The single-purpose Mössbauer effect analyser for fast determination of Fe<Superscript>2+</Superscript>/Fe<Superscript>3+</Superscript> ratio in ilmenite

Ilmenite (FeTiO₃) is an initial raw material in titanium white manufacture. The Fe²⁺/Fe³⁺ ratio in ilmenite is measured before the start of sulfate technological process. Possibility of fast measurements of this ratio could help to control manufacturing process. The single-purpose Mössbauer effect analyzer for fast determination of Fe²⁺/Fe³⁺ ratio in ilmenite was built.     

Application of 57Fe Mössbauer spectroscopy as a tool for mining exploration of bornite (Cu5FeS4) copper ore

Nuclear resonance methods, including Mössbauer spectroscopy,are considered as unique techniques suitable for remote on-line mineralogical analysis. The employment of these methods provides potentially significant commercial benefits for mining industry. As applied to copper sulfide ores, Mössbauer spectroscopy method is suitable for the analysis noted. Bornite (formally Cu₅FeS₄) is a significant part of copper ore and identification of its properties is important for economic exploitation of commercial copper ore deposits. A series of natural bornite samples was studied by ⁵⁷Fe Mössbauer spectroscopy. Two aspects were considered: reexamination of ⁵⁷Fe Mössbauer properties of natural bornite samples and their stability irrespective of origin and potential use of miniaturized Mössbauer spectrometers MIMOS II for in-situ bornite identification. The results obtained show a number of potential benefits of introducing the available portative Mössbauer equipment into the mining industry for express mineralogical analysis. In addition, results of some preliminary ^63,65Cu nuclear quadrupole resonance (NQR) studies of bornite are reported and their merits with Mössbauer techniques for bornite detection discussed.     

Evolution of Raman and Mössbauer spectra at high pressures up to 75 GPa and a phase transition in CaSnO<Subscript>3</Subscript> perovskite

Raman and Mössbauer spectra from ¹¹⁹Sn nuclei in CaSnO₃ perovskite have been studied at high pressures up to 75 GPa. A linear increase in the frequency of the main Raman modes and a monotonic decrease in the isomer shift in Mössbauer spectra in the pressure range of 0–40 GPa are established. It is shown that the pressure-induced increase in Raman frequencies can be associated with the variation of the angle between the Sn–O–Sn bonds in chains of oxygen octahedra SnO₆ along the c axis. The sharp variation of the parameters of the Raman and Mössbauer spectra is observed in the pressure region of 40–55 GPa, indicating the structural phase transformations, which can be associated with the transition into the post-perovskite state. Raman spectra of CaSnO₃ samples with the ilmenite structure have been obtained for the first time.     

Quantitative analysis of phases of iron ore and its reduced products

The Mössbauer effect was used to monitor the direct reduction of iron ore in hydrogen gas at temperatures above 800°C. Processes of transformation of iron oxides go through substituted magnetite phase, followed by metallisation with two kinds of iron. One kind is strongly influenced by impurities as the reduction temperature is increased. Concentration of the fayalite phase obtained from the reduced gangue is small aboveT=900°C.     

Mössbauer spectroscopy study of iron oxide nanoparticles obtained by spray pyrolysis

Mössbauer spectroscopy was used in this study to investigate magnetite nanoparticles, obtained by spray pyrolysis and thermal treatment under H₂ reduction atmosphere. Room temperature XRD data indicate the formation of magnetite phase and a second phase (metallic iron) which amount increases as the time of reduction under H₂ is increased. While room temperature Mössbauer data confirm the formation of the cubic phase of magnetite and the occurrence of metallic iron phase, the more complex features of 77 K-Mössbauer spectra suggest the occurrence of electronic localization favored by the different crystalline phase of magnetite at low temperatures which transition to the lower symmetry structure should occur at T ～120 K (Verwey transition).     

Ilmenite: mantle reservoir for nitrogen?

Five ilmenite megacrysts from the Premier kimberlite pipe have been analysed by Mössbauer spectroscopic and electron microprobe techniques for their Fe³⁺/Fe²⁺ ratio; and by nuclear bombardment techniques for their N₂ content. Comparison of the Fe³⁺/Fe²⁺ ratio determined from electron microprobe analysis for total Fe assuming stoichiometry with the direct determination of this ratio by Mössbauer techniques is sufficiently good that the former method which is very widely used may be considered reliable. The N₂ content of the ilmenite samples ranges from 0.5 to 2.0%, considerably in excess of that for most minerals. Heating experiments suggest that the N₂ is not located in liquid inclusions. Isotopic analysis of ilmenite is not available and so its origin is uncertain, but these results suggest that ilmenite could be a major reservoir for N₂ in the earth's mantle.     

Phase transformations of the FePO4 catalyst in the oxidative dehydrogenation to form an alkyl methacrylate

A phase specific iron orthophosphate catalyst, FePO₄, was synthesized and subjected to oxidative dehydrogenation reactions to form an alkyl methacrylate. The phases of the catalyst, before and after the reactions, were characterized by Mössbauer spectroscopy. The Mössbauer spectra show the change of the catalyst precursor FePO₄, tridymite-like phase (tdm), to the reduced form, iron(II) pyrophosphate, Fe₂P₂O_7, and thereafter the phase change is governed by the temperatures of oxidation. X-ray diffraction and Mössbauer measurements on the spent catalyst, after using organic and water co-feeds, show a transformation of the catalyst to a mixture of phases which are condition specific.     

In situ characterisation of supported iron-iridium catalysts by iron-57 and iridium-193 Mössbauer spectroscopy

Some silica-supported iron-iridium catalysts with different iron to iridium ratios and formed by the incipient wetness technique have been examined in situ by⁵⁷Fe and¹⁹³Ir Mössbauer spectroscopy following pretreatment in hydrogen. The results show that the reduction of the iron component is enhanced by the presence of iridium metal. The pretreated catalysts were evaluated for the hydrogenation of carbon monoxide at 270°C and 50 atmospheres pressure. The presence of iridium, which was shown by Mössbauer spectroscopy to result in the formation of reduced iron, iridium, and iron iridium alloy in the pretreated catalyst, was found to increase the catalytic activity and also influence selectivity. However, the systematic variation of the iridium content which was shown by Mössbauer spectroscopy to determine the exact phase composition of the pretreated catalysts, appeared to have little effect on catalytic performance. The⁵⁷Fe Mössbauer spectra recorded from all the used catalysts showed the formation of large and small particle ε′-Fe_2.2C under the high pressure of the catalytic reaction. The results suggest that the formation of iron carbides on reduced iron, perhaps at the surface, and the adsorption of hydrogen on reduced iridium are important features of this catalytic system.     

Hyperfine interaction characterization of iron minerals in Brazilian monazite sands

The Mössbauer spectra both at 300 K and 84 K of the naturally occuring monazite sands from the beaches of Marataizes (40°49′ 11.4″ LW and 21°02′ 26.7″ LS) indicate a paramagnetic phase of nearly 21% absorption and three other phases, including magnetic phases, of nearly 4% absorption each. Chemical analysis, X-ray diffraction and X-ray fluorescence alongwith Mössbauer spectra confirm these to be ilmenite, hematite, magnetite and orthopyroxene (chain silicate) minerals. Presence of a number of other non-iron minerals was also detected.     

X-ray diffraction and Mössbauer spectroscopy of high energy ball-milled α-Fe2O3/TiO2 composite powders

α–Fe₂O₃/TiO₂ Composite powders have been prepared by high energy ball-milling for different times. The composites were studied using Mössbauer Spectroscopy (MS) and X-ray diffraction (XRD). The patterns of XRD show broadening in the diffraction peaks, indicating a decrease in the particle size of the composites with milling time. Also, the XRD patterns show an evolving new structural phase correlated with an evolving Titanium ferrite species with milling time. Mössbauer Spectroscopy shows the evolving titanium ferrite species characterized by a quadrupole doublet at the expense of the α–Fe₂O₃ represented by the magnetic sextet. The doublet corresponding to the Ti-ferrite phase dominates the Mössbauer spectra at long milling time (greater than 100 h of milling).     

Mössbauer effect study of fly and bottom ashes from an electric generating plant

Samples of a fly ash and a bottom ash, each before and after ignition, have been investigated by X-ray diffraction and Mössbauer spectroscopy in order to explain the observed negative loss of ignition (LOI). It is shown that the ashes after ignition contain more maghemite resulting from newly formed magnetite. Moreover, the fly ash which contained already magnetite exhibited an increase of hematite after ignition. Both oxidation processes can be responsible for a weight gain which compensates the loss due to the burning of the remaining carbon. The magnetite and the alpha-iron formed after ignition is originated by an unidentified compound represented by a Fe^2?+? doublet in the Mössbauer spectrum.     

Magnetic and Mössbauer spectroscopic studies of NiZn ferrite nanoparticles synthesized by a combustion method

The properties of nanocrystalline Ni_0.5Zn_0.5Fe₂O₄ synthesized by an auto-combustion method have been investigated by magnetic measurements and Mössbauer spectroscopy. The as-synthesized single phase nanosized ferrite powder is annealed at different temperatures in the range 673–1,273 K to obtain nanoparticles of different sizes. The powders are characterized by powder X-ray diffraction, vibrating sample magnetometer, transmission electron microscopy and Mössbauer spectroscopy. The as-synthesized powder with average particle size of ~9 nm is superparamagnetic. Magnetic transition temperature increases up to 665 K for the nanosized powder as compared to the transition temperature of 548 K for the bulk ferrite. This has been confirmed as due to the abnormal cation distribution, as evidenced from room temperature Mössbauer spectroscopic studies.     

A Mössbauer effect investigation of the magnetic behaviour of (Fe,Co)S1 + x solid solutions

The Mössbauer spectra of (Fe, Co)S_{1 + x} were recorded at room temperature and 4.2 K for samples of varying composition to study the magnetic behaviour of the solid solutions. The Mössbauer spectra are split magnetically at iron concentrations above 16% Fe. For samples with less than 16%Fe, the Mössbauer spectra show no evidence of magnetic splitting down to 4.2 K. The room temperature centre shift data appear to vary continuously with composition and the hyperfine magnetic field decreases with decreasing Fe²⁺ concentration. A Mössbauer spectrum of ⁵⁷Fe:CoS at 4.2 K in an external field of 25 kOe showed no evidence of magnetic splitting beyond that caused by the applied field, indicating a net zero internal field.A high spin to low spin transition in Fe²⁺ is ruled out as being responsible for the observed magnetic behaviour on the basis of the centre shift data. The Mössbauer data are interpreted to indicate a substantial increase in electron delocalization towards the ligands as the 〈M-S〉 distance decreases with decreasing Fe²⁺concentration. This causes a reduction in the magnitude of the internal magnetic field contributions as well as a decrease of shielding of the nucleus, giving rise to the observed Mössbauer parameters.The Mössbauer spectrum of ⁵⁷Fe:CoS at room temperature is compared with the spectrum of FeS above the 6.7 GPa phase transition at room temperature. The similarities of the centre shift and the 〈M-S〉 distance in the two phases indicate that covalency may also be responsible for the observed high pressure behaviour of FeS, and not the presence of Fe³⁺ as was originally suggested.     

197AU,57Fe and121Sb Mössbauer study of gold minerals and ores

A survey of the¹⁹⁷Au Mössbauer spectra of naturally occurring gold species is given. Gold minerals have been studied as natural specimens or as synthetic analogues. Gold impurities have been identified in pyrites and arsenopyrites. An example of the use of¹²¹Sb, and⁵⁷Fe Mössbauer spectroscopy in characterizing gold-bearing ore minerals is given.     

Magnetic and Mössbauer studies of fucan-coated magnetite nanoparticles for application on antitumoral activity

Fucan-coated magnetite (Fe₃O₄) nanoparticles were synthesized by the co-precipitation method and studied by Mössbauer spectroscopy and magnetic measurements. The sizes of the nanoparticles were 8–9 nm. Magnetization measurements and Mössbauer spectroscopy at 300 K revealed superparamagnetic behavior. The magnetic moment of the Fe₃O₄ is partly screened by the Fucan coating aggregation. When the magnetite nanoparticles are capped with oleic acid or fucan, reduced particle-particle interaction is observed by Mössbauer and TEM studies. The antitumoral activity of the fucan-coated nanoparticles were tested in Sarcoma 180, showing an effective reduction of the tumor size.     

Iron ore processing -- in-situ monitoring

Mössbauer spectroscopy has been used to provide detailed quantitative insight into the reaction products present at different stages of the processing/reduction of iron ores. Typical phases include: α-Fe₂O₃; Fe₃O₄; Fe_1-xO; Fe₃C and Fe, which are in agreeement with the results obtained using standard wet chemistry methods. As an extension of this post reaction examination of products, we have used Mössbauer spectroscopy for in-situ measurements during the week long operation of an iron ore processing pilot plant. Such in-situ measurements allow optimal control of the plant to be obtained with, depending on the strength of the source, a time constant of ~ 1 h.     

On the Néel temperature of βFeOOH: Structural dependence and its implications

Using ⁵⁷Fe Mössbauer thermoscanning, the magnetic transition temperature T_N of a number of synthetic βFeOOH samples (akaganeite) is studied. Mössbauer spectra in external fields up to 6 T reveal the sextet-doublet transition to be a true magnetic order-disorder transition and that no superparamagnetic relaxation occurs. It is shown that the value of T_N in particular and the strength of the magnetic interactions in general are highly variable and depend upon the chemical circumstances during synthesis; this is due to spin reduction, induced by a variable amount of interstitial water molecules in the compound.     

Atmospheric corrosion in the Gulf of México

The corrosion products on steels exposed at two sites in Campeche, México and one site at Kure Beach, USA, have been investigated to determine the extent to which different marine conditions and exposure times control the oxide formation. The corroded coupons were analyzed by Mössbauer, Raman and infrared spectroscopy as well as X‐ray diffraction, in order to completely identify the oxides and map their location in the corrosion coating. The coating compositions were determined by Mössbauer spectroscopy using a new parameter, the relative recoilless fraction (F-value) which gives the atomic fraction of iron in each oxide phase from the Mössbauer sub‐spectral areas. For short exposure times, less than three months, an amorphous oxyhydroxide was detected after which a predominance of lepidocrocite (γ-FeOOH), and akaganeite (β-FeOOH) were observed in the corrosion coatings with the fraction of the later phase increasing at sites with higher atmospheric chloride concentrations. The analysis also showed that small clusters of magnetite (Fe₃O₄), and maghemite (γ(Fe₂O₃), were seen in the micro-Raman spectra but were not always identified by Mössbauer spectroscopy. For longer exposure times, goethite (α-FeOOH), was also identified but little or no β-FeOOH was observed. It was determined by the Raman analysis that the corrosion products generally consisted of inner and outer layers. The protective layer, which acted as a barrier to slow further corrosion, consisted of the α-FeOOH and nano-sized γ-Fe₂O₃ phases and corresponded to the inner layer close to the steel substrate. The outer layer was formed from high γ-FeOOH and low α-FeOOH concentrations.     

A Mössbauer and X-ray investigation of ξ-Fe2N

Mössbauer spectra of ξ-Fe₂N at 4·2°K, 78°K and room temperature are presented. The Mössbauer parameters are compatible with the nitrogen donor model previously found to be applicable to the lower iron nitrides. X-ray analyses of this phase confirm the structure previously reported. Mössbauer and X-ray spectra for partially nitrided iron foils containing γ′, ε and ξ iron nitrides are also discussed.