In recent years, the Knelson Concentrator has become the predominant unit used for primary gold recovery by gravity. However, its application potential as a cleaner in the final gravity concentration stage and its separation mechanisms have not been well studied. In most Canadian gold mills, rougher gravity concentrates produced by the Knelson (operating at 60 gravity acceleration or 'g') are further upgraded with shaking tables, non-centrifuge units operating at one 'g'. As a result, tabling results in significant gold losses (mainly fines). It is also labour intensive and represents a security risk because of the high gold content of the material being processed. The main objectives of this research, therefore, are to study the separation mechanisms of the Knelson Concentrator and to explore its potential application as a cleaner to supplement, or even to replace, tabling. / A methodology based on the use of synthetic feeds (tungsten, silica and magnetite) was designed to study the performance and mechanisms of a 7.5 cm Knelson. Four types of gangue were used, both fine and coarse silica and magnetite. The fluidization water flowrate was optimized for each gangue type, and the Knelson was then overloaded by feeding a high-grade feed. The extent of overload was found to be highly dependent on the size distribution and density of the gangue processed, from severe and immediate with coarse magnetite to non-existent with fine silica. A two-parameter model to describe the overload was derived, which showed a good fit with the experimental data. / Two separable bowls were designed to analyze the nature of the material recovered in the five grooves of the Knelson bowl. It was found that the densest minerals, such as tungsten and gold, were mainly recovered in the inner groove sections, closest to the rotating axis. The outer sections packed at the beginning of the recovery cycle and served mostly to distribute fluidization water evenly. / Three modified Knelson bowls, a cleaner bowl equipped with a filter at the bottom of each groove, the other two with different groove depth and fluidization water flowrate distribution, were tested using the synthetic feed methodology. Results generally confirmed the soundness of the basic designs of the conventional and cleaning bowls. The importance of superficial fluidization water velocity was also demonstrated. / Recovery of gold with the 7.5 cm Knelson from seven samples of gold table tails from different Canadian gold mills was tested. Recovery significantly increased when the coarser, largely barren fraction was removed prior to processing. Concentrate grade significantly increased, up to 60% Au, by feeding a magnetite "filler" prior to the recovery cycle, and removing it magnetically at the end of the test, to mimic the effect of the cleaner bowl.

In the present study, a fundamental theoretical and experimental investigation has been carried out on the mechanisms of heterogeneous nucleation of grains and pores in aluminum castings. A direct addition technique has been developed to introduce known types and quantities of inoculants into liquid aluminum alloys, irrespective of their wettability and chemical reactivity while preserving the surface characteristics and melt chemistry. Many different types of inoculants such as: $ rm Al sb2O sb3$, SiC, MgO, $ rm Mg sb2AlO sb4$, TiB$ sb2$, TiC, SrO and Sr(OH)$ sb2$ have been successfully added into liquid aluminum alloys, yielding single particulate distributions while avoiding incorporation of naturally occuring oxide films. / The commercial grain refining practice of Al and its alloys has been experimentally simulated by introducing synthetic TiB$ sb2$ and TiC crystals into melts containing dissolved Ti. Experimental findings indicate that in the absence of dissolved Ti, TiB$ sb2$ crystallites alone do not nucleate $ alpha$-Al. TiC particles which are generally believed to be the nucleating substrate are unstable and form various complex carbides. In the presence of dissolved Ti even below the peritectic level, an interfacial layer of TiAl$ sb3$ is formed at the TiB$ sb2$/melt interface which subsequently nucleates the $ alpha$-Al. A 'duplex' nucleation mechanism is proposed based on the solute segregation phenomenon to the substrate/melt interface. In the case of hypoeutectic Al-Si alloy, this interfacial layer was found to be a ternary compound of Al-Si-Ti, however, a drastic drop in the peritectic solidification temperature presumably reduces its grain refining potency at higher Si content. / Particles which do not nucleate the solid phase and/or do not get engulfed by the growing solid, are continuously rejected by the solid/liquid (S/L) interface until the end of local solidification. These substrates act as a barrier to the fluid flow as well as to the diffusion field at the S/L interface, giving rise to enhanced gas segregation and viscous pressure drop. A novel theoretical mechanism for the heterogeneous nucleation of pores has been proposed, based on this behaviour of foreign particles at the advancing S/L interface. Mathematical analyses have been employed to predict the gas segregation and pressure drop in the gap between the particle and the S/L interface. An order of magnitude analysis is done, and it is shown that pressures in the range of the activation barrier can be obtained in normal castings. To substantiate the mechanism further, experimental studies were carried out by introducing various possible inclusions into liquid aluminum. The experimental findings are in line with the theoretical predictions.

A computational modelling study has been undertaken for analysing the complex turbulent melt flow, involving convective and conductive heat transfer in the melt pool and the solidifying mushy-region within the wedge-shaped region of a vertical twin-roll stainless steel caster. / An essential feature of the model is the incorporation of the body-fitted curvilinear coordinate transformation method for correctly modelling the arbitrary shaped region of the caster in the presence of non-isothermal solidification. The model can also treat an obstacle such as a submerged nozzle within the domain. The dendritic columnar solidification of molten metal has been modelled through the implementation of the enthalpy-porosity technique. The transformed curvilinear momentum and energy transport equations were suitably non-dimensionalized and later solved for the physical variables, using a control-volume finite difference scheme. The pressure-velocity coupling of the momentum equations were resolved through a modification of the well-established SIMPLER algorithm. / In the absence of such previous studies for stainless steel, either mathematical or experimental, comparisons have been made of the present model with the experimental work carried out within Nippon Steel Corporation as well as with experimental studies of a similar system for a tin-lead alloy. Predictions based on the present model are in good agreement with experimental results. / Various parametric studies have been carried out for the important variables of the twin-roll casting system, and their effects on the turbulent velocity fields, temperature distributions within the melt, solidification profiles and the extent of the mushy region, have been predicted. The present results lead to many suggestions for the design of twin-roll casting systems.

The distribution of fluidization flow of a variable speed 3-in (7.5 cm) Knelson centrifugal concentrator was first studied. Its performance was then investigated for different types of synthetic ores, and the effect of rotation speed, fluidizing water, gangue density and size distribution, and feed rate determined. To obtain some fundamental information relative to the recovery mechanisms of the Knelson Concentrator, the percolation or migration of dense particles in a gangue bed was investigated in the gravitational field, using a fluidization column. The equations for determining the instantaneous radial settling velocity of a spherical particle in the dilute zone of the inner bowl were derived for the Stokes' Law region. / Recovery of tungsten from magnetite and silica gangues and recovery of magnetite from silica gangue were studied. Results indicate that rotating speed of the Knelson Concentrator affected both the movement of mineral particles and fluidization of the separation zone, which in turn affected KC performance. For all types of feeds studied, the fluidizing water flow rate needed to achieve a maximum recovery increased with increasing rotating speed. The study of capture kinetics showed that tungsten recovery from silica decreased gradually and almost linearly as the feed rate increased from 0.5 to 5 kg/min; this decrease became smaller with increasing Gs. However, the recovery drop (or the corresponding capture rate constant) was small for coarse (>106 mum) tungsten and moderate for fine (<106 mum) tungsten, suggesting that for typical applications of gold recovery from the circulating load of a grinding circuit, feed rate should be maximized. Further, accelerations of 30 to 60 Gs were adequate for very good recovery of tungsten above 25 mum. The recovery of magnetite from a -425 mum silica gangue was limited to a d 50c of 40 mum. Rotation velocity had little impact on how fine magnetite could be recovered, especially above the velocity corresponding to 60 Gs. / At 1 Gs (i.e. in the gravitational field), an optimum fluidization flow was also observed for all systems studied. High density gangue, especially when coarse, had a detrimental effect on percolation and migration. Particles of high density percolated or migrated faster than those of lower density. When the gangue bed was well fluidized, the migration rate of coarse particles was higher than the percolation rate of fines. These observations are basically consistent with the effect of fluidization water, gangue size distribution and density on Knelson performance.

Applicability of Monte-Carlo method for the textural and microstructural changes during annealing processes is studied. Models that are developed for explaining annealing processes are incorporated into Monte-Carlo procedures and the textural and microstructural changes are then followed. The simulated results are compared with the experimental observations to validate the proposed models, The three exemplary annealing processes selected for the present work are industrially important and diversified in their annealing characteristics, These are: (I) Cube {100}&lang;001&rang; texture development during recrystallization of Al, (II) Abnormal grain growth of Goss {110}&lang;001&rang; grains in Fe-Si steels, (III) The competition between (111) and (100)-fiber textures during annealing of nanocrystalline Ni and Ni-Fe alloy electrodeposits. The model for each annealing process is developed by analyzing the relevant metallurgical information obtained from the corresponding metals and alloys prior to annealing, / In Application-I, the texture and orientation-dependent stored energy measurements have been made on the cold rolled can-body aluminum alloy and the detailed discussion is presented, These results are analyzed for developing nucleation and growth models that would lead to the final cube texture formation during primary recrystallization of aluminum. The proposed recrystallization model for aluminum predicts the development of cube texture and equiaxed grain structure as observed in experiments. / For the abnormal grain growth in Fe-Si steel (Application-II), the role of high energy grain boundaries has been studied in detail using separate computer experiments to understand the importance of fraction of high mobility grain boundaries. From these experiments, it has been proved that the assumption of high mobility to CSL boundaries for the abnormal growth of Goss grains is not valid. / Finally, the Monte-Carlo procedure is employed for testing the proposed model for the texture competition between (111) and (100)-fibers in nanocrystalline Ni, Ni-20% Fe and Ni-45% Fe electrodeposits (Application-III). Based on the texture analysis, the proposed model assumes high mobility to the grains boundaries, which are in high non-equilibrium states. The results of the Monte-Carlo simulation using the proposed model are compared with experiments, The simulated results show that (111)-fiber grows faster than (100)-fiber as annealing progresses. (Abstract shortened by UMI.)

A novel process to efficiently produce anhydrous, oxide free MgCl 2 feed to a magnesium electrolysis cell is presented and examined in this thesis. The process involves reducing MgCl2 activity within a fused salt mixture via complexation and reacting any contaminating oxides that are mostly MgO with HCl(g). This process would not be possible nor economical if the MgCl2 activity was not reduced via complexation. This system is modeled here by treating the complexation as decreasing the MgCl2 activity coefficient in a fused salt mixture. / The chlorination was modeled thermodynamically using recently published thermodynamic data presented by Thompson et al. The model was experimentally verified. A kinetic model was also developed for the heterogeneous reaction between injected HCl gas and the MgO in fused salt. It was found that the rate of MgO elimination was proportional to C2/3MgO in the fused salt during the course of a kinetic batch test. The implication is that the rate of transport per unit area of MgO, of HCl to the MgO surface is constant for any particular condition which is only possible if the mass transport of HCl to the MgO particle surface is rate limiting. / The kinetics of MgO conversion were measured as a function of superficial gas velocity and mixing power per unit volume. The following relation empirically describes the conversion rate per unit area of MgO particle surface area wt%/min/m 2 for a given operating condition. km=0.001895Pg Vm0.8 3Vst 0.92 / Analysis of the kinetic information, confirmed the hypothesis for the mechanism of MgO elimination. / Measurements of the solubility of HCl(g) in fused salts were also taken. The findings show that the solubility of HCl varied between 270 ppmw to 2760 ppmw when the MgCl2 concentration was increased between 20 wt% to 70 wt% at 600°--650°C and a NaCl to CaCl2 weight ratio of 2.5--3.1. A mechanism for dissolution is presented.

The study emphasizes the effects of multi-pass narrow gap (NG) Submerged-Arc welding (SAW) on the Heat Affected Zone (HAZ). The differences between the HAZ of single-pass welds and multi-pass welds are radical and are caused by the subsequent thermal cycles which only exist in multi-pass welding. / A series of experiments were carried out to establish the relationship between the welding thermal cycles and the mechanical and microstructural properties of a HAZ. Both real and Gleeble simulated HAZ's were investigated. / Theoretical and experimental analyses indicate that the HAZ's in multi-pass welds can be improved significantly by the welding process itself. However, multi-pass welding does not always improve its HAZ. To realize the improvement, some special criteria must be met. The most important parameters are heat input, welding speed, deposit thickness and inter-pass temperature. This study establishes the relations between these parameters and the effects of HAZ refinement. There are some domains of the parameters in which HAZ refinement can be realized. The research reveals that in multi-pass NG welding, a set of welding parameters can always be found to fulfill the conditions for HAZ improvement. / Real welding processes, with welding parameters optimized according to the analyses, were performed and a totally refined HAZ was achieved under laboratory conditions. It is suggested that the method can be applied to in situ welding situations. / High heat input does not necessarily lead to inferior microstructural and mechanical properties in multi-pass NG welding. As long as the conditions for HAZ refinement are satisfied, an improved HAZ will be obtained no matter how high the heat input is. This also leads to the conclusion that the HAZ in multi-pass NG welding is less sensitive to heat input than that in single pass welding. / The tempering parameter, which has been used to evaluate the tempering effects at constant temperature, is successfully introduced into welding (non-isothermal) conditions. The effects of precipitates in 2.25Cr-1Mo steel are theoretically investigated.

The evolution of advanced ceramic materials over the past two decades has not been matched by improvements in ceramic joining science and technology, particularly for high temperature applications. Of the techniques being evaluated for joining ceramics, brazing has been found to be the simplest and most promising method of fabricating both ceramic/ceramic and ceramic/metal joints. A key factor in ceramic brazing is wetting of the ceramic by the filler metal. / This study deals with the application of brazing for the fabrication of $ rm Si sb3N sb4/Si sb3N sb4$ and $ rm Si sb3N sb4/Mo$ joints using Ni-Cr-Si brazing alloys based on AWS BNi-5 (Ni-18Cr-19Si atom%). Thermodynamic calculations were performed to predict wetting at $ rm Si sb3N sb4$/Ni-Cr-Si alloys interfaces. By using some simplifying assumptions and suitable scaling of the reaction, the model predicted that Ni-Cr-Si alloys with Ni/Cr = 3.5 and X$ sb{ rm Si}$ $<$ 0.25 would react chemically with and wet $ rm Si sb3N sb4$. Good agreement was found between the theoretical calculations and experimental results. / Brazing experiments were carried out to study the joinability of $ rm Si sb3N sb4$ with various Ni-Cr-Si filler metals which had already shown good wetting characteristics on $ rm Si sb3N sb4$. The $ rm Si sb3N sb4/Si sb3N sb4$ joints formed with a 10 atom% Si brazing alloy exhibited the highest strength ($ approx$120 MPa) which was mainly due to the presence of a CrN reaction layer at the ceramic/filler metal interface. The high temperature four-point bend strengths of $ rm Si sb3N sb4/Si sb3N sb4$ joints were markedly higher than the room temperature values. A high strength of about 220 MPa was achieved when the joints were tested at 900$ sp circ$C. / From the results of the $ rm Si sb3N sb4/Mo$ joining experiments it was found that the joint quality and microstructure were strongly influenced by the composition of the filler metal and such brazing variables as time and temperature. Of all the $ rm Si sb3N sb4$/Mo joints, those made with the S10 brazing alloy at 1300$ sp circ$C for 1 min. exhibited the highest strength of 55 MPa. / Finally, in all the cases, the shear strength of all the joints was found to be lower than their four-point bend values.

No description available.

The present work attempts to determine the feasibility of the accelerometer as an end-point control sensor for the basic oxygen steelmaking process. In this study, two series of laboratory scale BOF simulations were performed. In the first series, the accelerometer was sampled at low frequency to obtain an amplitude-time relation that can be related to the rate of decarburization during the oxygen blow. In contrast, the accelerometer was sampled at high frequency in the second series in order to discern the presence of specific vibrational frequencies that can be related to the process. As a final aspect of the research work, several high frequency simulations were terminated prematurely in an attempt to elucidate the carbon content of the bath at the point in time when the accelerometer can clearly detect the final change in the rate of decarburization.