Naoki Hiroyoshi

Enhancement of chalcopyrite leaching by ferrous ions in acidic ferric sulfate solutions

The effects of ferrous ions on chalcopyrite oxidation with ferric ions in 0.1 mol dm−3 sulfuric acid solutions were investigated by leaching experiments at 303 K in nitrogen. With high cupric ion concentrations, the chalcopyrite oxidation was enhanced by high concentrations of ferrous ions and copper extraction was mainly controlled by the concentration ratio of ferrous to ferric ions or the redox potential of solutions. Ferrous ions, however, suppressed the chalcopyrite oxidation when cupric ion concentrations were low. A reaction model, which involves chalcopyrite reduction to intermediate Cu2S by ferrous ions and oxidation of the Cu2S by ferric ions, was proposed to interpret the results.

A model for ferrous-promoted chalcopyrite leaching

Oxidative leaching of chalcopyrite with dissolved oxygen and/or with ferric ions is promoted by high concentrations of ferrous ions in sulfuric acid solutions containing cupric ions. This paper proposes a reaction model to interpret this phenomenon and the thermodynamics of the leaching is discussed. The model considers the leaching to take place in two steps: (1) reduction of chalcopyrite to Cu2S by ferrous ions in the presence of cupric ions and (2) oxidation of the Cu2S to cupric ions and elemental sulfur by dissolved oxygen and/or by ferric ions. The intermediate Cu2S is more amenable to oxidation than chalcopyrite, causing enhanced copper extraction. The model predicts that the formation of intermediate Cu2S and ferrous-promoted chalcopyrite leaching occur when the redox potential of the solution is below a critical potential that is a function of the ferrous and cupric ion concentrations. To confirm this, flask-shaking leaching experiments were carried out with 0.1 mol dm−3 sulfuric acid solutions containing known concentrations of ferrous, ferric, and cupric ions at 303 K in air. The results agreed well with the predictions, i.e. copper extraction was enhanced at solution potentials below the critical potential predicted with the model.

A case of ferrous sulfate addition enhancing chalcopyrite leaching

It is generally accepted that ferric ions as an oxidant are effective for leaching chalcopyrite but ferrous ions contribute to the leaching only as a source of ferric ions. However, this paper found that several chalcopyrite samples were more effectively leached in ferrous sulfate solution than in ferric sulfate solution. Chalcopyrite samples from four different sources were leached in 0.1 mol dm−3 sulfuric acid solution containing 0.1 mol dm−3 ferrous sulfate or ferric sulfate at 303 K in air for 168 h. Three samples were more effectively leached in the ferrous sulfate solution than in the ferric sulfate solution. Especially, with the sample from the Akenobe mines, Hyogo, Japan, the amount of copper extracted with ferrous sulfate was about five times larger than that with ferric sulfate. By using the Akenobe sample, leaching experiments and oxygen consumption measurements were carried out under various conditions. The amount of extracted copper increased markedly with increasing ferrous sulfate addition and decreasing pH. During the leaching experiments, most of the soluble iron was present in the ferrous form. By adding ferrous sulfate, proton consumption increased. The mole ratio of elemental sulfur to extracted copper was about 2. When the leach solution was purged with nitrogen, the amount of copper extracted was negligible even with ferrous sulfate. By adding ferrous sulfate, dissolved oxygen consumption on the sample surface increased. From these results, it was concluded that ferrous ions enhance the following reaction for the Akenobe sample: CuFeS2 + O2 + 4H+ = Cu2+ + Fe2+ + 2S0 + 2H2O. The importance of this effect in the bacterial leaching of chalcopyrite is discussed.

A new reaction model for the catalytic effect of silver ions on chalcopyrite leaching in sulfuric acid solutions

Abstract Chalcopyrite leaching in sulfuric acid solutions depends on the redox potential determined by the concentration ratio of ferric to ferrous ions, and the leaching rate is higher at redox potentials below a critical value. Previously, the authors have proposed a reaction model to interpret this phenomenon. The present study applied the model to interpret the catalytic effect of silver ions on chalcopyrite leaching. The model assumes that at lower potentials, chalcopyrite leaching proceeds in two steps: first, chalcopyrite is reduced by ferrous ions to form Cu 2 S that is more rapidly leached; next, the intermediate Cu 2 S is oxidized by ferric and/or dissolved oxygen to release cupric ions. During the chalcopyrite reduction, hydrogen sulfide is released to the liquid phase. Silver ions react with the hydrogen sulfide to form silver sulfide precipitate and decrease the concentration of hydrogen sulfide in the liquid phase, causing a rise in the critical potential of Cu 2 S formation and broadening of the potential range where rapid copper extraction takes place. To confirm the model, the redox potential dependence of chalcopyrite leaching was investigated in the presence of various concentrations of silver ions with 0.1 kmol m −3 sulfuric acid containing known concentrations of ferrous and ferric ions at 298 K in air. The critical potential increased with increasing concentrations of silver ions. This agrees with the model proposed here but cannot be explained by the conventional model proposed by Miller et al.

Inhibitory effect of iron-oxidizing bacteria on ferrous-promoted chalcopyrite leaching

It is generally accepted that iron-oxidizing bacteria, Thiobacillus ferrooxidans, enhance chalcopyrite leaching. However, this article details a case of the bacteria suppressing chalcopyrite leaching. Bacterial leaching experiments were performed with sulfuric acid solutions containing 0 or 0.04 mol/dm3 ferrous sulfate. Without ferrous sulfate, the bacteria enhance copper extraction and oxidation of ferrous ions released from chalcopyrite. However, the bacteria suppressed chalcopyrite leaching when ferrous sulfate was added. This is mainly due to the bacterial consumption of ferrous ions which act as a promoter for chalcopyrite oxidation with dissolved oxygen. Coprecipitation of copper ions with jarosite formed by the bacterial ferrous oxidation also causes the bacterial suppression of copper extraction. Copyright 1999 John Wiley & Sons, Inc.

Subcritical crack growth in rocks in an aqueous environment.

Subcritical crack growth is one of the main causes of time-dependent fracturing in rock. In the present study, we investigated subcritical crack growth in rock in distilled water (pH = 5–7) and in an aqueous solution of sodium hydroxide (NaOHaq, pH = 12), comparing the results to those in air. We also investigated the effect of the pH in an aqueous environment. We used andesite and granite for all our tests. We determined the relationship between the crack velocity and the stress intensity factor using the double-torsion test under conditions of controlled temperature. We showed that crack velocities in water were higher than those in air, in agreement with other research results indicating that crack velocity increases in water. When we compared our results for NaOHaq with those for water, however, we found that the crack velocity at the same stress intensity factor did not change even though the pH of the surrounding environment was different. This result does not agree with the accepted understanding that hydroxide ions accelerate subcritical crack growth in rocks. We concluded that the pH at the crack tip influences subcritical crack growth, and not the bulk pH, which has little effect.

IMPROVED CHALCOPYRITE LEACHING THROUGH OPTIMIZATION OF REDOX POTENTIAL

Abstract In the leaching of chalcopyrite with sulphuric acid solutions, the copper extraction rate reaches a maximum at a given redox potential. This article reviews our studies on the determination of the optimum redox potential for chalcopyrite leaching. Shaking flask and column leaching experiments for a chalcopyrite concentrate were performed under various conditions and the results were analyzed using a normalized redox potential defined from a reaction model assuming the formation of an intermediate Cu2S from chalcopyrite. Factors, such as metal ion concentrations, solid/liquid ratio and the presence of iron-oxidizing bacteria, caused significant variations in the copper extraction versus time plots. However the copper extraction rate versus normalized redox potential plots were independent of the above factors and the copper extraction rate reached a maximum at the normalized redox potentials around 0.43. Converting the normalized redox potential to the solution redox potential gives the optimum redox potential for chalcopyrite leaching as a function of cupric and ferrous ion concentrations. This semi-empirical equation can be used to predict the optimum redox potential during leaching operation and is useful to design the redox-controlled heap leaching for chalcopyrite. Lors de la lixiviation de la chalcopyrite avec des solutions d’acide sulfurique, la vitesse d’extraction du cuivre atteint un maximum à un potentiel rédox donné. Cet article examine notre étude de la détermination du potentiel rédox optimal de lixiviation de la chalcopyrite. On a effectué, sous diverses conditions, des expériences d’agitation du flacon et de lixiviation en colonne d’un concentré de chalcopyrite et l’on a analysé les résultats en utilisant un potentiel rédox normalisé défini à partir d’un modèle de réaction assumant la formation d’une forme active intermédiaire Cu2S à partir de la chalcopyrite. Des facteurs, comme les concentrations de l’ion métal, le rapport solide/liquide et la présence de bactéries oxydantes du fer, avaient pour conséquence des variations importantes dans l’extraction du cuivre par rapport au temps. Cependant, la vitesse d’extraction du cuivre par rapport aux graphes du potentiel rédox normalisé était indépendante des facteurs ci-dessus et elle atteignait un maximum aux potentiels rédox normalisés autour de 0.43. La conversion du potentiel rédox normalisé en potentiel rédox de la solution donne le potentiel rédox optimal pour la lixiviation de la chalcopyrite en fonction de la concentration des ions cuivriques et ferreux. On peut utiliser cette équation semiempirique pour prédire le potentiel rédox optimal lors de l’opération de lixiviation et cette dernière est utile pour concevoir la lixiviation en tas de la chalcopyrite contrôlée par le rédox.

Short and long term release mechanisms of arsenic, selenium and boron from a tunnel-excavated sedimentary rock under in situ conditions

Sedimentary rocks of marine origin excavated from tunnel construction projects usually contain background levels of hazardous trace elements, but when exposed to the environment, they generate leachates with concentrations of arsenic (As), selenium (Se) and boron (B) exceeding the WHO guideline for drinking water. In this study, the leaching of As, Se and B was evaluated under in situ conditions at various flow patterns, particle size distributions and column thicknesses. The results showed that these trace elements were leached out of the rock via short and long term mechanisms. In the short term, all three elements were rapidly and simultaneously released due to the dissolution of soluble evaporite salts formed from entrapped sea water of the Cretaceous. After their rapid release, however, these trace elements behaved differently as a result of their contrasting adsorption affinities onto minerals like clays and Fe-oxyhydroxides, which were further influenced by the pH, presence of coexisting ions and speciation of the trace elements. Selenium was quickly and easily transported out of the columns because it was mostly present as the very mobile selenate ion (Se[VI]). In comparison, the migration of As and B was hindered by adsorption reactions onto mineral phases of the rock. Boron was initially the least mobile among the three because of its preferential adsorption onto clay minerals that was further enhanced by the slightly alkaline pH and high concentrations of Ca(2+) and Na(+). However, it was gradually re-mobilized in the latter part of the experiments because it was only weakly adsorbed via outer sphere complexation reactions. In the long term, the rock continued to release substantial amounts of As, Se and B via pyrite oxidation and adsorption/desorption reactions, which were regulated by the temperature and rainfall intensity/frequency on site.

Suppression of Pyrite Oxidation by Carrier Microencapsulation Using Silicon and Catechol

Acid mine drainage (AMD) is formed from the natural oxidation of sulfide minerals such as pyrite and FeS2. Prevention of AMD is very important and several techniques are currently being investigated for the treatment and abatement of AMD. This paper proposes carrier-microencapsulation (CME) using Si–catechol complex – for preventing pyrite oxidation. In CME, the water soluble organic carrier, catechol, and metal ion, Si, make a complex, e.g., tris-catecholato complex of Si4+, which oxidatively decomposes on pyrite surface and forms a stable oxide or hydroxide, e.g., Si(OH)4 or SiO2 layer on the pyrite surface as a protective coating against pyrite oxidation. To demonstrate the effect of CME using on pyrite oxidation, shaking flask leaching experiments of pyrite without and with CME treatment were performed. Significant halt in pH drop was observed after the CME treatment of pyrite. The CME coating was found very effective even at low concentration of 1 and 5 mol m−3. The amount of Fe and S leached were lower from the pyrite with CME treatment than without CME treatment, indicating that CME using Si and catechol is effective in suppressing pyrite oxidation. The effect of pH and presence of microorganism were also evaluated. The CME coating was found very effective even in acidic pH range. The CME treatment suppressed pyrite oxidation even in the presence of iron oxidizing microorganism. Presence of Si and O on the residue pyrite surface in energy-dispersive X-ray spectroscopy analysis supported the sustainability of CME coating at acidic pH.

On the Use of Magnetite for Gold Recovery From Chloride Solution

The sorption of gold chloride ( ) from chloride solution on synthetic and natural magnetite powders was investigated by batch sorption experiments. The effects of different parameters on the recovery were studied. The results showed that Gold (Au) uptake by magnetite was influenced by pH, contact time, chloride concentration, and initial Au concentration. Gold uptake by synthetic and natural magnetite was maximum at pH 6–7 with sorption amounts of 4.4 μmol/g and 5.0 μmol/g, respectively, after 24 hr at an initial Au concentration of 0.05 mol/m3. The Scanning Electron Microscopy–Energy Dispersive X-ray and Back Scattering Electron analyses of the magnetite particles after treatment confirmed the presence of Au precipitates on the magnetite (Fe3O4) surface.