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© 2017 Canadian Institute of Mining, Metallurgy and Petroleum The technology to selectively leach gold, silver and base metals from ores, concentrates, and wastes, using an alkaline glycine solution, has initiated significant interest in using the new technology in various environments such as in situ, in-place, dump, heap, and vat and agitated tank leaching. It has been shown that glycine acts synergistically with a number of other lixiviants to lower the net consumption of the other lixiviants while allowing low-cost glycine solution recovery. Glycine is a non-toxic, stable, environmentally benign reagent that is available in bulk industrial quantities. It has the ability to dissolve most copper oxide and sulphide minerals, as well as native copper, whilst not interacting with acid-consuming gangue nor dissolving iron and various or ubiquitous gangue elements. It acts synergistically with small amounts of cyanide to leach gold-copper ores at leach rates higher than either glycine or cyanide on their own, while significantly reducing cyanide consumption and eliminating detoxification requirements. The leaching behaviour of various glycine-based systems will be reviewed, followed by the evaluation of the adsorption of gold onto activated carbon. Where the economics merit it, copper can be easily recovered using solvent extraction or sulphide precipitation.

Glycine solutions in an alkaline environment have been shown to be able to leach copper from its oxide, native and sulfide minerals as a copper glycinate complex. This paper explores the recovery of copper from its aqueous glycinate complexes at alkaline pH through solvent extraction. Solvent extraction of copper from synthetic glycinate liquor (with background sulfate anions) containing 2 g/L Cu has been studied using Mextral 84H (2-hydroxy-5-nonylacetophenone oxime) and Mextral 54–100 (1-benzoyl-2-nonyl ketone) extractants with a kerosene diluent. The effect of different parameters such as pH of aqueous phase, glycine and extractant concentrations, equilibrium time, temperature, extractant loading capacity as well as the concentration of sulphuric acid as stripping reagent were investigated. Over 99.9% Cu was extracted with 5% Mextral 84 H at room temperature at an Aqueous-to-Organic (A:O) ratio of 2:1. Under similar conditions, 95.87% Cu was extracted by 10% Mextral 54–100. HPLC analysis for free glycine indicated that neither extractants take up glycine during the extraction process. Stripping experiments with spent electrolyte solution showed that copper loaded on Mextral 54–100 can be completely stripped in single stage at an O:A ratio of 10:1 while only 79.95% Cu can be stripped from Mextral 84H with same conditions applied. Copper extraction from leachates of malachite and chalcopyrite ore samples was performed in order to validate extraction results from the synthetic solutions. Over 99% copper was extracted from the malachite leachate (containing 2620 mg/L Cu) in a single stage by both 10% Mextral 54–100 (raffinate pH of 9.76) and 5% Mextral 84H (raffinate pH of 10.0) at an O:A of 2:1; with no impurity elements picked up by either extractants. At an O:A ratio of 2:1, single stage copper extraction from chalcopyrite leachate by 10% Mextral 54–100 (raffinate pH of 9.87) was 93.20% and 95.57% by 5% Mextral 84H (raffinate pH of 9.91) with the rejection of all

© 2017 Elsevier B.V.The presence of cyanide soluble copper in the cyanidation of gold-copper ores and concentrates significantly increases the cyanide consumption in order to achieve sufficient gold recovery. In addition, cyanide recovery or cyanide detoxification/destruction processes are also required which adds extra cost to the process. This research introduces a leaching approach to extract gold, silver and copper from gold-copper ores and concentrates using a synergistic lixiviant leaching process using glycine in the presence of low concentrations of cyanide. The effects of glycine and cyanide concentrations on gold, silver and copper leaching kinetics and recovery were studied. It is shown that, in the presence of glycine, gold, silver and copper extraction increase significantly in solutions containing copper-cyanide species at a very low, or zero, free cyanide concentration. It has also been shown that the gold dissolution rate in glycine-cyanide system is almost three times higher than the gold dissolution rate in the conventional cyanidation. Kinetic studies were conducted to evaluate the effects of glycine concentration, pH, and CN/Cu ratio on the dissolution of gold and silver. It has also been shown that the gold dissolution rate increases by increasing the glycine concentration up to 2.0 g/L (Gly:Cu molar ratio of 2.2:1) and any further glycine addition has no significant effect on the dissolution of gold. It is shown that the presence of glycine in solutions containing copper (I)-cyanide species can significantly enhance the dissolution of precious and base metals. The proposed leaching approach significantly reduces the cyanide consumption by at least 75% and most of the copper is present as cupric glycinate in the final leach solution. In addition, gold, silver and copper extraction was higher than the conventional cyanidation process utilising similar cyanide dosages for all the gold-copper sources studied.

A conceptual flowsheet is proposed and the main processing steps are evaluated for the alkaline processing of chalcopyrite where glycine is the complexing agent. Glycine is utilised in an oxidising, alkaline environment to leach chalcopyrite at atmospheric pressure and mildly elevated temperatures. Process steps to recover copper and glycine from alkaline aqueous solutions were also investigated. The leaching of chalcopyrite flotation concentrate in glycine solutions was conducted at different leach conditions in a 1.25 L leach reactor with an agitated slurry and controlled dissolved oxygen (DO) concentration. In the presence of air, oxygen or hydrogen peroxide or a mixture thereof, glycine can dissolve copper from chalcopyrite at either ambient or elevated (40–60 °C) temperatures and atmospheric pressure. Increasing temperature, pH, glycine concentration and DO concentration all increase the rate and extent of copper extraction. The extraction of copper from “as-received” chalcopyrite flotation concentrate, at a particle size of 100% -45 µm, in solutions containing 0.4 M glycine at 60 °C with 25 ppm DO, was 40.1% after 24 h. If the chalcopyrite concentrate underwent an ultrafine grind to100% -10 µm with prior alkaline atmospheric pre-oxidation, 92% of the copper is leached within 17 h at 60 °C, atmospheric pressure and 9% solids during a batch leach. Pyrite associated with the chalcopyrite remained unreacted during leaching of chalcopyrite and iron concentration in the final pregnant solution was found to be less than 20 mg/L. Copper recovery by sulfide precipitation from the leach solution as pure covelite was up to 99.1% at a Cu:S2- molar ratio of 1:1. Solvent extraction (SX) experiments with LIX 84-I demonstrated that copper can be extracted into the organic phase up to 99.4% in a single stage at an equilibrium pH range 8.8–10.0. It is shown that copper can be stripped from the organic phase in a single stage acidic strip using conventional acidic return electro

Copper oxide minerals are normally extracted by acidic leaching followed by copper recovery with solvent extraction and electrowinning. However, copper oxide deposits often containing large amounts of acid consumable gangue leads to a very high acid consumption. In addition, if the oxide deposit also contains precious metals and iron bearing minerals, significant (lime) neutralisation costs are incurred to establish the conditions for subsequent cyanidation, with concomitant production of gypsum, potential formation of silver-locking jarosites and potential gel formation when acids interact with layer silicate minerals. In this study, an alternative aqueous alkaline glycine system has been employed to evaluate the batch leaching behaviour of copper oxide mineral specimens of the minerals azurite, chrysocolla, cuprite and malachite. The effects of glycine concentration and pH were investigated at ambient temperature and atmospheric pressure. Glycine concentration and pH both had a major effect on the copper extraction. Complete extraction of copper from azurite was achieved in < 6 h when glycine to copper ratio was 8:1. However, further investigation established the optimum leaching conditions as being pH 11 and glycine to copper ratio of 4:1. Under such conditions 95.0%, 91.0%, 83. 8% and 17.4% of copper was extracted after 24 h from the azurite, malachite, cuprite, and chrysocolla mineral specimens respectively. While the dissolution rates of the copper oxide minerals are markedly slower than acid leaching, the selective dissolution of copper over acid-consuming gangue minerals shows much potential. It was shown through UV–Vis spectroscopy that dissolved copper is in the cupric state and forms a neutral copper-glycinate complex under alkaline conditions. The study has shown that copper extraction from malachite, azurite and cuprite in aqueous alkaline glycine solutions is fast, whereas Cu extraction from chrysocolla was found to be poor and slow.

The leaching of gold and silver in a pH modified glycine solution in the presence of hydrogen peroxide was investigated. It was found that this lixiviant system can dissolve gold and/or silver under neutral and alkaline conditions at ambient to moderately elevated temperatures (23–60 °C). The gold leaching rate in solutions containing 0.5 M glycine and 1% peroxide at pH 11 after 48 h of leaching was found to be 0.322 μmol/m2 s, which is comparable to gold leaching rate after six days in thiosulphate–EDTA or thiosulphate–oxalate systems in the presence of thiourea (0.22–0.25 μmol/m2 s). In alkaline glycine–hydrogen peroxide solutions, it has been found that gold leach rate from gold–silver (50% Ag) alloy is about 6 times higher than rate from pure gold and the silver leach rate was 0.247 μmol/m2 s. An induction period of 48 h was noted for rolled precious metal foils, after which leaching rapidly accelerated. The reaction rate sensitivity to temperature can be explained by a mechanism that is chemical reaction controlling rather than mass transfer controlling as found for cyanide leaching. Precious metal dissolution rates increased with increased glycine concentration, increased silver content, and increasing pH (up to pH 11). The gold–glycinate complex was found to effectively load on activated carbon up to 13.2 g Au/kg carbon in 4 h.

The leaching of gold using alkaline amino acids–hydrogen peroxide solutions at low concentrations has been studied. The application of alkaline amino acid–hydrogen peroxide system may offer an alternative and environmentally benign process for gold leaching, particularly in the context of leaching low grade gold ores in an in-situ or in heap leach processes. In the presence of an oxidant or oxidants, it was found that amino acids can dissolve gold at alkaline condition at low and moderate temperature. Heating the leach solution between 40 and 60 C was found to enhance the gold dissolution significantly in alkaline amino acid–peroxide solutions. It was also found that gold dissolution increases by increasing amino acid concentration, peroxide and pH. Amino acids acts synergistically to dissolve gold. Although glycine showed the highest gold dissolution as a single amino acid compared to histidine and alanine, histidine was found to enhance gold dissolution when used in equimolar amounts with glycine. The presence of Cu2+ ion enhances gold dissolution in the glycine–peroxide solutions. The process will propose an environmentally benignprocess for gold treatment in order to replace the use of cyanide in heap or in-situ leaching. In the presence of pyrite, the amount of gold leached was lower due to the peroxide consumption in sulphide oxidation.

© 2015 Elsevier B.V. Abstract In the cyanidation of copper-gold ores, the presence of copper minerals can lead to soluble gold losses, the production of weak acid dissociable (WAD) cyanide, as well as a number of operational challenges in CIP/CIL circuits with regard to competitive adsorption, and subsequent difficulties associated with elution, electrowinning and smelting. In addition, copper minerals are significant cyanide consumers, leading to higher cost in ore treatment. This paper presents a process to enhance the dissolution of gold using copper-cyanide solutions in the presence of glycine where the solution is cyanide starved. The effect of glycine addition on gold leaching kinetics in copper-cyanide solutions under different leaching conditions was studied. The results show that, in the presence of glycine, gold dissolution rate increases significantly in solutions containing copper-cyanide species at a very low, or zero, free cyanide concentration. In the presence and absence of glycine, gold dissolution rates in solutions containing 10 mM Cu(CN)32 - were 11.1 µmol/m2·s and 0.65 µmol/m2·s, respectively. It is shown that the average gold dissolution rate in a Cu-CN--glycine system is about 6.5 times higher than the gold dissolution rate in the conventional cyanidation, the presence of cyanide being similar in each system. Kinetic and electrochemical studies were conducted to evaluate the effects of glycine concentration, pH, CN/Cu ratio, and initial copper concentrations on the dissolution of gold. It has also been shown that the gold dissolution rate increases by increasing the glycine concentration up to 1 g/L and any further glycine addition has a negative effect on the dissolution of gold. Increasing leaching pH up to 12 enhances gold dissolution in the copper-cyanide-glycine solutions. However, increasing leaching pH to 13 has an adverse

In the cyanidation process, copper–gold deposits containing significant amounts of cyanide soluble copper can lead to high cyanide consumption with low gold extraction. The significant levels of weak acid dissociable (WAD) cyanide must then be destroyed prior to tailings disposal. This increases the cost to the gold mining companies to cover both the extra cyanide consumption in the leaching stage and the additional cost for cyanide recovery/destruction. Therefore, in this study, a selective cyanide leaching process of gold over copper from copper–gold concentrate (also see Oraby and Eksteen, 2014) containing 490 g/t Au and 0.97% Cu present as metallic copper, oxides and sulfides has been investigated. To decrease the cyanide consumption, cyanide is added into the leach solutions to maintain a ratio of cyanide to total reactive copper (CN/Cu) below 2. At low CN/Cu ratio, increasing the pH of solution can lead to the precipitation of solubilized copper as CuO/Cu(OH)2, releasing cyanide ions for further both gold and copper dissolution. A comparison of leaching gold in cyanide–caustic, cyanide–ammonia and conventional cyanidation processes has been made. Higher gold extraction and lower copper concentration in the final leach was achieved in the cyanide–caustic system than in either the cyanide–ammonia or conventional cyanidation processes. The effects of the caustic soda (pH) concentration or cyanide concentration on gold extraction and copper suppression have also been studied. The results show that at high pH (>12), the gold dissolution rate increases significantly in solutions containing caustic soda and cyanide at zero, or very low free cyanide concentration.

The presence of copper minerals with gold is known to lead to many challenges during the cyanidation of gold ores, such as high consumption of cyanide with low gold extraction and undesirable impacts on gold recovery during the downstream processes. An alternative selective leaching process for copper minerals from copper–gold gravity concentrate (3.75% Cu, 11.6% Fe, 11.4%S and 0.213% Au) using alkaline glycine solutions was studied and evaluated. The lixiviant system containing glycine and peroxide showed that total copper dissolution of 98% was obtained in 48 h at ambient conditions and a pH of 10.5–11. The results show that 100% of chalcocite, cuprite, metallic copper, and about 80% of chalcopyrite in the concentrate were also dissolved. Pyrite remained intact during the leaching time and iron concentration in the final pregnant solution was found to be 12 mg/L when copper is solution is at 4745 mg/L, whilst the gold concentration was limited to 0.8 mg/L Au. QEMScan analysis indicated that unleached copper in the leach residue was mostly distributed amongst larger chalcopyrite grains and covellite.The effects of single versus two-stage leaching, pH, oxidant concentration, pulp density and glycine concentration on copper extraction rate and extent were explored

One promising alternative to cyanide for the leaching of gold is thiosulfate due to its low toxicity and efficiency in leaching gold from low-grade and carbonaceous ores and concentrates. Much of the published research focuses to optimizing the reagent concentrations to develop a suitable process for industrial applications. However, there are few studies on the effect of silver content on gold dissolution rate in the thiosulfate leaching system. In this paper, the dissolution rate of pure gold, pure silver, and gold–silver alloys of silver compositions, 4, 8, 20, and 50 wt% are reported. The effects of various parameters on the rate of gold and silver dissolution have been determined. The results show that the pure silver dissolution rate is around 6.89 times greater than pure gold. The results show that silver enhances gold dissolution in thiosulfate solutions. The dissolution rate of gold from pure gold is higher than that obtained from 4% and 8% silver alloys, but less than that obtained from 20% and 50% silver alloys.

Recently, many studies have focused on thiosulfate as one of the promising alternatives to cyanide for gold leaching. However, few studies have evaluated the leaching of silver in thiosulfate solutions. In this study, the leaching of pure silver (99.99% Ag) in thiosulfate solutions containing copper and ammonia is investigated. The effects of various parameters on the silver leaching kinetics were evaluated using a rotating disc technique in open and closed vessels. It was found that the leaching rate of silver in the closed vessel was greater than that obtained in the open vessel. Silver leaching rates were affected positively by increasing ammonia, thiosulfate, and copper(II) concentrations. An electrochemical study showed that increasing thiosulfate and ammonia concentrations has positive effects on the anodic dissolution of silver, however, the presence of thiourea has a negative effect on the anodic dissolution of silver. Finally the study showed that the optimum leaching conditions for silver is: 10 mM Cu(II), 0.8 M NH3 , 0.2 M (S2O3)2-, 30 ºC, 300 rpm, and pH =11.5.