Cyanide Solutions Research Articles

Dissolution of gold in chalcopyrite-containing cyanide solutions

The gold dissolution in anoxic cyanide solution in the presence of chalcopyrite was studied with a rotating disc electrode. It was found that the presence of chalcopyrite reduced and enhanced the dissolution activity of pure gold in the low and high potential regions, respectively. The dissolution of gold was diffusion-controlled at low potentials (≤177 mV) and low cyanide concentrations (≤980 mg/L); however, above the cyanide concentration of 980 mg/L, the current density of gold decreased and the dissolution of gold changed from diffusion-control to electrochemical reaction-control. At high potentials (>177 mV), gold dissolution was always controlled by diffusion. In cyanide solution containing chalcopyrite, appropriate increase of pH value and temperature could accelerate the dissolution of gold, but high pH value would promote the generation of passivation, which was harmful for the dissolution of gold in cyanide solution.

Electron-Transfer Driven Cyanation of Aryl Iodides

Aryl cyanides are widely utilized as important intermediates in the research fields of pharmaceuticals, natural products and also organic materials, because the nitrile group can easily be transformed into a wide variety of functional groups, such as amides, carboxylic acids, imines, ketones, and amines. In the preparation of aryl cyanides, Rosenmund–von Braun reaction, in which an aryl halide reacts with copper cyanide, is often used; however, harsh reaction conditions are required, and the separation of cupper complexes in a workup process is often problematic, especially in a large-scale synthesis. We initiated our study to develop a copper free method to prepare aryl cyanides, and found that a catalytic amount of 1,10-phenanthroline nicely promoted the reaction of aryl iodides with tetraethylammonium cyanide to afford aryl cyanides (Scheme 1). 1,10-phenanthroline would work as a SET initiator on the first stage to generate radical anion. We next tried to use the electrochemical reaction for the initiation step. Electricity was simply passed to the mixed solution of aryl iodides and tetraethylammonium cyanide to afford the corresponding aryl cyanides in moderate yields. Notably, the reactions proceeded with a catalytic amount of electricity. Figure 1

Effect of PbO on cyanidation leaching behavior of bornite

During the cyanidation of gold‑copper ores or concentrates, the presence of cyanide-soluble copper significantly increases the cyanide consumption. However, the consumption of cyanide and dissolved oxygen, the concentration of sulfide ions in the leaching solution decreased with the increase of PbO amount. The SEM-EDS analyzed that there was no obvious corrosion on the surface of the bornite leaching residue in the presence of 1.5 g/L PbO, but the surface was getting very sleek. It was known from thermodynamic calculation that PbO would partially dissolve to form plumbite ions in the alkaline cyanide solution, and plumbite ions would react with sulfide ions to form PbS precipitate. The XPS analysis also proved that a large amount of undissolved PbO and PbS precipitation were adsorbed on the surface of bornite. The adhesion of PbO and PbS on the surface of bornite inhibited dissolution of bornite in the cyanide solution.

Gold extraction via cyanide leaching using alkaline-based empty fruit bunch activated carbon

This research is carried out to extend the knowledge on the modernized method of modifying the physical and structural properties of empty fruit bunches (EFB), a low-cost, highly accessible and abundant agricultural waste in Malaysia, for gold adsorption process. To complete this aim, EFB was modified by using an alkaline-based chemical to improve the surface area, porosity volume and surface chemistry of sorbents. The structure of the empty fruit bunch is characterized by using characterization techniques such as Thermogravimetric analysis (TGA) and Energy-Dispersive X-ray spectroscopy. The gold solution was prepared by mixing gold with a sodium cyanide solution to undergo a leaching process for 24 hours. The solution was then used for the adsorption experiment. The performance of the alkaline-based EFB-AC to separate gold from the cyanide leaching solution was further studied on the effect of contact time, pH and agitation rate. The gold adsorption capacity achieved by the alkaline-based EFB-AC was measured by calculating the difference between the initial and final gold concentrations using Atomic Absorption Spectrometry (AAS). The maximum gold adsorption capacity findings of contact time, pH, and agitation rate were found to be 2.5 hours, pH of 10 and 60 rpm, respectively. This study reveals that the alkaline-based EFB-AC has the potential to extract gold as an alternative activated carbon in the gold adsorption process.

A mechanism of metastable sulfur speciation and the adsorption on a gold surface in the presence of sulfidic ore and lead in cyanide medium

Metallic gold or gold minerals have high affinity towards aqueous sulfur species during gold leaching process in cyanide solutions, thus the gold surface can often become passivated by the reaction between the different sulfur species and gold. During the cyanide leaching of sulfidic gold ores, the sulfide minerals dissolve into the leach solution and produce metastable sulfur species, such as sulfide, bisulfide, polysulfide, elemental sulfur, and thiocyanate. Most of the above sulfur species can directly passivate the gold surface, and significantly reduce the gold leaching efficiency.This paper pertains to studying the sulfur species formation in cyanide solutions and their adsorption chemistry and mechanism on gold surface. It was found that the lead salts, often added to the cyanidation process to improve the gold leaching efficiency, act as a sulfide oxidant and accelerates its oxidation to different metastable sulfur species. It was shown by electrochemical linear sweep voltammetry (LSV) tests that the gold oxidation current density, i.e. oxidation kinetics, was decreased 32%, from 2.20 to 1.50 mA/cm2, in the presence of a high sulfide ore by increasing the amount of lead salt from 0 to 50 mg/L at 300 mg/L free cyanide (300 mg/L CN). Monosulfide, disulfide and polysulfide sulfur species were detected on the gold surface, by X-ray photoelectron spectroscopy (XPS) method, confirming the adsorption of different sulfur species on the gold surface. At high concentration of added lead, 50 mg/L Pb2+, even elemental sulfur was shown to adsorb on the gold surface. The formation of different sulfur species was explored by thermodynamic modeling, which showed very good agreement with the results of the XPS study. This study helps to develop a better insight on the mechanism of gold surface passivation caused by sulfur species and the findings help to reduce gold passivation by better control of the process condition.

Features of gold cementation in percolation mode by electrolytic zinc powders

The study covers physicochemical features of dendritic zinc powders and their effect on gold cementation from cyanide solutions. Three zinc powders were obtained in a laboratory environment by electroextraction at different conditions, and these powders featured various particle size and specific surface area. The properties of zinc powders obtained and powder currently used for gold cementation were evaluated using SEM (Jeol JSM-6390LA), BET (Gemini VII 2390) and laser diffraction (Sympatec HELOS & RODOS) methods. It is shown that electrolytic powders have high specific surface area (1.3–2.6 times more) and a low bulk density (3.1–3.8 times less), relative to zinc powder currently used for gold cementation. It was found that due to specific physical properties electrolytic powders have low hydraulic resistance, which eliminates the need for inert additives introduced during cementation, increases unit capacity and reduces the load on equipment. Inert additives elimination will additionally increase the gold content in the resulting product. The dendritic morfology of zinc powders obtained compensates high particle size resulting in the high efficiency of gold precipitation. At the long cementation cycle the effective gold deposition area (with gold extraction of more than 97 %) turned out to be shorter for electrolytic powder compared to fine powder currently used. However, in practice, the cementation cycle is always limited by fine powder throughput and it is not possible to achieve the full zinc potential. The resulting cementation product usually contains 25–35 % of unused zinc. These studies show the effectiveness of using electrolytic zinc powder for gold cementation from cyanide solutions.

Extraction of silver from a refractory silver ore by sono-cyanidation.

In this study, effect of ultrasound on silver extraction from a refractory silver ore containing both native silver and various silver sulphide minerals was investigated. Main effects and interaction effects of pulp density, ultrasonic frequency, cyanide concentration, air flow rate and agitation speed on the extraction rate of silver were studied by a two-level fractional factorial experimental design. A few additional cyanidation tests were also conducted to verify the findings of the designed experiments. It was found that the overall extraction yield was varied from 67% to 90% depending on the operating conditions used in the sono-cyanidation tests (48h). However, it was varied from 63% to 80% by same operating conditions used in the direct cyanidation tests at the same cyanidation time. It was observed that an increase in the ultrasonic frequency has a negligible effect on the silver recovery. More importantly, it was determined that there were insignificant differences between the 24-h sono-cyanidation results and the 48-h direct cyanidation results for each cyanidation conditions. This finding, which is very important from the cyanidation practice standpoint, indicates that the cyanidation time can be reduced up to 50%, or the capacity of an operating silver extraction plant can be increased up to 100% by the sono-cyanidation by the refractory silver ores. In order to describe the rate of silver dissolution in the cyanide solutions, the experimental data were analysed using shrinking core models. It was found that there is a good fit between the experimental data and the models, indicating the rate of silver dissolution in cyanide can be described by a two-stage, porous layer diffusion controlled, shrinking core model.

Production and Characterization of Activated Carbon from Coal for Gold Adsorption in Cyanide Solutions

In this work, activated carbons were produced using coal as raw matter from seven Colombian carboniferous zones. Physical activation was performed in two stages: a carbonization stage with Nitrogen at a temperature of 850 °C and a residence time of 2 h, followed by an activation stage using steam at temperatures of 700 and 850 °C with residence times of 1,5 h and 2,5 h. From the pore volume characterization for the adsorption of gold, two activated carbons from Cundinamarca, obtained at 850 °C (1,5 h), 850 °C (2,5 h), and a commercial carbon (GRC 22) were selected. Gold adsorption tests were performed with those three activated carbons using synthetic aurocyanide solutions and a gold waste solution. The data of the adsorption isotherms were adjusted using the Freundlich adsorption model for the synthetic solution, as well as Langmuir for the waste solution. The results showed that, using a solution of 1 ppm, the activated carbons C-850-2.5 and C- 850-1.5 produced the higher maximum gold loading capacities in the equilibrium (8,7 and 9,3 mg Au/g, respectively) in comparison to the commercial activated carbon (4,7 mg Au/g). Gold adsorption test using a waste solution (21 ppm of gold) showed that the activated carbon C-850-1.5 had the highest value of adsorption capacity (4,58 mg Au/g) compared to C-850-2.5 (2,95 mgAu /g).

Интенсификация извлечения дисперсного и тонковкрапленного серебра при цианидном кучном выщелачивании золота

Since the cost of gold has increased by an order of magnitude over the past 20 years, gold mining began to be carried out from ores that are not previously acceptable for heap leaching (HL) technology – these are refractory ores with thin and ultrafine inclusions of precious metals, ore with an increased silver content. In the Russian Federation, classical representations in the field of gold and silver HL prevail in design solutions. At the same time, the problem of silver leaching is considered in terms of the associated extraction of the main component. This approach is determined by a limited set of technological measures that have become an integral part of the classic technological regulation: ore crushing to the class –200–40 mm, sodium cyanide consumption at the level of 0,5 kg/t; pH value – 10,5; cycle duration 65 days or more; gold concentration in production solutions – fractions, mg/l; using activated carbon for sorption of noble metals. Processing ores with silver contents of 30 g/t and a more classical approach to gold HL does not allow to increase the degree of extraction. There is a need to modernize the circuit or change the parameters of the technological regulations. The object of the study was a sample of poor quartz ore provided by the customer developing one of the gold and silver deposits of the Far Eastern Federal District, the object was to increase the degree of leaching of silver from poor gold-bearing ores without changing the technological scheme and without attracting additional mining equipment. Critical analysis of literary data was carried out and the main directions of gold production from poor gold-bearing ores were identified. Chemical and mineralogical compositions of silver-containing minerals and ore rocks were investigated. Forms of association of silver with ore minerals and rocks have been established. The option of physical and chemical activation of increase of reactivity of cyanide solutions during heap leaching of silver from poor gold-bearing ores is chosen. In laboratory conditions, it was found that an increase in the concentration of sodium cyanide by 6 times or more contributes to a satisfactory extraction of gold both in agitation and percolation leaching of silver. However, in the latter case, the positive result was obtained only with the use of solutions with increased reactivity

Recycling of gold from waste electronic components of devices

Tremendous generation of e-waste and its illegal recycling are causing immense threat to environment as well as the loss of precious metals. The present research reports a novel hybrid process for the total recovery of gold from small depopulated components of e-waste. Connectors and integrated circuits (ICs) liberated from printed circuit boards (PCBs) were pulverized and processed for gold leaching using 10 g/L sodium cyanide solution at 40 °C and mixing time 15 min, where more than 95% gold was found to be leached out in single stage. From the obtained leach liquor, gold metal was recovered by charcoal adsorption followed by heat treatment. The raffinate left after adsorption of gold was found to contain ~10 mg/L gold, which was also recovered using Amberlite IRA 400Cl at equilibrium pH 9.6 in 30 min maintaining aqueous/resin (A/R) ratio 25 mL/g. The raffinate solution was enriched to 882.41 mg/L and the solution was further processed to get metal/salt using cementation/ evaporation. Obtained leached residue is processed for non-ferrous metals recovery and finally disposed-off after treatment and TCLP test. The effluent left after leaching could be easily decomposed and treated in ETP using standard environmental procedure.

Au@Ag Core/Shell Nanorods Based Colorimetric Assay for the Rapid and Selective Detection of Cyanide Anions

A colorimetric assay based on Au@Ag core/shell nanorods was developed to rapidly determine cyanide in aqueous solution. The aspect ratios of Au@Ag core/shell nanorods were changed with the dissolution of metals in cyanide solution, which led to the decrease of Ag shell absorption spectra and the blue shift of longitudinal plasmon absorption band of Au core, as well as the color change of Au@Ag core/shell nanorods. In contrast to spherical Au@Ag nanoparticles with less sample analysis time (1 min per sample), the distinguish ability of this assay was proven to be higher. Under optimal conditions, the developed colorimetric assay gave a linear range of 1–200 μM for cyanide in water solution and a naked-eye detection limit of 0.5 μM. On the side, the colorimetric assay exhibited high selectivity towards cyanide over other common anions. The developed Au@Ag nanorods probes have been successfully applied to determine the level of cyanide in local lake, river and industrial water samples.

Precipitation of zinc from cyanide leach solutions using Trimercapto-s-triazine (TMT)

Cyanide leach solutions produced from gold/silver ores can be contaminated with zinc due to the gold/silver cementation in Merrill-Crowe process and/or the dissolution of zinc minerals present in the ore. Zinc levels in cyanide solutions should be controlled to avoid potential problems in leaching and downstream processes associated with the accumulation of zinc. In this study, the application of a novel method was demonstrated for the precipitation of zinc from synthetic/real cyanide solutions using Trimercapto-s-triazine (TMT), a non-toxic organic reagent. Experimental studies have shown that precipitation of zinc with TMT is a fast process occurring in a short period of time i.e., ≤15 min. Increasing the concentration of TMT (33–90 mM) and pH (10–12.5) was found to improve the precipitation of zinc, which was unaffected by increasing the temperature (20–60 °C). Response surface methodology was adopted for detailed investigation of the effect of [TMT]/[Zn] ratio (2.70–16.25), [CN−]/[Zn] ratio (3.00–8.00) and initial concentration of Zn (50–950 mg/L) on the precipitation of zinc. High recoveries (≤95.4%) were obtained under suitable conditions. Precipitation tests were also performed using synthetic multi-metal and real leach solutions. The complete recovery/dissolution of zinc from the precipitates was also demonstrated using different acids (HCl and H2SO4). The findings have shown that TMT can be effectively used as an environmentally friendly reagent for removal of zinc from cyanide leach solutions.

Removal of copper cyanide by precipitate flotation with ammonium salts

Cyanidation is the most commonly used method for leaching and recovering Au and Ag from ores. In the process, some copper species may dissolve and react with cyanide to form cyanide–copper complexes in a cyanide solution. Waste cyanide solutions must be treated to reduce their cyanide concentrations before they may be safely discharged into tailing storage facilities. In this preliminary work, precipitate flotation techniques were used to remove cyanide–copper complexes from synthetic solutions similar to those found in gold cyanidation plants. Cetyltrimethylammonium bromide (CTAB) and octadecyltrimethylammonium bromide (OTAB) were both employed as precipitants and frothers. Multiple parameters, including reaction time, bubbling time, pH, and surfactant dosage, were investigated. The results show that it is possible to remove up to 84.51% of the cyanide and 81.62% of the copper with a 7.60% water loss when adding 3.75mg/mg Cu CTAB at a pH of 6. While further decreasing the pH may increase removal efficiencies, the contribution of hydrogen cyanide (HCN) volatilization and stripping will also be greatly increased. Tests performed with an industrial solution corroborated the results obtained using synthetic solutions. This study reveals that precipitation flotation can be used in the gold mining industry as a treatment for cyanidation solutions with high cyanide concentrations discharged into tailings facilities.

Recrystallization Texture of Electrodeposited Zinc

Electrodeposited zinc layers help prevent the corrosion of steel surfaces. The microstructure, surface morphology, and texture of the electrodeposits are related to the deposition variables. Herein the surface morphology and microstructure are investigated as functions of the special electrodeposition variables, such as electrolyte composition, over-potential, current density, electrolyte temperature and pH. Upon heat treatment, the texture of the electrodepositd layers changes into a recrystallization texture. The texture of the as-electrodeposited and recrystallized zinc layers are studied. The //normal direction (ND) texture of the zinc electrodeposit can be obtained from zinc oxide, sodium hydroxide, potassium chloride, and sodium cyanide solution baths. The //ND texture of the zinc electrodeposit did not change even after recrystallization. This phenomenon of zinc eletrodeposits could be described by the strain-energy release maximization model.

Thermodymanic Considerations on Gold Dissolution in Different Lixiviants (I)

The paper presents the main thermodynamic aspects related to gold dissolution in different lixiviants. We analyzedthe thermodynamic stability of the gold complexes with the most representative lixiviants with potential for use in the gold industry. Based on the stability constants we determined the electrode potentials of the gold dissolution reactions in variouslixiviants and we presented the electrochemical mechanism of gold dissolution is alkaline cyanide solutions.

A mechanism and thermodynamic model for the interaction of silicate minerals with lead-assisted gold electro-oxidation in a dilute cyanide medium

Gold (Au) oxidation kinetics in cyanide (CN) solutions can be increased by the addition of low concentrations of aqueous lead [Pb(II)]. However, the Pb(II) addition is not always effective in improving Au oxidation kinetics. The present study provides a mechanistic and thermodynamic view of the Pb-assisted Au oxidation process in the presence of silicate minerals and presents evidence that Pb adsorption onto the silicate minerals plays a substantial role in the oxidation kinetics. AuCN(ads) film formation on the Au surface, in an Au-CN-H2O system at a low overpotential range of linear sweep voltammetry (LSV), was confirmed through both electrochemical study and XPS analysis. Once Pb(II) was added into the system, AuCN(ads) film formation was hindered, and the Au oxidation kinetics were significantly enhanced. However, the improving effect of Pb(II) on the Au oxidation process was reduced with the addition of quartz, due to Pb adsorption onto the quartz surface. As demonstrated through UV/Vis analysis and thermodynamic modeling, the cationic and neutral species, PbOH+, PbCN+ and PbO(a), were likely adsorbed onto the quartz surface. At lower CN concentration, the effect of quartz in reducing the Pb-assisted Au oxidation kinetics was more severe, and the Au oxidation was chemically controlled.

Characterization of a refractory arsenical silver ore by mineral liberation analysis (MLA) and diagnostic leaching

In this study, a detailed characterization of an extremely complex arsenical silver ore using mineral liberation analysis and diagnostic leaching for identification and occurrence of silver phases is presented. Mineralogical analysis of the ore has shown that silver is present mainly as acanthite and to a small extent as Fe-Pb-Ag-Zn arsenate/oxide. Acanthite was observed to occur as very fine grains (53% finer than 7 μm, d80: 11 μm) associated with various Pb-Fe-Sb arsenates and silicates (quartz, feldspar, kaolinite, mica/chlorite and clay). Around 33.1% of acanthite grains are present as free grains whilst 12% are locked-up within associated phases. Cyanide leach tests indicated that the extraction of silver from the ore was limited to 71% Ag over a cyanide leaching period of 24 h. This low extraction was presumably due to the limited exposure of fine acanthite grains to cyanide solution since ultra-fine grinding of the ore led to an enhanced silver extraction. It can be inferred from the findings of diagnostic leaching tests that the refractoriness of the ore can be attributed essentially to the encapsulation of acanthite within HCl-soluble phases (in particular, complex Pb-Fe-Sb-Zn oxides, arsenates and sulphates) (22%) and HF-soluble silicates (6%). It can be inferred from these findings that ultrafine grinding (even finer than few microns) or acid leaching can be used for achieving high silver extractions. This study also highlights the effective utilization of MLA in combination with diagnostic leaching for comprehensive characterization of refractory gold/silver ores and providing invaluable insights into their refractory behaviour.

Kinetics and Energy Consumption for a Three-Stage Electrocoagulation Process for the Recovery of Au and Ag from Cyanide Leachates

The most common processes used for the recovery of gold and silver from cyanide leachates are Merril-Crowe, activated carbon in pulp, and ion exchange resins; the process of electrocoagulation (EC) also is a promising new technique. EC is an electrochemical process whose mechanisms include oxidation, reduction, decomposition, deposition, coagulation, absorption, flotation, and precipitation. It has been used for the treatment of water and wastewater with different degrees of success. This study aimed to determine the kinetics of the reaction and the energy consumption at constant voltage, and at constant current using aluminum electrodes with two different distances between electrodes. EC was run in three stages for the removal of gold and silver from aqueous cyanide solutions from samples supplied by a Mexican mining company. Characterization of the sample showed initial concentrations of 49.48 and 383 mg/L of gold and silver, respectively. Results showed the effectiveness of the process by achieving removals up to 98.59% of gold and 99.43% of silver. Additionally, it was determined that the kinetics of the reaction is of zero order and that the lowest energy consumption can be achieved when working at constant voltage and with a separation of 0.8 cm between electrodes.

Carbon Sorbent Based on Special Fine Coke for the Extraction of Gold

The production of carbon sorbents based on special fine coke using a combination of steam and gas activation (by gas after the carbonization of Shubarkol coal with carbon dioxide and superheated water vapor) and chemical activation (by sodium hydroxide) was studied. Their textural and sorption properties were characterized. The necessity of separating the carbon and mineral parts of the special fine coke by elutriation in water was shown. The high adsorption capacity of the samples for gold ions was established (the recovery was 99.4–99.9%). The method for the production of a sorbent based on special fine coke in order to extract gold from cyanide solutions and the optimal conditions for its adsorption were recommended.

The effect of thermal and acid treatment of kaolin on its ability for cyanide removal from aqueous solutions

The removal of free cyanide from aqueous solutions was investigated by kaolin, calcined kaolin (metakaolin), and acid-treated metakaolin. The kaolin in raw form had no ability for cyanide adsorption, but calcination and acid treatment significantly improve its adsorptive properties. The best calcination conditions were determined to be at temperature of 600 °C and calcination time of 1 h. The scanning electron microscope (SEM), wavelength-dispersive X-ray (WDX), X-ray fluorescence (XRF), and X-ray diffraction (XRD) analysis methods were used to explain the adsorptive properties of the adsorbents. The loading capacity of metakaolin increased from 0.66 to 5.32 mg/g, and the loading capacity of acid-treated metakaolin increased from 1.33 to 10.67 mg/g by increasing the initial cyanide concentration from 25 to 531 mg/L. The kinetics of cyanide adsorption onto both adsorbents increases by increasing the initial cyanide concentration (25–531 mg/L) and solution temperature (30–50 °C). The equilibrium and kinetic data on both adsorbents are best fitted by the Freundlich isotherm and pseudo-second-order kinetic models, respectively. The thermodynamic study showed that the cyanide adsorption onto both adsorbents is endothermic and spontaneous in nature. The activation energy for the cyanide adsorption onto metakaolin and acid-treated metakaolin was determined to be 16.68 and 5.79 kJ/mol which indicates that the adsorption of cyanide ions onto metakaolin is chemisorption and onto the acid-treated metakaolin is a physicochemical process.