The selective chlorination of iron from llmenite ore by CO-Cl2 mixtures: Part I. intrinsic kinetics

The intrinsic kinetics of the selective chlorination of iron from ilmenite ore using carbon monoxide as the reducing agent were studied in a shallow fluidized bed. Experiments on the effects of chlorination temperature, carbon monoxide and chlorine gas partial pressures, and particle size were conducted in the absence of mass- and heat-transfer influences. Results indicate that the kinetics in the temperature range 923 to 1123 K are represented by the following pore-blocking rate law: λ[ exp (X_Fe/λ) − 1 ] = 33.7 exp (− E/RT)p _co ^0.52 ₂ ^0.32 t where E is 37.2 kJ/mol and p and t are in atm (=101.3 kPa) and minutes, respectively. The partial pressure of carbon monoxide was found to affect the chlorination rate more strongly than that of chlorine. A reaction mechanism in which iron in ilmenite reacts with chlorine before the liberated oxygen is removed by carbon monoxide is proposed. Formerly Graduate Student at the Department of Metallurgical Engineering, University of Utah     

A mathematical model for the solution mining of primary copper ore: Part I. leaching by oxygen-saturated solution containing no gas bubbles

An unsteady-state, one-dimensional model which simulates the solution mining of a rubblized copper ore body deeply buried below the water table has been developed. Leaching is accomplished by pumping water saturated with oxygen into the bottom of the flooded rubble chimney. The physical processes incorporated in the present model include the axial convective transport of mass and heat, axial dispersion of mass, mass transfer between the bulk fluid and solid surface, and pore diffusion within the ore fragments. The solution withdrawn from the top of the chimney is recycled through the bottom of the chimney, and the temperature of the chimney is allowed to build up by means of the heat generated during leaching. The coupled model equations are solved numerically by an implicit finite-difference method. Model calculations for the leaching process are made for two different modes of operation: (1) constant flow-rate and (2) variable flow-rate of the leach solution during leaching. The calculated results from both modes of operation indicate that thefractional recovery of copper increases with decreasing ore particle size, ore grade, pyrite/chalcopyrite molar ratio, and shape factor. Copper recovery is rather insensitive to the chimney porosity under typical operating conditions.     

Dissolution kinetics of chromium ore in slag system for stainless steelmaking

Abstract

To reveal the smelting reduction mechanism of chromium ore for producing stainless steel in converter, the dissolution behaviour of chromium ore in CaO–SiO₂–MgO–Al₂O₃ slag system was studied by laboratory experiments, and the effects of different temperatures and slag compositions on the dissolution rate of chromium ore in slag were investigated. A kinetic model for the dissolution process of chromium ore was developed for the first time. The dissolution process of chromium ore is controlled by the surface dissolution reaction under the conditions of the present experiments. The effects of temperature, slag composition and particle size of chromium ore on the dissolution rate of chromium ore are relatively great. The chromium ore sand is suggested to be used as raw material in the smelting reduction converter.

Afin de révéler le mécanisme de réduction de la fusion du minerai de chrome pour la production d'acier inoxydable dans un convertisseur, on a étudié le comportement de dissolution du minerai de chrome dans le système de laitier CaO-SiO₂-MgO-Al₂O₃, par des expériences en laboratoire. On a examiné l'effet des différentes températures et compositions de laitier sur la vitesse de dissolution du minerai de chrome dans le laitier. Pour la première fois, on a développé un modèle cinétique du procédé de dissolution du minerai de chrome. Le procédé de dissolution du minerai de chrome est contrôlé par la réaction de dissolution à la surface sous les conditions des présentes expériences. Les effets de la température, de la composition du laitier et de la taille de particule du minerai de chrome sur la vitesse de dissolution du minerai de chrome sont relativement importants. On suggère que le sable de minerai de chrome soit utilisé comme matériau brut dans le convertisseur de réduction de fusion.     

A mathematical model for the solution mining of primary copper ore: Part II. leaching by solution containing oxygen bubbles

The one-dimensional model developed previously to simulate thein situ leaching of copper from deeply-buried low-grade copper ore deposits is used to simulate thein situ operation in which the oxygen-saturated solution containing oxygen bubbles is introduced at the bottom of the chimney. The physical and chemical processes incorporated in the present model include the axial convective transport of mass and heat, axial dispersion of mass, mass transfer between the liquid and gas phases, fluid-solid mass transfer, diffusion of oxygen in the pores of ore fragments, and the dissolution of sulfide minerals. The coupled model equations are solved numerically by an implicit finite-difference method. Calculations have been made for various values of the volume fraction of oxygen bubbles (up to 0.1) in the fluid just downstream of the oxygen sparging nozzle. Calculated results indicate that, for a specific chimney considered, the total amount of copper extracted increases with increasing volume fraction of undissolved oxygen bubbles in the inlet fluid and increasing superficial velocity of the solution (up to 20 m per day). However, a further increase in the superficial velocity of liquid or undissolved oxygen bubbles does not enhance copper extraction. Calculated results also indicate that the total fractional recovery of copper increases with decreasing pyrite to chalcopyrite molar ratio, ore grade, particle size, and shape factor.     

Growth kinetics of solid-liquid Ga interfaces: Part I. Experimental

A technique based on the Seebeck effect was used to determine directly the solid-liquid (S/L) interface supercooling and toin situ monitor the interfacial conditions during growth of high-purity Ga single crystals from a supercooled melt. Using this nonintrusive technique, the growth kinetics of faceted (111) and (001) interfaces were studied as a function of the interface supercooling in the range of 0.2 to 4.6 K, corresponding to bulk supercoolings of about 0.2 to 53 K. In addition, the growth kinetics have been determined as a function of crystal perfection related to the emergence of dislocations at the S/L interface. The results show that at low super-coolings, the faceted interfaces grow with either of the lateral growth mechanisms: two-dimensional nucleation-assisted (2DNG) or screw dislocation-assisted (SDG), depending on the perfection of the interface. At increased interfacial supercoolings, however, both growth rates (2DNG and SDG) become a linear function of the supercooling. Application of the existing growth theories to the experimental results gives only qualitative agreement and fails to predict the observed deviation in the kinetics at high supercoolings. A theoretical treatment of the growth of faceted interfaces will be given in Part II of this series.¹ Formerly Research Assistant with the Department of Materials Science and Engineering, University of Florida. This paper is based on a presentation made in the symposium “The Role of Ledges in Phase Transformations” presented as part of the 1989 Fall Meeting of TMS-MSD, October 1–5, 1989, in Indianapolis, IN, under the auspices of the Phase Transformations Committee of the Materials Science Division, ASM INTERNATIONAL.     

Dissolution of calcified gallstones. Part I. Correlation of in vitro dissolution kinetics in methyl tert-butyl ether with patterns of calcification by computed tomography

RATIONALE AND OBJECTIVES: The authors evaluated the relationship between stone computed tomography (CT) attenuation patterns and the kinetics of dissolution with methyl tertbutyl ether (MTBE). METHODS: Single moderately and heavily calcified gallstones from 40 patients were selected from a gallstone library and classified for pattern of calcification by in vitro CT scan (dense, rim, core, and laminated). Each stone was placed in a 10-mL aliquot of MTBE for 24 hours. Stone residue was blotted dry and weighted at 8, 16, and 24 hours. Results were normalized with respect to stone size. RESULTS: Only 1 of 40 (4%) specimens dissolved to particulate matter that was smaller than 2 mm. All (6 of 6) stones that were densely calcified showed virtually no dissolution. The rate of gallstone dissolution varied temporally within the rim, core, and laminated stone categories and was related to the composition of the layer exposed to the solvent at any given time. CONCLUSION: The success and rate of dissolution may be predicted by the pattern of calcification as determined by computed tomography (CT).     

Dissolution kinetics of zinc silicate (hemimorphite) in ammoniacal solution

The influence of ammonia-ammonium ratio, specific surface area of particles, associated anions, temperature and total ammonia concentration on the dissolution kinetics of zinc silicate (hemimorphite) has been investigated in ammoniacal solutions. The results show that the initial dissolution rate increases with the specific surface area, temperature and total ammonia concentration. The associated anions have a significant influence on the zinc extraction from hemimorphite due to their different complexing affinity with zinc ion. SEM analysis shows that cleavage of hemimorphite appears during dissolution. The silica formed in the dissolution process is absorbed back and covers some active sites of the particles. The Elovich equation typifies the dissolution behavior of hemimorphite in ammoniacal solution with an activation energy of 57.6 kJ/mol which is characteristic for a chemically controlled process.     

Dissolution of uranium from silicate-apatite ore by Acidithiobacillus ferrooxidans

Bioleaching of a low-grade Indian silicate-apatite uranium ore containing 0.024% U₃O₈ and 10.6% iron with minor amounts of base metals has been reported. The studies involved extraction of uranium using enriched culture containing Acidithiobacillus ferrooxidans (A. ferrooxidans) derived from the source mine water employing bio-chemically generated ferric ion as an oxidant. Parameters such as particle size of the ore, pulp density, and pH of lixiviant media were optimised. Maximum uranium bio-dissolution of 98% was achieved using ore of mixed particles of < 76 μm size. Uranium bio-recovery was found to be 96% at the pulp density (PD) of 10% (w/v) and 20% (w/v) with the particles of < 76 μm size in 40 days at 2.0 pH and 35 °C temperature. At 1.7 pH and 20% (w/v) PD, 98% uranium bio-recovery was achieved with a rise in redox potential from 595 mV to 715 mV in 40 days. Uranium bio-dissolution may be correlated with the generation of ferric ions through the bio-chemical action on the ore. The work illustrated the efficacy of leaching of uranium by the involvement of bacteria by indirect mechanism.     

Milling dynamics: Part I. Attritor dynamics: Results of a cinematographic study

The motions of grinding media and powder in an attritor canister were studied by means of filming the agitated charge and frame-by-frame scrutiny of the footage. In conjunction with auxiliary experiments, this permitted semiquantitative analysis of the milling action. In particular, the mill can be divided into several regions characterized by different balances between direct impacts and rolling/sliding of the grinding media. Simple calculations suggest that impacts are more capable of effecting mechanical alloying (MA) than are rolling or sliding events in an attritor. Powder circulation within an operating mill was also investigated. Based on the results and the accompanying analysis, concepts for improved attritor design are presented. D. Maurice is formerly a Graduate Student, Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22903     

Reduction of phosphate ore by carbon: Part II. Rate limiting steps

Considerations of intraparticle mass transport processes are presented for the reduction of phosphate ore by carbon in the presence of silica. Under conditions of temperature and composition which produce reaction rates of commercial interest, the conversion is limited by product mass transport away from the reaction sites. This results in a pseudo-first order dependence of conversion on reaction time which is explained by our model. Thermochemical data derived from conversion is consistent with published data on heat of formation. At lower temperatures, where the reaction is limited by reactant diffusion in a silicate melt, contained within the overall matrix, diffusivities and activation energy were measured. Detailed diffusion studies were conducted in a graphite crucible where the interfacial reaction between the apatite-silica mixture and the graphite boundary could be detected. A mathematical model based on diffusion theory was developed, and it was found to be consistent with the diffusion data and the overall conversion at lower temperature.     

Optimization and continuous casting: Part I. Problem formulation and solution strategy

Optimization strategies determine process parameters that maximize or minimize some aspect of a process (the objective), while ensuring that the process operates within established limits or constraints. The optimization problem is formulated as a constrained, nonlinear programing (NLP) problem, is solved using successive quadratic programing (SQP), and is applied to the continuous casting of steel. The process status and constraints are evaluated with the aid of a heat flow and solidification model. In this study, we also present the casting objectives of maximum casting rate and formulate the casting constraints. These constraints can represent product quality and process feasibility through limits on strand shell thickness, metallurgical length, maximum heating and cooling rate, casting rate, and reheating of the strand surface. The constraints formulated are representative of a range of typical caster mechanical, thermal, and solidification constraints. To illustrate the capabilities of the method, a small casting example previously reported in the literature is solved using SQP. It is shown that considerable attention must be paid to the formulation in order to derive smooth objective and constraint functions for SQP. An important advantage of this solution technique is that the formulation contains casting rate and spray cooling parameters and that the solution satisfies all constraints. The results are achieved in a single solution to the nonlinear constrained optimization problem. The solution of more complex caster optimization problems is reported in Part II.^[22] Formerly with the Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University.     

Macrotransport-solidification kinetics modeling of equiaxed dendritic growth: Part I. Model development and discussion

An analytical model that describes solidification of equiaxed dendrites has been developed for use in solidification kinetics-macrotransport modeling. It relaxes some of the assumptions made in previous models, such as the Dustin-Kurz, Rappaz-Thevoz, and Kanetkar-Stefanescu models. It is assumed that nuclei grow as unperturbed spheres until the radius of the sphere becomes larger than the minimum radius of instability. Then, growth of the dendrites is related to morphological instability and is calculated as a function of melt undercooling around the dendrite tips, which is controlled by the bulk temperature and the intrinsic volume average concentration of the liquid phase. When the general morphology of equiaxed dendrites is considered, the evolution of the fraction of solid is related to the interdendritic branching and dynamic coarsening (through the evolution of the specific interfacial areas) and to the topology and movement of the dendrite envelope (through the tip growth velocity and dendrite shape factor). The particular case of this model is the model for globulitic dendrite. The intrinsic volume average liquid concentration and bulk temperature are obtained from an overall solute and thermal balance around a growing equiaxed dendritic grain within a spherical closed system. Overall solute balance in the integral form is obtained by a complete analytical solution of the diffusion field in both liquid and solid phases. The bulk temperature is obtained from the solution of the macrotrasport-solidification kinetics problem.     

Oxidation of cobaltite: Part II. kinetics

A kinetics study has been performed on cobaltite to understand the oxidation processes over a temperature range of 573 to 1173 K using a thermogravimetric method. The results show that oxidation of cobaltite occurs in two stages. In the first stage, which occurs between 823 and 913 K, the majority of the sulfur is removed. However, the arsenic remains in the lattice of the reacted region. A pore-blocking kinetic model yields a satisfactory fit to these experimental data. At higher temperatures, there is a concurrent release of As and S from the crystal lattice of CoAsS. The shrinking-core kinetic model is applicable. Complementary X-ray diffraction and scanning electron microscopic analyses on these partially oxidized samples support the kinetic models. The effects of partial pressure of oxygen and particle size on roasting have been evaluated.     

Infiltration of fibrous preforms by a pure metal: Part I. Theory

General expressions are derived to describe fluid flow and heat transfer during infiltration of fibrous preforms by a pure metal. Analytical solutions to the problem are given for the case of unidirectional infiltration into a uniform preform of aligned fibers under constant applied pressure. Calculations are carried out for infiltration kinetics (including total infiltrated length) and temperature distribution, using as an example alumina fiber/aluminum composites. Limiting cases leads to very simple expressions. Initial fiber temperatures both above and below the metal melting point are considered. In the case of fibers at a temperature significantly below the metal melting point, it is concluded that the factor most strongly influencing infiltration is the solidification of metal in the interfiber region. In the calculations, it is assumed that this solidification is in the form of a uniform solid metal sheath around the fibers. Metal superheat, when present, serves to progressively remelt the solidified sheath from the upstream end of the preform. Fiber volume fraction and initial temperature are predicted to have a major effect on infiltration kinetics, while metal superheat exerts a relatively minor influence. When no external heat extraction is present and a constant pressure is applied to the metal, flow through the preform continues indefinitely. For the case of external heat extraction, flow ceases when sufficient solidification occurs to block flow. L.J. MASUR, formerly a Graduate Student with the Department of Materials Science and Engineering, Massachusetts Institute of Technology     

Infiltration of fiber preforms by a binary alloy: Part I. Theory

During infiltration of a fiber preform by a binary hypoeutectic alloy, solid metal can form in the composite because of cooling at the fibers or at the mold wall. Contrary to the case of an unalloyed matrix, temperature, composition, and fraction solid may vary in the composite. This results in macrosegregation and microstructural heterogeneity within the composite casting. It is shown that solid metal that forms because of cooling at the fibers grows gradually behind the infiltration front, while the local temperature increases. Metal superheat, when present, serves to progressively remelt solid metal in the composite during infiltration and increases compositional and microstructural heterogeneity within the composite. General expressions are derived to describe heat, mass, and fluid flow during the infiltration process. In the case of unidirectional adiabatic infiltration driven by a constant applied pressure, a similarity method can be used to reduce the mathematical complexity of the problem. Numerical solution of the resulting equations then allows us to predict temperature, fraction solid, and composition profiles within the composite. With the further assumption of negligible thermal conduction, the problem lends itself to an analytical solution. The analysis is performed for the case of unidirectional adiabatic infiltration under constant applied pressure of 24 vol pct δ-alumina preforms by Al-4.5 wt pct Cu. Results indicate that there is significant latitude for control of macro-segregation and microstructure within cast fiber-reinforced alloys.     

Mechanical behavior and damage kinetics in nodular cast iron: Part I. Damage mechanisms

Numerous authors consider that in nodular cast iron, the interface between the graphitic nodule and the α-matrix is so weak that nucleation appears suddenly at the beginning of plastic strain. Nevertheless, the present experimental study indicates that the nucleation of voids is a continuous process as a function of the imposed mechanical parameters (Σ _m , ɛ _peq ). This result is explained in terms of interaction effects between closely spaced particles. A critical parameter for interaction is determined. Growth kinetics are also investigated and comparison with the Gurson-Tvergaard law allows identification of the q1 and q2 parameters of the potential. A critical void volume fraction f ^c is finally defined and is shown to depend on the stress triaxiality level. At last, the role of the various microstructural heterogeneity levels on the damage kinetics is emphasized. Then, it is clearly demonstrated that all these heterogeneity levels must be considered to estimate a critical interfacial stress.     

Pressure acid leaching of arid-region nickel laterite ore: Part II. Effect of ore type

A comparison of the leach chemistry and residue mineralogy has been carried out on the pressure acid leaching of nontronite, limonite and saprolite ores, using hypersaline water. Results are also compared with a typical arid-region laterite feed from Bulong, which consists of a blend of these ore types. Particular emphasis is placed on the influence of ore type on liquor analysis of iron, aluminium and magnesium, residue mineralogy and nickel extraction. Microprobe evidence is presented that incomplete nickel extraction results from the presence of unreacted minor phases present in the original ore, or from the presence of nickel in the amorphous silica, in apparent association with magnesium.     

Pressure acid leaching of arid-region nickel laterite ore: Part I: effect of water quality

The effect of water salinity on the reactions occurring during pressure acid leaching of an arid-region laterite ore, using hypersaline water, seawater, sub-potable water and tap water, is examined. Particular emphasis is placed on the mineralogy of the residue and its implications with regard to residue volume/mass, overall acid consumption and nickel extraction. Analysis of a pressure acid leach residue by electron microprobe indicates that the residual nickel is present in phases that contain silicon and varying concentrations of aluminium, but are deficient in sulphur. Incomplete extraction of nickel from the ore may not be attributed to any one mineral phase.