Recovery of Carbon Black from Scrap Rubber - American Chemical

544

Energy & Fuels 1999, 13, 544-551

Articles Recovery of Carbon Black from Scrap Rubber J. Piskorz,† P. Majerski,† D. Radlein,† Torsten Wik,‡ and D. S. Scott* Department of Chemical Engineering, University of Waterloo, Waterloo, Ontario, Canada, N2L 3G1 Received March 17, 1998

A hydrogenation process has been developed to recover carbon black from rubber crumbs prepared from scrap tires. A semi-batch 2 L autoclave was used, with hydrogen continuously bubbled through a slurry of rubber crumbs and a paraffinic dissolution oil under pressures from 300 to 1500 psig and temperatures of about 400 °C. The dissolution oil dissolved the rubber and allowed the carbon-black particles to be freed from the polymer matrix. The carbon black then acted as a hydrogenation/cracking catalyst. Under optimal reaction conditions, the yield of carbon black + inorganic additives was approximately 36%, the gas yield 1-2%, naphtha yield 8%, and the balance a product oil, part of which can be recycled as the dissolution reagent. The carbonblack product after filtering and drying had properties which were an average of those of the various grades added during tire manufacture. One-half of the inorganics could be removed by a simple acid wash if desired. The carbon black has a value more than twice that of the other products and could be the factor which would allow this process to be economically viable without subsidies.

Introduction Disposal of waste rubber tires is increasingly becoming a problem in most developed countries all over the world. Especially in North America, where the tire consumption is higher than in any other continent, a long-term solution to the waste problem is needed. In North America alone, an estimated 2.5 million tonnes of scrap tires are generated each year and constitute a major environmental problem.1 Several options for dealing with waste tires exist, such as sending to landfills, storing in piles, reuse and retread, grinding into rubber crumbs for use in different consumer products, tertiary processing into petrochemicals, fuels, and other products, or simply burning them for energy.2 The first two options are used for the majority of waste rubber tires. Storage in piles poses a hazard for diseases and accidental fires. Retread of old rubber tires is mainly performed on truck and aircraft tires, which, on average, are retreaded 4 times. Newer car tires, in general, do not have side walls strong enough for retreading. Grinding the tires into rubber crumbs of different sizes, usually in ranges from 4 to 200 mesh, and then using the crumbs in various products is becoming more * Author to whom correspondence should be addressed. † Present address: Resource Transforms International Ltd., Unit 5, 110 Baffin Place, Waterloo, Ontario N2V 1Z7. ‡ Present address: Dr. Liboriusg. 20, 413 23 Go ¨ teborg, Sweden. (1) Williams, P. T.; Besler, S.; Taylor, D. T. Fuel 1990, 69, 14741482. (2) Eastman, A. L. 144th Meeting of the Rubber Division, American Chemical Society, Orlando, FL, October 26, 1993; paper 5.

and more common. Several full-scale plants for making rubber crumbs are operating all over the world, but it appears that the market for rubber crumbs, at present, is limited. Using the crumbs together with virgin rubber decreases the properties of the rubber product significantly. Still, the crumbs are used successfully in products such as rubber carpets, running tracks, bumpers, and other rubber products where the physical properties are not crucial. However, these markets are unlikely to become big enough to swallow the excess of waste tires produced. To find a market adequate for large-scale disposal, it has been suggested that rubber crumbs should be used as a filler in asphalt. In the United States, legislation requires a minimum of 2% rubber crumbs in all new federally funded highways. The problem is that in order to get high-quality asphalt, very fine mesh rubber crumb has to be used, preferably less than 60 mesh, which increases the price of the asphalt significantly. Mixing with rubber crumbs also makes the asphalt more difficult to work. One type of tertiary recycling of rubber tires is rubber reclaiming, i.e., devulcanization of some sort followed by re-curing together with virgin rubber, the most common process. Rubber reclaiming has been widely used for a long time, but as rubber recipes have become more and more sophisticated, reclaiming has greatly diminished as a way of reusing scrap rubber. Pyrolysis of used tires has been the subject of research studies for many years, and published literature in the area has increased tremendously during the past decade. Three pyrolysis products are typically derived from the rubber, i.e., gas, oil, and char. However, the product

10.1021/ef980054i CCC: $18.00 © 1999 American Chemical Society Published on Web 03/19/1999

Recovery of Carbon Black from Scrap Rubber

compositions are dependent on the pyrolysis conditions used and on the tire composition. The pyrolysis is usually conducted under oxygen insufficiency or in an inert gas such as nitrogen or helium, at atmospheric pressures and at temperatures ranging from 300 to 1000 °C. The rubber feed can be rubber crumbs, ground rubber, or even whole tires. The process is highly endothermic with a typical energy consumption of 4.05.7 MJ/kg of rubber. More than 26 patents for processes concerning pyrolysis of rubber tires exist, mostly Japanese, German, and U.S. patents. Several pilot plants in various countries are running, and some full-scale plants are also operating in Japan, the United States, and Italy. However, very few research results from these plants have been published in the literature, except for possibly two groups of investigators.3-5 The pyrolytic oil produced from rubber has a relatively high calorific value, around 43 MJ/kg, and a low sulfur content of around 1%, while the heating values of the gas are in the range of 29.3-49.8 MJ/m3, depending on the pyrolysis temperature.1,4,5 Hence, these products are considered good fuels, but their value is not sufficient to give an economically viable operation, and other products of high value are required if the conversion process is not to be heavily subsidized. This has led to several investigations of the use of rubberderived pyrolysis chars as carbon blacks for rubber compounding (pyro-blacks) or for conversion to activated carbons.6-10 In general, the chars require some secondary processing, e.g., grinding, extraction, activation, etc., to give a potentially marketable product. Although a number of companies are offering such pyrolysis-derived carbons, they do not seem to have gained general acceptance by the rubber or other industries, except in a few very limited applications. They tend to possess very high ash contents, low surface areas, and organic impurities, and the carbon blacks do not seem to have the particulate structure desired for rubber compounding. Another approach to thermal degradation, developed by Japanese workers, makes use of various solvents at high pressures and temperatures in both subcritical and supercritical states.11,12 However, detailed product evaluation has not yet been reported. A variation of this approach has recently been reported by Hydrocarbon (3) Mirmiran, S.; Roy, C. J. Anal. Appl. Pyrol. 1992, 22, 205-215. (4) Kaminsky, W.; Sinn, H. In Thermal conversion of solid wastes and biomass; Jones, J. L., Radding, S. B., Eds.; American Chemical Society Symposium Series No. 130; American Chemical Society: Washington, DC, 1980; pp 423-439. (5) Roy, C.; Labrecque, B.; de Caumia, B. Resour., Conserv., Recycl. 1990, 4, 203-213. (6) Darmstadt, H.; Roy, C.; Kaliaguine, S. Carbon 1994, 32, 13991406. (7) Beck, M. R. Pyrolysis carbon blacksan overview; 144th Meeting of the Rubber Division, American Chemical Society, Orlando, FL, October 26, 1993; paper 4. (8) Ogasawara, S.; Kuroda, M.; Wakao, N. Ind. Eng. Chem. Res. 1987, 26, 2552-2556. (9) Teng, H.; Serio, M. A.; Bassilakis, R.; Morrison, P. W.; Solomon, P. R. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1992, 37 (2), 533547. (10) Akbar, A. M.; Retrich, M. A. Environ. Energy Eng. 1993, 8, 1370-1376. (11) Funazukuri, T.; Takanashi, T.; Wakao, N. J. Chem. Eng. Jpn. 1987, 20, 23-27. (12) Funazukuri, T.; Ogasawara, S.; Wakao, N.; Smith, J. M. J. Chem. Eng. Jpn. 1985, 18, 455-460.

Energy & Fuels, Vol. 13, No. 3, 1999 545 Table 1. Example of a Rubber Compounding Composition component

wt %

natural rubber styrene-butadiene rubber carbon black zinc oxide sulfur other additives

52.2 13.0 29.4 3.3 2.1 small

Table 2. Characteristics of Some Commercial Rubber Reinforcing Carbon Black Grades ASTM designation

mean particle diameter, nm

mean aggregate size, nm

surface area sq m/g

N110 N220 N326 N330 N339 N351 N550 N660 N774

27 32 41 46 39 50 93 109 124

93 103 108 146 122 159 240 252 265

143 117 94 80 96 75 43 36 28

Technologies Inc.13 In this method, waste oil is heated with scrap rubber pieces and additives such as CaO to high temperatures to form a solid carbonaceous residue, a light oil, and gas. The oil was subsequently hydrogenated, while the solid residue was tested as an asphalt component. Our study of scrap tire recycling with carbon-black recovery was initiated because it was apparent that a carbon product of higher value would be necessary to give an economically viable process. Carbon black constitutes 20-35% of the mass of scrap tires, and this use represents the major market for commercial carbon blacks. Table 1 shows an approximate composition for one type of rubber tire compounding.11 However, as the formulas used by commercial producers are considered to be proprietary, the exact amounts of the various components in any scrap tire are not known. Further, different compositions are used for different parts of the tire. The commercial carbon blacks used as reinforcing agents for rubber are commonly prepared in a number of grades. For those used in rubber tires, a number of standard ASTM grades are employed, from the finest grade, N110, to the coarsest, N774. Table 2 gives selected representative data for some of the commonly used grades.20 It should be noted that although the carbon-black particles are roughly spherical in shape, they are normally connected together to form aggregates. If the carbon black is not well-dispersed (e.g., by high mechanical shear) in compounding, these aggregates will form larger agglomerates due to van der Waals forces or due to added binder. The size and shape of the aggregates is one of the principal features that determines the properties of the carbon black as a reinforcing agent. An electron microscope picture of a typical N339 carbon black is shown in Figure 6.20 The (13) Comolli, A. G.; Stalzer, R. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1997, 42 (4), 1053-1057. (14) Radlein, D.; Piskorz, J.; Majerski, P.; Scott, D. S.; de Bruijn, T. T. W. Submitted to Can. J. Chem. Eng. (15) Farcasiu, M.; Smith, C. Energy Fuels 1991, 5, 83-87. (16) Farcasiu, M.; Smith, C. M.; Ladner, E. P. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1991, 36 (4), 1869-1877. (17) Farcasiu, M. Another use for old tires; coprocess them with coal to make liquid fuels. CHEMTECH 1993, 22-24.

546 Energy & Fuels, Vol. 13, No. 3, 1999

Piskorz et al.

Figure 1. Diagram of the experimental setup.

Figure 2. Ash content as a function of the yield: (- - -) measured ash content, (s) calculated ash content.

Figure 4. TEM of carbon-black product (acid-washed). Typical overview-note bead-like structures; large inorganic crystal in the lower right quadrant, run 7.

appearance of the aggregates is very individual, with some being very branched and others more clustered. Within each aggregate, the particles are very uniform in size, but for each carbon black grade, there is a very large distribution in aggregate size. The highest grades of carbon black are used in the tread cap, with lower grades in the casing and side wall, and the lowest grades in the liner. Because used tires will have a high degree of tread wear, there will be a less-than-expected amount of the highest grade carbon black in scrap tires. In addition, other fine particulates are present as additives, such as zinc oxide, silica, or titanium dioxide, and these inorganic materials could be expected to be found also in a recycled carbon black. Work in our laboratories in which activated carbon was successfully used as a hydrogenation/hydrocracking

catalyst for upgrading of a bitumen dissolved in a paraffinic solvent suggested that the large amount of high surface area carbon black in rubber tires might also serve as an effective upgrading catalyst.14 Similar studies in recent years by Farcasiu and co-workers15-17 and by others18 have shown that carbon black is an effective and highly selective cracking catalyst and can act as a “hydrogen shuttle” for hydrogenation of heavy oils, scrap rubber, or rubber-coal mixtures. In our work, two requirements had to be met. A solvent was needed that was capable of dissolving the rubber sufficiently to free carbon-black particles and to allow their catalytic action, and the solvent also had to be a highly saturated hydrocarbon to give a maximum hydrogenation effect, as demonstrated in recent years for activated carbon catalysts.19 Both were satisfied by the use of a recycled paraffinic lubricating oil stock. Also, work in our laboratory21 with bitumen/paraffinic solvents/ hydrogen/activated carbon systems has shown that if the solvent is a highly saturated paraffinic or naphthenic hydrocarbon, then very little decomposition of this solvent occurs in the presence of hydrogen and an activated carbon catalyst at temperatures less than about 420 °C. This is due to the hydrogen shuttling concept which has been defined as the facilitated transfer of hydrogen between the gaseous phase and the catalyst surface through an intermediate hydrogen-rich species.22

(18) Mastral, A. M.; Murillo, R.; Calle´n, M.; Perez-Surio, M. J.; Mayoral, M. C. Energy Fuels 1997, 11, 676-680. (19) Dockner, T. Angew. Chem., Int. Ed. Engl. 1988, 27, 679-682. (20) Othmer, K. Encyclopedia of Chemical Technology, 4th ed.; John Wiley and Sons Inc.: New York, 1992; Vol. 4, pp 1037-1074.

(21) Piskorz, J.; Radlein, D.; Majerski, P.; Scott, D. S. Hydrotreating of Heavy Hydrocarbon Oils in Supercritical Fluids. U.S. Patent No. 5,496,464, March 5, 1996. (22) Aimoto, K.; Nakamura, I.; Fujimoto, K. Energy Fuels 1991, 5, 739-744.

Figure 3. BET surface area versus measured yield.


Energy & Fuels, Vol. 13, No. 3, 1999 547

Figure 5. TEM comparison of high-severity carbon-black product run 2 (left) and low-severity product run 7 (right). Not acidwashed.

Figure 6. Electron microscope picture of grade N339 carbon black.20

Experimental Section The primary object of this experimental work was to recover the carbon black in the scrap rubber in its original form if possible. Any other inorganic materials added to the rubber during compounding would also be present in the recovered solids. Commercial rubber crumb from scrap automobile tires is largely free of steel belting and polymeric fibers and was therefore a suitable raw material for our tests. Pieces of crumb

used were approximately 6 mm in size and prepared from waste tires by a commercial firm. The concept of the work was based on a complete dissolution of the rubber into a liquid phase and conversion of the organic portion of the scrap into good-quality liquid or gaseous products by hydrogenation. The dissolution step frees the carbon-black particles from the rubber polymer matrix and allows them to act as a hydrogenation/hydrocracking catalyst. On the basis of the results of other work done with carbon catalysts and hydrogen-rich solvents as discussed above, the dissolution oil used would not be significantly consumed in the process and could largely be recycled provided it was a highly saturated hydrocarbon originally. In this way, it was expected that the carbon black could be recovered in nearly its original form, as well as the majority of the dissolution oil. In our work, the dissolution oil used was stated by the manufacturer (Safety-Kleen Canada Ltd.) to be a re-refined highly paraffinic lubricating oil base stock prepared from recycled motor oil. Two lots of this oil were used, a lighter one with a mean boiling point of 370 °C for 11 of the 13 runs and a slightly heavier one with a mean boiling point of 415 °C for the remainder. A SimDis of this latter oil is shown by a dashed line in the results presented in Figure 8. All SimDis analyses were done with the same program and at the same conditions, so the SimDis for the dissolution oil can be superimposed on the product SimDis as shown in Figure 8. It can be seen from Figure 8 that these heavy oils had a relatively narrow boiling range. Experiments were done in a laboratory 2-L pressure vessel with continuous hydrogen feed. A schematic drawing of the apparatus is shown in Figure 1. After charging the reactor with rubber crumb and dissolution oil, the vessel was heated to an operating temperature of approximately 400 °C. After initial runs, a liner was inserted into the reactor which gave easy handling but reduced the rate of heating of the reactor. As a result, heating times ranged from 2 to 3 h. Holding times


Piskorz et al.

Figure 7. Typical; GC results for the naphtha fraction of liquid product, run 9.

pressure reduction. After metering, the gases were collected in large gas bags for future analysis by GC. After the desired reaction time, the hydrogen flow was shut off and the reactor allowed to cool to room temperature, when it was depressurized through the condenser followed by purging with hydrogen. The naphtha was then collected from the condenser, and the reactor contents were removed as a “mud” which contained the carbon black, inorganic materials, dissolution oil, and product oil. The solids in this mud were washed several times by decantation after mixing with a volatile solvent (acetone, naphtha, and trichlorethylene were all used in various runs) and then centrifuged and dried. The decanted oil/solvent mixture was vacuum evaporated to remove solvent. Each fraction was weighed and yields calculated on a dissolution-oil-free basis, on the assumption that very little dissolution oil was decomposed. Further analysis of liquid composition was done by SimDis, and the final carbon-black product was also analyzed for ash content and BET surface area. The structure and particle size of the carbon black were determined by a TEM (transmission electron microscope). Some samples of the carbon black were also washed with dilute HCl to reduce their ash content, and the properties of these blacks are reported as “acid-washed”. In the various runs, system pressures were varied from 1500 to 300 psig and dissolution oil/rubber ratios were 1.2 to 1.7 by weight, with a majority of the final tests being done at a ratio of 1.4. A number of repeat tests were done at conditions close to optimal (1000 psig; 400 °C; 30-40 min holding time; 1.4/1 oil/rubber weight ratio) in order to prepare a sufficiently large sample of carbon black for rheological and compounding tests.

Results and Discussion

Figure 8. SimDis of composite liquid product from run 9. at a reaction temperature of 400 °C varied from 0 to 4 h, with the optimum time being about 0.5 h, a time that was used in later runs. During both the heating and holding periods, hydrogen was continually fed to the reactor and liquid volatiles were collected in the condenser during the run and after

A total of 13 runs in all were done in the development of this process. The first five runs were exploratory in nature. The balance of the runs were done at more standardized conditions. The reaction conditions, as well as the product yields as a percentage of the total initial charge, are given in Table 3. Reaction time is the time the charge was held at the final operating temperature and does not include heating-up time. Runs 10, 11, and 12 were replicate tests carried out primarily to produce enough carbon black for testing in rubber compounding. Nature of the Carbon-Black Products. The most important and valuable product is the carbon black, if it can be recovered in its original submicron, high surface area form. Two types of carbon black are produced in this process, the “primary” carbon black which is an original component of the rubber and a “secondary” carbon black which is a product of the

Table 3. Mass Recoveries as a Percent of Total Initial Charge reaction conditions

charge

mass recoveries

run (RR no.)

press (psi)

max temp (°C)

holding time (min)

rubber crumb (%)

diss oil (%)

total (%)

carbon (%)

naphtha (%)

prod oilc (%)

gas (%)

1 2 3 4 5 6 7 8 9 10 11 12 13

1500 1500 1500 1410 1010 1000 1000 1000 980 1000 1000 1000 300

405 426 411 387 415 397 404 415 394 410 410 407 406

130 285 124 90 130 0 20 20 60 40 35 35 40

39.9 36.9 36.6 36.9 36.8 45.6 45.4 45.5 40.9 45.5 45.5 45.5 45.5

60.1 63.1 63.4 63.1 63.2 54.4 54.6 54.5 59.1 54.5 54.5 54.5 54.5

ndd 77.5 91.7 88.1 94.1 98.3 95.6 95.8 98.7 92.4a 96.6 97.5 99.1

18.3 20.2 14.1 16.0 18.6 16.3 18.0 17.8 16.0 nd nd 17.2 21.5

nd 3.1 3.5 8.9 10.9 3.5 5.9 5.0 3.6 nd 5.5 4.6 19.9

nd 56.5 76.7 59.6 52.6 77.4 73.3 69.2 77.9 nd nd 76.0 52.4

nd 20.2 5.6 15.4 17.9 2.8 2.8 6.0 1.9 4.4 2.5b 4.2 9.5

a

Value too low due to the loss of all naphtha in the condenser. b Value low due to gas loss. c By difference.

d

Not determined.



Table 4. Carbon Characteristics, Yield, and Yield Estimation wt % of Rubber Charged run (RR no.)

yieldmeas (wt %)

1 2 3 4 5 6 7 8 9 12 13 6-12

yieldesta (wt %)

45.8 44.7 54.7 51.7 38.6 36.9 43.5 40.4 50.6 48.1 35.7 35.8 39.7 37.7 39.1 39.1 39.0 35.0 37.9 39.5 c 47.4 composite sample

ashcarbon (wt %) 14.1 12.2 17.1 15.6 13.1 17.6 (9.7)b 16.7 16.1 18.0 15.0 13.7 16.4 (8.3)b

BETmeas (m2 /g)

BETashfree (m2 /g)

43.5 35.9 35.5 44.1 39.4 nd 45.2 (50.6)b 45.6 46.5 47.2 36.4 48.2 (50.6)b

50.6 40.9 42.8 52.2 45.3 nd 54.3 54.3 56.7 55.6 42.2 57.7 (55.2)b

iodine no. 121 157 95.5 115 126 nd nd nd nd nd nd

a Yield b est are based on a measured ash content of 6.30 for rubber batch no. 1 and 5.92 for batch no. 2. Acid-washed values given in parentheses. c 27% of solid yield was +60 mesh pieces of unreacted rubber.

Table 5. Results from Elemental Analysis

a

sample

carbon (%)

Cashfree (%)

hydrogen (%)

Hashfree (%)

sulfur (%)

non acid washeda acid washeda RR no. 2 Commercial Carbon Black20

78.28 86.84 83.11 97.3-99.3

93.6 94.7 94.7 98.3-99.4

0.81 0.86 1.67 0.20-0.40

0.97 0.94 3.00 0.20-0.40

3.62 1.29 2.63 0.20-1.20

Composite samples, runs 6-12.

reaction of rubber or oil depolymerization and decomposition. The two differ in two wayssfirst, the primary carbon contains ash and sulfur because of the inorganic materials added during compounding, which are also submicron in size and incorporated into the rubber matrix but which are not present in the secondary carbon, which is a product of the decomposition of liquid organic molecules. The secondary carbon also has a significantly lower surface area than the original carbon black. At “low” severity conditions (that is, a minimum of hydrocracking), the carbon recovered will be essentially primary carbon, and this material is about 35% of the rubber, on a wire-free basis. Table 4 shows the carbon-black yields, ash contents, and BET surface areas for all runs, as well as calculated ash-free values for the surface areas. Measurement of the ash contents of the original rubber charged, as well as those of the solid product recovered from the reaction mud, are easy to do and relatively unambiguous. On the reasonable assumption that no ash appears in the gaseous or liquid products, it should be possible to predict the yield of carbon product from the ratios of ash contents of solid charge and solid product. While the validity of this assumption is to be expected in most cases, we have confirmed it from our experimental values. The carbon-black product yield estimated from the ash content only is shown in Table 4 also and is graphically illustrated in Figure 2 as an ash content calculated from the experimental solid product yields and the initial ash content of the rubber crumb. It is apparent that a correlation valid to within about 5% accuracy exists. Because much of the inorganic additives are calcium carbonate or zinc sulfide (the majority of the sulfur in the rubber crumb appears in the solid product mud as zinc sulfide), these can be readily removed by washing the carbon black with a dilute acid, which results in cutting the ash content about in half, that is from 17.69.7%, for example. The bracketed numbers in Table 4 give some values for acid-washed samples. Table 4 also shows clearly that the BET surface area decreases as the carbon-black yield increases, because of the genera-

tion of secondary carbon at high-severity (that is, more extensive hydrocracking) conditions. This behavior is also shown graphically in Figure 3. Similarly, the ash content decreases at high carbon-black yields because the carbon generated from hydrocracking reactions has essentially no ash content. The elemental analysis of the carbon black is shown in Table 5, for the 1 kg composite sample made up from all the carbon black collected from runs 6-12, both as made and after acid washing to reduce ash content. The results for one high-severity run (run 2), which would contain a significant amount of secondary carbon, are also given. It should be noted that the BET surface areas shown in Figure 3 for the carbon blacks containing mainly primary carbon (35-40% carbon-black yield) on an ash-free basis are close to the range of those commercial carbon blacks of N300-N500 grade used for rubber compounding. The balance of the solids remaining in the carbon black after acid washing were found to be silica and titanium dioxide, both of which are added in compounding. The presence of these inorganics in the scrap rubber emphasizes that this product is a true recycled material, essentially as originally present in the rubber. The resulting carbon-black product is, of course, a composite of the various grades of carbon black added in different parts of the tire and will also contain the inorganic additives, such as zinc oxide (which appears in the recycled carbon black as zinc sulfide), calcium carbonate, and silica and titanium dioxide, all of which have been identified in the TEM X-ray diffraction scattering mode. While about 50% of these inorganics can be removed by acid washing, it may not be advisable to do so, since they were added originally for a purpose and are still in their original form. Hence, the recycled carbon-black ash content is, in fact, made up of useful compounding additives. The results shown in Table 5 indicate, however, that the carbon-black product, even on an ash-free basis, does not have quite as high a carbon content as does commercial carbon black, for which the carbon content generally exceeds 97%. The values for the hydrogen and sulfur content in the acid-washed composite sample are


within the ranges of these values for commercial carbon blacks. The high hydrogen content for the high-severity (high yield) carbon black (run 2) suggests that the secondary carbon contains a considerable amount of hydrocarbon. A number of transmission electron microscope pictures were made of typical carbon-black products. Examples are shown in Figures 4 and 5. From these photos, particle sizes in the aggregates and aggregate structures were determined. Particle sizes can be summarized as shown below:

less than 50 nm ≈ 30% 50-100 nm ≈ 30% 100-200 nm ≈ 30% greater than 200 nm ≈ 5% These values are within the ranges of particle size for N300-N600 commercial grades of carbon blacks. From Figure 4, for the acid-washed sample, the typical bead structure of a carbon black is evident, as well as the occasional large particle of an additive (lower quadrant). Figure 4 also shows the aggregates (and their agglomerates) formed by the individual beads, a desirable characteristic also typical of commercial carbon blacks. In our work, it was observed that the size of these aggregates depended to some extent upon the solvent used in decanting and washing the carbon-black product. Figure 5 shows enlarged views of an aggregate for a nonacid-washed, high-severity sample and for a nonacid-washed, low-severity sample. The typical bead structure is still evident in the high-severity sample, but there appears to be some increase in particle and aggregate size and additional amorphous material. This is consistent with the concept that the secondary carbon is deposited on the existing carbon particles rather than forming discrete entities. The data in Table 4 and Figure 3 show that the high-yield carbon has a lower overall surface area, so that the secondary carbon deposits on the original carbon-black particles serve to significantly reduce the surface area of the agglomerates. The carbon product samples, both as-produced and acid-washed, were subjected to X-ray analysis. The occurrence in decreasing order of the inorganic constituents of the as-produced particulate agglomerates was Si, Zn, Al, S, and traces of Fe and Ti, while that for the acid-washed was Si, Al, S, and traces of Fe, Zn, Ti, and Cl. For the as-produced sample, the zinc peaks and the sulfur peaks accompanied each other, which supports (together with the generation of considerable H2S) the concept that one of the major ash components removed by the acid washing was zinc sulfide. The Si/Al presumably originates from acid-washed clays used as additives in compounding. Gaseous and Liquid Products. Significant amounts of gas were produced only under high-severity conditions (high pressure, long reaction times). Under lower severity conditions when little coke or secondary carbon was formed, there was very little gas production, usually 10% or less of the weight of the rubber charged. A typical analysis of this gas is shown in Table 6. Note that all gas components have been accounted for and that they are all saturated light hydrocarbons, with only traces of olefins. This result is due to the catalytic action

Piskorz et al. Table 6. Typical Gas Analysis (Hydrogen-Free) CO2 CH4 C2H4 C2H6 C3H8 C4-C6

(wt %)

(vol %)

6.1 16.3 0.4 16.7 22.3 38.2

5.0 36.8 0.6 20.1 18.3 19.2

of the carbon black, which has both a cracking and a hydrogenation function. The fact that the carbon black is present in the scrap rubber in such large amounts means that no catalyst needs to be added to achieve hydrogen addition to the organic decomposition fragments formed. The C3-C6 fraction is over 60% of the weight of the gas produced. This process yields two liquid fractions, one a light naphtha in the gasoline range which distills from the reactor during a run and is condensed in the external condenser. This product is called the “Condenser” liquid. A heavier liquid (product oil) remains in the reactor. Much of this is the dissolution oil, but a significant amount of product is in the distillate-heavy gas oil range and is recovered in the final product “mud” when the reactor is drained at the end of a run. The naphtha fraction contains mostly saturated hydrocarbons up to about C15, but some alkyl-substituted aromatics are also present. A typical GC for a naphtha product from run 9 is shown in Figure 7. The most plentiful aromatic present is a C10 fraction, which may include isoprene dimer products such as dipentene, p-cymene, or pmethenes. The GC for the reactor liquid shows less aromatic content and appears to consist mostly of saturated hydrocarbons with larger amounts of heavy components and lesser amounts of light ends than are present in the naphtha fraction. Figure 8 shows a typical SimDis chromatogram of the composite oil product from a relatively low-severity run (run 9). The plot shows clearly that little decomposition of the dissolution oil took place in this run, because of the hydrogenating action of the carbon black. The naphtha-kerosene fraction is about 60% of the total liquid product (solventfree basis) and the distillate-heavy gas oil fraction about 35%. The “pitch” fraction (>525 °C) is normally less than 5%. In general, in low-severity runs, about 50-60% of the rubber appears in the products as liquid hydrocarbons and 35-45% as carbon black. For high-severity runs (higher temperature, longer times), it is apparent that the product oil yield is less than the amount of dissolution oil added, so that some decomposition of the dissolution agent has obviously taken place. It is likely that this cracking of the heavy oil added as a rubber solvent is one of the sources of the secondary carbon formed at high severity, i.e., at long reaction times. Run 13, done at a very low hydrogen pressure of 300 psig, also suggests that excessive cracking and coke formation can result from too low a hydrogen pressure. A graphic representation of the rubber degradation process occurring is presented in Figure 9 (all data is on a solvent-free basis). It is assumed to be essentially a two-step reaction, with the first stage being a dissolution/depolymerization which is primarily thermal and is aided by the presence of the dissolution oil. This step opens the polymer matrix and frees the submicron


Figure 9. Suggested scheme for waste rubber decomposition.

particles of carbon black and other inorganic solids. The liquids and gases produced in this step are then hydrocracked in the presence of carbon black, now acting as a catalyst, to give lighter and more saturated liquid and gaseous products. If reaction conditions are “severe” (temperature of 400 °C or above and longer reaction times), increasing amounts of secondary carbon (coke) will be formed as a consequence of the increased cracking and a reduction in liquid yield and increase in gas yield will also result. This secondary carbon (expressed as a parameter “x” in Figure 9) does not have as fine a particle size as the original carbon black and has a smaller surface area. It has not yet been established what level of secondary carbon can be tolerated by rubber compounders. Summary A standard test of rheological properties important in acceptable compounding of rubber, of a large 1 kg composite sample of the carbon black recovered in runs 6-12, was done in the laboratory of an independent compounder and manufacturer of rubber products. According to the results obtained, this recycled carbon black has rheological properties equivalent to an N300grade commercial carbon black. Hence, this carbon black should be acceptable for rubber compounding as an alternative to some of the carbon black grades normally


used. Such recycling would not only give a high value product for the process, but would replace a significant quantity of hydrocarbon feedstock used for the manufacture of virgin carbon black. The recycling method tested in our work for scrap rubber tires and other rubber wastes appears to be able to reclaim the carbon black content of scrap rubber in essentially its original form. It also converts the organic part of the rubber polymer into a potentially highquality synthetic crude oil. It recovers the inorganic additives used in compounding the tire rubber in a recyclable form as well. Although in our work a clean rubber crumb was used, there appears to be no reason shredded whole tire pieces could not be processed equally well. The steel wire would be freed from the elastomer matrix in the dissolution step, while the polymeric fibers would be converted to liquid or gas in the hydrocracking reaction. The steel wire can be easily removed by a simple screening of the reactor “mud” product. There are other advantages to this process also; for example, pressures and temperatures are moderate, conventional reaction and processing equipment can be used, a flexible batch process is employed and readily scaled-up, sulfur is removed as ZnS or as H2S using a dilute acid wash, and the only waste products are steel and some inorganic salts if acid washing is used. Further, a preliminary economic estimate of the process developed in our work indicates that it is unique in that it could be potentially profitable without any subsidy for scrap tire disposal because of the recovery of the high-value carbon-black product. Acknowledgment. The authors acknowledge the initial financial support of this work by Giltek Ltd. (Israel). We also extend our thanks to the New Energy Development Organization, RITE program of MIDI (Japan), and the Office of Research of the University of Waterloo, for additional funding. Some of the measurements reported were carried out by Orin Paddock, and his contribution is acknowledged also. EF980054I

Recovery of Carbon Black from Scrap Rubber - American Chemical

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