Carbon Materials in Large Volume Applications

9 Carbon Materials in Large Volume Applications E R L E I. SHOBERT, II

Petroleum Derived Carbons Downloaded from pubs.acs.org by UNIV OF TEXAS AT AUSTIN on 08/28/17. For personal use only.

Stackpole Carbon Co., St. Marys, Pa. 15857

I.

Introduction

A. Economics. The size and relative distribution of the business in the carbon field is shown in Table 1. These numbers include a l l but the carbon used in the manufacture of aluminum and magnesium in which Soderberg electrodes are used. Carbon and Graphite Market 1974 - $400,000,000 Annually Product

Percent

Brushes Electrodes (Furnace) Anodes (Chlorine) Electrical Uses (Welding, Power Tubes, Illuminating) Mechanical Uses (Seals, Discs, Vanes, Nozzles, etc.) Nuclear (Piles and Moderators) and Aerospace Chemical, Metallurgical Refractories, etc.

14 45 5 8 8 8 12

13,000 people; 2,000 technical Table I The percentages are based on industrial and government statistics. The number of people involved in the industry is an estimate as is the number of technical people which includes technical people in production and sales as well as those in research and engineering activities. B. Raw Materials. Practically a l l of the raw materials used in the production of carbon and graphite are byproducts of 91


92

PETROLEUM DERIVED CARBONS

some other industry, Petroleum coke is the material left at the end of the refining process of oil. Coal tar pitch comes from the production of coke for the steel industry. The carbon black and lampblack used in the carbon industry are but small percentages of these materials which are produced primarily for the rubber and printing industries. Natural graphite and various resins and plastics are representative of the few materials used by the carbon industry which are not just byproducts. This puts a definite constraint on the carbon industry in that some changes which can take place in product quality are not under the strict control of the people making carbon and graphite. For example, the properties of hard pitch change with the rate of production in the steel industry since this determines the manner in which the coke ovens are operated. The quality of petroleum coke varies from o i l field to o i l field and from refinery to refinery. It i s also somewhat different depending upon the mix of products from the refinery. A major part of the art as well as the science of the carbon industry is involved in producing uniform products from the variable raw materials which are available. These raw materials include: Fillers Binders Petroleum Coke of Various Kinds Hard Pitch Carbon Black Petroleum Pitch Lampblack Resins Natural Graphite Plastics Artificial Graphite Charcoal Metallurgical Coke Impregnations Other Additives Metals: Cu, Ag, Sn, Babbit, Lead Solid Lubricants: M 0 S 2 , etc. Lubricants Abrasives Resins and Plastics Metal Powders for Sintered Ma Linseed Oil terials: Cu, Ag C. Processes. The processes involved in the production of tonnage amounts of carbon and graphite are those involved, in general, in dealing with fillers and binders. The basic processes are as follows: 1. Raw Material Control and Test 2 . Raw Material Preparation: (a) Calcining; (b) Grinding 3. Mixing: Hot and Cold 4. Molding: Hot and Cold 5. Extrusion 6. Baking 7. Graphitizing: Continuous and Batch 8. Impregnating 9. Finishing, Shapes and Hardware Each of the variables must be under control to provide materials


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Carbon Materials in Large Volume Applications

93

which are uniform within themselves as well as from lot-to-lot in a short time scale and from lot-to-lot over a long time scale. The carbon industry must be able to produce carbon brush materials , for example, which are the same now as they were when they were produced forty years ago since they must operate on the same machines. Process flow charts for extruded and molded materials are shown in Figures 1 and 2. D. Operating Methods, The development and manufacture of materials for commercial applications requires a great deal of coordination within the various operating areas of the manufacturing organization. In general, these operating methods are as follows: 1. Determine requirements of the application; 2. Choose the raw materials which best will provide these properties within cost limitations; 3. Test full scale shapes and sizes; 4. Change raw materials or modify processes to get the best results; 5. Determine measurable properties of best material; 6. Set up Q.C. to insure same raw materials» processing procedures, and final properties. Final properties alone will not determine in most applications whether a material is suitable. The same set of physical properties—strength, hardness, density, porosity, conductivity, etc.—can be achieved in many different ways, considering the available materials and process variations, but the results on a given application will not be the same; in fact, they could be disastrous. The philosophy of carbon and graphite application takes into account the fact that each of the various raw materials reacts differently to the processes and each has its own characteristic properties. This experience with a l l of these materials tells the developer what to try first and then which direction to take when improvement is required. These methods have resulted in low resistivity and high resistivity; low thermal conductivity and high thermal conductivity; low friction and high friction; high thermal shock resistance; good resistance to oxidation; high contact voltage drop and low contact voltage drop; and many more property ranges. In the short time available, we will touch on some of the key technical aspects of each application, pointing out the properties required as well as the problem involved. II.

Applications

A. Electrodes. 1. Steel Arc Furnaces. The electric arc steel furnace, Figure 3, is a dramatic use of graphite. Three large electrodes enter the top of a pot and generate the threephase arc with the charge. The furnaces are charged hot or cold, depending upon the application, and the cycle of operating

94


COALTAR PITCH

CALCINED COKE

1 PROCESS OIL

[MILL]—|TEST|


1EXTRUDE frùr] I BAKE I iTESTl

|GRAPHITIZE[

[MACHINE I |INSPECT| I SHIP I Figure 1. Typical process for the manufacture of extruded graphite materials

LAMP BLACK MIXER

SLUG

OVEN

MILL

TEST

BLEND

COAL TAR

CALCINE FLOUR

MIXER

TEST

BLEND

TEST

CALONE FLOUR

PLATE MOLD

PLATE H TEST STOCK

SULFUR

KILN

IGRAPHITIZING OVEN

PITCH

PLATES

IMPREGNATING TANKS

Figure 2.

CURING OVEN

TEST

BRUSH CUTTERS

BRUSH BLANKS

Typical process for the manufacture of electrographitic brushes


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95

includes meltdown in which the arc is heating the metal to the melting point. During this part of the cycle, i t is possible for large chunks of metal to f a l l onto the electrodes, causing breakage* When the charge is completely melted, there is a refining period where the temperature is held and during which the metal composition is adjusted. Adjustment to pouring temperature may then be made, after which the electrodes are raised and the melt is poured. The diagram, Figure 4, shows the essential elements of the electrode system with water cooled electrical connections to the electrodes. The electrode sections are joined by means of graphite nipples which must also be good thermal, electrical, and mechanical connections. Because of the high currents, there are thermal gradients, both radially and longitudinally in the electrode which set up thermal stresses. Mechanical stress comes from the charge. The diagram shows the electrode tapered; this is because of the oxidation on the sides of the electrode. The area at the tip is of the order of one-half the area of the new electrode, which means that about one-half of the material is lost by oxidation from the sides. Arc spot temperatures of about 4000°C cause erosion of the tip where the arc takes place. Current densities in the arc cathode and anode spots are estimated between 10,000 and 50,000 apsi. Suitable materials are those which have low electrical resistance, high thermal shock resistance, and good oxidation resistance. Thermal shock resistance includes the factors of thermal conductivity and thermal expansion as well as heat capacity. These materials are produced by the extrusion of large sections, baking, graphitizing, and subsequent machining. 2. Aluminum Refining. The diagram of an aluminum pot is shown in Figure 5. In this case, the anode is the Soderberg electrode. The steel case is made in sections and the carbon mix is tamped in at the top. As the material wears away at the face, steel and the anode material are dropped down and the different electrical connections are made. The cathode is usually made from a petroleum coke pitch mix which is tamped into the furnace shell against the insulation. Steel rods are used for connections. In some cases, graphite rods and plates are used to improve the conductivity of the cathode. As the anode material moves toward the hot part of the pot, the pitch is carbonized, conductivity increases, and the carbon material is available for conduction to the electrolyte. The material in the cathode is cured in a similar manner by operation. The use of carbon and graphite in the aluminum industry is important since i t takes just somewhat less than one pound of



Figure 3. Electric arc steel furnace in operation

WATER COOLED ELECTRICAL CONNECTIONS CONTACT RESISTANCE TO BOTH ELECTRIC + HEAT FLOW

I I

/

r— THERMAL.ELECTRICAL AND MECHANICAL CONNECTIONS

^ — A R C EROSION SPOT TEMR 40OO*C

Figure 4. Diagram of electric arc steel furnace electrodes

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97


carbon to produce a pound of aluminum. The production of the mix for the Soderberg cells involves control over the raw materials, mixing, and on the tamping operation into the steel shells. B. Anodes. Until recently, the primary use of graphite in anodes was for the manufacture of chlorine. While there are s t i l l some cells in operation using graphite, there has been a major change in the case of mercury cells from the use of graphite to the use of a titanium metal screen holding precious metal oxides. While the initial cost of these so-called dimensionally stable anodes is high, depending upon the maintenance and power costs, there are installations where this is economical. There are s t i l l some installations, however, where graphite is used in the conventional cells. In the mercury cell, chlorine is formed at the anode; and sodium, which dissolves in the mercury, is formed at the cathode. The mercury is circulated through a so-called dénuder in which the sodium is reacted with water to form hydrogen and sodium hydroxide. The mercury is then returned to the cell. In the mercury cell, Figure 6 , the anodes must be very low in vanadium content since this builds up in the system to the extent that the electrochemical relations at the cathode are changed and some hydrogen is formed which can react with the chlorine to cause an explosion. Graphite materials from these cells as well as for the diaphragm cells are produced by extrusion, baking, graphitizing, and machining. A diaphragm cell is shown in Figure 7 in which the graphite anodes are cast in a lead base, standing vertically in the cell. The cathode is an iron screen covered with an asbestos diaphragm which separates the two halves of the cell. The chlorine i s collected over the graphite and the hydrogen over the cathode. C. Brushes. Brush materials are made by conventional carbon and graphite processes, using either extrusion or molding as the forming process. Metal graphite brushes are made by molding metal powders with graphite additions or by impregnating porous carbon and graphite materials with molten metal. A conventional process specification for brush materials is shown in Figure 2. There are several key points involved in the application of brushes which we may refer to the basic theory of contacts. In the first place, a brush sliding on a metal surface produces a film which is partly oxide, partly graphite, partly lubricant (such as water), and which contains metallic conducting regions as shown in Figure 8. Current densities in the metallic conducting areas are of the order of amp/cm. The resistance of these brushes, based on contact theory, depends upon the contact area as determined by the film, the specific resistance, and the effect of the hardness of the material as shown in Figure 9.

200,000

2

RETURNED TO CELL

DENUDER

Figure β. Mercury cell for manufacture of chlorine from sali



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Carbon Materiab in Large Volume Applications

99

Figure 7. Diaphragm cell for the manufacture of chlorine from salt

Figure 8. Schematic sliding contact surface


100


The mechanical contact area under a brush is determined by the yield point and contact hardness. The electrical conducting regions are small parts of this mechanical surface. The final contact resistance is the result of an equilibrium between the tendency of the metal to oxidize and decrease these contact areas and the effect of the breakdown or fritting of the film which tends to increase these areas when the voltage at the im mediate contacts reaches voltages of the order of .2 of a volt. The position of this equilibrium is further influenced by the abrasive nature of the brush and its materials which tend to increase the conducting areas and the presence of other lubri cants which tend to decrease these areas. This equilibrium makes i t appear that the brush might be acting as a semiconduc tor i f tests are run for relatively long periods at each cur rent. However, instantaneous changes in current indicate that the conduction is metallic as shown by the straight lines A and Β in Figure 10, which are taken very quickly. The necessity for lubrication is shown in Figure 11 in which brushes are run on a ring and the air is being dried. At a certain dryness, the wear increases drastically as the brushes essentially disappear in a cloud of dust, friction increases, and the contact resistance decreases. For high altitude and space applications, i t is necessary to provide lubrication in another manner. The conduction mechanism is purely metallic within the collector and the brush, that is, by means of the normal conduc tion electrons. The transfer takes place across the water or other lubricating film by means of the tunnel effect. The lu bricating film separates the two material, carbon and metal, enough to prevent the mingling of their electron clouds which would result in welding or seizure, but i t introduces a voltage drop only of the order of 0.1 volt from the electrical point of view. Brush wear is shown in the general context of wear in Fig ure 12 which shows wear rates all the way from hydrodynamically lubricated bearings to chalk on the blackboard. Commutation is an important aspect of brush operation, and the details of the electrical nature of commutation are shown in Figure 13. One of the most important aspects of brushes has to do with mechanics. Various brush holder designs, shown in Figure 14, provide suitable means of holding brushes on the commutator, but the surface of the brush must adapt to the minute irregu larities to provide this contact, as shown in Figure 15. D. Seals, Bearings, and Brakes. Friction was not men tioned under brushes although this is an important part of successful brush operation. It is also important in seals and bearings that the friction be low, and that in the case of brakes, the friction be high. Figure 16 shows various seal


100%,

DROP IN FILM- TUNNEL EFFECT , j/ CONSTRICTION RESISTANCE IN COLLECTOR 1/

A-FRITTING .3 VOLTS.

BFRITTING 1-2 VOLTS 50% NORMAL CURRENT LOAD

100%

Figure 10. Schematic of contact resistance

102



\—

0

6 0

i—ι

0

5

1

1

K>

15

1

1

1

20 25 30 TIME, MINUTES

1

1

1

35

40

45

1—r

50

55

60

Figure 11. Effect of eliminating water vapor on sliding contacts

9.


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—ι

103

η—ΓΊ

CHALK

ON B L A C K B O A R D


A U T O TIRE DRY SKID

SOFT L E A D PENCIL ON P A P E R

AUTO

TIRE 0.01% SLIP

AUTO

TIRE 0.1%

4-+

SLIP

AIRCRAFT GEN.-DUSTING AUTO

TIRE

AUTO

GENERAT

AUTO

STARTER

AIRCRAFT

1.0% SLIP;

GEN. - A L T F.H.R-

DIESEL AIRCRAFT

BEST

BRUSHES GEN.-SEA

LEVEL

POWER B R U S H E

/OIL LUBRICATED S L E E V E BEARING

.0001

.001

.01 .1 COEFFICIENT OF FRICTION

K>

Figure 12. Schematic of wear rates and coefficients of friction



Figure 13. Circuits involved in commutation

Figure 14. Examples of brush holders

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105

BRUSH FORCE . 4 LB


BRUSH ELASTICITY « 100X10 a/cm

RADIUS OF BRUSH 4.04 IN. 4.125 IN.

ELASTIC PENETRATION 0.000035 IN. 0.00006 IN.

RATE OF RECOVERY OF ELASTIC PENETRATION ABOUT l/IOTH THE VELOCITY OF SOUND IN THE BRUSH MATERIAL OR ABOUT IÔ SEC. FOR 0.001 IN. FASTEST COMMUTATOR BAR ABOUT IÔ SEC. 7

9

Figure 15.

Elasticity of the brush face

Figure 16.

Carbon sealringsfor various purposes


106


materials that are used i n a wide variety of applications. The largest seals are those used i n jet aircraft engines between the various stages including those at the highest temperatures. The smaller seals are various types of face seals used for sealing liquids and gases. Figure 17 shows three jet engine seals and the machining which must be provided on the surface to provide lubrication and the removal of wear particles. Figure 18 i s a diagram of a carbon face seal which shows the elements used i n a shaft seal. This type of seal i s used for many different gases and liquids, and the materials of a seal and the face plate must be adjusted for these different materials. In the case of a face seal, f r i c t i o n and wear must be kept as low as possible to keep wear particles from interfering with the sealing action. The film which prevents the seizure of the carbon and metal face plate must be developed either from the liquid or material being sealed, or must be provided i n the seal and face plate materials. For example, i n sealing very dry gases, many of the same additives are used that are used i n carbon brushes for high a l t i tudes . Carbon and graphite, while having low f r i c t i o n , under certain circumstances can also have high f r i c t i o n . This i s developed i n brake applications in which the constant f r i c t i o n , low wear, high thermal capacity, and high shock resistance of graphite materials are used for aircraft brakes. Materials for these applications have been developed i n several different ways, some of which include graphite cloth and pyrolytic carbon and graphite materials. E. Refractories. The refractory nature of carbon and graphite i s used i n many applications, particularly where oxidation i s not present. Carbon brick i s used for blast furnace liners. Glass mold faces are made with graphite; bearings that must run at high temperatures are made of graphite. Graphite i s used for furnace boats and supports to a great extent i n the semiconductor f i e l d and has the further advantage that i t can be machined to very close tolerances for the support of these parts. Figure 19 shows some of the detail which can be achieved. III.

SUKMARV

In i t s e l f , the carbon industry i s sizeable. Its materials are used in an essential way by many other industries with results that provide useful applications throughout our entire economy. The technical nature of these applications i s better understood, and the carbon industry can respond to application requirements more quickly and i n t e l l i g e n t l y now than i t could i n the past. We have learned more about the nature of carbon and graphite, and we understand the requirements of the applications

Figure 19. Furnace brazing support


108



better. With some few exceptions, most of the carbon used commer cially has come through the life cycle. Because of its high binding energy, i t tends to retain some of the structure of the organic compound from which i t was formed. This, together with the processing possibilities, provides an infinite variety of material properties. New technical opportunities are chal lenging the industry. It can and will respond constructively. Literature Cited. 1. Shobert II, Erie I., "CARBON BRUSHES, The Physics and Chemistry of Sliding Contacts," Chemical Publish ing Co., Inc., New York (1965). 2. Holm, Ragnar (in collaboration with Else Holm), "ELECTRIC CONTACTS, Theory and Application," 4th Ed., Springer-Verlag New York, Inc., (1967). 3. Walker, Philip L., Ed. "CHEMISTRY AND PHYSICS OF CARBON," Marcel Dekker, Inc., New York (1965-73), 1-11. 4. "Proceedings of the Conferences on Carbon," University of Buffalo, Buffalo, No. 1-4. Waverly Press, Inc., Baltimore, publishers of Conf. 1 and 2. Conference No. 5 Held at The Pennsylvania State University, University Park. Pergamon Press, New York, publishers of Conf. 3 to 5. (1953-1961). 5. Mrozowski, S., Ed., CARBON, Pergamon Press, New York, (1963 through present). 6. Houseman, D. H., "Some Factors Affecting Electrode Consumption in the Electric Arc Furnace," J. of the Iron and Steel Institute, (May, 1956), 183, 48-53. 7. Hearne, K. R., S. A. Nixon, and D. Whittaker, "Axial Temperature Distributions Along Thin Graphite Elec trodes," J. Phys. D. Appl. Phys., (1972), 5, 710. 8. Bello, J. R., "Fundamentals of the Electric Arc Fur nace," Paper No. EFC-12, The Metallurgical Society of AIME, New York. 9. Yavorsky, P. J. (American Smelting and Refining Co., S. Plainfield, N.J.) and J. F. Elliott, (Professor, Dept. of Metallurgy and Materials Science, MIT, Cambridge,) "Observed and Predicted Temperatures in Large Arc Furnace Electrodes", from MIT Thesis, (1970) 10. Ciotti, J. Α., "A New Era in Melting," J. of Metals, (April, 1971), 30-35. 11. Sundberg, "The Power Circuit of Arc Furnaces," Elektrowarme International (April, 1972) 30, B2, B93-B99. 12. Schwabe, W. E., "Measuring Problems and Techniques at A-C Furnace Arcs," National Carbon Research Laborator ies, Cleveland (1954).


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13. Holm, R., "Calculation of the Temperature Development in a Contact Heated in the Contact Surface, and Appli cation to the Problem of the Temperature Rise in a Sliding Contact," J. App. Phys., (April, 1948), 19, 361-366. 14. Schwabe, W. E., "The Mechanics of Consumption of Graphite Electrodes in Electric Steel Furnaces," Presented at the 29th Electric Furnace Conference of the Metallurgical Society of AIME, Toronto, (December, 1971). 15. Rykalin, Ν. N., Nikolaev, Α. V., and Goronkov, O.A., "Current Density at the Anodic Spot of an Arc," High Temperature (Sept. - Oct., 1971), 9, (5), 893-6. 16. "Some Factors Affecting the Wear of Graphite Elec trodes in the Electric Arc Furnace," J. of the Iron and Steel Institute, (February, 1954), 176, 159-165. 17. Cosh, Τ. Α., "Graphite Electrode Consumption in Electric Arc Furnaces," J. of the Iron and Steel In stitute, (March, 1957), 185, 328-332. 18. Hering, C. "Laws of Electrode Losses in Electric Furnaces," Trans. Electrochemical Society, (1909), 16, 265-316. 19. Schwabe, W. E., "Experimental Results with Hollow Electrodes in Electric Steel Furnaces," Iron and Steel Engineer, (June, 1957), 34, (61), 84-91. 20. Schwabe, W. E., "Steelmaking in Ultra High Power Electric Arc Furnaces," Stahl u. Eisen, (August, 1969), 89, (17, 21), 927-937. 21. Ravenscroft, J., "Distribution of Electrode Consump tion in an Electric Arc Furnace," Metallurgia V. (December, 1959), 60, 253-259. 22. Shobert II, Erie I., CARBON AND GRAPHITE, Chapter Printed in Modern Materials, Academic Press, New York, (1964), 4, 1-99.

Carbon Materials in Large Volume Applications

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