- Email: [email protected]
The manufacture of high-value carbon from coal-tar pitch
The manufacture coal-tar pitch* Jiirgen
W.
Stadelhofer,
Riitgerswerke AG, t Verkaufsgesellschaft
of high-value
Rolf
Marrett
and
Castrop- Rauxel, West Germany fiir Teererzeugnisse ( VfT)mbH,
carbon
Walter
from
Gemmeket
Essen,
West Germany
World-wide, 17 million t a~’ of coal-tar are obtained as a by product in the chamber coking process for the production of metallurgical coke. Refining of this aromatic raw material yields coal-tar pitch which is the traditional coal-derived starting material for the manufacture of carbon precursors and carbon artefacts. Considerable progress has been made in the elucidation of the physical and chemical nature of this material by means of chromatography, n.m.r. spectroscopy, thermal analysis and chemical reactions schemes. The dominant fields of application of pitch are the manufacture of pitch coke and electrode binders. Delayed coking and horizontal chamber coking are the technologies currently used for the production of cokes with low sulphur and metal content, for anodes for the aluminium-refining industry and the electric steel process. Coal-tar pitch, low in quinoline-insolubles (al), is an excellent raw material for the manufacture of needlecokes with a low coefficient of thermal expension (CTE). The separation of inherent 01 can be performed via gravity settling in aliphatic hydrocarbon mixtures, by centrifugation in a disc separator or by filtration. The possible co-carbonization with aromatic petroleumderived residues yields premium coke suitable for the manufacture of UHP-electrodes. New developments in the production of coke from coal-tar pitch aim to improve coke yields and increase anisotropy (i.e. low CTE and high electrical conductivity values). Further technological progress has been made in the manufacture of hard pitch which can be used as a starting material for the production of pitch coke in the chamber coking process and for the production of electrode binders by means of a continuous flash process with optimized thermal and pressure treatment of pitch, thus facilitating the ‘tailored’ manufacture of binder pitches of different qualities.
Coal has very significant potential as a raw material for the manufacture of a whole spectrum of basic materials ranging from pure hydrogen to almost pure carbon. Thus, coal can be converted into gaseous products by gasification, into liquid hydrocarbons by means of liquefaction in an atmosphere of hydrogen or into a solid, non-fusable coke by a pyrolysis process. Therefore, it is, from the chemical point of view, a suitable substitute for petroleum. Despite the many attempts to improve the methods for upgrading the coal for the manufacture of gaseous and liquid products to pass the economic threshold for the substitution of petroleum products, the long-established process of coking coal in horizontal chambers at temperatures of 1200°C is still the only economically feasible process in coal chemistry. World-wide, the present production capacity for metallurgical coke amounts to 440 million t a- ‘. The capacity is evenly distributed all over the world (Figure I), the US, the Soviet Union, Japan and West Germany being the largest producers of coke’. The majority of the metallurgical coke produced, which amounts to approximately 390 million t a- I, is used for the manufacture of pig iron in the blast furnace where the coke plays a multipurpose role: (1) it acts as the skeleton to carry the burden, (2) it is burnt in order to supply the necessary heat for the smelting process of the ore, and (3) it is used as a raw .~_ * Presented at the Conference: ‘Industrial Conversion of Coal and Carbon to Gas, Liquid and High-Value Solid Products’, organized by the Industrial Carbon and Graphite Group of the Society of Chemical Industry, and held at the Society of Chemical Industry, 14, Belgrave Square, London SWIX EPS, UK, 7-9 April 1981
0016~ 2361/81/090877~06$2.00 @ 1981 IPC Business Press
material for generating the reduction medium, being complemented in this function by heavy fuel oil. Owing to the increasing price of heavy fuel oil, which is injected into the tuyeres of the blast furnace as a substitdte to metallurgical coke, the resubstitution of oil by coke is occurring in many countries producing pig iron. This gives coke an increased importance in the iron production process. However, metallurgical coke is not the exclusive product of the coal carbonization process for a number of by-products are also obtained. Amongst these by-
West
Germany
states
Pig lmn capacity Japan Coking capacity Rg iron capacity Figure
1
Production
440
mllllon tons/year 630 mllllon tons/yew capacities
for metallurgical
coke and pig iron
FUEL, 1981, Vol 60, September
877
Manufacture Tab/e 7 Number
of high-value
Distribution
carbon
from coal-tar
pitch:
of isomers of PAH
of PAH of n hexagons kata-annelated PAH
peri-condensed
1 1 2 5 12 37 123 446
0 0 1 2 10 45 210 1002
PAH
z 1 1 3
7 22 82 333 1448
products, coal-tar is the most significant in terms of quantity and suitability as a chemical raw material. Thus, 17 million t a-l of coal-tar are currently processed in some 125 tar refineries’ all over the world. This paper describes the physical and chemical nature, and the chemical potential of the heavy bottom of coaltar, viz. coal-tar pitch, and some of the most important bulk products made therefrom. CHEMICAL
AND PHYSICAL
NATURE
OF PITCH
It is well-established that coal-tar pitch is composed of a very large number of polycyclic aromatic compounds. When considering the theoretical number of isomers3 of aromatics containing more than 3 rings it becomes clear that full identification of the individual constituents of pitch is unlikely. For only 6 hexagons there are 82 isomers (Tahte I) and it is reasonable to assume that a great number of these isomers occur. With the development of high-pressure liquid chromatography (h.p.1.c.) the identification of these highly fused polycyclics has been facilitated. However, because of the complexity of this aromatic mixture the use of statistical analytical techniques is especially rewarding. Thus, it is highly desirable to gauge the average molecular weight and the statistical distribution of functional groups of the high molecular conglomerate. Gel-permeation chromatography (g.p.c.) was first applied by Petro and Edstrom as a means to study the molecular distribution of pitch constituents. Unfortunately, the pitch molecules behaved in a rather unorthodox manner because it was not the hydrodynamic volume but other factors such as primarily molecular shape and polarity which governed the separation, thus thwarting the construction of calibration curves. A novel g.p.c. technique employing quinoline as a solvent and using reductive hydrogenation with lithium in ethylenediamine base has been applied recently to study petroleum pitch5. This technique proved successful for obtaining molecular weight distributions of petroleum pitch mesophases and semicokes which have quinolineinsoluble contents approximating 100 wty& For coal-tar pitch a similar separation can be expected. 13C nuclear magnetic resonance (n.m.r.) spectroscopy is the method of choice for the determination of the statistical carbon skeleton of carbonaceous materials. Hence, the application of this technique to coal-tar pitch yields unambiguous evidence as to the aromatic structure of this material both in the solid and soluble state. No detectable alkyl side-chains6 greater than the ethyl moiety were found by this sensitive method.
878
FUEL,
1981,
Vol 60, September
J. W. Stadelhofer
et al.
The isolation of reactive centres of coal-tar pitch was described recently by Blumer et a/.‘. According to this scheme the highly reactive moieties of pitch (approximately 10%) are separated from the less reactive matrix by adduct formation with picric acid. The increased reactivity of these species was confirmed by a kinetic study which yielded an activation energy for the reactivecentres of68.0kJ mol-’ tiersus 136.5 kJ mol-’ for the whole pitch in carbonization experiments*. The great number of individual compounds present in pitch also govern its physical behaviour. Franckg was among the first to use the term ‘undercooled liquid’ for describing the crystallization inhibition observed when cooling coal-tar pitch. In more modern terminology this phenomenon is explained by the glass-like behaviour of characteristic Thus, materials. the amorphous temperature of the glass transformation is now used to describe the glass-like behaviour of coal-tar pitch. Both Differential Scanning Calorimetry (DSC)” and n.m.r. accurate techniques for the spectroscopy’ ’ are determination of the temperature of the glass transition. Results obtained by these techniques confirm the general rule that the viscosity of amorphous materials is approximately 10 I2 Pa s at their respective glass transition temperature (20-30°C for coal-tar pitch). The viscosity behaviour of liquid pitch has also found considerable interest in the last few years. The vexed question whether pitch is a Newtonian liquid has been extensively discussed. Thus Nazem12 and Briggs13 found that despite the invariance of viscosities over a wide range of shear rate, the classification as Newtonian liquid must be considered with caution because of Weissenberg effects exhibited by the pitches investigated. In summarizing the progress made recently in elucidating the chemical and physical composition of coal-tar pitch, a further successful step in solving the endless puzzle of fully identifying the chemical composition and the physical behaviour can be claimed, though the deepening of the understanding of this material remains a permanent challenge to the coal chemist’.
PITCH AS RAW MATERIAL PRODUCTION OF CARBON
FOR THE
Because of its high carbon content, pitch is the material of choice for the production of industrial carbon. In a comparison of the elemental compositions of various competing petroleumand coal-derived residues (Table 21, it is evident that coal-tar pitch has by far the highest carbon content.
T&,/e 2
Elemental
composition
of residual feedstock,
Element
Bitumen
SRC-pitch
Coal-tar pitch
Heavy fuel oil
c H N s 0
86.7
89.1 5.8 2.2 0.5 2.4
93.0 4.5 0.9 0.6 1 .o
86.6 11.5 0.2 0.9 0.5
1.3
1.7
0.6
(wt%) (wt%) (wt%) (wt%) (wt%)
C/H
9.7 0.6 2.0 1 .o 0.8
Manufacture
of high-value
carbon
from coal-tar pitch: J. W. Stadelhofer
et al.
425°C +
+
__Yapo”r,
Closed
Iall
8
I
fl CnkP __. ._
I
40°C
Open
valves
t
fl
Wet gas
+r
1 Combinationn tower
1
:--_
L 1: -_ _
Compressor
t
a ‘Top
ref Iux
b Wild gasoline to VRU
--,___ 4
drums
205°C
Light coker gas 011 to furnace oil desulphurlzer
Heavy coker gas oil to FCCU
CoGter Figure 2
Scheme of the delayed
Residual
feed
coking process
In order to produce pure carbon, the non-carbon elements of these precursors must be removed by a series of heat-treatment steps up to 3000°C. Coke is the first intermediate product on the pathway to industrial carbon. High-purity coke has two main areas of application in the aluminium and steel industries. In the first, it is used as the skeleton for the production of anodes for the HallHerault-Process; in the latter it serves as starting material for the production of the carbon electrodes for the production of steel especially in ultra-high-power (UHP) furnaces. In terms of quality, high anisotropy and a low coefficient of thermal expansion are, along with low sulphur contents and ash yields, the most important quality criteria of cokes for UHP electrodes. These electrodes are produced by baking a moulded mixture of calcined coke and coal-tar binder pitch and graphitizing the baked carbon stock at temperatures up to 3000°C. This market for carbon is promising for the electric steel production has experienced a considerable growth in the last decade and contributes, at present, 21% of the world steel production. As approximately 667 kg of graphite are consumed for the manufacture of 1 ton of steel in the electric steel furnace process the total consumption of graphite amounts to nearly 1.1 million t a-‘. For aluminium anodes, however, the quality criteria are less stringent as only the contents of sulphur, iron and silicon are relevant. Various technologies exist for the production of coke from heavy residues. Amongst the conversion processes, the delayed coking process clearly is the most significant with a production capacity of approximately 16 million t a - ‘. In this process a petroleum- or coal-derived residue is fed via a tubular furnace into a steel-lined coking drum where the carbonization process takes place under
moderate pressure and at temperatures of 45&5OO”C over an average period of l&12 h (Figure 2). Volatile components are driven off at the end of coking cycle by blowing steam into the coke drum. The yield of the green coke produced in this lowtemperature process is directly related to the Conradson carbonization residue of the feedstock (Table 3)i5. This, however, is the crucial point for the economy of the delayed coking process of coal-tar pitch. As coke is the main product of pitch carbonization (owing to its high carbonization residue of 35-60x) and the liquid byproducts are not upgraded in the conversion process the pitch coke must be marketable at an economic price. The boundary conditions for the petroleum refiner, however, are completely different as the costs of the conversion process can be distributed mainly to the expensive middle distillates and crude petrol-fractions because the major portion of the product spectrum comprises these products. (The yield of petroleum coke normally doesn’t exceed 20% compared with a 5&60% yield of pitch coke.) Despite this economic difficulty a number of versions for the production of coke from coal-tar pitch by delayed coking have been developed; the aim of producing coke of the highest quality is a common feature of these schemes. This type of coke is usually referred to a needle-coke because of the needle-like appearance of its optical texture and its crystallinity. However, pretreatment of crude coal-tar pitch is necessary for the production of needle-coke from coal-tar pitch. According to the Nittetsu16 process, soft pitch is treated with an aliphatic petroleum cut in order to reduce the content of material insoluble in quinoline (QI) which hampers growth and coalescence of the mesophase as it remains at the periphery of the developing spherules. High-quality coke of a coefficient of thermal expansion
FUEL,
1981,
Vol 60, September
879
of high-value carbon from coal-tar pitch: J. W. Stadelhofer et al.
Manufacture Tab/e 3
Delayed
coking of various feedstocks
California Feed Gravity (15Or.Z. g cme3) Conradson carbon (wt%) Sulphut fwt%)
0.973 12.1 1 .l
Product yields C3 and lighter (wt%)
(wt%)
7.1 27.6 22.9
Comparison
of petroleum
CQ and gasoline Coke (wt%)
Table 4
Oklahoma
1.223 31.2 0.48
0.986 9.4 1.6
14.1 23.3 19.1
3.0 10.7 60.9
and pitch-derived
Petroleumderived coke
Sulphur fwt%) Ash (wt%) Vanadium (ppm) Nickel (ppm) Water fwt%) Volatile matter (wt%) Real density (g cmp3) Coefficient of thermal expansion of graphite (l/K)
Coal-tar pitch
coke
Coal-tar pitchderived coke
Raw
Calcined
Raw
Calcined
1.5 max 0.35 max 150 rnax 150 max 7
1.5 max 0.4 max 15omax 150 max 0.1
0s 0.2 <5 <30 7
0.3 0.2 <5 <30 0.1
8
0.2
8
0.2
-
2.17
-
2.14
-
5.10-‘max
-
5.10-7
(35@-45O”C) of 8 x lo-’
K-’ can be produced by this method. In the Rutgers process’ 7, however, medium coaltar pitch is fed together with aromatic residues from steam cracking processes to a separator or a filtration unit in order to reduce the content of QI. The production of need&cokes of a quality comparable to the Japanese standard is possible from this mixture of petroleum- and pitch-derived hydrocarbons. A comparison of the most important quality requirements demonstrates that high-quality coke from coal-tar pitch has noteworthy advantages in terms of its low metal content when compared with cokes of similar quality (Table 4) derived from petroleum cuts such as pyrolysis oil or decant oil. In addition to delayed coking, the carbonization of pitch in horizontal brick-lined chambers is still common in West Germany and a number of Comecon States. In this process, hard pitch is fed at the top of the chamber into the oven through a loop passing over the top of the slot-type oven; the hard pitch (softening point 160-185°C R and B) may be produced either by oxidizing medium pitch with air at temperatures up to 400°C or by flash distillation (Figure 3). A continuous flash distillation unit for the production of 250000 t a-i hard pitch was put on stream in 1979 at the CastropRauxel works of Riitgerswerke AG. After a coking period of 15-20 h in the horizontal coke chamber an isotropic coke can be obtained in 80% yield. The low sulphur, ash and metal content of this coke are noteworthy assets. In addition to the pitch coke, coke oven gas rich in hydrogen and condensate is produced in this process. This condensate can be redistilled yielding secondary pitch.
880
FUEL,
1981,
Vol 60, September
The quality of the chamber coke can be varied by adding differing amounts of secondary pitch. Thus, highly isotropic coke for graphite production for applications in the nuclear industry (gas-cooled reactor) can be produced by using up to 100% secondary pitch as feedstock for the carbonization process. In both the delayed coking and chamber coking processes the residence time of the pitch in the reaction zone is not uniform, as pitch is continuously charged to the coking drum whereas in the latter process it is charged in a number of stages. However, a novel approach has recently been proposed by Glaser et ~1.‘~ According to this invention, a temperature programme is applied to the pitch in order to secure a uniform residence time for all components of hard pitch. Thus, a highly anisotropic coke can be obtained by carbonizing hard pitch at temperatures up to 450°C in an insertion oven. The optical texture of this anisotropic coke is shown in Figure 4. This experiment indicates that a needlelike structure of coke can be obtained under quiescent conditions. PITCH AS A BINDING
AGENT
At present, the most important product of coal-tar refining is binder pitch for the manufacture of anodes for the aluminium-refining industry. These anodes are used for the electrolysis of refined bauxite; they are produced by moulding and baking a mixture of electrode coke with almost 20% binder pitch into blocks typically 70 cm wide, 125 cm long and 50 cm high at temperatures of lo& 1200°C (pm-baked anodes) or by performing this process in the aluminium-refining process itself with a mixture of 30% pitch and 70% coke, supplying the heat for the baking process by the electric current (selfbaking Siiderberg anode). Thus, for every kilogram of aluminium produced approximately 0.4 kg coke and 0.1 kg binder are consumed. Despite many attempts to replace coal-tar pitch by petroleum products or possibly, solvent-refined coal (SRC) products the traditional pitch has firmly maintained its position for this application owing to its unmatched quality. Both the growing tar production and unexploited pitch reserves which are, at present, not marketed in this field, ensure the supply of pitch for the aluminium industry which has shown an outstanding growth in the last decade (Figure 5). The theoretical understanding of the behaviour of electrode pitch in the manufacture of anodes is still in its infancy. Thus, the production and application of binder
Gas sepamtor I
I b
Tubular f urnoce
. Medrum Figure3
CentrZuql pump
1 Hard ,..+.-L.
patch Thermal
reformer
for the production
1 P^^~___..^
of hard pitch
kkwufacture
Figure 4 Optical texture pitch in thin layers
of coke obtained
by carbonizing
hard
11
_ c 14c
of high-value
f
I
carbon from coal-tar
1970 Figure 5
I
I
I
71
72
73
Development
I
I
74 75 Year
of production
I
I
I
76
77
78
I
79
1
80
of aluminium
pitch is based on experience rather than on theoretical considerations. However, certain rules have been developed by the pitch producer and the consumer as to the specification of a suitable binder pitch. A breakdown of typical specifications is given in T&e 5 along with the respective data of coal-tar and straight-run medium pitch. In addition to these requirements a suitable wettability, which is measured via the contact angle and an optimum viscosity, are prerequisites for a high-quality binder pitch. As the composition of the medium pitch in terms of the relevant solubilities (i.e. toluene, quinoline) varies considerably with the properties of crude coal-tar, it is advantageous to treat the medium pitch in a thermal reforming step subsequent to the distillation process. This results in a uniformity of the amount of p-resins and the coke yield.
et al.
In this process, medium pitch is exposed to a thermal treatment under pressure up to 1.5 MPa which leads to the desired increase in material insoluble in quinoline and toluene. The final adjustment is usually carried out in a flash distillation process where the volatile portions are driven off until the desired softening point is reached. In principle, this process is similar to the production of hard pitch by flash distillation. Owing to the flexibility of this process concerning soaking time, temperature and pressure in the flash drum, the tar refiner is in a position to ensure the supply of products of a wide-range of specifications in terms of softening point, ratio of inherent/secondary QI, Conradson coking residue and further physical and chemical requirements. Various attempts have been made to circumvent this thermal reforming step by oxidizing pitch with air or by adding carbon black or mesophase QI in order to reach the necessary level of QI of the binder pitch. However, the modified pitches proved unsatisfactory to the aluminium producer because of reduced mechanical strength or increased consumption of pitch for the manufacture of electrodes. In addition to its binding capability, the impregnating effect of coal-tar pitch is of considerable industrial importance for many high-performance applications of graphite electrodes. Impregnation with coal-tar pitch results in the deposition additional pitch coke in the pores of the baked stock, thus bringing about a higher density of carbon and a significant improvement in mechanical strength. Characteristic data of a typical impregnating pitch are given in Table 6. The amount of insolubles, the viscosity and the wettability of the pitch must be well balanced for during the impregnation process the insolubles form a filter cake of low permeability on the surface of the stock which reduces the penetrability of the impregnating agent. Tab/e 5
II
pitch: J. W. Stadelhofer
Characteristic
Density (20°C, g cm-? Softening point (OC) (Kraemer-Sarnow) Softening point (‘C) (MettIer) Quinoline-insoluble (wt%) Toluene-insoluble (wt %I p-Resins (wt%) Carbonization residue (wt%) (Conradson) Carbon (w/t%) Hydrogen fwt%) C/H-ratio
Tab/e 6
Characteristic
data of coal-tar and coal-tar Coal-tar
Medium
1 .18
1.28
pitch
pitches Electrode 1.32
<0
68
92
86
110
3
7
14
8 5
18 11
39 25
45 92.5 4.5 1 .J
56 94 4.3 1.8
20 91 5 1.5
data of impregnating
pitch
Softening point (R and B) (“C) Quinoline-insoluble (wt%) Toluene-insoluble (wt%) Carbonization residue Conradson (wt%) Alcan (wt%) Filtration rate (5 g min-‘1 Viscosity (1 50°C, mPa s)
FUEL,
1981,
pitch
75 3 19 38 42 2.0 55
Vol 60, September
881
Manufacture of high-value carbon from coal-tar pitch: J. W. Stadelhofer et al.
The pick-up of impregnating pitch usually is up to 30% in the first impregnating step. A further impregnation may follow after having the stock rebaked. Moreover, for electrode nipples a third impregnation step can be advantageous.
SUMMARY AND FUTURE OUTLOOK It has been shown in this paper that coal-tar pitch is a very versatile raw material for the production of high-quality carbon and binding materials. In the past this basic material has experienced many changes regarding its main form of application. In former times it was used mainly as a fuel, then as a road building material and later for the production of pitch coke. Recently, more sophisticated applications have been found in the aluminium and steel industry. As still considerable portions of pitch are unexploited for these purposes a further growth of this market can be expected in the future. The tendency towards more sophisticated areas of application is further underlined by the production of high-modulus carbon fibres starting from coal-tar pitch. Additionally, other applications, for which it is not the purity of the carbon derived from the pitch but, for example, the agglutinating capability of the pitch which is used, offer a considerable market potential in the future. Thus, coal-tar pitch has proved a useful additive to noncoking coals. Moreover, the production of smokeless briquettes by means of special coal-tar pitches is a
882
FUEL, 1981, Vol 60, September
valuable contribution to the substitution of liquid fuels by solid domestic fuels. Thus the coal-tar-refining industry has proved to be highly flexible to changing markets in the past. As to the future, this branch of coal chemistry is confident to cope with all coming challenges. REFERENCES I
2
3
4 5 6 7 8
9 10 11
12 13 14 15 16 17 18
Nashan, G. Stahl und Eisen 1980, roo, 982 Collin, G. 'Proc. World Conf. Future Sources Org. Raw Mater., Chemrawn r, Toronto, Canada, 10--13 July 1978, Pergamon Press 1979, p. 283 Collin, G. and Zander, M. ErdDI Kohle, Erdgas Petrochern. 1980, 33, 557 Edstrom, T. and Petro, B. A. J. Polyrn. Sci. 1968, 21 (C), 1971 Greinke, R. A. and O'Connor, L. H. Anal. Chern. 1980, 52, 1877 Fischer, P., Stadelhofer, J. W. and Zander, M. Fuel 1978, 57, 345 Bliimer, G.-P., Haase, 1., Zander, M. and Zellerhoff, R. B. Erdijl Kohle, Erdgas Petrochern. Erganzungsband 80/81, p. 159 Stadelhofer, J. W. Fuel 1980,59,360 Franck, H.-G. BrennstojJ-Chern. 1955, 36, 12 Stadelhofer, J. W. Carhon 1979, 17,301 Hayward, J. S., Ellis, B. and Rand, B., Preprints Carbon 80, Int. Carbon Conference, Baden-Baden, West Germany, 1980, p. 338 Nazem, F. F. Fuel 1980,59, 851 Briggs, D. K. H. Fuel 1980,59,201 Bartle, K. D., Collin, G., Stadelhofer, J. W. and Zander, M. J. Chern. Tech. Biotechnol. 1979, 29, 531 Rose, K. E. Hydrocarhon Processing 1971,85 Gambro, A. 1., Shedd, D. T., Wang, H. W. and Yoshida, T. Chern. Eng. Progr. 1969,65, 75 Franck, H.-G. et al. Riitgerswerke AG, German patents No. 2016276 and 2.013927 Glaser, H. et al. Riitgerswerke AG, German patent pending
W.
Stadelhofer,
Riitgerswerke AG, t Verkaufsgesellschaft
of high-value
Rolf
Marrett
and
Castrop- Rauxel, West Germany fiir Teererzeugnisse ( VfT)mbH,
carbon
Walter
from
Gemmeket
Essen,
West Germany
World-wide, 17 million t a~’ of coal-tar are obtained as a by product in the chamber coking process for the production of metallurgical coke. Refining of this aromatic raw material yields coal-tar pitch which is the traditional coal-derived starting material for the manufacture of carbon precursors and carbon artefacts. Considerable progress has been made in the elucidation of the physical and chemical nature of this material by means of chromatography, n.m.r. spectroscopy, thermal analysis and chemical reactions schemes. The dominant fields of application of pitch are the manufacture of pitch coke and electrode binders. Delayed coking and horizontal chamber coking are the technologies currently used for the production of cokes with low sulphur and metal content, for anodes for the aluminium-refining industry and the electric steel process. Coal-tar pitch, low in quinoline-insolubles (al), is an excellent raw material for the manufacture of needlecokes with a low coefficient of thermal expension (CTE). The separation of inherent 01 can be performed via gravity settling in aliphatic hydrocarbon mixtures, by centrifugation in a disc separator or by filtration. The possible co-carbonization with aromatic petroleumderived residues yields premium coke suitable for the manufacture of UHP-electrodes. New developments in the production of coke from coal-tar pitch aim to improve coke yields and increase anisotropy (i.e. low CTE and high electrical conductivity values). Further technological progress has been made in the manufacture of hard pitch which can be used as a starting material for the production of pitch coke in the chamber coking process and for the production of electrode binders by means of a continuous flash process with optimized thermal and pressure treatment of pitch, thus facilitating the ‘tailored’ manufacture of binder pitches of different qualities.
Coal has very significant potential as a raw material for the manufacture of a whole spectrum of basic materials ranging from pure hydrogen to almost pure carbon. Thus, coal can be converted into gaseous products by gasification, into liquid hydrocarbons by means of liquefaction in an atmosphere of hydrogen or into a solid, non-fusable coke by a pyrolysis process. Therefore, it is, from the chemical point of view, a suitable substitute for petroleum. Despite the many attempts to improve the methods for upgrading the coal for the manufacture of gaseous and liquid products to pass the economic threshold for the substitution of petroleum products, the long-established process of coking coal in horizontal chambers at temperatures of 1200°C is still the only economically feasible process in coal chemistry. World-wide, the present production capacity for metallurgical coke amounts to 440 million t a- ‘. The capacity is evenly distributed all over the world (Figure I), the US, the Soviet Union, Japan and West Germany being the largest producers of coke’. The majority of the metallurgical coke produced, which amounts to approximately 390 million t a- I, is used for the manufacture of pig iron in the blast furnace where the coke plays a multipurpose role: (1) it acts as the skeleton to carry the burden, (2) it is burnt in order to supply the necessary heat for the smelting process of the ore, and (3) it is used as a raw .~_ * Presented at the Conference: ‘Industrial Conversion of Coal and Carbon to Gas, Liquid and High-Value Solid Products’, organized by the Industrial Carbon and Graphite Group of the Society of Chemical Industry, and held at the Society of Chemical Industry, 14, Belgrave Square, London SWIX EPS, UK, 7-9 April 1981
0016~ 2361/81/090877~06$2.00 @ 1981 IPC Business Press
material for generating the reduction medium, being complemented in this function by heavy fuel oil. Owing to the increasing price of heavy fuel oil, which is injected into the tuyeres of the blast furnace as a substitdte to metallurgical coke, the resubstitution of oil by coke is occurring in many countries producing pig iron. This gives coke an increased importance in the iron production process. However, metallurgical coke is not the exclusive product of the coal carbonization process for a number of by-products are also obtained. Amongst these by-
West
Germany
states
Pig lmn capacity Japan Coking capacity Rg iron capacity Figure
1
Production
440
mllllon tons/year 630 mllllon tons/yew capacities
for metallurgical
coke and pig iron
FUEL, 1981, Vol 60, September
877
Manufacture Tab/e 7 Number
of high-value
Distribution
carbon
from coal-tar
pitch:
of isomers of PAH
of PAH of n hexagons kata-annelated PAH
peri-condensed
1 1 2 5 12 37 123 446
0 0 1 2 10 45 210 1002
PAH
z 1 1 3
7 22 82 333 1448
products, coal-tar is the most significant in terms of quantity and suitability as a chemical raw material. Thus, 17 million t a-l of coal-tar are currently processed in some 125 tar refineries’ all over the world. This paper describes the physical and chemical nature, and the chemical potential of the heavy bottom of coaltar, viz. coal-tar pitch, and some of the most important bulk products made therefrom. CHEMICAL
AND PHYSICAL
NATURE
OF PITCH
It is well-established that coal-tar pitch is composed of a very large number of polycyclic aromatic compounds. When considering the theoretical number of isomers3 of aromatics containing more than 3 rings it becomes clear that full identification of the individual constituents of pitch is unlikely. For only 6 hexagons there are 82 isomers (Tahte I) and it is reasonable to assume that a great number of these isomers occur. With the development of high-pressure liquid chromatography (h.p.1.c.) the identification of these highly fused polycyclics has been facilitated. However, because of the complexity of this aromatic mixture the use of statistical analytical techniques is especially rewarding. Thus, it is highly desirable to gauge the average molecular weight and the statistical distribution of functional groups of the high molecular conglomerate. Gel-permeation chromatography (g.p.c.) was first applied by Petro and Edstrom as a means to study the molecular distribution of pitch constituents. Unfortunately, the pitch molecules behaved in a rather unorthodox manner because it was not the hydrodynamic volume but other factors such as primarily molecular shape and polarity which governed the separation, thus thwarting the construction of calibration curves. A novel g.p.c. technique employing quinoline as a solvent and using reductive hydrogenation with lithium in ethylenediamine base has been applied recently to study petroleum pitch5. This technique proved successful for obtaining molecular weight distributions of petroleum pitch mesophases and semicokes which have quinolineinsoluble contents approximating 100 wty& For coal-tar pitch a similar separation can be expected. 13C nuclear magnetic resonance (n.m.r.) spectroscopy is the method of choice for the determination of the statistical carbon skeleton of carbonaceous materials. Hence, the application of this technique to coal-tar pitch yields unambiguous evidence as to the aromatic structure of this material both in the solid and soluble state. No detectable alkyl side-chains6 greater than the ethyl moiety were found by this sensitive method.
878
FUEL,
1981,
Vol 60, September
J. W. Stadelhofer
et al.
The isolation of reactive centres of coal-tar pitch was described recently by Blumer et a/.‘. According to this scheme the highly reactive moieties of pitch (approximately 10%) are separated from the less reactive matrix by adduct formation with picric acid. The increased reactivity of these species was confirmed by a kinetic study which yielded an activation energy for the reactivecentres of68.0kJ mol-’ tiersus 136.5 kJ mol-’ for the whole pitch in carbonization experiments*. The great number of individual compounds present in pitch also govern its physical behaviour. Franckg was among the first to use the term ‘undercooled liquid’ for describing the crystallization inhibition observed when cooling coal-tar pitch. In more modern terminology this phenomenon is explained by the glass-like behaviour of characteristic Thus, materials. the amorphous temperature of the glass transformation is now used to describe the glass-like behaviour of coal-tar pitch. Both Differential Scanning Calorimetry (DSC)” and n.m.r. accurate techniques for the spectroscopy’ ’ are determination of the temperature of the glass transition. Results obtained by these techniques confirm the general rule that the viscosity of amorphous materials is approximately 10 I2 Pa s at their respective glass transition temperature (20-30°C for coal-tar pitch). The viscosity behaviour of liquid pitch has also found considerable interest in the last few years. The vexed question whether pitch is a Newtonian liquid has been extensively discussed. Thus Nazem12 and Briggs13 found that despite the invariance of viscosities over a wide range of shear rate, the classification as Newtonian liquid must be considered with caution because of Weissenberg effects exhibited by the pitches investigated. In summarizing the progress made recently in elucidating the chemical and physical composition of coal-tar pitch, a further successful step in solving the endless puzzle of fully identifying the chemical composition and the physical behaviour can be claimed, though the deepening of the understanding of this material remains a permanent challenge to the coal chemist’.
PITCH AS RAW MATERIAL PRODUCTION OF CARBON
FOR THE
Because of its high carbon content, pitch is the material of choice for the production of industrial carbon. In a comparison of the elemental compositions of various competing petroleumand coal-derived residues (Table 21, it is evident that coal-tar pitch has by far the highest carbon content.
T&,/e 2
Elemental
composition
of residual feedstock,
Element
Bitumen
SRC-pitch
Coal-tar pitch
Heavy fuel oil
c H N s 0
86.7
89.1 5.8 2.2 0.5 2.4
93.0 4.5 0.9 0.6 1 .o
86.6 11.5 0.2 0.9 0.5
1.3
1.7
0.6
(wt%) (wt%) (wt%) (wt%) (wt%)
C/H
9.7 0.6 2.0 1 .o 0.8
Manufacture
of high-value
carbon
from coal-tar pitch: J. W. Stadelhofer
et al.
425°C +
+
__Yapo”r,
Closed
Iall
8
I
fl CnkP __. ._
I
40°C
Open
valves
t
fl
Wet gas
+r
1 Combinationn tower
1
:--_
L 1: -_ _
Compressor
t
a ‘Top
ref Iux
b Wild gasoline to VRU
--,___ 4
drums
205°C
Light coker gas 011 to furnace oil desulphurlzer
Heavy coker gas oil to FCCU
CoGter Figure 2
Scheme of the delayed
Residual
feed
coking process
In order to produce pure carbon, the non-carbon elements of these precursors must be removed by a series of heat-treatment steps up to 3000°C. Coke is the first intermediate product on the pathway to industrial carbon. High-purity coke has two main areas of application in the aluminium and steel industries. In the first, it is used as the skeleton for the production of anodes for the HallHerault-Process; in the latter it serves as starting material for the production of the carbon electrodes for the production of steel especially in ultra-high-power (UHP) furnaces. In terms of quality, high anisotropy and a low coefficient of thermal expansion are, along with low sulphur contents and ash yields, the most important quality criteria of cokes for UHP electrodes. These electrodes are produced by baking a moulded mixture of calcined coke and coal-tar binder pitch and graphitizing the baked carbon stock at temperatures up to 3000°C. This market for carbon is promising for the electric steel production has experienced a considerable growth in the last decade and contributes, at present, 21% of the world steel production. As approximately 667 kg of graphite are consumed for the manufacture of 1 ton of steel in the electric steel furnace process the total consumption of graphite amounts to nearly 1.1 million t a-‘. For aluminium anodes, however, the quality criteria are less stringent as only the contents of sulphur, iron and silicon are relevant. Various technologies exist for the production of coke from heavy residues. Amongst the conversion processes, the delayed coking process clearly is the most significant with a production capacity of approximately 16 million t a - ‘. In this process a petroleum- or coal-derived residue is fed via a tubular furnace into a steel-lined coking drum where the carbonization process takes place under
moderate pressure and at temperatures of 45&5OO”C over an average period of l&12 h (Figure 2). Volatile components are driven off at the end of coking cycle by blowing steam into the coke drum. The yield of the green coke produced in this lowtemperature process is directly related to the Conradson carbonization residue of the feedstock (Table 3)i5. This, however, is the crucial point for the economy of the delayed coking process of coal-tar pitch. As coke is the main product of pitch carbonization (owing to its high carbonization residue of 35-60x) and the liquid byproducts are not upgraded in the conversion process the pitch coke must be marketable at an economic price. The boundary conditions for the petroleum refiner, however, are completely different as the costs of the conversion process can be distributed mainly to the expensive middle distillates and crude petrol-fractions because the major portion of the product spectrum comprises these products. (The yield of petroleum coke normally doesn’t exceed 20% compared with a 5&60% yield of pitch coke.) Despite this economic difficulty a number of versions for the production of coke from coal-tar pitch by delayed coking have been developed; the aim of producing coke of the highest quality is a common feature of these schemes. This type of coke is usually referred to a needle-coke because of the needle-like appearance of its optical texture and its crystallinity. However, pretreatment of crude coal-tar pitch is necessary for the production of needle-coke from coal-tar pitch. According to the Nittetsu16 process, soft pitch is treated with an aliphatic petroleum cut in order to reduce the content of material insoluble in quinoline (QI) which hampers growth and coalescence of the mesophase as it remains at the periphery of the developing spherules. High-quality coke of a coefficient of thermal expansion
FUEL,
1981,
Vol 60, September
879
of high-value carbon from coal-tar pitch: J. W. Stadelhofer et al.
Manufacture Tab/e 3
Delayed
coking of various feedstocks
California Feed Gravity (15Or.Z. g cme3) Conradson carbon (wt%) Sulphut fwt%)
0.973 12.1 1 .l
Product yields C3 and lighter (wt%)
(wt%)
7.1 27.6 22.9
Comparison
of petroleum
CQ and gasoline Coke (wt%)
Table 4
Oklahoma
1.223 31.2 0.48
0.986 9.4 1.6
14.1 23.3 19.1
3.0 10.7 60.9
and pitch-derived
Petroleumderived coke
Sulphur fwt%) Ash (wt%) Vanadium (ppm) Nickel (ppm) Water fwt%) Volatile matter (wt%) Real density (g cmp3) Coefficient of thermal expansion of graphite (l/K)
Coal-tar pitch
coke
Coal-tar pitchderived coke
Raw
Calcined
Raw
Calcined
1.5 max 0.35 max 150 rnax 150 max 7
1.5 max 0.4 max 15omax 150 max 0.1
0s 0.2 <5 <30 7
0.3 0.2 <5 <30 0.1
8
0.2
8
0.2
-
2.17
-
2.14
-
5.10-‘max
-
5.10-7
(35@-45O”C) of 8 x lo-’
K-’ can be produced by this method. In the Rutgers process’ 7, however, medium coaltar pitch is fed together with aromatic residues from steam cracking processes to a separator or a filtration unit in order to reduce the content of QI. The production of need&cokes of a quality comparable to the Japanese standard is possible from this mixture of petroleum- and pitch-derived hydrocarbons. A comparison of the most important quality requirements demonstrates that high-quality coke from coal-tar pitch has noteworthy advantages in terms of its low metal content when compared with cokes of similar quality (Table 4) derived from petroleum cuts such as pyrolysis oil or decant oil. In addition to delayed coking, the carbonization of pitch in horizontal brick-lined chambers is still common in West Germany and a number of Comecon States. In this process, hard pitch is fed at the top of the chamber into the oven through a loop passing over the top of the slot-type oven; the hard pitch (softening point 160-185°C R and B) may be produced either by oxidizing medium pitch with air at temperatures up to 400°C or by flash distillation (Figure 3). A continuous flash distillation unit for the production of 250000 t a-i hard pitch was put on stream in 1979 at the CastropRauxel works of Riitgerswerke AG. After a coking period of 15-20 h in the horizontal coke chamber an isotropic coke can be obtained in 80% yield. The low sulphur, ash and metal content of this coke are noteworthy assets. In addition to the pitch coke, coke oven gas rich in hydrogen and condensate is produced in this process. This condensate can be redistilled yielding secondary pitch.
880
FUEL,
1981,
Vol 60, September
The quality of the chamber coke can be varied by adding differing amounts of secondary pitch. Thus, highly isotropic coke for graphite production for applications in the nuclear industry (gas-cooled reactor) can be produced by using up to 100% secondary pitch as feedstock for the carbonization process. In both the delayed coking and chamber coking processes the residence time of the pitch in the reaction zone is not uniform, as pitch is continuously charged to the coking drum whereas in the latter process it is charged in a number of stages. However, a novel approach has recently been proposed by Glaser et ~1.‘~ According to this invention, a temperature programme is applied to the pitch in order to secure a uniform residence time for all components of hard pitch. Thus, a highly anisotropic coke can be obtained by carbonizing hard pitch at temperatures up to 450°C in an insertion oven. The optical texture of this anisotropic coke is shown in Figure 4. This experiment indicates that a needlelike structure of coke can be obtained under quiescent conditions. PITCH AS A BINDING
AGENT
At present, the most important product of coal-tar refining is binder pitch for the manufacture of anodes for the aluminium-refining industry. These anodes are used for the electrolysis of refined bauxite; they are produced by moulding and baking a mixture of electrode coke with almost 20% binder pitch into blocks typically 70 cm wide, 125 cm long and 50 cm high at temperatures of lo& 1200°C (pm-baked anodes) or by performing this process in the aluminium-refining process itself with a mixture of 30% pitch and 70% coke, supplying the heat for the baking process by the electric current (selfbaking Siiderberg anode). Thus, for every kilogram of aluminium produced approximately 0.4 kg coke and 0.1 kg binder are consumed. Despite many attempts to replace coal-tar pitch by petroleum products or possibly, solvent-refined coal (SRC) products the traditional pitch has firmly maintained its position for this application owing to its unmatched quality. Both the growing tar production and unexploited pitch reserves which are, at present, not marketed in this field, ensure the supply of pitch for the aluminium industry which has shown an outstanding growth in the last decade (Figure 5). The theoretical understanding of the behaviour of electrode pitch in the manufacture of anodes is still in its infancy. Thus, the production and application of binder
Gas sepamtor I
I b
Tubular f urnoce
. Medrum Figure3
CentrZuql pump
1 Hard ,..+.-L.
patch Thermal
reformer
for the production
1 P^^~___..^
of hard pitch
kkwufacture
Figure 4 Optical texture pitch in thin layers
of coke obtained
by carbonizing
hard
11
_ c 14c
of high-value
f
I
carbon from coal-tar
1970 Figure 5
I
I
I
71
72
73
Development
I
I
74 75 Year
of production
I
I
I
76
77
78
I
79
1
80
of aluminium
pitch is based on experience rather than on theoretical considerations. However, certain rules have been developed by the pitch producer and the consumer as to the specification of a suitable binder pitch. A breakdown of typical specifications is given in T&e 5 along with the respective data of coal-tar and straight-run medium pitch. In addition to these requirements a suitable wettability, which is measured via the contact angle and an optimum viscosity, are prerequisites for a high-quality binder pitch. As the composition of the medium pitch in terms of the relevant solubilities (i.e. toluene, quinoline) varies considerably with the properties of crude coal-tar, it is advantageous to treat the medium pitch in a thermal reforming step subsequent to the distillation process. This results in a uniformity of the amount of p-resins and the coke yield.
et al.
In this process, medium pitch is exposed to a thermal treatment under pressure up to 1.5 MPa which leads to the desired increase in material insoluble in quinoline and toluene. The final adjustment is usually carried out in a flash distillation process where the volatile portions are driven off until the desired softening point is reached. In principle, this process is similar to the production of hard pitch by flash distillation. Owing to the flexibility of this process concerning soaking time, temperature and pressure in the flash drum, the tar refiner is in a position to ensure the supply of products of a wide-range of specifications in terms of softening point, ratio of inherent/secondary QI, Conradson coking residue and further physical and chemical requirements. Various attempts have been made to circumvent this thermal reforming step by oxidizing pitch with air or by adding carbon black or mesophase QI in order to reach the necessary level of QI of the binder pitch. However, the modified pitches proved unsatisfactory to the aluminium producer because of reduced mechanical strength or increased consumption of pitch for the manufacture of electrodes. In addition to its binding capability, the impregnating effect of coal-tar pitch is of considerable industrial importance for many high-performance applications of graphite electrodes. Impregnation with coal-tar pitch results in the deposition additional pitch coke in the pores of the baked stock, thus bringing about a higher density of carbon and a significant improvement in mechanical strength. Characteristic data of a typical impregnating pitch are given in Table 6. The amount of insolubles, the viscosity and the wettability of the pitch must be well balanced for during the impregnation process the insolubles form a filter cake of low permeability on the surface of the stock which reduces the penetrability of the impregnating agent. Tab/e 5
II
pitch: J. W. Stadelhofer
Characteristic
Density (20°C, g cm-? Softening point (OC) (Kraemer-Sarnow) Softening point (‘C) (MettIer) Quinoline-insoluble (wt%) Toluene-insoluble (wt %I p-Resins (wt%) Carbonization residue (wt%) (Conradson) Carbon (w/t%) Hydrogen fwt%) C/H-ratio
Tab/e 6
Characteristic
data of coal-tar and coal-tar Coal-tar
Medium
1 .18
1.28
pitch
pitches Electrode 1.32
<0
68
92
86
110
3
7
14
8 5
18 11
39 25
45 92.5 4.5 1 .J
56 94 4.3 1.8
20 91 5 1.5
data of impregnating
pitch
Softening point (R and B) (“C) Quinoline-insoluble (wt%) Toluene-insoluble (wt%) Carbonization residue Conradson (wt%) Alcan (wt%) Filtration rate (5 g min-‘1 Viscosity (1 50°C, mPa s)
FUEL,
1981,
pitch
75 3 19 38 42 2.0 55
Vol 60, September
881
Manufacture of high-value carbon from coal-tar pitch: J. W. Stadelhofer et al.
The pick-up of impregnating pitch usually is up to 30% in the first impregnating step. A further impregnation may follow after having the stock rebaked. Moreover, for electrode nipples a third impregnation step can be advantageous.
SUMMARY AND FUTURE OUTLOOK It has been shown in this paper that coal-tar pitch is a very versatile raw material for the production of high-quality carbon and binding materials. In the past this basic material has experienced many changes regarding its main form of application. In former times it was used mainly as a fuel, then as a road building material and later for the production of pitch coke. Recently, more sophisticated applications have been found in the aluminium and steel industry. As still considerable portions of pitch are unexploited for these purposes a further growth of this market can be expected in the future. The tendency towards more sophisticated areas of application is further underlined by the production of high-modulus carbon fibres starting from coal-tar pitch. Additionally, other applications, for which it is not the purity of the carbon derived from the pitch but, for example, the agglutinating capability of the pitch which is used, offer a considerable market potential in the future. Thus, coal-tar pitch has proved a useful additive to noncoking coals. Moreover, the production of smokeless briquettes by means of special coal-tar pitches is a
882
FUEL, 1981, Vol 60, September
valuable contribution to the substitution of liquid fuels by solid domestic fuels. Thus the coal-tar-refining industry has proved to be highly flexible to changing markets in the past. As to the future, this branch of coal chemistry is confident to cope with all coming challenges. REFERENCES I
2
3
4 5 6 7 8
9 10 11
12 13 14 15 16 17 18
Nashan, G. Stahl und Eisen 1980, roo, 982 Collin, G. 'Proc. World Conf. Future Sources Org. Raw Mater., Chemrawn r, Toronto, Canada, 10--13 July 1978, Pergamon Press 1979, p. 283 Collin, G. and Zander, M. ErdDI Kohle, Erdgas Petrochern. 1980, 33, 557 Edstrom, T. and Petro, B. A. J. Polyrn. Sci. 1968, 21 (C), 1971 Greinke, R. A. and O'Connor, L. H. Anal. Chern. 1980, 52, 1877 Fischer, P., Stadelhofer, J. W. and Zander, M. Fuel 1978, 57, 345 Bliimer, G.-P., Haase, 1., Zander, M. and Zellerhoff, R. B. Erdijl Kohle, Erdgas Petrochern. Erganzungsband 80/81, p. 159 Stadelhofer, J. W. Fuel 1980,59,360 Franck, H.-G. BrennstojJ-Chern. 1955, 36, 12 Stadelhofer, J. W. Carhon 1979, 17,301 Hayward, J. S., Ellis, B. and Rand, B., Preprints Carbon 80, Int. Carbon Conference, Baden-Baden, West Germany, 1980, p. 338 Nazem, F. F. Fuel 1980,59, 851 Briggs, D. K. H. Fuel 1980,59,201 Bartle, K. D., Collin, G., Stadelhofer, J. W. and Zander, M. J. Chern. Tech. Biotechnol. 1979, 29, 531 Rose, K. E. Hydrocarhon Processing 1971,85 Gambro, A. 1., Shedd, D. T., Wang, H. W. and Yoshida, T. Chern. Eng. Progr. 1969,65, 75 Franck, H.-G. et al. Riitgerswerke AG, German patents No. 2016276 and 2.013927 Glaser, H. et al. Riitgerswerke AG, German patent pending