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.

A theoretical and experimental study is presented of texture evolution in hcp zirconium deformed at high temperatures. It is shown that, within the temperature range in which nuclear components are fabricated, the development of crystallographic texture is controlled by the simultaneous activation of $ langle$a$ rangle$ and $ langle$c + a$ rangle$ slip. This is demonstrated by experiments on Zr-2.5 Nb and unalloyed Zr deformed along three of the four elementary strain paths: axisymmetric elongation and compression and pure shear. Further experimental work on the axisymmetric deformation of unalloyed Zr and Zr-20 Nb shows that the influence of the bcc $ beta$-Zr phase present in Zr-2.5 Nb at the temperatures of interest is not due to the formation of crystallographic textures in this phase. / Texture evolution modelling was implemented for the case of polycrystalline hcp metals by making use of the duality principle between the generalized Taylor and Bishop-Hill theories of polycrystal plasticity and with the help of linear programming. The high temperature experimental textures for the three elementary strain paths mentioned above were successfully predicted using the relaxed constraint theory. It is shown that, for the cases of pure shear and compression, the so-called pancake version of this approach gives a better approximation to the observed textures than the full constraint calculations. For axisymmetric elongation, the curling version of the relaxed constraint model gives the best results. / By the analysis of slip activity maps, it is shown that the limitations of the full constraint model reside in the high activity level for $ langle$c + a$ rangle$ slip associated with this approach. When the relaxed constraint model is applied, the activity level of $ langle$c + a$ rangle$ slip is considerably reduced, leading to much better agreement of the predicted textures with the experimental results. The full constraint model is also applied to simulate texture evolution in $ alpha$-Zr deformed in simple shear at room temperature. It is demonstrated that the level of agreement is at least as good as that obtained when full constraint calculations were carried out. Finally, a high temperature texture rosette is proposed for prediction of the textures developed during tube forming operations for Zr alloy nuclear components. (Abstract shortened with permission of author.)

The digestion (dehydration and dissolving), in a molten salt containing MgCl2, of partially hydrated MgCl2 in the form of 'prills' was visually observed. The 'prills' were prepared by Magnola Metallurgie Inc. Pictures of the digestion are presented and two types of bubbling behaviours were observed. / It was concluded that the digestion process occurred in two steps: (1) dehydration/hydrolysis of prills in the solid state and (2) dissolution of the dehydrated products. Each step was accompanied by a distinctive bubbling behaviour as the gas products of the step were evolved. / The type of bubbling behaviour associated with the first step was termed 'Primary Bubbling' and was due to the release of the waters of hydration, hereinafter termed dehydration. The second type of bubbling behaviour observed was termed 'Secondary Bubbling' and was due to the decomposition of dissolved oxide in the molten salt according to the reaction: MgOHCldiss→MgOs+HCl g / The solubility of the dissolved oxide was seen to be a function of temperature and MgCl2 activity in the melt. Analysis of the gas evolved during Secondary Bubbling revealed a gas composition of about 51 vol.% HCl and 49 vol.% H2O, indicating that dissolved oxide was also being chlorinated by the reaction: MgOHCldiss →MgCl2l+HO g / Injection of Ar decomposed the dissolved oxide by offsetting the first reaction above to the left and injection of HCl chlorinated solid MgO by the reaction: MgOs+2H Clg→MgCl2l +H2Og / Surprisingly, it was found the HCl atmosphere in the freeboard of the reactor had no significant effect to suppress the hydrolysis reaction occurring during dehydration.

The purpose of the present work was to remove selectively manganese from a neutral leach zinc-rich solution at 80ºC using the gas mixture of sulphur dioxide (SO2) and oxygen (O2) as oxidizing agent. In order to determine the optimum conditions for manganese removal using SO 2/O2, several semi-batch experiments were performed, where the effects of pH, ORP, SO2/O2 ratio, mixing intensity, etc. were investigated. Results of these tests showed that SO2/O 2 was a fast and effective oxidant for removing manganese down to ppm level provided that the appropriate reactor design, agitation and SO 2/O2 ratio were employed. In an attempt to improve the precipitate's characteristics, e.g. crystallinity and solid/liquid separation, a new technique called Step-Wise Oxidative Precipitation (S.W.O.P) was investigated using a two-reactor continuous circuit employing pH and ORP control and precipitate recycling. These tests revealed that a birnessite-like phase with general formula (Na0.7Ca0.3)Mn7O14·2.8H 2O was produced with co-precipitation of a significant amount of zinc apparently via substitution. The applied technique (S.W.O.P combined with recycling) proved effective in producing dense particles but not on lowering zinc losses. The biggest advantage of this novel oxidation technique was the total elimination of scaling. (Abstract shortened by UMI.)

The aggregation behaviour of sphalerite suspension and the role of zinc and magnesium ions are investigated. Aggregation is monitored by suspension analysis (turbidity) and optical microscopy and, in particular, a conductivity-settling technique. To probe the mechanisms, electrophoretic mobility (zeta potential) measurements and field emission scanning electron microscopy (FE-SEM) imaging are used. / In the case of sphalerite alone, for samples from a variety of sources, aggregation occurred at pH 8--10, well above the range in iso-electric point (pH 2--6). The aggregation is attributed to the presence of Zn(OH) 2, the dominant species over this pH range. To test whether zinc hydrolysis products promote aggregation silica and chalcopyrite suspensions were doped with Zn2+ ions; aggregation over the same pH range was found. This observation is similar to that of Healy and Jellet (1967) for zinc oxide, ZnO. They suggested aggregation was due to release of Zn2+ ions to form Zn(OH)2 which polymerizes and flocculates the particles. The same mechanism is proposed for sphalerite. / Aggregation due to magnesium ions was determined using the settling rate of sphalerite and silica suspensions (individually) as a function of Mg 2+ concentration, pH and suspension density (%v/v solids). Aggregation at pH > 10 was found for both minerals corresponding to magnesium hydroxide. However, the mineral's response to the three variables suggests the mechanism for each is different. The proposed mechanism of aggregation by Mg(OH) 2 for sphalerite is chemical bridging and for silica, electrostatic bridging. Electrostatic bridging is revealed by aggregation passing through a maximum as a function of both coagulant concentration and pH. For sphalerite, while there is a maximum with [Mg2+] (Mg2+ concentration) there is none with pH (after allowing for the self-aggregation of sphalerite). Further, electrostatic bridging requires surface patches of the bridging material (Mg(OH)2) and the FE-SEM images showed no such evidence. The interpretation for silica aggregating with Mg2+ follows that proposed by Krishnan and Iwasaki (1986). The pH, [Mg2+] and solid concentration effects are compatible with electrostatic bridging, as is the morphology as hydroxide patches were identified by FE-SEM. / In certain cases the conductivity-settling data suggested the particles were more conductive than the liquid. The conductivity-settling technique was adapted to measure the electrical conductivity of particles dispersed in water. The conductivity was estimated at the iso-conductivity point where the solution and the particles have the same conductivity. The technique was tested on chalcocite, chalcopyrite, galena, pyrite and sphalerite. The order of mineral conductivity followed that of their electrochemical rest potential, as expected. It is observed that the adsorption of xanthate significantly reduced the conductivity of chalcopyrite and copper activation increased the conductivity of sphalerite but treatment with lead had no effect.