Sintering behavior of TiO2-supported model cobalt Fischer-tropsch catalysts under H2 reducing conditions and elevated temperature

The sintering of model TiO2-supported cobalt catalysts has been studied. The TiO2 supports used were anatase, P25 (85% anatase-15% rutile) and rutile. The catalysts were characterized at each stage of treatment. These treatment stages were calcination, reduction and sintering. It was found that the TiO2 support does not influence the Co3O4 crystallite and particle size as shown by powder x-ray diffraction (XRD) and transmission electron microscopy (TEM) respectively. The reduction of the cobalt catalysts and bare supports was studied by temperature programmed reduction (TPR). It was found that the bare supports were not as inert as expected. The supports showed minor reduction as seen in the H2 consumption. All the cobalt catalysts showed a two-step reduction. Sintering of anatase-supported cobalt was shown to be the most substantial with P25- and rutile-supported showing a lower sintering tendency; P25- supported cobalt being the most stable based on TEM measurements. Sintering kinetics based on the Generalized Power law Expression (GPLE) model, showed that sintering of anatase-supported cobalt is the most rapid with a large sintering rate constant. Sintering of P25-supported cobalt is the lowest, shown by the lowest sintering rate constant. The study has conclusively shown the effect of the catalyst support phase. The study has also shown that the use of high surface area supports is not necessarily the only answer to preventing sintering. The phase of the catalyst support is also important.Sasol Group Technology, National Research Foundation and the University of Pretoria.http://pubs.acs.org/journal/iecred2017-09-30hb2016Materials Science and Metallurgical Engineerin

The thermal expansion of 3C-SiC in TRISO particles by high temperature X-ray diffraction

The lattice parameter change of SiC in TRISO particles prepared by chemical vapour deposition (CVD) was measured using high temperature X-ray diffraction, across a temperature range of 25–1400 C. Al2O3 was used as the internal standard and the SiC temperature corrections were calibrated using its two independent lattice parameter values along the a- and c-axes. Experimental unit cell values of SiC atlow temperatures corresponded well with those published in previous literature, but deviated systematically at higher temperatures. Thermal expansion coefficients of the CVD prepared SiC shell material are considered the most accurate and follow a linear trend with increasing temperature (a11 = 2.7706 10 9 - T +3.3048 10 6 K 1). The TRISO particles are described best using non-lin ear expansion coefficients. Apparent is the deviation in the SiC lattice constants of the shell material and the TRISO particles from which it originated.This could indicate a residual strain in the TRISO particles or a difference in sample displacement between the TRISO particles and the surrounding alumina standard.The room temperature lattice constant for the shell material is 4.36030 Å(SD0.00006 Å) as compared with that of the TRISO sample of 4.35835 Å(SD0.00006 Å) after adjusting the sample displacement toget alumina lattice constants as close to the accepted values. Residual stress of 300 MPa is calculated from the lattice constant differences.http://www.elsevier.com/locate/jnucmathb2013ai201

Refinement of iron ore sinter phases : a silico-ferrite of calcium and aluminium (SFCA) and an Al-free SFC, and the effect on phase quantification by X-ray diffraction

Crystals of a silico-ferrite of calcium and aluminium (SFCA) and an Al-free SFC were prepared from the melt by slow cooling of synthetically prepared mixtures and examined by single-crystal diffraction methods. Both crystals belong to the space group P-1. SFC has lattice parameters a = 9.1255(3) Å, b = 10.1189(3) Å, c = 10.6183(2) Å, α = 63.9554(9)°, β = 84.4964(11)°, γ = 65.6706(9)° with a final R(|F|) = 0.024. SFCA has a cell with a = 9.0738(9)Å, b = 10.0474(10)Å, c = 10.5611(10) Å, α = 64.061(3)°, β = 84.356(3)°, γ = 65.722(3)° with a final R(|F|) = 0.030. The SFC structure was transformed to the cell used by Hamilton et al. (1989) and refined to an R(|F|) = 0.024. All the atomic positions are equivalent to those reported by Hamilton et al. (1989) with the exception of one (Ca,Fe) position and two oxygen positions that are displaced from the published positions by 0.5y (Ca,Fe1), 0.5z (O4), or 0.5x (O12). This is ascribed to transcription errors in the published crystal structure data. The calculated powder pattern of SFCA (this study) was compared with the experimental data and it shows that the low angle peak intensities agree significantly better than those calculated from the published atomic positions. Additional electron density is located in proximity to the octahedral and tetrahedral cation sites of the main structure. These positions, coupled with the partially occupied cation sites of the main structure, suggest a minor sharing of cations between the main cation sites and the additional sites.http://link.springer.com/journal/710hb2017ChemistryMaterials Science and Metallurgical Engineerin

The effect of titanium oxide additions on the phase chemistry and properties of chromite-magnesia refractories

Please read abstract in the article.The THRIP (Technology and Human Resources for Industry Program) of South Africahttp://www.mdpi.com/journal/ceramicshj2021Materials Science and Metallurgical Engineerin

The oxidation behavior of a selection of South African chromites

Please read abstract in the article.Samancor Chrome.https://link.springer.com/journal/116632023-09-28hj2023Materials Science and Metallurgical Engineerin

Stacking disorder in silicon carbide supported cobalt crystallites : an X-ray diffraction, electron diffraction and high resolution electron microscopy study

Supported cobalt Fischer–Tropsch catalysts are characteristically nanoparticulate and the reduced SiC supported catalyst was found to contain both HCP and FCC polymorphs. This is reflected in the powder XRD patterns and generally there is a poor fit between the experimental and calculated diffractograms. This was ascribed to small crystallite sizes and the occurrence of disorder, manifested as peak broadening and peak shifts. Selected area electron diffraction data of suitably oriented cobalt catalyst grains on silicon carbide supports show non-periodic disorder in the zone axis orientations that contain the common (001) (HCP) and (111) (FCC) reciprocal lattice planes. Both FCC and HCP polymorphs are present in the same grains and these show disorder mainly in the HCP component. The disorder is further examined using high angle annular dark field (HAADF) scanning transmission electron microscopy at atomic resolution and the stacking sequences elucidated. Random sequences of mainly FCC are interrupted by HCP sequences and twin surfaces with reverse stacking sequences are also present. This study highlights the presence of significant disorder in cobalt catalyst grains confirmed by HAADF microscopy.We acknowledge Sasol for funding.http://www.rsc.org/journals-books-databases/about-journals/PCCP2017-11-30hb2016Chemistr

Partial slag solidification within an ilmenite smelter

Within ilmenite smelters, the slag is at a temperature just above its liquidus (because the furnace operates with a slag freeze lining), and the metal bath is at a temperature which is approximately 150°C lower than the slag temperature. As a result, solidification of the slag in contact with the metal is expected, and was indeed detected with sounding bars in operating furnaces. Samples of the solidified layer were obtained from a DC ilmenite smelter which had cooled with its contents intact. Analysis showed parts of the layer to be close to anosovite (Ti3O5) in composition; in all cases, the solid contained much less FeO than the primary pseudobrookite which forms during bulk slag solidification after tapping. Equilibrium calculations show that these compositions can result from reaction of initially FeO-rich pseudobrookite with dissolved carbon in the metal bath. The presence of this layer of near-anosovite material in the furnace has possible implications for settling of metal and for the furnace energy balance; the latter was explored with thermodynamic calculations.http://maney.co.uk/index.php/journals/mpm/ai201

Variation of the redox conditions and the resultant phase assemblages during iron ore sintering

The oxygen potential prevailing during iron ore sintering wasmeasuredwith a zirconia sensor in a series of sinter pot experiments. Thiswas done to get a better indication of the redox conditions during commercial sintering. It was found that the pO2 is appreciably more oxidizing than previously assumed, with a minimum value of ~0.01 atm. It is concluded that this value represents the oxygen potential of the gas phase and it is therefore a mixture of combustion gas and downdraft air. The contents of a quenched sinter pot where the reactions were interrupted with the flame front situated midway through the sinter bed were investigated. X-ray diffraction analysis, using an internal standard to quantify the amorphous slag phase, revealed that at the flame front only magnetite and slag were present. SFCA phases only formed at the top of the bed after the flame front had passed. Thermodynamicmodeling of the phases at equilibriumagree qualitativelywith the phase analysis and explained the extensive presence of magnetite and melt, as well as the formation of calcium ferrite phases during cooling below 1100 °C.Kumba in the form of a bursary.http://www.elsevier.com/locate/ijminpro2017-05-31hb2016Materials Science and Metallurgical Engineerin

β-alumina-14H and β-alumina-21R : two chromic Na2-δ(Al,Mg,Cr)17O25 polysomes observed in slags from the production of low-carbon ferrochromium

The crystal structures of unknown phases found in slags from the production of low-carbon ferrochromium were studied by powder and single-crystal X-ray diffraction. Two phases of Na2−δ (Al, Mg, Cr)17O25 composition were found to be composed of an alternating stacking of a spinel-type and a Na-hosting block. Similar structures are known for β-alumina and β”- alumina, NaAl11O17. However, the spinel-type block in Na2-δ(Al, Mg, Cr)17O25 is composed of five cation layers in contrast to three cation layers in the β-alumina spinel-block. The two new phases, β-alumina-14H, P63/mmc, a=5.6467(2), c=31.9111(12) Å, and β-alumina-21R, R m, a=5.6515(3), c=48.068(3) Å have a 14-layer and 21-layer stacking with a 2 × (cccccch) and a 3 × (ccccccc) repeat sequence of oxygen layers in cubic and hexagonal close packing, respectively.http://www.elsevier.com/locate/jssc2017-09-30hb2016Electrical, Electronic and Computer Engineerin

Pyrometallurgical upgrading of PGM-rich leach residues from the Western Platinum Base Metals Refinery through roasting

The production of Platinum Group Metals (PGMs) normally entails the smelting of PGM flotation concentrates, converting of the furnace matte and removal of the bulk of the Ni, Cu, Co, S and Fe through atmospheric and pressure leaching in a base metals refinery to produce a PGM-rich concentrate. A number of impurities, mostly Se, Te, As, Bi, Os and Pb, are not removed significantly during the oxidising leach process in sulphuric acid media. In addition slag inclusions in matte leads to contamination of the PGM residues with silica, fayalite, magnetite and trevorite phases. Furthermore some Cu, Ni, Fe and S also remain. For this reason a typical Precious Metal Refinery (PMR) feed material contains less than 65% PGMs. The PMR is based on a chloride process and requires contaminants to be within narrow specification limits to prevent the formation of PGM residues that must be reprocessed or tolled, leading to poor first pass metal efficiencies and extending the duration of the production pipeline for efficient recovery. A process has been developed to significantly upgrade the BMR leach residues through pyrometallurgical processing, which include a multistep process of roasting under oxidising atmospheres, a two-step smelting process of the roasted calcine (with engineered slag chemistry and slag-refractory interactions) and subsequent atomisation of the molten alloy which can be fed as a slurry into the HCl/Cl2 dissolution reactors in the precious metals refinery. These pyrometallurgical steps upgrade the BMR residue from a 45–50% grade up to an alloy grade of ca. 90% PGMs, whilst removing the most deleterious elements with major process impacts on the PMR. This paper will focus primarily on the roasting step and it will investigate the thermochemical and mineralogical changes occurring during roasting. These changes were evaluated through a combination of thermochemical modelling and experimental investigation. The roasting step needs to be in an oxidative environment in order to achieve the vapourisation of Se, Te, As, Os and S. The speciation of PGMs and their vapourisation behaviour are presented, as well as the sensitivity of precious metals deportment to changes in roast conditions.http://www.elsevier.com/locate/minenghb2014ai201