Mechanism of enargite pressure leaching in the presence of pyrite
Research Highlights
►The presence of pyrite enhances the pressure leaching of enargite. ►Ferric ions are the main oxidant in enargite leaching. ►Non linear behaviour was observed in copper dissolution from pyrite rich enargite concentrates.
Introduction
Arsenic, usually in the form of enargite (Cu3AsS4), is present in most Chilean copper deposits. In the froth flotation process used for concentrating sulfide copper ores, most of the enargite reports to the final copper concentrate. The increasing presence of enargite in the copper concentrates is starting to complicate the traditional pyrometallurgical treatment of the sulfide minerals. In addition, there are large copper ore resources in Chile, where enargite is not an impurity but one of the main copper minerals. Enargite rich copper concentrates cannot be treated by conventional smelting/converting technology because of the environmental risk of arsenic emissions to the atmosphere and because of a deteriorated quality of the final copper product. Therefore, nonconventional hydrometallurgical methods such as pressure leaching must be sought for the processing of these arsenic contaminated concentrates.
Previous investigators have found that enargite is a refractory mineral in aqueous media, especially in acidic solutions. The leaching rate of enargite in sulfate media at atmospheric pressure is very slow using either oxygen or ferric ions as oxidants. On this matter, Dutrizac and MacDonald (1972) leached discs of synthetic and natural enargite in acidified sulfate solutions under atmospheric conditions. They found that the rate of dissolution of enargite was extremely slow and the dissolution kinetics was linear in all cases. The activation energy associated with the dissolution of the synthetic sample was 13.3 kcal/mol, in the temperature range of 60 to 95 °C. They also found that pure natural enargite samples dissolved at approximately the same rate as the synthetic enargite. Flynn and Carnahan (1989) found that silver or mercury sulfate salts could be used to catalyze the ferric sulfate leaching of enargite at atmospheric pressure. They found that by adding 0.25 g/l of silver sulfate to a leaching solution containing 0.8 mol/l Fe2(SO4)3 and 1 mol/l H2SO4, they could leach 97% of the enargite in 6 h at the boiling point of the solution. They also claimed that mercury sulfate was less effective as a catalyzer than silver sulfate.
The use of chloride ions to accelerate the leaching of enargite has also been studied. Padilla et al. (2005) leached natural enargite particles in H2SO4–NaCl solutions using oxygen as oxidant at atmospheric pressure. These investigators found that the leaching rate was also slow in this system, although it was considerably faster than in the absence of chloride ions. In a 2 M NaCl and 0.25 M H2SO4 solution, about 7% of the enargite was dissolved in 7 h leaching at 100 °C.
On the other hand, pressure leaching has been found to be a more effective method for the dissolution of enargite in acid media, although the leaching rate is still slow as compared to other copper sulfides, including chalcopyrite. Padilla et al. (2008) studied the kinetics of pressure dissolution of enargite in sulfuric acid oxygen media in the temperature range 160 to 220 °C and oxygen partial pressures of 303 to 1013 kPa. They determined that enargite dissolution occurred with total oxidation of the sulfide sulfur according to the following reaction:Cu3AsS4 + 8.75O2 + 2.5H2O + 2H+ = 3Cu2+ + H3AsO4 + 4HSO4−
These investigators also found that enargite leached with linear kinetics and that the dissolution rate increased substantially with increasing temperature. Complete dissolution of enargite with particle size −75 + 53 μm was obtained in 120 min at 220 °C and 689 kPa O2 partial pressure. Riveros and Dutrizac (2008) also investigated the pressure leaching of enargite with ferric sulfate–sulfuric acid solutions in the temperature range of 130 to 180 °C without oxygen addition, and with an overpressure of oxygen of 100 psi. They found that in the latter case the rates were relatively faster than those obtained with only ferric sulfate. They also reported that in both cases the dissolution of copper was incomplete at temperatures below 170 °C because of the formation of coatings of elemental sulfur; while at higher temperatures the sulfide sulfur oxidized to sulfate and complete dissolution of copper was obtained.
Rivera-Vásquez and Dixon (2009) studied the dissolution of enargite mixed with various amounts of pyrite at atmospheric pressure and 80 °C, using a solution containing sulfuric acid, ferric and ferrous ions with a redox potential of 435 mV vs. SCE which was controlled with oxygen gas. They reported enhanced dissolution of enargite in the presence of pyrite which they attributed to the formation of a galvanic couple between enargite and pyrite — similar to the well established interaction that occur between chalcopyrite and pyrite (Berry et al., 1978, Murr, 1980, Mehta and Murr, 1982, Metha and Murr, 1983, Dixon et al., 2008) with pyrite providing the cathodic surface for the anodic dissolution of enargite.
Nadkarni and Kusik (1988) reported that the addition of pyrite also increased the rate of dissolution of enargite under pressure leaching conditions. These authors mentioned a leaching process for enargite concentrates where enargite and pyrite were mixed in an approximately ratio 10/1 and leached at a temperature of about 225 °C and 150 psi O2 partial pressure. The copper dissolution was about 98% from this mixture as compared to less that 70% in the absence of pyrite. Unfortunately, the leaching time was not given.
Considering that pyrite is a common impurity in copper–arsenic concentrates, the effect of pyrite on enargite dissolution rate is of great practical interest. Therefore, the main objective of this work was to study the pressure leaching of a pyrite rich enargite concentrate in H2SO4–O2 and to compare the results with the pressure leaching of a pure enargite concentrate in order to elucidate the reason for the rate enhancement observed.
Section snippets
Materials
The primary material used in the experimental work was an enargite concentrate with a high content of pyrite, which was prepared from large particles (about 11″ in size) with enargite and pyrite mineralization from El Indio Mine (Barrick Corp.). This material was crushed manually to a size of approximately 5 mm, and a pre-concentrate was obtained by hand sorting the sulfide rich particles. This pre-concentrate was ground in an agate mortar to a size smaller than 150 μm, and the particles smaller
Results and discussion
Most of the experiments were carried out with the enargite–pyrite concentrate, to study the main variables that could affect the pressure leaching of this material: i.e. stirring speed, concentration of sulfuric acid, particle size, oxygen partial pressure, temperature, and time. Some experiments were also carried out using the pure enargite concentrate, in order to compare the leaching behaviour of this material with the behaviour of the enargite–pyrite concentrate.
Mechanism and kinetic considerations
The rapid pressure leaching rates observed for the enargite in the presence of pyrite in the concentrate could be explained by two alternative mechanisms:
- i)
The formation of a galvanic couple between enargite and pyrite, with pyrite as the cathodic surface which would lead to an accelerated anodic dissolution of the enargite.
- ii)
The generation of ferric ions during the leaching which dissolved enargite faster than oxygen.
The first mechanism was proposed by Rivera-Vásquez and Dixon (2009) to explain
Conclusions
From the experimental data on the acid pressure leaching of enargite–pyrite concentrate and pure enargite sample the following can be concluded:
- −
The pressure leaching of enargite in the presence of 40% w/w pyrite is considerably faster than the dissolution of pure enargite and all the copper can be dissolved in 15 min at 200 °C depending upon other conditions.
- −
Temperature, oxygen partial pressure and particle size had significant influence on the rate of enargite dissolution from the
Acknowledgments
The authors acknowledge The National Fund for Scientific and Technological Development, FONDECYT, of Chile for the financial support of this study through project no. 1080289. The collaboration of O. Jerez in some of the experiments is also acknowledged.
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