COKING METHODS AND PRODUCTS
I
S. W. MARTIN and F. L. SHEA, Jr. Research and Development Department, Geat Lakes Carbon Corp., Morton Grove, 111.
Microstructures of Carbon Products Carbon microscopy will contribute significantly to a more complete correlation between commercial performance of carbon products and inherent physical properties
T H E manufacture of amorphous carbon products usually involves preparation of carbonaceous aggregate particles, which are first coated with a pitch binder. This mix is shaped and subsequently baked to form an amorphous article having an over-all porosity of 20 to 30%. If a graphitic material is desired, the amorphous based article is subjected LO temperatures in excess of 2500' C. (72) * T o specify the quality of such carbonaceous materials, data relative to the chemical composition, certain physical, mechanical, and electrical properties, and reactivity to certain gases and liquids may be obtained. Even with complete data, the information is insufficient to characterize amorphous and graphitic carbon product3 satisfactorily. The technologist can readily measure the external physical properties of individual aggregate particles. However, once these are bonded into a finished product, information on relative arrangement of the particles, internal and interparticle porosity, and presence
Literatuire Review Ramdohr (80)
Kuhlwein and Abramski (13) Abramski ( 1 ) Abramski and Mackowsky (3) Lichtenberg-Strunk (14) Mackowsky (16)
First article on use of polished sections of coke impregnated prior to polishing Detailed method Microscopic study of special cokes Review, 1935 to 1951
Review, 1951'to 1955
of microcracks and flaws is extremely limited. The chief difficulty was encountered in preparing polished samples by a technique that would not destroy or alter the finer cellular structures, particularly in relatively soft graphitized products. Application and further development of this technique should reduce to a minimum the wide gap of uncertainty in knowledge of the microstructures of the diverse products of pilgt or commercial carbon plants. Sample Preparations
The technique of preparing samples of metallurgical cokes has been described by Abramski and Mackowsky (3). Certain modifications have been developed in extending the technique to carbon products. The following steps are involved in the microscopy of carbons. Selection of Sample, Because the field of the polished sections is necessarily limited, great care must he exercised in the selection of samples. The exact procedure varies considerably, depending on the type of material to be investigated. The procedure in the case of foundry coke has been adequately described ( 3 ) . In the examination of relatively large electrodes or anodes, samples must be representative of the coke structure from the periphery to the center of the piece. The samples needed for a study are best determined by first inspecting a number of sections under low power magnification, to comprehend visually the over-all structure of the material.
The magnification at which photographs are taken is likewise dependent on the type of material. Usually photographs should cover an area which is large relative to the maximum diameter of the particles present. Based on the examination at low power, special areas of interest may be examined at higher magnifications, to afford better resolution of microstructural details. Preparation of Sample Prior to Impregnation. Relatively hard materials such as amorphous carbon, calcined coke, and foundry cokes, are cut by a rotary diamond saw; graphites are normally sampled by a metal band saw. Specimens approximately 2 inches square and about inch thick are used. Information concerning the orientation of the specimens relative to the main sample is recorded. Specifically the position and location of a specimen are defined relative to a large section of graphite electrode, anode, etc. After cutting, the specimens are ground flat on both surfaces, by use of a coarse abrasive. The edges of the specimen are also ground or filed, to prevent gouging of the cloth during later polishing. At this point, the specimens are thoroughly washed in water and placed in a drying oven for approximately 1 hour at 140' to 150' C. This drying period also preheats the sample for the impregnation process. Impregnation. The impregnating resin or Schneiderhohn mixture consists of 3 parts by weight of No. 1 Singapore dammar resin, 2 parts by weight of high grade orange flake shellac, and 1 part by weight of true Venice turpentine. The resin mixture is prepared by heating in a cast iron pot with a Meker burner. The dammar resin, which is VOL. 50, ,NO. 1
JANUARY 1958
41
the ingredient of highest melting point, is added to the pot first in increments as it melts. The shellac and finally the turpentine are added with sufficient agitation to yield a homogeneous mixture. As developed in Germany, the impregnation procedure for metallurgical cokes involved placing the dried preheated carbonaceous specimen in a dish filled with the molten resin: the sample was allowed to remain in the resin several hours, preferably overnight. However, with commercial carbons and graphites it is necessary to use vacuum impregnation, particularly where the sample has a very fine pore structure. This ensures much better penetration by the resin mixture. After impregnation and cooling, the resin hardens and the impregnated specimens are ready for polishing. Grinding. After the resin has hardened, the sharp edges are removed with a file and the samples are hand-ground with five different grades of silicon carbide grit, on glass plates placed inside stainless steel trays 12 X 12 X 1 1 / 4 inches deep. Five separate plates and trays provided ,with covers to prevent contamination with other abrasives are used, and water is used as the vehicle to prevent possible flow or deformation of the resin. The sizes of silicon carbide grit are used in the order: 54, 220, 2F. 500, and 1000. No. 54 grit is used primarily to remove the resin coating from the surface of the specimen, with a4 little contact as possible. The time or amount of grinding done with each grit size varies considerably, depending upon the hardness of the samples. Over-
(10x1
grinding should be avoided, as impregnation of the specimen with the resin is most complete at or near the surface. Usually sufficient grinding is performed with each grit size to remove the scratches from the previous coarser grit. Polishing. The results of microscopic examination of polished sections are highly dependent upon the polishing technique, which to a very great extent is considered an art. Consequently presentation of a routine technique for this procedure is very difficult. The basic criterion which determines whether a specimen is properly prepared depends upon ability to distinguish between the various components under the microscope and introduction of no significant surface roughness or porocitv in the polishing step. A standard metallographic polishing table equipped with wheels 8 inches in diameter having two speeds-550 and 1100 r.p.m.-is used. The polishing abrasives in the order in which they are used are: chromic oxide. polishing alumina No. 1 (particle size 5 microns), and polishing alumina No. 3 (particle size 0.1 micron). All types of polishing cloths are used: billiard cloth micro cloth. Metcloth, Sdvyt, red felt, and silk velvet. For polishing the harder specimens such as amorphous carbon. billiard cloth ivith chromic oxide is satisfactorv I n the usual polishing procedure two stages are conducted on the wheel: the first with chromic oxide and the next with alumina 1. The final polishing is done by hand. using polishing alumina 3, on a glass plate in a stainless steel tray. Greatest difficulty in polishing is encountered with the soft samples. such as graphite. which exhibit a tendency toward smearing; thus detail may be obliterated. Extreme care must be exercised in the final polishing. Upon completion of polishing, the sample is thoroughlv washed with water and after drying is ready for mounting. Modeling clay is used as a mounting media to affix the specimens to a glass slide. a hand press being utilized to ensure exact leveling of the sample. Such samples may be examined with any suitable microscope equipped with a vertical illumination system.
Figure 2. High density U. S. foundry coke (10x1
Figure 3. High density U. coke (400x1
Figure 1. Typical U. s. foundry coke from blend of coals in by-product oven
42
INDUSTRIAL AND ENGINEERING CHEMISTRY
S. foundry
Table I. Comparison of Standard and High Density Foundry Coke High
Density Foundry Coke Property
u. s.
(Fig. 1)
Foundry Coke (Fig. 2)
0.5 0.7 8
0.5 0.7 3.5
Volatile matter, wt. 91,
sulfur, wt. % Ash, wt. % Tumbler (7) (1400 revolutions) +1 inch, wt. % +I/d inch, wt.
%
Tumbler (400 revolutions) $ 1 inch, wt. % +l/4 inch, wt. % Shatter (6) $ 4 inch, wt. $ 3 inch, wt. % $ 2 inch, wt. yo Apparent density (61, g*/cc. Real density, g./cc. Porosity, vol. %
55
...
51 52
aa
76
93
77
74
79 91 96
...
94
0.9 1.92 53
1.27 2.00
36
A Model K P M Leitz microscope for coal petrography is recommended. Photographs in the range of 1 x to lox are taken with a Model L Bausch & Lomb camera. In this range of magnification, the main problem is to obtain uniform lighting over the relatively large sample area. The illumination system employed is a Leitz hlonla Universal lamp with focusing collimating lens and a 120-mm. condensing lens. Although alignment is extremely critical, satisfactory illumination can be obtained. Photographs at high power magnification are taken on a Leitz Model K P M petrographic microscope with vertical bellows camera M A TVB. Prior to taking photographs, all specimens are scanned visual1)- to determine over-all structure and locate areas of special interest. Photographs are usually taken of a typical area or one that shows an unusual feature. Kodak Panatomic sheet film is used for all photomicrographs w-ith a green filter (Wratten B 58). The negatives obtained at low
Figure 4.
(200 x )
Calcined coal tar pitch coke
COKING METHODS A N D PRODUCTS
,
Figure
(200 x)
5.
Calcined
petroleum coke
or high power are used to make contac,t or projection prints. Microstructures of Metallurgical Cokes
Figures 1 through 3 illustrate the wide range of structures encountered in metallurgical cokes. T h e special high density U. S. foundry coke (78) is characterized by thick cell walls and a small number of large voids (Figure 2), in contrast to the thin cell wall structure and many fine as well as coarse voids. characteristic of standard foundry coke (Figure 1). Typical physical and mechanical properties summarized in Table I and the photomicrographs may be used to establish correlations between basic properties and performance in cupolas (2, 77). Figure 3, a photomicrograph of high density U. S. foundry coke in a region where anthracite coke is visible, was taken 20' from crossed Nicols. It shows that the anthracite particles are well bonded into the surrounding coke structure. More detailed information on analysis of coke structures and interpretation of photoniicrographs is given by Mackowsky and Abramski (2, 3, 77). Microstructures of Base Cokes
The manufacture of amorphous carbon and graphite products involves a wide
Figure 6. Calcined bonded fluid petroleum coke (200X)
Figure 7. Calcined anthracite, crossed Nicols (200X )
variety of carbonaceous raw materials: calcined petroleum cokes, calcined anthracite, coal tar pitch coke, etc. A calcined coal tar pitch coke (Figure 4) exhibits a high over-all structural anisotropy; calcined petroleum coke (Figure 5 ) , characterized by an intermediate degree of structural anisotropy, is a widely used commercial coke produced by delayed coking (8, 70, 77, 75, 22). The petroleum industry has developed a continuous coking process ternled fluid coking (27), which produces a rather fine powder approximating 20 to 100 mesh in size. To facilitate examination, fluid coke particles were pitch bonded and carbonized into a massive coke structure. A photograph of a polished section (Figure 6) clearly differentiates between the isotropic base aggregate particles and anisotropic pitch binder coke which serves as the binding agent. Although this is a relatively simple system, it clearly demonstrates the utility of microscopic examination of polished sections. Calcined anthracite finds wide application in cathode liners for aluminum reduction cells, amorphous prebaked electrodes, and certain types of structural products. This material under crossed Nicols is characterized by a uniformly distributed fine void system and by large areas of uniform anisotropy as indicated by the dark and light areas (Figure 7). This optical phenomenon (4) is useful in identifying base cokes and other components in complex carbonaceous systems.
Soderberg or continuous self-baking type and prebaked. As carbon consumption is an important' cost item in the production of aluminum. considerable attention has been devoted to the factors that influence the rate. In general, carbon consumption is directly related to the apparent density of the carbon anode at the bath level and is dependent on microstructural factors which heretofore have defied analysis. The anode of Figure 8 was responsible for a much lower carbon consumption and superior performance, compared to the product shown in Figurc 3. The anode butt of Figure 9 contains numerous flaws or fissures as wcll as areas of low density which are not detectable in Figure 8. Both the coke aggregate particles and pitch binder are from different sources. The final product differences point to the conclusion that air oxidation was a major factor in accounting for the higher consumption of the fissured product. Soderberg anodes, used in the aluminum industry, have somewhat lower baked apparent densities than those of the prebaked variety. Inasmuch as Soderberg pastes are continuously carbonized and consumed, the evaluation of final baked carbon quality is subject to a variety of difficulties. Carbon microscopy has proved extremely useful in correlating information on Soderberg
Amorphous Carbon Products
Figure 8. Prebaked anode butt after removal from a commercial cell
-
(lox)
Apparent density 1.60 g./cc. Porosity 20% Compressive strength 4 3 0 0 Ib./sq. inch Electrical resistivity a . 0 0 2 1 ohm-inch
Amorphous carbon is used extensively as both an anode and cathode liner in the electrolytic reduction of alumina (79). T h e consumption of carbon in the cathode is extremely low. A normal life for a cathode pot liner is about 1000 days; however, the anodes are consumed a t about 0.5 pound of baked carbon per pound of alumirium metal. In general, two types of anodes are used-i.e.,
Figure 9. Prebaked anode butt after removal from a commercial cell (IOX)
-
Apparent density 1.54 g./cc. Porosity 25% Compressive strength 2900 Ib./sq. inch Electrical resistivity 0.0036 ohm-inch
VOL. 50, NO. 1
JANUARY 1958
43
Figure 10.
Baked Soderberg anode
(10x1
Figure 11. Baked Soderberg anode Figure 12. exhibiting numerous microcracks (1 0 X ) (10x1
-
Apparent density 1.44 g./cc. Porosity N 28%
Figure 13. liner ( 1 O X )
Baked, rammed cathode
-
Apparent density 1 S 26% Porosity
Figure 14. (10x1
O
g./cc.
Prebaked cathode block
-
Apparent density 1.60 g./cc. 20% Porosity
Apparent density 1.46 g./cc. Porosity 28%
Baked Soderberg anode
Apparent density 1.58 g./cc. Porosity N 2370
Figure 15.
(lox)
Blast furnace liner block
-
Apparent density 1.52 g./cc. 2570 Porosity
paste and Soderberg baked carbon performance at the potline level. Figure 10, although representative of low apparent density Soderberg carbons, constitutes a well bonded system. The Soderberg of Figure 11, although of higher apparent density, exhibits definite evidence of cracks. fissures, etc., and this in turn was largely responsible for high paste consumption as compared to the performance realized with the carbon of Figure 10. A Soderberg anode baked carbon with an apparent density of 1.58 grams per cc. is shown in Figure 12. This carbon system is characterized by a relatively low porosity and, because of excellent bonding by the pitch binder carbon. is an anode of high strength and excellent performance. Carbon is also the essential material for making the cathode liner of an alumina
reduction cell. I n addition to having low electrical resistivity, a cathode liner should have high compressive strength and excellent resistance to penetration and erosion by the molten bath constituents. I n Figure 13, a rammed cathode liner baked in place, the large dense particles are calcined anthracite, well bonded into the surrounding coke structure. Cathode liners are also made from prebaked carbon blocks (Figure 14). The higher proportion of coarse particles and generally denser structure of the prebaked block are apparent from a comparison of Figures 13 and 14 Structural carbon is finding increasing use as a refractory material for lining blast furnaces (Figure 15). The continuous phase has a fine compact structure and the calcined anthracite particles are well bonded into the over-all structure, Microscopic examination can
Figure 16. Highly anisotropic coke from impregnating pitch ( 4 0 0 X )
Figure 17. Typical electrode binder Figure 18. Identical field to that of pitch coke showing dense narrow bands Figure 17 except crossed Nicols ( 4 0 0 X ) of isotropic coke ( 4 0 0 x 1
Crossed Nicols
44
INDUSTRIAL A N D ENGINEERING CHEMISTRY
be applied to samples removed from a liner upon completion of a campaign, to increase knowledge of the factors that contribute to blast furnace carbon liner performance. Microstructure of Pitch Binder Cokes
I n the study of many carbon products, the pitch binder coke produced during baking is an all-important factor in bonding coke aggregate particles into an over-all structure possessing high strength, high apparent density, and low electrical resistivitv. Three photomicrographs of binder coke, produced by ldboratory coking coal tar pitches in the absence of aggregate over a 4-dav c \ c k to a maximum temperature of 1000c C.. are shown in Figures 16, 17, and 18. Figures 17 and 18 are identical except that Figure 18 was taken Lvith crossed Nicols.
COKING METHODS AND PRODUCTS
Figure 19.
Hardwood charcoal (600X ) Figure 20.
(600X 1
Highly reactive coal char
Figure 2 1. Longitudinal section of 10 X 60 inch graphite electrode ( 2 0 x 1
-
Apparent density 1.60 g./cc. Porosity 27%
Figure 22. Transverse section of 24 X 72 inch graphite electrode ( 2 0 X )
-
Apparent density 1.53 g./cc. Porosity 30%
These photographs are representative of coke from standard binder pitch showing an area where dense narrow bands of highly isotropic coke are visible. Microstructures of Special Carbons
Carbon finds application as a reductant in chemical reactions where high reactivity is desired. Materials which have been established aftrr years of trial and error appear under the microscope as a very fine, permeable pore structure) inherent in which is a high surface area. In Figure 19 is shown a highly magnified photomicrograph of hardwood charcoal, which in granular form has a bulk density of about 13 pounds per cu. foot and is employed extensively in the production of carbon disulfide. Another highly reactive char (Figure 20) is produced from specially processed coal and, in spite of high microporosity, its bulk density is 30 pounds per cu foot. Wherever reactivity is a prime consideration, microscopic examination has proved useful in evaluating the performance of established and new carbonaceous materials. Microstructure of Synthetic Graphites
The largest tonnage of synthetic graphite is consumed as electrodes used in electrothermal proceses, particularly
23. Longitudinal section of Figure 24. Transverse section of 12!h 1 2 E X 14 inch graphite nipple ( 2 0 X ) X 14 inch graphite nipple (20X)
Figure
-
Apparent density 1.74 g./cc. Porosity 2 1%
electric steel furnaces. Their diameter may vary from several to 24 inches or more. Their manufacture involves the preparation of an aggregate system from petroleum coke particles and pitch binder, which is then formed to the desired size by extrusion. After baking in gas-fired furnaces to a temperature of about 1000' C., the amorphous article is, charged to an Acheson-type furnace and heated to a temperature in excess of 2500' C. (72). In general, the maximum size aggregate particle used is increased as the diameter of the electrode increases; thus electrodes of small diameter are finer grained than those of large diameter. A typical graphite electrode 10 inches in diameter and 60 inches long (Figure 21) has a maximum particle size observable of about 10 mesh. In Figure 22 is shown a transverse section of a graphite electrode 24 inches in diameter and 72 inches long; the maximum size of aggregate particle observable in this field is about 4 mesh. Individual lengths of such electrodes are fitted with a tapered threaded socket and these sections are connected by threaded tapered nipples. Differences in porosity and porosity distribution, as well as in alignment of the larger particles, are evident in Figures 23 and 24. These differences in the structure of longitudinal and transverse sections due to the extrusion process are a well established effect in carbon technology,
-
Apparent density 1.74 g./cc. Porosity 2 1%
This preferential alignment of particles and difference in porosity may be detected by other physical measurements such as electrical resistivity, coefficient of thermal expansion, and permeability (9). Synthetic graphite finds extensive application in electrolytic anodes, mold stock, corrosion-resistant equipment, and as a moderator in nuclear reactors. The structure of a typical electrolytic anode, not pitch-impregnated, used in the production of chlorine and caustic, is shown in Figure 25. Mold stock (Figure 26) and graphite used for corrosion resistant equipment have a much finer grain structure than that used for electrothermal electrodes. Transverse and longitudinal sections of a much finer grained nuclear graphite, produced by pressing, are shown in Figures 27 and 28. The transverse specimen is a section taken perpendicular to the direction of pressing, and differences in porosity distribution between longitudinal and transverse are pronounced. This stock is characterized by a maximum dimension of the pores of the order of 0.3 mm. A laboratory-developed graphite having a much finer pore size than previously available (Figures 29 and 30) was not pitch-impregnated and has a lower apparent density than the nuclear graphite of Figures 27 and 28; yet, because of the extremely fine pore VOL. 50, NO. 1
JANUARY 1958
45
tional trends of both the carbon aggregate and the void systems. Microscopy of carbon products is still in its infancy and undoubtedly offers the carbon technologist a fertile field for fundamental and industrial investigations. Acknowledgment
Figure 25. Longitudinal section graphite electrolytic anode ( 2 0 X )
of
Figure 26. Transverse section of extruded graphite mold stock ( 2 0 x 1
-
Apparent density 1.68 g./cc. Porosity 23%
Apparent density 1.56 g./cc. Porosity N 2970
The authors wirh to express their appreciation and indebtedness to M. T. Mackowskv, Steinkohlenbrrgbauverein, Essen. Germany, for her many contributions, and to Harlan Fritz, microscopiqt. Research Department, Great Lakes Carbon Corp. Literature Cited
Figure 27. Transverse section of molded fine-grained, nuclear graphite
Figure 28. Longitudinal section of molded fine-grained, nuclear graphite
(20x1
(20x1
-
-
Apparent density 1.72 g./cc. Porosity 2270
Apparent density 1.72 g./cc. Porosity 22%
Figure 29. Longitudinal section of extremely fine-grained graphite mold stock (20x1
Figure 30. Longitudinal section of extremely fine-grained graphite mold stock ( 6 0 0 x 1
Apparent density 1.45 g./cc. Porosity 34%
Apparent density 1.45 g./cc. Porosity 34%
system, it appears at low magnification to be of higher apparent density.
result, ought to provide a more suitable basis for theoretical treatment of carbon bonded products. Specifically, average over-all porosity is calculable from real and apparent densities. A finer degree of resolution is obtainable by porosimeters which permit calculation of pore size distributions. It is also possible to determine the percentage of available and closed pores. Severtheless, such information is based on assumptions which definitely limit the validity of the final data, and conclusions derived therefrom. Carbon microscopy, as a supplemental research tool, makes possible reduction of the foregoing uncertainties to a minimum. Furthermore, in studying anisotropic effects in finished products, carbon microscopy is unique in that it permits visually observing direc-
-
Possibilities of Carbon Microscopy
The technique of carbon microscopy as applied to amorphous and graphite products, in the opinion of the writers, will contribute significantly to a more complete correlation between the performance of carbon products in commercial applications and their inherent physical properties. In fundamental studies, the carbon technologist has often attempted to set u p structural concepts to serve as a basis for correlation with observed physical properties. Such efforts have been rewarded with limited success. Carbon microscopy should prove an effective research tool, as it permits observing actual physical structures and, as a
46
-
INDUSTRIAL A N D ENGINEERING CHEMISTRY
(1) Abramski, C., Bergbau-Aich. 516, 153-63 (1947). ( 2 ) Abramski, C., Gluckauf, Supplement (Beiheft) to 91st yr., 195-201 (August 1955). (3) Akamski, C., Mackowsky, M. T., Handbuch der Mikroskopie in der Technik,” ed. by H. Preund, vol. 11, Pt. I, pp. 311-410, Umschau Verlag, Frankfort, Germany, 1952. Upern, B., Brennstoff-Chem. 36, 216-17 (1955). Am. SOC. Testing Materials, Standards, D 141-48, Part V (1955). Ibid., D 167-24. Zbid., D 294-50. Breese, F., Petrol. Processing 8, 1170 (August 1955). Currie, L. M., Hamister, V. C., MacPherson, H. G., Proceedings International Conference on Peaceful Uses of Atomic Energy, vol. VII, Paper 534, United Nations, New York. 1955. (10) Foster, A. L.,Petrol. Engr. 23, C53C62 (April 1951). (11) Fuchs, 0. A., Petrol. Processing 5 , 1058 (October 1950). (12) Hader, N., Gamson, B. W.,Bailey, B. L.. IKD.ENG. CHEM.46, 2--11 (1954j. Kuhlwein. F. L.. Abramski. C.. Gluckauf 75, 685-’90 (1939). Lichtenberg-Strunk, G., “Handbuch der Mikroskopie in der Technik,” vol. 11, Pt. I,-p. 443-81, Umschau Verlag, Frankfort, Germany, 1952. Maass. R.. Lauterbach. R. E., Petrol. Engr. 19, C110-26 (February 1947’i. Mackokky,, M. T., Brennstoff-Chem. Mackokky 36, 304-14 (1955). nMackowsky, M. T., Giesseri 20, 540 (19:4). Martin, S. W.,Gamson, B. W., Bowers. T. G.. Iron A p e 174. 97-9 (Aug. 5 , 1954): Martin, S. W.,Nelson, H. W., J . Metals 7 , 540-3 (1955). Ramdohr, P., Arch. Eisenhuttenw. 1, Hll,S609-72 (1928). Voorhies, A., Jr., Martin, H. Z., Petrol. Refiner 32, 127-30 (December 1953). ” (22) Ward: J. W., Holocek, J. M., Ibid., 33, NO. 2,157-9 (1954). I
,
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RECEIVED for review March 5 1957 ACCEPTED November 15, 1957 Division of Gas and Fuel Chemistry, Symposium on Coking Methods and Products, Joint with Division of Petroleum Chemistry, 131st Meeting, ACS, Miami, Fla., April 1957.
