The mechanism of ultrasound oxidation effect on the pyrite for refractory gold ore pretreatment

3.1.1 The microstructure of pyrite

In this section, the microstructure of pyrite slice and powder is analyzed. Before ultrasonic treatment, the pyrite slice surface is smooth and with no scratch (Fig. 3a). after 4 h’ ultrasonic treatment at 5 °C in deionized water, the pyrite slice is seriously corroded (Fig. 3b). Not only that, pyrite particle has integrity crystal structure before ultrasonic treatment (Fig. 3c), compared with severely corroded pyrite particle after ultrasound which crystal structure is severely damaged (Fig. 3d). This indicates that ultrasound has the ability to break the gold inclosure (pyrite).

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Fig. 3

The images of pyrite slice and pyrite powder before(a,c) and after ultrasound(250 W) for 4 h (b,d).

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In order to investigate the influences of different treatment condition on pyrite surface, several experiments are conducted to determine the influences of time, ultrasonic power, alkalinity and temperature, and the experiment results is shown in Fig. 4. It could be seen that the pyrite slice has a very smooth surface before ultrasonic treatment under SEM (Fig. 4a). With the increase of the ultrasonic power and temperature, the surface corrosion of pyrite becomes also more and more obvious (Fig. 4d and c). This result indicates that ultrasound can effectively damage pyrite structure under neutral condition. The increases of alkalinity have slightly improvement on pyrite corrosion (Fig. 4e and f). The EDS analysis indicates that some oxidant have formed on the pyrite surface under alkaline condition after ultrasound (Fig. S1 and Fig S2). The hematite or feroxyhite could be the oxidant base on previous study (Caldeira et al., 2003).

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Fig. 4

SEM of the polished pyrite slice before (a) and after ultrasound at different condition. (Condition: (b) $T=5^{°}\mathrm{C}$ , $W_U=250W$ , t = 1 h, $C_{\mathit{NaOH}}=0mol/L$ ; (c) $T=5^{°}\mathrm{C}$ , $W_U=250W$ , t = 4 h, $C_{\mathit{NaOH}}=0mol/L$ ; (d) $T=5^{°}\mathrm{C}$ , ${\mathit{W}}_{\mathit{U}}=50\mathit{W}$ , t = 4 h, $C_{\mathit{NaOH}}=0mol/L$ ; (e) $T=5^{°}\mathrm{C}$ , $W_U=250W$ , t = 4 h, ${\mathit{C}}_{\mathit{N}\mathit{a}\mathit{O}\mathit{H}}=0.5\mathit{m}\mathit{o}\mathit{l}/\mathit{L}$ ; (f) $T={80}^{°}\text{C},W_U=250W$ , t = 4 h, ${\mathit{C}}_{\mathit{N}\mathit{a}\mathit{O}\mathit{H}}=0.5\mathit{m}\mathit{o}\mathit{l}/\mathit{L}$ .)

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3.1.2 The chemical changes of pyrite surface

In this section, XRD and XPS were used to determine the chemical changes of pyrite powder and slices, respectively. The oxidized pyrite powder after drying was analyzed by XRD (Fig. 5). Under neutral condition, no new phases appear. However, Fe₂O₃ were detected in the dried slag when alkalinity or temperature rises (Fig. 5(b)). The formation of iron oxide under alkalinity or high temperature condition might attribute to the hydrolyzation of iron ion produced by pyrite (Caldeira et al., 2003). The ultimate product of iron oxide(Fe₂O₃) might be formed by hematite or feroxyhyte (Caldeira et al., 2003).

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Fig. 5

(a) XRD image of pyrite powder before and after ultrasound; (b) partial magnified XRD image of pyrite powder before and after ultrasound.

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Fig. 6 indicates the Fe2p and S2p core level spectra of the polished pyrite slice surface before and after ultrasound. The three multiplet peaks at 706.7 eV (Fe2p_3/2A), 707.6 eV (Fe2p_3/2B) and 708.6 eV (Fe2p_3/2C) correspond to high spin Fe(II) ions bonded to sulfur (Fig. 6a) (Demoisson et al., 2007). The spectrum of sulfur has two main peak at 163.8 eV(S2p_1/2) and 162.6 eV(S2p_3/2), which indicate the presence of disulfide (Fig. 6b) (Demoisson et al., 2007; Liao et al., 2013; Scheinost et al., 2008; Thomas et al., 2003). The small peak at 161.7 eV(S2p_3/2) contribute to monosulfide (Demoisson et al., 2007). After ultrasound for 4 h, the multiplet peaks of Fe(II) ions barely changed and there is a new peak at 164.4 eV in the spectrum of sulfur, which attribute to elemental sulfur (Demoisson et al., 2007). This is due to that pyrite decomposes to ferric ion and elemental sulfur in the presence of oxidizer.

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Fig. 6

XPS spectra of iron(a) and sulfur(b) of pyrite slices.

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3.2.1 The iron species distribution

ICP and qualitative chemical analysis was used to determine the concentration of iron ions in solution. The saturated potassium thiocyanate solution (KSCN) and hydrogen peroxide (H₂O₂) was used to determine the chemical valence of iron ions (Video.1). It is obvious that few Fe(III) existed in liquid, because the color barely changed after adding KSCN. Oppositely, when H₂O₂ was firstly added, the liquid became blood red instantly after adding KSCN. Which proved that the iron was mainly existed as Fe(II), few are in the form of Fe(III). It has been reported that the oxidation of pyrite in aqueous solution is mainly carried out through the following chemical reactions(Eq1-3) (Caldeira et al., 2003; Garrels and Thompson, 1960; Singer and Stumm, 1970). This also suggests that Fe(II) is the dominant species in the solution. Further experiment indicated that iron ions were only existed at neutral condition, the Fe(II) ions deposited when the NaOH concentration was raised (Fig. 7a). This is contributed to hydrolyzation of Fe(II) ion (Caldeira et al., 2003). This result is coordinate with XRD analysis from Section 3.1.1. The Fe(II) concentration will increase as the power increases (Fig. 7b). A higher ultrasonic power can motivate stronger cavitation, strip the deposited iron oxide and expose the new reaction interface, which promotes the oxidation of pyrite (Ince et al., 2001; Villeneuve and Alberti, 2009).

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Fig. 7

Effect of NaOH concentration and ultrasonic power on the iron concentration in the solution. (Conditions: (a) $T=5^{°}\mathrm{C}$ , $W_U=250W$ ; (b) $T=5^{°}\mathrm{C}$ , $C_{\mathit{NaOH}}=0mol/L$ ).

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3.2.2 Sulfur species distribution in water

Fig. 8 a is the image of HPLC of the sulfur species. From Fig. 8a, only SO₄^2- and S₂O₃^2- were detected, no other sulfur ions are detected (S^2-, SO₃^2-, polysulfide, etc.). This result indicates that pyrite will only be oxidized to SO₄^2- and S₂O₃^2- under ultrasound. More specific experiment was conducted to determine the influences of different reaction condition. Fig. 8 (b to d) is the effect of NaOH concentration, temperature and ultrasonic power on the sulfur species. Under ultrasound (250 W) for 4 h at 5 °C (Fig. 8b), the SO₄^2- and S₂O₃^2- concentration increases significantly after adding NaOH, but decreases when NaOH concentration is too high. This result indicates pyrite oxidation is in mild alkalinity under ultrasound. Moreover, the concentration of S₂O₃^2- under alkaline condition is much higher than neutral condition, which is contributed to a higher stability of S₂O₃^2- under alkaline condition (Aylmore and Muir, 2001; Ha et al., 2014). Because the cavitation effect increases along with ultrasonic power, the oxidation ability of the system increased (Villeneuve and Alberti, 2009; Hu et al., 2008). The concentration of SO₄^2- and S₂O₃^2- increases along with ultrasonic power (Fig. 8d, conditions: $C_{\mathit{NaOH}}=0M$ , temperature = 5 °C). The SO₄^2- and S₂O₃^2- concentration decreases tremendously when temperature rises (Fig. 8c, condition: $C_{\mathit{NaOH}}=0.5mol/L$ , $W_U=250W$ ). This might contribute to hydrolyzation of iron ions which diminished the oxidation ability. The effect of temperature shows that the oxidation of pyrite under ultrasound should be carried out at low temperature.

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Fig. 8

The image of HPLC of the sulfur species(a), Effect of NaOH concentration(b), temperature(c) and ultrasonic power(d) on the sulfur species.

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3.2.3 pH changes under ultrasonic cavitation

The changes of H⁺ concentration under ultrasound were measured by pH meter. Fig. 9a indicated the increases of ultrasonic power could significantly increase H⁺ concentration (initial pH = 7). This is due to the decomposition of H₂O into the hydrogen and hydroxyl radicals under ultrasonic treatment. The hydroxyl radicals participate in the pyrite oxidation. The oxidation of pyrite under ultrasound is a reaction that releases hydrogen ions. Based on the previous analysis, the chemical reaction of pyrite oxidation under ultrasound should be carried out as follow:

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Fig. 9

pH changes after ultrasonic oxidation under different condition. (condition: (a) $T=5^{°}\mathrm{C}$ , $C_{\mathit{NaOH}}=0mol/L$ ; (b) $T=5^{°}\mathrm{C}$ , $W_U=250W$ ; (c) $W_U=250W$ , $C_{\mathit{NaOH}}=0.5mol/L$ ;).

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This result consists with previous analysis, which proved that ultrasound could oxidize pyrite effectively. The hydrogen radicals might also formats as hydrogen accord as Draganic’s study (Draganic and Draganic, 1971). The pH changes of each experiment are shown in Table S1; Table S2 and Table S3. It could be seen that under neutral condition (Fig. 9b, Table.S2), the pH after ultrasonic oxidation decreases significantly. However, the changes of the final pH decrease rapidly with the rises of the initial pH. This result is similar with the change of sulfur and iron species, which proved that oxidation was weakened with the increase of the initial alkalinity. Under 0.5 M NaOH, the pH hardly changed after temperature rises (Fig. 9c, Table.S3). This indicates the rises of temperature have slightly influences to ultrasonic oxidation.