3.2. Nickel Concentrate Leaching
Pressure oxidation of 10%
w/
w fresh nickel concentrate at 250 °C with 700 kPa O
2 overpressure was rapid and nickel extraction reached 95% after just five minutes, as shown in
Figure 1. The free acidity rises only slowly after this time while the total iron concentration decreases slowly and stabilizes at a concentration of 4.7 g/L. Ferrous iron was barely detectable after 10 min though a trace amount of pyrite was still present in the residue taken at this time. Otherwise, the changes in mineralogy for the sulfide minerals are rapid and essentially complete within five minutes. The copper extraction curve is not shown as this is virtually identical to that for cobalt.
The initial iron hydrolysis products are hematite and hydronium jarosite, however, the attendant rapid increase in free acidity during the first five minutes and further iron hydrolysis thereafter results in significant formation of basic ferric sulfate, FeOHSO
4, as shown in
Figure 2. The data also suggest that the solids have neared equilibrium between 30 and 45 min. Here, the normalized mineral content represents the percentage of minerals in the sample relative to the amount of solid used for the experiment. This allows the absolute change in a mineral’s content to be identified.
The data described above, while indicating that rapid extraction of the nickel concentrate can be achieved, provide little insight into the chemical and mineralogical changes that accompany the process. For this reason, the reaction was slowed by regulation of the rate at which oxygen was transferred into the reactor. Nominally the value set using a mass flow controller was ~2.4 L/min maximum (subject to an oxygen overpressure of ~100 kPa) and this was consistent with the calculated rate at which oxygen was consumed during a second reaction conducted with 10% w/w fresh nickel concentrate in site water at 250 °C. The metal extraction, free acidity and ferrous iron concentration results are shown in
Figure 3.
The data in
Figure 3 confirm a slower rate of metals extraction, that nickel, cobalt and copper have different extraction profiles and suggest that the iron concentration has not achieved its equilibrium value by 90 min. The latter is, in part, a consequence of a significant residual ferrous iron concentration at the conclusion of the reaction though this also suggests that oxygen consumption by this time has slowed substantially. The changes in the mineralogy during this test are presented elsewhere [
17].
The pressure oxidation of 9.2%
w/
w “aged” nickel concentrate at 250 °C as a function of time, with 100 kPa O
2 overpressure and maximum oxygen flow rate of ~1.2 L/min., was investigated several years after the initial tests. These tests were conducted to examine the sulfide minerals transformations in greater detail and the impact of chemical additions upon the iron-containing phases formed. The extractions of nickel, cobalt, copper and iron, and iron(II) and free acid concentrations using deionized water are plotted in
Figure 4. The attendant increases in Eh and decreases in pH values, as the dissolved iron(II) is oxidized and the iron(III) hydrolysed to generate sulfuric acid, are shown in
Figure 5. Notably, the Eh increases rapidly and most of the ferrous iron is oxidized. Similar trends were noted for the pressure oxidation of 10%
w/
w fresh nickel concentrate in site water under the same conditions.
The metals extraction data indicate that nickel, cobalt and copper extractions are asynchronous. The nickel sulfide minerals are extracted preferentially while the copper sulfide minerals dissolved only after near complete nickel extraction. Copper extraction only became significant when the Eh increased above 350 mV (versus Ag/AgCl). This value compares with optimum potential range of 410 to 430 mV (versus Ag/AgCl) proposed for chalcopyrite dissolution in acidic sulfate medium [
18].
Interestingly, cobalt extraction does not mirror that of nickel extraction and the extraction rate increases somewhat after around 60 min. This suggests that a significant amount of the cobalt is associated with minerals other than nickel sulfides, a conclusion also reached in a previous leaching-based study [
19]. The mineralogy of the leach residues, discussed below, indicates the dominant sulfide mineral present by that time is pyrite which is the only such mineral detected by XRD analysis in residues taken from time 77 min onwards. Notably, the cobalt extraction was still increasing at the conclusion of the reaction as a trace amount of pyrite was still present. Based upon the amounts of pyrite present in these residues, it was estimated that the average cobalt content of the pyrite was 0.4–0.5%. Watmuff [
20] noted that the cobalt content of pyrite from Mt. Windarra varied quite substantially, even within individual grains, and from the analyses obtained, averaged 1.02%. When referenced back to the pyrite content of the feed, somewhere between 50% and 60% of the cobalt is estimated to be hosted by this mineral. In comparison, the study of Lu et al. [
19] was conducted using lower temperature conditions for which the dissolution of pyrite is expected to be slow. Based upon the composition provided for their nickel concentrate and extraction data for Ni, Co and Fe, the cobalt content of the pyrite can be estimated, ~0.8%, assuming the unleached Fe is associated with the pyrite.
Metal extractions can be readily understood from the mineralogical characterization of the leach residues. The curve for the extraction of nickel, shown in
Figure 6, demonstrates that pentlandite (Fe
4.5Ni
4.5S
8) is rapidly oxidized with the subsequent formation and consumption of violarite (FeNi
2S
4) and subsequently vaesite (NiS
2) according to the following step-wise (Equations (6)–(8)) and overall (Equation (9)) reactions:
The required acid is generated by the oxidation and subsequent hydrolysis of ferrous sulfate (Equation (10)) primarily generated from leaching of the nickel sulfide minerals and, from leaching of the iron sulfide minerals as discussed below.
The transformation of pentlandite to violarite has been confirmed to also occur in the study of Tenailleau et al. [
21] who employed mild hydrothermal conditions (80 or 120 °C) with buffering of the pH between 3 and 5. A dissolution-reprecipitation mechanism for the transformation was indicated and subsequently studied in greater detail by Xia et al. [
22]. The data in
Figure 6 indicate that conversion of pentlandite to violarite occurs during the heating period and prior to commencement of pressure oxidation. However, as the amount of sulfuric acid present during this period is low, the difference in pentlandite and violarite contents compared to the data given in
Table 2 suggests instead that the “aged” concentrate sampled for the tests was no longer homogenous in relation to its mineralogy. Furthermore, the formation of hematite during the heating period (as discussed later) is expected to occur from the dehydration of goethite (not from the hydrolysis of ferric iron which would generate acid).
The hydrothermal transformation of pyrrhotite (Fe
7S
8) to pyrite (FeS
2) or marcasite (FeS
2) has been known for some time [
23]. Several studies have indicated that the onset of marcasite formation occurs between pH 4 and 6 [
24,
25]. However, it was the study of Murowchick [
26] that first proposed the preservation of crystallographic relationships between pyrrhotite and pyrite or marcasite implied a solid-state mechanism was involved, whereas the absence of such relationships suggested a dissolution–reprecipitation mechanism. Subsequent work by Qian et al. [
27], who studied the transformations under both anaerobic and oxic conditions, confirmed that the iron disulfide phase formed depended both upon the pH and the degree of supersaturation relative to that phase. The study concluded that both marcasite and pyrite were formed from pyrrhotite by a dissolution-reprecipitation mechanism regardless of crystallographic relationship. More recently, the manner that pH influences the phase formed was investigated using
ab initio calculations [
28]. This modeling was able to predict rapid nucleation of marcasite below pH 4 and that crystal growth of the marcasite and pyrite phases requires a critical nucleus size to be achieved.
In the present study, marcasite was the primary transformation product, including during the heating period before oxygen injection commenced, though a small amount of pyrite appears to have formed once the reactions commenced, as shown in
Figure 7. The mechanism for pyrrhotite replacement has been proposed to follow Equations (11) and (12) under anaerobic and oxic conditions, respectively [
27]:
Notably, no marcasite was detected when fresh concentrate was heated up under anaerobic conditions. Although the liquor was acidic, pH ~2.5, the amount of sulfuric acid available would be insufficient to facilitate pyrrhotite replacement. In contrast, for the “aged” concentrate this does not appear to have been the case.
Once pressure oxidation commenced, the remaining pyrrhotite was almost exclusively transformed to marcasite within the first 15 min, while both marcasite and pyrite were mostly oxidized over a period of 77 min. The rapid oxidation of pyrite between 62 and 77 min, corresponds to a noticeable change in the extraction of cobalt, confirming this mineral as a significant host for cobalt, a point previously discussed. Additionally, the persistence of pyrite (given there are no other detectable sulfide minerals) in the residue after 77 min is consistent with the incomplete leaching of cobalt. Based upon SEM/EDS data provided below, pyrite present in the concentrate feed is expected to have a higher cobalt content than neoformed pyrite generated by the oxidation of pyrrhotite.
Examination of the mineralogy of the iron hydrolysis products indicates that hematite is the dominant phase up to 71 min (
Figure 8). At this time, the amount of residue relative to the added initial solids is lowest, which is consistent with the fraction of dissolved iron being highest, as shown in
Figure 4. Thereafter, the oxidation of ferrous to ferric iron with attendant hydrolysis results in the decrease in dissolved iron and increases in the amounts of hematite (Fe
2O
3) and basic ferric sulfate (FeOHSO
4) present in the residue. The formation of basic ferric sulfate occurs as the free acidity exceeds the value referred to by Tozawa and Sasaki [
29] as the breakpoint value. The data also indicate that some of the hematite formed is dissolved and the iron reprecipitated as basic ferric sulfate. This phase is environmentally unstable and will degrade to generate iron oxyhydroxide phases and sulfuric acid, which in addition to the free acidity of the liquor, will require neutralization. Calculations indicated that 30% of the sulfuric acid generated was locked up in the basic ferric sulfate.
The acidity in the reaction at high temperatures can be buffered in the presence of soluble metal sulfates [
29], though few studies since have taken advantage of this behavior to mitigate the formation of basic ferric sulfate [
30,
31]. The addition of magnesium sulfate to buffer the acid during the pressure oxidation of the nickel concentrate has little impact upon the metal’s extraction behavior, as shown in
Figure 9. The main impact was to increase the free acidity in the system and at the end of the experiment, this value increased from 54 g/L when conducted in deionized water (
Figure 4) to 103 g/L in the presence of 1 molal (i.e. mole per kg solvent) magnesium sulfate solution (
Figure 9). In comparison, Sasaki et al. [
32] noted that the breakpoint acidity for iron hydrolysis in the presence of 27–33 g/L Mg, i.e., 1.0–1.2 molal, was ~105 g/L and this is consistent with the fact that no basic ferric sulfate was detected in the leach residue samples (
Figure 10). As before, hematite was formed during the leaching of the sulfide minerals; the sharp increase in normalized hematite content corresponds with ferrous iron oxidation and this was followed by a slower increase as the system approaches an equilibrium state. It is also notable in
Figure 10 that the normalized mineral content in the first two residue samples exceeds 100%. This was due to the presence of precipitated kieserite (MgSO
4·H
2O) in the samples that had been incompletely washed out during sample preparation.
Whether sulfuric acid generated is locked up in the residue and/or is present in the leach liquor, it can be employed to leach other minerals such as those present in nickel laterite ores. For a low-grade nickel concentrate, such as that used in this study, a significant fraction of the sulfide is associated with iron. Assuming each of nickel, cobalt and copper is associated with sulfide with ratio 1:1, which here is a good approximation given the identities the base metal sulfide minerals, ~87% of the sulfide was estimated to be associated with iron. The amount of acid that can be generated was calculated to be ~800 kg/t concentrate using the data given in
Table 1; this value is also consistent with the experimental analyses assuming that all dissolved sulfate content at the conclusion of pressure oxidation, except that associated with base metal dissolution, derives from or is associated with sulfuric acid generation. This acid must either be neutralized in a commercial process or can potentially be used to leach further metal values, such as from a nickel laterite ore [
33]. The latter is the topic of a subsequent study [
34].