Cyanide Solutions Research Articles

Enhanced leachability of gold and silver in cyanide media: Effect of alkaline pre-treatment of jarosite minerals

Composite samples of tailings containing gold (1.35 g/t) and significant amounts of silver (155 g/t) were subjected to batchwise cyanide leaching to assess the feasibility of extracting gold and silver. The tailings are waste solids arising from flotation and leaching operations whereby the flotation product (sphalerite concentrate) is calcined and then solubilised into dilute sulphuric acid solution and eventually sequestered from the electrolyte by electrowinning. Silver and gold are part of the zinc refinery residue, flotation tailings and to a limited extent the calcine leach tailings. Mineralogical results showed that composite tailings are refractory in nature (44% quartz, 17% silico aluminates and 12% jarosites). The concept of enhancing gold and silver recovery from the tailings focused on firstly decomposing the jarosite minerals by alkaline pre-treatment and then secondly leaching with cyanide solution. These two steps ensured that free gold and silver found in the zinc refinery residue and in the jarosite minerals could be leached simultaneously. The composite tailings were treated with Ca(OH) 2 solutions and then heated to 90 °C for 2 h to decompose the silver-bearing mineral (Ag,PbFe 3(SO 4) 2(OH) 6). The alkaline pre-treated tailings were then subjected to cyanide leach tests at different NaCN dosages (2.5–10 kg/t) and particle size (96–200 μm). Without an alkaline pre-treatment stage, leach efficiencies achieved were 41% and 25% for gold and silver, respectively at 40 °C and 8 h mixing time. But, better leach efficiencies (55% for Au, 81% for Ag) were achieved after the feed was pre-treated with Ca(OH) 2. The leaching mechanism of gold was explained by the shrinking sphere model denoted by surface chemical reaction.

The use of the advanced oxidation process in the ozone + hydrogen peroxide system for the removal of cyanide from water

The results of laboratory tests are presented in this paper, which demonstrate a possibility of using the advanced oxidation process for the removal of cyanide from the water. A comparative analysis was carried out for the processes of ozonation, oxidation with hydrogen peroxide and advanced oxidation in the O 3 + H 2O 2 system. The paper presents experimental test results that indicate a very clear effect of the value of pH on the efficiency of the ozonation process. The performed tests have confirmed an increase in cyanide reduction with increasing reaction of water treated. This may suggest a higher susceptibility of cyanide to indirect oxidation by hydroxyl radicals than to direct oxidation by ozone. In this connection, tests were carried out on the use of advanced oxidation methods for the removal of cyanide. These methods are based on the generation of hydroxyl radicals being the most active reagents. In most processes, •OH radicals are produced with the help of an oxidant added. In view of the fact that hydrogen peroxide is used for waste-water treatment, the tests were carried out for the O 3 + H 2O 2 system. The paper gives optimal ozone doses in conjunction with hydrogen peroxide for water prepared with a cyanide solution. The effect of oxidizer dosing order of on the reduction of cyanide is described.

Electrocatalysis by adatoms at the gold and silver dissolution in cyanide solutions

The peculiarities of the effects of upd thallium, lead, bismuth, and mercury on the dissolution rates of gold and silver in cyanide electrolytes are compared. In general, they feature the abrupt acceleration of the dissolution of gold and, to a lesser extend, silver in the chemisorption range of mentioned ions. As the potential increases, the gold dissolution rates passes through a maximum the height of which is comparable with the limiting current of this process associated with limitations in the delivery of cyanide ions to the electrode surface. The current decay after the maximum is due to desorption of catalytically active adatoms. The chemisorption rate of thallium, lead, and bismuth ions at potentials more negative than the current peak is controlled by their diffusion to the gold surface, whereas the chemisorption rate of mercury is controlled by the adsorption kinetics. With the increase in the surface coverage with adatoms θ, the catalytic activity of all considered adatoms passes through a maximum. The sharp increase in the effective transfer coefficient in the presence of these adatoms makes the main contribution into the acceleration of the gold dissolution, while the increase in the exchange current has a smaller effect. The chemisorption of mentioned atoms on gold not only accelerates the dissolution but also changes its mechanism. For gold dissolution, the catalytic activity of upd thallium, lead, and bismuth increases in the following sequence: Tl ≪ Pb < Bi and the effect is additive in their simultaneous presence. For silver, the increase in the exchange current makes the main contribution into the acceleration of dissolution, whereas the transfer coefficient and the reaction order with respect to the ligand change insignificantly. Explanation of the observed peculiarities is given.

光触媒およびオゾンを用いたＡｇ（Ｉ）含有シアン溶液の分解反応

光触媒およびオゾンを用いたＡｇ（Ｉ）含有シアン溶液の分解反応

Cementation of gold by magnesium in cyanide solutions

Contact deposition of gold by a magnesium powder in a hydrodynamic mode in 0.001–0.005 M solutions of the system [Au(CN)2]−-CN− is investigated. It is found that the cementation rate of gold (V c ) is directly proportional tot he concentration of the [Au(CN)2]− complex and the area of the cementing agent. As the temperature is increased in the ranges 20–30 and 30–40°C, the increase in V c is 20–25 and 8–10%, respectively. The time dependence of degree of gold extraction on magnesium has an extreme character due to the increasing rate of the inverse process of leaching. In an ultrasonic field, the cementation rate and degree of gold extraction increase. This phenomenon is conditioned by destruction of the hydroxide film on the magnesium surface. The deposited gold has a film nature.

Sensitive determination of total aliphatic amines in water samples by spectrofluorimetry using the new fluorogenic probe 3-(4-fluorobenzoyl)-2-quinolinecarboxaldehyde

A rapid and highly sensitive fluorogenic method has been developed for the determination of total primary aliphatic amines with a novel fluorogenic derivatizing reagent, 3-(4-fluorobenzoyl)-2-quinolinecarboxaldehyde (FBQCA). FBQCA selectively reacts with amines to form a highly fluorescent isoindole in buffer of pH 9.7 and in the presence of potassium cyanide solution. The fluorescence intensity of derivatized products are detected at λex/λem = 480/546 nm. The results of the experiments showed a linear dynamic range of 1 × 10−8 to 6 × 10−6 mol L−1 with a detection limit (S/N = 3) of 6 nmol L−1. The method was successfully used for the direct spectrofluorimetric determination of aliphatic amines in water samples with recoveries of 95–104%.

Si cleaning method without surface morphology change by cyanide solutions

Hydrogen cyanide (HCN) aqueous solutions can remove copper contaminants from Si surfaces more effectively than hydrochloric acid/hydrogen peroxide mixture (HPM) and sulfuric acid/hydrogen peroxide mixture (SPM). When pH of the HCN solutions is adjusted at 9, Si surface morphology is not changed, while when pH exceeds 10, the Si surfaces are considerably roughed. AFM measurements show that Cu contaminants are present in the form of particles on the bare Si surfaces. XPS measurements show that the particles consist of metallic Cu. The Cu particle height decreases almost linearly with the cleaning time, and the Cu surface concentration decreases exponentially with it. It is concluded that Cu particles gradually dissolve into the HCN aqueous solutions by the direct reaction with cyanide ions at the surface of the Cu particles.

Continuous Laboratory Gold Solvent Extraction from Cyanide Solutions using LIX 79 Reagent

AbstractContinuous laboratory solvent extraction of gold from cyanide solutions has been investigated by using LIX 79 guanidine‐based extractant. Different variables that affected the extraction included aqueous pH, extractant concentration and modifier concentration. Extraction isotherms of the aurocyanide complex with respect to the other cyanoanions were compared, and the following order of selectivity was observed: Au &gt; Ag &gt; Cu &gt; Zn &gt; Fe. According to the pH isotherms, aurocyanide can be extracted in alkaline media, and a better separation with respect to other cyano anions was obtained in the pH range 10.5–11.2. From the McCabe‐Thiele diagrams, better recovery was observed when using LIX 79 and tridecanol at 10 vol.‐%. Stripping gold from the loaded organic was carried out at pH &gt; 12 by using NaOH and NaCN solutions. The pilot plant tests indicate that a two‐stage extraction followed by one strip step are more than adequate to obtain an overall process efficiency of 92 %. However, for those cases where copper is present significantly, a copper wash stage is recommended before gold stripping. In this case, stripping of copper is accomplished at a pH 10.8, whereas the gold stripping was done at a pH of 12.0.

A review of effects of silver, lead, sulfide and carbonaceous matter on gold cyanidation and mechanistic interpretation

Despite the wealth of published data on the beneficial or detrimental effects of silver, lead, sulfide, and carbonaceous matter on the rate of gold cyanidation at an anode or by dissolved oxygen, the lack of comparative studies on relative effects has hampered rationalisation of the role of these activators or passivators of gold. In the present study, the published rate data per unit surface area of gold, silver, and gold–silver alloys based on electrochemical or chemical dissolution of rotating discs or foils of constant surface area in aerated cyanide solutions at ambient temperatures are analysed on the basis of the Levich equation. The current status of the reaction mechanism is also reviewed and updated on the basis of species distribution and potential–pH diagrams, stoichiometric factors, and interim chemical species of gold(I), silver(I), and lead(II). The anodic peak potentials of reported voltammograms closely follow the potential–pH lines of Au(I)/Au(0) and Pb(II)/Pb(0) couples. Despite the formation of stable complexes between lead(II), nitrate, and hydroxide ions, the total calculated soluble lead(II) in alkaline solutions of pH range 10–11 saturated with lead hydroxide is shown to be < 0.1 g/m 3. A comparison of the reported diffusion coefficients of cyanide ions and dissolved oxygen with the values based on the Levich plots of reported rates reveals the rate-controlling stoichiometric M/CN or M/O 2 molar ratios. The difference between some of these ratios and the generally accepted ratios of M/CN = 1/2 and M/O 2 = 1/0.5 or 1/0.25 based on the formation of M(CN) 2 −, H 2O 2 or OH − in the overall cyanidation reaction is attributed to the oxidation of cyanide to cyanate and passivation due to the formation of gold hydroxides/oxides. The alloyed or dissolved silver and lead eliminate passivation due to the involvement of mixed hydroxo–cyano complexes of silver and lead ions in the surface reaction. Gold dissolution by oxygen in cyanide-rich solutions is limited by oxygen diffusion, but enhanced by the presence of a low concentration of sodium sulfide due to the involvement of hydrosulfide ion in the surface reaction. However, excess lead or sulfide retards gold cyanidation due to surface blockage by metallic lead, lead hydroxide, or due to passivation by Au 2S/S. Even low concentrations of hydrosulfide passivate gold–silver alloys due to the formation of Ag 2S. This can be eliminated by adding stoichiometric quantities of lead(II) to precipitate sulfide as PbS. Large stoichiometric ratios of O 2/M for the cyanidation of graphite coated gold appears to be a result of the enhanced oxidation of cyanide by oxygen or hydrogen peroxide, leading to a cyanide deficiency at the surface and passivation of gold by hydroxide/oxide. The presence of excess cyanide or lead(II) does not override this effect.

Pressure cyanide leaching for precious metals recovery

A novel method demonstrates that the oxidation and dissolution of gold and silver in alkaline cyanide solution can be conducted simultaneously in the same autoclave in less than 90 minutes with a recovery that exceeds 96%. Because mild operating conditions of 80°C and 0.6 MPa oxygen pressure are used in this process, low cost materials of construction can be utilized for the autoclave.

Ferrate(VI) oxidation of zinc–cyanide complex

Zinc–cyanide complexes are found in gold mining effluents and in metal finishing rinse water. The effect of Zn(II) on the oxidation of cyanide by ferrate(VI) ( Fe VI O 4 2 - , Fe(VI)) was thus investigated by studying the kinetics of the reaction of Fe(VI) with cyanide present in a potassium salt of a zinc cyanide complex (K 2Zn(CN) 4) and in a mixture of Zn(II) and cyanide solutions as a function of pH (9.0–11.0). The rate-law for the oxidation of Zn ( CN ) 4 2 - by Fe(VI) was found to be - d [ Fe ( VI ) ] / d t = k [ Fe ( VI ) ] [ Zn ( CN ) 4 2 - ] 0.5 . The rate constant, k, decreased with an increase in pH. The effect of temperature (15–45 °C) on the oxidation was studied at pH 9.0, which gave an activation energy of 45.7 ± 1.5 kJ mol −1. The cyanide oxidation rate decreased in the presence of the Zn(II) ions. However, Zn(II) ions had no effect on the cyanide removal efficiency by Fe(VI) and the stoichiometry of Fe(VI) to cyanide was approximately 1:1; similar to the stoichiometry in absence of Zn(II) ions. The destruction of cyanide by Fe(VI) resulted in cyanate. The experiments on removal of cyanide from rinse water using Fe(VI) demonstrated complete conversion of cyanide to cyanate.

Breaking bad news

The objective of the present study was to ascertain whether cyanide shares the properties of methylene blue as a selective inhibitor of vascular smooth muscle relaxation elicited by agents that stimulate the formation of cyclic GMP. Experiments were performed with endothelium-intact rings prepared from bovine intrapulmonary artery. Methylene blue, a good inhibitor of soluble guanylate cyclase, antagonized both arterial relaxation and cyclic GMP accumulation in response to sodium nitroprusside, glyceryl trinitrate, S-nitroso-N-acetylpenicillamine and acetylcholine. In contrast, cyanide inhibited only the responses to sodium nitroprusside. Increasing concentrations of methylene blue depressed resting arterial levels of cyclic GMP and caused slowly developing but marked contractions whereas cyanide was without effect. Contractile responses to phenylephrine, potassium and U46619 were potentiated by methylene blue but not by cyanide. Preincubation of dilute solutions of cyanide containing sodium nitroprusside in oxygenated Krebs9 buffer at 37 degrees C for 15 min before addition to bath chambers depressed relaxation and cyclic GMP accumulation caused by sodium nitroprusside markedly. Similar treatment of glyceryl trinitrate, however, failed to alter its effects in arterial rings. A chemical inactivation of sodium nitroprusside by cyanide appears to account for the specific inhibitory action of cyanide on arterial responses to sodium nitroprusside. This study indicates clearly that cyanide does not share the properties of methylene blue as an inhibitor of arterial relaxation elicited by vasodilators that stimulate cyclic GMP formation.

Au(I) extraction by LIX-79/ n-heptane using the pseudo-emulsion-based hollow-fiber strip dispersion (PEHFSD) technique

The extraction of Au(I) from alkaline cyanide solutions through PEHFSD was conducted in a microporous hydrophobic polypropylene hollow-fiber membrane contactor. The organic extractant LIX-79 ( N, N-bis(2-ethyl hexyl)guanidine) in n-heptane was used. The alkaline cyanide feed solution containing Au(I) was passed through the tube side, and pseudo-emulsions of LIX-79/ n-heptane and NaOH were passed through the shell side in counter-current mode, using a single microporous hydrophobic polypropylene hollow-fiber contactor for extraction and stripping. In PEHFSD the aqueous strip (0.2 M NaOH) solution is dispersed in the organic (12% LIX-79/ n-heptane) membrane solution in a container with a mixing arrangement (impeller stirrer) designed to form a strip dispersion. The strip dispersion is circulated from the mixer to the membrane module to provide a constant supply of the LIX-79/ n-heptane solution to the membrane pores. Various hydrodynamic and chemical parameters, such as variation in feed pH; gold cyanide concentration in feed; variation in concentrations of NaOH, LIX-79/ n-heptane and Au(I); variation in feed, strip and LIX-79/ n-heptane volume ratios and variation in feed and strippant flow rates, were investigated. Mass transfer modeling was performed and the validity of the model was evaluated with experimental data and found to tie in well with theoretical values. It was possible to concentrate Au(I) in product (NaOH) using the PEHFSD technique.

Characterization and treatment of artisanal gold mine tailings

The solid waste generated by artisanal gold mining, with high mercury and gold contents, can be found in several areas in the South America. The present study focused on the tailings of an artisanal gold mine area located in the Brazilian northeastern. Samples of the mine tailings were taken and used to perform a physical and chemical characterization study using X-ray diffraction, scanning electron microscopy, neutron activation, X-ray fluorescence, induced coupled plasma-mass spectrometry, among others analytical methods. The results indicate that the material is composed mainly by quartz and goethite, the characteristic size of the particles ( d 50) is about 150 μm, and the density is close of that of quartz. The main constituents are silicon, iron, and aluminum. The tailings gold content is of about 1.8 mg/kg and the mercury content is of about 10 mg/kg. A remarkable feature of this solid waste is that the gold and mercury are both concentrated in both the fine and the coarse particles, but not in particles of intermediary size. Leaching studies indicated that the tailings are stable in weak organic acids, but soluble in alkaline and aired cyanide solutions, in which 89% of gold and 100% of mercury are extracted in 24 h. Electroleaching experiments, performed using sodium chloride as electrolyte, indicated that mercury and gold are extracted simultaneously and the recovery of both metals can be as high as 70% in 4 h. In addition, chromium, nickel, and lead are found in relatively large amounts in the solution, which indicate an effectively action of the electroleaching method to clean up solid wastes contaminated with metals.

Ion-interaction Chromatography of Several Metallocyanide Complexes Stabilized by Adding Cyanide in a Neutral Mobile Phase

Standard solutions (at 10(-5) M levels) of Cu(I)- and Fe(II)-cyanide complexes were stabilized for at least 5 h using 0.5 mM cyanide solution (around pH 9) as a medium. Complexes of Cu(I)- and Fe(III)-cyanide also could be stabilized without any dissociation by adding 1 mM cyanide to an acetonitrile-water (18:82, v/v) mobile phase (pH 7.0) containing 10 mM tetra-n-propylammonium salt (TPA). Under the optimal conditions, the six complexes of Cu(I)-, Ag(I)-, Ni(II)-, Fe(II)-, Fe(III)- and Au(I)-cyanides were resolved from their mixtures within about 45 min, with well-shaped chromatographic peaks.

Leaching of silver sulfide with ferricyanide–cyanide solution

Significantly lower silver extraction than gold has been commonly observed in traditional gold-silver cyanidation process, especially in heap leaching operations. The different mineralogy of gold and silver in the ores is suspected to be the main reason, e.g., the occurrence of low solubility acanthite (Ag 2S) typically results in low overall silver extraction. Leaching of silver sulfide with ferricyanide–cyanide has been investigated with the purpose of increasing the silver extraction. Ferricyanide proved to be an effective oxidant to dissolve silver sulfide in cyanide solution. The main reaction involved under the experimental leaching conditions was determined, indicating the formation of thiocyanate as the sulphur reaction product. The dissolution rate of Ag 2S in ferricyanide–cyanide leaching system (0.5 g/L Fe as ferricyanide) in a rotating disc experiment gives the value of 21 μmol m − 2 s − 1 (0.78 μm/h), much higher than that in aerated cyanide solution (13 μmol m − 2 s − 1 , or 0.38 μm/h). The effect of pH, temperature, ferricyanide and cyanide concentration, and rotating speed on the dissolution rate of Ag 2S were examined. A diffusion-controlled process was confirmed with an activation energy of 6.7 kJ/mol. Study on the leaching kinetics of several mixed sulfide minerals in ferricyanide–cyanide solution shows that the presence of non-silver sulfide minerals may decrease silver extraction from silver sulfide to different extents. A significant decrease in silver extraction is observed in the presence of chalcopyrite (CuFeS 2) and pyrrhotite (Fe 7S 8). It is believed that chalcopyrite and pyrrhotite consume ferricyanide, making it unavailable for silver sulfide leaching.

Toxicity of cobalt-complexed cyanide to Oncorhynchus mykiss, Daphnia magna, and Ceriodaphnia dubia

Cobalt cyanide complexes often result when ore is treated with cyanide solutions to extract gold and other metals. These have recently been discovered in low but significant concentrations in effluents from gold leach operations. This study was conducted to determine the potential toxicity of cobalt-cyanide complexes to freshwater organisms and the extent to which ultraviolet radiation (UV) potentiates this toxicity. Tests were also conducted to determine if humic acids or if adaptation to UV influenced sensitivity to the cyanide complexes. Rainbow trout (Oncorhynchus mykiss), Daphnia magna, and Ceriodaphnia dubia were exposed to potassium hexacyanocobaltate in the presence and absence of UV radiation, in the presence and absence of humic acids. Cyano-cobalt exposures were also conducted with C. dubia from cultures adapted to elevated UV. With an LC50 concentration of 0.38 mg/L, cyanocobalt was over a 1000 times more toxic to rainbow trout in the presence of UV at a low, environmentally relevant irradiance level (4 microW/cm2 as UVB) than exposure to this compound in the absence of UV with an LC50 of 112.9 mg/L. Toxicity was immediately apparent, with mortality occurring within an hour of the onset of exposure at the highest concentration. Fish were unaffected by exposure to UV alone. Weak-acid dissociable cyanide concentrations were observed in irradiated aqueous solutions of cyanocobaltate within hours of UV exposure and persisted in the presence of UV for at least 96 hours, whereas negligible concentrations were observed in the absence of UV. The presence of humic acids significantly diminished cyanocobalt toxicity to D. magna and reduced mortality from UV exposure. Humic acids did not significantly influence survival among C. dubia. C. dubia from UV-adapted populations were less sensitive to metallocyanide compounds than organisms from unadapted populations. The results indicate that metallocyanide complexes may pose a hazard to aquatic life through photochemically induced processes. Factors that decrease UV exposure such as dissolved organic carbon or increased pigmentation would diminish toxicity.

Oxidation of Elemental Gold in Alcohol Solutions

Gold is designated as the noblest metal because of its chemical inertness. It is known to dissolve in cyanide solutions in the presence of air or H2O2 or in halogen-containing solutions, aqua regia being the most famous example. Herein, we report a unique thiol, especially 4-pyridinethiol (4-PS), assisted dissolution of Au in alcohol solutions. Although dissolution was found to be very selective for pyridinethiols, such a phenomenon is astonishing since thiols are commonly used as etch resists for Au and even 4-PS is extensively used as a surface modifier for Au. To gain further understanding of the dissolution process, the influence of the reaction conditions was extensively studied. On the basis of the obtained results, a mechanism for the dissolution reaction is proposed. Fascinatingly, by tuning of the reaction conditions, this phenomenon can be applied in selective preparation of self-supporting nanometer-thick Au foils.

Application of Ag 2X (X = SO 32−, Cr 2O 72−, C 2O 42− and CO 32−) solid-phase reagents for indirect determination of cyanide in the industrial effluent using FIA-FAAS system

Four solid-phase reagents have been tested for indirect determination of cyanide using flow injection analysis-flame atomic absorption spectrometry (FIA-FAAS). The method is based on insertion of aqueous cyanide solutions into an on-line Ag 2X (where X are SO 3 2−, Cr 2O 7 2−, C 2O 4 2− and CO 3 2−) packed column (25%, m/m suspended on silica gel beads) and re-distilled water or sodium hydroxide are used as the carrier stream. The eluent containing the analyte as silver cyanide complexes, produced from reaction between Ag 2X and cyanide, measured by flame atomic absorption spectrometry. The method is simple, fast and selective than other published FIA procedures. A relative standard deviation (R.S.D.) better than 1.12% was obtained in a repeatability study. The method was applied to the determination of cyanide in industrial electrolytic baths.

An electrogenerative process for the recovery of gold from cyanide solutions

Traditional methods for the recovery of gold from electronic scrap by hydrometallurgy were cyanidation followed by adsorption on activated carbon or cementation onto zinc dust and by electrowinning. In our studies, a static batch electrochemical reactor operating in an electrogenerative mode was used in gold recovery from cyanide solutions. A spontaneous chemical reaction will take place in the reactor and generate an external flow of current. In this present work, a static batch cell with an improved design using three-dimensional cathodes namely porous graphite and reticulated vitreous carbon (RVC) and two-dimensional cathode materials, copper and stainless steel plates were coupled with a zinc anode. The electrogenerative system was demonstrated and the performance of the system using various cathode materials for gold recovery was evaluated. The system resulted in more than 90% gold being recovered within 3h of operation. Activated RVC serves as a superior cathode material having the highest recovery rate with more than 99% of gold being recovered in 1h of operation. The morphology of gold deposits on various cathode materials was also investigated.