Sulfur Product Layer Research Articles

Leaching Chalcopyrite Concentrate with Oxygen and Sulfuric Acid Using a Low-Pressure Reactor

This article presents a copper leaching process from chalcopyrite concentrates using a low-pressure reactor. The experiments were carried out in a 30 L batch reactor at an oxygen pressure of 1 kg/cm2 and solid concentration of 100 g/L. The temperature, particle size and initial acid concentration were varied based on a Taguchi L9 experimental design. The initial and final samples of the study were characterized by chemical analysis, X-ray diffraction and particle size distribution. The mass balance showed that 98% of copper was extracted from the chalcopyrite concentrate in 3 h under the following experimental conditions: 130 g/L of initial sulfuric acid concentration, temperature of 100 °C, oxygen pressure of 1 kg/cm2, solid concentration of 100 g/L and particle size of −105 + 75 μm. The ANOVA demonstrated that temperature had the greatest influence on copper extraction. The activation energy was 61.93 kJ/mol. The best fit to a linear correlation was the chemical reaction equation that controls the kinetics for the leaching copper from chalcopyrite. The images obtained by SEM showed evidence of shrinking in the core model with the formation of a porous elemental sulfur product layer.

The role of silver-enhanced pyrite in enhancing the electrical conductivity of sulfur product layer during chalcopyrite leaching in the Galvanox™ process

The rate of chalcopyrite leaching in ferric sulfate media during the Galvanox™ process increases significantly in the presence of silver-enhanced pyrite. The increase in the kinetics of leaching is attributed to the galvanic interaction between pyrite and chalcopyrite. Pyrite provides an additional surface area for the cathodic reaction. The oxidation reaction rate increases to compensate for the increased consumption of electrons. Pyrite is more noble than chalcopyrite; hence chalcopyrite leaches preferentially under oxidizing conditions. This leads to the acceleration of the rate of copper extraction. However, an insulating product layer of elemental sulfur forms on the surface of chalcopyrite as leaching proceeds. Formation of this non-conductive layer limits the electrical contact between pyrite and unleached chalcopyrite. In the presence of silver-enhanced pyrite, a very small proportion of the added silver to pyrite dissolves in the leaching solution and reacts with chalcopyrite particles to form silver sulfide. In this study, increases in the conductivity of the sulfur layer sufficient to allow the rapid transfer of electrons are observed, in spite of very low levels of silver in the sulfur product layer (60–100ppm). Hence, it is shown that in the presence of silver-enhanced pyrite, the resistivity of the sulfur layer is lowered, allowing the electrical contact between pyrite and chalcopyrite to be maintained.

The mechanism of chalcopyrite leaching in the presence of silver-enhanced pyrite in the Galvanox™ process

A dramatic improvement in the leaching kinetics of chalcopyrite in ferric sulfate media was observed upon the addition of silver-enhanced pyrite. In this process, pyrite samples are pretreated with a miniscule amount of silver and used as a catalyst in atmospheric leaching of chalcopyrite. The aim of this study was to understand the mechanism of this process and to reveal the role of silver and pyrite in accelerating the rate of chalcopyrite leaching. It is shown that this process, known as Galvanox™, is the galvanically assisted leaching of chalcopyrite, where pyrite provides an alternative surface for ferric reduction. As the leaching reaction proceeds, in the absence of silver-enhanced pyrite, a non-conductive sulfur layer forms on the surface of chalcopyrite. This layer prevents electrical contact between pyrite and chalcopyrite particles in the slurry. However, it is observed that, in the presence of silver-enhanced pyrite, a small proportion of the added silver to the pyrite dissolves and reacts with the elemental sulfur around chalcopyrite particles to form silver sulfide. In spite of the very low silver level in this layer, its conductivity increases significantly and allows the transfer of electrons from chalcopyrite to pyrite. In this study, it is shown that pyrite and silver have critical roles in enhancing the kinetics of chalcopyrite leaching. Pyrite provides an alternative surface area for ferric reduction. Silver increases the catalytic properties of pyrite and therefore, the rate of ferric reduction on the surface of silver-enhanced pyrite is faster than this rate on the surface of natural pyrite. Another role of silver is to decrease the resistivity of the sulfur product layer and to facilitate the electrical contact between pyrite and chalcopyrite.

Bioleaching kinetics and multivariate analysis of spent petroleum catalyst dissolution using two acidophiles

Bioleaching studies were conducted to evaluate the recovery of metal values from waste petroleum catalyst using two different acidophilic microorganisms, Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans. Various leaching parameters such as contact time, pH, oxidant concentration, pulp densities, particle size, and temperature were studied in detail. Activation energy was evaluated from Arrhenius equation and values for Ni, V and Mo were calculated in case of both the acidophiles. In both cases, the dissolution kinetics of Mo was lower than those of V and Ni. The lower dissolution kinetics may have been due to the formation of a sulfur product layer, refractoriness of MoS 2 or both. Multivariate statistical data were presented to interpret the leaching data in the present case. The significance of the leaching parameters was derived through principle component analysis and multi linear regression analyses for both iron and sulfur oxidizing bacteria.

Sulfur Product Layer in Sphalerite Biooxidation: Evidences for a Mechanism of Formation

The sphalerite-pyrite oxidation by Acidithiobacillus ferrooxidans was studied to analyze how the formation of the elemental sulfur layers occurs around sphalerite grains. Two possible mechanisms of formation have been raised. One mechanism corresponds to the formation of sulfur pseudomorphs where, both, iron and zinc have been leached of the sphalerite, whereas compound sulfur is oxidized, in situ, to elemental sulfur, leaving an unreacted core of sphalerite that remains in the center. Another mechanism consists in the dissolution of iron, zinc and sulfur presents. When the attack by Fe3+ as by H+ broken the S-metal bonds, sulfur is then oxidized to a series of sulfur intermediate compounds. These compounds in solution then are oxidized to elemental sulfur, which precipitates on sphalerite grains.

Relative importance of diffusion and reaction control during the bacterial and ferric sulphate leaching of zinc sulphide

A study has been undertaken on the bacterial and ferric sulphate leaching of sphalerite, with the aim of observing the relative importance of both diffusion and reaction control, across a range of operating conditions. To facilitate this kinetic analysis a mixed-control rate equation was developed based upon the shrinking particle and shrinking core models, featuring both an intrinsic rate of zinc extraction and a product layer diffusion coefficient. The ferric sulphate oxidation of a pure sphalerite sample was first studied, and the process was found to be mainly controlled by surface diffusion, thought to be due to formation of the elemental sulphur product layer. The mesophilic bacterial oxidation of a high-grade zinc ore was studied at 35 °C, and the process was observed to be controlled by both chemical reaction and diffusion. However, during the mesophilic bacterial oxidation of a low-grade zinc ore at 25 °C, diffusion was found to be rate limiting. It was proposed that this diffusion resistance arises from an unreacted layer of gangue material.

Leaching of zinc sulfide by Thiobacillus ferrooxidans: bacterial oxidation of the sulfur product layer increases the rate of zinc sulfide dissolution at high concentrations of ferrous ions.

This paper reports the results of leaching experiments conducted with and without Thiobacillus ferrooxidans at the same conditions in solution. The extent of leaching of ZnS with bacteria is significantly higher than that without bacteria at high concentrations of ferrous ions. A porous layer of elemental sulfur is present on the surfaces of the chemically leached particles, while no sulfur is present on the surfaces of the bacterially leached particles. The analysis of the data using the shrinking-core model shows that the chemical leaching of ZnS is limited by the diffusion of ferrous ions through the sulfur product layer at high concentrations of ferrous ions. The analysis of the data shows that diffusion through the product layer does not limit the rate of dissolution when bacteria are present. This suggests that the action of T. ferrooxidans in oxidizing the sulfur formed on the particle surface is to remove the barrier to diffusion by ferrous ions.

A kinetic study on the acid pressure leaching of pyrrhotite

Natural monoclinic pyrrhotite particles (Fe 1− x S) were subjected to pressure leaching by oxygen in sulphuric acid solutions at temperatures ranging between 353 and 453 K (80–180°C). For temperatures below the melting point of sulphur (392 K), the rate of pyrrhotite oxidation shows a moderate dependence on temperature, while it is totally independent of sulphuric acid concentration. Nonetheless, in the absence of oxygen, as much as 30% of the mineral can be dissolved in 0.5 mol/l H 2SO 4. The conversion data were found to fit well to a shrinking-core model with mixed control by half-order surface reaction and oxidant diffusion though a product layer. Despite the high initial reactivity of pyrrhotite, complete oxidation of the mineral was never achieved at temperatures below 393 K, apparently due to an impervious sulphur product layer covering the particles. Complete pyrrhotite oxidation was achieved at temperatures above the melting point of sulphur and only with the use of lignin sulphonate as dispersant of molten sulphur. By analysing the conversion data with the shinking-core model, pyrrhotite oxidation in the high temperature range (403–453 K) was found to be surface-reaction controlled and of first order with respect to oxygen pressure.

An electrochemical study of the oxidative dissolution of synthetic pentlandite in aqueous media

Pentlandite (Fe 4.5Ni 4.5S 8) and violarite (FeNi 2S 4) were synthesised from their elements by dry in vacuo techniques. The products were analysed by reflected light microscopy, powder X-ray diffraction and electron microprobe analysis. The synthetic pentlandite was found to have an average stoichiometry of Fe 4.35Ni 4.65S 8. A partial phase segregation of pentlandite into heazlewoodite (Ni 2.7Fe 0.2S 2) and monosulphide solid-solution (Fe 0.74Ni 0.21S) was observed. The synthetic violarite grains showed a zonal separation into a Fe 1.2Ni 1.8S 4 core, and a Fe 0.5Ni 2.5S 4 rim. Trace amounts of pyrite (FeS 2) and millerite (NiS) were also detected. A study of the oxidative dissolution of pentlandite by electrochemical techniques including potentiometry, linear sweep cyclic voltammetry, intermittent galvanostatic polarisation, chronopotentiometry and chronoamperometry was made to clarify the mechanism by which pentlandite is leached in acidic iron(III) chloride solution. The products were analysed using scanning electron microscopy, powder X-ray diffraction, electron microprobe analysis, atomic absorption spectroscopy and gravimetric analysis. A critical comparison of similar studies by other workers on metal sulphide minerals is included. The fit of experimental results to a simple electron transfer model via the Sand equation was tested and found to follow non-Arrhenius behaviour. A mechanism for the oxidative dissolution of pentlandite is proposed. In acid solution, pentlandite decomposes spontaneously, liberating aqueous metal ions and hydrogen sulphide. Under potentiostatic conditions akin to iron(III) chloride leaching, pentlandite is oxidised directly to elemental sulphur, without the formation of any intermediate phases. The lack of formation of violarite indicates that the system is substantially perturbed from equilibrium due to slow solid-state diffusion of metal atoms within the sulphur sub-lattice. The formation of metastable amorphous sulphur as the alternative product is further evidence for this perturbation. The physical properties of the sulphur product layer cause an impediment to mass transport between the bulk aqueous solution and the mineral surface. However, the oxidation involves an intrinsically slow electron transfer for the S,Fe 2+, Ni 2+ Fe 4.5 Ni 4.5 S 8 couple which, within the potential range relevant to iron(III) chloride leaching, is rate-determining for an appreciable part of the reaction. The implications for extractive hydrometallurgy are discussed.

Electrochemical processes in recovering metals from ores

Chalcopyrite (CuFeS{sub 2}) is one of the most abundant copper-bearing minerals in the U.S. Oxidative leaching and smelting and refining are the most common methods used for recovering copper from chalcopyrite. One of the problems associated with oxidative leaching is the formation of an elemental sulfur product layer around the unreacted chalcopyrite core. The sulfur coating slows the reaction by inhibiting both the diffusion of the oxidant to the unreacted core, and the diffusion of the copper and iron species to the bulk solution. Another problem with leaching is that the iron and copper are oxidized simultaneously. Both appear in the bulk solution in their most oxidized states. The direct electrodissolution of copper sulfide ore slurries could reduce the number of steps involved in the copper recovery process, possibly leading to significant reductions in energy consumption and operating costs. The potential application of electrodissolution processes in hydrometallurgy has been reviewed. This paper reviews investigations of the electrochemical dissolution of chalcopyrite, digenite (Cu{sub 1.8}S), chalcocite (Cu{sub 2}S) and covellite (CuS).

Kinetic effects of particle-size and crystal dislocation density on the dichromate leaching of chalcopyrite

Relatively pure, polycrystalline samples of Golden, New Mexico and Transvaal, South Africa chalcopyrite were ground and polished to fit into a density-compatible titanium alloy sandwich assembly and shock loaded (the Golden at 1.2 GPa, the Transvaal at 18 GPa peak pressure). The recovered samples were examined in the transmission electron microscope and observed to contain roughly 103 and 104 times more dislocations respectively than the natural, unshocked materials which contained roughly 107 cm−2. These materials were ground to three size fractions and leached in an acid-dichromate lixiviant at 50 and 70 °C (323 and 343 K). Corresponding size fractions were prepared from the unshocked material and leached for comparison; and these experiments were extended to a range of temperatures up to 90 °C (363 K) using a Bingham Canyon, Utah chalcopyrite. An unambiguous effect of dislocations was observed at the largest size fraction leached at 50 °C (323 K): the reaction rate constant increased from 1.44 × 10−3 to 1.62 × 10−3 to 2.73 × 10−3 min−1 for dislocation densities increasing from roughly 103 cm−2 to 1011 cm−2. A linear relationship in the leaching rate was observed between 25 and 60 °C (298 and 333 K) and the activation energy was calculated to be approximately 12 kcal/mol (50 kJ/mol). Above 60 °C (333 K) a conspicuous rate decrease was observed. This was related to a densification of the sulfur product layer, which at 50 °C (323 K) was a loose, porous precipitate while at 90 °C (363 K) it was a continuous, tenacious, glassy coating on the chalcopyrite particles. In addition, and more importantly, the high-temperature decline in leaching rate was inferred to be associated with adsorption of Cr(VI) ion which decreases with increasing temperature. The anomalous and unpredictable behavior of chalcopyrite leached in acid-dichromate lixiviant was therefore observed to be related to variations in a surface chemisorption mechanism as well as variations in the nature of the sulfur reaction product layer. These effects also mask the influence of crystal defects and other surface-related crystallographic effects in leaching.