conditions for electrowinning were found to be as: an electrode distance of 4.9 mm and a current density of 73.78 mA·cm−2. The anodic polarization results showed that an Ir-based mixed metal oxide (MMO) anode could more efficiently oxidize the free cyanide because of its high current density than other types of anodes (Ir-based MMO: 299.3 mA·cm−2 > SS304: 3.5 mA·cm−2 > Pt-coated Ti: 1.3 mA·cm−2). Through the demonstrated hydrocyclone electrowinning test, the gold concentration could be decreased from 100 to 0.24 mg·L−1 within 12 h while destroying almost total amount of free cyanide in a day. [ABSTRACT FROM AUTHOR]

Recovery of gold and gold-containing sulfide mineral particles requires multiple-stage recovery processes starting from the grinding circuits to avoid over-grinding of the liberated gold particles. Due to high density, these gold grains tend to follow the hydrocyclone underflow, and a significant amount of metal increases in the circulating load. Therefore, in recent years the grinding circuits have been designed to improve the recovery of free gold/gold-containing particles in the grinding circuit. Gravity separation (centrifugal gravity separators) and flash flotation processes are commonly used for gold recovery in the grinding circuit. This study used a methodology based on modeling-simulation studies to assess various flowsheet configurations involving flash flotation, gravity separation, and the conventional sulfide mineral flotation process. The standard GRG, flash flotation, and rougher kinetic tests were used for the model development of each process. The laboratory tests and simulation studies showed that gold and sulfur recoveries in flash flotation were approximately 7% and 17% higher than that of the gravity separation process. However, the grade of the gravity concentrate was considerably higher. Therefore, one of these unit processes or their combinations can be selected depending on the ore characteristics and the aim of the recovery process. Simulation studies were performed to illustrate the gold recovery performance of various flowsheet configurations. This methodology could be used effectively for flowsheet development, particularly for greenfield projects. [ABSTRACT FROM AUTHOR]

• A porous hydrogel was synthesized by incorporating a polymer network into a β -CD-based MOF structure. • The β -CD-MOF hydrogel demonstrated increased structural stability. • The β -CD-MOF hydrogel exhibited excellent adsorption properties for Au3+, Ag+, and Pb2+ ions. • The β -CD-MOF hydrogel showed preferential adsorption of Au3+ over Ag+, potentially useful for gold recovery. Wastewater containing heavy metal ions produced by electroplating, smelting and other industries poses a threat to human safety. Recently, metal–organic frameworks (MOFs) have emerged as porous crystalline adsorbents for the removal of heavy metal ions. Starch-derived cyclodextrins (CDs), a series of cyclic oligosaccharides, have been explored for fabrication of cost-efficient MOF materials. However, the coordination bonds in CD-based MOFs can be destroyed by water, which limits their application as absorbent materials for the removal of heavy metal ions from wastewater. In this work, a β -CD-MOF-based porous hydrogel was developed by incorporating a crosslinked hydrogel network based on acrylic acid (AA) and N,N'-methylenebis(acrylamide) (MBA) monomers into a β -CD-based MOF structure. The synthesized porous hydrogel, denoted A/M−CDMOF−gel, not only showed enhanced structural stability but also exhibited excellent properties for simultaneous adsorption of Au3+, Ag+, and Pb2+ ions. A kinetic study revealed that the A/M−CDMOF−gel had a satisfactory binding rate within 60 min of contact, and the higher fit coefficient from the pseudo-second-order model indicated its chemical adsorption property. Moreover, according to the Langmuir isotherm model fitting results, the maximum capacity of the A/M−CDMOF−gel reached 316.4, 60.9, and 414.2 mg/g for Au3+, Ag+, and Pb2+, respectively. Therefore, the regenerable A/M−CDMOF−gel adsorbent is expected to provide ideas for the development of renewable absorbent materials in the field of wastewater treatment and remediation. [ABSTRACT FROM AUTHOR]