Effects of grinding on the preg-robbing behaviour of pyrophyllite

Hydrometallurgy 146 (2014) 154–163

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Effects of grinding on the preg-robbing behaviour of pyrophyllite Sima Mohammadnejad a, John L. Provis a,b,⁎, Jannie S.J. van Deventer a a b

Department of Chemical & Biomolecular Engineering, University of Melbourne, Victoria 3010, Australia Department of Materials Science and Engineering, Sir Robert Hadfield Building, Mappin St, University of Sheffield, Sheffield S1 3JD, UK

a r t i c l e

i n f o

Article history: Received 18 December 2013 Received in revised form 31 March 2014 Accepted 8 April 2014 Available online 18 April 2014 Keywords: Pyrophyllite Grinding Preg-robbing XPS

a b s t r a c t Mechanical activation of the 2:1 layer lattice aluminosilicate pyrophyllite is investigated, with particular regard to the implications of this process for gold preg-robbing behaviour. Short-term grinding in a high energy laboratory ring mill increases the adsorption of gold by pyrophyllite in an acidic chloride medium, which would be problematic in the operation of a hydrometallurgical process. However, prolonged grinding (30 minutes or more) inhibits the sorption of gold anions onto the surface of pyrophyllite, reducing the extent of gold losses from solution. Analysis of surface chemistry and structure of ground pyrophyllite reveals significant distortion of the mineral by grinding. The dehydration and dehydroxylation of pyrophyllite during prolonged grinding are accompanied by changes in Al coordination number and the formation of an amorphous Al-rich phase covering the surface of pyrophyllite to a depth of around 50 nm. Approximately 41% coverage of the pyrophyllite surface by this newly formed Al-rich phase is calculated from XPS data after 30 minutes of intense grinding. The local structural alteration of Si tetrahedra as well as Al octahedra is directly identified for the first time in our experimental results, indicating some incorporation of Si in the newly formed amorphous phase on the surface of intensely ground pyrophyllite, and this plays a significant role in influencing gold sorption. The highly reactive amorphous material on the surface of particles cements the particles together, which results in lower surface reactivity and pregrobbing. These findings demonstrate the important role of surface activation of ore constituents during fine grinding and the need for careful adjustment of grinding parameters to achieve the highest overall recovery. © 2014 Elsevier B.V. All rights reserved.

1. Introduction The two most common reasons for refractoriness of an ore are (a) gold locked in sulphide minerals such as pyrite, arsenopyrite and pyrrhotite, and (b) gold locked in silicates (Marsden and House, 2006), so that pre-treatment is essential to expose the gold during leaching. For many years the high cost of ultra-fine grinding has prevented its application to refractory silicate ores. However, in the last 10–15 years, the development of new ultra-fine grinding technologies has significantly reduced costs and has replaced chemical pre-treatment techniques (Flatman et al., 2010). Therefore, the preg-robbing of gold by silicates during fine grinding is worthy of investigation. Pyrophyllite is one of the common silicate minerals in gold ores, sometimes encapsulating the precious constituents (Powers, 1993; Rapprecht, 2010) and has been identified as a preg-robbing mineral (Urban et al., 1973). It has been proposed that pyrophyllite and other Mg-rich phyllosilicates have the same potential for gold adsorption as that of carbonaceous matter (Baum, 1988). Our experiments also ⁎ Corresponding author at: Department of Materials Science and Engineering, Sir Robert Hadfield Building, Mappin St, University of Sheffield, Sheffield S1 3JD, UK. Tel.: +44 114 222 5490. E-mail address: j.provis@sheffield.ac.uk (J.L. Provis).

http://dx.doi.org/10.1016/j.hydromet.2014.04.007 0304-386X/© 2014 Elsevier B.V. All rights reserved.

showed it to be the strongest preg-robber compared to other minerals (Mohammadnejad et al., 2011). This paper is focused on the effect of grinding on the surface reactivity and the extent of gold adsorption by this common mineral in gold ores. Pyrophyllite (Al2Si4O10(OH)2) is a layered hydroxy-aluminosilicate, containing an Al octahedral sheet condensed between two tetrahedral Si sheets, i.e. a 2:1 structure (Wardle and Brindley, 1972). Each aluminium atom in the octahedral unit shares four oxygens with adjacent Si tetrahedra, and is bonded to two structural hydroxyl groups. In pyrophyllite as well as other 2:1 silicates only limited substitution occurs in the tetrahedral layers (Temuujin et al., 2003), with an upper limit of about 7% Al3+ in the tetrahedral sheet and 5% Si4+ in the octahedral sheet (Lee and Guggenheim, 1981). Pyrophyllite is also very resistant to acid attack because of the unstrained structure and the location of the oxygen atoms solely between the layer surfaces (Maqueda et al., 1987). Above 550 °C, pyrophyllite loses the structural hydroxyl groups associated with its octahedral sheet, and above 1200 °C mullite and cristobalite are formed (Frost and Barron, 1984; Sánchez-Soto and Pérez-Rodríguez, 1989; Sánchez-Soto et al., 1997). In contrast with the high chemical stability of pyrophyllite, grinding of this mineral results in significant structural distortion (Pérez-Rodríguez et al., 1988; Sánchez-Soto et al., 1997; Temuujin et al., 2003; Wang et al., 2002; Wiewióra et al., 1993). Dehydroxylation and mechanical transformations of pyrophyllite have been researched for many years in

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the mineral processing, ceramic and refractory industries. Hayashi et al. (1962) showed that pyrophyllite can transform to an amorphous substance under fine grinding. Nemecz (1984) showed that pyrophyllite1Tc is distorted into a monoclinic modification via prolonged grinding. Sugiyama et al. (1994) compared the structural changes in pyrophyllite induced by dry grinding with similar processes in kaolinite, and found that the SiO4 tetrahedron remains essentially unchanged in both clays, while the coordination number and corresponding interatomic distances around aluminium decrease. They noted that the variation induced by dry grinding is similar to that which takes place during the formation of metakaolin by calcination of kaolinite, where the local ordering structure around aluminium is changed toward 4 and 5 coordination (White et al., 2010a,b). Sánchez-Soto and co-workers researched the effects of dry grinding on pyrophyllite (Pérez-Rodríguez et al., 1988; Sánchez-Soto and Pérez-Rodríguez, 1989; Sánchez-Soto et al., 1994, 1997, 2000), and proposed that high energy milling results in distortion of the crystal structure and conversion of the 6-coordinated aluminium to 4 and 5-coordinated, in agreement with the literature. However, their results did not show any triclinic-monoclinic transformation as claimed by Nemecz (1984). They also revealed that low-temperature amorphous mullite is formed from ground activated pyrophyllite via combined mechanical and thermal effects. In analysis of different clays including pyrophyllite, illite and kaolinite, the formation of this new mullite phase was only associated with pyrophyllite, and was attributed to the change of aluminium coordination as a result of structural breakdown of pyrophyllite during prolonged grinding (Sánchez-Soto et al., 1994). Temuujin et al. (2003) investigated the effect of grinding of pyrophyllite on its leaching behaviour in HCl solution, where the structural distortion and breakdown of pyrophyllite during grinding resulted in enhanced leaching of Al3+. However, extended grinding inhibited the leaching process, and this could not be explained solely by agglomeration effects; it was proposed that new strong Al-O-Si bonds are probably formed during prolonged grinding. MacKenzie et al. (2008) reported the disruption of the pyrophyllite lattice structure by severe grinding, resulting in four-, five- and six-fold coordinated Al, similar to the situation in metakaolin, which gave high reactivity with alkaline solutions at moderate temperatures. From this body of literature it is evident that grinding causes major structural disorder via distortion or breakage of the pyrophyllite crystalline network, while the sequence of reaction from pyrophyllite to the dehydroxylated amorphous phase has not been fully elucidated. There is even uncertainty about the identities of the intermediate phases which form during thermal treatment of pyrophyllite, despite extensive studies. This research aims to shed light on the effect of mechanochemical activation on the structural and surface properties of pyrophyllite in the context of gold processing. Our previous investigations on mechanochemical activation of quartz revealed the formation of point defects including low valence silicon and non-bridging oxygen centres playing an important role in the surface reactivity of the quartz and the extent of gold loss during preg-robbing (Mohammadnejad et al., 2013). Laboratory experiments to determine the preg-robbing propensity of pyrophyllite also showed that this mineral had the highest capacity for adsorption of gold per unit surface area, among all silicates evaluated (Mohammadnejad et al., 2011). In this paper we reveal that mechanochemical activation and other processes taking place during grinding play a significant role in determining the extent of gold adsorption onto pyrophyllite and its mechanically-modified derivatives. Despite the co-occurrence of gold and pyrophyllite in some deposits, whenever fine grinding of silicates is required for liberation of precious constituents, the undesired effect of surface activation of other minerals such as pyrophyllite has to be considered carefully. This paper will explain the role of surface reactivity of ore components in leaching and will justify the need for careful adjustment of grinding conditions to optimise overall recovery.

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In this study, as-received and ground pyrophyllite samples are subjected to adsorption tests in gold chloride medium under dynamic acidic conditions. The samples are also characterised using nitrogen sorption with Brunauer–Emmett–Teller (BET) surface area calculation, dynamic light scattering (DLS), Fourier transform infrared (FTIR) spectrometry, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) with depth profiling. The results are discussed in comparison with the preg-robbing ability of freshly ground and aged pyrophyllite surfaces, with implications drawn regarding the role of the mechanically activated pyrophyllite in gold hydrometallurgical processes. 2. Materials and methods A pyrophyllite sample in the size range 0.1–2.5 μm was provided by Geological Specimen Supplies, Australia. The sample showed some trace quartz impurities and a small amount of muscovite, as identified by X-ray diffractometry (Bruker D8 Advance). The chemical composition of this sample was reported previously (Mohammadnejad et al., 2011). The sample was dried at 50 °C overnight before grinding and spectroscopic studies. No further treatment was applied to the sample before experiments, and all experiments were conducted at room temperature. The sample was dry ground in a Labtechnics LM1 series Standard ring mill for surface activation. The ring mill, with a power intensity of 2750 kW/m3 (400 cm3 grinding chamber and 1.1 kW power), was loaded with only 20 g of sample per run to maximise the extent of mechanochemical activation. Before each grinding run, the 125 cm 3 Labtechnics steel bowl, including ring and puck, was loaded with coarse quartz sand (0.3–2.5 mm) to remove oxide scale and any other contamination from the surface, and then rinsed with water and dried. A quantity of 20 g of pyrophyllite was ground in each run, for durations of 30 seconds, 1, 5, 15 and 30 minutes. During grinding, the temperature of the rings and bowl of the mill rose significantly. Preg-robbing experiments were carried out at room temperature and pressure, using an acidic chloride solution with 50 g/L of asreceived or ground pyrophyllite, and 10 ppm gold solution (supplied as 1000 ppm HAuCl4 solution, Sigma–Aldrich Australia, and diluted with RO-grade purified water), which were mixed immediately after grinding of the pyrophyllite. The sorption testing was performed in glass beakers with 500 mL of chloroauric acid per test, under dynamic conditions using a magnetic stirrer. The initial pH of the solution with 10 ppm chloroauric acid was 2.2–2.5, and was not readjusted after adding the silicates. Samples of 10 mL of the solution were taken every 30 min for the first 2 h, then every hour until 5 h, and finally at the end of each run. The Eh and pH of the solution were measured with every sample collection. The specimens were then centrifuged, filtered with 0.2 μm polycarbonate membrane filters, and finally diluted (10 ×) with distilled water for inductively coupled plasma optical emission spectroscopy (ICP-OES; Varian Instruments) analysis. All of the ground samples as well as the as-received pyrophyllite were characterised using DLS particle size measurement (Malvern HTTP dynamic light scattering instrument equipped with a He–Ne laser operating at a wavelength of 633 nm) and FTIR spectrometry (Bruker T27, KBr pellet technique). The sample after 15 minutes of grinding was also studied using XRD (Bruker D8 Advance). The specific surface areas of the samples were also measured by N2 sorption and the BET method using an ASAP2010 instrument (Micromeritics, USA) after degassing at 110 °C for 6 hours. To investigate the modification of the surface structure by grinding, analysis was conducted using XPS, with destructive depth profiling by Ar+ gas sputtering. This was conducted on the original sample, as well as freshly ground (b24 h) samples after 30 seconds, 5 and 30 minutes of grinding. Samples were mounted in a 1 mm powder sample holder and scanned using a Thermo K-alpha X-ray Photoelectron Spectrometer equipped with a micro-focused monochromated source of Al Kα

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radiation (1486.6 eV) and a PHI-5500 spherical capacitor analyser. The spectra were recorded under a vacuum of 10−9 torr. After the collection of a spectrum from each as-milled surface, the samples were bombarded with a 3 keV ion beam with a current of 54 nA and 400 μm spot size, across an area of 2 × 2 mm2. Argon ion sputtering was applied for 10 s per step in 14 steps (140 s in total), with an XPS measurement taken for each step. The thickness of the material removed in each step was estimated to be around 3.5–4 nm, based on the known performance of this specific sputtering system. All spectra collected were calibrated using the carbon 1s peak at a binding energy of 284.6 eV to correct for sample charging. 3. Results and discussion 3.1. Morphology of ground pyrophyllite Fig. 1 shows the DLS particle size and surface area measurement results for the as-received and ground pyrophyllite samples. Table 1 presents the size reduction ratio, represented as the ratio of feed to product size, for each incremental time period of grinding (dn/d(n + 1)), and it is observed that no further size reduction takes place beyond a point between 5 and 15 minutes of grinding. This is seen by the size reduction ratio increasing beyond 1.0, which indicates that a grinding limit exists where increasing grinding time no longer results in breakage of the particles. This grinding limit occurs at a particle size of about 500 nm, corresponding to a surface area of 37 m2/g for this pyrophyllite. Different values have been reported for the grinding limit of pyrophyllite in the literature, which is related to the fact that these studies have analysed different types of pyrophyllite with different levels of impurities (Pérez-Rodríguez et al., 1988; Sánchez-Soto et al., 1997; Sugiyama et al., 1994; Temuujin et al., 2003). Quartz impurities result in a lower particle size (higher surface area) at the grinding limit, via generation of a higher degree of structural distortion and particle breakage prior to agglomeration (Temuujin et al., 2003). In contrast, the presence of mica minerals results in a less effective breakage mechanism and stronger agglomeration (Santos et al., 2011). A high degree of agglomeration is observed for pyrophyllite here, compared to our previous study of quartz (Mohammadnejad et al., 2013). The surface area of pyrophyllite after 30 minutes of grinding is actually 10% lower than the as-received material. The rapid decrease in the surface area and agglomeration of particles in pyrophyllite is due to the structural distortion and collapse of the layers, as will be discussed later, and is in agreement with previously reported data for pyrophyllite (Pérez-Rodríguez et al., 1988; Sugiyama et al., 1994; Temuujin et al., 2003). This result shows that even extensive high energy milling does not generate a high surface area in pyrophyllite, and this is an important factor in determining the extent of preg-robbing from leaching solutions.

Fig. 1. Average particle size and surface area of pyrophyllite in each cycle of grinding.

Table 1 Particle size reduction of pyrophyllite in a ring mill. Grinding time

Size reduction ratio dn/d(n + 1)

30 sec 1 min 5 min 15 min 30 min

0.66 0.78 0.74 1.58 1.41

3.2. Preg-robbing due to ground pyrophyllite The preg-robbing capacity of unmodified pyrophyllite has been reported to be about 0.03–0.14 μmol/m2 surface area in our previous study (Mohammadnejad et al., 2011); Fig. 2 compares the adsorption onto the as-received and ground pyrophyllite samples studied here. In agreement with previously reported results on pyrophyllite as well as other silicates, the adsorption starts instantly after the introduction of pyrophyllite into the solution, and equilibrium conditions are reached in the first 30–60 minutes. The total amount of gold lost from solution is significantly higher in the samples ground for 1 and 5 minutes. However, the gold loss significantly decreases (lower than the as-received samples) in the samples ground for 15 minutes, and remains almost constant up to 30 minutes of grinding. Comparing the adsorption results with the particle characterisation data to plot the curve in Fig. 2 on a per-surface area basis, the effects of grinding on the surface properties of the silicates can be divided into three different regimes as follows: • In the first 5 minutes of grinding, the main mechanism of grinding is particle breakage, as the size and surface area of the particles change significantly. However, the steady rate of adsorption per unit surface area reveals that the major cause of increased total adsorption is higher available surface area. In other words, the surface properties of pyrophyllite have not changed considerably in the first 5 minutes of grinding. In this stage of fine grinding, the total effect on pregrobbing is significant, as the surface area increases drastically until it reaches the grinding limit, but the gold uptake per surface area is not changed. • From 5–15 minutes of grinding, agglomeration starts, as reflected by the decreased surface area and increased particle size. However, both the total amount of gold and the gold lost per surface area increase significantly in this period. If agglomeration was the only effective mechanism here, this could not explain the significant increase in gold loss per surface area in this period. Even if a portion of agglomerated grains in the adsorption solution became de-agglomerated

Fig. 2. Gold adsorption by as-received (shown as 0 minutes) and ground pyrophyllite samples.

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through the mechanical forces imposed during stirring, a significant rise in the total gold sorption compared to the sample ground for 5 minutes can only be explained by surface activation as the dominant mechanism in this period. The surface properties of pyrophyllite have been altered by mechanical treatment, and the material now shows a stronger tendency toward adsorption of gold chloride anions. • In the last stages of grinding, agglomeration is the dominant mechanism, as reflected in the decreasing trend in surface area. Compared to the second stage, there is also a transition from surface activation to passivation, resulting in a decrease in gold adsorption per surface area between 15 and 30 minutes of grinding. The prolonged grinding has inhibited the sorption of gold anions onto the surface of pyrophyllite. This might be the result of a highly agglomerated particle structure during prolonged grinding, but the possibility of chemical and/or structural changes still requires further investigation. It has been suggested that edge surfaces of clays are responsible for the majority of preg-robbing of gold from chloride solutions in acidic conditions (Mohammadnejad et al., 2011). The most reactive surface functional group on the edge surfaces of pyrophyllite is the hydroxyl group exposed on the periphery of the mineral (Keren and Sparks, 1994). This functional group can be linked to two types of sites: Al(III) and Si(IV), which are sited in the octahedral and tetrahedral sheets respectively. The hydroxyl group associated with Al(III) can interact with a proton at low pH values and form a Lewis acid site, while at the edge of the tetrahedral sheet, the hydroxyl group is singly coordinated to a silicon site (Keren and Sparks, 1995). Thus, the respective increases and decreases in the preg-robbing effect observed in adsorption experiments may reflect modification of the hydroxyl functional groups of the pyrophyllite surface. Similar results have been reported before regarding the leaching properties of pyrophyllite (Temuujin et al., 2003), where it was observed that the structural breakdown of the ground pyrophyllite facilitated leaching of Al3+, but this effect diminished at longer grinding times. However, the exact reason controlling those observed effects was not established. The adsorption results clearly show surface modification of the ground pyrophyllite. However, the observed results cannot simply be explained via changes in the morphological properties of the ground pyrophyllite in the different stages of grinding. Hence, to understand the nature of the surface alteration taking place during grinding, the samples have been subjected to different characterisation methods including XRD, FTIR and XPS. 3.3. XRD analysis Fig. 3 presents the XRD diffractogram of as-received pyrophyllite, as well as the sample ground for 15 minutes. Grinding has caused the

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intensity of the pyrophyllite reflections to decrease radically, and the level of background increased slightly. However, our data show, in contrast with the results of Temuujin et al. (2003) but in agreement with some earlier studies (Nemecz, 1984; Pérez-Rodríguez et al., 1988), that the non-basal XRD reflections are not affected as much as basal reflections by grinding. This shows that the alteration of the structure occurs mostly in the c-axis of the pyrophyllite crystals, as expected based on the layered structure of the mineral. The structural modification of pyrophyllite, conceptualised as partial amorphisation, can be clearly observed in the X-ray diffractograms in Fig. 3. Mechanochemical alteration of the structure can be identified by observing the decrease in intensity of the XRD peaks as well as peak broadening effects after 15 minutes of high energy milling. However, XRD shows average rather than local structure, and cannot detect surface-specific processes in these samples. So, to investigate the sequence of structural modification and type of chemical alteration during grinding, samples after 0.5, 1, 5, 15 and 30 minutes grinding are also studied using FTIR and XPS methods. 3.4. FTIR analysis The infrared spectra of the as-received pyrophyllite, as well as the ground products, are depicted in Fig. 4. Pyrophyllite is characterised in particular by an infrared absorption peak at 3675 cm−1 attributed to an Al-OH vibration band (Farmer, 1974). In comparison with the original pyrophyllite, the FTIR spectra of the samples ground for 0.5, 1 and 5 minutes show almost no difference in peak shapes and positions. However, there is a small shift to higher wavenumbers for Al-related bands of pyrophyllite, which is attributed to minor alteration of Al environments. The alteration of the pyrophyllite structure starts sometime after 5 minutes of grinding, and is strongly evident in the spectrum after 15 minutes of grinding. The bands attributed to quartz impurities, including 461, 695, and the doublet at 779 and 797 cm −1 (Bandopadhyay, 2010), remain unchanged over 30 minutes of grinding. The feature at 695 cm−1 is particularly used as a measure of quartz crystallinity (Saikia et al., 2008), and reveals that quartz remains stable over 30 minutes of intensive grinding here. In contrast, structural alteration of the pyrophyllite by grinding results in broadening of the 950–1200 cm−1 bands, and a decrease in all other characteristic bands of pyrophyllite. The bands at 539 and 576 cm−1 are ascribed to Si-O-Al vibrations (Wang et al., 2002). The samples ground for 15 and 30 minutes only show one band assigned to a Si-O-Al vibration, 542 cm−1, and its occurrence indicates that the Si-O-Al linkages still exist after prolonged grinding, while its frequency change (Fig. 4) reveals a change in the bond length. Aluminium usually occupies one of three different geometries with respect to oxygen: octahedral (AlO6), pentahedral (AlO5) or

Fig. 3. Changes in the XRD diffractogram of pyrophyllite by grinding.

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30 min

Adsorption

15 min 5 min 1 min 0.5 min 0 min

Fig. 4. FTIR spectra of original and ground pyrophyllite.

tetrahedral (AlO4). When aluminium ions are present in octahedral coordination, the Al-O stretching and bending modes appear in the region 500–750 cm−1, while if they have tetrahedral symmetry (AlO4), the Al-O stretching and bending modes instead appear in the range 750–850 cm−1 (Shek et al., 1997). The hydroxyl-related bands in pyrophyllite appear as a sharp peak at 3675 cm−1 for the OH stretching vibration, a sharp band at 950 cm−1 for the in-plane OH bending vibration of Al-OH, and a less distinct band, or shoulder, due to OH bending at 854 cm−1. The spectrum of the material ground for 15 minutes shows that the intensities of molecular water bands (adsorbed H2O, at 3639 and 1639 cm−1) increase, and these bands then decrease with longer grinding times. All other band groups decrease significantly with 15 minutes of grinding, and both 854 and 950 cm−1 features almost disappear after 30 minutes of intensive grinding. It is expected that size reduction and the increase in surface area help OH groups to leave the strained crystal structure (Wiewióra et al., 1993), but this only appears to happen after 15 minutes of intensive grinding, where mechanical treatment has stressed the crystal structure to the point that the bond energy of the OH groups decreases, and these can be lost at the lower temperatures prevailing during grinding, compared to the much higher temperatures required for thermallyinduced dehydroxylation. Another effect observed in the samples ground for 15 and 30 minutes is a shift of the Si-O-Si asymmetric stretching vibration bands to higher wavenumbers (from 1070 to 1078 and 1076 cm−1 respectively). The asymmetric vibration mode of pure Si-O-Si is at about 1085 cm−1, while for Si-O-H (surface hydroxyl groups) it shifts to 975 cm−1 (Yu et al., 2003). The shift of this band to higher wavenumbers in pyrophyllite samples is thus attributed to the loss of hydroxyl groups, consistent with the dramatic decrease of the 3676 cm−1 band after 15 minutes of grinding. It has also been reported that there is an increasing degree of covalent Si-O-Al bonding due to the condensation of the OH groups (Yu et al., 2003) giving a shift to lower wavenumber of this band, but this cannot be clearly detected in our spectra. In the last stage, after prolonged grinding for 30 minutes, decomposition and total destruction of the pyrophyllite structure occurs. For the samples ground for more than 15 minutes, the decomposition is seen as a significant decrease in the intensity of the IR signals of pyrophyllite, which are replaced by a group of broad peaks in Fig. 4. These broad spectral features imply that the products of decomposition of pyrophyllite are mainly amorphous in nature. The spectrum of the sample ground for 30 minutes mostly contains quartz peaks with some weak bands of the original pyrophyllite. The marked reduction – almost disappearance – of the pyrophyllite bands at 576, 835, 854 and 949 cm−1, all of which are related to bonds involving Al, supports a significant structural transformation around Al cations. A shift from octahedral coordination of Al in pyrophyllite to tetrahedral coordination has previously been reported during thermal

transformation of pyrophyllite at high temperatures (Pérez-Rodríguez et al., 1988; Sánchez-Soto et al., 1994, 2000). However, no detectable band at 842 cm−1 related to tetrahedral Al (Boumaza et al., 2009; Percival et al., 1974) could be observed in Fig. 4 even after 30 minutes of grinding. Temuujin et al. (2003) also suggested the formation of an amorphous phase containing Al-O-Si linkages in mechanically activated pyrophyllite, based on DTA data, although the exact nature of this phase could not be characterised. However, the FTIR spectra in Fig. 4 appear to suggest a simple breakdown of the crystal structure of pyrophyllite, without major frequency shifts or the appearance of additional bands, which would be expected if significant cation migrations or displacements were involved. 3.5. XPS analysis XPS analysis is sensitive to the chemical bonding environments on the surfaces of materials, and so is ideally suited to the analysis of the mechanically modified samples here. Table 2 shows the measured XPS peak positions in the as-received and ground samples on the first level no sputtering). In this surface level, there is a small shift of the Al 2p peak to lower binding energies in the samples ground for 30 seconds and 5 minutes, from 74.3 to 73.8 and 73.9 eV respectively. However, the sample ground for 30 minutes shows a shift back to higher binding energies by about 0.7 eV, as well as peak broadening in the surface level of the sample. The O 1s peak position shows only a slight increase under prolonged grinding, while the Si 2p binding energy increases continuously. A shift in the Al KLL Auger peak to lower binding energy has been reported for kaolinite during thermal as well as mechanical treatment (Torres Sánchez et al., 1999), attributed to the development of an Al tetrahedral component in the mechanically treated kaolinite. However, their Al 2p photoelectron peak did not show any detectable shift. The difference in the Al 2p binding energy between octahedral and tetrahedral Al in dioctahedral phyllosilicates has been calculated to be as small as 0.8 eV (Ebina et al., 1997), and so it appears that the position of Al core level peaks is not very sensitive to crystal chemistry. The differences in binding energy of Al among Al-O, Al-OH and Al-OOH are very small, generally in the order of 0–0.5 eV (Kloprogge et al., 2006), which is on the same order of magnitude as the experimental precision of XPS. Table 2 XPS peak positions of original and ground silicates without sputtering. Time of grinding (min)

0

0.5

5

30

Al 2p O 1s Si 2p

74.3 532.1 102.9

73.8 532.2 102.9

73.9 532.4 103.0

74.2 531.6 103.4

S. Mohammadnejad et al. / Hydrometallurgy 146 (2014) 154–163

Nonetheless, the deconvolution of Al 2p peaks via Gaussian fitting to spectra collected at different depths does reveal some systematic changes on the surface of pyrophyllite, which can explain observed features in our experimental results as well as some reported data in the literature.

Centre

FWHM

73.7

2.3

74.3

1.8

74.6

2.2

The deconvolution of Al 2p spectra collected for the as-received sample using a Gaussian peak shape function, median baseline mode and the second derivative method for peak position identification, results in the identification of three major peaks, at 73.6–73.9, 73.9–74.4 and

a)

b) Centre

FWHM

73.9

2.8

74.3

1.9

74.6

3.0

c) Centre

FWHM

73.9

2.6

74.3

2.1

d) Centre 74.3

159

FWHM 2.3

Fig. 5. The deconvolution of the Al 2p peak in the original pyrophyllite. a) level 0 (no sputtering), b) level 1, c) level 2, d) levels 3–14.

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74.5–75.0 eV, where the precise position of each sub-peak differs slightly from sample to sample. The spin-orbit separation of the Al 2p doublet (2p1/2 and 2p3/2) was too small to resolve (~ 0.4 eV), so each site was fitted with only one Gaussian peak, following previous work in this area (Matsuo et al., 1999). The area ratios of the three peaks vary as a function of depth (i.e. the duration of ion sputtering), and the lower (73.6–73.9 eV) and higher (74.5–75.0 eV) bonding energy peaks are not present at any depth below the 4th level of sputtering (removal of ~ 10 nm) from the surface of the original sample. The Al 2p peak deconvolutions for the first four levels are shown in Fig. 5, and the peak fitting results are presented in Table 3. The peaks have been labelled as Low, Med and High for the three different binding energy ranges of the peaks because of the small variations in the peak positions. The peak located at 74.3 eV (marked as Med) is attributed to octahedral Al in the undamaged pyrophyllite structure, as it is the only peak appearing at depths of more than 10 nm from the surface of the original sample, although the peak position is slightly lower than reported data for octahedral Al in phyllosilicates (Ebina et al., 1997; Naumkin et al., 2012). The broad nature of the other two peaks indicates low crystallinity of these phases, and they are attributed to other Al compounds either present as impurities on this aged surface, or formed by alteration of the Al octahedral and Si tetrahedral units in pyrophyllite during grinding. Obviously the original pyrophyllite sample has been ground to achieve 1–2.5 μm particles. The Al 2p binding energy of low crystallinity Al hydroxide (Al(OH)3) is reported to be 75.0 eV, while other Al oxide/ hydroxide phases are in the range of 73.7–74.5 eV (Kloprogge et al., 2006). Based on the reported data in the literature, we assign the higher binding energy peak at 74.6− 75.0 eV to chemical environments very similar to amorphous Al(OH)3. The lower binding energy peak may be partly due to muscovite (73.7 eV), as was detected in XRD. However, the systematic change in the prevalence of this peak cannot be explained by muscovite alone. Therefore, based on the area percentages of the deconvoluted peaks (Table 3), we identify that the original pyrophyllite is about 40% covered with an amorphous Al phase which only appears in the first few nanometers of the surface of the particles. The deconvolution of the Al 2p peaks for samples ground for 30 seconds and 5 minutes (at the surface as well as lower levels) shows only a single peak in the middle range of binding energies, similar to spectrum (d) in Fig. 5. The assignment of this peak to octahedral Al in pyrophyllite is in agreement with FTIR results, as the pyrophyllite structure was unchanged at this stage of grinding. The consistency of the spectra at the surface and lower levels of these samples indicates a low rate of surface hydration exposed to the air. The ground samples were measured within 24 hours after grinding, but the original sample was aged for a few months after it was received from the supplier. However, the sample after prolonged grinding (30 minutes) again shows another peak at lower binding energy. The shape of this peak is different to the lower binding energy peak of the original sample, being narrow with 1.4–1.6 eV FWHM, and located at a slightly lower binding energy of 73.6–73.7 eV. The area percentage of the low binding energy peak, as an indication of concentration of the corresponding phase, has been plotted as a function of depth in Fig. 6, for the 30 minutes ground sample. There is a smooth decreasing trend from a maximum of 41% at the surface, and this peak is still present at 5% after removal of about 50 nm of material from the surface. The peak

Table 3 Area percentage of each deconvoluted Al 2p sub-peak in the original pyrophyllite. Depth level

0

1

2

3–14

Low Mid High

22 40 38

19 56 25

23 77 0

0 100 0

Etching depth (nm)

160

Fig. 6. The area percentage of the lower Al 2p binding energy peak in the sample ground for 30 minutes at the surface, and in the 14 lower levels after etching.

deconvolution results are presented in Fig. 7 for the 1st, 2nd, 7th and 15th levels (~50 nm) to demonstrate the trend observed in these peaks. The Al/Si ratio of 0.48 calculated from XPS data on the surface of the original sample is in good agreement with the chemical composition of pure pyrophyllite. However, the Al/Si atomic ratio of the surface layer of the ground samples decreases in the first 30 seconds of grinding, and then significantly increases with grinding time, particularly in the 30 minutes ground sample (Fig. 8). This effect is much less notable in the sub-surface levels of the 30 minutes ground sample, in agreement with qualitative discussions based on the Al 2p peak shape as presented above. Torres Sánchez et al. (1999) also reported up to 30% Al enrichment on the kaolinite surface during mechanical activation, while Temuujin et al. (2003) observed that a greater extent of Al depletion from the surface of ground pyrophyllite than unground material was possible by HCl leaching, which was attributed to the strain in the structure of pyrophyllite induced via grinding which facilitated Al leaching. This effect reduced at longer grinding times, similar to our results here. The change of Al/Si atomic ratio can be explained only by the structural breakdown and separation of stacked layers. The Al enrichment taking place through prolonged grinding can be attributed to a newly formed amorphous Al-rich phase, detected at lower binding energy in the Al 2p spectra. This phase can be formed via splitting of the Al and Si layers of the original pyrophyllite during grinding. However, it cannot be concluded from the Al 2p peaks whether the newly formed phase shows incorporation of Si tetrahedra (forming Si-O-Al bonds) as was claimed by Temuujin et al. (2003), or is solely formed through amorphisation of Al units. In contrast, the higher Al/Si ratio after prolonged grinding indicates the formation a more Si-free phase on the surface. It has been claimed that Si tetrahedra are quite resistant against structural alteration, and remain unchanged and stable during mechanochemical activation (Sugiyama et al., 1994) while the coordination number and interatomic distance around aluminium decrease. If the alteration of the structure is solely taking place in the Al octahedral units we would not expect any alteration in the Si 2p peak shape and position in the ground pyrophyllite samples, and correspondingly, the Si 2p peak is almost identical in the original and 30 seconds ground samples. However, a small shoulder appears at higher binding energy in the 5 minutes ground sample. This peak is very strong in the 30 minutes ground sample, shifting the whole peak to a position 0.3 eV higher, as shown in Fig. 9. The concentration of this peak decreases with depth, in agreement with the formation of the lower binding energy peak in the Al 2p spectra, and consistent with the identification of this as being a newly formed phase. The alteration of the Si 2p peak shows that the Si tetrahedral environment in pyrophyllite also changes under the mechanical force of grinding, in agreement with FTIR and XRD results.

S. Mohammadnejad et al. / Hydrometallurgy 146 (2014) 154–163

Centre

FWHM

73.7

1.9

74.0

2.9

161

a)

b) Centre

FWHM

73.7

1.8

74.0

2.6

c)

Centre

FWHM

73.6

1.5

73.9

2.5

d)

Centre

FWHM

73.1

1.4

73.7

2.6

Fig. 7. The deconvolution of the Al 2p peak in the 30 minutes ground pyrophyllite. a) level 0 (no sputtering), b) level 1, c) level 7 and d) level 15.

Dehydration and dehydroxylation of pyrophyllite occur after 30 minutes of intense grinding under mechanical and thermal treatment induced by grinding. During dehydroxylation, one molecule of water is eliminated per structural formula unit of pyrophyllite. On this basis, lower coordination states of Al can be expected to form, similar to what

happens during thermal treatment of pyrophyllite (Fitzgerald et al., 1996; Frost and Barron, 1984; Sánchez-Soto and Pérez-Rodríguez, 1989; Sánchez-Soto et al., 1997; Wang et al., 2002): AlO4(OH)2 → AlO5 + H2O

162

S. Mohammadnejad et al. / Hydrometallurgy 146 (2014) 154–163

Fig. 8. The Al/Si ratios calculated from XPS data at different depths in the original and ground pyrophyllite.

During progressive dehydroxylation of the pyrophyllite, the number of anions bonded to the Al ions is reduced. Ideally, Al3+ is entirely in octahedral coordination in pyrophyllite, but it can most likely enter tetrahedral sites in structurally disordered or amorphous ground material. The appearance of the new band at lower binding energy in the Al 2p spectrum is attributed to the alteration of the Al coordination number on the surface of pyrophyllite after 30 minutes of intense grinding. The formation of new Al-O-Si linkages during prolonged grinding can be proposed based on the observed alteration of Si 2p and Al 2p peaks in the XPS results presented here. It is proposed that this highly reactive amorphous phase on the surface of particles cements the particles together, accentuating the agglomeration process at extended grinding times, which results in lower surface reactivity during hydrometallurgical processes. This structural alteration is therefore responsible for inhibition of the preg-robbing by pyrophyllite after prolonged grinding, as well as the lower leaching rate of Al observed in previous investigations (Temuujin et al., 2003). 4. Conclusions Grinding of pyrophyllite results in structural as well as surface chemistry alterations, which influence the rate of surface activation and consequently the extent of gold preg-robbing in chloride media, with important implications for the potential wider use of this process in gold hydrometallurgy. Mechanical forces imposed during fine grinding lead to a significant structural reorganisation when the size of the particles reaches the grinding limit. The spectroscopic results show systematic changes, first by surface activation (5–15 minutes grinding), and then surface passivation and a reduced preg-robbing effect after prolonged grinding (15–30 minutes). Based on spectroscopic results, the mechanochemical activation of pyrophyllite can be divided into three stages: • Particle breakage and surface area increase (unchanged state of pyrophyllite)

Fig. 9. Si 2p peak of original and ground pyrophyllite (no sputtering).

• Structural breakdown and splitting Si and Al layers of pyrophyllite (amorphisation of pyrophyllite) • Formation of new Si-O-Al linkages (chemical alteration of pyrophyllite) Structural breakdown of the ground pyrophyllite increases the observed extent of preg-robbing, but the formation of new Al-O-Si bonds as well as agglomeration of particles after prolonged grinding then reduced the tendency towards preg-robbing. Surface characterisation of pyrophyllite reveals that the newly formed amorphous phase is Al rich and covers up to 41% of the surface. This chemical alteration extends up to 50 nm in depth into the pyrophyllite particles. We have observed, for the first time, direct spectroscopic evidence for formation of an amorphous Al-rich silicate phase during the mechanochemical activation of pyrophyllite. This study therefore reveals the effect of grinding and mechanical activation on the surface reactivity of the layer silicate pyrophyllite, and in particular its influence on the preg-robbing effect which is observed in gold hydrometallurgy. Acknowledgements This work was funded by the Australian Research Council (ARC) through Discovery grant DP1095477 and was supported by the Particulate Fluids Processing Centre, a Special Research Centre of the ARC. References Bandopadhyay, A.K., 2010. Determination of quartz content for Indian coals using an FTIR technique. Int. J. Coal Geol. 81, 73–78. Baum, W., 1988. Mineralogy-related processing problems of epithermal gold-silver ores. In: Carson, D.J.T., Vassilon, A.H. (Eds.), Process Mineralogy VIII. The Minerals, Metals and Materials Society (TMS), Warrendale, Pennsylvania, USA, pp. 3–20. Boumaza, A., Favaro, L., Lédion, J., Sattonnay, G., Brubach, J.B., Berthet, P., Huntz, A.M., Roy, P., Tétot, R., 2009. Transition alumina phases induced by heat treatment of boehmite: An X-ray diffraction and infrared spectroscopy study. J. Solid State Chem. 182, 1171–1176. Ebina, T., Iwasaki, T., Chatterjee, A., Katagiri, M., Stucky, G.D., 1997. Comparative study of XPS and DFT with reference to the distributions of Al in tetrahedral and octahedral sheets of phyllosilicates. J. Phys. Chem. B 101, 1125–1129. Farmer, V.C., 1974. The infrared spectra of minerals. Mineralogical Society, London 534p. Fitzgerald, J.J., Hamza, A.I., Dec, S.F., Bronnimann, C.E., 1996. Solid-state 27Al and 29Si NMR and 1H CRAMPS studies of the thermal transformations of the 2:1 phyllosilicate pyrophyllite. J. Phys. Chem. 100, 17351–17360. Flatman, S., Battersby, R.M., Imhof, R., Battersby, M., Ibrayev, S., 2010. The Leachox refractory gold process - the setting, design, installation and commissioning of a large scale plant at the VASGOLD gold mine, Kazakhstan (Precious Metals 2010, Falmouth, Cornwall, UK, 15–16 June 2010). Frost, R.L., Barron, P.F., 1984. Solid-state silicon-29 and aluminum-27 nuclear magnetic resonance investigation of the dehydroxylation of pyrophyllite. J. Phys. Chem. 88, 6206–6209. Hayashi, H., Koshi, K., Hamada, A., Sakabe, H., 1962. Structural change of pyrophyllite by grinding, and its effect on toxicity of the cell. Clay Sci. 1, 99–108. Keren, R., Sparks, D.L., 1994. Effect of pH and ionic strength on boron adsorption by pyrophyllite. Soil Sci. Soc. Am. J. 58 (43), 1095–1100. Keren, R., Sparks, D.L., 1995. The role of edge surfaces in flocculation of 2:1 clay minerals. Soil Sci. Soc. Am. J. 59, 430–435.

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