Thiosulphate Leaching Research Articles

Oxidative pre-treatment in thiosulphate leaching of sulphide gold ores

Abstract A diagnostic leaching showed that partial oxidation of the sulphide minerals in a gold ore was beneficial for thiosulphate leaching of gold. A pre-treatment process with oxidative ammoniacal solution enhanced the thiosulphate leaching of the sulphide ore, while the thiosulphate consumption was substantially reduced. The sulphide minerals partially decomposed in the pre-treatment process, exposing gold to the leach solution. Oxygen input by air bubbling and a longer contact time enhanced the oxidative ammonia pre-treatment process and hence accelerated subsequent thiosulphate leaching of the sulphide ore. Gold extraction in 0.8 M ammonia and 0.1 M thiosulphate solution after 24 h increased from 69% without pre-treatment to 81%, 84%, 90% and 94% respectively after 1, 3, 7 and 22 h pre-treatment. The consumption of sodium thiosulphate was 2.37 kg/t after 24 h leaching without pre-treatment, but was negligible after over 1 h oxidative ammonia pre-treatment. A counter-current leaching process was conducted in the leaching of the sulphide ore. The fresh leachant still gave higher leaching rates in contact with the pre-leached ore, while the pre-used leachant had significantly lower leaching kinetics and overall gold extraction in contact with the fresh ore. This 2-step counter-current leaching process proved that the leachant, other than the passivation, was the determinant factor causing the gold leaching rates to decrease after a certain time of leaching. The findings enable the thiosulphate leaching of high sulphide containing gold ores to be more efficient at lower thiosulphate consumption following the oxidative ammoniacal pre-treatment.

Study of the silver ions cementation after mechanical activation of cementator

Thiosulphate leaching of silver is a proposed alternative to cyanide or thiourea leaching for certain types of refractory silver ores and secondary sources. Traditional method for the recovery of rare metals from various sources by hydrometallurgy is leaching followed by cementation onto zinc powder and electrowinning. Silver cementation is an inexpensive and simple way to deposit thin metal films. Cementation of silver onto zinc powder treated by mechanical activation from thiosulphate solutions was studied by measuring of Ag + ions concentration in this work. The high energy attrition and planetary ball milling in methanol was applied for the mechanical activation of zinc powder. The changes of the particle size distribution and surface area of cementator, occurring during mechanical treatment have been registered. The observed changes in the physico-chemical properties illustrate the possibility of the applied activation to modify the surface and/or volume properties of zinc powder and its further application in well-known hydrometallurgical process of cementation. The study was also focused on the changes in the concentration of zinc and pH as a function of reaction time as well as on the morphology of cementation products on zinc powder.

EXTRACTION OF GOLD AND SILVER FROM TURKISH GOLD ORE BY AMMONIACAL THIOSULPHATE LEACHING

Although cyanide is a widespread leaching reagent for the recovery of precious metals, there still are continuing investigations on alternative processes due to the related environmental problems concerned. Thiosulphate leaching of precious metals is one of the processes that were developed as an alternative and nontoxic technique to conventional cyanidation. The aim of this experimental study was to investigate the possibilities of leaching gold and silver in ammoniacal thiosulphate solutions on a laboratory scale. Samples used in the experiments were taken from Ovacık Gold Mine, located in the Bergama region of Turkey. The influence of temperature, copper sulphate concentration, ammonia concentration, thiosulphate concentration, sodium sulfite concentration, and solid–liquid ratio on gold and silver leaching recoveries was investigated and optimum leaching conditions were determined. At these conditions, 99.57% Au and 95.87% Ag leaching recoveries were achieved.

Interactions between sulphides and manganese dioxide in thiosulphate leaching of gold ores

The interactions between manganese dioxide and pyrite or chalcopyrite were investigated in both the ammoniacal and ammoniacal thiosulphate leaching systems. Pyrite and chalcopyrite dissolved at enhanced rates in the presence of manganese dioxide in both ammoniacal and ammoniacal thiosulphate solutions. The interactions between manganese dioxide and sulphides were also applied in the thiosulphate leaching of gold from a pyrite concentrate and a sulphide ore. The addition of a small amount of manganese dioxide in the thiosulphate leaching of the sulphide bearing gold ores improved both the kinetics and the overall gold extractions without much impact on thiosulphate consumption. A pre-treatment process with copper ammoniacal solutions enhanced gold extractions from the sulphide bearing ores and reduced thiosulphate consumption to a large extent. The sulphides partially degraded in the pre-treatment process likely exposing gold to leach solutions. SEM images of the surface corrosion of pyrite and chalcopyrite in contact with manganese dioxide showed that the pre-treatment with manganese dioxide enhanced the degradation of the sulphide matrices, thus achieved a better gold leaching from the sulphide bearing ores.

The role of oxygen in thiosulphate leaching of gold

The role of oxygen in thiosulphate leaching of gold has been investigated in both pure gold and ore systems. The gold dissolution decreased in the presence of oxygen in the pure gold system. Air or oxygen bubbling caused higher consumption of thiosulphate and leaching passivation. Nitrogen bubbling largely increased gold dissolution and this beneficial effect appeared to be more noticeable at a higher nitrogen flowrate. Nitrogen bubbling stabilised thiosulphate and prevented the passivation of gold. The use of oxygen pre-saturated water had little effect on gold dissolution. The counter current leaching tests showed that both gold passivation and the products due to thiosulphate decomposition affected the gold dissolution. The leaching solutions could be re-used without any impact on gold dissolution under a nitrogen atmosphere. Mineralogy affected the thiosulphate leaching system under different atmospheres. The leaching behaviour of a sulphide ore with only a small amount of sulphides was determined by the leaching behaviour of gold. However, air bubbling gave a higher overall recovery in the leaching of a pyrite concentrate, while nitrogen bubbling only marginally increased the overall gold recovery. Oxygen injection enhanced the dissolution of sulphides, releasing more gold from the sulphide matrices. Nitrogen injection hindered the corrosion process of the sulphides, affecting the overall gold recovery. Nitrogen bubbling decreased the thiosulphate consumption in the leaching of the pyrite concentrate, while air bubbling increased the thiosulphate consumption. It is suggested that high sulphide containing ores should be pre-treated to fully or partially break down the sulphide matrices to liberate gold, followed by gold leaching under a nitrogen atmosphere.

The effect of sulphur species on thiosulphate leaching of gold

Thiosulphate is a metastable anion that tends to readily undergo chemical decomposition in aqueous solutions. The presence of tetrathionate, trithionate and sulphide is unavoidable in the ammoniacal thiosulphate system, as they are products of thiosulphate decomposition. Thiosulphate decomposition is of great importance in the thiosulphate leaching of gold. The effect of three typical sulphur species tetrathionate, trithionate and sulphide on thiosulphate leaching has been investigated in both pure gold and ore systems. The gold dissolution increased at low concentrations of the sulphur species, but decreased at high concentrations. The presence of these sulphur species significantly reduced thiosulphate decomposition, and this beneficial effect became more prominent at high levels of the species. The Eh–pH diagram for the Au–N–S–H 2O system indicated that the sulphide ion could form HS − to complex gold, enhancing gold leaching. Tetrathionate, trithionate and sulphide would convert to thiosulphate with some elemental sulphur being formed via oxidation or reduction routes. The presence of the sulphur species shifted the thiosulphate decomposition reactions, and hence stabilised thiosulphate. Raman spectra demonstrated that the gold surfaces were readily passivated in the presence of the sulphur species, and the passivation tended to be more severe at higher levels of the species.

Effect of hematite on thiosulphate leaching of gold

Hematite, as a typical iron oxide slime in sulphide ore slurries, was artificially added into the leaching systems of pure gold and a sulphide ore respectively, in an attempt to investigate the effect of iron oxide slimes on the ammoniacal thiosulphate leaching of gold. The presence of hematite significantly reduced the dissolution of gold and this detrimental effect became more pronounced with increasing hematite concentration. Hematite formed coatings on gold surfaces, which could prevent leach solution from diffusing to the gold surfaces and hence, inhibit gold dissolution. Hematite catalysed the oxidative decomposition of thiosulphate to polythionates with oxygen present. XPS studies indicated a thin layer of iron oxide coating as well as the deposition of some copper and sulphur species on gold surfaces. SEM images revealed a lesser extent of corrosion for gold after leaching in the presence of hematite. The gold extraction from the sulphide ore was reduced with the addition of hematite and this effect became more noticeable with an addition of hematite at a higher concentration. A natural guar type surfactant (Gempolym M47) reduced the detrimental effect of hematite on gold extraction likely due to the prevention of hematite coating on gold and mineral particles and the dispersion of the mineral slurry. Gempolym M47 stabilised thiosulphate by weakening the interaction between cupric ions and thiosulphate and by minimising the catalytic effect of hematite on thiosulphate decomposition.

Ammoniacal thiosulphate leaching of gold in the presence of pyrite

The effect of pyrite on gold dissolution was investigated in the ammoniacal thiosulphate leaching system using pure gold foils. Special emphasis was placed on gold leaching in association with pyrite dissolution, thiosulphate decomposition and gold leaching passivation. The presence of pyrite retarded gold dissolution and this detrimental effect became more pronounced at higher pyrite contents. Pyrite catalysed the decomposition of thiosulphate to trithionate. The addition of sulphate enhanced gold leaching by retarding the dissolution of pyrite in ammoniacal thiosulphate solutions. XPS analysis indicated the presence of iron hydroxide species at the gold surfaces in the presence of pyrite, which was likely responsible for the reduced gold dissolution. SEM analysis indicated a lesser extent of gold dissolution occurring in the presence of pyrite.

Aplicación de modelos cinéticos en la cementación de oro con cinc desde soluciones con tiosulfato y amoniaco

Being thiosulphate leaching of gold one of the proposed alternatives to conventional cyanide leaching of several gold ores, different approaches for metal recovery from pregnant thisulphate solutions had been considered and from these, gold cementation by zinc had not been widely study. In the present work, several kinetics models had been tested on experimental results of gold cementation by powdered zinc, and under varying conditions results are best fitted to the passivation model. Overall, the cementation of the precious metal by zinc from ammoniacal thiosulphate solutions was concluded to be under mass transfer control, higher temperatures and pH/ammonia concentration were established to positively affect the rate of gold cementation.

Optimization of extraction of silver from silver sulphide concentrates by thiosulphate leaching

A novel method of extracting silver from silver sulphide concentrates is provided in this study. The method is selective for silver in the presence of 1.0 M of thiosulphate solution with acetic acid–ammonium acetate buffer at pH 4.15. The conditions are calculated using related equilibria constants and then tested with experiments.

Combined mechanochemical and thiosulphate leaching of silver from a complex sulphide concentrate

The intensification of the thiosulphate leaching of silver from a silver-bearing complex sulphide concentrate (Casapalca deposit) using mechanochemical alkaline leaching as the pretreatment step was investigated. The leaching of “as-received” concentrate with the thiosulphate solution afforded only 6% recovery of Ag into leach. The mechanochemical pretreatment resulted 85% amorphization of tetrahedrite as a silver-bearing mineral in the concentrate as well as an increase in specific surface area from the original value of 0.26 m 2 g −1 to a maximum value of 15.7 m 2 g −1. The pretreatment was performed in a stirred ball mill using variable milling times and sample weights. The physico-chemical changes in the concentrate as a consequence of mechanochemical pretreatment had a pronounced influence on the subsequent silver extraction. The optimum results from mechanochemical pretreatment and subsequent leaching of the concentrate with ammonium thiosulphate were achieved by using a milling time of 60 min and a weight of sample of 60 g. In this case, 99% recovery of Ag was achieved after only 3 min of leaching.

Gold recovery from arsenopyrite refractory concentrates using hydrometallurgical technology

Gold recovery from arsenopyrite carbon-containing concentrates obtained by ore flotation concentration has been studied. The breakdown of concentrates was carried out with HNO3 (1·5–3 mol/l) at 80–85°C. Thiosulphate leaching (0·5 M Na2S2O3 + 0·5 M NH3) of gold from the broken down concentrates was investigated.

Influence of mechanochemical pretreatment on leaching of silver in cyanide and non-cyanide medium

The leaching of silver from a mechanochemically pretreated silver-bearing complex sulphide concentrate of Peruvian origin (Casapalca) has been studied. The results with as-received concentrate have shown very low extraction of silver (up to 5.7%) in all cases of leaching. The mechanochemical pretreatment has caused 85% amorphization of tetrahedrite as silver-bearing mineral and an increase in the specific surface area of the concentrate from 0.3 m2 g−1 to a maximum value of 15.7 m2 g−1. This fact manifested itself in the subsequent process of silver extraction. By application of thiosulphate and thiourea leaching 99% recovery of Ag was reached already after 3-5 min of leaching.

Leaching behaviour of sulphides in ammoniacal thiosulphate systems

Systematic studies have been conducted to understand the leaching behaviour and dissolution mechanisms when common gold-host sulphides such as chalcopyrite, pyrite, arsenopyrite and pyrrhotite are treated by oxidative ammoniacal thiosulphate leaching. Leach solution composition strongly influenced the sulphide leaching, and the presence of sulphides also enhanced the decomposition of thiosulphate. The XRD patterns of the binary mixtures before and after leaching indicated that the relative leaching rates of the sulphides in the ammoniacal thiosulphate system were in the order chalcopyrite>pyrrhotite>arsenopyrite>pyrite, which was in accordance with the observations made from the leaching tests. SEM analysis with the aid of EDAX indicated the formation of iron oxide at the chalcopyrite, pyrrhotite and pyrite surfaces and the formation of iron arsenate at the arsenopyrite surface after leaching. SEM analysis also demonstrated that the high-energy defect sites and crystal boundaries favoured the sulphide leaching. Raman spectroscopy indicated that haematite was formed during the leaching of chalcopyrite. Iron and arsenic concentrations in the leach solutions were very low due to the formation of iron oxide and iron arsenate during the leaching reactions. Pyrite enhanced chalcopyrite and sphalerite dissolution. Chalcopyrite and sphalerite also enhanced pyrite dissolution.

Preg-robbing phenomena in the thiosulphate leaching of gold ores

The ability of various sulphide and gangue minerals and gold ores to adsorb gold during thiosulphate leaching was examined. The effect of free thiosulphate concentration, metal species, gold thiosulphate concentration, ammonia concentration, particle size and cupric addition on preg-robbing was examined in detail, and likely preg-robbing mechanisms were proposed as well. It was found that all the gold ores were highly preg-robbing in thiosulphate deficient systems and that the preg-robbing process was fairly fast. The presence of free thiosulphate could greatly decrease or eliminate preg-robbing in both mineral and gold ore systems, depending on its concentration. Physical adsorption of gold was reversible in the presence of thiosulphate. Lead and zinc ions could reduce gold adsorption to some extent, and competitive adsorption of the gold thiosulphate with the lead and zinc thiosulphate species on the minerals or ores was expected. The gold elution tests demonstrated that preg-robbing was dominated by physical adsorption for the gangue minerals and was controlled by both chemical and physical adsorption for the sulphide minerals. The ability of an ore to adsorb the gold thiosulphate species from solution was found to be intimately associated with the free thiosulphate concentration. Surface topological studies by SEM coupled with EDS showed that gold was adsorbed at defect sites on the surfaces of the minerals.

Sorption recovery of gold from thiosulphate solutions after leaching of products of chemical preparation of hard concentrates

Gold recovery from arsenopyrite coaly hard concentrates obtained by ore flotation concentration has been studied. The thiosulphate leaching of the products obtained after the chemical preparation of the concentrates was carried out in the system Na 2S 2O 3–NH 3–H 2O at 20–22°C. The gold recovery on ion exchangers and carbon adsorbents from the thiosulphate solutions was investigated. It was found that the use of strong basic or polyfunctional anion exchangers of macroporous or porous structure is the most applicable for hydrometallurgical technology.

A study of the factors affecting copper cementation of gold from ammoniacal thiosulphate solution

Thiosulphate leaching of gold is a proposed alternative to cyanide leaching for certain types of refractory gold ores. The most promising method for recovering gold from thiosulphate leach solutions is by copper cementation. This study focused on determining how the factors of pH/ammonia concentration, copper concentration, thiosulphate concentration, sulphite concentration, and temperature, commonly manipulated in thiosulphate leaching studies, affect gold cementation performance using copper. These factors were varied in artificial leach solutions according to the ranges typically found in the literature. Both powder cementation and rotating disk electrode cementation were used to study the kinetics of gold cementation. Overall, the cementation of gold by copper from ammoniacal thiosulphate solution was concluded to be under mass transfer control. Higher temperatures and pH/ammonia concentration were determined to positively affect the rate of gold cementation. Conversely, sulphite and copper in solution were established to negatively affect cementation performance.

Gold extraction from a copper bearing ore by thiosulphate leaching.

Gold extraction by cyanidation is reduced to less than 30% as the copper content in the ore is higher than 0.3% Cu. Thiosulphate leaching process could overcome this problem. The effect of the variables on the gold extraction have been investigated. Under the optimun leaching conditions: 0.4 M thiosulphate, 0.2 M ammonia, 0.03 M cupric ion and pH 11, 90% of gold was extracted. The possible processes for gold recovery from the pregnant solution was also discussed.

Thiosulphate leaching for gold hydrometallurgy

Thiosulphate leaching of precious metals has been developed as an alternative and non-toxic technique to the conventional cyanidation. This process has advantages over cyanide in a decreased interference from foreign cations and a lower environmental impact. Ammoniacal thiosulphate solution allows the solubilization of gold as stable anionic complex. Leaching of gold occurs at appreciable dissolution rates. The aim of this work is devoted to a preliminary evaluation of the feasibility of thiosulphate leaching for the extraction of gold from precious metals ores on a laboratory scale. The experimental work has allowed the authors to point out the influence of temperature, thiosulphate concentration, ammonia concentration and copper sulphate concentration on the gold dissolution from an ore (51.6 g/t Au) that originated from the Dominican Republic. Gold was recovered from the leach liquors by adsorption onto activated carbon or by electrowinning. With the optimisation of the process parameters about 80% gold recovery has been attained, as in the case of conventional cyanidation.

5366092 Separation of valuable minerals from material found in the beds of bodies of water

The ability of various sulphide and gangue minerals and gold ores to adsorb gold during thiosulphate leaching was examined. The effect of free thiosulphate concentration, metal species, gold thiosulphate concentration, ammonia concentration, particle size and cupric addition on preg-robbing was examined in detail, and likely preg-robbing mechanisms were proposed as well.It was found that all the gold ores were highly preg-robbing in thiosulphate deficient systems and that the preg-robbing process was fairly fast. The presence of free thiosulphate could greatly decrease or eliminate preg-robbing in both mineral and gold ore systems, depending on its concentration. Physical adsorption of gold was reversible in the presence of thiosulphate. Lead and zinc ions could reduce gold adsorption to some extent, and competitive adsorption of the gold thiosulphate with the lead and zinc thiosulphate species on the minerals or ores was expected.The gold elution tests demonstrated that preg-robbing was dominated by physical adsorption for the gangue minerals and was controlled by both chemical and physical adsorption for the sulphide minerals. The ability of an ore to adsorb the gold thiosulphate species from solution was found to be intimately associated with the free thiosulphate concentration. Surface topological studies by SEM coupled with EDS showed that gold was adsorbed at defect sites on the surfaces of the minerals.