The extractive metallurgist in an emerging world of...

The 1989 Extractive Metallurgy Lecture The Minerals, Metals & Materials Society

The Extractive Metallurgist in an Emerging World of Materials

J.K. BRIMACOMBE

Extractive metallurgy was born in fire some 6000 years ago, but it is only in the last 50 years that knowledge of the chemical and physical aspects of metals has flowered. During this latter period, a beginning has been made to probe the dynamics of metals processes and, spurred by advances in computer technology, to quantify complex process behavior with mathematical models. But this process engineering activity is, in many respects, at an early stage, especially when compared to physical metallurgy. The application of process engineering to both extractive and thermomechanical processes unifies the broad flow sheets by which metals are produced and, at last, provides the quantitative linkage between the metal product (intermediate or final) and each process stage. Owing to the complexity of metals processes, whether treating metal ~n the liquid or solid state, process engineering is, by definition, interdisciplinary; and it is as dependent on measurements on the full or pilot scale as it is on mathematical models to characterize process behavior quantitatively. There are lessons to be learned from the extractive metallurgist by the materials scientists and engineers who are renaming our Metallurgy Departments and restructuring curricula as well as research. Process engineering must be taught as a discipline to undergraduates and graduate students alike, particularly those planning a career in industry. The reality, however, is frequently the opposite at many universities, where the characterization of materials properties dominates the curriculum and isolates students unnecessarily from the needs of industry. Likewise, process engineering research must be boosted in the metals and materials field to replace descriptive information on processes with quantitative knowledge. The competitiveness of our industry depends on it.

The Extractive Metallurgy Lecture was authorized in 1959 to provide an outstanding individual in the field o f nonferrous metallurgy as a lecturer at the annual AIME meeting.

J. KEITH BRIMACOMBE is the Stelco/NSERC Professor and Di- rector o f the Centre for Metallurgical Process Engineering at the Uni- versity of British Columbia. After receiving an honors Bachelor ' s Degree in Metallurgical Engineering at the University o f British Columbia in 1966, he was awarded a Commonweal th Fellowship and studied under Professor F.D. Richardson, F.R.S. in the John Percy group at the Royal School of Mines, London. He earned his Ph.D. Degree in mass transfer and interracial phenomena in 1970, where- upon he returned to the University of British Columbia to establish teaching and research programs in process metallurgy. He has since been awarded the Doctor of Science in Engineering Degree by the University of London in 1986. His research interests have centered

on the quantitative characterization of metallurgical processes for the production o f nonferrous and ferrous metals; these processes range from the rotary kiln, copper converter, and flash smelting processes to continuous casting and microstructt~'al engineering. This research has involved extensive mathematical modeling and industrial trials as well as laboratory studies and has led to the publication of more than 150 papers, including two books and three patents. He and his colleagues have received numerous honors for their research including the Extractive Metallurgy Science Award in 1979 and 1987 and the Extractive Technology Award in 1983 from The Metallurgical Soci- ety. He has also been awarded the Champion H. Mathewson Gold Medal in 1980 and is a Distinguished Member o f the Iron and Steel Society as well as a Fellow of the Royal Society of Canada. From 1985 to 1986, he was President of the Metallurgical Society of the Canadian Institute of Mining and Metallurgy. Currently, he serves as Chairman of the newly formed TMS Extractive and Processing Division.

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I. A N E A R L I E R T I M E

W H E N I entered the field of metallurgy twenty-five years ago, it was a different time. North America dominated the world production of metals like steel and nickel; metal prices generally were buoyant, and predictions were being made of unlimited growth in steel demand worldwide. Optimism prevailed. As a fresh undergraduate at UBC, I was unaware of the ebb and flow of events in the metallurgical world. My immediate concern was to ne- gotiate my way through the metallurgical engineering curriculum, grappling with new (to me) concepts of chemical thermodynamics, pyrometailurgy, hydrometal- lurgy, crystal structure, dislocations, and microstructural phenomena. Interestingly, it was just before the time that calculators were introduced to replace the slide rule or, as we called it, the "slipstick." Much has changed since then.

In the intervening years, I have witnessed the rapid expansion of steelmaking in Japan and, latterly, in Korea and so-called "developing nations" like Brazil, while the steel industry has shrunk in North America and Europe (and, more recently, in Japan). Global realignment has not been confined to steel but also has been the case for nonferrous metals like copper and nickel. I have watched with others in dismay as metallurgical research labora- tories have closed, particularly in the U.S. nonferrous industry, while others cut back their activities. This occurred during the same period in which OPEC taught us the value of energy upon which our processes are so dependent. It has been a time of recognition that metals processes must work in harmony with the environment and, finally, that our planet is finite. It also has been a time in which the metals industry came to be regarded as "smokestack" and "low tech" by a sector of our society.

More recently, the spotlight of attention has shifted away from metals toward new advanced materials, driven in part by aerospace programs fueled by defense and commercial interests. So strong has the movement become that the words "metallurgy" and "metals" are dis- appearing rapidly from North American university campuses to be replaced by "materials science and engineering." These advanced materials, metals included, increasingly compete in the global marketplace with products from technologically sophisticated corporations in Japan and Europe as world trade shifts. At the same time, competition is growing between different Metals and materials in numerous applications like electronics, automobiles, beverage cans, and construction. Domi- nating the scene over the last two decades has been the computer, which has placed enormous computational power in our hands. We have entered the Knowl- edge Age.

With such major changes, it is little wonder that the extractive metallurgist would ponder his future and ask what role he has to play in the metals and materials field as it evolves. Is he a dinosaur, at least in the North American context, or does he possess a unique background ideally suited to the development of new processes and the improvement of existing operations? If it is the latter, are we pursuing extractive metallurgy or, more broadly, the process engineering of metals and materials as vigorously as needed? In this lecture, I will

probe these questions and, hopefully, provide some di- rections. What emerges is the definition of metals and materials processing as a broad interdisciplinary field rooted in chemical metallurgy, physical metallurgy, and the different engineering disciplines. The application of integrated process analysis will be seen to be as neces- sary for existing metals and materials processes as it is for new processes. But first, to gain historical perspec- tive, let us look back a long way in time.

II. T H E E V O L U T I O N OF E X T R A C T I V E M E T A L L U R G Y

Born in fire, extractive metallurgy has shaped civili- zations for millennia. It began with Neolithic man who, probably by accident, reduced malachite with charcoal to form copper 6000 years ago and, subsequently, established a copper smelting industry at Timna in the Southern Sinai. Extractive metallurgy has a history dominated by the same factors that influence pyrometallurgi- cal production today: raw materials availability, melting points, composition, furnace design, ceramics, and metal product properties. The addition of tin to copper, whether initially by accident or design, around 3000 B.C. low- ered the melting point of the resulting bronze alloy by several hundred degrees Centigrade and facilitated casting while imparting new properties to the metal. The Chinese, in particular, elevated bronze casting to an advanced state with superior ceramics and furnace design. Bronze gave way to iron in Western Civilization, possibly because the tin trade was disrupted by war in the eastern Mediterranean basin around 1200 B.C. The first iron produced by fire was probably a serendipitous event, a consequence of the use of iron ore as a flux in early copper smelting. This iron by-product was not liquid, owing to its high melting point and the relatively low maximum temperatures attainable in the crude furnaces. Separation of the iron from the slag was achieved by blacksmiths, who hammered the metal bloom into wrought iron after repeatedly reheating it in a charcoal fire. Owing to its inherently inferior properties, iron would not have replaced bronze so readily were it not for the discovery in about 1500 to 1000 B.C. in Mesopotamia that during the charcoal forge reheating, the metal carburized to form steel with properties closer to those of bronze. This was followed by the equally important discovery that the steel could be made harder by quenching it in water, although the metal also became more brittle and prone to crack- ing. This problem was solved by raising the temperature of the steel to about 700 ~ for a short time and then allowing it to cool more slowly. Thus, the quench-and- temper process, the beginning of steel physical metallurgy, came into being to make a product with superior properties for tools and weapons.

But it was the Chinese who became the leading ferrous metallurgists in about 600 B.C. With superior furnace design coupled to horizontal and double-acting box bellows, they were able to smelt iron ore with an excess of carbon to produce the first molten "pig iron" that, like bronze, could be cast. This major breakthrough, built on early technology and innovation, was not repeated in Europe, which remained tied to the forge and anvil for another 2000 years. The Chinese went further and learned

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how to decarburize the surface of cast iron objects and create a tough steel skin by heating them to 800 ~ to 900 ~ in air. By the first century A.D., these ingenious metallurgists were converting liquid cast iron directly to wrought iron containing less than 1 pct carbon by stirring or "puddling" the molten metal. The process was un- known in Europe for centuries until Henry Cort rein- vented it in England in 1784. Finally, it was Henry Bessemer who took the oxidation of molten cast iron a stage further in 1856 with the injection of air into the metal. With the consequent acceleration of the reaction rate and productivity, the steel industry, as we know it today, began to evolve and take shape.

Up to the time of Bessemer, serendipity, ingenuity, and tenacity alone made the difference between discovery and stagnation, because science and engineering, as applied to processes, were in their infancy. In the middle of the nineteenth century, metallurgical knowledge was limited to the overall chemistry of processes and to their thermal requirements. The engineering of processes and the properties of metals were largely descriptive. Al- though knowledge of the solid state of metals was relatively scarce, ,this was to change with an increasing emphasis on metallography. By the 1930's, the development of X-ray diffraction and improvements to optical microscopy ensured that physical metallurgy was established as a strong discipline which has continued to the present day. By comparison, the growth in knowledge of the chemical aspects of metallurgical processes was slower and, for reasons not altogether clear, confined to a smaller group of dedicated individuals. Perhaps the ex- perimental difficulties of working with complex solutions at high temperature deterred all but the brave, foolhardy, or ambitious, but progress in chemical metallurgy eventually was made, set on the foundations of thermodynamics. Thus, particularly in the last fifty years,_ the thermodynamics of many pure phases, solutions, andi metallurgically important chemical reactions have been determined. Perhaps the epitome of that knowledge is the free energy diagram, an example of which is shown for oxides in Figure 1.[~] This plot of Gibbs free energy against temperature is an elegant summary of the relative stability of the pure mater ia ls - -as useful to the materials scientist as to the extractive metallurgist. The concepts on which the diagram is based are at once brilliant and di f f icul t - -as any bewildered undergraduate student will tell you! Looking back at these developments, the names of a cadre of great men prevail, including Richardson (for whom, as my Ph.D. supervisor, I had the deepest admiration), Schenck, Chipman, Darken, Wagner, Kubaschewski, Lumsden, and Kelley.. More recently, the important work of Kellogg, Yazawa, Elliott, Turkdogan, Alcock, and Gaskell, amongst others, can be cited. Similar advances have been made in hydro- metallurgy by notables such as Pourbaix, Burkin, Wadsworth, and Peters. Changes in computer technology, particularly with respect to personal computers, have facilitated the organization of this thermodynamic information into data banks and have spurred the creation of "user-friendly" software packages to compute every- thing from phase diagrams to complex equilibria. We have come a long way from the days of Bessemer.

But, for all these achievements, one area of extractive

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metallurgy has been badly neglected: process engineering or process metallurgy. It remains a mystery to me that an activity at the heart of extractive metallurgy, and upon which the metallurgical industry depends utterly, has not been given the emphasis it d~serves in both research and undergraduate curricula. As I shall show later, this failing is the Achilles heel of the materials revolution as well and has impacted adversely on process development for both metals and materials production. Our approach to processing sometimes seems uncomfortably similar to that of Neolithic Man. This is not to say process engineering is dead or dying; rather, it is not being pursued with the vigor that its importance warrants.

Richardson was one of the strongest proponents of the need for enhanced research and teaching in process engineering, as well as in chemical metallurgy. In his 1971 Extractive Metallurgy Lecture, TM he lamented: "Unfor- tunately in many institutions it (Process Engineering) is being unreasonably starved of men and money." The sit- uation is only marginally better today, with strong university research teams sprinkled sparsely over the globe in Canada, the United States,~Europe, and Japan.

The neglect of process engineering has had consequences not only for the development of extractive metallurgy but for mechanical metallurgy (solid state processing of metals) as well. There has been a singular lack of effort to apply physical metallurgy knowledge on phenomena ranging from phase transformation kinetics to precipitation, in a quantitative manner, to thermomechanical processing. Our emphasis has been on the characterization of metals at the expense of processing. Thus, our understanding of these processes remains nearly


as qualitative today as it was when I was an undergraduate. As will be shown in the next section, this is a inajor problem crying out to be addressed.

HI. A VISION OF M E T A L S PROCESS ENGINEERING

Our failure to develop metals process engineering as a discipline at the universities has caused us to lose rel- evance to the metals industry. Unlike the evolution of industrial chemistry to chemical engineering early in this century, the development of metals process engineering is only gradually taking place in the 1980's. Today, many graduating metallurgical engineers, or their materials counterparts, can barely perform a heat balance, have only rudimentary computational skills, and have not been oriented to think in terms of process analysis and design like chemical or mechanical engineers. Little wonder then that industry, dependent as it is on processes, frequently turns to chemical and mechanical engineering departments for their "new blood."

One might argue, like the mechanical engineering professor from an illustrious east coast university I recently encountered, that this is the way it should be. Leave the science-oriented research and teaching to the metallurgist or materials scientist on the scale of crucible and microscope, and vacate the processing field in favor of other engineering disciplines. My answer to this person, only half in jest, was that I have spent a considerable part of my career correcting the design flaws of mechanical engineers in continuous-casting machines which have caused a host of quality problems (an example will be given later). The process engineer has to understand the links between a given process and the product it makes. In this particular instance, he must comprehend the mechanical behavior of metal at elevated temperature. But, in a broader context, he must have a thorough grounding in the properties of metals, both physically and chemi- cally, or be working jointly with someone who has this background.

There is an even more compelling reason to develop metals process engineering as a discipline, and this can be found in the nature of the processing itself. The flow sheets of metals production are broad, spanning raw materials preparation, extraction, refining, alloying, casting, and hot and cold working. An example for iron and steelmaking is shown in Figure 2. In this chain o f processes, what happens in the early stages can influence the operation of later stages and, even more importantly, the properties of the final product. For example, refer- ring to Figure 2, poor desulfurization of steel in the ladle could lead to difficulties in continuous casting and to adverse performance of a product like pipe for arctic service. Thus, the process engineer needs to have a strong grasp of both chemical and physical processes. Process engineering, or process metallurgy, is a much broader field than "high temperature chemical engineering," as it was called when I was a graduate student.

If mechanical and chemical engineering by themselves do not encompass metals process engineering ade- quately, they are, at the same time, essential components of the discipline. With such breadth of activity, it is hardly surprising that metals process engineering is character-

istically interdisciplinary in nature, drawing together engineers from these fields with chemical and physical metallurgists. The former are imbued with principles of analysis and design; they are at home with fluid flow, heat transfer, mass transport, dynamics, stress analysis, and numerical methods. The latter are active in the characterization of the chemical and physical behavior of metals, such as chemical thermodynamics, chemical kinetics, microstructural phenomena, and mechanical properties. Metals process engineering, then, naturally is a bridge, providing the link between the macroscopic behavior of processes and the characteristics of the products made at each stage in a flow sheet. This is shown pictorially in Figure 3.

That the process engineering discipline has not evolved in this manner on any sort of scale in the academic community has impeded the quantitative understanding and development of metallurgical processes, as I have suggested earlier. One might have supposed that such "mature" processes as rotary kiln calcination of limestone or the fuming of zinc from lead blast-furnace slag, which have been in operation for decades, would be understood in great detail. With respect to process dynamics, such is not the case, as shall be seen, and similar arguments could be mounted for other processes, both chemical and physical. Thus, while looking to the development of new processes in the future, the process engineer has the daunting challenge of grappling with existing operations, trying to determine the rate-limiting steps, and then prod- ding reactors to reach new levels of productivity or learning how to engineer the microstructure into a metal product.

With all that there is to be done, what tools does the process engineer have at his disposal? For the answer to this question, we need only turn to the 1972 Extractive Metallurgy Lecture of Nick Themelis, r41 who delineated these clearly with numerous examples from the copper industry. According to Themelis, there are four such tools:

(1) measurements in an existing process; (2) pilot plants; (3) physical models; and (4) mathematical models.

To these, I would add a fifth tool- -measurements in the laboratory as required to probe the mysteries of a process. Looking broadly over metals process research in the last two decades, one sees that the first two tools have been utilized primarily by industry, while the latter two in the list have been favored by academic research- ers. Although perhaps understandable, this is a pity, because the real strength of process engineering lies in a combination of models and plant measurements, as indicated in Figure 3.

If metallurgical processes, involving either liquid or solid metal, can be characterized by a single word, it is "complexity." Thus, the study of a process inevitably must include, at an early stage, measurements of one kind or another in a full-scale operation or in a pilot plant. More often than not, the measurements must be made under difficult, even hostile, conditions. The hostility may stem as much from the wrath of a plant superintendent, who bears the burden of meeting daily production quo- tas, as it does from noise, heat, dust, noxious gases, and

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Fig. 2--Flow sheet of iron and steelmaking processes (from The Making, Shaping and Treating of Steel[S~).

an element of danger! Ultimately, there are limits to what can be measured, imposed by economics, accessibility, safety, and urgency. We have not, for example, been able to talk a copper company into injecting a process engineer bodily into a Peirce-Smith converter through a tuyere so that he could obtain "first-hand information on the trajectory o f the air jet and its physical and chemical

interaction with the matte," as Themelis [4] has suggested in the ideal limit. However, we have been permitted to observe the growth of accretions at the tuyere tip, in a converter through a tuyerescope inserted into the back of the pipe, and to measure pressure fluctuations during the discharge o f air into the bath at numerous smelters in North America.tS] These are the kinds o f measurements

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Fig. 3--The elements of metals process engineering and its relation to the process-product linkage.

METALLURGICAL TRANSACTIONS B VOLUME 20B, JUNE 1989-- 295

that need to be made to give us insight into an aspect of process behavior and to provide the knowledge required to proceed with the development of a mathematical model or the construction of a physical model. There is nothing quite like going to a process, asking it important questions, and learning how to interpret the answers one re- ceives in reply.

Measurements of all kinds have been facilitated by the development of sensors and high-speed data acquisition systems. This is driven home to me every time I think about early measurements we made in the laboratory, of gas fraction and bubble frequency in submerged gas jets horizontally injected into a mercury bath. Greg Oryall connected a single-tip electroresistivity probe and the mercury bath to a dc power supply and then measured the fraction of time the circuit was open (probe tip sur- rounded by gas) and the number of circuit interruptions at different locations in the jet. t~] Laboriously, the data were recorded in a lab book for subsequent input to a computer and contour mapping. In more recent times, Humberto Castillejos constructed a two-element electroresistivity probe and coupled it to a microcomputer equipped with specially developed hardware and software which could analyze signals from the probe in real time. t6,7) Thus, he was able to measure the distributions of gas fraction and bubble frequency in aqueous and me- tallic baths with much greater precision and also to determine the spectra of bubble velocity and pierced length.

Similarly, we have evolved from the use of a bulky data acquisition system based on a paper tape punch for recording mold temperatures in continuous casting ESJ to portable, high-speed digital systems employing discs or magnetic tapes. Moreover, commercial software packages now facilitate the treatment of data including graph- i c s - gone are the days of transposing data from a chart onto graph paper by hand. There is less excuse to under- take plant trials than ever before.

Measurements on processes give us understanding and knowledge, but mathematical models provide the frame- work to assemble the knowledge and to apply it quantitatively to link process behavior to product properties. Thus, mathematical modeling, which was the subject of the 1987 Extractive Metallurgy Lecture by Julian Szekely, t9) is a vital component of process engineering research and teaching. Thirty years ago, the development of many of the models we take for granted today was virtually unthinkable, but galloping computer technology has been the vehicle for change. In this period, there has been considerable modeling activity which has led nowhere, other than possibly to raise our awareness of the potential of this powerful tool. All too frequently, the modeling work has become derailed by an excessive interest in the model itself, forsaking the original objective of process development. This is akin to the gardener who takes up a hoe to weed his vegetables, but becoming so enamored of the tool, never enters the garden. In many instances, mathematical models are left untested by the trial of f i re - -compar ison to measurements made on full-scale or pilot-scale plants. A model that has not been validated can lead to disastrous consequences if er- roneous predictions are made that create confusion, or worse, lead to a waste of money and manpower in a plant. Better, then, not to model at all.

There also is much fruitful modeling that has been done both in academic and industrial research circles. It is fair to say that modeling has come of age, at least in research, but as already stated, greater emphasis needs to be placed on the combination of models and measurements. Concerted efforts also need to be made to include mathematical modeling and numerical methods in the undergraduate curricula of metallurgy/materials departments, as I shall emphasize later.

IV. PROCESS ENGINEERING APPLIED

To illustrate several of the points argued in this lecture, four examples drawn from our work are presented in which a process engineering approach, involving models and plant measurements, has been applied. The first two, the rotary kiln and zinc slag fuming furnace, have been spotlighted, because they are both so-called "mature" reactors whose fundamental secrets have eluded us until recently. The third example, the continuous casting of steel billets, is worthwhile examining briefly, because it shows the close ties between process design, in particular of the mold, and the formation of defects in the cast product. The final example, the controlled cooling of steel rod on a Stelmor line, showcases the application of process engineering to a solid-state process, on which the principles of physical metallurgy are brought to bear.

A. The Rotary Kiln

A fixture on the industrial scene since the last century, the rotary kiln remains a workhorse today for the drying, heating, calcining, reducing, roasting, or sintering of a range of materials. A photograph of four rotary kilns utilized for the reduction of iron ore with coal via the S L / RN process is presented in Figure 4. The continued use of this venerable reactor owes as much to its ability to treat solids feed with a broad particle size distribution as it does to the plug flow behavior of the solids. More- over, it can be fired with different fuels, including some that are dirty, without unduly contaminating the product. Although some research has been undertaken to unravel the workings of the rotary kiln (many of these studies have been reviewed by Barr et al.,~11] Gorog et al. ,[12.13] and Henein et al.(141), full characterization of the complex heat-exchange processes, which largely govern the productivity of a rotary kiln, had to await the investi- gation of BAIT. [11'15'16]

Owing to its size, rotary motion, and high temperature, detailed measurements of temperature in the moving solids, freeboard gas, and refractory wall of an operating rotary kiln are difficult in the extreme. Thus, the reactor is an ideal candidate for study on the reduced scale of a pilot plant, which is where Barr began his work at UBC.

The UBC pilot rotary kiln is shown in Figure 5. The kiln is 5.5 m long x 0.4 m I.D., lined with castable refractory inside a 0.61 m O.D. steel shell, and fired with natural gas. t~7] The unique feature of the pilot plant is its heavy instrumentation, as shown in Figure 6. For his study, Barr installed a total of 66 thermocouples, which included sliding suction thermocouples to measure gas

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Fig. 4--Photograph of SL/RN rotary kilns producing direct-reduced iron at Iscor, South Africa (from Formanek et al.tl~

temperature, bare thermocouples to sense bed temperature, and wall probes, consisting of thermocouples cast in refractory, which could be inserted into the kiln lining to measure the cyclical temperature change of the inside wall and the steady-state thermal profile deeper in the wall. Then, feeding either inert material (Ottawa sand or calcined petroleum coke) or reactive solids (limestone) under steady-state conditions, the temperature fields in the gas, solids, and wall were measured. Heat flows were calculated from heat balances on the gas and solids phase; together with a finite-difference analysis of the wall.

Barr confirmed three observations that had been made in an earlier study: t18]

(1) the bed temperature and inside wall temperature of the kiln were virtually the same over the entire measurement zone, as can be seen for limestone in Figure 7; (2) within the initial 1.5 m of kiln length, the solids heated rapidly (Figure 7), then the rate of heat transfer to the bed declined until calcination commenced at about the 3-m point (Figure 8); and (3) at the onset of the calcination reaction, the net heat input rate to the solids increased sharply and rose by up to 700 pct over the precalcination value.

To explain these results and to understand their impli- cations for industrial rotary kilns, Barr turned to the mathematical model.

Thus, he set out to characterize mathematically the different heat-flow paths that exist among, and in, the

Fig. 5--Photograph of UBC pilot rotary kiln (from Brimacombe and Watkinson07]).


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solids, wall, and gas phases; these are shown in Figure 9. The model consisted essentially of two modules, one to simulate heat flow within the rotating refractory wall, including covered wall-to-bed heat transfer, and the other to characterize the radiative exchange in the freeboard among the wall, gas, and refractory. Only a cross-sectional slice was considered, because owing to both geometry and gas absorption, upstream and downstream radiation have only a small effect on local heat transfer. The wall heat conduction was modeled with the aid of a finite- difference grid, which extended into the stationary layer of solids in the bed that move with the rotating lining. Thus, complications arising from covered wall-bed heat- transfer coefficients were skirted in this effort to describe the regenerative action of the wall mathematically. The freeboard radiative exchange model was formulated from the zone method of Hottel ug] and incorporated real gas behavior as well as reflections from the solid surfaces. Complete details of the model are to be published early this year. l"]

The model was verified by inputting measured gas and solids temperatures and comparing predictions of heat flow via the different paths shown in Figure 9 to the val- ues determined at 180 cross-sections in the pilot kiln trials. In proceeding through this critical phase of the model development, Barr made a critical discovery when he observed that even small variations in input bed temperature had an exceedingly strong influence on the predicted net heat transfer from the gas and to the bed. An example of this effect is shown in Figure 10, where the net rates of heat input to the wall and bed are plotted

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Fig. 9 - - P a t h s for heat transfer at the cross-section of a rotary kiln.

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Fig. 10--Predicted net heat-transfer rates for the freeboard gas (Qg), solids bed (Qb), and refractory wall (Q~=) as functions of relative bed temperature under pilot kiln conditions for two freeboard gas tern- peratures (from Barr et al.J'J).

against the relative bed temperature, defined as the difference between the inside wall temperature as it is initially exposed at the top of the bed, Twso, and the temperature of the exposed surface of the bed, Teb, for two gas temperatures. Thus, the net rate of heat input to the bed, Qb, is seen to change dramatically with relatively small variations in Tw~o - TCb; moreover, for rapid solids heating, the relative bed temperature should have a large positive value.

This, of course, is an artificial calculation, because the relative bed temperature cannot be manipulated in a real rotary kiln, where it is a dependent variable changing with axial position and a function of feed conditions, firing rate, and kiln dimensions. But it does explain two of the observations made in the pilot kiln. The net rate of heat transfer is high at the feed end of the kiln and during an endothermic reaction such as limestone calcination, because then the temperature of the bed, TCb, is especially lower than that of the wall; i.e., the relative bed temperature is largest. At the charge end, the cold feed material itself causes TCb tO be low, while in the endothermic reaction zone, the bed temperature is de- pressed relative to the inside wall, as a significant fraction of the heat is absorbed by the reaction instead of raising the sensible heat and the temperature of the solids.

With the model, Barr went on to explain the close coupling of the inside wall and bed temperatures observed in the pilot kiln trials (Figure 7). The model showed very simply and elegantly that the thermal resistance for heat transfer from wall to bed was much lower than that either for gas to wall or through the wall. Thus, the driv- ing force for heat flow between the wall and bed, the

temperature difference, must be much lower. Model predictions for a larger kiln with a 4-m I.D. exhibited the same trends.

What are the practical consequences of these findings? First, they tell us that in the absence of an endothermic reaction, the rotary kiln is a poor heat exchanger. This conclusion would hardly surprise the rotary kiln designer or operator who has come to accept riigh temperature and energy content in the waste gas. In the SL /RN direct reduction process, for example, the waste gas carries 32 to 59 pct of the total heat input, depending on the coal type utilized, i~~ This has led to the use of the waste gas to preheat the feed ore and reductant external to the kilns at New Zealand Steel. Preheaters utilizing waste gas also are installed routinely with short lime kilns. From Barr 's study, the use of preheaters makes good sense, because it transfers the solids heating function outside the rotary kiln to a more efficient heat exchanger and leaves for the kiln the job of carrying out the endothermic reaction, which it does best, owing to the increased relative bed temperature and accelerated net heat transfer to the bed. At least now we know why.

But the study has an even more powerful outcome. It places in our hands a tool with which rotary kiln processes can be studied, changed, optimized, and de- signed. With minor modification and the inclusion of data on reaction kinetics (e.g., References 20 through 22 for iron ore reduction), the cross-section model can be run in a marching scheme from one end of the kiln to the other to predict the kiln performance quantitatively. It removes much of the guesswork and mystery shrouding this reactor and opens up new possibilities flowing from the fundamental understanding that has been achieved. One wonders if the ancient Chinese had had such tools in their possession, where extractive metallurgy would be today.

B. The Zinc Slag Fuming Process

The recovery of zinc from slag, where it resides as an oxide, was developed as a commercial process over sixty years ago. It has changed little in the intervening decades. The zinc fuming process is conducted in a rect- angular, water-jacketed furnace, as shown in Figure 11, where 45 to 90 tonne batches of molten slag are treated with pulverized coal and air. The coal is conveyed to the furnace by primary air and is injected into the furnace

C,CO ~CO~ Terto'y elf

Z%~-1/2 0 2 ~ZnOis ) . (unreguloted)

Przmory otr 0 5-3 3 Nm3/s Cool I- 15 kg/s

[ S e c - - y a,r I l~ ~Oo ZnO(~'og)~Zn(g)- ok , , - / ----II W~176

vo,,, -, ~ - / ) , ,5o-,3oo*c Tuyere ~I" . . . . I l

Slog Fuming Process

Fig. 11 --Schematic diagram of the zinc fuming process (from Richards and coworkers [23-26]).


through opposing banks of tuyeres by a blast of secondary air introduced behind the coal stream. Typically, the injection pressure of the air is about 100 kPag or slightly lower. The primary reaction is the reduction of ZnO either by C or CO to zinc, which is a vapor at the bath temperatures of 1150 ~ to 1300 ~ and fumes from the slag. Ferrous and ferric iron dissolved in the slag (total iron content of 25 to 30 pct) also participate in the oxidation/ reduction reactions in the process. Above the slag, the zinc vapor is oxidized to ZnO by leakage air. Typically, lead blast furnace slags containing 11 to 18 pct Zn are reduced to 1.5 to 2.5 pct Zn in a total fuming cycle time of about three hours.

The process ran unmolested by scientific inquiry, at least as would appear in the published literature, for twenty-five years. Then the process began to yield up some of its secrets to the analytical power of thermodynamics applied first by Bell e t al., [27] followed by Kellogg t281 and Grant and Barnett. t29j Assuming equilibrium in the bath, mathematical models of increasing so- phistication were developed and were applied to explain the behavior of the process. While the predictive capa- bilities of the models were reasonably good, one could question the assumption of the fuming process operating at, or very near to, equilibrium. If the assumption, indeed, was wholly valid, one would be left with the dreary prospect of having little room for change; the only levers of control would be chemical potential and temperature. The nagging doubt lurking behind the thermodynamic approach, however, was whether or not three phases - - gas, coal, and s l ag - -cou ld attain equilibrium throughout the fuming cycle.

Thus, Richards and coworkers t23-261 began afresh by going to the process at the Cominco Trail Smelter and four other smelters and asking it questions. Is the injected coal completely combusted at the tuyeres with the primary and secondary air, or does some of it penetrate into the bath? Samples taken of the slag during fuming cycles at the five smelters, when analyzed (Figure 12), revealed the presence of carbon in concentrations typically of 0.4 to 0.7 pct. Acid digestion of slag samples

t I D I I I I I I I 28 O ~ O _ ~

-~ o ~ o ~ 14 ~26 $102

. . . . / _ . . . . . o . . . . o _ . t - / '

o,i- I Cm; , ~ "-o f 6L I 6o

i L . . . . . . . " ~ L 4 o - ~ . . . . - - V ~ V ~ v 50 1200 L 0.3~-- 'O F O u ~ O , T . m p - ~7~V~V

2 ~ .o----~ ,,8o

0.I Fe " ~"o " " "

0 I0 20 30 40 50 60 70 Elapsed time (rain)

Fig. 12--Slag composition, coal injection rate, and slag temperature as a function of time for a slag fuming cycle at the Cominco Trail Smelter (from Richards et al.t23J).

and, later, X-ray analysis of micro filtration residues in the laboratory indicated further that the carbon was in the form of solid particles. Clearly then, a fraction of the coal was finding its way into the slag without being combusted.

This finding provided an answer to the most fundamental question: does the process operate at equilibrium? The response logically had to be negative, because, thermodynamically, carbon cannot coexist with ZnO in the concentrations typical of zinc fuming slags. Other evidence of Zn and Fe concentration gradients adjacent to pores in the slag samples, as well as carbon passing through the bath unconsumed and reporting to the ZnO fume, confirmed this conclusion. Consequently, there had to be kinetics phenomena at work limiting the process with all the promise of finding a way to accelerate fuming rates.

The observation of coal particles in the slag away from the injected air stream also suggested that the zinc fuming process could be viewed rather simply as consisting of two reaction zones depicted schematically in Figure 13. In the bath, a reduction zone existed in which entrained coal (F) reacted directly with slag; while in the coal- depleted, tuyere gas stream rising up the walls of the furnace, there was oxidation of coal (Y1) and ferrous iron in the slag to generate heat. If this picture is essentially correct, the rate of zinc fuming would be limited by the quantity of coal that is entrained in the slag.

Next came the task of putting this new knowledge and these ideas to work in the formulation of a kinetics model of slag fuming. Richards started by considering what happens to a coal particle, or a clump of particles, that penetrates into the molten bath. He reasoned that it is unlikely the particles would be wet by the slag, because the reaction between the two phases generated gaseous products, CO and Zn vapor. Therefore, one could imag- ine coal particles entering the slag, immediately pyro- lyzing, and becoming encapsulated in a small bubble, as

co2 I 17 1

illlil i Coal

,.,..Heat

Solidified slag

Fuming furnace model schematic

Fig. 13--Basic conception of reaction processes in zinc slag fuming (from Richards and Brimacombet24J).

30Of--VOLUME 20B, JUNE 1989 METALLURGICAL TRANSACTIONS B

shown in Figure 14. The reduction of the slag then pro- ceeds via the Boudouard reaction and the gaseous inter- mediates, CO and CO2. Initially, only ZnO and F e 3 0 4 reduction and diffusion in the slag were considered, but this was extended later to include PbO reduction as well as oxidation of liquid lead prills by ferric iron. [261 The entrained particles were assumed to rise with the up- welling liquid and to release their gaseous contents at the bath surface. To estimate the residence time of the particles in the slag, Richards had to formulate another model which drew upon measurements he had made of bubble discharge frequency at the tuyeres and an assumed bath circulation velocity. Other submodels were needed to characterize the partitioning of coal, the air- slag reaction, and the thickness of the freeze on the water- cooled furnace walls. Finally, all the models were tied together with overall heat and mass balances on the slag.

The result of this exercise was a model which was capable, at least in theory, of predicting the change in bath composition and temperature with time during a fuming cycle from a given set of operating conditions. However, Richards faced a final hurdle not uncommon to the process engineer: he was not able to determine several parameters needed for the model, viz., the re- spective fractions of injected coal which entrained in the slag, combusted in the rising gas stream, or, short-circuited the bath without reacting appreciably, nor did he know the fraction of oxygen in the injected air that was consumed by reaction with the slag and coal. Thus, he was forced to fit his model to the industrial data, an example of which was shown earlier in Figure 12, from fuming cycles at the five different smelters. Remarkably, he found that the fraction of coal entrained, predicted by the model, for eleven fuming cycles fell into a relatively narrow range of 0.32 -+ 0.05. Moreover, a clear relationship emerged between the fraction of oxygen consumed from the injected air and the bath height, as would be expected. The later refinement of the model by Cockcroft to include PbO reduction and liquid lead oxidation by ferric iron gave essentially the same results but, in addition, revealed a greater fraction of coal entrainment with increasing air exit velocity from the tuyeres. [26] An example of the fit is shown in Figure 15 for one fuming cycle.

. . . . . Fe20 3

! i ~ C02 ~ s Slog

/ / CO,H2 Zn,C021 D-S,u,,on < ,.;o .o,-,,-o... / -

//Pbl _ _ PbO

Liquid lead Fig. 1 4 - - Entrained char particle reacting with slag (from Cockcroft et al326I).

O.

I Cycle CI I

2xPb

0.0 ,O.O 20 O 30 0 40 0 $~ O 60 0

, t I

700

~o! w<5 w o L,.o~ 0d I . IJo ~SZ-

O0

0 o o . . ~ . . . . . . .

Fe 2+

Fe 3+ ....... ~ ......... ; ......... ; ......... ~ ......... ~ ......... ~ ........

I0 0 20 0 ~0.0 40,0 50 0 60 0 70 0

o.o ,~ o 2;) o sb o 4o.0 ~o o 6o o ~o o ELAPSED TIME (MINS)

Fig. 1 5 - - M o d e l fit of transient slag composition and temperature to measurements from a slag fuming cycle at the Cominco Trail Smelter (from Cockcroft et al.[26]).

With this tool, the depths of the fuming process could be plumbed to reveal the importance t)f the Boudouard reaction kinetics for the fuming rate. Thus, it was possible to show that the best coal for zinc fuming should have low moisture and ash content, high fixed carbon (or volatiles), and high reactivity. The model also revealed how fuming kinetics are influenced adversely by an increasing concentration of ferric iron. The level of ferric iron is not constant but depends on its consumption by reduction with entrained coal particles and its generation by the oxidation of ferrous iron with air surging upward through the slag. Because both this source and sink of ferric iron are affected by bath depth, the model indicated that the latter also was a significant factor in fuming kinetics. But the most important prediction, perhaps, was that if the fraction of coal entrained could be increased, the slag fuming rate could be enhanced. The model pointed to the sought-after acceleration in the fuming rate.

The next step was easy conceptually but difficult to implement in practice. How better to enhance the entrainment of coal in the slag than to inject it through one or more of the tuyeres with a high gas pressure and solids loading so that with greater momentum, the particles would penetrate into the bath more readily. This concept was attractive for another reason: it permitted a form of control to be exercised on the fuming furnace which hitherto was not possible. Under the new scheme, the conventional tuyeres with low pressure and lower solids


loading could be operated to ensure adequate heat was being generated by oxidation, while the high-pressure injectors were run to maintain the fuming rate. A new variable was being given to the process engineer, at least in theory.

To test the concept, a high-pressure injection system was installed on a trial basis at the Trail Smelter.tZs] Pul- verized coal was injected at slightly lower than 690 kPag through a 6.83 mm I.D. injector positioned concentrically inside an existing conventional tuyere, as shown in Figure 16. The coal-to-air ratio in the high-pressure injector was 25 to 38 (wt/wt) as compared to 0.11 in the conventional tuyeres. The estimated coal exit velocity from the high-pressure injector was 116 to 119 m/s, more than double the 47 m/s of its low-pressure counterpart. The momentum increase of particles injected at high pressure was estimated to be 130 to 196 times.

In the plant trials, up to 29 pct of the total coal nor- mally blown through the conventional tuyeres was injected over an interval of time through the high-pressure injector to maintain a roughly constant overall coal rate. Analyses of slag samples taken at intervals are shown in Figure 17 for one of the trials. Thus, it is evident that the fuming rate is accelerated by the increased coal entrainment during high-pressure injection, while ferrous iron rises and ferric iron falls slightly. For this particular run, the fuming rate increased by 83 pct, while the fuming efficiency (Zn/coal) rose by 99 pct relative to standard operations. Analysis with the mathematical model suggests that the fraction of coal entrained from the high-pressure tuyere ranged from 0.65 to 0.90, two- to threefold greater than for the conventional tuyere. Encouraged by these results, Cominco has proceeded to install a permanent high-pressure system, which is currently being commissioned�9

Once more, the strength of the process engineering approach is evident, in this case, leading again to major improvements to a long-established process. It is also clear that the fundamental knowledge gained on the process dynamics of slag reduction can be applied, with modifications, to other systems�9 It is all part of building the discipline of metals process engineering.

C. The Continuous Casting of Steel Billets

Now we move downstream to a process that is at the boundary between extractive and mechanical metallurgy: the continuous casting of steel�9 Compared to slag fuming and the rotary kiln, it is a relative newcomer to the metallurgy landscape. Its origins go back to the last century, but it was not until about thirty years ago that continuous casting began to take root in the steel industry. Since that time, it has largely replaced ingot casting for the production of steel billets, blooms, and slabs owing to its inherent advantages of low cost, high yield, flexibil- ity of operation, and ability to achieve a superior cast product�9

Continuous casting has not been free of problems, however, as can be seen in the transverse section of the steel billet shown in Figure 18. Originally, the billet was meant to be square and free of defects, but, instead, it has assumed a rhomboid shape, and internal cracks are present in the off-comer regions�9 What is worse, de-

From SlOfOge hopptr |

Prl rno;'y o l r - c o o l

Secondory ~. ~ \

3.,+;; Inject~ ~ + f ~ [TTP'+ "? '

=, "3 Fig. 16- -Scbemat ic diagram of the pneumatic system employed to inject pulverized coal at high pressure into the slag fuming furnace (from Cockcroft et al.12s~).

pending on the severity of the shape and crack problems, defects may appear in the rolled product.

The problem seen in Figure 18 has dogged the billet industry for twenty-five years, but only in the last decade has it been tackled with process engineering tools�9 Em- pirically, it was known, for example, that the cast defects originated in the mold, because the obtuse-angle corners of off-square billets always appeared much hotter (brighter) than the acute-angle comers, as the billet

Z N

1,.--

. . . . . . # . . o + *

Run 2

L i n e s - model P o i n t s - data

2 x P b

' ~ 1 4 9 . . . . . . . . - % - - - + * . . s . o . ~ . . . o # . . . . . .

o. o.

+

0 0 10.0 20.0 30.0 40.0 50.0 60 0 70.0 80.0 90.0

~ o 2+ t~ o. Fe

i++ o . r ~ - - * ~ ' * * ~ ' * * ~ L d o+ * * - ~ ' ~ 1 7 6

+" r " e + + ~ o ' "r " * "<P * * ' r * * ~ . . . . 9 . . . ~ . ~ e . . . o . . . . . " + * * ' ~ " " "o" <~

o+o ,b o +o.0 +o.o ,~ o +o+o m+o to.0 +o.o +o o

++l . . . , . . . , . . . , . . . . t " ~ �9 . . . . . . . . . . . . . �9 . . . . . . . . ~ + . o q l ++| [

+J , S t a r t , Finish

- . . . . . e . . . ~ - - ~

. * - - - ' + o - - 0 . . . O . o .

0 0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 gO 0

ELAPSED TIME (MINS)

Fig. 1 7 - - M o d e l fit of transient slag composition and temperature to measurements from a high-pressure injection trial at the Cominco Trail Smelter. Vemcal lines denote the beginning and end of high-pressure injection (from Cockcroft et al.[2~]).

302--VOLUME 20B, JUNE 1989 METALLURGICAL TRANSACTIONS B

Fig. 18 - -Macroe tch of transverse section of a 125-mm square steel billet.

emerged from the mold. Moreover, the problem could be alleviated frequently by simply replacing the mold with a new one. At the same time, practices were developed without any sound theoretical basis to minimize the defects; one technique, euphemistically called "soft cooling," required a reduction in cooling water flowing through the mold. Another practice involved adjustment of the spray cooling on different faces of the billet below the mold to cool the steel differentially and somehow maintain a square shape. The problem with this approach was that the orientation of the off-squareness usually" changes frequently throughout the casting of a heat, so that an operator would have to remain below the casting floor and continually make adjustments to the spray water - -c lear ly not a solution!

To gage the confusion existing in the industry, one need look no further than the mold conditions employed at different companies around 1980. At that time, a sur- vey was conducted of a number of North American steel companies, and some of the results are shown in Table I. [3~ Thus, great variation is seen in such key variables as cooling water velocity (4.5 to 13.9 m/s) , mold oscillation stroke length (9.53 to 25.4 mm) and frequency (0.75 to 2.33 Hz), and liquid level or "meniscus" relative to the top of the mold (38 to 200 mm). In the absence of standards based on a knowledge of mold behavior, the mold system in each company was different. In the parlance of the day, everybody was doing his own th ing - -and many were doing it badly (Figure 18).

The billet mold has a deceptively simple design, as can be seen in Figure 19. Its basic components are a copper tube (usually square with rounded comers), a steel jacket in which the tube is located concentrically, and an outer steel housing to which the tube and jacket are attached. The copper mold tube is unsupported over its length but is secured to the housing by steel plates fitting into slots milled near the top of two or four sides; in a

different design, the copper tube is held at the top and bottom via expansion plates. The cooling water is introduced at the bottom of the mold housing at about 500 kPag and flows up the water channel between the copper tube and the steel jacket. As indicated earlier in Table I, the mold is oscillated and, further, is lubricated, usually with oil fed down the walls, in order to prevent sticking of the steel to the copper. The mold tube typically is 700 to 800 mm long, has a wall thickness of 6 to 18 mm, is tapered positively to compensate for billet shrinkage, and is made from highly conductive copper.

To understand the thermal behavior of the mold, Samarasekera formulated a mathematical model to predict the temperature distribution in the vertical midplane through the copper wall. t31,32j Data were taken from the mechanical engineering literature to characterize the heat transfer between the copper and the cooling water and from a published U.S. Steel paper t33~ to describe the heat flux mathematically from steel to mold. The axial heat- flux profile, corrected by Samarasekera, is shown in Figure 20. Adopting a finite-difference method to solve the heat-conduction equation, she computed the temperature distribution in the mold midplane for a number of cases, one of which is shown in Figure 21. With this, she made two important discoveries. First, even though the maximum heat flux is at the meniscus, the peak mold temperature is several centimeters below it, owing to the strong vertical component of heat conduction in this region, which assumes major importance when thermal distortion of the mold is considered. Second, at a water velocity of about 7 m/s and local cooling water pressure of 200 kPag, typical of a number of steel plants at the time, nucleate boiling occurs at the mold wall near the meniscus. Given that nucleate boiling causes the heat extraction by the cooling water to fluctuate markedly with local copper wall temperature, it is an tmcontrollable and, therefore, undesirable phenomenon. Thus, immediately, the need was seen to raise the cooling water velocity sufficiently to snuff out the boiling for mold stability. Model calculations suggested that a value of 9 to 10 m/s would be sufficient. Thus, the rationale for minimum cooling water flow was established in contrast to most of the operations observed in Table I.

Figure 21 shows that following the heat-flux profile (Figure 20), the upper part of the mold wall is hotter than the lower regions toward the mold exit. This is particularly important, because the mold is unconstrained over most of its length, as mentioned earlier, and is free to expand transversely (outward) in response to the local rise in temperature. Obviously, the thermal expansion cannot be uniform over the length of the mold, and it must change its shape during operation. To gain an understanding of the distortiota process, Samarasekera again turned to the mathematical model but this time a three-dimensional, finite-element, elastoplastic analysis of stresses. [34] A model prediction of the mold distortion at the vertical plane is presented in Figure 22. It is seen that the mold tube expands outward away from the steel by a fraction of a millimeter near the top of the mold, where it is hottest. Most significant is that the resulting negative taper of the upper mold extends below the meniscus. Consequently, the gap between the mold tube and the solidifying shell, which strongly alters heat


T a b l e I . M o l d a n d C a s t i n g C o n d i t i o n s *

Casting Mold Speed

Number ( m m / s )

Observed Water Method Meniscus Flow Water Water of

Level Rate Velocity AT Con- (nun) (1 /s) (m/s) (~ s t ra in(

Number Oscillation of

Stroke Heats Length Freq. before (mm) (s - 1) Reform**

Total Heats

Break- outs

A l l l 24.2 A l l 4 24.2 A162 24.2 B633 25.4 B682 25.4 B692 25.4

25.4 CI00 - 3 3 . 8

D31 29.6 E4 38.1 E9 38.1

16.9 F10 - 19.05 F28 F31 F39

19.05 F47 - 2 0 . 7 4 G189 40.2 G12811 40.2 G12823 40.2 H142 50.8

12 80.4

13 80.4 J5368 34.0 J249 32.4 K l l 36.8 K17 36.8 L303 50.8 L309 50.8 L212 50.8 L221 50.8 M174

60 28.4 7.9 5 3 60 28.4 7.9 5 3 50 28.4 7.9 5 3 55 29.0 7.0 5 2 55 29.0 7.0 5 2 57 29.0 7.0 5 2

20.5 8.9 45 - 2 2 . 1 - 9.6 - - 1

15.7 5.1 50 - 18.9 - 6.2 10 2

150 37.8 10.5 6.6 2 150 37.8 10.5 6.6 2

38 20.2 4.5 6.1 2 50 20.8 4.5 5.6 2 38 29.0 6.5 5.6 2

- - 20.5 4.6 6.1 2

30 22.7 5.1 5.0 2 150 22.1 10.9 24.0? 1 150 22.1 10.9 24.0? 1 150 22.1 10.9 24.0? 1

50 22.1 8.2 8.5 1 150

- 2 0 0 - - - - 8.3 1 150

- 2 0 0 - - - - 8.3 1 100 25.2 8.2 8.5 2 120 25.2 8.2 8.5 2 100 22.1 10.7 7.2 1 100 22.1 10.7 7.7 1 100 25.2 13.9 - - 2 100 25.2 13.9 - - 2 100 25.2 13.9 - - 2 100 25.2 13.9 - - 2 100 - - - - 1 0 2

12.7 1.18 273 273 - - 12.7 1.18 first 320 0 12.7 1.18 206 661 2 12.7 1.08 224 535 2 12.7 1.08 first 223 - - 12.7 1.08 first 229 0

19.1 1.5 first 93 5 reformed

19.1 1.5 4 times 600 4 23.0 0.75 first 61 6 23.0 0.75 first 73 13

9.53 2.33 first 140 - - 9.53 2.33 first 57 - - 9.53 2.33 first 51 - - 9.53 2.33 first 184 - -

9.53 2.33 first 177 - - 9.53 2.0 first 27 4 9.53 2.0 first 20 4 9.53 2.0 first 265 10

19.1 1.5 - - - - - -

19.1 - - first 54 3

1 9 . 1 - - first 44 2 19.05 1.1 121 121 2 19.05 1.1 28 159 2 19.05 1.3 first 161 1 19.05 1.3 first 763 5 25.4 1.0 first 46 - - 25.4 1.0 first 110 - - 25.4 1.0 first 123 - - 25.4 1.0 first 136 - -

- - - - first - - - -

* Used molds only. ** "First" signifies first campaign on the mold.

~Method of constraint (see Figure 2).

Type 1: Side constraint--slots near top, 4 sides, with set screws to center mold tube in liner. Type 2: Side constraint--slots near top, 2 sides, with set screws or spacers to center mold in liner. Type 3: Top and bottom constraints.

t ransfer , is o p e n i n g in the first f e w c e n t i m e t e r s b e l o w the men i scus .

But this was al l theory , and it was c l ea r that m a n y o f the p ieces in t he p u z z l e o f b i l le t de fec t s and thei r re la- t ionsh ip to m o l d b e h a v i o r c o u l d o n l y be f o u n d f r o m m e a s u r e m e n t s on an ope ra t ing b i l le t cas te r . M o r e o v e r , the m o d e l p red ic t ions n e e d e d to be ve r i f i ed . C o n s e - quen t ly , a m o l d at the S te l co E d m o n t o n W o r k s was out- f i t ted wi th th ree L V D T d i s p l a c e m e n t t r ansducers to measu re m o l d w a l l m o v e m e n t dur ing casting.t32] T h e re- suits c o n f i r m e d the m o d e l p red ic t ions o f t he rma l dis tor- t ion and p r o v i d e d s o m e e v i d e n c e o f bo i l i ng jus t b e l o w the men i scus .

Nex t , a m o l d at W e s t e r n C a n a d a S tee l was instru-

m e n t e d wi th axial rows o f t h e r m o c o u p l e s i m b e d d e d wi th c o p p e r p lugs in ad jacen t wa l l s o f the c o p p e r tube , and t empe ra tu r e s w e r e m e a s u r e d dur ing the rou t ine cas t ing o f a l a rge n u m b e r o f heats , c81 In addi t ion , s a m p l e s o f b i l l e t s w e r e cut in each tr ial hea t fo r subsequen t m e t a l - lu rg ica l e x a m i n a t i o n o f de fec t s , cast s t ructure , and os- ci l lat ion marks on the bil let surface. The pr imary var iables s tud ied w e r e steel c o m p o s i t i o n and c o o l i n g w a t e r v e l o c - i ty; the la t ter was m e a s u r e d wi th Pi to t tubes and was v a r i e d t h roughou t s o m e heats .

T i m e - a v e r a g e d axia l p ro f i l e s o f m e a s u r e d m o l d t e m - pe ra tu re at the m i d f a c e and o f f - c o m e r loca t ions fo r one set o f cond i t ions are s h o w n in F i g u r e 23.[351 T h u s , the t e m p e r a t u r e o f ad jacen t f aces is seen to be d i f f e r e n t at

3IM--VOLUME 20B, JUNE 1989 METALLURGICAL TRANSACTIONS B

I Water

f OUt

Wa,er f al ~ " /

I /

|

Fig. 19--Typical mold design for a billet caster (from Brimacombe e t al.[~]).

I Mold 2 Steel jacket 3 Housing 4 Support plate 5 Lubricator plate 6 Cover plate 7 Water channel

E

"1-

300(

200(

\ \ \

\

J Carbon content J 0.9 %

. . . . 0 .1" / ,

x

0 ~ 0 4 8 12 16 2 0 2 4

Dwell time (s)

Fig. 2 0 - - H e a t flux from steel to mold as a function of time below the meniscus (data from Singh and Blazek mi corrected by Sammasekera and Brimacombe t32]).

5 m m I

T , /

I > ,oo ] ,ooZ/i "~176

,,olht,!ol I t II ".'ol . I I ' ~ l 2 ,0tJ

i i l l l i l l | l l

l't~ il N N

530 I

I

N N

9O

8 0 I

I " I

M e n i s c u s

Fig. 21--Model-predicted isotherms (~ in the wall of the billet mold for the casting of a high-carbon steel and a cooling water velocity of 7 m/s (from Samarasekera and Brimacombet32]).

the midplane, an early indication o f dissimilar heat- extraction rates. The temperature profiles then were con- verted with a mathematical model into heat-flux profiles, such as presented in Figure 24, for subsequent input as a boundary condition to solidification models.

The real clue to the thermomechanical behavior o f the mold causing the defects in Figure 18 emerged with the measurement of the depth of oscillation marks. Then, it was found that the average o~cillation mark depth frequently was greater in the off-corner region than at the midface, and increased with decreasing cooling water velocity and higher levels o f phosphorus in the steel, as shown in Figure 25. t351 This made little sense by itself, but combined with knowledge of the thermally distorted mold shape (Figure 22) and, in particular, the existence of a negative taper extending well below the meniscus, it became clear that during its oscillation cycle, the mold was interacting mechanically with the newly solidifying

M E T A L L U R G I C A L T R A N S A C T I O N S B V O L U M E 20B, J U N E 1 9 8 9 - - 3 0 5

C

IO0

2OC

E E 30C '10

~ 4 0 0

50C o

6or g

o 70C

8 0 0

' I

I I I I I I 0 0.04 0.08 0.12 0.16 0.20

Distortion (mm)

Fig. 2 2 - - M o l d wall distortion at the vertical midplane of the hot face predicted by the three-dimensional elastoplastic model (from Samarasekera and BrimacombeP2}).

shell at the meniscus. This is possible because the oscillation stroke and frequency are adjusted in practice to ensure that for a fraction of each cycle, the downward velocity of the mold exceeds that of the descending steel. This period of "negative strip" has been found to be nec- essary to prevent sticking between the steel and the mold.

0

I00

E 200

:E

300

g ~- 400 E

5OO i

n A ~ n __

P~tol Le~ll ~ O- r

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/~ Of f -c~x~er S ~ ~11 I"1

/ o

,/ / | |

50 I00 150 Temperature (*C)

200

Fig. 2 3 - - T i m e - a v e r a g e d mold temperature profiles from a heat containing 0.35 pct carbon cast with a cooling water velocity of 5.0 m / s (from Samarasekera et al.PS]).

5OOO

4 0 0 0

E

_g u .

o

~- zooo

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,.' ~., ,,.," ,.,.," ~., ~., =,SI !~' ~'~ ~t~ - - 2 4 2 6 8 014 002~055 00~9C'14 19 92 21

24259 016 0021 051 0033012 r 19 - - - 24240 020 0022 IO0 O0~OtO I 27') 93 23 ---- 24254 036 OOZE iOIB60~E 012 [132) 97 Zl

"~.."- .- \ \

o,.. t o~o ~o ~o ~5o= zoo Ttme ( s )

Fig. 2 4 - - T h e influence of steel carbon content on mold heat extraction (from Samarasekera et a1.[35}).

Thus, during negative strip, the negatively tapered region of the mold would bear down mechanically on the shell and cause it to buckle close to the meniscus, where the solid steel is thinnest, hottest, and weakest. Low- ering the cooling water velocity has the effect of raising the mold wall temperature and increasing mold distortion, as shown in Figure 26, which, in turn, enhances the extent of mold-shell interaction and shell buckling

0 27(3

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0 230

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(%p, AT) O <O026% P

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I I I (0017,29), 0

6 8 I0 Ve locny (m/s)

Fig. 25 - - T h e influence of mold water velocity on the average depth of oscillation marks on the surface of billets for two ranges of phos- phorous content (from Samarasekera et a/.PSl).


0

I00

200

E 3O0

" 0

0

400

Q.

~ 5OO

E

6O0

0 .~_

"~ 7o0

8 0 0

900

Metal LeVel -....~.

/

, i I Heat No I / 24264 k i 24265

Vw(m/s) %C 92 036 50 035

I I 0-I 0.2

Dynamic Distortion (mm) 0.3

Fig. 26--Model-predicted distortion of a billet mold for different mold water velocities (from Samarasekera e t al.OS]).

during negative strip. Consequently, the oscillation marks become deeper, as observed in Figure 25.

This mechanism readily accounts for the effect of" phosphorus, because raising the concentration of this solute in medium-carbon steel from 0.017 to 0.033 pct reduces its tensile strength and ductility markedly over the temperature range of 930 ~ to 1450 ~ As a re- suit, when the mold squeezes down on the shell during negative strip, the steel with the higher phosphorus level, being weaker, buckles more to generate a deeper oscillation mark.

The off-squareness and off-comer cracks seen in Figure 18 then are linked to the fact that the oscillation marks vary in depth across the face of a billet; that is, the mechanical mold/shell interaction is not uniform in the transverse plane. Nonuniform oscillation marks cause the gap between the shell and mold and the gap- dependent heat transfer to vary, so that the shell grows like the section shown in Figure 27, with thin hot corners and thick cold comers. Depending on mold taper, the shell may bulge in the mold to cause off-comer cracks 1361 and, finally, upon entering the spray cooling zone, will be differentially cooled to an off-square shape (Figure 27).

It all boils down then to a simple concept: billet quality fundamentally is linked to the control of mold distortion and mold/shell interaction at the meniscus. Armed with this knowledge, it has been possible to examine each of the mold design and operating parameters systemat-

icaUy and to set specifications for minimum cooling water velocity, mold wall thickness, meniscus level, dimensional tolerances, cooling water quality, and so o n - - a far cry from the muddled empiricism and general confusion evident in Table I.

Having reached such a satisfactory outcome, one would think that the billet producers would clamor after this new knowledge, thirsting for novel ideas to implement on their casting machines in the quest for improved quality. Indeed, many companies have embraced these concepts and seen the benefits, [39,4~ but many others, either through human inertia or crushing ignorance, cling al- most religiously to the past, as if it were a tabernacle holding sacred truths. Education, then, must be the vehicle for change, and so we have organized short courses for the industry to facilitate technology t ransfer- -another aspect of process engineering. We are beginning to see the benefits.

D. The Controlled Cooling o f Steel Rod on a Stelmor Line

Finally, we end up very close to the final product and examine a process which was developed originally by Stelco and Morgan over twenty years ago to replace lead baths for the controlled cooling (~patenting') of hot-rolled steel rod. Now, globally, there are over 150 Stelmor lines producing about 20 million tons/year of rod. Hll The lay- out of a modem Stelmor installation is shown in Figure 28.

f

-rA / x \ i \ ~ / / /

/

N

Upper Sprays

v

II

Off- square billet contomin 9 off-corner internal cracks

%-

Fig. 2 7 - - S c h e m a t i c diagram showing a billet with nonuniform shell thickness being distorted into an off-square shape by spray cooling (from Bommaraju e t a l .p6j ) .


I I

J ~- A

\ / " g ' t ~

Fig. 28--Schematic drawing of the Stelmor process: (A) delivery pipe, (B) wheel guide, (C) vertical laying head, (D) conveyor, (E) plenum chambers, (F) tractor chains, and (G) coil forming chamber (after Morgan Engineering).

Conceptually, it is a simple process in which steel rod from the last rolling stand is passed through a water box to control temperature (and the nature of the surface oxide) and then is formed into loops by a rotating laying head (C in Figure 28). The loops lie on one another, as seen at the top of Figure 28, and are moved horizontally by a traveling chain or rolls. Air is blown from under the moving bed of loops (E in Figure 28) to cool the rod at a controlled rate and, thereby, to achieve the desired microstructure and mechanical properties. Allowance is made for the higher density of steel at the edges of the moving loops by locally blowing additional air. Finally, the loops are gathered into coils (G in Figure 28) for shipping or further processing.

Working closely with a faculty colleague, Bruce Hawbolt, and several talented students, [42,43'44] the Stelmor process has been a most interesting foray into what we now call "microstructural engineering." It has been a fascinating experience, because it revealed to me, first- hand, the unity of the process engineering approach to both extractive and thermomechanical processes, which I emphasized earlier. Our objective in this study was very simply to develop a mathematical model capable of predicting the thermal and microstructural evolution, as well as mechanical properties, of the rod for a given steel composition and Stelmor conditions. After eight years, on and off, of laboratory measurements, software development, and plant trials, we can claim success with a user-friendly model capable of running on a personal computer.

This was an exercise which also demonstrated a lesson I had learned earlier: the mathematical component of a modeling study is usually much less demanding of human and financial resources than the laboratory and plant measurements needed for the model. In the case of the Stelmor process, the model was based straightforwardly on one-dimensional, unsteady-state heat conduction in cylindrical coordinates. The only real mathematical complication was the evolution of latent heat during the decomposition of austenite to pearlite and ferrite, which had to be included as a separate heat generation term in the governing heat-flow equation. Because the austenite transformation kinetics are temperature dependent and, at the same time, alter the temperature field with the

latent heat evolution, the kinetics and heat-flow equations are coupled and need to be solved simultaneously.

The Stelmor process clearly involves continuous cooling, but from a fundamental standpoint, it was de- sirable to characterize the kinetics of austenite-to-pearlite and austenite-to-ferrite transformations isothermally with a mathematical expression such as the Avrami equation. 145'46'47] This raised two questions, the In'st con- cemed with the adequacy of the isothermal transformation data in the literature for hypoeutectoid and eutectoid steels, and the second with the procedure to be followed for applying isothermal data to calculate the incubation and growth kinetics during continuous cooling.

Taking up the second question first, the application of the Additivity Principle in combination with a finite- difference solution of the governing heat-conduction equation was examined.t48~ The Additivity Principle is a convenient mathematical tool which states that a continuous-cooling transformation can be represented by a series of isothermal transformation events. An analogy which springs immediately to mind is the descent of a banistered staircase. One can either slide down the ban- ister in a continuous motion (accepting the dangers of slivers and an inadequate braking system) or more gently walk down, taking one step at a time. Whether the descent is continuous or in discrete steps, one ends up at the same place (hopefully). To be valid, the Additivity Principle requires that the transformation rate at any in- stant is a function only of the local temperature and the amount transformed. In other words, the steel must not "know" its history but only its present state. Fortunately, from a comparison of measured continuous-cooling transformation kinetics to those computed from isothermal data, the Additivity Principle was found to hold for decomposition of austenite to both ferrite and pearlite over a broad range of plain carbon steel compositions provided that empirical incubation times were input to the calculations.

Concerning isothermal transformation data, an early attempt t42] to utilize T-T-T (Temperature-Time- Transformation) curves from the literature was only par- tially successful and showed us that we had to make our own transformation kinetics measurements under carefully controlled thermal conditions. Thus, Hawbolt constructed the resistance-heated, diametral dilatometer shown in Figure 29 to measure the dimensional changes accom- panying austenite decomposition to ferrite and pearl- i teJ 49,s~ The steel samples were machined into hollow cylinders having an 8-mm diameter and wall thickness of just under 1 mm to minimize radial temperature gradients. A thermocouple was spot welded to the sample at the same location where a quartz tipped extensometer was located to sense changes in diameter during the phase transformation. The variation in diameter under both isothermal and continuous cooling conditions was con- verted to fraction austenite transformed, X, which finally could be fit to the Avrami equation with a term introduced by Umemoto e t a l . t51] to account for grain size effects:

I--

X = 1 - e x p / - 0t"/ [1] L dmJ

308--VOLUME 2013, JUNE 1989 METALLURGICAL TRANSACTIONS B

Fig. 29 - -Schemat i c diagram of diametral dilatometer used to measure austenite decomposition kinetics in plain carbon steels (from Hawbolt and coworkerst48,491).

where t = the transformation time; d = the prior-austenite grain diameter; and

b, n, and m = empirically determined constants.

Thus, the empirical constants were obtained under isothermal conditions for a number of plain carbon steels, while the incubation times for the start of both austenite- to-ferrite and pearlite transformations were obtained from continuous-cooling experiments. These data were incorporated in the form of correlations into the mathematical model.

With incorporation of grain growth effects to characterize d m in Eq. [1] and inclusion of the external heat extraction rate due to convection and radiation, the model could be run to predict the evolution of temperature and phase transformation during the air cooling of steel rods. Examples of the model predictions of the centerline temperature in two steel rods (nominal 0.70 pct carbon) cooled under laboratory conditions are shown in Figure 30. Thus,

the effect of the evolution of latent heat during transformation to cause recalescence in the undercooled rods is readily apparent. Also, the influences of rod diameter and the crossflow velocity of air on the transformation start time (close to the beginning of recalescence) and the extent of undercooling (which influences pearlite interlamellar spacing) can be seen.

To test the model at this stage in its ~tevelopment, the air delivery system shown in Figure 31 was constructed to cool steel rods under carefully controlled laboratory conditions. The apparatus was capable of blowing air with uniform velocity, in crossflow, over the length of preheated steel rods that had been instrumented with a centerline thermocouple. Measured transient temperature responses from two experiments are shown in Figure 30 and compared to model predictions for iden- tical conditions. The agreement is satisfactory.

If the model is to be taken a step further and given the capability of predicting the mechanical properties of the cooled rod, considerable microstructural information is needed, because properties and microstructure are linked. Specifically, data are required on the interlamellar spacing of pearlite, which depends on undercooling of the rod; also, the diameter and fraction of ferrite, which are functions of steel composition and, to a lesser extent, of cooling rate, need to be known. More- over, a program of mechanical testing of control cooled rods with known compositions and microstructures must be mounted to establish the empirical relationships among these variables. This is classical physical metallurgy, now in the harness of process engineering and driven by the requirements of the mathematical model.

An example of the microstructural work, now completed, is the pearlite interlamellar spacing shown in Figure 32 for a 1070 steel, t44] Regression equations were fit to these data and to those for ferrite~ as well as to mechanical properties (U.T.S. and Y.S.)- microstructure-composition measurements and incorporated in the Stelmor model to give it the full predictive capability sought.

Fig. 30- -Compar i son of model predictions to measurements of centerline temperature for the air cooling of two 1070-grade steel rods in the laboratory (from Campbell et al.[a4j).

Fig. 31 - -Schemat i c diagram of the apparatus employed to control cool instrumented steel rods with a cross flow of air in the laboratory (from Campbell et al.144]).


" 7

c

" - - v Measured , , . . , S ~ I / / x Me.=,.,=.~m~ I o o / /

(3 (~ 16 / ~- . ~ o~

12 g o .-~

n 8 * ~ " ~ -- {r-" x x x x x �9

s 4 #" ~ - p ~ , , �9 Q. �9 ~ - ~ ol l /~ / i~" "

0 I I I I I I I I I I I I I I I I I

0 20 40 60 80 100 120 140 160 180

Undercooling Below TA1 deg.C

Fig. 32- -P lo t of the reciprocal mean and minimum pearlite spacing against undercooling (below TA~ on phase diagram) measured in a 1070 steel (from Campbell et al3441).

The final stage in the model development was vali- dation with measurements made on an operating Stelmor line. Consequently, with the cooperation of Stelco, trials in the No. 2 Rod Mill of Hilton Works were organized and successfully completed. The plant work consisted of three activities: measurement of the cooling air velocity distribution with a Pitot tube; characterization of cooling rates on the Stelmor line during routine operation by in- sertion of preheated, instrumented rods (similar to that in Figure 31) into the moving loops and recording of temperature response with a hand-held data logger; and sampling of rods for subsequent mechanical testing and microstructural examination. In all, 75 tests were conducted on three rod diameters and three grades of steel. Model predictions of Y.S. and U.T.S. are compared to measured results from all the plant and laboratory trials in Figures 33 and 34, respectively. Thus, the model has a respectable predictive capability and warrants transfer

650 12. 11 -Lab 1070

2-PlantJLab 1039 / 600 3-Plant/Lab 1019 /

"~ ~,-Lab 1040 / , r 550 5-Plant1018 . . ~ _ 7 77_

S-Plant 1037 1 - j l~ f" ~ ' r r 7-Plant1075 . 1 ~ / r '

500 B-Plant 1033 1~ ~ / 10 9-Plant 1020 - i " -~ 450

-o 400 4 422

350 1 ~ 9 9 �9 ~- 300 s

250 , 250 35o 4~o 55o 650

Measured Yield Strength MPa

Fig. 33- -Mode l predictions plotted against measurements of yield strength for a range of steel compositions and cooling conditions (from Campbell p2j ).

1.1 0 .t= 1 c~

.~ 0.9

03 0.8

E 0.7 +...

0.6 "10 , - 0.5 O "0 a~ 0.4 L -

Q. 0.3

0.4

6 2

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. . , ~ ~ 3-Plant/Lab 1019 t-Lab 1040

~ _ 5-Plant 1018 3-Plant 1037 7-Plant 1075 3-Plant 1033 9-Plant 1020

0.6 0.8 1 Measured Ultimate Strength G P a

Fig. 34- -Mode l predictions plotted against measurements of ultimate tensile strength for a range of steel compositions and cooling conditions (from Campbell ml ).

to industry as well as generalization to other grades of steel, such as low alloy.

Many broad conclusions can be drawn from this modeling exercise. Certainly, it reinforces the interdisciplinary nature of process engineering in its combination of elements of physical metallurgy and mechanical engineering. And the need for professionals from the different disciplines to cooperate fairly shouts out. It also points to the future, because as computing power con- tinues to burgeon, placing the equivalent of a mainframe computer on our desks, a user-friendly mathematical model that can link the properties and behavior of metals quantitatively to process parameters will become increasingly attractive to the process engineer, who is at- tempting to remove the guesswork from processing and replace it with science. With this tool, he can sit in his office and study the effects of rod diameter, steel composition, air flowrate, laying head temperature, or line speed on the mechanical properties of the rod. He can assess changes to existing Stelmor lines or begin to design new ones to exploit the process-properties relationship to the full. He can apply the model to control the process. Whatever his objective, the process engineer now has the means to make rational change quantitatively, the same as the engineer at the front end of the flowsheet who has a rotary kiln model in his possession.

V. T H E C H A L L E N G E OF M A T E R I A L S

If these four illustrations could be magnified to a much greater level of activity at the major universities globally, process engineering would hold the promise of pro- foundly changing the course of the extractive and physical processing of metals. But with the shifts in the academic community, which is embracing and pushing the mate- dais revolution, we must develop a broader vision, built on metals processing, of materials process engineering as well. This is an enormous task, because the transition

31'0--VOLUME 20B, JUNE 1989 METALLURGICAL TRANSACTIONS B

from metallurgy to materials science and engineering has occurred before metals process engineering was ever ac- cepted by the universities and developed as a discipline with full-fledged teaching and research programs. In some ways, we are starting from scratch, and there is much intellectual inertia to overcome.

At least we all agree that processing is an essential activity, together with properties characterization and design, of materials science and engineering. This basic anatomy of materials can be represented by the matrix shown in Figure 35, where these three components are set out in rows cutting across the individual classes of materials. Thus, we all appreciate that the application of process engineering fundamentals and tools of analysis, generally, to chemical and physical processes, as I have described and illustrated in this lecture, is as valid for composite polymers and ceramics as it is for metals. Even when entering nonstructural materials fields, such as magnetic and electronic materials, where processing may be pursued at the atomic scale, the process engineer has a contribution to make. Certainly, we have no trouble with this generic concept when we consider physical metallurgists, chemical metallurgists, and materials scientists pursuing the characterization of materials properties. The same basic principles of structure and form must apply across the board to each class of materials, and the tools for their study are frequently common whether it be on the atomic, microscopic, or macroscopic levels. This is no less true when we consider the third component of ma te r i a l s - -des ign- -which , by definition, brings together different materials for a broad range of new products and applications. The subject has opened new vistas for research for mechanical and civil engineers.

Looking at Figure 35 brings sharply into focus the need to strike some kind of balance among the three materials activities. Certainly, within the context of this lecture; we could ask if the changes currently being wrought on university curricula and research have been to balance, even roughly, materials characterization and materials process engineering. Regrettably, this does not appear to be the case. Materials science and engineering is being shaped primarily by physical metallurgists and materials scientists whose focus, traditionally, has been materials properties and not their production. For those who doubt

Electronic/ Others Metals Ceram=cs Polymers Compos=tes Opt=caP (B=omatermls

Magnet=c etc ) Matermls

Z / / / / / / i / / i / / r /

Property/ CHEMICAL METALLURGY Behav=our MATERIALS SCIENCE Characte- PHYSICAL METALLURGY

~ I I I Design MECHANICAL ENGINEERING with CIVIL ENGINEERING

Materials MATERIALS SCIENCE

I I t

Fig. 35 - -Ana tomy of the materials field and the role of process engineering.

these words, I invite you to scan the most recent edition of the ASM Metallurgy~Materials Education Y e a r b o o k [531 and compare the faculty population of properties re- searchers to process engineers in academia. It would seem that Materials Science and Engineering has a very large "S" and a very small "e," at least as far as the universities are concerned. As I stated earlier, this, to me, is the Achilles heel of the discipline. ~ would seem we have learned little from the lessons of the past.

There is much at stake here. If we do not take up the challenge and build strong teaching and research programs in materials process engineering, just who is going to engineer the processes to make materials (including metals!) products with the desired properties, price, and reliability in service? Who is going to run the growing materials industry and to manage the technological change needed to sustain it? Are we going to leave this field, which impacts directly on the competitiveness of our industry, to the chemical, mechanical, and industrial engineers?

If I focus briefly on the metals industry, which I know best, problems of recruitment already are cropping up in such countries as the United States, Australia, and Great Britain. Admittedly, only part of this problem rests on the shoulders of the universities, because the metals industry itself has not done enough, especially when the economic climate was difficult, to assure a secure supply of its most valuable resource--people. In the longer term, even the so-called ~high-tech" materials industries, which have a stronger attraction for many graduating professionals, may experience difficulty in hiring the people they need to work in the production climate, because too few have been oriented in this direction.

What is needed, first, is a change in the undergraduate and graduate curricula of Materials Science and Engi- neering Departments to include a greup of courses that focus on materials process engineering. Emphasis must be on computation, problem solving, and the development of analytical skills (learning to think, not regurgi- tate!). Subjects would include transport phenomena as well as simple overall heat and material balances, process thermodynamics, numerical methods, mathematical modeling, and process analysis/design. At the undergraduate level, courses on statistics, economics, man- agement, and communication would enhance the education of the student seeking a career in the materials industry. Obviously, the inclusion of new courses would require some specialization in Materials Property Characteriza- tion and Materials Process Engineering streams in the latter half of an undergraduate program, but options are hardly a new concept. If we do not vigorously pursue the development of curricula along these lines, the needs of industry and society will not be met, and we will have a crisis on our hands. Some would say we are reaching this point now in sectors of the metals industry.

Already I have said much about the need for a height- ened effort in process engineering research. We require the research not only for the development and improvement of processes but also to supply knowledgeable people to teach courses and to excite students about the challenges and opportunities that lie ahead in the materials industry. Excellence breeds excellence. Thus, we must redouble our efforts to assemble interdisciplinary


groups which are capable of tackling "real world" process problems with all the rewards that can accrue.

In all this, what can be the role of the extractive metallurgist? Simply put, he is more important now than ever before to the metals industry and to the materials field as well. At a time when academic extractive metallurgy programs are being pared or closed, a strong voice is needed to champion the retention of chemical metallurgy subjects and the strengthening of process engineering. The extractive metallurgist must show the continuity in the transition from metallurgy to materials science and engineering, as surely as we have seen the evolution of metals and mankind. He is at home with diversity and, schooled in the fundamentals of chemical metallurgy and process engineering, is an example to the materials engineer of the need to wed property characterization and process engineering. The extractive metallurgist is familiar with reactors, refractories, molten slags, metals and sulfides, drops, bubbles, fluid flow, and much more - -knowledge that fairly spills over into the processing of other materials. He is used to the difficulties imposed by searing temperatures, high pressure, the complexity of ores, and the vagaries of the marketplace. Adversity is not confined to metals.

The current mood on many university campuses is to abandon extractive metallurgy in the pursuit of a new grail. These academics would do well to listen to what the subject has to teach for their own benefit and that of mankind.

VI. T H E R O A D N O T T A K E N

So finally, we have come to a fork in the road stretch- ing out before us, and the choice comes down to this. Is process engineering going to receive the boost in money and manpower needed to establish it as a discipline, to link metals/materials properties quantitatively to processes? Or will it remain a poor cousin to physical metallurgy and materials science in the clamor to find new properties and materials? Which road shall we take? Per- haps we should heed the words of the great American poet, Robert Frost:

"I shall be telling this with a sigh Somewhere ages and ages hence: Two roads diverged in a wood, and I - - I took the one less traveled by, And that has made all the difference."

VII. AN A P P R E C I A T I O N

Each of us is the product of our forebears and our experience. Some of us have been truly fortunate to have included in that experience the wise counsel, advice, and support of colleagues and friends at critical stages in our careers. Thus, I would like to acknowledge Clarence Samis, Ernest Peters, and Margaret and Bill Armstrong, who gave me a good start; the late Denys Richardson, who showed me excellence in all that he did; Nickolas Themelis, Peter Tarassoff, Jock McKay, and Ray Meadowcroft, who opened my eyes to the opportunities in process engineering; and later, people with whom I

have enjoyed working, including Bob Lee, Gerry Hatch, Charlie Sutherland, Gerry Toop, Terry Burgan, Carlos Diaz, Carlos Landolt, Malcolm Bell, Peter McGeer, Bob Pugh, Alex McLean, Paul Watkinson, Martha Salcudean, Fred Weinberg, my faculty colleagues mentioned in this lecture, and, finally, but most importantly, our research staff and students. It has been a rich experience.

R E F E R E N C E S

1. F.D. Richardson and J.H.E. Jeffes: J. Iron Steel Inst., 1948, vol. 160, p. 261.

2. F.D. Richardson: Metall. Trans., 1971, vol. 2, pp. 2747-56. 3. The Making, Shaping and Treating of Steel, 10th ed. (Latest

Technology), W.T. Lankford, N.L. Samways, R.F. Craven, H.E. McGannon, eds., U.S. Steel, AISE, 1985, p. 2.

4. N.J. Themelis: Metall. Trans., 1972, vol. 3, pp. 2021-29. 5. G.N. Oryall and J.K. Brimacombe: Metall. Trans. B, 1976,

vol. 7B, pp. 391-403. 6. A.H. Castillejos and J.K. Brimacombe: Metall. Trans. B, 1987,

vol. 18B, pp. 649-71. 7. A.H. Castillejos and J.K. Brimacombe: Metall. Tram. B, in press. 8. J.K. Br I.V. Samarasekera, N. Walker, I. Bakshi,

R. Bornmaraju, F. Weinberg, and E.B. Hawbolt: ISS Tram., 1984, vol. 5, pp. 71-77.

9. J. Szekely: Metall. Trans. B, 1988, vol. 19B, pp. 525-40. 10. L. Formanek, H. Eichberger, and H. Serbent: Metall. Plant and

Technology, 1988, vol. 11, pp. 7-16. 11. P.V. Barr, J.K. Brimacombe, and A.P. Watldnson: Metall.

Trans. B, in press. 12. J.P. Gorog, J.K. Brimacombe, and T.N. Adams: Metall. Trans. B,

1981, vol. 12B, pp. 55-70. 13. J.P. Gorog, T.N. Adams, and J.K. Brimacombe: Metall. Trans. B,

1982, vol. 13B, pp. 153-63. 14. H. Henein, J.K. Brimacombe, and A.P. Watldnson: Metall.

Trans. B, 1983, vol. 14B, pp. 191-220. 15. P.V. Barr: Ph.D. Thesis, The University of British Columbia,

Vancouver, BC, 1986. 16. P.V. Barr, J.K. Brimacombe, and A.P. Watkinson: Pyrometal-

lurgy 87, IMM, London, 1987, pp. 53-90. 17. J.K. Brimacombe and A.P. Watkinson: Metall. Trans. B, 1978,

vol. 9B, pp. 201-08. 18. A.P. Watkinson and J.K. Brimacombe: Metall. Trans. B, 1978,

vol. 9B, pp. 209-19. 19. H.C. Hottel and A. Sarofim: Radiative Transfer, McGraw-Hill,

New York, NY, 1967. 20. G.A. Sucre, D. Ablitzer, and J.K. Brimacombe: Proc. Int. Symp.

on Phys. Chem. of lron and Steelmaking, CIM, Montreal, 1982, pp. II-16-II-25.

21. G.J. Roman-Moguel and J.K. Brimacombe: Philbrook Memorial Syrup. on Fundamentals of lron and SteelmaMng, ISS, Warrendale, PA, 1988, pp. 39-60.

22. R.J. de Carvalho and J.K. Brimacombe: Pyrometallurgy 87, IMM, London, 1987, pp. 223-54.

23. G.G. Richards, J.K. Brimacombe, and G.W. Toop: Metall. Trans. B, 1985, vol. 16B, pp. 513-27.

24. G.G. Richards and J.K. Brimacombe: Metall. Trans. B, 1985, vol. 16B, pp. 529-49.

25. S.L. Cockcroft, G.G. Richards, and J.K. Brimacombe: Metall. Trans. B, 1989, vol. 20B, pp. 227-35.

26. S.L. Cockcroft, G.G. Richards, and J.K. Brimacombe: Can. Metall. Quart., 1988, vol. 27, pp. 27-40.

27. R.C. Bell, G.H. Turner, and E. Peters: TMS-AIME, 1955, vol. 203, pp. 472-77.

28. H.H. Kellogg: TMS-AIME, 1967, vol. 239, pp. 1439-49. 29. R.J. Grant and L.J. Barnett: in South Australia Conference 1975,

Australas IMM, Melbourne, 1975, pp. 247-65. 30. J.K. Brimacombe, E.B. Hawbolt, and F. Weinberg: ISS Trans.,

1982, vol. 1, pp. 29-40. 31. I.V. Samarasekera and J.K. Brimacombe: Can. Metall. Quart.,

1979, vol. 18, pp. 251-66. 32. I.V. Samarasekera and J.K. Brimacombe: lronmaking and Steel-

making, 1982, vol. 9, pp. 1-15.


33. S.N. Singh and K.E. Blazek: Proc. AIME Open Hearth Conf., 1976, vol. 59, pp~ 264-83.

34. I.V. Samarasekera, D.L. Anderson, and J.K. Brimacombe: MetaU. Trans. B, 1982, vol. 13B, pp. 91-104.

35. I.V. Samarasekera, J.K. Brimacombe, and R. Bommaraju: ISS Trans., 1984, vol. 5, pp. 79-94.

36. R. Bommaraju, J.K. Brimacombe, and I.V. Samarasekera: ISS Trans., 1984, vol. 5, pp. 95-105.

37. R.P. Sopher: Welding J., 1958, vol. 37, pp. 481S-92S. 38. J.K. Brimacombe, I.V. Samarasekera, and R. Bommaraju:

Steelmaking Proc., ISS, Warrendale, PA, 1986, vol. 69, pp. 409-23.

39. L. Wolfe, B. Wales, J. Delgado, and A. Rodriguez: Iron and Steelmaker, 1988, vol. 9 (7), pp. 33-41.

40. V. Krujelis and J. Cook: Proc. Steelmaking Conf., ISS, Warrendale, PA, 1988, vol. 71, pp. 349-52.

41. A. Tendler: Wire J., 1981, vol. 14, pp. 84-91. 42. P.K. Agarwal and J.K. Brimacombe: Metall. Trans. B, 1981,

vol. 12B, pp. 121-33. 43. J. Iyer, J.K. Brimacombe, and E.B. Hawbolt: Proc. Mechanical

Working and Steel Processing XXll, ISS, Warrendale, PA, 1984, pp. 47-58.

44. P.C. Campbell, E.B. Hawbolt, and J.K. Brimacombe: Proc. Int. Symp. on Accelerated Cooling of Rolled Steel, G.E. Ruddle and A.F. Crawley, eds., CIM, Montreal, 1987, pp. 309-30.

45. M. Avrami: J. Chem. Physics, 1939, vol. 7, pp. 1103-12. 46. M. Avrami: J. Chem. Physics, 1940, vol. 8, pp. 212-24. 47. M. Avrami: J. Chem. Physics, 1941, vol. 9, pp. 177-83. 48. M.B. Kuban, R. Jayaraman, E.B. Hawbolt, and J.K. Brimacombe:

MetaU. Trans. A, 1986, vol. 17A, pp. 149~-503. 49. E.B. Hawbolt, B. Chau, and J.K. Brimacombe: Metall. Trans. A,

1983, vol. 14A, pp. 1803-15. 50. E.B. Hawbolt, B. Chau, and J.K. Brimacombe: Metall. Trans. A,

1985, vol. 16A, pp. 565-78. 51. M. Umemoto, N. Komatubara, and I. Tamura: J. Heat Treating,

1980, vol. 1, pp. 57-64. 52. P.C. Campbell: Ph.D. Thesis, The University of British Columbia,

Vancouver, BC, 1989. 53. Metallurgy~Materials Education Yearbook, K. Mukherjee and J.P.

Nielsen, eds., ASM, June 1988.

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