The Kinetics of Pyrite Dissolution in Nitric Acid Solution

Abstract

Refractory sulphidic ore with gold captured in pyrite has motivated researchers to find efficient means to break down pyrite to make gold accessible and, ultimately, improve gold extraction. Thus, the dissolution of pyrite was investigated to understand the mechanism and find the corresponding kinetics in a nitric acid solution. To carry this out, the temperature (25 to 85 °C), nitric acid concentration (1 to 4 M), the particle size of pyrite from 53 to 212 µm, and different stirring speeds were examined to observe their effect on pyrite dissolution. An increase in temperature and nitric acid concentration were influential parameters to obtaining a substantial improvement in pyrite dissolution (95% Fe extraction achieved). The new shrinking core equation (1/3ln (1 - X) + [(1 - X)^-1/3 - 1)]) = kt) fit the measured rates of dissolution well. Thus, the mixed-controlled kinetics model describing the interfacial transfer and diffusion governed the reaction kinetics of pyrite. The activation energies (E_a) were 145.2 kJ/mol at 25-45 °C and 44.3 kJ/mol at higher temperatures (55-85 °C). A semiempirical expression describing the reaction of pyrite dissolution under the conditions studied was proposed: 1/3ln(1 - X) + [(1 - X)^-1/3 - 1)] = 88.3 [HNO₃]^2.6 r₀^-1.3 e^-44280/RT t. The solid residue was analysed using SEM, XRD, and Raman spectrometry, which all identified sulphur formation as the pyrite dissolved. Interestingly, two sulphur species, i.e., S₈ and S₆, formed during the dissolution process, which were detected using XRD Rietveld refinement.

Keywords: mechanism; mixed kinetic model; nitric acid; oxidative leaching; pyrite; sulphur species.

Figure 1

The XRD pattern of the phase composition of a pyrite sample.

Figure 2

The particle size distribution (PSD) of the pyrite sample.

Figure 3

The proposed pyrite dissolution mechanism in nitric acid solution.

Figure 4

The energy band diagram of a pyrite–solution interface adopted from [21].

Figure 5

The extraction of iron from pyrite at different temperatures from 25 to 85 °C. Conditions: 3 M nitric acid, S/L ratio 1 to 20, particle size +75–106 µm.

Figure 6

Time dependence of iron extraction from pyrite with different nitric acid concentrations (1–4 M). Conditions: temperature 65 °C, S/L ratio 1 to 20, particle size +75–106 µm.

Figure 7

The effect of particle size on Fe extraction as a function of time. Conditions: 3 M nitric acid, temperature 65 °C, S/L ratio 1 to 20.

Figure 8

The effect of stirring speed on Fe extraction as a function of time. Conditions: 3 M nitric acid, temperature 65 °C, S/L ratio 1 to 20, particle size +53−75 µm.

Figure 9

Graphs of 1/3ln (1 − X) + [(1 − X)^−1/3 −1] = k.t vs. time (min) at various temperatures of 25 to 85 °C for the dissolution of pyrite in 3 M nitric acid.

Figure 10

Arrhenius plot of lnk vs. 1000/T (k⁻¹).

Figure 11

Graph with curves of 1/3ln (1 − X) + [(1 − X)^−1/3 − 1)] vs. time (min) at different concentrations of HNO₃ at a temperature of 65 °C.

Figure 12

Plot of lnk as a function of ln(HNO₃).

Figure 13

Plot of lnk as a function of ln(r₀).

Figure 14

Relation of the applied kinetic equation and the acquired semiempirical expression.

Figure 15

The backscattered SEM-EDS images of (a) pyrite before leaching and (b) after leaching in nitric acid.

Figure 16

SEM images of pyrite residue at different temperatures of 25, 45, 65, and 85 °C.

Figure 17

SEM images of leached pitted pyrite particles in 3 M nitric acid at a temperature of 65 °C.

Figure 18

The XRD pattern of pyrite samples before leaching and the residue of leached pyrite in 3 M nitric acid at different temperatures of 25, 45, 65, and 85 °C.

Figure 19

The Raman spectra of pyrite residue leached in 3 M nitric acid at various temperatures of 25, 45, 65, and 85 °C, showing the Raman spectra of pyrite and sulphur separately.

Figure 19

The Raman spectra of pyrite residue leached in 3 M nitric acid at various temperatures of 25, 45, 65, and 85 °C, showing the Raman spectra of pyrite and sulphur separately.