A BRIEF LESSON IN THE CHEMISTRY OF PIG IRON G . T. FRANKLIN. LANE TECHNICAL HIGHSCHOOL, CHICAGO, ILLINOIS
It is doubtless the concern of many teachers of general chemistry how to attack the problem of iron and steel. T o neglect the problem means a chance lost to r & m numerous chemical principles and a n opportunity wasted to put into w e knowledge pra~iouslystudied. To organize the material suitable to the purpose has been the writer's difficulty. The school texb are of necessity limited in their discussions. Most of the technical works are too difficult for the beginner and the popular ones tend to submerge the chemistry in details of construction, etc. The @per represents a n effort to ptlt into brief s@ce the work of considerable reading, obsemtion of factories, and senreral years of experience in attempting to teach the topic to boys in a technical high school.
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The blast furnace used in the manufacture of pig iron is not a total stranger to many people. The numerous chemical reactions involved in the process are not generally known and much less understood. A comprehensive grasp of the essentials of the subject may be had by keeping in mind clearly the materials involved and the final products obtained. Numerous applications of a few principles of elementary chemistry are essential. I t is common knowledge that pig iron contains such elements as iron, carbon, silicon, sulfur, phosphorus, and manganese. From the outlines in Figure 1 it is noted that the iron comes from the iron ore and unavoidably from the same source come silicon, phosphorus, and manganese. The sulfur comes from the coke. One might guess that some of the sulfur might come from the ore and some of the phosphorus from the limestone. Diligent search of the literature failed to find any authority for such statements. No free elementary solid substances are introduced into the blast furnace except the carbon of the coke. The f a d that elements exist in the furnace indicates a reducing action. The chief reduang agent is carbon monoxide. In Figure 1 is shown a series of chemical reactions, beginning near the top of the stock line and terminating in the zone of fusion. The list includes some of the most important; it is by no means complete. Some reactions oppose others. Of course it is important to provide the conditions to carry forward to an end reactions for the production of free iron and repress those which tend toward the reoxidation of the iron when formed. At some particular temperature and concentration, these reactions are all equilibrium reactions. As these factors change some of the reactions are repressed and others camed to an end. The reactions may be compared with those involving the preparation of a volatile acid. At room temperatures the reaction rapidly comes to equilibrium. The addition of heat carries forward the reactions to an end. The purpose of the limestone is to furnish material which forms a fusible 143
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mixture of substances insoluble in molten pig iron. Calcium carbonate decomposes to form infusible calcium oxide. The calcium oxide reacts with acid oxides to form fusible salts which a d as solvents of other substances. Thus the slag includes the silicates, phosphates, and most of the sulfides. The metals that are set free are found in the pig iron. This does not necessarily mean that these elements are in the free form in the LIMESTONE
+
(4) COZ C-2C0
FeO + CO-Fe + Cop + C-Fe + CO I6)5) FeO CaC0,-CaO + COz (7)
pig iron. The presence in the pig iron of phosphides and silicides means that phosphorus and silicon were set free in the furnace and then recombined. The outline in Figure 1 gives substances that may be found in pig iron. They may not all be present in any one sample. Sulfur exists in soft coal as F&. In coking about half of the sulfur is lost. In the upper part of the furnace the sulfur is all converted into
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FeS (Figure 2). The FeS would pass into the pig iron if it were not for two powerful reducing actions, that of carbon shown in equation 9, Figure 1, and manganese shown in Figure 2. Manganese sulfide distributes itself between slag and pig iron, the larger percentage in the slag. The route of the manganese is shown in Figure 3. Aluminum and calcium are unreduced and pass out with the slag. Some of the pbosphorus is reduced in the zone of fusion (equation 12, Figure 1) and is dissolved by the pig iron as phosphide of manganese or iron or as eutectic. A basic furnace tends to decrease the sulfur in pig iron but increases COAL the pbosphorus and manganese. Why? An acid furnace increases the sulfur in pig iron and causes much of the manganese to dissolve in the slag as manganese silicate. Why? High temperatures tend to decrease the sulfur in pig iron but increase the silicon and phosphorus. Why? Many important details, not discussed so far, are suggested in the questions to follow. To answer the questions standard works should be consulted. A thorough review of many chemistry principles will doubtless be necessary in many cases. Outlmes and diagrams included in Figures 1, 2, and 3 areintended to help. Many statements are made in techFIGURE 2 nical works as facts to be remembered that are easily deduced from well-known chemical principles. Questions What solid materials are used in the blnst furnace? What gaseous substance is introduced into the blast furnace? What liquid substances are taken from the furnace? What gaseous substance is obtained from the furnace? List the metals that enter the blast furnace in the combined form in the order of their activity. Which of the metals would be most liiy to be
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reduced? If phosphorus and silicon were set free where would they be found, in the slag or the pig iron? Review methods for the commercial preparation of silicon and phosphorus and compare the conditions with those existing in the blast furnace. Do the conditions in the fumace warrant a guess that silicon and phosphorus would be reduced? Is it possible that a small portion of these elements might be reduced? Review the chemistry involved in the manufacture of ordinary window glass. How do the conditions as to temperature and materials in the blast furnace compare with those of the furnace used in the manufacture of ordinary window glass? What is an acidic oxide? What is a basic oxide? What class of cornpounds is formed by reaction between a basic and an acidic iron ore oxide? Do sulfides of manganese and calcium dissolve in the slag or the pig iron? Would you expect free manganese to exist in the presence of iron compounds? Would you expect slag or pig iron to dissolve iron sulfide? Under what conditions would iron sulfide exist in pig iron? Are conditions favorable to the formation of free manganese in the blast furnace? If the element were set free, would it dssolve in the slag or the pig iron? Review the methods used in the manufacture of producer FIGURE 3 and water gases and apply the principles to the reactions that take place when air is introduced into a blast furnace. Do the substances obtained from the furnace as gas corroborate your mental analysis? Does a mixture of water and chloroform separate into layers because they are not of the same density, or because they are not miscible? Why do slag and pig iron in the molten condition separate into layers? Some potassium iodide solution is mixed with chloroform and shaken. What happens after a time? Now suppose a crystal of iodine is added to the mixture and shaken, where does the iodine go? Where would most of the iodine go, if chloroform and pure water were used? Suppose some salt crystals were shaken in a mixture of chloroform and pure water, where
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would the salt go? Try to get a mental picture of the zone of fusion of the blast furnace, containing two immiscible solvents and a number of solutes. Apply the old saying "like dissolves like" and trace the route of each element through the furnace. Review the methods used in the preparation of the carbides of calcium and silicon. Could other carbides be prepared in this way? Are conditions in the blast furnace favorable to the formation of metallic carbides? Suppose a sample of pig iron were dissolved in nitric acid and no carbon remained, how would you explain it? If carbon remained from the nitric add solution, what allotropic form would be expected? How would you explain the odor of phosphine often obtained from this reaction? How would you explain the odor of hydrocarbons from the reaction? Remembering that when a compound is decomposed energy equivalent to the heat of formation is absorbed, from a table of heats of formation determine which of the equations in Figure 1 are endothermic and which exothermic. (Suggestion: Write the equations in the following form and observe whether energy absorbed is greater or less than energy liberated-
+ +
+
FeO CO +Fe COz (65,700 c. 29,000 c . ) 97.000 c. (absorbed) (liberated).
It is readily seen from this equation that the energy absorbed is slightly less than energy liberated and consequently the reaction is exothermic.) Would it be desirable to use dry air in the furnace? Why is i t necessary for blast furnace gas to contain approximately 25% carbon monoxide? Of three iron ores, hematite, limonite, and magnetite, which is most valuable for pig iron manufacture if each contained the same percentage of iron compound?
References The following works were consulted and the f a d s used in this paper were taten from them: 1. AUSTIN,"Metallurgy of Common Metals," 6th edition. John Wdey & Sons. Inc., New York City, 1926. 2. BOYLSTON, ''Iron and Steel," John Wiley & Sons, Inc., New York City, 1928. 3. BRAY,"The Principles of Metallurgy," Ginn & Co., Boston, Mass., 1926, chap. 15. "The Manufacture and Properties of Iron and Steel." 4th edition. 4. CAMPBELL, McGraw-Hill Book Co., Inc., New York City. 1907. 5. H n ~ w n m ,"Outline of Metallurgical Practice," D. Van Nostrand Co.. New York City, 1929, chap. 22. 6. H o w , "The Metallmgy of Steel and Cast Iron," McGraw-Hill Baok Co., New Pork City, 1916. 7. HUDSON and BENGOUGR, "Iron and Steel," D. Van Nostrand Co.. New York City, 1916, chap. 2, p. 11.
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PORTEE,"A B C of Iron and Steel." 5th edition, Penton Publishing Co., Cleveland, Ohio, 1925, chap. 7.p. 77. ROGERS,"Manual of Industrial Chemistry," 4th edition, volume 1, D. Van Nostrand Co., New York City, 1926, chap. 18. S A W E ~"The , Metallurgy and Heat Treatment of Iron and Steel." 3rd edition, McGraw-Hill Book Ca., Inc., New York City, 1926, chap. 22. S-c, "Non-Technical Chats on Iron and Steel," Frederick A. Stokes Co.. New York City, 1917, chap. 4, p. 52. STO~GHTON, "The Metallurgy of Iron and Steel," 2nd edition. McGraw-Hill Book Co., Inc., New York City, 1911, chap. 1. STOUGHTON and B m s , "Engineering Metallurgy," McGraw-Hill Book Co., Inc., New York City, 1926. T n o m , "Industrial Chemistry," 3rd edition, The Macmillan Co., New York City, 1916, p. 601. "Encyclopaedia Britannica," 14th edition, Encyclopaedia Brittanica, Inc., New York City, volume 12, 1929. "New International Encyclopaedia," Dadd Mead and CJ., New York City. volume 12, 1923.
