Search Documents

E. P. Pfleider, University of Minnesota I would like to ask Bob Merrill whether he considers that horizontal concave curvature of a slope has any stabilizing effect, such as Jenike 1 suggested several years ago. R. H. Merrill, U. S. Bureau of Mines, Denver Federal Center The stabilizing effect is due to the restraint offered to radial movement by the curvature. We found in laboratory tests that if we constrained the rock with the same stresses as those measured in the pit then it would sustain a maximum stress about two or four times greater than when the same rock type was unconstrained. We are therefore definitely of the opinion that a constrained situation in a slope is favorable towards stability. G. A. Leonards, Purdue University I was interested in Mr. Lane's observations concerning the correspondence between rate of slide movement with rise of reservoir. Perhaps the most interesting feature is that the rate of movement at the time the rock slope failed was less than the rate at an earlier time when it did not fail. This is difficult to explain from current slope stability theory. K. S. Lane, U. S. Army Corps of Engineers I believe that reduced rate of movement is the result of the progressive type of failure, such as described by Patton and Deere. which occurs along the sliding surfaces. It is difficult to analytically define the progressive failure condition for inclusion in slope stability conditions, but its existence should be recognized in design. We should also realize that many apparently stable slopes undoubtedly contain overstressed regions. To design a slope with no overstressed region would be uneconomical and conservative. D. H. Deere I would pose the question-"What is the meaning of a factor of safety of one?" We were involved in the design of a large, irregularly shaped, underground cavity with an unsupported span of several hundreds of feet. Tests on rock specimens indicated a strength of 1300 lb per sq in. The RQD would have been between 90 to 100 percent indicating excellent structural continuity. In-situ stress determinations indicated an approximately hydrostatic state of stress at a level of 1400 lb per sq in. Calculation of stress concentrations at the walls indicated that the stresses there would be about 3000 to 4000 lb per sq in. Admittedly the rock is in a confined condition everywhere except at the very skin of the wall and the rock walls would thus on the average be somewhat stronger. Allowing for the effect of restraint the factor of safety was found to be less than 0.5 over a region several feet into the wall. Construction was started, using rockbolts only to support the walls. Failure around the immediate skin occurred after each

Jan 1, 1967

By W. T. Bishop

For many years pine oil has been used extensively as a frother for the flotation of minerals, particularly sulphides, yet the published information available to the average user on its composition, chemical and physical properties, has been rather meager. It is the purpose of this paper to discuss these points and their relationship to the frothing properties of pine oil used in flotation processes. Pine oil, as the name implies, is a liquid obtained from the processing of wood of the longleaf and other southern pines. It is a complex mixture of naturally occurring terpene derivatives, primarily alcohols, and a smaller amount of terpenes, which has found many uses in various industries. Its use as a frother in flotation processes was the first large-scale application in industry and today the mining industry still consumes large quantities of pine oil but the textile and disinfectant industries consume comparable volumes. Much is used in soap manufacture and laundry operations. The surface-active properties of pine oil are in a large degree responsible for this versatility. History The name "Naval Stores" goes back to the days when pine tar and pitch were used to caulk leaky ships and 'Oat their ropes and rigging. Today this term is still used to include not only the oleoresinous products of the pine tree—rosin and turpentine— but other products such as pine oil, pine tar, and the many terpene and resinous compounds obtained by extracting the pine wood or tapping the tree. That part of the industry known as "wood naval stores" dates from about the beginning of this century and today produces a large part of the country's rosin and turpentine by processing pine wood. In addition, all of our natural pine oil is supplied by the wood naval stores industry, as tapping of the live tree gives only rosin and turpentine. This latter phase is known as the "gum naval stores " industry. The first successful commercial operation of the steam and solvent process of the wood naval stores in industry was carried out by Yaryan at Cadillac, Mich., in 1909.1 This operation did not prove profitable, so Yaryan went south to utilize the longleaf pine, and built a plant at Gulfport, Miss.2 Soon after, another plant was built at Brunswick, Ga., which is still in operation, and in 1914 was reported to have processed about 500 tons of wood per day.' In 1921 Hercules Powder Co. acquired the Yaryan interests, at which time the production of pine oil was only 3 to 5 million pounds per year and flotation was the largest customer. As time went on, technological advances were made, other companies entered the field, and the production of pine oil increased many fold. The growth of the industry was especially marked during the past 10 years, and in

Jan 1, 1947

By W. T. Bishop

For many years pine oil has been used extensively as a frother for the flotation of minerals, particularly sulphides, yet the published information available to the average user on its composition, chemical and physical properties, has been rather meager. It is the purpose of this paper to discuss these points and their relationship to the frothing properties of pine oil used in flotation processes. Pine oil, as the name implies, is a liquid obtained from the processing of wood of the longleaf and other southern pines. It is a complex mixture of naturally occurring terpene derivatives, primarily alcohols, and a smaller amount of terpenes, which has found many uses in various industries. Its use as a frother in flotation processes was the first large-scale application in industry and today the mining industry still consumes large quantities of pine oil but the textile and disinfectant industries consume comparable volumes. Much is used in soap manufacture and laundry operations. The surface-active properties of pine oil are in a large degree responsible for this versatility. History The name "Naval Stores" goes back to the days when pine tar and pitch were used to caulk leaky ships and 'Oat their ropes and rigging. Today this term is still used to include not only the oleoresinous products of the pine tree—rosin and turpentine— but other products such as pine oil, pine tar, and the many terpene and resinous compounds obtained by extracting the pine wood or tapping the tree. That part of the industry known as "wood naval stores" dates from about the beginning of this century and today produces a large part of the country's rosin and turpentine by processing pine wood. In addition, all of our natural pine oil is supplied by the wood naval stores industry, as tapping of the live tree gives only rosin and turpentine. This latter phase is known as the "gum naval stores " industry. The first successful commercial operation of the steam and solvent process of the wood naval stores in industry was carried out by Yaryan at Cadillac, Mich., in 1909.1 This operation did not prove profitable, so Yaryan went south to utilize the longleaf pine, and built a plant at Gulfport, Miss.2 Soon after, another plant was built at Brunswick, Ga., which is still in operation, and in 1914 was reported to have processed about 500 tons of wood per day.' In 1921 Hercules Powder Co. acquired the Yaryan interests, at which time the production of pine oil was only 3 to 5 million pounds per year and flotation was the largest customer. As time went on, technological advances were made, other companies entered the field, and the production of pine oil increased many fold. The growth of the industry was especially marked during the past 10 years, and in

Jan 1, 1947

By F. V. Lenel

Two years ago in Chicago a seminar was held on the theory of sintering of pure metal powders. As an introduction to this seminar D,.. Rhines1 gave an excellent survey of the literature on this subject, His method of presentatioll was to summarize the experimental observations on sintering and from them to develop a composite theory of sintering in which he combined all the important contributions to the theory into one organic whole. In contrast to the mechanism of sintering of pure metal powders, which, of course, al\vays takes place in the solid phase, sintering j,, the presence of a liquid phase, cannot be treated as one unified mechanism, such as the sintering mechanism of pure metals. The reason is that there are really several mechanisms depending upon the type of alloy system which is sintered and the field of its con. stitutional diagram ill which the sintering takes place, Two mechanisms have heen investigated and will be reviewed in this paper. Ill the first lnechanism the liquid is presellt during the entire time while the compacts are at the sintering temperature, In other words they are sintered between the liquidus and the solidus of the alloy system and are ~e~erogelleous during the entire sintering cycle. The second mechallism applies to alloys in which the liquid phase is formed during the sintering process, but disappears before the sintering process is completed through diffusion and formation of a solid solution. These alloys are therefore homogeneous at the end of the sintering cycle. This second mechanism is more complicated because of the two stages of sintering, the liquid and the solid, as they may be called. It is therefore not surprising that the few detailed investigations of sintering in the presence of a liquid phase have been concerned with heterogeneous sintered alloys, namely the tungsten-nickel-sinter ing which is the so-called "heavy alloy," and the cemented carbides. The mechanism of sintering of these systems is characterized by the fact that theoretical or near theoretical density is attained during sintering, while simultaneously a distinct grain growth takes place through solution of the smallest grains of the solid phase in the liquid phase and reprecipitation on the larger grains. In the first part of the survey this sintering mechanism which is called for short the heavy alloy mechanism is treated in detail. In the second part Of the survey experimental observations for the sintering of homogeneous sintered alloys where the liquid phase disappears before the completion of sintering are discussed. Much less systematic work has been done on these alloys to which the conlmercially important Porous bronzes and the iron-nickel-aluminum permanent magnet alloys belong. Emphasis will be laid throughout this survey upon the microstructural and density changes during sintcring because the Sintering mechanism can usually be described directly by these changes. Other

Jan 1, 1949

By E. M. Tory, P. T. Shannon

An analysis of batch and continuous thickening in terms of the movement of planes of constant concentration (i.e. continuity waves) is presented. Use of solids flux as a primary variable greatly facilitates the analysis and clarifies the relationship between batch and continuous thickening. An "ideal" slurry is defined as one whose settling rate is a function only of the local solids concentration. An analysis of continuous thickening of this ideal slurry in an ideal thickener explains many aspects of the steady — and unsteady — state behavior of real slurries, such as the main features of a thickener's "solids profile" (including the break attributed to rake action), the sudden appearance at abnormally high feed rates of a zone of constant concentration and the velocity with which it rises, and the effect of detention time at constant weight of sludge per unit area. Where deviations exist, they are explained in terms of the batch settling behavior of real slurries. The purpose of this paper is to interpret continuous thickening phenomena in terms of batch settling behavior. Both types of thickening may be considered as the process of propagating concentration changes, due to the downward movement of the solids, from the bottom of the settling vessel upward through the slurry.' These concentration changes may be either finite or infinitesimal as in the case of the propagation of a continuous concentration gradient. The slurry itself may be moving downward as in the case of continuous thickening. The downward velocity of the slurry may be such that the concentration changes being propagated up from below remain stationary relative to the thickener walls. This is the case for steady-state thickening. It seems appropriate to analyze continuous thickening in terms of the movement of planes of constant concentration. This has already been done for batch settling."3 Although our main interest is in real slurries, there are certain aspects of both types of settling which can be deduced from a theoretical analysis of highly idealized systems. To this end, an "ideal slurry" is defined as one for which the dimensionless settling rate at any point is a function only of the local solids concentration.1,2 An "ideal thickener" is one in which there is no radial variation in either the solids concentration or in the downward component of slurry velocity. The underflow outlet is a point sink. It is to be expected that the behavior of non-ideal systems will deviate in some measure from theoretical results. However, the same characteristics which cause deviations in continuous thickening should be evident in batch settling, permitting a rational explanation of the former in terms of the latter. PREDICTED RESULTS FOR AN IDEAL SLURRY IN AN IDEAL CONTINUOUS THICKENER General Theory: In continuous thickening, the downward solids flux (mass/area-time) relative to the thickener, G, results from two different

Jan 1, 1967

By F. V. Lenel

INTRODUCTION Two years ago in Chicago a seminar was held on the theory of sintering of pure metal powders As an introduction to this seminar Dr Rhines1 gave an excellent survey of the literature on this subject His method of presentation was to summarize the experimental observations on sintering and from them to develop a composite theory of sintering in which he combined all the important contributions to the theory into one organic whole In contrast to the mechanism of sintering of pure metal powders, which, of course, always takes place in the solid phase, sintering in the presence of a liquid phase, cannot be treated as one unified mechanism, such as the sintering mechanism of pure metals The reason is that there are really several mechanisms depending upon the type of alloy system which is sintered and the field of its constitutional diagram in which the sintering takes place Two mechanisms have been investigated and will be reviewed in this paper In the first mechanism the liquid is present during the entire time while the compacts are at the sintering temperature In other words they are sintered between the liquidus and the solidus of the alloy System and are heterogeneous during the entire sintering cycle The second mechanism applies to alloys in which the liquid phase is formed during the sintering process, but disappears before the sintering process is completed through diffusion and formation of a solid solution These alloys are therefore homogeneous at the end of the sintering cycle This second mechanism is more complicated because of the two stages of sintering, the liquid and the solid, as they may be called It is therefore not surprising that the few detailed investigations of sintering in the presence of a liquid phase have been concerned with heterogeneous sintered alloys, namely the tungsten-nickel-copper alloys, which is the so-called "heavy alloy," and the cemented carbides The mechanism of sintering of these systems is characterized by the fact that theoretical or near theoretical density is attained during sintering, while simultaneously a distinct grain growth takes place through solution of the smallest grains of the solid phase in the liquid phase and reprecipitation on the larger grains In the first part of the survey this sintering mechanism which is called for short the heavy alloy mechanism is treated in detail In the second part of the survey experimental observations for the sintering of homogeneous sintered alloys where the liquid phase disappears before the completion of sintering are discussed Much less systematic work has been done on these alloys to which the commercially important porous bronzes and the iron-nickel-aluminum permanent magnet alloys belong Emphasis will be laid throughout this survey upon the microstructural and density changes during sintering because the sintering mechanism can usually be described directly by these changes Other

Jan 1, 1948

By R. W. Nelson

The theoretical basis is presented for energy dissipation methods of measuring permeability in saturated heterogeneous media. Analysis starts with the equation for single-phase flow in heterogeneous porous media which is a first-order partial differential equation in the unknown permeability. This equation is reduced to a system of characteristic equations useful in deriving a differential expression for the permeability as a function of the known potential distribution. For steady flow systems the expression is integrable, giving the permeability integral for direct calculation of the distribution along successive streamlines. The boundary condition in permeability is presented and includes a discussion of the special requirement to assure uniqueness. Theoretical results and necessary computations are incorporated into two general computer programs capable of determining the permeability distribution in two- and three-dimensional steady flow systems. Results are then presented for an energy dissipation analysis using the methods and soltware to determine a field permeability thickness distribution. INTRODUCTION Theoretical work leading to the description of fluid flow in heterogeneous porous media has developed slowly and somewhat sporadically, although the areas of engineering applications predominantly involve flow in nonhomogeneous media. The lack of accurate and economical methods for measuring field permeability distributions has been a major limitation to realistic description of natural flow systems. The economical measurement of permeability using energy dissipation methods may overcome this limitation. Among other things, any permeability measurement method involves consideration of energy dissipation; as used here, "energy dissipation method" denotes the following general approach. The areal energy distribution is measured on an existing flow system in porous media. Then through appropriate analysis of the dissipation of energy in the flow system, the permeability distribution is determined throughout the region using the minimum boundary conditions required to assure a mathematically unique result. This paper presents the theoretical basis for such a method, as well as the techniques and computer software needed to carry out the analysis. The method and techniques are then used in a preliminary field application. EQUATIONS DESCRIBING FLOW IN HETEROGENEOUS MEDLA Heterogeneous as used here implies differing in kind, having unlike quantities or possessing different characteristics. Accordingly, a heterogeneous porous medium has unlike quantities or differing characteristics at different locations. More precisely, if a porous medium is heterogeneous with respect to some property, then that property is functionally dependent on the spatial location. (Media properties are considered here as macroscopic.) Specifically considering permeability k,** then k = G(x, y, z) ,............(1) or the permeability is a scalar function G of the location (x, y, z). Spatial variation of several other properties, of which porosity, dispersivity and medium compressibility are but a few, is implied by the broad title of heterogeneous porous media. The term will be used hereafter to imply only heterogeneity with respect to permeability. The porous medium is considered isotropic as implied by G being only a scalar function. Anisotropy

Jan 1, 1969

By E. H. Burdick

The difficulties inherent in table concentration operations as applied to gold, silver, lead and zinc ores, are accentuated in the scheelite mill, which has a flowsheet that is similar in general principles. The narrow "spread" in the specific gravity between scheelite and the usual gangue in which it occurs often makes its clean separation extremely difficult and in some cases impracticable. The problem is further complicated by the extreme friability of the mineral and its inordinate tendency to "slime," particularly where fine grinding is necessary and wet grinding methods are employed. The physical aspect of the occurrence of scheelite in the ores, the kind and character of the rock in which the mineral is present, its hardness, friability and the specific gravity of its constituents, as well as like characteristics of the associated minerals— pyrite, pyrrhotite, sphalerite, and others, all have a bearing on the successful milling of crude ores and the production of a concentrate that is marketable with minimum price penalty or none at all. Most tungsten deposits are considered low grade when the limiting factor of low percentage recovery by gravity methods is borne in mind. The great majority of deposits developed or in process of development carry from 0.4 to 1.0 per cent tungstic oxide. Ore carrying I per cent is considered good grade; 0.7 to 0.8 per cent, fair, and below that figure, low grade. Steady mill feed averaging upward of I .o per cent WO3 is exceptional. Inasmuch as buying specifications demand a 60 per cent W03 content, the mineral content of the ores as mined must be concentrated to at least that degree. A few companies have constructed chemical plants for the treatment of their complex concentrates and low-grade flotation product, but the smaller operator must rely on table concentration, borrowing the tools and to a large extent the methods of his predecessors in the gold, silver, lead and zinc mills. Whether these are the best tools and methods that can be devised is an open question. Except in rare instances, table concentration, however carefully conducted, does not yield a satisfactory final recovery of the mineral content in headings as indicated by chemical determinations of tungstic oxide content. A saving of 60 to 65 per cent is considered fair, and an average 80 per cent recovery, exceptional. Each scheelite ore body is a separate problem. Details of handling that give best results in one case may not be successfully operative at a property on the same contact zone and but a short distance removed. Supplementary treatment of tailings by flotation may be successful in producing a low-grade concentrate of around 15 per cent W03, which is adapted to further treatment chemically at a given property and entirely unsuccessful at another. This problem constitutes another of the unanswered questions. The following paragraphs relate solely to the beneficiation of scheelite ores by table concentration and supplementary methods.

Jan 1, 1943

By E. H. Burdick

The difficulties inherent in table concentration operations as applied to gold, silver, lead and zinc ores, are accentuated in the scheelite mill, which has a flowsheet that is similar in general principles. The narrow "spread" in the specific gravity between scheelite and the usual gangue in which it occurs often makes its clean separation extremely difficult and in some cases impracticable. The problem is further complicated by the extreme friability of the mineral and its inordinate tendency to "slime," particularly where fine grinding is necessary and wet grinding methods are employed. The physical aspect of the occurrence of scheelite in the ores, the kind and character of the rock in which the mineral is present, its hardness, friability and the specific gravity of its constituents, as well as like characteristics of the associated minerals— pyrite, pyrrhotite, sphalerite, and others, all have a bearing on the successful milling of crude ores and the production of a concentrate that is marketable with minimum price penalty or none at all. Most tungsten deposits are considered low grade when the limiting factor of low percentage recovery by gravity methods is borne in mind. The great majority of deposits developed or in process of development carry from 0.4 to 1.0 per cent tungstic oxide. Ore carrying I per cent is considered good grade; 0.7 to 0.8 per cent, fair, and below that figure, low grade. Steady mill feed averaging upward of I .o per cent WO3 is exceptional. Inasmuch as buying specifications demand a 60 per cent W03 content, the mineral content of the ores as mined must be concentrated to at least that degree. A few companies have constructed chemical plants for the treatment of their complex concentrates and low-grade flotation product, but the smaller operator must rely on table concentration, borrowing the tools and to a large extent the methods of his predecessors in the gold, silver, lead and zinc mills. Whether these are the best tools and methods that can be devised is an open question. Except in rare instances, table concentration, however carefully conducted, does not yield a satisfactory final recovery of the mineral content in headings as indicated by chemical determinations of tungstic oxide content. A saving of 60 to 65 per cent is considered fair, and an average 80 per cent recovery, exceptional. Each scheelite ore body is a separate problem. Details of handling that give best results in one case may not be successfully operative at a property on the same contact zone and but a short distance removed. Supplementary treatment of tailings by flotation may be successful in producing a low-grade concentrate of around 15 per cent W03, which is adapted to further treatment chemically at a given property and entirely unsuccessful at another. This problem constitutes another of the unanswered questions. The following paragraphs relate solely to the beneficiation of scheelite ores by table concentration and supplementary methods.

Jan 1, 1943

By Louis E. Toth, Alan W. Searcy

A modification of Le Claire's microscopic model for self-diffusion is developed in a form suitable for prediction of activation energies for diffusion in disordered substitutional solutions as well as in pure metals Bonding is considered as a localized interaction, and the energy of bonding between atoms of different types is taken as the arithmetic mean of the energies in the pure elements. The activation energies for vacancy formation and migration in substitutional alloys are shown to depend on the empirical constants developed for self-diffusion when the equations are adjusted for the mole fractions of the two elements. The differences in activation energies between alloys and pure metals are calculated and compared with the experimental differences. MaNY different correlations have been proposed for use in predicting activation energies for self-diffusion or diffusion in dilute alloys. Usually theories for self-diffusion cannot be applied to alloys nor can theories for dilute alloys be satisfactorily adapted to self-diffusion. We present here a correlation scheme which can approximate activation energies for diffusion, vacancy formation, and atom migration in substitutional-alloy systems of any concentration. The activation energy for self-diffusion QD has been correlated to the melting temperature Tm,1, 2 the sublimation energy L,,l the heat of fusion Lf,3 crystal structure, and valence.4 Working with mechanical parameters Le claire5 proposed that the activation energy for vacancy migration Qm could be related to appropriate shear moduli. He also proposed that the activation energy for vacancy formation 62, is proportional to Ls. THEORY We consider that Le Claire's expression for 4, is satisfactory. But an alternate expression for Qm in terms of the bulk modulus is just as satisfactory from a theoretical point of view for the prediction of the energy of vacancy migration in pure metals and can more easily be used for alloys. In order for Qm to be a minimum, not only the moving atom but also adjacent and nearby atoms must undergo distortions. These distortions are neither isotropic as is implied by a correlation with the bulk modulus nor unidirectional as implied by a correlation with a shear modulus, but are complex and multidirectional. A complete analysis in terms of the bulk modulus and all the shear moduli is not yet available. In a first approximation in which only one parameter is used, there is no obvious theoretical reason to prefer the shear modulus over the bulk modulus. The bulk modulus is a much more convenient parameter for use in predicting Qm. Shear moduli are known for less than half of the metallic elements while bulk moduli are usually available. Furthermore, because of the directional dependence of shear moduli, their estimation for alloys appears inherently more difficult than is the estimation of bulk moduli. We assume that the bulk modulus Bs reflects an average value of the shear moduli such that the strain energy at the point of maximum distortion of the lattice during vacancy migration can be expressed as

Jan 1, 1964

By Jack White, Ralph B. Adair

THE Mufulira mine in Northern Rhodesia is 13° south of the Equator and at an altitude of 4100 ft above sea level. The concentrator was planned in 1930 to treat about 10,000 tons of ore per day, but before construction work had gone very far the size of the plant was reduced to 1500 tons daily capacity. Then just as the construction was finished in December 1931, the price of copper did not warrant the commencement of operations and it was not until October 1933 that the mine re-opened. Extensions have been made to the plant in 1034, 1935, 1936 1937, 1940 and finally in 1945. The capacity of the plant when the 1945 extension is completed will be 9500 short tons per day. The various extensions have been carried out with the minimum confusion and with great skill on the part of the designers and the erection engineers. In dealing with the plant in this paper it is proposed to give an outline of the present practice and only to refer to past practice when it is considered of particular interest. ORE TREATED The ore consists of fine grained highly siliceous, felspathic and argillaceous quartzites, the latter containing graphitic carbon and locally termed graywacke. These rocks are considerably altered near the surface and on the fringes of the ore body. Despite extensive efforts at mixing, both underground and in the crushing plant, many sudden changes of ore take place in the mill feed. The minerals are chalcocite, bornite and chalcopyrite with relatively unimportant amounts of malachite, native copper, azurite, covellite and chrysocolla. Gold is present at about 1 oz to a thousand tons, and silver at about 2 dwt. per ton. With increasing depth the nonsulphide content or, as commonly termed locally, the oxide content of the ore is decreasing and is now less than 0.25 pct against a high of 0.77 pct. CRUSHING OPERATIONS The first stage of crushing is done underground where the oversize from 12-in. grizzleys is fed to a 56 X 72-in. jaw crusher. At the Selkirk shaft two 10-ton skips deliver the run of mine ore at a rate of 450 tons per hour to the 400-ton capacity shaft bins. One 6 ft wide Ross chain feeder and one locally made air-operated finger feeder feed the two 30-in. McCully crushers. Ahead of each crusher is a 6 X 9-ft spring grizzley spaced at 5 in. on a 35° slope. Neither the Ross chain feeder nor the finger feeder is entirely satisfactory and for the wet, muddy and sticky ore that has to be handled it is planned to replace these with two 6-ft wide apron pan feeders. The McCully crushers deliver a minus 5-in. product, at a rate of over 500 tons per

Jan 1, 1947

By D. F. Atkins, B. A. Rogers

The constitutional diagram presented herein is relatively simple. Complete mutual solid solubility exists for an interval below the solidus line, a continuous curve with a flat minimum near 22 pct Cb and 1740°C. Upon cooling, the solid solution breaks up, except at the columbium-rich side, from two causes: zirconium-rich alloys transform under the influence of the ß-a transformation in zirconium; alloys of intermediate composition decompose into two solid solutions below 1000°C. The combined effect is the formation of a eutectoid at a temperature of 610°C and a composition of 17.5 pct Cb. The eutectoid horizontal extends from 6.5 to 87.0 pct Cb. Some age hardening effects have been observed in the zirconium-rich alloys but the positions of the solvus lines remain uncertain. IN recent years, zirconium has been produced in much larger quantities than were available previously. Correspondingly, the incentive for studying its alloy systems has increased, as the number of recent publications on alloy systems testifies. However, only a partial diagram of the Zr-Cb system has been published and relatively few references have been made to alloys of the two metals. Hodge' investigated the Zr-Cb system up to about 25 pct Cb. His data on melting points were not sufficiently numerous to distinguish with certainty between the alternatives of a narrow eutec-tic horizontal and a wide flat minimum in the solidus curve. Although Hodge considered his results on transformations in the solid state to be only tentative, he suggested that the eutectoid in the zirconium-rich alloys lay at about 625 °C and 10 pct Cb and estimated that the solubility of colum-bium in zirconium at 625 °C was near 6 pct. According to Simcoe and Mudge,2 less than 0.5 pct Cb is soluble in zirconium at 800°C. These authors observed an increased strength in both the 0.5 and I pct Cb alloys made with hafnium-containing zirconium. According to Keeler,3 the strength of zirconium is increased by addition of columbium to a content of at least 3 pct. Keeler' also observed a maximum in hardness at about 10 atomic pct Cb and commented on the brittleness of alloys of this composition. Anderson, Hayes, Rober-son, and Kroll5 investigated the tensile properties of Zr-Cb alloys containing 5.1 and 12.9 pct Cb at room temperature and at 343°C. The 12.9 pct alloy had a high tensile strength at room temperature but also a low percentage of elongation. All alloys had high elongation at 343 °C. Littona measured strength and elongation values of annealed alloys containing up to 27.5 pct Cb and found low elongation values for all of the alloys of high columbium content. Some observations on the resistance of Zr-Cb alloys to corrosion in water at high temperature have been published by Lustman, De Paul, Glatter, and Thomas' who found that additions of columbium up to 1 pct had only a minor effect on the corrosion resistance of zirconium. Preparation of the Alloys Raw Material: Zirconium of a relatively good grade was available for making the alloys. It was obtained as scrap pieces that had been left over from an operation that included production by the iodide process, melting under a protecting atmosphere, and fabrication to plates. The individual pieces had hardness values of 24 to 32 Ra and a typical analysis is shown in Table I. The columbium also was scrap trimmed from sheets. It was furnished by the Fansteel Metallurgical Corp. and had a high ductility but its analysis was known only approximately. The metal probably contained about 0.5 pct Ta, perhaps 0.25 pct C, and a few hundredths percent each of iron, silicon, and titanium. Melting: The alloys were melted in a tungsten-electrode copper-crucible arc furnace similar to units that have been described recently in the metallurgical journals.'.' The crucible of this furnace is provided with a cavity in which a getter charge can be melted before the melting of the alloy charges. Hardness measurements on the ingots indicate that the getter charge takes up a considerable fraction of the oxygen and nitrogen from the helium atmosphere of the furnace. The alloys used in the investigation are given with their intended compositions, hardness, and melting points in Table 11. Fabrication: All alloys of the Zr-Cb system appear to be amenable to fabrication. At least, all of the compositions listed in Table II could be reduced to wires in a rotary swaging machine. The starting material was either slabs cut from ingots and ground by hand to rough cylinders or narrow strips trimmed from sheets made by cold rolling slabs. However, not all of the alloys could be fabricated satisfactorily by the same method. From 0 to 4 pct Cb and from 20 to 30 pct Cb or more, the alloys could be swaged cold from ¼ in. cylinders to 0.80 mm wires with only one intermediate annealing, sometimes with none. From 40 to 90 pct Cb, the alloys were difficult to swage either hot or cold but could

Jan 1, 1956

By Henry R. Linden

The demand for natural gas continues to increase at higher than anticipated rates, partly because of its widening price advantage over most other fossil fuels when the cost of air-pollution control is included. However, there are clear indications that the natural gas supply from the conliguous 48 states and continental shelves will not keep up with this rapid growth in demand indefinitely. Projections are presented which define the extent of potential deficiencies from the 1970's to the year 2000. Among the sources of supplemental gas - imported pipeline natural gas from Canada and Mexico, tanker import of liquefied natural gas, and synthetic pipeline gas from coal and oil shale -by far the most abundant at potentially competitive costs is pipeline gas from coal. The state of development and relative economics of the various coal gasification processes are reviewed. It is shown that synthetic pipeline gas could become a very substantial market for bituminous coal and lignite at current mine-mouth prices - 60-70 million tons of coal for each trillion cubic feet of synthetic pipeline gas produced. This corresponds to only slightly more than the current annual increase in gas demand. Although annual discoveries (gross additions to proved reserves) of natural gas in the United States are still on a general upward trend from the current level of 22 trillion cu ft annually, most forecasters do not expect this to increase substantially in the foreseeable future. For example, the updated (to include 1966 and 1967 data) mathematical model of natural gas discovery and production in the U.S. developed by the Institute of Gas Technology (IGT)' projects that discoveries will level out at about 25 trillion cu ft annually in the late 1970's and during the 1980's and then decline to about 21 trillion cu ft by the year 2000 (Fig. 1). This adds up to a new supply for the period 1968-2000 of about 790 trillion cu ft. Experts who usually reflect the producers' viewpoint, such as Radford L. Schantz of Foster Associates,* are relatively more pessimistic. In contrast, a forecast just made by the U.S. Dept. of the Interior is much more optimistic.3 It assumes an increase in gas discoveries of 2.2% per year over the period 1965-80, reaching about 30 trillion cu ft in 1980. If this rate of increase were extended to the year 2000, annual discoveries would reach 46 trillion cu ft at that time, for a total over the period 1968-2000 of about 1100 trillion cu ft. To these forecasts of new gas discoveries must be added proved reserves of roughly 290 trillion cu ft,4 bringing total U.S. supplies for the rest of the century to nearly 1100 trillion cu ft (IGT) and possibly as high as 1400 trillion cu ft (U.S. Dept. of the Interior). This is approximately the same range as that of two estimates of total remaining recoverable natural gas supply: Potential Gas Committee, 980 trillion cu ft5 and IGT, 1450 trillion cu ft.6 Only the 1965 estimate by the U.S. Geological Survey7 suggests that economically recoverable natural gas supplies will not be exhausted around the end of the century. These forecasts are, naturally, based on the assumption that changes in technological, economic, and regulatory environment as they affect the gas industry will be of an evolutionary, not revolutionary, nature. The various forecasts of potential natural gas supply must now be compared to forecasts of natural gas demand (Table I). The general consensus is that the recent Future Requirements Committee projection to 1990' (extended to the year 2000 by the most recent U.S. Bureau of Mines (USBM) projection9) represents the minimum gas requirements (Table 11). They add up to a total of 1030 trillion cu ft for the period 1968-2000. Even this minimum anticipated gas demand exceeds the total remaining supply estimate by the Potential Gas Committee and would nearly exhaust the proved reserves plus new discoveries projected by IGT. The supply situation would appear much tighter if the demand projections of the Texas Eastern Transmission Gorp.10 and the American Gas Assn.(A.GA.)'' were used (Table I). Yet, these higher forecasts probably do not include the effects of such new markets as gas fuel cells, use of liquefied natural gas as a transport fuel, etc. They also may not fully reflect the impact of air quality control on the fuel market. Obviously, the probable discrepancy between projected supply and demand can only be accommodated in four ways. 1) Rapid increase in exploration and drilling activity to provide new supplies in the amount projected by the optimistic U.S. Dept. of the Interior forecast, coupled with an increase in net pipeline imports from Canada and Mexico from the present 0.5 trillion cu ft per year

Jan 1, 1970

By A. F. Henderson, C. F. Cornell, A. F. Dunyon, D. A. Dahlstrom

Ossi E. Palasvirta (Development Engineer, Oliver Iron Mining Diu., U. S. Steel Gorp.)—The authors are to be congratulated for their interesting article, which thoroughly illustrates the variables inherent in filtration of taconite concentrate. The work and the conclusions based thereon largely parallel the test work done by the writer at the Pilotac plant" and the experience gained with a commercial size agitating disk filter in the same plant. At Pilotac, however, a thorough study was also made of the effect of depolarizing (demagnetizing) the filter feed, and it is the purpose of this discussion to comment on the merits of depolarization of the magnetite concentrate prior to filtering. The work at Pilotac was done in three phases: 1) preliminary laboratory testing with a circular filter leaf of 0.047 sq ft, followed by 2) plant testing using a 4-ft diam, single-disk agitating filter that was purchased on the basis of the pilot tests on the 4-ft model. In the laboratory tests depolarization was achieved by slowly withdrawing' batches of thickened concentrate from a coil producing an alternating field of about 300 oersteds. In plant tests the standard Pilotac procedure' was employed, wherein the pulp falls freely through the depolarizing coil. The preliminary tests in the laboratory at first seemed to indicate that although depolarization of the filter feed decreases the cake moisture, it also tends to decrease the thickness of the cake, thus decreasing filtering rate. The tests with the 4-ft disk filter soon showed, however, that the compactness of the cake, attained during the form period because of depolarization, permitted a considerable decrease in drying time without any sacrifice in final moisture content. Thus, the filter could be operated at a much higher speed, and the overall capacity was higher than with magnetized feed. Because of the great compactness of the cake there was little shrinkage during the drying period, which prevented cracking and subsequent loss in vacuum. This in turn permitted operation with as thick a feed pulp as the diaphragm pumps could handle, eliminating the necessity of pulp density control. On the basis of these findings, the 6-ft agitating disk filter has been operated at 2 rpm, using feed pulps varying from 65 to 73 pct solids. Initially Saran 601 was used as medium, but it was later replaced with a relatively open, tight-twist nylon cloth. Filtering rates up to 750 lb per ft- er hr can be attained with feeds averaging about 70 pct- 270 mesh, and there is no trouble because of cracking. The cake moistures vary between 8.5 and 9.5 pct. To recapitulate, the merits of depolarizing the filter feed may be summed up as follows: 1) The well dispersed pulp shows less tendency to settle in the filter tank. 2) The homogeneous filter pool results in more even cake formation. 3) Because of absence of flocs, great compactness of cake is attained during the form period. 4) The cake does not tend to crack during the drying period. 5) A drier cake is produced. 6) A shorter drying period is necessary, permitting higher operating speed, which in turn results in increased capacity. 7) Density of the feed pulp can be kept as high as the equipment can handle. This increases capacity, since it is directly proportional to the percentage of solids in the pool. A few tests were also made to study the effect of chemical flocculants on filtration efficiency. Results showed that the effects of chemical and magnetic floc-culation were quite similar. Thus the use of a floccu-lant would impair rather than improve the filtering of magnetite concentrate. A. F. Henderson, C. F. Cornell, A. F. Dunyon and D. A. Dahlstrom (authors' reply)—We want to thank O. E. Palasvirta for his comments, particularly in view of the encouraging results obtained with demagnetized taconite concentrate. In our studies an attempt was made to evaluate the effects of depolarizing the feed to the plant filters by passing the slurry through a coil, similar to the method described by Palasvirta. Unfortunately, in our experiments there were no startling improvements in performance level, neither cake rate increase nor cake moisture reduction. However, when slow filter cycle speeds were employed, the filter cake tended to crack and the vacuum level dropped, resulting in an increase in cake moisture content. When demagnetized feed was used during slow speeds, no cake cracking was evidenced and the vacuum level remained constant. Thus the depolarizing coil was found necessary only in cases of cracking. It should be noted that most of our test work concerned a feed of 85 to 90 pct —335 mesh and about 60 pct by weight solids concentration. This contrasts with 70 pct —270 mesh and 65 to 73 pct by weight solids as noted by Palasvirta. Reviewing both sets of results, it might be concluded that depolarizing may be successfully employed to alleviate cake cracking tendencies and may markedly improve cake rates and moistures on the coarser taconite concentrates. Further investigations may disclose the exact relationship of grind and pulp density to the depolarizing action.

Jan 1, 1959

By J. J. Frawley, W. J. Childs, W. R. Maurer

The object of this investigation was to determine the growth habit of bismuth and bisrrtuth alloy dendrites as a function of supercooling. To do this, techniques were developed to increase the amount of supercooling in bismuth and bismuth alloys. For pure bismuth, the growth habit was dependent on the amount of supercooling. At low amounts of supercooling, about 10" C, prismatic dendrites were obtained. With increased supercooling, about 20 C, a hopper growth habit was observed. In many cases where hopper growth had occurred, the hopper dendrites were twinned during the growth process. This twinned surface enable prismatic dendrites to nucleate and grow by a twin plane mechanism. When the amount of supercooling was increased to about 25 °C, the growth habit was a triplanar growth. With still greater supercooling, about 3s°C, a branched growth habit occurred. The exposed planes on the prismatic, hopper,, triplane, and branched dendrites have been determined. The growth habit of the dendrites which grew along the crucible wall was found to have the (111) as the exposed plane, with <211> growth direction. It is apparent that dendritic growth of a metal is dependent on its purity and the solidification variables present. One of the solidification variables is the degree of supercooling. Supercooling, although often observed, has not been studied extensively until recent years. For dendritic growth to occur in a pure metal, the metal must be thermally supercooled. After the dendrites grow into the supercooled melt, the heat of solidification raises the temperature of the specimen to the melting point of the material and the remaining liquid will solidify at this temperature. Decanting is the removal of this remaining liquid before complete solidification. This removal of the remaining liquid after recalescence had occurred is a great aid in the study of dendritic growth. In this investigation, decanting was accomplished by a vacuum-decanting technique . Other investigators1-5 have studied the growth characteristics of various low-melting-temperature pure metals and alloys as a function of supercooling. However, large degrees of supercooling were not included. For their study of dendritic growth of lead, Weinberg and chalmersl employed a decanting technique which was achieved by pouring off the remaining liquid, exposing the solid/liquid interface. This method was employed later by Weinberg and Chalmers2 for the investigation of tin and zinc dendrites. The method for obtaining a solid/liquid interface was improved by Chalmers and Elbaum. They employed a triggered spring which was attached to the solidifying section of the specimen. Upon activation, the spring jerked the solid interface away from the liquid melt. In the study of growth from the supercooled state, a metal of low melting point which exhibited a high degree of supercooling was desired. Bismuth gave very consistent supercooling when a stannous chloride flux was employed. The maximum supercooling obtained was 91°C, with an average supercooling of between 65" and 75°C. The consistency of supercooling greater than 50°C was very high. The use of vacuum to aid in the rapid decanting of molten metal has proven to be very successful in this investigation. The vacuum gives a rapid decantation, usually leaving the solidified metal structure sharply defined. The purpose of this investigation was to study the effects of supercooling and the effects of alloy additions on the growth habit of bismuth dendrites. The structure of bismuth has been variously defined as face-centered rhombohedral, primitive rhombohedral, and hexagonal. However, bismuth has only one plane with threefold symmetry, the (111) plane, and the crystal-lographic structure is considered a 3kn structure. MATERIALS The bismuth which was employed in this investigation was obtained from the American Smelting and Refining Co. of South Plainfield, N. J. The accompanying spectrographic analysis data indicated the bismuth to be 99.999+ pct pure. The tin was obtained from the Vulcan Materials Co., Vulcan Detinning Division, Sewaren, N. J. It was classified as "extra pure". Nominal analysis was 99.999+pct. In order to prevent contamination of the bismuth melt from the atmosphere, an anhydrous stannous chloride (Fisher certified reagent grade) was added to each melt. The fluxing action obtained from the use of the chloride provided a large amount of supercooling in the specimen. APPARATUS A 30-kw, 10,000-cps motor-generator set, connected to a 6+-in.-diam air induction coil, was employed to melt and superheat the specimens. The temperatures were recorded by means of a chromel-alumel thermocouple and a potentiometric recorder. The thermocouples were 0.003 in. in diam, and were encapsulated with Pyrex glass to prevent the thermocouple from acting as a nucleating agent and also from contaminating the melt. Fig. 1 illustrates the vacuum-decanting apparatus when a liquid flux was employed. A standard 30-ml Pyrex beaker was placed on top of an asbestos insulating block. A 5-mm-ID Pyrex tube with aA-in. spacer tip attached to its end was used for the decanting tube. The spacer tip contributed significantly to a successful decanting operation. The tip located the opening of the decanting tube about -^ in. from the bottom of the

Jan 1, 1969

By Louis Moyd

In the preparation of illustrations to accompany reports of investigations concerning particle shapes of various natural and manufactured materials proposed for use as he aggretates in concrete structures, it was found that stereoscopic views of such materials presented the information much more effectively than the usual two-dimensional photographs. With the whole-hearted cooperation of the Photographic Section, a technique was developed in the petro-graphic laboratory of the Concrete Research Division, U.S. Corps of Engineers, at Clinton, Miss., for simple and rapid preparation of paired stereoscopic photographs and micrographs. The method requires only equipment which is usually part of the general stock of any laboratory, and it is adaptable for all purposes in which three-dimensional views would be found superior to ordinary photographs. Stereoscopic Photographs Where the subject or subjects to be photographed are small but not of microscopic size, an ordinary camera having a ground-glass focusing screen is set up vertically over the field. The material to be photographed is placed on a card, the surface of which is suitable as a background. The card is mounted so that it can be tilted about 5&apos; above and below the horizontal plane, on an axis that coincides with the north-south diameter of the viewing screen. A simple way of achieving this is to mount the card on an object having a convex lower surface. In our laboratory, a watch-glass has been used, with plasticene as the mounting medium. The convex surface should be mounted on a support having a round opening on top and a flat bottom. A roll of tape, a petri dish, a beaker, or any other piece of equipment having the proper shape to give stability to the setup will be satisfactory. The arrangement of the required equipment is shown in Fig 1. For the best results, the subject to be photographed should be mounted in the plane of the axis of rotation. To achieve this effect, the depth of the mass of plasticene between the background and the watch-glass is varied so that when the watch-glass is tilted back and forth on the axis, the subject to be photographed does not appear to move back and forth across the viewing screen. A permanent piece of equipment, embodying an adjustable vertical screw, with lock-nuts, could be made for this purpose. The subject may be illuminated in any manner desired. It was found that the presence of shadows intensifies the effect of depth. In taking the photographs, the concave surface is mounted so that one side of the background card is about 5&apos; below the horizontal, while the axis remains in the horizontal plane. The subject is focused carefully on the ground glass and a picture is taken. The card is then tilted so that the opposite side is about 5&apos; below the horizontal, while the axis still remains in the horizontal plane. The subject is again put in focus and another picture is taken. After both pictures are developed and printed, they are placed side by side in the same orientation and examined with a stereoscopic viewer. If depressions appear where there ought

Jan 1, 1950

By E. H. Crabtree, Neil S. Parker

The custom milling plant of the Eagle-Picher Mining and Smelting Co. is at Sahuarita, Arizona, approximately 20 miles south of Tucson. It is connected by a 2-mile railroad spur to the main line of the Southern Pacific Railroad between Tucson and Nogales. The capacity of the plant is 500 tons per day. The mill is conventional except perhaps in the rather wide variety of ores treated. These vary from straight copper ores, from which only one concentrate is produced, and lead-zinc or copper-zinc ores from which two concentrates are produced, to complex gold-silver-copper-lead-zinc ores, which require the simultaneous production of three or lour concentrates. Custom ores are received in lots varying in size from a few truckloads to several carloads. Similar ores, after crushing and sampling, may be commingled, or individual lots may be milled individually. The ores are first dumped either into one of two 200-ton railroad bins, or into one of the two 75-ton truck bins. From these bins the lot of ore is individually crushed in a 24 by 16-in. jaw crusher to 3-in. size, and then to ½-in. size in a 3-ft Symons shorthead crusher. After crushing to ½-in., the ore is conveyed to the automatic sampling plant. This consists of three Vezin samplers in series, each successively cutting out 10 pct, 10 pct and 5 pct of its feed. After No. I sampler, the material passes through a mixing barrel and is then crushed to -in. in a set of rolls before passing to No. 2 sampler. Between No. 2 and No. 3 samplers, the ore is again mixed in a mixing barrel. The final sample, consisting of one pound per ton of ore, is taken to the laboratory, where it is further crushed to 1/8-in. in a coffeemill, and then riffled down to the size required for grinding in a pulverizer. The capacity of the crushing and sampling plant is up to 100 tons per hour, depending upon the character of the ore. After sampling, the ore is stored in five ore-storage bins, consisting of four 200-ton bins and one 175-ton bin. Each of these bins is discharged by means of belt feeders and conveyor to the ball mills, so that ores may be composited for milling in any required ratio, or milled separately by themselves. Fine grinding is done in one 6 by 4-ft Allis-Chalmers ball mill and one 8-ft by 36-in. Hardinge mill. These two mills both discharge to one 54-in. Akins Simplex High weir classifier. The classifier sands can be returned to either or both mills, so that flexibility in grinding for different ores is easily obtained. Flotation equipment consists of four 66-in. Fagergren machines for copper-lead roughing and six similar machines for zinc roughing. Lead-copper bulk concentrate is cleaned and recleaned in eight No. 18-S Denver machines, and zinc concentrate is cleaned in three 66-in. Fager-grens. Copper-lead differential separation

Jan 1, 1949

By L. S. Darken

The manufacture and treatment of metals comprises operations whose effectiveness depends in large measure upon diffusion phenomena. The significance of such phenomena has, for a few simple cases, long been understood, but in more complex cases it has not been adequately recognized. This paper presents a mathematical analysis of some of the cases in which diffusion leads to the formation of a new phase within the metal, and this analysis provides a basis for interpreting quantitatively many apparently diverse experimental observations recorded in the literature, some of which are outlined in the following pages. OBservt1ons Recorded in Literature An outstanding instance is the general type of phenomenon in which diffusion of an element into an alloy is followed by a phase change, which most frequently is a reaction (precipitation) of the diffusing element with one or more components of the alloy to form a compound. Thus in certain alloys heated in contact with oxygen not only the usual oxide scale appears but, in addition, beneath the surface, a well-defined zone in which free oxide particles are dispersed with notabIe uniformity. This zone, commonly referred to as the subscale or zone of internal oxidation, is remarkable in that the visible boundary between it and the unaltered metal is usually sharp. Such a subscale, which appears to have been recognized first in oxidized copper alloys, has been described and studied by C. S. Smith,l by Rhines,2 and by Rhines, Johnson and Anderson;3 but it appears also in other alloys when they are exposed to conditions that bring about the precipitation of some compound, not necessarily an oxide. Another clear case of this same kind is the diffusion of carbon into a steel containing an element such as molybdenum, chromium, vanadium or tungsten, whose carbide is less soluble in iron than iron carbide, the result being that some part of that element is precipitated as a dispersed carbide. A similar phenomenon may occur on the welding of two alloys each of which contains one of a pair of elements that react to form a compound or a new phase of any sort. But in addition to the precipitation of a compound formed from the diffusing element, other effects may result merely from the presence of the diffusing element. For instance, if homogeneous austenite, nearly saturated with respect to carbon, is heated in contact with silicon in the absence of oxygen, one would expect to find, behind the several iron-silicon phases analogous to a scale, a zone of austenite containing dispersed carbide, because of the fact that silicon decreases the solubility of carbon in iron. As still another example, oxidation of an alloy may result in diffusion toward the surface of one or more of the constituents; for instance, during oxidation of a steel containing an element such as "copper, which is less prone to oxidation than iron, there is a notable enrichment of copper just beneath the scale-metal interface. AS an example of a yet more

Jan 1, 1942

By L. S. Darken

The manufacture and treatment of metals comprises operations whose effectiveness depends in large measure upon diffusion phenomena. The significance of such phenomena has, for a few simple cases, long been understood, but in more complex cases it has not been adequately recognized. This paper presents a mathematical analysis of some of the cases in which diffusion leads to the formation of a new phase within the metal, and this analysis provides a basis for interpreting quantitatively many apparently diverse experimental observations recorded in the literature, some of which are outlined in the following pages. OBservt1ons Recorded in Literature An outstanding instance is the general type of phenomenon in which diffusion of an element into an alloy is followed by a phase change, which most frequently is a reaction (precipitation) of the diffusing element with one or more components of the alloy to form a compound. Thus in certain alloys heated in contact with oxygen not only the usual oxide scale appears but, in addition, beneath the surface, a well-defined zone in which free oxide particles are dispersed with notabIe uniformity. This zone, commonly referred to as the subscale or zone of internal oxidation, is remarkable in that the visible boundary between it and the unaltered metal is usually sharp. Such a subscale, which appears to have been recognized first in oxidized copper alloys, has been described and studied by C. S. Smith,l by Rhines,2 and by Rhines, Johnson and Anderson;3 but it appears also in other alloys when they are exposed to conditions that bring about the precipitation of some compound, not necessarily an oxide. Another clear case of this same kind is the diffusion of carbon into a steel containing an element such as molybdenum, chromium, vanadium or tungsten, whose carbide is less soluble in iron than iron carbide, the result being that some part of that element is precipitated as a dispersed carbide. A similar phenomenon may occur on the welding of two alloys each of which contains one of a pair of elements that react to form a compound or a new phase of any sort. But in addition to the precipitation of a compound formed from the diffusing element, other effects may result merely from the presence of the diffusing element. For instance, if homogeneous austenite, nearly saturated with respect to carbon, is heated in contact with silicon in the absence of oxygen, one would expect to find, behind the several iron-silicon phases analogous to a scale, a zone of austenite containing dispersed carbide, because of the fact that silicon decreases the solubility of carbon in iron. As still another example, oxidation of an alloy may result in diffusion toward the surface of one or more of the constituents; for instance, during oxidation of a steel containing an element such as "copper, which is less prone to oxidation than iron, there is a notable enrichment of copper just beneath the scale-metal interface. AS an example of a yet more

Jan 1, 1942

By R. F. Mehl, M. L. Swanson, J. P. Hirth, G. M. Pound

The sorption of zinc in a-brass was investigated. By analyzing the elliPtically polarized light reflected from the surfaces of a-brass solid-vapor couples, their optical constants and surface compositions were determined. It was shown that the surface compositions approached an equilibrium value exponentially with time. The bulk diffusion coefficient, surface diffusion coefficient and condensation coefficient for a-brass solid-vapor couples were calculated near 350°C from the experimental results. It has been assumed during various diffusion experiments with bimetallic solid-vapor couples (Dunn,1 Seith and Kraus,2 Jenkins,3 Balluffi and Seigle,4 and others) that the surface composition of such couples reaches its equilibrium value immediately. Although this assumption is probably valid at elevated temperatures, it is certainly not expected to be confirmed at lower temperatures.* consequently, it was decided that a fundamental experiment, designed to solve this problem as well as to examine all aspects of bimetallic sorption, would be the investigation of the rate of approach to equilibrium of the surface composition of solid-vapor couples. The sorption (i.e., adsorption, absorption, and desorption) of metals is obviously dependent on the state of the metallic surface and substrate. Much recent theoretical8"9 and experimental10-12 work on condensation and evaporation of pure metals has shown that the surface contour in atomic dimensions is a critical factor in sorption, so that any deformation or contamination13,14 of the surface exerts a profound influence. It is also known that absorption or diffusion occurs much more rapidly along grain were and in other deformed regions. Thus an effective sorption experiment must utilize a strain-free and clean surface, or at least a clearly defined and reproducible surface state. A mechanically polished surface is in a highly deformed17-l9 state. Because the degree of this deformation is uncertain, it is clear that for most experimental purposes a strain-free (i.e., etched, chemically polished, or electropolished) surface is desirable. Although adsorbed gas molecules on metallic surfaces are unavoidable even in the highest vacuum,20 oxide films can in some cases be removed by suitable vacuum annealing. Electrolytically polished and annealed surfaces, being relatively without strain, smooth and free of bulk oxides, were considered adequate for the present sorption experiments. Measurement of the surface composition of a solid-vapor couple presents a unique problem. Since experiments to determine the approach to equilibrium of the surface composition must be performed at temperatures where diffusion is very slow, solute penetration is so small that conventional analytical methods are unsuitable. An optical reflectivity method utilizing a polarization spectroscope was employed because of the relatively small penetration (~200A) of visible light in metals. By this method, elliptically polarized light reflected from the metal surface was analyzed and compared with that reflected from samples of known composition. The a phase of the copper-zinc system was

Jan 1, 1962