Kinetics of the zinc slag-Fuming process: Part i. industrial measurements

A study involving industrial measurements and mathematical modeling has been conducted to eluci-date kinetic phenomena in the zinc slag fuming process. In the first part of this three-part paper, the results of industrial measurements and observations are presented. In Part II a mathematical model of the process is developed, and finally in Part III the implications of a kinetic conception of the process for process improvement are explored. The industrial work consisted primarily of slag sampling through the fuming cycles of five different fuming operations. In addition, tuyere back-pressure mea-surements, tuyere photography using a tuyerescope, and sampling of the fume product were under-taken at one operation. Analysis of the slag samples has shown that, in general, the zinc elimination curve is linear with time and that a portion of the injected coal entrains in the slag. Analysis of tuyere back-pressure fluctuations and movie photographs of the tuyere tip indicate that the coal-air mixture enters the slag in the form of discrete bubbles. From these results it can be deduced that the fuming furnace consists of two reaction zones which are created by the division of coal between the slag and the tuyere gas stream. The coal entrained in the slag reduces ZnO and Fe₃O₄ in a “reduction zone” which is responsible for fuming. The coal remaining in the tuyere gas stream combusts in an “oxidation zone” although a fraction passes through the bath unconsumed and reports to the solid products. The oxidation zone supplies heat to the endothermic reduction reactions and heat losses. Formerly Graduate Student     

Kinetics of the zinc slag-Fuming Process: part II. mathematical model

A mathematical model of zinc slag fuming has been formulated based on the kinetic conception of the process developed in Part I of this paper. Each of the major reaction zones in the furnace — the slag bath where reduction of zinc oxide and ferric oxide takes place and the tuyere gas column where oxidation of coal and ferrous oxide occurs — have been characterized mathematically. The two zones and the water-jacketed furnace wall have been linked by overall heat and mass balances. Insufficient information is available, however, to characterize quantitatively two of the important kinetic processes occurring in the furnace: the division of coal between entrainment in the slag, combustion in the tuyere gas column and bypass; and oxygen utilization. To overcome this problem the model has been fitted to the data from eleven industrial fuming cycles. Consistent values have been obtained for these kinetic parameters over five different fuming operations indicating that the kinetic conception of the process is sound. The results indicate that about 33 pct of the injected coal is entrained in the slag, 55 pet combusts in the tuyere gas column, and 12 pct bypasses the bath completely. Oxygen utilization has been found to be high and can be correlated to bath depth. Formerly Graduate Student     

Manganese metallurgy review. Part III: Manganese control in zinc and copper electrolytes

Manganese is often associated with zinc and copper minerals, and can build up in the processing circuits. Part III of the review outlines the current practice and new developments to get a better understanding of manganese behaviour and control in electrowinning of zinc and copper, and identifies suitable methods and processes to control manganese.In zinc electrowinning, the presence of small amounts of manganese (1–5 g/L) can minimise the corrosion rate of the anodes and reduce the contamination of the cathodic zinc with lead, but excess manganese results in significant decreases in the current efficiency. The neutralized zinc feed solution that contains little acid is considered to be the best place to implement manganese control. Various methods and processes for manganese control in zinc electrowinning have been developed. Oxidative precipitation and solvent extraction are the most important methods. For the neutralized zinc solution at pH 5, oxidative precipitation using a strong oxidant such as Caro's acid and SO₂/O₂ can selectively precipitate manganese as insoluble MnO₂ or Mn(OOH), leaving other impurities, e.g., Mg, Cl⁻, F⁻, etc. in the circuit. Solvent extraction of zinc using D2EHPA (di-2-ethylhexyl phosphoric acid) can selectively recover zinc from the solution and leave other impurities including manganese in the raffinate.In copper solvent and electrowinning circuits, the problem of manganese is mainly associated with the decrease in the current efficiency and degradation of the solvent caused by the higher valent manganese species generated on the anode. The prevention or minimisation of Mn(II) oxidation during the electrowinning is critical. This can be achieved by adding ferrous ions or sulfur dioxide to control the cell potential.     

Correlational tests of predictions from a process model of the interview.

We conducted a field study to test eight propositions derived from a process model of the selection interview (Dipboye, 1982; Dipboye & Macan, 1988). According to the model, interviewers' preinterview impressions of an applicant bias the subsequent conduct of the interview and processing of information in the direction of confirming these initial impressions. To test predictions from the model, we surveyed managers and the applicants they interviewed in each of 164 interviews. In support of the model, interviewers' preinterview evaluations were positively related to postinterview evaluations of applicant qualifications and process variables predicted to mediate this relation. Results also supported the model in that interviewers with favorable preinterview impressions were more likely to attribute good interview performances to the applicants' qualifications for the job and poor performances to external factors. Contrary to the model, confidence failed to moderate the above findings, and preinterview impressions were not predictive of applicant reports of interviewers' time spent in questioning. Some possible implications of the model for future research and for improving interview practice are discussed. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Microstructural engineering applied to the controlled cooling of steel wire rod: Part III. Mathematical model-formulation and predictions

In this final part of the study, a mathematical model incorporating heat flow, microstructural phenomena, and structure-composition-mechanical property relationships has been developed to compute the yield strength (YS) and ultimate tensile strength (UTS) of steel rod control cooled on a Stelmor line. The predictive capability of the model, in terms of temperature response, microstructural evolution, and strength of the rods, has been tested by comparison to measurements from an extensive set of laboratory and plant trials. Thus, the model has been shown to simulate the complex heat flow and microstructural phenomena in the steel rod very well, although improvements need to be sought in the characterization of the austenite-ferrite transformation kinetics and of pearlite interlamellar spacing. The latter variable has a significant influence on the strength of eutectoid steels. Nonetheless, the model consistently is capable of predicting the strengths of plain-carbon steel rods ranging from 1020 to 1080 to within ± 10 pet. Formerly Graduate Student, The University of British Columbia.     

A general mechanism of martensitic nucleation: Part III. Kinetics of martensitic nucleation

The growth of martensitic fault embryos in the fault plane, the development of their in-terfacial structure, and the thickening of the embryos normal to the fault plane are ex-amined as possible rate limiting steps in the total martensitic nucleation process. Growth of the embryos in the fault plane appears the most probable rate limiting step, capable of accounting for both the observed isothermal and athermal kinetic behavior depending on the parameters (such as activation volume) which control the motion of the transformational dislocations. The thermally activated nucleation of dislocation loops responsible for lattice invariant deformations is a possible rate limiting step for some isothermal transformations, though such deformations are not required for all marten-sitic transformations. Embryo thickening by the nucleation of discrete loops of trans-formation dislocations appears improbable in bulk material; instead, a plausible pole mechanism for embryo thickening is presented which incorporates existing “forest≓ dislocations intersected by embryos growing in the fault plane. Lattice softening phe-nomena may lower the critical chemical driving force for nucleation, but are not essen-tial for martensitic nucleation by the proposed faulting mechanism. This paper is Part III of a three-part series based on a thesis sub-mitted by G. B. Olson for the degree of Sc.D. in Metallurgy at the Massachusetts Institute of Technology in June 1974.     

A general mechanism of martensitic nucleation: Part III. Kinetics of martensitic nucleation

The growth of martensitic fault embryos in the fault plane, the development of their interfacial structure, and the thickening of the embryos normal to the fault plane are examined as possible rate limiting steps in the total martensitic nucleation process. Growth of the embryos in the fault plane appears the most probable rate limiting step, capable of accounting for both the observed isothermal and athermal kinetic behavior depending on the parameters (such as activation volume) which control the motion of the transformational dislocations. The thermally activated nucleation of dislocation loops responsible for lattice invariant deformations is a possible rate limiting step for some isothermal transformations, though such deformations are not required for all martensitic transformations. Embryo thickening by the nucleation of discrete loops of transformation dislocations appears improbable in bulk material; instead, a plausible pole mechanism for embryo thickening is presented which incorporates existing “forest” dislocations intersected by embryos growing in the fault plane. Lattice softening phenomena may lower the critical chemical driving force for nucleation, but are not essential for martensitic nucleation by the proposed faulting mechanism. This paper is Part III of a three-part series based on a thesis submitted by G. B. Olson for the degree of Sc.D. in Metallurgy at the Massachusetts Institute of Technology in June 1974.     

The modified quasi-chemical model: Part III. Two sublattices

The modified quasi-chemical model in the pair approximation for short-range ordering (SRO) in liquids is extended to solutions with two sublattices. Short-range ordering of nearest-neighbor pairs is treated, and the effect of second-nearest-neighbor (SNN) interactions upon this ordering is taken into account. The model also applies to solid solutions, if the number of lattice sites and coordination numbers are held constant. It may be combined with the compound-energy formalism to treat a wide variety of solution types. A significant computational simplification is achieved by formally treating the nearest-neighbor pairs as the “components” of the solution. The model is applied to an evaluation/optimization of the phase diagram of the Li,Na,K/F,Cl,SO₄ system.     

Thermal analysis of the arc welding process: Part I. General solutions

An analytical solution for the temperature-rise distribution in arc welding of short workpieces is developed based on the classical Jaeger’s moving heat-source theory to predict the transient thermal response. It, thus, complements the pioneering work of Rosenthal and his colleagues (and others who extended that work), which addresses quasi-stationary moving heat-source problems. The arc beam is considered as a moving plane (disc) heat source with a pseudo-Gaussian distribution of heat intensity, based on the work of Goldak et al. It is a general solution (both transient and quasi-steady state) in that it can determine the temperature-rise distribution in and around the arc beam heat source, as well as the width and depth of the melt pool (MP) and the heat-affected zone (HAZ) in welding short lengths, where quasi-stationary conditions may not have been established. A comparative study is made of the analytical approach of the transient analysis presented here with the finite-element modeling of arc welding by Tekriwal and Mazumder. The analytical model developed can determine the time required for reaching quasi-steady state and solve the equation for the temperature distribution, be it transient or quasi-steady state. It can also calculate the temperature on the surface as well as with respect to the depth at all points, including those very close to the heat source. While some agreement was found between the results of the analytical work and those of the finite-element method (FEM) model, there were differences identified due to differences in the methods of approach, the selection of the boundary conditions, the need to consider image heat sources, and the effect of variable thermal properties with temperature. The analysis presented here is exact, and the solution can be obtained quickly and in an inexpensive way compared to the FEM. The analysis also facilitates optimization of process parameters for good welding practice.     

A process model for the microstructure evolution in ductile cast iron: Part I. the model

In the present investigation, the multiple phase changes occurring during solidification and subsequent cooling of near-eutectic ductile cast iron have been modeled using the internal state variable approach. According to this formalism, the microstructure evolution is captured mathematically in terms of differential variation of the primary state variables with time for each of the relevant mechanisms. Separate response equations have then been developed to convert the current values of the state variables into equivalent volume fractions of constituent phases utilizing the constraints provided by the phase diagram. The results may conveniently be represented in the form of C curves and process diagrams to illuminate how changes in alloy composition, graphite nucleation potential, and thermal program affect the microstructure evolution at various stages of the process. The model can readily be implemented in a dedicated numerical code for the thermal field in real castings and used as a guiding tool in design of new treatment alloys for ductile cast irons. An illustration of this is given in an accompanying article (Part II).     

Mathematical modeling of sulfide flash smelting process: Part III. Volatilization of minor elements

The mathematical model described in Part I^[14] was extended to include the minor element behavior inside a flash-furnace shaft during flash smelting of copper concentrate. The volatilization of As, Sb, Bi, and Pb was computed, and experiments were carried out for Sb and Pb in a laboratory flash furnace. Satisfactory agreement between the predicted and measured results was obtained for antimony and lead. From the computational results, the behavior of each minor element was predicted for various target matte grades. The model predictions show that the elimination of As and Bi to the gas phase increases sharply at about 0.3 m from the burner; however, that of the Sb increases gradually along the flash-furnace shaft, and that of lead occurs suddenly at about 0.6 m from the burner. The predicted results also show that the elimination increases for Bi and Pb as the target matte grade increases; however, it is relatively independent of the target matte grade between 50 and 60 pet Cu for As and Sb. At higher target matte grades above 60 pet Cu, the elimination of As and Sb decreases as matte grade increases. formerly Graduate Student, Department of Chemical Engineering, University of Utah,     

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.     

Oxidation kinetics of zinc vapor in CO:CO2 mixtures: Part II. Application of plug flow concepts

In the first of two articles on the subject, it was shown that the oxidation kinetics of Zn vapor in CO:CO₂ mixtures cannot be understood on the basis of previously proposed rate expressions. In this second article, an alternative interpretation, which appears capable of reconciling the apparent discrepancies of the past literature, is proposed. Evidence is provided to the effect that competing reactions occur in the system Zn−CO−CO₂ under the majority of conditions investigated. Analysis on the basis of a simple “plug flow” model reveals that oxidation by CO₂ can take place indirectly by a combination of the reactions Zn_(g)+CO_(g)→ZnO_(s)+C_(s) and C_(s)+CO_2(g)→2CO_(g) Zinc is also oxidized by direct reaction with both CO and CO₂. It is proposed that the distinctive morphologies observed (coarse, intermediate, and fine) can be related to the degree of direct and/or indirect oxidation occurring in the system.     

Mathematical modeling of the argon-oxygen decarburization refining process of stainless steel: Part I. Mathematical model of the process

Some available mathematical models for the argon-oxygen decarburization (AOD) stainless steelmaking process have been reviewed. The actual situations of the AOD process, including the competitive oxidation of the elements dissolved in the molten steel and the changes in the bath composition, as well as the nonisothermal nature of the process, have been analyzed. A new mathematical model for the AOD refining process of stainless steel has been proposed and developed. The model is based on the assumption that the blown oxygen oxidizes C, Cr, Si, and Mn in the steel and Fe as a matrix, but the FeO formed is also an oxidant of C, Cr, Si, and Mn in the steel. All the possible oxidation-reduction reactions take place simultaneously and reach a combined equilibrium in competition at the liquid/bubble interfaces. It is also assumed that at high carbon levels, the oxidation rates of elements are primarily related to the supplied oxygen rate, and at low carbon levels, the rate of decarburization is mainly determined by the mass transfer of carbon from the molten steel bulk to the reaction interfaces. It is further assumed that the nonreacting oxygen blown into the bath does not accumulate in the liquid steel and will escape from the bath into the exhaust gas. The model performs the rate calculations of the refining process and the mass and heat balances of the system. Also, the effects of the operating factors, including adding the slag materials, crop ends, and scrap, and alloy agents; the nonisothermal conditions; the changes in the amounts of metal and slag during the refining; and other factors have all been taken into account. []—metal phase; ()—slag phase; {}—gaseous phase; and 〈〉—solid phase     

A process model for the microstructure evolution in ductile cast iron: Part II. Applications of the model

In the present investigation, the process model developed in Part I has been implemented in a dedicated numerical code to reveal the evolution of the coupled thermal and microstructural fields during directional solidification of ductile iron. In a calibrated from, the model predicts adequately both the variation in the graphite nodule count and the resulting microstructural profiles (i.e., graphite, iron carbide, ferrite, and pearlite) in the length direction of the bar. At the same time, the model has the required flexibility to serve as a research tool and predict behavior under conditions that have not yet been explored experimentally. In this article, the aptness of the model to alloy design and optimization of melt treatment practice for ductile iron is illustrated in different case studies and numerical examples.     

Leaching of chalcopyrite with ferric ion. Part III: Effect of redox potential on the silver-catalyzed process

This study examines the effect of redox potential on silver-catalyzed chalcopyrite leaching. Leaching tests were carried out in stirred Erlenmeyer flasks with 0.5 g chalcopyrite mineral, 1 g Ag/kg Cu and 100 mL of a sulphate solution of Fe³⁺/Fe²⁺ (with redox potential ranging between 300 and 600 mV Ag/AgCl) at pH 1.8, 180 rpm and 35°C or 68 °C. Unlike uncatalyzed leaching, an increase of the redox potential increased copper dissolution in the presence of silver ions, as the regeneration of Ag⁺ requires a high concentration of oxidizing agent, Fe³⁺. Additionally, the high reactivity of the mineral surface when silver was present could have been responsible for inhibiting the nucleation of hydrolysis products of Fe³⁺ on it. Excessive addition of silver transformed the chalcopyrite surface into copper-rich sulphides such as covellite, CuS, and geerite, Cu₈S₅, preventing the formation of CuFeS₂/Ag₂S galvanic couple and the recycling of silver ions.     

Oxidation kinetics of zinc vapor in CO∶CO2 mixtures: Part I. Comparison with past literature

The methods employed and the results obtained during a recent investigation of the oxidation of gaseous zinc by CO∶CO₂ mixtures are described. The kinetic data are tested against the predictive models derived and employed by previous investigators of this reaction. Although the experimentally determined reaction rates are highly reproducible, no consistent agreement can be found with the models presented in the prior literature. Efforts to account for the kinetics on the basis of the oxygen potential of the gas mixture are also unsuccessful. It is concluded that previous attempts to understand the kinetics in terms of the reaction Zn _(g) +CO _2(g) ⇒ZnO _(s) +CO _(g) are oversimplistic. A novel interpretation which appears to resolve the discrepancies of the literature is detailed in the second of two articles on the subject.     

Deriving exact predictions from the cascade model.

Discusses J. L. McClelland's (see record 1979-32860-001) cascade model and demonstrates that the model does not have a well-defined RT distribution function because it always predicts a nonzero probability that a response will never occur. By conditioning on the event that a response does occur, RT density and distribution functions are derived, thus allowing most RT statistics to be computed directly and eliminating the need for computer simulations. Using these results, an investigation of the model reveals that (a) it predicts mean RT additivity in most cases of pure insertion or selective influence, (b) it predicts only a very small increase in standard deviations as mean RT increases, and (c) it does not mimic the distribution of discrete-stage models that have a serial stage with an exponentially disturbed duration. (12 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

A mathematical model for the dynamic behavior of melts subjected to electromagnetic forces: Part I. model development and comparison of predictions with published experimental results

A mathematical model was developed to predict electromagnetically driven flow and (particularly) free surface behavior in melts subject to electromagnetic forces. Such melts appear in electromagnetic casters, induction furnaces, and other metal processing units. The calculations started with Maxwell’s equations and Ohm’s law, which were solved by a novel “modified hybrid technique.” The instantaneous continuity and Navier-Stokes equations (rather than their time-averaged versions) were then solved with electromagrretic forces as input. The calculations allowed for the dynamic behavior of the free surface of the melt, and electromagnetic fields were recomputed as the free surface changed. In this first part of a two-part article, the model predictions are compared with the experimental measurements of induced current, magnetic field, melt velocity, and free surface deformation reported by others.     

A process model for the heat-affected zone microstructure evolution in duplex stainless steel weldments: Part I. the model

The present investigation is concerned with modeling of the microstructure evolution in duplex stainless steels under thermal conditions applicable to welding. The important reactions that have been modeled are the dissolution of austenite during heating, subsequent grain growth in the delta ferrite regime, and finally, the decomposition of the delta ferrite to austenite during cooling. As a starting point, a differential formulation of the underlying diffusion problem is presented, based on the internal-state variable approach. These solutions are later manipulated and expressed in terms of the Scheil integral in the cases where the evolution equation is separable or can be made separable by a simple change of variables. The models have then been applied to describe the heat-affected zone microstructure evolution during both thick-plate and thin-plate welding of three commercial duplex stainless steel grades: 2205, 2304, and 2507. The results may conveniently be presented in the form of novel process diagrams, which display contours of constant delta ferrite grain size along with information about dissolution and reprecipitation of austenite for different combinations of weld input energy and peak temperature. These diagrams are well suited for quantitative readings and illustrate, in a condensed manner, the competition between the different variables that lead to structural changes during welding of duplex stainless steels.