Constant true strain rate (2 s$ sp{-1})$ compression tests with one or two interruptions were carried out on a series of steels over the temperature range 900-1050$ sp circ$C. Based on a 0.07% C, 1.25% Mn and 0.006% N reference steel, the three microalloyed steels contained the following: (i) 0.08% Al, (ii) 0.04% Nb, or (iii) 0.045% Nb and 0.08% Al. The degree of fractional softening during the interpass was used to assess recrystallization start (R$ sb{ rm s})$ and finish (R$ sb{ rm f})$ times as well as those for the onset (P$ sb{ rm s})$ and termination (P$ sb{ rm f})$ of precipitation. / Relative to a plain C steel, the static recrystallization kinetics show that the single solute retarding effect of Al is small but measurable while that of Nb is substantially larger. The influence of each element is enhanced when they are present jointly. The retardation effects observed at 1000$ sp circ$C are due solely to the presence of these elements in solution. Additional delays observed at or below 950$ sp circ$C in the two Nb bearing steels are ascribed to strain induced precipitation of Nb(CN). / The P$ sb{ rm s}$ times were estimated from the fractional softening data as well as from stress relaxation type tests. The presence of high levels of Al retarded the onset of Nb(CN) precipitation while it accelerated its progress. The Nb precipitates were cube shaped and heterogeneously distributed, suggesting that they were formed on dislocation networks. / A thermodynamic model was developed for the alloy system studied. The calculations indicated that there is a higher chemical driving force for precipitation in the Nb steel than in the Nb-Al steel at a given temperature. Consequently, this produces a higher equilibrium mole fraction of Nb(CN) in the former. The P$ sb{ rm s}$ times calculated based on the classical nucleation theory are in good agreement with the values observed. / Three hit test results further showed that increased hold times between the first two hits decrease the rate of softening between the second and third hits. Residual strain in the partially recrystallized structure is more effectively accumulated at lower temperature due to slower softening rates. These data enabled the development of a model for calculation of residual strains following a hold in a partially recrystallized structure. (Abstract shortened by UMI.)

Three microalloyed steels were tested under continuous and interrupted loading conditions. These materials consisted of a base steel containing 0.06%C and 1.31%Mn and two further alloys containing single additions of 0.29%Mo and 0.04%Nb. / Double hit compression tests were performed on cylindrical specimens of the three steels at 820, 780 and 740$ sp circ$C within the $ alpha$ + $ gamma$ field. A softening curve was determined at each temperature using the offset method. It was observed that, in the base steel, $ alpha$ recrystallizes more slowly than $ gamma$. The addition of Mo steel retards the recrystallization of the base material; it has a greater influence on $ gamma$ than on $ alpha$ recrystallization. Nb addition has an even greater effect on the recrystallization of the two phases. In this steel, the recrystallization of ferrite was incomplete at the three intercritical temperatures. Furthermore, the austenite remained completely unrecrystallized up to the maximum time involved in the experiments (1 hr). / The progress of ferrite recrystallization was followed on quenched specimens of the three steels by means of quantitative metallography. Most of the data fit an Avrami equation with exponents in the range 2 to 2.5. A departure from this equation was observed for the final stages of the recrystallization process. Growth rate analysis showed that the average growth rate decreased with time for all the experimental materials, this decrease being greatest for the base steel and least for the Nb-containing grade. The nucleation of recrystallization was observed to occur heterogeneously in the microstructure. The interface between ferrite and austenite was the preferred site for nucleation. Measurement of the number and size of recrystallized ferrite grains showed that some grains begin to recrystallize very early, growing to a large size by the end of the process. The total number of grains increased continuously until almost 80% of the volume fraction was recrystallized. After that, due to coalescence, the number of recrystallized grains decreased very rapidly. (Abstract shortened by UMI.)

One critical issue in chloride hydrometallurgy is the regeneration of the lixiviant HCl that is commonly achieved by pyrohydrolysis. It is the scope of this thesis to investigate a low temperature HCl regeneration process, which regenerates HCl and produces saleable gypsum. Experiments were performed to determine the optimum conditions of the chemical reaction that involved the reactive mixing of calcium chloride (CaCl2) solution with sulfuric acid (H2SO4) solution at low temperature and atmospheric pressure. / The results showed that the crystallization chemistry of the CaCl 2-H2SO4 reaction can be controlled in such away that favors the production of well grown gypsum crystals with the simultaneous regeneration of practical strength HCl. High strength HCl (= 4 M) was regenerated by the reaction of (3.5 M) CaCl2 leach solutions with (8 M) sulfuric acid in the temperature range of 20-60°C. Acid addition in stages to simulate the titration procedure, seeding and recycling, and increasing temperature were found to favor crystal growth and filterability. Finally, this research showed that there is negligible metal uptake with the exception of lead, making the production of marketable gypsum possible.

This thesis reports the results of an investigation of scorodite long-term solubility and stability conducted at fixed pH (5-9) and temperature (22°C, 50°C and 75°C). The decomposition of scorodite at 22°C was found to be extremely slow. At neutral pH, the arsenic concentration stabilized at 5.8 mg/L. As scorodite dissolved, the iron reprecipitated as 2-line ferrihydrite. The growth and re-crystallization of ferrihydrite was delayed by apparent arsenate adsorption. The solubility product of scorodite was calculated as 10-25.5. The dissolution rate of scorodite in a pure system was modeled with a decreasing exponential equation. The pre-exponential factor approached first order vis-a-vis OH- concentration and the activation energy suggested that scorodite dissolution is chemically controlled. A thermodynamic investigation indicated that the stability field of scorodite extends up to pH 6.75 for a molality of the aqueous species of 6.7x10 -5 (5 mg/L As) when in equilibrium with ferrihydrite but only up to pH 2.3 when in equilibrium with goethite. The presence of gypsum led to lower arsenic concentrations; this was attributed to the formation of calcium-iron-arsenate compounds, such as yukonite (Ca2Fe3(AsO4) 4(OH)·12H2O).

Nonoriented electrical steels are mainly used as the core materials in motors and generators. The magnetic properties required for these materials are low core loss, high magnetic induction and high magnetic permeability. These magnetic properties of nonoriented electrical steels are mainly influenced by grain size and crystallographic texture. All research to improve the magnetic properties so far has focused on grain growth through the control of chemical compositions and various processing parameters. However, in the manufacturing techniques of nonoriented electrical steels, the control of grain size is approaching its limit. Since there is still enough room to improve the magnetic properties through the control of texture, future research in nonoriented electrical steels should be concentrated on the optimization of crystallographic texture. / Recrystallization texture itself has long been a subject of research because the origin of recrystallization texture provides a high level of scientific interest. Despite much research on recrystallization texture, the mechanism of the formation of recrystallization texture is still disputed. This dispute is associated with the complexity of microstructural inhomogeneity formed in the deformed state, different nucleation rates for different orientations of deformed grains and different growth rates of nuclei into the neighboring deformed matrix. However, the advent of Orientation Imaging Microscopy (OIM) has contributed to a remarkable progress in the area of recrystallization texture, and stimulated research on the annealing texture in low carbon and interstitial free steels. Conversely, no systematic research on the evolution of the annealing texture in nonoriented electrical steels has yet been done. / The mechanism of texture evolution during recrystallization in the nonoriented electrical steels containing 1% and 2% Si respectively, a main objective of this thesis, is investigated from electron backscatter diffraction (EBSD) measurements on the materials with different recrystallization fractions. Based on obtained results, the mechanism for the formation of the recrystallization texture is proposed. The behavior of deformed grains of specific orientations is also discussed in terms of image quality value introduced by EBSD techniques. In addition, the origin of nuclei of specific orientations is suggested from observed EBSD results. / The texture development during grain growth, another main objective of this thesis, is investigated. Furthermore, a mechanism of texture development during grain growth is proposed based on the misorientation angle distribution with adjacent grains and the average size of grains of specific orientations. / Some useful means to optimize the magnetic properties through texture control in nonoriented electrical steels are studied. The effect of initial grain size prior to cold rolling on texture evolution during both the recrystallization stage and the grain growth stage is clarified. In addition, the effects of temper rolling reduction and annealing time on texture development during final annealing are examined. A mechanism of texture transformation by strain induced boundary migration (SIBM) is suggested by means of the introduction of image quality value in EBSD techniques. Finally, the effect of heating rate on texture development during final annealing is investigated, and the reason for texture change due to different heating rates is discussed.

In flotation, bubbles are the transport medium of mineral particles and water. The recovery of fine hydrophilic gangue is related to the recovery of water, which impacts the grade of the concentrate. In the solution (pulp) phase, water is transported as a layer on the bubble surface and as a trailing wake. The amount of water depends on bubble size, gas rate and, it is hypothesized, frother type. / Frothers are surface-active reagents that reduce bubble size by retarding coalescence. To isolate the effect of frother type (chemistry) on water transport from that of bubble size and gas rate an appropriate independent variable had to be established. Bubble surface area flux and gas holdup were proposed and correlated against the overflow rate of water from a bubble column at a given foam depth which was used as a measure of water carrying rate. / Gas holdup is established as the independent variable and it is shown that the common frothers can be grouped into families based on the gas holdup-carrying rate relationship. No relation was found between water carrying rate and bubble surface area flux. Possible reasons are considered.

A three-dimensional mathematical model for the simulation of vertical direct chill (DC) slab casting of aluminum alloys has been developed. The model is based on the solution of the 3D time-averaged turbulent momentum (Navier-Stokes) and energy equations. The momentum equations are modified with a Darcy-type source term to simulate motion of the melt in the mushy region. The buoyancy force term is implemented in the model through the Boussinesq approximation. The low Reynolds number k-ε turbulence model of Launder and Sharma is used to calculate the Reynolds stresses and the turbulent heat fluxes. A variable heat transfer coefficient is used along the ingot surface to account for the different cooling regions. The mathematical model is qualitatively and quantitatively verified by comparing the computed results with a physical water model and a real casting experiment, respectively of independent researchers. Each of the comparisons showed a good agreement. The quantitative verification of the solidification front depths is improved when the thermal buoyancy force effect is included in the model. / A parametric study has been carried on two casters of variable aspect ratio each using a different type of inlet melt distribution system. In the case of the small aspect ratio caster, the physical properties of aluminum Al-3104 are used. For this caster, the studied parameters are the casting speed, the primary cooling rate, the melt superheat and the combo-bag dimensions. Also, the effect of complete blockage of the bottom windows of the distribution bag is studied. An in-depth understanding of some behaviors of the melt flow and solidification profile in the steady state operational phase of the DC casting process is gained. For example, the roles played by the angle flow and the upward component of the vertical recirculation at the wide symmetry plane in controlling both the solidification front depth and the mushy layer thickness at the slab center are ascertained. This study has revealed the influence of the melt stream issued from the bottom window of the bag on the depth and uniformity of the solidification front. The model has successfully identified a faulty design of the short combo bag. This industrially favorite design causes what is called the reverse flow, that is, the melt from the surrounding sump enters the combo bag through the bottom window. (Abstract shortened by UMI.)

Different types of zinc bicrystals were tested under creep conditions. Observations of grain boundary sliding along boundaries with different misorientations were made. It was established that Sigma = 9 coincidence boundary slides faster than boundaries of general type. Grain boundary sliding along coincidence boundary was activated at higher stress and only in the presence of intragranular slip. Sliding along coincidence and near-coincidence boundaries is accompanied by regular boundary migration whereas sliding along general boundary is not. The sliding-migration behavior of coincidence boundaries was explained in terms of DSC-dislocations associated with steps. Sliding along general boundaries was described in terms of non-DSC grain boundary dislocations having infinitesimal Burgers vector. / It is established in bicrystals with individual boundaries that crystallographic slip can increase significantly the rate of grain boundary sliding. Two mechanisms of stimulation of sliding by slip are proposed. Intragranular slip can make a direct contribution to grain boundary sliding in incompatible conditions of deformation. Identical deformation of grains can increase the rate of sliding only by its accommodation at boundary irregularities. The close relationship between sliding and slip at incompatible deformation makes intragranular slip strongly dependent on boundary structure and geometry of slip and independent of the Schmid factor. / It is demonstrated that deformation of textured fine-grained Zn-1.1%Al alloy in the direction favorable for intragranular slip decreases the contribution of grain boundary sliding to the total strain. Deformation in the direction unfavorable for slip increases its contribution. This shows that grain boundary sliding and intragranular slip are concurrent and independent processes during superplastic deformation.

Visualization and quantitative oxidation kinetic experiments on 100 g samples of Pb-As at 600°C; thermal analysis and phase-equilibrium measurements of Pb-PbO-As2O3 samples under argon over the temperature range of 420°C to 875°C; computational thermodynamic solution modeling; and phase diagram and equilibrium calculations using FACTSage(TM) were performed to elaborate the poorly documented thermodynamics of the slags in the lead softening stage in the pyrometallurgical refining of lead. In the softening stage, the minor element impurities: arsenic, antimony and tin are removed from lead bullion by oxidation and are transferred to a skimmable oxide slag phase. / It was found that optimizing an ionic molten oxide solution model that was conceptualized to contain Pb2+ and O2- with AsO3-4 and AsO3-3 ions, or with SbO3-4 and SbO3-3 ions in the respective PbO rich regions of the Pb-As-O and the Pb-Sb-O systems, was able to accurately reproduce the measured and published thermodynamic data. It was also found that the subsystems in the PbO-As2O 3-As2O5 and PbO-Sb2O3-Sb 2O5 systems showed small deviation from the ideal ionic solution model and small magnitude excess Gibbs energy parameters were sufficient to fit the predicted liquidus curves to the experimental measurements. / Arsenic in the +3 and +5 oxidation states was measured in the PbO rich region of the Pb-As-O liquid solution in the temperature range of 420°C to 875°C. The variability in the ratio of trivalent arsenic to the total arsenic content, as well as the complex variation of arsenic distribution between metal and oxide phases found strong interaction between the lead, arsenic and oxygen atoms at the 3PbO to 1AS2O3 molar ratio thus suggesting a short range ordering corresponding to the formation of AsO3-3 groupings, and indicating that the Pb3(AsO3) 2(l) species was likely to be present in the PbO rich region of the Pb-As-O system and contributing to an understanding of the Pb-As-O liquid oxide structure. Also, two new compounds (Pb3(AsO3) 2(s), Pb2AsO4(s)) were identified in the Pb-PbO-As 2O3 quenched samples via wavelength-dispersive spectrometry using the electron microprobe. The present work has application in commercial oxygen partial lead softening (OPLS), as uniquely practiced at Teck Cominco Ltd., British Columbia. There, pure oxygen gas is injected into the bath of impure bullion through a number of submerged lances in order to oxidize only part of the arsenic, antimony and tin into a slag phase. For such an operating practice, it was concluded from the visualization and quantitative oxidation experiments that the formation of solid oxides as the product of oxidation produced a physical barrier to the progress of oxidation and resulted in the commercially observed, highly-problematic, process initiation issues. When the product was liquid, there was much less of a barrier to rapid oxygen mass transfer to the minor element impurities and the softening reactions were easy to initiate. Such a change in the physical state of the products of oxidation was correlated to the optimized ternary Pb-As-O and Pb-Sb-O phase diagrams. / A current point of interest in partial lead softening is to increase the arsenic content of the slag phase. Arsenic distribution between lead bullion and slag calculated by the optimized solution model of the Pb-As-O system suggests that this can be achieved in a counter-current contacting of the slag and bullion.

Magnesium additions to aluminum enhance strength and hardness, lowering density, and improve corrosion resistance. However, during melting and casting processes, significant amounts of magnesium are lost due to selective oxidation. Preventing these losses would reduce the production cost and improve the quality of the final product. / The effects of various levels of strontium addition on the oxidation behaviour of aluminum-magnesium alloy melts were investigated by monitoring sample weight gains with time using a thermogravimetric balance at 750°C. Sample cross-sections have been examined in detail using Hitachi S-4700 Field Emission Gun Scanning Electron Microscope, and phases formed on the oxide layer and in the alloy morphology were identified by EDS, WDS and low angle X-ray diffraction techniques. It was observed that in the absence of Sr, the Al-Mg samples gained substantial amounts of weight by formation of spinel (MgAl2O4) phase at the oxide-metal interface. Samples containing Sr had significantly lower weight gains. The drop in total weight gain by Sr additions was about 98% in low Mg-containing Al-Mg alloys. This change in oxidation behavior was linked to the presence of a Sr-enriched liquid phase underneath the first formed MgO layer, suppressing the formation of spinel crystals. In addition, spherical equilibrium shapes are found in the different as-cast and oxidized alloys of Al-Mg-Sr.