Dissolution of alumina particles in CaO-Al2O3-SiO2-MgO slags

Abstract

The dissolution behaviour of alumina particles in CaO-Al₂O₃-SiO₂-MgO slags was studied by confocal scanning laser microscopy. Three different slag systems were investigated to evaluate the effect of changing the MgO content in a high silica tundish slag, and the effect of decreasing the silica content when dissolving the particles in a ladle slag. A particle of 80 mum diameter dissolved in 50 s in a simulated ladle slag but needed 100 s to dissolve in a high silica slag at the same temperature. When the silica content was decreased, a decrease in the alumina particle dissolution time was noted. During dissolution of alumina particles in a slag containing MgO, the alumina particle reacts with the slag to form an MgAlO₄ layer. No reaction layer was observed during the dissolution of alumina particles in slags that did not contain MgO. This work suggests that the dissolution time of large alumina particles is significant, increasing with particle size and with decreased temperature.     

Effect of Mg on sintering phenomenon of aluminium alloy powder particle

Abstract

The effect of Mg on the sintering phenomenon of aluminium alloy powder particles has been examined using XPS analysis of the chemical reaction at the top most surface of the particle during heating. The relative density of the sintered material increases by 9% according to the increase of Mg content. The mechanical properties of the sintered material also increase remarkably as the Mg content in the particle increases. The ratio of the dimple patterns observed at the fractured surface after the tensile test also increases. It is considered that Mg acts to deoxidise the Al₂O₃ film that covers the particle surface as a barrier and helps sintering between the particles.     

Microstructure of TiC–TiN–Ni–(B) cermet systems

Abstract

TiC-TiN-Ni-(B) systems were investigated to understand particle coarsening and morphology changes as a function of sintering time and compositions. In addition, the variations in the C/(C+N) ratios of Ti(C_1-xN_x) solid solutions formed in the system were studied. As expected, the particle size becomes larger with an increase in the duration of sintering. However, added boron significantly reduced the growth of particles. X-ray diffraction analysis shows that the binder phase consists largely of Ni₃B in TiC-TiN-20Ni-1B (in wt-%) system. The presence of boron in the liquid Ni seems to interfere with the dissolution of TiC and TiN and/or transport of Ti, C, and N. The effect of boron decreases as the amount of Ni binder increases. The particle morphology is found to change with variations in the C/(C+N) ratio.     

Structural characteristics and desorption properties of nanostructured MgH2 synthesised by high energy mechanical milling

Abstract

The effect of particle size, lattice strain and crystallite size on the hydrogen desorption properties of nanocrystalline magnesium hydride powder was investigated. Commercial MgH₂ powder was milled in a Spex 8000M up to 16 h and its structural evolution and desorption characteristics at different time intervals were examined using various analytical techniques. At the early stage of milling, the formation of metastable γ-MgH₂ phase was noticed. While the crystallite size gradually decreased to 12 nm with increasing the milling time, the accumulated lattice strain gained a maximum value of 0·9% after 4 h milling. The highest drop in the desorption temperature (～100°C) was attained at the peak strain with the average grain size of 20 nm. The particle size of MgH₂ varied in the range of 20–50 μm without profound effect of the desorption properties.     

Water model study on inclusion removal from liquid steel by bubble flotation under turbulent conditions

Abstract

Inclusion removal from liquid steel by bubble flotation under turbulent conditions is analysed using a water model. Turbulence is realised by impeller stirring in a water containing vessel. First, the effects of variables such as filter pore size, gas flowrate, NaCl concentration, and stirring intensity on bubble size are investigated. Second, particle removal by bubble flotation is studied using the water containing vessel system. The results indicate that particle removal rate by bubble flotation is controlled by non-first order kinetics. The factors affecting the particle removal rate constant k ₁ are discussed and a final empirical equation is derived as follows: -dc/dt = k ₁ c^1·3665 and k ₁ = A(d _p/d _B)^2·65?0·104 Q _g^1·630, where c is particle number density, t is time, A is a constant parameter, d _p and d _B are the particle and bubble diameter respectively, ? is the turbulent energy dissipation rate, and Q _g is the gas flowrate.     

Production of bronze powders by water jet cooled rotating disc atomisation

Abstract

New type of water jet cooled rotating disc atomisation unit was designed and constructed. The raw material was melted in graphite crucible with high frequency induction heating, and atomisation was performed in high purity argon gas atmosphere. Cu–10Sn alloy was atomised to investigate the effect of production parameters, such as disc speed, disc surface condition, liquid metal flowrate, disc fin number and superheat of liquid metal with respect to mean particle size and powder yield rate. The produced powders appeared spherical, rounded, ligamentous, irregular and flaky, depending on particle size. The mean particle size of produced powders was in the range of 100–250 μm with 65–85% powder yield rate depending of atomisation parameters. The ZrO₂ material coated disc with four fins gave the finer mean particle size and higher powder yield rate in comparison with uncoated disc with two fins.     

The Effect of Bubble Size on Fine Particle Flotation

Abstract

Expressions for the probability of collision (P_c) and adhesion (P_a) have been derived for fine particle flotation by calculating the trajectory of particles as they flow past a bubble in streamline How. Three different flow regimes have been considered in the present work, i.e., Stokes, potential and intermediate. For the intermediate flow conditions in which most flotation operations are carried out, the particle trajectories have been determined using an empirical stream function derived in the present work. For the case of a very hydrophobic coal sample, the values of the probability of collection (P) determined experimentally have been found to be in close agreement with the theoretically predicted P_c values over a range of bubble and particle sizes

The expression for P_a has been derived by determining the time it takes for a particle to slide along the surface of a bubble after collision. It has been assumed that the bubble-particte adhesion occurs when the sliding time is equal to or exceeds the induction time, which varies with the particle hydrophobicity. P_a is shown to be a function of particle size, bubble size and induction time. The values of P_a predicted in the present work are in good agreement with the results of microflotation tests conducted on a coal sample.     

AN INVESTIGATION INTO METHODS FOR THE PARTICLE-SIZE ANALYSIS OF UO2 POWDER AND THE EFFECT OF BALL-MILLING ON SIZE DISTRIBUTION

Abstract

Techniques for the particle-size analysis of “ceramic-grade” UO₂ in the 4–40 μm size range are assessed, with a view to selecting the most suitable method for analysis of UO₂/PUO₂ mixtures under glove-box conditions.The β-backscatter method is considered to show some advantages over the Coulter counter or optical microscopy and the mean coefficient of variation is ± 1.2% at the 1σ level.

The effects of various forms of pretreatment, e.g., high-speed stirring, manual mixing, and ultrasonic shaking, are examined and discussed with relation to the agglomerative properties of UO₂. Ultrasonic shaking is the most successful form of dispersion and the necessity of standardizing the pretreatment stage is emphasized.

β-backscatter is used to compare particle-size distribution after wet and dry ball-milling. Whilst little difference is observed in the ultimate particle size produced by the two treatments, the effect of agglomeration in dry milling is clearly shown. The increased effectiveness of milling UO₂ powder in a suitable dispersing medium, rather than water, is demonstrated.     

Improving pore morphology of PM 316L stainless steels by prealloyed powder prepassivation in 20% nitric acid

Abstract

The effect of prepassivation of prealloyed powder of 316L stainless steels on pore morphology and powder particle shape was investigated. Image analysis technique was used to study the effect of prepassivation, compaction and sintering temperature on the pore morphology of powder metallurgy 316L stainless steels. Porosity, dimension and morphology of the pores were characterised by means of four basic parameters: fraction of surface porosity, equivalent circle diameter, shape factor and elongation factor. In addition, SEM macrographs of powder particles were also investigated by applying the image analysis technique. The Feret’s average diameter and elongation factor were employed to describe the size and roundness of powder particles respectively. Annealing treatment reduced the equivalent circle diameter of pores and simultaneously improved f_shape and f_elong towards higher values. It was proposed that the prepassivation treatment reduced irregularity of powder particles through elimination of sharp corners of powder particles by exposure to acid environment.     

Wear behaviour of Fe3 Al intermetallic particle reinforced PM based iron metal matrix composites

Abstract

The wear behaviour of unreinforced and reinforced PM based iron metal matrix composite, the latter containing 10 and 20 vol.-% nano sized Fe₃Al intermetallic particles, was studied as a function of sliding distance under two different loads and dry lubricated conditions. The intermetallic Fe₃Al nanoparticles were prepared by mechanical alloying and used as particle reinforcement with 10 and 20 vol.-% in the matrix. The processing of the composites included mixing and cold compaction followed by sintering at 1120°C. The influence of Fe₃Al additions on the dry sliding wear behaviour was studied at loads 20 and 40 N over sliding distances 2160, 3240, 4320 and 6480 m. The study showed that the composite exhibited a lower wear rate than that of the unreinforced matrix and the wear rate was influenced by the volume percentage of Fe₃Al particles. It is understood that iron aluminide reinforcement has a beneficial effect on the wear properties. Delamination and microcutting were the chief mechanisms of wear for the composites.     

Characterisation of electric arc furnace dust generated during plain carbon steel production

Abstract

Electric arc furnace (EAF) dust is a waste generated in the EAF during the steel production process. Among different wastes, EAF dust represents one of the most hazardous, since it contains heavy metals such as Zn, Fe, Cr, Cd and Pb. The goal of the present work is to characterise the waste through chemical analysis, particle size distribution, X-ray diffraction (XRD), scanning electron microscopy coupled with energy dispersive spectroscopy detection and thermal analysis. The waste sample is composed essentially of spherical particles and has a very small particle size and the majority of the identified elements were Fe, Zn, Ca, Cr, Mn, K and Si. The XRD has presented compounds such as ZnO, ZnFe₂O₄, Fe₂O₃, MnO, SiO₂, FeFe₂O₄ and MnAl₂O₄. According to the thermal analysis results, up to 1000°C the total weight loss was ～5%. The results of waste characterisation are very important to these further investigations.     

THE TECHNOLOGY OF UAl3–Al IRRADIATION TEST-PLATES

Abstract

The existing literature concerning UAl₃ and UAl₃-Al dispersions is reviewed. Calculations of the strength of the dispersions have also been made. The experimental work deals with the preparation of UAl₃ by induction melting, its chemical analysis, and crushing to the correct particle size for use in reactor-fuel elements. The blending of UAl₃-Al dispersions and their compaction by cold pressing to densities suitable for roll-bollding are described, and their properties have been determined. The picture-frame technique has been used for the aluminium cladding of the UAl₃-Al dispersions and to produce irradiation test-plates.     

Combustion synthesis of TiB2 based hard material cemented by Fe–Al intermetallics

Abstract

TiB₂ based hard material has been fabricated by combustion synthesis from elemental powders using Fe–Al intermetallics as the binder phase. Both the crystal orientation map and the pole figures by electron backscatter diffraction analysis showed that the TiB₂ particles have no preferential orientation. Preheating of the power compact is helpful to increase the relative density and to reduce the particle size of the synthesised product, which increases the hardness of the product. The highest hardness and relative density were achieved for a composition of TiB₂–20 vol..-%FeAl.     

Effect of atomisation medium on sintering properties of austenitic stainless steel by eliminating influence of particle shape and particle size

Abstract

Gas and water atomised 316L stainless steel powders with similar powder morphology and particle size were injection moulded and sintered. The results show that compacts prepared from the gas atomised powder exhibit higher density and tensile strength, whereas those prepared from the water atomised powder exhibit higher elongation, finer grain size and superior corrosion resistance. Chemical analysis shows that the water atomised powder has a higher Si and O content, and microstructural analysis of the sintered compacts reveals that SiO₂ particles disperse as a second phase in the compacts prepared from the atomised powder, which accounts for the property behaviour. Due to the presence of SiO₂, the porosity increases, whereas the pore coarsening and grain growth are inhibited. Besides, SiO₂ particles can also improve the passivation effect of stainless steel, and hence increase the corrosion resistance.     

Generation of small batch high quality metal powder

Abstract

As part of a project to develop a small batch metal atomisation system, based on free fall atomisation and close-coupled atomisation, the influence of the process parameters on various powder quality features has been investigated, using copper–tin alloys as feedstock material. Particle size distribution, appearance of satellite particles, particle circularity and the flowability were recorded as criteria of particle quality. From the data obtained in these experiments, the effects of the main process parameters (atomisation pressure, mass melt flow, and height of the spray chamber as well as the atomiser system) have been evaluated with respect to powder quality features. The atomisation system was optimised to produce high quality powder with narrow particle distributions (d_84·3/d_50·3?=?1·6 for free fall) and high circularity with mass melt flows in the range of 100 kg h^?1 using melt volume between 100 and 1000 mL.     

Thermodynamic and Kinetic Flotation Criteria

Abstract

Flotation thermodynamics and notation kinetics are discussed and compared with chemical thermodynamics and chemical kinetics. While chemical thermodynamics can predict whether a reaction will take place under a given set of conditions (temperature and pressure), and can also predict the direction in which the equilibrium will be shifted in response to variation in these parameters, flotation thermodynamics can predict how likely it is for the mineral particle and gas bubble to attach in a moment of their collision. While thermodynamics can predict the probability of particle-to-bubble attachment, it cannot predict the rate of this process, for the energy barrier, G_attach, opposing particle-to-bubble attachment, is not interrelated with the free energy change accompanying the attachment

The thermodynamic (Δ G < 0) and kinetic (τ_i; < τ_c c) flotation criteria are examined by a study of the effect of frothers on flotation. Various hypotheses that explain how the frother can affect flotation rate are discussed with emphasis on unsolved problems.     

HIGH-STRENGTH SINTERED IRON-BASE ALLOYS BY USING TRANSITION METAL CARBIDES

Abstract

A simple method of producing alloy steels by mixing, pressing, and sintering iron powder with carbides of transition metals is described. The carbides must have a high carbon content and be soluble in iron. These characteristics are found in the carbides of chromium, tungsten, molybdenum, and vanadium. Tensile strengths of ～800 N/mm² have been achieved in annealed specimens having carbide additions of 1–12 wt.-%, depending on the carbide. The additions produce steels with partly air-hardening properties.

The effects of concentration and particle size of the carbides, sintered density, sintering conditions, and cooling rate on mechanical properties have been determined. The alloys are not very sensitive to the sintering atmosphere. The effect of heat-treatment on the mechanical properties of Fe-Cr₃C₂ alloys is reported. A tensile strength >1300 N/mm² was observed with 4 wt.-% Cr₃C₂. Dilatometric measurements were conducted with different carbide concentrations and heating conditions to study the sintering process. The important process of homogenization was investigated by electron microprobe analysis.     

Morphology and growth of alumina inclusions in Fe–Al alloys at low oxygen partial pressure

Abstract

The degree of supersaturation is a factor that influences the Al₂O₃ inclusion characteristics in steel. The influence of the addition of a large amount of Al in the molten steel on the formation, growth and morphology of Al₂O₃ inclusions was investigated by laboratory scale experiments. Consecutive steel samples were taken during the deoxidation process and subjected to chemical analysis (ICP-AES), automated image analysis (AIA) and scanning electron microscopy (SEM) assessment with respect to the extracted inclusions. The characterisation and quantification of Al₂O₃ particles show different growth processes, leading to variations in particle size distribution as well as in the morphology.     

Characteristics of rapidly solidified Cu–10%Sn alloy powders produced by water jet cooled rotating disc atomisation

Abstract

In the present work, an experimental water jet cooled rotating disc centrifugal atomiser was designed and constructed and used to produce rapidly solidified Cu–10%Sn alloy powders. The characteristics of rapidly solidified Cu–10%Sn alloy powders have been investigated with respect to powder size and disc surface condition. Uncoated and ZrO₂ coated copper discs were used to investigate the effect of disc surface conditions on the microstructure and cooling rate of the powders. The produced powders appeared in the shape of sphere, rounded, ligament, irregular and flaky, depending on the particle size. The powders exhibited fine grained microstructure, cell size increased with increasing powder size and higher cooling rates were obtained using uncoated disc. The results indicated that cooling rates of 20 μm powder produced with uncoated and ZrO₂ material coated discs were estimated as 5·82×10⁵ and 1·44×10⁵ K s^?1 respectively.