Miki et al.
[4][38] studied the effect of the redox potential in a Cu
2S solution (synthetic) with the use of a 0.2 M HCl solution, 0.2 g/L of Cu (II), and 2 g/L of Fe (III)/Fe (II) at a temperature of 35 °C. The researchers, in their findings, reported that the dissolution of Cu
2S occurs rapidly at a potential of 500 mV but then stops when 45% copper is removed (end of the first stage). For potentials of 550 mV, there is then an increase in the dissolution until reaching 50% extraction of copper. Subsequently, the copper mineral present is mainly covering, which requires potentials of at least 600 mV to be able to dissolve. However, Niu et al.
[28][39] point out that these results were not determined in a range of industrial redox potentials. In their experiments for the dissolution of Cu
2S, Niu et al.
[28][39] worked in mini glass columns (30 cm long and 6 cm in diameter), adding Fe
2(SO
4)
3 as a leaching agent. In their results, the researchers note that the dissolution rate of the second stage of Cu
2S leaching was insensitive to the redox potential at moderate temperatures (30–40 °C) in the industrial range of 650–800 mV. In the study conducted by Hashemzadeh et al.
[6][63], the researchers modeled the dissolution kinetics of Cu
2S in chloride media using leaching data obtained under fully controlled temperature, pH, and solution potential. In their results, the researchers mentioned that an increase in the chloride concentration and temperature generated an increase in the redox potential, increasing from 680 to 830 mV with the addition of 0.1 chlorides and 3 mol/L of NaCl, respectively, and consequently higher dissolution kinetics, mainly in the second leaching stage.
The results obtained from the aforementioned studies are directly related to the formation of a layer of elemental sulfur on the surface of the covellite in the second stage, which decreases with the increase in the potential of the solution; however, the layer in the solid surface is a mixture of sulfur and polysulfides (CuS
n)
[13][42], where these could be responsible for a slow reaction during this stage.