Showing papers in "Metallurgical and Materials Transactions B-process Metallurgy and Materials Processing Science in 1976"

Factors affecting the grain-refinement of aluminum using titanium and boron additives

Abstract: The development of grain-refining techniques for the casting of aluminum, and the results achieved using additions of ternary Al-Ti-B master-alloys, are briefly described. A test procedure is given for assessing the relative effectiveness of these master-alloys, and the results are related to the various kinds of alloy treated, including those with constituents which “poison” the grain-refining agent. The equilibrium conditions which affect the behavior of the constituents of the grain-refined alloys are examined quantitatively from a thermodynamic viewpoint. One theory about the mechanism of nucleation of aluminum on titanium diboride is examined and found inapplicable. New data on the solubility of TiB2, ZrB2, TiC, ZrC, Cr3C2, CrB2 in liquid aluminum are presented graphically. Practical aspects of alloy behavior in grain-refining practice are analyzed and discussed on the basis of the quantitative data.

Aluminum matrix composites: Fabrication and properties

Abstract: Aluminum alloy matrix composites containing 1 to 30 wt pct of fibrous and particulate nonmetals varying in size from 0.06 μm to 840 μm were fabricated. The composites were cast into cylindrical molds for friction and wear tests, hot extrusion and tensile tests. The distribution of the nonmetals in the cast ingots was homogeneous. Friction and wear tests were done on a pin (52100 bearing steel) and dish type machine without lubrication. It was found that composites containing ∼10 wt pct or more of SiC, TiC, Si3N4, Al2O3, glass, solid waste slag, and silica sand wear less than the pure matrix alloy, but have slightly higher average coefficients of friction. Wear in composites containing soft particles, especially MgO and boron nitride was higher than the pure matrix alloy. The average coefficient of friction of all the composites was in the range of 0.35 to 0.58. Increasing the sliding velocity reduced this range to ∼ 0.4 to 0.45. The longitudinal tensile properties of the extruded composites (with the exception of loss of ductility in some cases) are comparable to that of the matrix alloys. Improvements in strength or ductility were noted. For example, addition of 15 wt pct of 3 μm size Al2O3 particles raised the yield and ultimate strength of the Al-4 pct Cu-0.75 pct Mg alloy matrix from 227 to 302 MPa, and 356 to 403 MPa, respectively. The corresponding percent elongation decreased from 25.8 to 12.5. The fact that the various composites can be readily cast and hot formed suggests a variety of engineering applications.

Direct Leaching of sulfides: Chemistry and applications

Abstract: The sulfide minerals associated with copper, nickel, zinc, lead, and molybdenum concentrates are described according to their thermodynamic stability zones on Eh-pH diagrams. From these zones, the chemistry associated with various thermodynamically feasible decomposition paths is discussed, and process developments associated with the most favorable decomposition paths are briefly reviewed.

The activity coefficient of oxygen in binary liquid metal alloys

Abstract: The Wagner model with one energy parameter,h, for describing the effect of alloying elements on the activity coefficients of nonmetallic solutes in liquid metals is extended to have two energy parameters,h 1andh 2. The validity of both the Wagner one-parameter equation and the newly derived two-parameter equation is tested using data available in the literature for twelve ternary metal-oxygen systems. In order to have consistent thermodynamic data, all the relevant binary, as well as the twelve ternary metal-oxygen systems are evaluated using the same thermodynamic values for the reference materials which were used in carrying out the experimental measurements. It is found that the twoparameter equation is capable of quantitatively accounting for the compositional dependences of the activity coefficients of oxygen in all twelve ternary systems while the Wagner one-parameter equation is not. A correlation between the Wagner parameter,h, and the thermodynamic properties of the respective binary metal-oxygen and binary metals systems is found, from which the value of this parameter may be predicted without referring to any ternary data. Accordingly, the two-parameter equation is more useful in evaluating ternary experimental data while the Wagner one-parameter equation in connection with the correlation betweenh and binary data is capable of predicting ternary data without any experimental investigation in the ternary region. Based on the one-parameter and the two-parameter equations, theoretical equations for the first-order and second-order free energy interaction parameters,(∈ 0 j )sand(ρ 0 j )s, are derived in terms of the model parameters. The values of(∈ 0 j )s and(ρ 0 j )s for all the systems are derived and are found to vary linearly with the reciprocal of temperature. Furthermore, linear relationships between these two interaction parameters and their slopes with 1/T are found, from which the temperature dependence of the interaction parameters may be estimated in the absence of experimental data.

Thermodynamics of rare earths in steelmaking

Abstract: The standard free energies of formation of the oxides, sulfides and oxysulfides of cerium and lanthanum under steelmaking conditions have been calculated and used to predict the behavior of rare earths in steelmaking. Deoxidation and desulfurization constants, expressed in terms of Henrian activities, have been used to construct a precipitation diagram which indicates the sequence of rare earth inclusion formation. An enrichment of lanthanum in (RE)-oxysulfide and cerium in (RE)-sulfide is predicted. It is also predicted that rare earths should be able to reduce the soluble oxygen and sulfur contents of liquid steel well below the contents presently found in most industrial and laboratory practices. A simple method of calculating steelmaking additions for complete rare earth control of inclusion composition is presented.

Interdendritic fluid flow in a lead-tin alloy

Abstract: Fluid flow through interdendritic channels of a partially remelted Pb-20 pct Sn alloy has been measured. The flow, resulting from gravity forces, was determined for columnar and equiaxed dendritic structures as a function of dendrite arm spacing. The initial interdendritic flow was found to be consistent with Darcy’s law with a tortuosity factor of 4.6. The initial permeability of the dendritic array was found to be a function of the square of the primary dendrite spacing, and a more complex function of secondary dendrite arm spacing. There was little difference in flow rates for columnar and equiaxed structures of similar size. After the initial flow period the flow rate was observed to increase above that defined by Darcy’s law for castings with small dendrite spacings. This was shown to be a result of coarsening of the liquid channels during flow. The observed coarsening process is considered in terms of diffusion controlled ripening.

Interfacial reaction kinetics in the decarburization of liquid iron by carbon dioxide

Abstract: The kinetics of decarburization of liquid iron have been studied between 1160 and 1600°C under conditions where mass transport of reactants is not rate determining. Studies with continuously carbon-saturated iron and of iron with varying carbon concentration have been used to show that the slow step at high concentrations of carbon is independent of carbon concentration and is first order with respect to the pressure of CO2. For high purity iron, the forward rate constant, in mole cm2 s-1 atm-1, is given by the equation ln kf = -11,700/T-0.48. It is concluded that the data are consistent with the chemisorption process as the rate limiting step. A marked sensitivity of the rate to trace amounts of sulfur has been found and it is shown that this is consistent with ideal adsorption of sulfur and is in fair accord with the existing measurements of the depression of the surface tension of iron-carbon alloys by sulfur.

Fluid velocities in induction melting furnaces: Part I. Theory and laboratory experiments

Abstract: Fluid flow in an induction furnace due to electromagnetic stirring forces is predicted theoretically from furnace design parameters by the simultaneous solution of the Maxwell and Navier Stokes equations. Streamline plots and velocity profiles are obtained and compared with surface velocities measured experimentally. The measurements were made on a mercury pool stirred inductively by a Tocco 30 kW 3 kHz induction melting unit. The agreement between the experimental measurements and theoretical predictions was good considering that no curve fitting by manipulation of adjustable parameters was involved. It is believed that such a model would be of value in the design and development of induction furnaces.

Heat flow, gap formation and break-outs in the continuous casting of steel slabs

Abstract: A theoretical investigation of heat flow and gap formation in the mold of a continuous slab caster has been undertaken using a mathematical model, with the ultimate purpose of predicting the casting conditions which can lead to break-outs. The mathematical model that has been developed for this study is capable of treating the heat flow and air gap as coupled phenomena, and can accept operating variables such as slab size, casting speed, mold taper, and thermal conductivity of the mold powder, as input. Four different cases of slab casting have been investigated; and it has been found that hot spots can form on the surface of the slab within a few centimeters of the corners. Depending on their temperature, it is suggested that these hot spots may give rise to the formation of break-outs off the corners of the slab. From an examination of the behavior of the hot spots, the susceptibility of the cases studied to break-outs has been evaluated. The usefulness of the present analysis in qualitatively ascertaining the location and extent of mold wear under different casting conditions has also been examined.

Langmuir adsorption, the gibbs adsorption isotherm, and interfacial kinetics in liquid metal systems

Abstract: The adsorption of strongly surface active solutes is considered in terms of the ideal site-fillageor Langmuir isotherm. The resulting equation for the depression of surface tension of a liquid metal by such a solute is σ P −σ = RTΓ i Emphasis>/o In (1 +Ka i) where σ p − σ is the depression of surface tension of the pure liquid metal, Γ i Emphasis>/o is the saturation coverage by the solute,a i is the bulk activity of the solute, andK is a coverage independent adsorption coefficient. The isotherm is found to give a very good description of the available data for Group VI solutes on several metals. The derived values of the adsorption coefficient are applied to several kinetic studies and they are found to give good relative and possibly absolute measures of the interference by such solutes on interfacial reaction rates. The iron-sulfur system appears to differ, but only by a factor of two. The application of these adsorption coefficients in aiding the interpretation of kinetic studies is considered.

The physical behavior of a gas jet injected horizontally into liquid metal

Abstract: The gas fraction and bubble frequency distributions in a submerged air jet, injected horizontally into mercury, have been measured under isothermal, nonreactive conditions for nozzle diameters of 0.325 and 0.476 cm and jet Froude numbers ranging from 20.5 to 288. The measurements reveal that the jets expand extremely rapidly upon discharge from the nozzle with an initial expansion angle of 150 to 155 deg. This value, which is over seven times greater than is found with air jets in water, indicates that the physical properties of the liquid exert considerable influence on the jet behavior. In conjunction with the rapid expansion, the air jets in mercury were also found to penetrate extensively behind the nozzle, and in many respects resembled a vertically injected jet. The extent of backward penetration of the jets was constant for all blowing conditions studied while the forward penetration increased with both increasing jet Froude number and nozzle diameter. The measured jet penetration in both the forward and backward directions were considerably larger than expected from model predictions. The core of the jets consists of a high concentration of gas bubbles. Both the gas volume fraction and bubble frequency in the core increase with increasing jet Froude number and nozzle diameter. The gas concentration and bubble frequency decrease with increasing distance along the jet trajectory due presumably to entrainment of liquid metal and bubble coalescence. On the basis of these findings, it is likely that process jets, such as are injected into copper converters, also expand rapidly and penetrate only a short distance into the bath. Thus rather than reacting in the middle of the bath, the jets may be impinging on the backwall refractory and contributing to the erosion observed there.

Flow pattern velocity and turbulence energy measurements and predictions in a water model of an argon-stirred ladle

Abstract: Experiments were carried out using a simplified water model of an argon-stirred ladle system. The flow patterns were determined by a flow visualization technique and the velocity and turbulence energy fields were quantitatively measured using hot-film anemometry. The latter quantities were predicted by solving the turbulent Navier-Stokes equations using Spalding’sk-W model for the turbulence viscosity. There is semiquantitative agreement between predictions and measurements. Mixing lengths also were computed. This agreement between measurements and predictions provides further evidence that modeling is a promising approach for the study of recirculating turbulent flows in steel processing operations.

An analytical description of the jet finishing process for hot-dip metallic coatings on strip

Abstract: An analytical study of the jet finishing of hot-dip metallic coated strip which relates the final solidified coating thickness to strip speed, jet nozzle operating parameters, and coating metal and finishing fluid properties is described. The results of laboratory and mill tests are compared with the predictions of the analytical model and engineering considerations for jet nozzle operation to reduce coating weight at increased line speed are discussed.

Reduction of tungsten oxide to tungsten metal

Abstract: Tungsten oxide, WO2.96, was reduced toα-W in hydrogen for various time periods over the temperature range 500°C to 900°C. Intermediate oxides were determined using X-ray diffraction analysis of the semireduced powders. Several dopant conditions were used, to find the effect on oxide structures and reaction kinetics of the usual dopants K, Si and Al. The scanning electron microscope was employed to determine the morphology of the crystallites at intermediate and final stages of reduction.

Evaluation of zinc sulfate electrolytes by cyclic voltammetry and electron microscopy

Abstract: A cyclic voltammetric technique has been developed for approximating the quantities of active chemical species present in zinc sulfate electrolytes. The experimental apparatus consisted of a Pyrex “H” cell, an aluminum cathode encased in a Teflon holder, a carbon anode and a mercurous sulfate reference electrode. Voltammograms were obtained using industrial, purified neutral leach solution (Cominco Ltd., Trail, BC) acidified to give a final concentration of 0.77 M Zn++ and 1 M H2SO4. The polarization curves were then evaluated and used as reference standards to compare with results obtained when various organic and inorganic additions were made. The deposit morphologies obtained for short-time cathodic cycles were also studied with the aid of a Scanning Electron Microscope. Changes in concentrations of glue in the 5 to 10 ppm range and of antimony in the 5 to 10 ppb range were detected using the techniques described.

On the Mechanism of Pore Formation in Metals

Abstract: The formation of different types of gas pores has been investigated by directional solidification experiments. A mathematical model of pore growth has been derived and the calculated pore growth has been compared with experimental data and a good correlation was found. The nucleation process of pores has also been treated. It was shown that micropores can be homogeneously nucleated in an interdendritic area according to the pressure drop caused by the solidification shrinkage.

The transfer of silicon from the gas phase to molten iron in the blast furnace

Abstract: The mechanism of silicon transfer in a blast furnace has been studied. It is concluded that the contribution of slag-metal reaction to silicon increase in the metal will not be very important because of the slow rate of the reaction and high oxygen potential of slag in a blast furnace. By making use of kinetic and thermodynamic data, it is shown that the reaction of SiO containing gas with liquid iron of high carbon content takes an important role in the silicon transfer in a blast furnace.

The removal of solid particles from molten aluminum in the spinning nozzle inert flotation process

Abstract: The removal of solid particles from molten aluminum by flotation was investigated based on theoretical fluid dynamics The energy spent for stirring the melt in the SNIF process accelerates the agglomeration of small particles into larger particle aggregates which can be removed from the metal by gas bubbles during the short residence time of the melt in the refining unit Theory suggests that supplementation of thermal agglomeration of the particles with turbulent agglomeration and small gas bubbles are the major factors which can lead to high particle collection efficiencies in molten aluminum

Structural changes and kinetics in the gaseous reduction of hematite

Abstract: The mechanism of the gaseous reduction of hematite grains to magnetite was studied. Gravimetric measurements were carried out for the reduction of Carol Lake hematite pellets and grains in CO-CO2 atmospheres over the temperature range 500 to 1100°C. The pore size distribution in the reduced magnetite was measured by mercury porosimetry. Partially reduced grains were examined by optical microscopy. At temperatures below 800°C, the reduction of a hematite grain to magnetite occurred at a well-defined shrinking-core interface. The average pore size in magnetite formed at 600°C was found to be 0.03 μm. An estimate of the rate of CO diffusion through pores of this size indicated that the reaction rate at 600°C was controlled by a step near the hematite-magnetite interface. At temperatures above 800°C, the reaction mechanism became altered due to the preferential growth of magnetite along a single direction in each hematite grain. The reduction rate decreased with an increase in temperature, and no microporosity was present in magnetite formed at 1000°C and above. It was postulated that the reaction rate was controlled by the rate of formation of fresh nuclei and by their rate of subsequent growth.

A thermodynamic analysis of the phase equilibria in the Fe-Mn-S system

Abstract: The experimental information on the phase equilibria in the Fe-Mn-S system is meager. An attempt has now been made to predict these phase equilibria by extrapolation of the free energy properties from the binary sides. The crucial point is the choice of a thermodynamic model for the liquid phase which could apply to the whole range of composition. Such a model is suggested. The calculations show some encouraging agreement with the experimental information available and they yield a better understanding of some aspects of the formation of FeS during the solidification of steel, containing manganese.

The Rate of reduction of MgO by carbon

Abstract: The rate of reduction of MgO by carbon within a carbon-bearing MgO refractory material was investigated by measuring the rate for an actual refractory material as well as for various powder mixtures of MgO and carbon in atmospheres of Ar, He, and CO from 1475 to 1600°C. The effective gas diffusivities within the brick were also determined in order to interpret the results. For small samples, for which gas-diffusion processes do not affect the rate, the rate is controlled by a slow chemical reaction. Indications are the slow reaction is the oxidation of carbon however, the exact mechanism is not known. The temperature dependence of the rate of reaction is large 157 kcal (657 kJ). For larger samples the initial rate is controlled by the diffusion of the products (Mg and CO) through the gas film boundary layer near the surface and in the later stages by diffusion through the pores of the reacted MgO. To model the actual geometry in a BOF an experiment in which the reaction products would be removed from a single surface was conducted and indicated that after a short period of time the rate is controlled by diffusion through the pores of the brick. It was concluded that in actual practice a pseudosteady-state thickness for the reacted decarburized zone will develop because of the balance between the diffusion-limited layer growth and normal refractory wear and that the layer would be relatively small (0.1 to 0.4 cm).

A kinetic study on the pressure leaching of sphalerite

Abstract: The dissolution of sphalerite (ZnS) in sulfuric acid solution under oxygen pressure was investigated. Effects of temperature, percent solids, agitation, sample size, oxygen partial pressure and foreign ions were evaluated. The effect of hydrogen pretreatment on sphalerite leaching rate was also examined. Leaching of sphalerite at 90°C and 150 psi oxygen pressure was found to occur at a constant rate. This rate was determined from the experimental data observed under the different leaching conditions mentioned above. The constant leaching rate was attributed to the chemical reaction occurring on the surface of the flat-plate type sphalerite sample. The rate-controlling step of the reaction was determined to be the oxidation of hydrogen sulfide to elemental sulfur. Oxidation of hydrogen sulfide was studied through the addition of iron and through the observation of the change in iron concentration during leaching. The oxidation was concluded to be by reaction with ferric ion rather than by direct oxygen oxidation. Leaching tests run with samples pretreated with hydrogen do not show any increase in the rate of zinc extraction.

Thermodynamic study of liquid aluminum-magnesium alloys by vapor pressure measurements

Abstract: Thermodynamic properties of liquid Al-Mg alloys have been determined by vapor pressure measurements over pure liquid magnesium and liquid Al-Mg alloys, in the composition range of 5 to 94 at. pct magnesium between 900 and 1245 K, employing transpiration technique. The results obtained have been compared with those reported by earlier workers and possible reasons for the observed inconsistencies have been discussed. The present investigations indicate that the liquid Al-Mg system nearly conforms to ideal solution behavior, with slight negative deviation. The minimum activity coefficient value for magnesium and the maximum integral molar heat of solution for the alloy have been found to be equal to 0.6 at 10 at. pct and -2.49 kJ/g atom at 50 at. pct magnesium respectively at 1000 K.

Measurement of the thermal diffusivity of liquid oxides and metallurgical slags

Abstract: The methods of heat transfer in stagnant bodies of glass and slag are reviewed, and it is shown that the effective thermal diffusivity determines the rate at which heat is transferred in glasses similar in composition to metallurgical slags. A new experimental apparatus is also described for measuring the effective thermal diffusivity of these glasses. The results of experiments performed on glasses containing between 10.9 and 26.8 pct FeO, with lime/ silica ratios,B, of 1.0 and 1.5, and at temperatures ranging from the liquidus temperatures of the glasses to 1750 K are represented by\({\rm K}_{eff} = 0.001 \cdot (1.5 - 0.5 \cdot B) + 0.018 \frac{{(T/1500)^3 }}{{(pct FeO)^{0.8} }}; (cm^2 /s).\) A theoretical basis is used to develop the form of this relationship, and consequently, it should yield reasonable estimates for glasses of other composition and at other temperatures.

The kinetics of nickel oxide reduction by hydrogen; measurements in a fluidized bed and in a gravimetric apparatus

Abstract: The suitability of a batch fluidized bed laboratory reactor for measuring the rates of gas-solid reactions was investigated. Experiments were carried out on the reduction of Falconbridge nickel oxide by hydrogen in a batch fluidized bed reactor within the tem-perature range 550 K to 650 K using particles in the range of 60 to 100 mesh. The reactor was operated at approximately atmospheric pressure and gas flow rates were in the range of two to four times the minimum fluidization velocity at temperature. The results showed internal consistency and rough agreement with the results of previous workers. The re-sults were interpreted and correlated by means of a structural model for gas-solid reac-tions. As a check on the fluidized bed measurements, experiments were also carried out using the conventional gravimetric technique to measure the rate of reduction of compac-ted pellets of nickel oxide by hydrogen. When due allowance was made for the change of surface area of the oxide during compaction, the results were in close agreement with the fluidized bed results. Rate measurements using hydrogen-nitrogen mixtures revealed that the reaction is not first order with respect to hydrogen, as usually assumed, but is ap-proximately of order two-thirds at one atmosphere hydrogen partial pressure.

A rapid electrochemical method for measuring the concentration of active glue in copper refinery electrolyte which contains thiourea

Abstract: Good cathode quality in copper electrorefining depends on maintaining an optimum level of glue and other leveling reagents in the electrolyte. A rapid electrochemical method has been developed for measuring the concentration of active glue in the electrolyte. It is based on the fact that glue strongly influences the polarization (currentvs potential) curve of a copper cathode in such electrolyte. The polarization curve is also influenced by the concentrations of copper, nickel, and thiourea (another leveling reagent) in the solution; the concentrations of these substances also can vary. With the present method a cathodic scan curve of a copper electrode is measured in the fresh, test electrolyte and again after the electrolyte is heated such as to destroy the active glue by hydrolysis; the heating does not affect the thiourea, Cu+2 or Ni+2 ions. The glue concentration is determined from the difference in the scan curves for the fresh and heated electrolytes. The difference may be quantified if needed by making standard glue additions to the cooked electrolyte and measuring the corresponding scan curves.