Session 08 – Mechanisms and Kinetics

Spodumene, LiAl(SiO₃)₂ in LCT pegmatite ores is the main source of lithium, which is a key component in the manufacture of Li batteries. The processing of spodumene concentrate usually involves a combination of elevated temperature and chemical treatment processing to extract lithium from silicate minerals. Heat treatment to convert a-spodumene to b-spodumene (~1100°C) is considered essential to expand the volume of spodumene to release lithium from its crystalline structure. 

The mechanism and intermediate phases formed during the conversion of a-spodumene to b-spodumene is known and related to heating rate and particle size. However, the effect of elemental (e.g., Mn, Fe, Ca, Mg) and mineral impurity inclusions on the thermal properties of spodumene and particle rheology are not well understood. As part of a research project evaluating the geometallurgical properties of different LCT pegmatite ores in Western Australian, the thermal properties of a suite of spodumene samples have been investigated. 

A combination of X-ray diffraction, including in-situ HT-XRD analysis, electron microscopy and thermal gravimetric techniques, have been used to track the composition changes of both spodumene and the partial melting of gangue materials at levelled temperatures. The findings and inferences on clinker formation and on lithium extraction process are reported from this work.

Mineral replacement reactions are ubiquitous in nature occurring at all scales and across geological settings. One such economically important mineral replacement reaction is the replacement of chalcopyrite (CuFeS₂) by digenite (Cu_1.8S)/covellite (CuS) known as the chemical metathesis of chalcopyrite. The chemical metathesis of chalcopyrite is an emerging method³ in the mineral industry to upgrade primary copper concentrates. We have performed a series of ex-situ quenched laboratory experiments and in-situ powder diffraction experiments (180 ^oC – 240 ^oC) at the Australian Synchrotron investigation the mineral evolution during the chemical metathesis of chalcopyrite. The in-situ experiments demonstrated the crucial role of aqueous copper Cu²⁺ (aq) during chalcopyrite metathesis as no chalcopyrite was observed to take place in experiments where Cu²⁺ (aq) was absent. Avrami modeling of chalcopyrite dissolution during the metathesis reaction revealed a two-stage mechanism suggesting chalcopyrite metathesis taking place via the three-dimensional growth of covellite in the first stage and subsequently digenite grows via the diffusion of Cu²⁺ (aq) into the chalcopyrite through the covellite layer. Our findings provide fundamental insight into the complex nature of mineral replacement reactions in the copper-sulfide system. The findings from these experiments can also be used as primary guidelines in designing engineered mineral systems aimed at the hydrometallurgical of chalcopyrite and other primary copper-sulfides.

References

1.        Putnis, A. Mineral Replacement Reactions. Rev. Mineral. Geochemistry 70, (2009).

2.        Xia, F. et al. Mechanism and kinetics of pseudomorphic mineral replacement reactions: A case study of the replacement of pentlandite by violarite. Geochim. Cosmochim. Acta 73, 1945–1969 (2009).

3.        Dunn, G. M., Saich, S. & Bartsch, P. J. Hydrometallurgical method for the removal of radionuclides from radioactive copper concentrates. (2017).

Pyrite and marcasite provide a wealth of information about the physical-chemical conditions of the Earth's evolution and formation of mineral deposits. However, a mechanistic and kinetic study on the phase transformation from the thermodynamically metastable polymorph marcasite to the stable polymorph pyrite is yet to be completed. This limits the application of marcasite and pyrite-marcasite pair as indicators of low-temperature geological environments. Here we report results from in situ synchrotron powder X-ray diffraction and ex situ anneal/quench experiments at 400-540 °C, demonstrating that the mechanism and kinetics of this transformation depend not only on temperature, but also on particle size, the presence of water vapor, and the presence of pyrite inclusions in marcasite. Under dry conditions, the transformation is limited by surface nucleation and occurs via epitaxial nucleation of pyrite on marcasite, with (100)_pyrite//(101)_marcasite and (001)_pyrite//(010)_marcasite. In contrast, in the presence of water vapor, there is little crystallographic orientation relationship between the two phases. The transformation is limited by surface nucleation, but modification of the surface properties by water vapor results in a different nucleation mechanism, and consequently different kinetics. Kinetic analysis estimates a half-life of 1.5 Ma at 300 °C for the transformation from marcasite to pyrite under dry conditions with small and pyrite-free marcasite grains. However, this estimation should be used with caution due to aforementioned complexity. Based on synchrotron X-ray fluorescence elemental mapping, trace elements (As and Pb) play no role during the transformation from marcasite to pyrite. This study highlights the importance of textural and structural analyses of pyrite-marcasite association during low-temperature precipitation from the (hydrothermal) fluid. A precise estimation of temperature and relative timing of mineral deposition (paragenesis) should include the impact of the analyzed reaction parameters.

Large scale energy storage is required to store the energy generated by renewable energy sources.¹ Li ion batteries are perfect for small scale energy storage but are generally expensive compared to fossil fuels, while Li is also a limited resource. Hydrogen is an energy vector that is perfect for large scale energy storage, although high density storage is a requisite. Metal hydrides can store hydrogen in high densities but often require increased temperatures to reversibly react with hydrogen in which absorption of hydrogen by the metal is an exothermic process.² Therefore, those materials that react at high temperature can be used store and release heat, which can be used to produce electricity. This heat can be produced as industrial waste heat (e.g. aluminium smelting plant), direct from concentrated sunlight (i.e. concentrated solar thermal) or produced by electrical heating.

The development of metal hydrides for technological application requires characterisation of the reaction process between the metal and hydrogen. Although hydrogen can’t be detected using X-ray techniques, the reaction mechanism can still be determined using X-ray powder diffraction (PXD), in particularly time-resolved PXD where the sample is heated and the reaction monitored over time.³ This will also determine whether by-products are produced or if sintering occurs. This presentation will outline the X-ray characterisation methods used to study metal hydrides including the development of advanced sample environments.

References

1  D. N. Harries, M. Paskevicius, D. A. Sheppard, T. E. C. Price and C. E. Buckley, Proc. IEEE, 2011, 100, 539.

2  D. A. Sheppard, M. Paskevicius, T. D. Humphries, C. E. Buckley et al., Appl. Phys. A, 2016, 122, 395.

3  T. D. Humphries, D. A. Sheppard, C. E. Buckley et al., J. Mater. Chem. A, 2016, 4, 12170.

Storage of renewable energy is one of the major technological challenges of our time. Focus is often directed towards lithium-ion batteries or hydrogen as an energy carrier. Although, a hidden gem for energy storage may be metal carbonates, readily available as minerals around the globe. Metal carbonates have great potential as thermochemical energy storage materials through the endo- and exothermic release and uptake of CO₂ with low cost and high energy density [1]. However, the major challenge is the loss of CO₂ capacity, which drastically decreases over multiple cycles [2,3].

Recently, it was established that dolomite, CaMg(CO₃)₂, dug straight out of the ground, works even better than a high-purity laboratory synthesized dolomite sample due to the positive effect of chemically inert impurities present in the sample [1]. However, its relatively low 550 °C operational temperature leaves room for improvement. Thus, BaCO₃ (also known as the mineral witherite) is the focus of this study. To enable utilisation of BaCO₃ (T_dec ~ 1400 °C) as a thermal energy storage material, the operational temperature is lowered to ~ 850 °C through a reactive carbonate composite of BaCO₃-BaSiO_3. Furthermore, addition of a second mineral, CaCO₃ (calcite), is shown to enhance the reaction kinetics significantly (>5 times). This presentation will give an overview of present research and an outline of future perspectives.

References

[1] T. Humphries, K. T. Møller, et al., J. Mater. Chem. A, 2019, 7, 1206. [2] G. S. Grasa, J. C. Abanades, Ind. Eng. Chem. Res. 2006, 45, 26, 8846. [3] J. Abanades, D. Alvarez, Energy Fuels 2003, 17, 2, 308.