Activation of sphalerite by lead ions in the presence of ethyl xanthate was investigated by microflotation, zeta potential measurements, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and extraction by ethylenediaminetetraacetate (EDTA). The source of lead, either from solution or contact with galena particles, proved immaterial. From EDTA extraction data flotation response was determined as a function of surface concentration of Pb ([Pb]surf, mg/cm2). A critical [Pb]surf ≃ 0.5 x 10-4 mg/cm2 causing activation was estimated. By combining with a Pb ion production model from the literature, the possibility of Pb activation for an ore of given Pb grade was explored. Ores with as little as 0.1% Pb are candidates for accidental activation. / A mechanism of lead interaction is proposed: at weakly acidic to mildly alkaline conditions Pb2+ and PbOH+ adsorb on sphalerite and react with xanthate to form PbX2 and Pb(OH)X; at high pH, hydrophilic Pb(OH)2(s) precipitates depress flotation. / Various candidate deactivators were compared using microflotation. The reagents were diethylenetriamine (DETA), sodium bicarbonate (NaHCO3), silica sol (SS), sodium phosphate (Na3PO4·12H 2O) and sodium polyphosphate (NaPO3)n. The latter had the strongest effect and was selected for detailed study. The mechanism of polyphosphate action was investigated by SEM and XPS. The results show that polyphosphate acted to remove Pb ("clean") from the sphalerite by forming soluble complexes. / The polyphosphate (PP) to lead (Pb) ratio in the complexes was determined from conductometric titration of lead nitrate vs. polyphosphate. It was found that initially a precipitate formed with PP/Pb of 1/3. The precipitate dissolved in excess polyphosphate, resulting in a variety of complexes with PP/Pb of 1/2, 1/1, 3/2, 2/1, and 3/1. Knowing the amount of Pb to be removed and taking the lowest PP/Pb ratio, 1/2, the quantity of polyphosphate required to solubilize the Pb can be calculated.

The susceptibility of steels to transverse cracking during continuous casting depends, in part, on the hot ductility of the steel. In the past, hot ductility has been measured in the laboratory by tensile testing specimens reheated to a high temperature (preferably melted), and then cooled down to the desired test temperature before isothermally testing to fracture. More recently, isothermal tensile testing has been performed after imposing different thermal histories prior to reaching the test temperature. The most relevant thermal history is that of the solidifying strand surface. Such thermal histories have been found to be quite complex, and invariably involve rapid cooling and heating cycles. This will therefore lead to high thermal gradients, which, in turn, can generate strains in the surface of the solidifying strand. Thus, the purpose of this thesis was to consider the effect of the thermomechanical history on the hot ductility of steel. / A Nb-Ti microalloyed steel was chosen to be studied, since microalloying often increases susceptibility to transverse cracking. After in-situ melting and solidification, tensile test specimens were subjected to the thermal history typical of a continuously cast billet. Different amounts of prior deformations in various schedules were imposed on the specimens at selected stages during the thermal history, before tensile testing to fracture at the time and temperature corresponding to the unbending stage of the billet casting. It was found that the hot ductility varied from 1% to 98%, depending on the stage in the thermal history at which prior deformation was executed. Microstructural studies showed remarkable changes in the precipitation pattern occurred due to the employed prior deformation. Similar hot ductility measurements executed on a low carbon steel, that was free from microalloy elements, exhibited an effect of prior hot deformation which was quite different to the Nb-Ti microalloyed steel. In particular, the effect on the hot ductility was not as dramatic. The effects were also postulated to be due to precipitation.

In this work, the kinetic and equilibrium profiles of each individual reaction step involved in the production of hematite at 200°C via oxydrolysis of ferrous sulphate in concentrated zinc sulphate media were established. Crystallization of ferrous sulphate was found to play a crucial role in the overall process due to its relatively low solubility and fast crystallization kinetics at elevated temperatures. In fact, the overall kinetics of the oxydrolysis process were found to be limited by the re-dissolution of ferrous sulphate. Pre-crystallization of ferrous sulphate prior to oxidation was found to result in enhanced overall kinetics, cutting down the required retention time from 3 to 2 hours. Enhanced kinetics were also achieved by performing oxydrolysis in two-temperature stages: a low temperature (T = 180°C) first stage with retention time >20 min and an elevated temperature (T = 200°C) second stage with retention time >100 min. / The typical composition of the hematite material produced in this work was 64.3% Fe, 1.3% S (as SO4), 0.6% Zn, and 4.6% H2O. The sulphur content was found to be predominantly (0.6--0.8%) due to formation of sodium jarosite with the remaining (0.3--0.5%) attributed to SO4 chemisorption, and (to less extent) basic ferric sulphate formation. In the absence of zinc sulphate, the majority of sulphate contamination was due to basic ferric sulphate formation. Hematite was found to form via a predominantly homogeneous nucleation mechanism with sub-micron crystallites clustered together as aggregates of 5--10 mum size and around 7 m 2/g specific surface area. In contrast, hematite produced by direct hydrolysis of ferric sulphate possessed one order of magnitude higher specific surface area. As for the industrial hematite product, its composition was found to be 52.6% Fe, 4.6% S, 1.0% Zn, and 8.8% H2O. The sulphur contamination in the industrial product was mainly due to co-precipitation of jarosite and basic ferric sulphate compounds. Hydrothermal trans-formation of the industrial hematite product at elevated temperatures (>200°C) and retention time of >60 min with solids loading as high as 16 wt.% proved to be effective in reducing the sulphur content to less than 1%.

Through the application of solution thermodynamic principles, brazing alloys based on the Ni/Cr ratio of AWS BNi-5 (Ni-18Cr-19Si (atom%); Ni/Cr = 3.46) were designed for brazing SiC to itself and Nb. / The optimum alloy composition for SiC-SiC joints was computed to be Ni-14.5Cr-35Si (atom%). Brazing experiments were conducted to assess the effect of changing the Si content of the brazing alloy away from the calculated optimum on the joint microstructures and strengths. For alloys containing less than 37 atom% Si excessive reaction was observed, resulting in the formation of a porous reaction zone at the brazing alloy/SiC interface and poor joint strengths, the thickness of the porous zone decreasing and the joint strength increasing as the brazing alloy composition approached 37 atom% Si. The most favourable joint microstructures and highest joint strengths ($ approx$100MPa) were observed for the 37 atom% Si brazing alloy, in excellent agreement with the thermodynamic model. For alloys containing greater than 37 atom% Si, the microstructures exhibited localized debonding of the brazing alloys from the SiC and poorer joint strengths, also in good agreement with the thermodynamic model. / For the SiC-Nb brazing study, the thermodynamic modelling procedure determined that a series of Ni-Cr-Si-Nb alloys were suitable for forming bonds. Brazing experiments were performed using two of the "optimum" brazing alloy compositions (i.e. Ni-11.6Cr-29Si-19Nb and Ni-11.2Cr-20Si-30Nb (atom%)) and two alloys of lower Si and Nb contents in order to assess the predictions of the thermodynamic model through evaluation of the joint microstructures. All of the joints exhibited porous reaction zones, with the thickness of the reaction zone decreasing as the Si and Nb content of the brazing alloys approached the computed optima. However, due to cracking and solidification porosity in the brazing alloy, these joints were found to be unsuitable for engineering applications. / It was concluded that the solution thermodynamic method developed in this thesis was very useful for selecting Ni-based (Ni/Cr = 3.46) brazing alloy compositions suitable for forming SiC-SiC-Nb bonds.

The main objectives of this Thesis were the determination of the reaction kinetics of the dissolution of zinc-ferrite particles ((Zn$ sb{1-x}$, Fe$ sbsp{x}{2+}$)$ rm Fe sbsp{2}{3+}O sb4$, x $ le$ 0.4) in hot aqueous solutions of sulphuric acid, and the development of a mathematical model for the prediction of the performance of a series of continuous stirred-tank reactors where zinc ferrite is leached. / Well-characterised, porous zinc-ferrite particles of industrial origin were subjected to controlled leaching experiments at temperatures close to 373 K in sulphuric acid solutions of concentration higher than 0.25mol L$ sp{-1}$. The dissolution process was found to be described most adequately by the grain model with surface reaction being the rate-controlling step. After analysing the experimental results through this model, a unique rate equation for zinc-ferrite dissolution as a function of temperature and solution composition, was obtained. / Based on this rate equation, a mathematical framework was built for the analysis of the start-up and the steady-state of reactor cascades where zinc ferrite is continuously leached. This framework consisted of population-balance and mass-balance equations, which were solved analytically or numerically. Computer simulation results, which were obtained by this reactor model, showed very good agreement with actual industrial data.

The role of gas nucleation and cavitation in flotation has been systemically studied. It was shown theoretically that about a 10 m high recovery zone is required to ensure at least one collision between a particle less than 10 $ mu$m and a bubble, under conventional column flotation conditions, indicating that the collision is the rate limiting step for fine particle flotation. Direct formation of small bubbles on the hydrophobic particle surfaces and in slurry by gas nucleation and cavitation has, therefore, been exploited as a way to accelerate the fine particle collection rate. / Experimentally, the existence of gas nuclei in water was demonstrated from coagulation, sedimentation and filtration tests of fine coal (d$ sb{50} approx 5 mu$m) and silica (d$ sb{50} approx 3.5 mu$m) particles. Small bubbles are generated, from the expansion of the pre-existing gas nuclei, by hydrodynamic cavitation induced by flow of liquid through a nozzle at a velocity of 8-15 m/s, depending on nozzle diameter and length. This velocity of bubble initiation is reduced 5-7 m/s when the liquid is gas-supersaturated. Surfactants do not affect the on-set of bubble formation by cavitation, but increase the amount of bubbles formed. The addition of a small amount of hydrophobic particles in gas-supersaturated systems increases the quantity of bubbles generated, while the presence of hydrophilic particles reduces bubble formation. / Dissolved air (or carbon dioxide) flotation of fine silica (d$ sb{50} approx 1.6 mu$m) has shown that recovery increases with the slurry flow velocity, regardless of the saturation pressures applied (102-310 kPA). Releasing the gas-supersaturated slurry into a solution gives a higher recovery than releasing the gas-supersaturated slurry into a slurry. These observations suggest that bubble nucleation is a better mechanism for enhancing particle collection compared with the direct particle-bubble contact. / Flotation results using a newly designed flotation reactor indicate that adding a cavitation tube and a small amount of air in the feed stream increases recovery significantly, and that bubbles formed by hydrodynamic cavitation play a role in improving flotation kinetics, even in the absence of added gas in the stream.

Four low carbon steels were drawn from rod to wire on a commercial multi-die wire drawing machine. Samples were obtained from between dies. Internal damage, as a function of cold work, was determined using precision density measurements. Nitrogen was observed to have the greatest effect on the ductility of low carbon steel. Interstitial nitrogen causes internal damage, which results in wire breaks. The mobility of nitrogen increases with temperature, which could account for the increased probability of wire breaks at high drawing speeds.

In order to understand and control the deportment of common impurity elements in the Pt(IV)-Pd(II)-HCl-8-hydroxyquinoline (TN 1911)$ sp{ rm a}$ system, their solvent extraction chemistry in chloride solution has been studied. The investigated elements were Fe(III), Cu(II), Ni(II), Zn(II), Pb(II), Sn(IV), Ag(I), As(V), Sb(V), Bi(III), Se(IV) and Te(IV). Extraction experiments carried out using multi-element and single element feed solutions helped to elucidate the extraction behaviour of the elements with the extractant TN 1911. From the collected data it was deduced that the majority formed anionic chlorocomplexes and extracted via the ion-pair mechanism involving the protonated extractant. Consequently, scrubbing procedures were developed to control the deportment of these elements. These scrubbing steps were then incorporated into a tentative flowsheet which was simulated in a batch-wise fashion in the laboratory. The most contaminating elements were found to be Zn(II), Pb(II), Ag(I), Te(IV), Se(IV) and Bi(III). The first five were effectively scrubbed with 2.0M HCl whereas Bi(III) could only be scrubbed with 2.0M H$ sb2$SO$ sb4$ at A/O ratios of $>$3:1. The element Sn(IV) which was found to build-up in the organic, could only be scrubbed with a 1.0M NaOH solution for long contract times (60 minutes). ftn$ sp{ rm a}$A proprietary alkylated derivative of 8-hydroxyquinoline (Schering Berlin AG).

The flow and beta recrystallization behaviour during thermomechanical processing of near-alpha titanium alloy IMI834 (Ti-5.8Al-4Sn-4Zr-1Nb-0.5Mo-0.35Si), with an initial bimodal alpha+beta microstructure, has been investigated. The effects of temperature and strain rate were characterized and modelled at beta and alpha+beta hot working temperatures near the beta to alpha+beta transition temperature (beta transus) to study the quantitative differences in one- and two-phase isothermal forging. The experimental work for characterization and modelling was based on compression testing of lab-scale specimens at temperatures of 975-1100°C, strain rates of 0.01-1s-1, and post-deformation annealing times of 5-420s. Supplementary interrupted compression testing was also performed at 975-1000°C to evaluate the applicability of fractional softening in the determination of static recrystallization kinetics. The stress-strain analysis, which employed corrections for friction and deformation heating, showed increasing stress at increasing strain rates and decreasing temperatures. The stress-temperature dependence increased below the beta transus due to the increasing alpha phase fraction with decreasing temperature. Microstructural observation through optical microscopy indicated dynamic recrystallization occurred, although complete grain refinement and homogeneity was only achieved following static recrystallization. Quantitative measurement via image analysis revealed static recrystallization kinetics increased with temperature for single phase beta pre-deformation microstructures (1060-1100°C). However, bimodal alpha+beta microstructures (1000-1025°C) displayed greater recrystallization rates with decreasing temperature. This behaviour was attributed to the associated increase in alpha phase fraction, which yielded a refinement in initial beta grain size and an increase in favourable nucleation sites. Interrupte / L'évolution de la contrainte d'écoulement et de la recristallisation de la phase beta (beta) de l'alliage de titane IMI834 (Ti-5.8Al-4Sn-4Zr-1Nb-0.5Mo-0.35Si), qui présente une microstructure bimodale, a été étudiée lors d'essais thermomécaniques. Des essais de compression ont été effectués à des températures comprises entre 975°C et 1100°C et à des vitesses de déformation allant de 0,01 à 1s-1. Les échantillons ont ensuite été maintenus à la température de déformation pendant 5 à 420 secondes. Ces essais ont permis de mieux comprendre l'influence de la température et de la vitesse de déformation sur le procédé de forgeage isothermique. Des essais de compression interrompus ont également été effectués pour évaluer l'effet de l'adoucissement sur la cinétique de la recristallisation statique. Un modèle mathématique a ainsi pu être déterminé pour permettre une meilleure compréhension du procédé de forgeage à des températures supérieures et inférieures à la température de transformation allotropique (beta transus). Les courbes de déformation, auxquelles un facteur de correction attribuable à la friction et à la transformation de l'énergie de déformation en chaleur a été appliqué, démontrent que la contrainte est proportionnelle au taux de déformation et inversement proportionnelle à la température de déformation. Lorsque la déformation s'opère à une température inférieure au beta transus, la relation contrainte-température devient plus importante. Dans ces conditions, la microscopie optique révèle la présence de grains recristallisés dynamiquement, bien qu'une microstructure complètement recristallisée ne puisse être atteinte que sous l'influence de la recristallisation statique. L'étude quantitative de la microstructure déformée au-dessus du beta transus, via un système d'analyse d'image, révèle que le taux de recristallisation

Cleanness and uniformity in steel properties are important for high quality steel. Physical and/or mathematical models can be used in order to achieve optimum conditions for the clean steel during steelmaking processes. In the present study, important metallurgical transport phenomena in steelmaking ladles and tundishes have been investigated using both mathematical and physical (water) models. / Through appropriate solutions of the Navier-Stokes equations, the intermixing of fluid within gas-stirred ladles can be modelled quite satisfactorily. It is shown that off-centered bubbling gives the most consistent results in terms of minimising mixing times, since angular velocity components intermix fluid across the width of a ladle. Comparisons between mathematical and experimental data are presented. / Fluid flow, heat transfer and inclusion flotation have been modelled mathematically for testing the behaviour of several tundish designs. Computations are presented to illustrate the importance of thermal natural convection currents in mixing the upper and lower layers of steel. Particle removal rates are also experimentally studied with the aid of the novel E.S.Z. (Electric Sensing Zone) system, and compared with computational results.