Spinel Solid Solution Research Articles

Determination of the Li Distribution in Synthetic Recycling Slag with SIMS

The recovery of technically important elements like lithium from slag of pyrometallurgical recycling of lithium traction batteries will be very important in future due to the expected increasing demand of this element with the upcoming world-wide implementation of electro mobility. Therefore, the investigation of possibilities to recover lithium from pyrometallurgical slag from the recycling of lithium traction batteries is mandatory. In this context, the EnAM (engineered artificial mineral) approach is very promising. Solidified melt of synthetic recycling slag with the compounds Li2O-MgO-Al2O3-SiO2-CaO-MnO contains various Li-bearing phases including spinel solid solution, Li-aluminate and eucryptite-like Li-alumosilicate. Most probably, the Ca-alumosilicate matrix (melilite-like solid solution) incorporates lithium as well. These compounds can be determined and calculated to an acceptable approximation with electron probe microanalysis (EPMA). Nevertheless, an adequate precise measurement of lithium is virtually impossible due to the extremely low fluorescence yield and long wavelength of Li Kα. Secondary mass spectrometry (SIMS) can be used to fill this gap in the analytical assessment of the slag. Therefore, the combination of these two analytical methods can distinctively improve the mineralogical and chemical characterization of lithium-containing solidified (slag) melt.

Defect cluster engineering in ZnGa2−x(Mg/Ge)xO4:Cr3+ nanoparticles for remarkably improved NIR persistent luminescence

AbstractRecently, study on Cr3+‐doped zinc gallate and zinc gallogermanate persistent phosphors has become a hot topic in persistent luminescence and bio imaging areas, because of their near infrared (NIR) emission and long afterglow. However, regulation of efficient traps and improvement of persistent luminescence through bottom‐up design are the key challenge. Here, we recommend a new paradigm of chemical unit co‐substitution with [Mg2+‐Ge4+] substituting for [Ga3+‐Ga3+] in ZnGa2O4, which contributes to the opposite charged and distorted octahedral defects of MgGa′ and GeGa· in pair around the CrN2 ions. The formed defect clusters of MgGa′‐CrN2‐GeGa·, which are closely related to the trap depth, can be accurately regulated through varying the doping content of Mg2+/Ge4+ in the resulting spinel solid solutions of ZnGa2−x(Mg/Ge)xO4:Cr3+ (x = 0–1.25). Moreover, the defect clusters cannot only store and recharge visible and UV radiations that contributes to the long lasting NIR persistent luminescence but also can enhance the NIR emission intensity at ~695 nm. The persistent luminescence induced by UV light excitation exhibits an improvement at a deeper trap depth, but it follows an opposite law through visible light excitation. The prepared nanoparticles have the advantages of intense NIR emission, long lasting afterglow, and excellent rechargeability for visible/UV radiations, so they are the potential nanoprobes for long‐term bio imaging in living animals.

Synthesis, structure and spectroscopy of Fe2+:MgAl2O4 transparent ceramics and glass-ceramics

We report on a comparative study of a transparent Fe:MgAl2O4 (spinel) ceramics and a transparent nanophase Fe:MgAl2O4-based glass-ceramics. The 0.1 mol% Fe:MgAl2O4 ceramics was synthesized by hot pressing (at 1500 °C/50 MPa) of powders obtained by the sol-gel method using LiF as a sintering aid. The Fe:MgAl2O4 ceramic is a single-phase material (cubic structure, sp. gr. Fd3‾m, a = 8.083 Å) with a mean grain size of ~50 μm. The ceramic exhibits a broadband transparency of 0.2–6.0 μm and a high in-line transmission at ~1 μm of 74.4%. The iron ions are presented in the ceramics in the single state of Fe2+ species in tetrahedral (Td) sites. A broad absorption band spanning from ~1.2 to 3.7 μm assigned to the 5E → 5T2 (5D) transition of Fe2+ ions in Td sites is observed, corresponding to a ground-state absorption cross section of 0.28 × 10−18 cm2 at 1.90 μm. The glass-ceramics were prepared by secondary two-stage heat-treatments of the magnesium aluminosilicate glass nucleated by titanium oxide and doped with 0.1 mol% FeO. Transparent Fe:MgAl2O4-based glass-ceramics obtained at the temperature of the second stage of 800–1000 °C were multi-phase materials containing two crystalline nanophases, i.e., spinel (mean size: 3.7–7.4 nm) and magnesium aluminotitanate solid solution (mean size: 6.4–20.6 nm), as well as residual silica-rich glass. Glass-ceramics obtained at the temperature of the second stage of 1050 °C were transparent and based on Fe-doped sapphirine. For glass-ceramics, absorption has a much more complex character as it is caused by interplay of iron and titanium ions in different valence states, coordination sites and locations. The iron ions enter the spinel nanocrystals but unlike the ceramic, in the form of both VIFe2+ and IVFe2+ species. The developed ceramics and glass-ceramics are promising for saturable absorbers of mid-infrared (2–3 μm) lasers.

Structural and Magnetic Properties of (Mg0.8Fe2+0.2)(Al0.4CrxFe3+1.6−x)O4 Spinel Solid Solution Obtained by Molten Salt Synthesis

(Mg0.8Fe2+0.2)(Al0.4CrxFe3+1.6−x)O4 (x = 0.0, 0.4, 0.8, 1.2, and 1.6) spinel particles are prepared by molten salt method at 1050 °C for 5 h. The solid solution structure is determined by X‐ray diffraction (XRD) as cubic spinel with Fd‐3m space group. The lattice parameter and crystalline size decrease with increasing Cr content. Cr3+ ions occupy the octahedral sites, as shown by combined Fourier transform infrared spectroscopy (FTIR) and XRD analysis. The magnetization as a function of field (±4.52 T) and field‐cooled (FC) and zero‐field‐cooled (ZFC) magnetization measurements under a field of 100 Oe is measured for temperatures ranging from 1.5 to 300 K. It is found from M(T) curves that the interparticle interactions are suppressed below 28.1–40.8 K, and the curves also shows the ferromagnetic from M(H) hysteresis for the samples with x = 0.0, 0.4, and 0.8. However, for x = 1.2 and 1.6, the samples show superparamagnetic, as also supported by the obtained electron paramagnetic resonance (EPR) and Mössbauer spectra.

Thermodynamics of Mg-Al Order-Disorder Reaction in MgAl2O4-Spinel: Constrained by Prolonged Annealing Experiments at 773-1123 K.

MgAl2O4-spinel has wide industrial and geological applications due to its special structural and physical–chemical features. It is presumably the most important endmember of complex natural spinel solid solutions, and therefore provides a structural model for a large group of minerals with the spinel structure. There exists a well known but still inadequately understood phenomenon in the structure of MgAl2O4-spinel, the Mg–Al cations readily exchanging their positions in response to variations of temperature, pressure, and composition. A large number of experiments were performed to investigate the Mg–Al cation order-disorder process usually quantified by the inversion parameter x (representing either the molar fraction of Al on the tetrahedral T-sites or the molar fraction of Mg on the octahedral M-sites in the spinel structure), and some thermodynamic models were thereby constructed to describe the x-T relation. However, experimental data at some key T were absent, so that the different performance of these thermodynamic models could not be carefully evaluated. This limited the interpolation and extrapolation of the thermodynamic models. By performing some prolonged annealing experiments with some almost pure natural MgAl2O4-spinel plates and quantifying the x values with single-crystal X-ray diffraction technique, we obtained some critical equilibrium x values at T down to 773 K. These new x-T data, along with those relatively reliable x values at relatively high T from early studies, clearly indicate that the CS94 Model (a model constructed by Carpenter and Salje in 1994) better describes the Mg–Al cation order-disorder reaction in MgAl2O4-spinel for a wide range of T. On the basis of the CS94 Model, a geothermometer was established, and its form is T-closure = 21362 × x3 − 12143 × x2 + 6401 × x − 10 (T-closure standing for the closure temperature of the Mg–Al cation exchange reaction). This geothermometer can be used to constrain the thermal history of the geological bodies containing MgAl2O4-spinel.

Speciation of Manganese in a Synthetic Recycling Slag Relevant for Lithium Recycling from Lithium-Ion Batteries

Lithium aluminum oxide has previously been identified to be a suitable compound to recover lithium (Li) from Li-ion battery recycling slags. Its formation is hampered in the presence of high concentrations of manganese (9 wt.% MnO2). In this study, mock-up slags of the system Li2O-CaO-SiO2-Al2O3-MgO-MnOx with up to 17 mol% MnO2-content were prepared. The manganese (Mn)-bearing phases were characterized with inductively coupled plasma optical emission spectrometry (ICP-OES), X-ray diffraction (XRD), electron probe microanalysis (EPMA), and X-ray absorption near edge structure analysis (XANES). The XRD results confirm the decrease of LiAlO2 phases from Mn-poor slags (7 mol% MnO2) to Mn-rich slags (17 mol% MnO2). The Mn-rich grains are predominantly present as idiomorphic and relatively large (>50 µm) crystals. XRD, EPMA and XANES suggest that manganese is present in the form of a spinel solid solution. The absence of light elements besides Li and O allowed to estimate the Li content in the Mn-rich grain, and to determine a generic stoichiometry of the spinel solid solution, i.e., (Li(2x)Mn2+(1−x))1+x(Al(2−z),Mn3+z)O4. The coefficients x and z were determined at several locations of the grain. It is shown that the aluminum concentration decreases, while the manganese concentration increases from the start (x: 0.27; z: 0.54) to the end (x: 0.34; z: 1.55) of the crystallization.

Thermodynamic study on the formation of Mg(Al1-x,Fex)2O4 between single-crystal MgAl2O4 and FetO-containing slag at 1550°C

Herein, the effects of FetO content and basicity (CaO/Al2O3) on MgO solubility in calcium aluminate slag were evaluated. The experimental results revealed that the dependence of MgO solubility on basicity reversed across the equilibrium region as the phase changed from MgO to spinel. Meanwhile, a dense layer of Mg(Al1-x,Fex)2O4 solid solution formed at the interface between the single-crystal MgAl2O4 and FetO-containing MgO-saturated slag. Thermodynamic evaluation of the spinel solid solution formation indicated that the solid solubility of ferric ions (Fe3+) in the reaction layer was dominated by the redox equilibria of ferrous (Fe2+) and ferric ions in the slag. The standard Gibbs free energy change (ΔG°) of the formation of Mg(Al1-x,Fex)2O4 solid solution was derived considering both the energy change in mixing (ΔG°Mix) from the configurational change and the cation distribution owing to the inversion in the spinel, as well as a chemical reaction (ΔG°Rxn) with the FetO-containing slag. The dissolution mechanism via the substitution of cations, as inferred from these calculations, is discussed in detail.

Formación y análisis termodinámico de inclusiones en metales de soldadura de acero que contienen Ti con diferentes contenidos de Al

Se prepararon muestras de metales de soldadura de acero que contienen Ti con contenidos de Al de 0,01-0,085%. Los efectos del contenido de Al en los metales de soldadura sobre la evolución de las inclusiones se investigaron en detalle mediante cálculos termodinámicos junto con microanálisis de sonda de electrones. Los resultados muestran que las inclusiones en el 0,01% de metal de soldadura de Al están compuestas principalmente de ilmenita con más cantidades de (Mn-Si-Al) -óxido y titanial-spinel. Cuando el contenido de Al aumenta hasta un 0,035%, se forma una mayor cantidad de corindón y una pequeña cantidad de pseudobrookita. En el 0,085% de metal de soldadura de Al. En el metal de soldadura de 0,085% Al, la fase de óxido (Mn-Si-Al) desaparece por completo y las inclusiones contienen una cantidad sustancial de corindón, además de una cantidad mínima de pseudobrookita. Ti3O5, MnTi2O4 y MnTiO3 son los componentes principales de las soluciones sólidas de pseudobrookita, titanial_spinel e ilmenita, respectivamente. Titanial-spinel e ilmenita tienen cantidades más altas de Mn, pero niveles más bajos de Ti en comparación con la pseudobrookita.

Understanding the solidification and leaching behavior of synthesized Cr-containing stainless steel slags with varying Al2O3/SiO2 mass ratios

This study investigated the effect of Al2O3/SiO2 mass ratios on the equilibrium crystallization behavior of synthesized CaO–SiO2–MgO–Al2O3–Cr2O3 stainless steel slags to understand the selective concentration behavior of Cr into a primary Mg(Cr,Al)2O4 spinel phase during slag solidification and to determine the leaching stability of Cr-containing slags. The spinel solid solution was precipitated within the temperature range of 1600-1400 °C, where the Cr/(Cr+Al) mole ratio in the Mg(Cr,Al)2O4 spinel phase gradually decreased for slags with higher Al2O3/SiO2 mass ratios. When the Al2O3/SiO2 mass ratio increased from 0.125 to 0.5, the Cr content in the amorphous glass phase gradually decreased, with a subsequent increase in the Cr content in the crystalline phase. For slags with a unit Al2O3/SiO2 mass ratio and MgO mole percent comprising less than the combined sum of the Cr2O3 and Al2O3 mole percents, the Cr content in the amorphous glass phase increased, which was correlated with the enhanced substitution of Cr3+ with Al3+ in the spinel. The trend of the amount of Cr-related ions in the leachate was consistent with the trend of Cr in the amorphous glass phase: the amount decreased for slags with Al2O3/SiO2 mass ratios from 0.125 to 5 and then increased for slags with an Al2O3/SiO2 mass ratio of 1. The results suggest that the addition of appropriate amounts of Al2O3 to stainless steel slags could be conducive to stabilizing Cr into the primary spinel phase to minimize Cr leaching into the environment.

High-pressure and high-temperature phase relations in the systems KAlSiO4-MgAl2O4 and CaAl2O4-MgAl2O4: Stability fields of NAL phases

Phase relations in the system KAlSiO4-MgAl2O4 were determined up to 28 GPa and 1500 °C. A hexagonal aluminous (NAL) phase is stable above 16 GPa with a narrow compositional range of (1-x)KAlSiO4·xMgAl2O4, x ≈ 0.61–0.65. The stability field of NAL phase in the CaAl2O4-MgAl2O4 system was also determined up to 29 GPa and 1200 °C, resulting in a compositional range of NAL phase of (1-x)CaAl2O4·xMgAl2O4, x ≈ 0.67–0.74. Compared with the above compositional widths of NAL phases, the stability field of NAL phase in the NaAlSiO4-MgAl2O4 system is much wider, (1-x)NaAlSiO4·xMgAl2O4, x ≈ 0.47–0.70 (Ono et al., 2009). The difference may be caused by the fact that both of Na+ and Mg2+ with similar cation sizes enter 6-fold trigonal prism sites in the NAL structure. In the system KAlSiO4-MgAl2O4, KAlSiO4 kalsilite and MgAl2O4 spinel coexist below ~8 GPa, above which an assemblage of MgAl2O4 spinel, corundum, K2Mg2Si2O7-rich phase X and pyrope becomes stable. The assemblage further changes into NAL phase at 16 GPa. These results and compositional similarity suggest that composite inclusions consisting of MgAl2O4-rich spinel and kalsilite-nepheline solid solution found in diamonds in the Juina-5 kimberlite, Brazil, were formed below ~8 GPa presumably by decomposition of K-rich NAL phase trapped in diamonds in the lower mantle conditions.

Toward High Voltage Mg-Ion Battery Cathode Via a Solid-Solution Cr-Mn Spinel Oxide

The capability of the tailored solid-solution spinel, MgCrMnO4, is evaluated by theoretical and experimental approaches. Lattice Mg2+ in the designed oxide is electrochemically utilized at high potentials in a non-aqueous electrolyte. Complementary evidence supports bulk Mg2+ (de)intercalation throughout the designed oxide frame where strong electrostatic interaction between Mg2+ and O2- exists. Mg/Mn antisite inversion in the spinel is lowered via post-annealing to further improve Mg+2 mobility. Spinel lattice is preserved upon removal of Mg2+ without any phase transformations, denoting structural stability at the charged state at a high potential. In the remagnesiated state, insertion of Mg2+ into interstitial sites in the spinel is detected possibly resulting in partial reversibility which needs to be addressed for structural stability. The observations constitute a first clear path to the development of a practical high voltage Mg-ion cathode using a spinel oxide.

Structural Study of MgyAl(8+x-2y)/3O4-xNx (0 < x < 0.5, 0 < y < 1) Spinel Probed by X-ray Diffraction, 27Al MAS NMR, and First-Principles Calculations.

The deep understanding of the crystal structure and composition-structure relationship is important for modifying and designing solids to obtain functional materials with customized properties. However, because of multiple compositions and complex structures in some spinel solid solutions, the composition-structure relationship is unknown, or becomes very complicated and difficult to be controlled. In this work, the solid-state magic-angle spinning nuclear magnetic resonance (MAS NMR) technique and powder X-ray diffraction (XRD) Rietveld refinement were combined to characterize the crystal structure of quaternary disordered MgyAl(8+x-2y)/3O4-xNx solid solutions in detail, which was supported by the first-principles density functional theory calculations. Diffraction data indicate that Mg ions preferentially enter the tetrahedron structure in MgyAl(8+x-2y)/3O4-xNx solid solutions because the sum of the bond valence in tetrahedron is closer to the atomic valence of Mg. With the compositional change, the coordination polyhedra in the crystal structure will also adjust the volume according to the changes in lattice parameter, anion parameter, and inversion parameter. In addition, the populations, chemical shifts, and quadrupole coupling constants of Al located in different coordinated environments of the solid solutions were detected through the simulation and integration of 27Al MAS NMR spectra, which were related to the structural parameters by the bond valence method. It turns out that 27Al MAS NMR parameters are highly sensitive to the subtle changes in the local environment of Al caused by the preferential occupation of Mg in the tetrahedron. These results provide deep insights into the crystal structural details of novel spinel materials with multiple disorders.

Support composition effect on the structures, metallic sites formation, and performance of Ni-Co-Mg-Al-O composite for CO2 reforming of CH4

The relationship among the bulk structure, Ni/Co reduction, site formation, and the catalytic performance of Ni-Co-Al-Mg-O composite synthesized by co-precipitation method was studied for CO2 reforming of CH4 (DRM). Three catalysts were prepared by changing the Mg/Al ratio of the composite from 0.3 to 2. The catalysts were denoted as CopCat-Mg/Al-0.3, CopCat-Mg/Al-1, and CopCat-Mg/Al-2. ICP-MS, XRD, Al-NMR, XPS, TEM, and XANES (Mg, Al, Ni/Co K-edge) were utilized to characterize the crystalline structure, the coordination number, degree of spinel inversion, and the Ni /Co reduction of the catalysts. Results showed that the bulk structure of the calcined precipitates changed from spinel-dominating to the co-existence of spinel and MgO solid solution phases as the Mg/Al ratio changed from 0.3 to 2. In the spinel-dominating structures, Ni and Co reduction was found to be difficult, and a small number of metallic sites with larger nanoparticles were generated after reduction. As the MgO solid-phase became significant, the reduction of Ni and Co was enhanced, and more metallic sites of smaller sizes were formed. These metallic sites performed differently for the activation of CH4 and CO2, as the catalyst with the lowest Mg/Al ratio (<0.5) experienced carbon deposition and produced synthetic gas of low H2/CO ratio, which led to lower catalytic activity and quicker catalyst deactivation during DRM. In contrast, the catalyst with the highest ratio of Mg/Al suppressed carbon formation, maintained a higher catalytic activity and stability due to the improved basicity that promoted the quick conversion of carbon to CO.

Comparative study of structure-catalytic activity relationship for Ni–Cr–Al–O and Cu–Ni–Cr–Al–O spinel oxide solid solutions

A spinel solid solution expressed by a composition formula of CuxNi1−xAl1.8Cr0.2O4 (0 ≤ x ≤ 0.2) was studied focusing on the role of Cu species in the catalytic activity toward NO reduction under a simulated three-way catalysis condition. X-ray Rietveld analysis revealed that Cu and Cr prefer to occupy the tetrahedral site and the octahedral site, respectively, whereas Ni and Al are distributed across both sites. Although NiAl1.8Cr0.2O4 (x = 0) showed negligible catalytic activity for NO reduction, the partial replacement of Ni by Cu significantly enhanced the activity, achieving the highest activity at x = 0.05 not only for NO reduction but also for CO and C3H6 oxidation. Based on the infrared spectra and pulsed reaction experiments, it was concluded that the monovalent Cu site in the tetrahedral site plays a key role in CO chemisorption, oxygen vacancy formation, and subsequent NO reduction via the Mars–van Krevelen mechanism.

Strong magnetic coupling between antiferromagnetic and ferromagnetic phases in polycrystalline hollow nanoparticles composed of spinel solid solution

Exchange bias is an interfacial phenomenon that is used to increase the magnetic anisotropy in order to surpass the superparamagnetic limit. Exchange bias has been effectively manipulated through variation of local arrangement of ferromagnetic and antiferromagnetic nanocrystals in an unusual morphology and for very specific compositions. For this purpose, polycrystalline hollow nanoparticles with spinel structure composed of Co3-δFeδO4 (with δ > 0) and Co1+γFe2-γO4 (γ > 0) crystallites, were prepared using = CoFe metal nanoparticles as precursors. These particles were oxidized to obtain the desired mixed-oxide nanoparticles with various compositions using the well-known Kirkendall effect. Spinel nanoparticles were composed of an antiferromagnetic phase, which was predominantly Co3-δFeδO4, and a ferromagnetic phase, Co1+γFe2-γO4. The antiferromagnetic phase showed a relative increase in its concentration with the increase in Co content. The magnetic response includes high field thermoremanent magnetization in antiferromagnetic phase, ferromagnetic crystal size, increased anisotropy due to antiferromagnetic and surface spin glass phases, etc. Antiferromagnetic phase has been demonstrated to be robust in comparison with the surface spin glass phase to generate exchange bias. Finally, a phase diagram of the magnetic phases as a function of Co concentration is constructed to identify ferromagnetic, antiferromagnetic and spin glass regimes in this system.

Thermodynamic Database Development for the ZrO2-MgO-MnOx System

Thermodynamic description of the ZrO2-MgO-MnOx was derived for the first time using available experimental data on phase relations in air and protective gas atmosphere. Solid solution phases were modelled using compound energy formalism. The liquid phase was described by the modified two-sublattice model for ionic liquid. Solubility of ZrO2 was modelled in cubic spinel and MgO-MnO solid solution (halite structure) and therefore the Gibbs energies of Zr-containing endmembers were introduced. Ternary interaction parameters were introduced for halite, cubic spinel and cubic ZrO2 to reproduce the stabilization of cubic ZrO2 at temperatures below its stability in bounding systems and stabilization of cubic spinel at temperatures above its stability limit in the bounding systems. The obtained thermodynamic database was used to interpret results of differential thermal analysis.

Cu1−xMgxAl3 spinel solid solution as a sustained release catalyst: One-pot green synthesis and catalytic performance in methanol steam reforming

A versatile and green method for the synthesis of an efficient catalyst applied in methanol steam reforming is promising in view of clean energy production. Cu1−xMgxAl3 ternary spinel oxide catalysts have been prepared by a one-pot solid-phase reaction method using Cu(OH)2, Al2O3·xH2O and MgCO3 as the raw materials. The structure, reducibility, surface chemical state, and the aluminum ion distribution are comprehensively characterized by XRD, UV–vis, XPS, H2-TPR, N2O chemisorption, and 27Al MAS NMR techniques. The performances of the catalysts in methanol steam reforming are evaluated. Compared with binary Cu-Al spinel, the incorporation of Mg into the spinel lattice results in the change of Al3+ cation distribution between tetrahedral and octahedral sites, and thus the variation of copper surrounding environment can be inferred. Consequently, the copper releasing rate from the Mg containing spinel structure declines substantially, which is believed to be in favor of maintaining a stable catalytic performance longer. Besides, the ternary spinel oxide might facilitate the in-situ formation of smaller nano copper metals compared to CuAl3 binary spinel catalyst. Among all the catalysts, Cu0.9Mg0.1Al3 presents the highest activity as well as the best catalytic stability. The findings of this report suggest that introducing a foreign cation into the Cu-Al spinel structure might be a promising way to regulate the copper releasing property for achieving a better sustained release catalyst system.

Cu-Ni-Al spinel catalyzed methanol steam reforming for hydrogen production: Effect of Al content

Cu-Ni-Al ternary spinel solid solution catalysts with different Al content are prepared by the solid-phase ball milling method. The characterizations with XRD, H2-TPR, BET and XPS, and catalytic performance testing are carried out to study the effects of Al content on the physicochemical properties of the Cu-Ni-Al spinels and their sustained release catalytic performances in methanol steam reforming for hydrogen production. Characterization results show a significant increase in the specific surface area and pore volume of the catalysts with increasing the Al content (Al = 2, 3, 4) at a constant Cu/Ni molar ratio of 0.95:0.05. At the same time, both the cell parameters and crystallite sizes of Cu-Ni-Al spinel solid solutions decrease, and the catalysts become difficult to be reduced. Furthermore, the content of spinel Ni2+ increases slightly while the spinel Cu2+ decreases significantly, leading to a declined total content of spinel Cu2+ plus Ni2+. The results also indicate that the presence of Ni2+ inhibits the formation of spinel Cu2+. Surface analysis results show that the increase of Al content transforms the catalyst surface composition from Cu-rich to Al-rich, and the surface spinel Cu2+ decreases, but it is still higher than the spinel bulk. The catalyst testing results show that as the Al content in the catalysts increases, the initial activity increases notably, and the CO selectivity decreases, but too much Al results in an inferior catalytic stability. In general, the catalyst with an Al = 3 shows a better catalytic performance in terms of activity and stability. The results of this paper demonstrate that there is an optimal Al content for the Cu-Ni-Al spinel solid solutions used as the sustained release catalysts, playing a crucial role in obtaining high catalytic stability.

Experimental study on the effects of H2O upon crystallization in the Lower and Critical Zones of the Bushveld Complex with an emphasis on chromitite formation

Mafic–ultramafic layered rocks of the Lower and Critical Zones of the Bushveld Complex are believed to have formed from a parental magma usually labelled B1 with SiO2 content at 55 wt.%, MgO at about 12 wt.% and Cr2O3 concentration at 0.15 wt.%. Previous experimental study at 300 MPa and nominally dry conditions showed that this composition had orthopyroxene on liquidus between 1250 and 1300 °C, and the mineral was joined by plagioclase and clinopyroxene at and below 1166 °C. The objective of this study was to investigate the effects of variable additions of H2O (2, 4 and 6 wt.% in starting compositions) on liquidus and sub-liquidus phase equilibria. Main attention was given to the solubility of Cr-spinel and Cr distribution between melt and crystalline phases. Six experiments were carried out in the internally heated pressure vessel (IHPV) at 300 MPa and in the temperature interval 1100–1300 °C. Redox conditions were not controlled and thermal dissociation of the H2O component followed by hydrogen diffusion through container walls resulted in a noticeable oxidation of run products. The effective values of fO2 in experiments were calculated using SPINMELT 2.0 software bases on the regression of experimental data on the spinel-melt equilibrium. According to the calculations, H2O additions in isothermal series of experiments increased fO2 by 0.8–1.6 log-units from the level in a nominally dry run. Overall, the experiments showed that H2O additions suppressed crystallization of silicates, shifted olivine-orthopyroxene peritectic equilibrium towards higher olivine stability (olivine first appears in H2O-bearing compositions at 1175 °C) and increased the proportion of magnetite component in spinel solid solution. The addition of 4 wt.% H2O to the starting composition at 1200 °C resulted in complete melting of orthopyroxene, which left Cr-spinel as the only liquidus phase. Equal amount of H2O at 1125 °C leads to complete melting of plagioclase, orthopyroxene and clinopyroxene, which leaves only olivine and spinel stable and increases the liquid mass fraction from 0.62 in the nominally dry compositions to 0.9. The results of this study appear to support the idea that chromitite layers in the Critical Zone of the Bushveld Complex were formed by episodes of hydration melting of silicate cumulates.

Steel Slag Characterisation—Benefit of Coupling Chemical, Mineralogical and Magnetic Techniques

Steel-making slag is largely used today in road construction and other applications, but significant volumes are landfilled and cannot be recycled for excessive contents in hazardous metals, such as chromium or vanadium. The long-term behaviour of this material is still little known, and the characterisation of large volume slag dumps remains an environmental challenge. In this study various analytical techniques are used to characterise Electric Arc Furnace (EAF) slag landfilled for several decades and exposed to chemical weathering and erosion. Coupling chemical, mineralogical and magnetic techniques helps to understand the relations between hazardous metals and mineral phases. A special interest is given to Fe-bearing minerals microstructure so as to link the magnetic properties of the material to its mineralogical composition. The studied slag presents high amounts of chromium (between 1 and 3 wt. %) and very high magnetic susceptibility values (near 60 × 10−6 m3/kg), explained by the presence of magnetite and a spinel solid solution. Some correlations are found between magnetic susceptibility and potentially hazardous metals, providing new perspectives for future environmental investigations.