Gujie Qian

Replacement of pyrrhotite by pyrite and marcasite under hydrothermal conditions up to 220° C: An experimental study of reaction textures and mechanisms

Abstract The transformation of pyrrhotite to Fe disulfide (pyrite and/or marcasite) under hydrothermal conditions was studied experimentally by probing the effects of temperature (up to 220 °C, vaporsaturated pressures), ΣS(-II) concentrations, pH, and availability of oxygen on reaction progress and on the resulting textures. The pyrrhotite to Fe disulfide reaction proceeded by a dissolution-reprecipitation mechanism under all conditions. Marcasite and pyrite formed under both oxic and anaerobic conditions, which is inconsistent with the traditionally assumed polysulfide route for FeS2 formation (oxidants required for polysulfide formation). The nature of the products was controlled by the level of supersaturation of the solution with respect to Fe disulfide minerals. Marcasite formed preferentially at low pH or S(-II)-deficient solutions (saturation index << 1000), while pyrite was the main product at saturation indices >1000. Different textures were obtained for the replacement of pyrrhotite by either pyrite or marcasite. Pyrite formation proceeded by direct replacement of pyrrhotite and, simultaneously, by overgrowth from solution. The pyrite crystals were >10 μm in size and randomly oriented. In comparison, marcasite crystals were <1 μm in size, and no significant overgrowth was observed. At pH21°C <3, the marcasite nanocrystals showed the well-known crystallographic relationship with pyrrhotite, but at pH21°C 3.96, the marcasite crystallites were randomly oriented. These experimental results confirm that the preservation of the crystallographic orientation is not a distinguishing feature between dissolution-reprecipitation and solid-state reactions. The different textures among pyrite and marcasite reflect the dominance of crystal growth (pyrite) vs. nucleation (marcasite) as a precipitation mechanism.

Physical characterization and in silico modeling of inulin polymer conformation during vaccine adjuvant particle formation

This study combined physical data from synchrotron SAXS, FTIR and microscopy with in-silico molecular structure predictions and mathematical modeling to examine inulin adjuvant particle formation and structure. The results show that inulin polymer chains adopt swollen random coil in solution. As precipitation occurs from solution, interactions between the glucose end group of one chain and a fructose group of an adjacent chain help drive organized assembly, initially forming inulin ribbons with helical organization of the chains orthogonal to the long-axis of the ribbon. Subsequent aggregation of the ribbons results in the layered semicrystalline particles previously shown to act as potent vaccine adjuvants. γ-Inulin adjuvant particles consist of crystalline layers 8.5 nm thick comprising helically organized inulin chains orthogonal to the plane of the layer. These crystalline layers alternate with amorphous layers 2.4 nm thick, to give overall particle crystallinity of 78%.

A thermosyphon-driven hydrothermal flow-through cell for in situ and time-resolved neutron diffraction studies

A flow-through cell for hydrothermal phase transformation studies by in situ and time-resolved neutron diffraction has been designed and constructed. The cell has a large internal volume of 320 ml and can operate at temperatures up to 573 K under autogenous vapor pressures (ca 8.5 × 106 Pa). The fluid flow is driven by a thermosyphon, which is achieved by the proper design of temperature difference around the closed loop. The main body of the cell is made of stainless steel (316 type), but the sample compartment is constructed from non-scattering Ti–Zr alloy. The cell has been successfully commissioned on Australias new high-intensity powder diffractometer WOMBAT at the Australian Nuclear Science and Technology Organization, using two simple phase transformation reactions from KAlSi2O6 (leucite) to NaAlSi2O6·H2O (analcime) and then back from NaAlSi2O6·H2O to KAlSi2O6 as examples. The demonstration proved that the cell is an excellent tool for probing hydrothermal crystallization. By collecting diffraction data every 5 min, it was clearly seen that KAlSi2O6 was progressively transformed to NaAlSi2O6·H2O in a sodium chloride solution, and the produced NaAlSi2O6·H2O was progressively transformed back to KAlSi2O6 in a potassium carbonate solution.

A LA-ICP-MS sulphide calibration standard based on a chalcogenide glass

Abstract The accurate measurement of trace element concentrations in natural sulphides by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) has been limited by the lack of matrix-matched calibration standards. The synthesis of a standard, IMER-1, by incorporating four minor and 34 trace elements into a chalcogenide glass matrix Ge28Sb12S60 is reported here. Chemical analysis by electron probe microanalysis (EPMA), LA-ICP-MS, solution ICP-MS, and inductively coupled plasma-optical emission spectroscopy (ICP-OES) confirmed the excellent homogeneity of major elements (1-σ relative standard deviation (RSD) <1% for S, Sb and Ge) and acceptable homogeneity of most trace elements (1-σ RSD <10%). The standard was validated by analysing trace-elements concentrations in three geological pyrite specimens using IMER-1 as the calibration standard and comparing the results to previously reported values also determined by LA-ICP-MS but using a different calibration standard, STDGL2b-2. The results suggest that IMER-1 may be an appropriate calibration standard for LA-ICPMS analysis of trace elements in natural sulphides.

The Formation of Silicate-Stabilized Passivating Layers on Pyrite for Reduced Acid Rock Drainage

Acid and metalliferous release occurring when sulfide (principally pyrite)-containing rock from mining activities and from natural environments is exposed to the elements is acknowledged as a major environmental problem. Acid rock drainage (ARD) management is both challenging and costly for operating and legacy mine sites. Current technological solutions are expensive and focused on treating ARD on release rather than preventing it at source. We describe here a viable, practical mechanism for reduced ARD through the formation of silicate-stabilized iron oxyhydroxide surface layers. Without silicate, oxidized pyrite particles form an overlayer of crystalline goethite or lepidocrocite with porous structure. With silicate addition, a smooth, continuous, coherent and apparently amorphous iron oxyhydroxide surface layer is observed, with consequent pyrite dissolution rates reduced by more than 90% at neutral pH. Silicate is structurally incorporated within this layer and inhibits the phase transformation from amorphous iron (oxy)hydroxide to goethite, resulting in pyrite surface passivation. This is confirmed by computational simulation, suggesting that silicate-doping of a pseudoamorphous iron oxyhydroxide (ferrihydrite structure) is thermodynamically more stable than the equivalent undoped structure. This mechanism and its controlling factors are described. As a consequence of the greatly reduced acid generation rate, neutralization from on-site available reactive silicate minerals may be used to maintain neutral pH, after initial limestone addition to achieve neutral pH, thus maintaining the integrity of these layers for effective ARD management.

A large volume cell for in situ neutron diffraction studies of hydrothermal crystallizations

A hydrothermal cell with 320 ml internal volume has been designed and constructed for in situ neutron diffraction studies of hydrothermal crystallizations. The cell design adopts a dumbbell configuration assembled with standard commercial stainless steel components and a zero-scattering Ti-Zr alloy sample compartment. The fluid movement and heat transfer are simply driven by natural convection due to the natural temperature gradient along the fluid path, so that the temperature at the sample compartment can be stably sustained by heating the fluid in the bottom fluid reservoir. The cell can operate at temperatures up to 300 °C and pressures up to 90 bars and is suitable for studying reactions requiring a large volume of hydrothermal fluid to damp out the negative effect from the change of fluid composition during the course of the reactions. The capability of the cell was demonstrated by a hydrothermal phase transformation investigation from leucite (KAlSi(2)O(6)) to analcime (NaAlSi(2)O(6)⋅H(2)O) at 210 °C on the high intensity powder diffractometer Wombat in ANSTO. The kinetics of the transformation has been resolved by collecting diffraction patterns every 10 min followed by Rietveld quantitative phase analysis. The classical Avrami/Arrhenius analysis gives an activation energy of 82.3±1.1 kJ  mol(-1). Estimations of the reaction rate under natural environments by extrapolations agree well with petrological observations.

Experimental study of chemical durability of fluorozirconate and fluoroindate glasses in deionized water

This paper investigates the chemical durability of a fluoroindate (IZSBGC) glass (developed by our previous research for low-loss fluoroindate fiber production) compared to the widely studied fluorozirconate (ZBLAN) system via leaching of glass samples in deionized water. The chemical stability of both glass systems is probed using a series of analytical techniques such as FTIR, XPS and SEM to study the sample surfaces (before and after leaching) and hydrated layer products, both of which reflected the nature of the leaching process. Our experimental results suggest that IZSBGC glass presented better chemical stability in water than ZBLAN. The absorption due to both OH- stretching and HOH bending vibrations for both glass types increased with increasing amounts of hydrated layers formed during the leaching. The investigation of hydrated layers using SEM suggests that the NaF content in fluoride glass accelerated the leaching significantly. XPS analyses suggest that (hydr)oxyfluorides and hydroxides formed on both fluorozirconate and fluoroindate glass surfaces after leaching, respectively. The degradation of fiber breaking strain in NaF-free IZSBGC glass is less than that of NaF-containing ZBLAN glass.

Single-pass flow-through reaction cell for high- temperature and high-pressure in situ neutron diffraction studies of hydrothermal crystallization processes

A large-volume single-pass flow-through cell for in situ neutron diffraction investigation of hydrothermal crystallization processes is reported. The cell is much more versatile than previous designs owing to the ability to control independently and precisely temperature (up to 673 K), pressure (up to 46 MPa), flow rate (0.01-10 ml min-1) and reaction-fluid volume ([greater-than or equal to]65 ml). Such versatility is realized by an innovative design consisting of a room-temperature and ambient-pressure external fluid supply module, a high-pressure reaction module which includes a high-temperature sample compartment enclosed in a vacuum furnace, and a room-temperature and high-pressure backpressure regulation module for pressure control. The cell provides a new avenue for studying various parameters of hydrothermal crystallizations independently, in situ and in real time at extreme hydrothermal conditions (e.g. supercritical). The cell was successfully commissioned on the high-intensity powder diffractometer beamline, Wombat, at the Australian Nuclear Science and Technology Organisation by investigating the effect of pressure on the hydrothermal pseudomorphic conversion from SrSO4 (celestine) to SrCO3 (strontianite) at a constant temperature of 473 K and flow rate of 5 ml min-1. The results show that the increase of pressure exerts a nonlinear effect on the conversion rate, which first increases with increasing pressure from 14 to 20 MPa, and then decreases when pressure further increases to 24 MPa.

Formation of Aluminum Hydroxide-Doped Surface Passivating Layers on Pyrite for Acid Rock Drainage Control

The aim of this study was to test the performance of a novel method for acid rock drainage (ARD) control through the formation of Al(OH)3-doped passivating surface layers on pyrite. At pH 2.0 and 4.0, there was no obvious inhibition of the pyrite oxidation rate on addition of 20 mg L-1 Al3+ (added as AlCl3·6H2O). In comparison, the pyrite oxidation rate at circumneutral pH (7.4 ± 0.4) decreased with increasing added Al3+ with ≈98% reduction in long-term (282 days) dissolution rates in the presence of 20 mg L-1 Al3+. Al3+ was added to the solution and allowed to equilibrate prior to pyrite addition (2 g L-1). Consequently almost all Al3+ (>99.9%) was initially present as aluminum hydroxide precipitates at pH 7.4. X-ray photoelectron spectroscopy analysis showed a significant concentration of Al3+ (20.3 at. %) on the pyrite surface reacted at pH 7.4 with 20 mg L-1 added Al3+, but no Al3+ on pyrite surfaces reacted at pH 2.0 and 4.0 with added Al3+. Transmission electron microscopy and synchrotron X-ray absorption near edge spectroscopy analyses indicated that compact surface layers containing both goethite and amorphous or nanocrystalline Al(OH)3 formed in the presence of 20 mg L-1 Al3+ at circumneutral pH, in contrast to the porous goethite surface layers formed on pyrite dissolved in the absence of Al3+ under otherwise identical conditions. This work demonstrates the potential for novel Al-based pyrite passivation of relevance to the mining industry where suitable Al-rich waste materials are available for ARD control interventions.

The Effects of Galvanic Interactions with Pyrite on the Generation of Acid and Metalliferous Drainage

Although the acid generating properties of pyrite (FeS2) have been studied extensively, the impact of galvanic interaction on pyrite oxidation, and the implications for acid and metalliferous drainage, remain largely unexplored. The relative galvanic effects on pyrite dissolution were found to be consistent with relative sulfide mineral surface area ratios with sphalerite (ZnS) having greater negative impact in batch leach tests (sulfide minerals only, controlled pH) and galena (PbS) having greater negative impact in kinetic leach column tests (KLCs, uncontrolled pH, >85 wt% silicate minerals). In contrast the presence of pyrite resulted consistently in greater increase in galena than sphalerite leaching suggesting that increased anodic leaching is dependent on the difference in anodic and cathodic sulfide mineral rest potentials. Acidity increases occurred after 44, 20, and 12 weeks in the pyrite-galena, pyrite-sphalerite, and the pyrite containing KLCs. Thereafter acid generation rates were similar with the Eh consistently above the rest potential of pyrite (660 mV, SHE). This suggests that treatment of waste rocks or tailings, to establish and maintain low Eh conditions, may help to sustain protective galvanic interactions and that monitoring of Eh of leachates is potentially a useful indicator for predicting changes in acid generation behavior.