Conversion of Waste Tyres into Carbon Black and their Utilization as Adsorbent

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1 Journal of the Chinese Chemical Society, 2006, 53, Conversion of Waste Tyres into Carbon Black and their Utilization as Adsorbent Jasmin Shah,* M. Rasul Jan, Fazal Mabood and M. Shahid Department of Chemistry, University of Peshawar, N.W.F.P., Pakistan Pyrolysis has the potential of transforming used tyres into useful recyclable products. Pyrolytic carbon black is one of the most important products of tyre pyrolysis. Waste tyres were pyrolysed at 450 Cin a batch reactor under atmospheric pressure. The recovered pyrolytic carbon black residues were studied to investigate their characteristics for use as a possible adsorbent. EDX elemental analysis and surface area determinations were used to investigate the distinctive features of pyrolytic carbon black. Due to various inorganic additives of the original tyre that contaminated the carbon black obtained, it was treated with acid for demineralization. The demineralized carbon black was activated at 900 C in a furnace. It was observed that acid treatment and activation increased the surface areas and decreased the concentration of contaminants. Furthermore, adsorption characteristics of methylene blue on acid-treated and activated carbon black (prepared via acid treatment) were compared with those of commercial activated carbon in liquid phase adsorption. It was found that adsorption capacity of methylene blue on acid-treated activated carbon black was greater. Keywords: Pyrolytic carbon black; Demineralization; Adsorption; Methylene blue. INTRODUCTION Scrap tyres represent a major environmental problem. When tyres are no longer usable, they are usually dumped in landfill sites, or burned in cement kilns or in brick kilns. 1 However, because of the elevated emissions of polyaromatic hydrocarbons (PAH) and of heavy metals (e.g. zinc), expensive gas cleaning systems have to be installed in order to perform this process in an environmentally responsible fashion. 2 Pyrolysis may be an environmentally friendly process to transform used tyres into useful products. This process transforms used tyres into gas, oil and pyrolytic carbon black. 3-5 Researchers have shown that the conversion of used tyres into oil is a feasible process. 6-9 The derived oils may be used directly as fuel or added to petroleum refinery feedstock. They may also be an important source of refined chemicals. 10 The gaseous products are also useful as fuel, and the carbon black may be used as reinforcing filler in rubber, especially in tyres, 11 or as activated carbon because of its high carbon content Activated carbon is widely used for water purification, air purification and also in batteries and fuel cells. The main purpose of the present work was to prepare activated carbon from waste tyres, which are suitable for adsorbing relatively large molecules. The adsorption characteristics of the obtained carbon were investigated after acid treatment and activation for organic molecules like methylene blue. EXPERIMENTAL Characteristics of the Sample Pyrolysed Tyres are composed of very different compounds (rubber, carbon black, fillers, stearic acid, zinc oxide, etc.). 17 In order to use representative samples of the whole tyre, cross section pieces of 5-10 mm wide of a commercial passenger car tyre were used for the pyrolysis experiments. Thermogravimetric Analysis The thermogravimetric experiment was performed in a Perkin-Elmer TGA 7 type instrument. The sample was heated from 40 C to a final temperature of 1000 C at a rate * Corresponding author. Fax: ; jasminshah2001@yahoo.com

2 1086 J. Chin. Chem. Soc., Vol. 53, No. 5, 2006 Shah et al. of 20 C/min. Pyrolysis Procedure The carbon black samples were produced from waste tyre pyrolysis. For pyrolysis about 10 g of waste tyre rubber particles were put into a batch reactor of glass tube. The reactor was heated from room temperature to a final temperature of 280, 300, 350, 400, 450 and 500 C for 2 h. The initial gas atmosphere in the reactor consisted of air. Upon heating, however, decomposition of the tyre sample started. The gases released by these reactions gradually replaced the air in the reactor. The gaseous products passed through a trap, where the liquid hydrocarbon was collected. Liquid and solid (carbon black) pyrolysis yields were determined in each experiment by weighing the amount of each obtained and calculating the corresponding percentage. The gas yields were determined by difference. Sample Characterization a. Surface area measurements Surface areas were measured using iodine adsorption number (ASTM 1510). b. EDX elemental analysis The elemental composition of carbon black was determined using an Inca 200 (UK) Energy Dispersive X-Ray (EDX) Analyzer. c. Acid demineralization An acid demineralization process was carried out to decrease the inorganic impurities as well as to remove undesirable (ash) contents from the carbon black. Hydrochloric acid (HCl) and sulphuric acid (H 2 SO 4 ) were selected for the acid demineralization. 1 N hydrochloric acid and sulphuric acid solutions were used, and demineralization was conducted at room temperature. The samples were demineralized for a period of 24 h, a length of time that was considered to be long enough not to limit the demineralization process. After 24 h the samples were filtered and thoroughly rinsed with distilled water to remove the residual acid. The samples were then dried in an oven at 110 C for 24 h. The carbon black sample was ground and sieved through a 45 m sieve. In the activation step, the known weight of an oven dried acid treated carbon black sample was placed in a furnace at 900 C for 2 h, which was already optimized. Liquid Phase Adsorption The adsorption characteristics of carbon black acid treated and activated were investigated. Methylene blue, which is a relatively large molecule, was employed as the adsorbate in the adsorption experiments. An aqueous solution with a concentration of 1000 mg/dm 3 was prepared by mixing an appropriate amount of methylene blue with distilled water. Adsorption experiments were conducted by placing 0.5 g of the adsorbent and 50 cm 3 of the aqueous solution in a 250 cm 3 glass stopper flask. The flasks were agitated at room temperature until the equilibrium was attained. The solutions were filtered and finally their unadsorbed concentrations were measured using a UV-Visible spectrophotometer (Unico 2100, USA) at wavelengths of 610 nm. The equilibrium adsorption capacities (q e ) at different methylene blue concentrations were determined according to the mass balance on the adsorbate: q e =(C i C e ) V/m where C i is the initial concentration, C e is the equilibrium concentration, V is the volume of the liquid phase, and m is the mass of the carbon black. RESULTS AND DISCUSSION Thermogravimetric analysis of waste tyre rubber was performed to determine the optimum temperature for pyrolysis using a batch reactor. A thermogravimetric curve for waste tyre rubber is shown in Fig. 1. In the first part 3.4% weight loss occurred at C to C and then 62.9% weight loss occurred from C to C. The pyrolysis completed in the temperature range of C to C with a constant weight (33.7%) of carbon black. The results obtained in the tyre pyrolysis experiment using a batch reactor at different temperatures are presented in Fig. 2. With an increase of temperature from 300 C to450 C the %wt of solid decreased to 34%, almost equivalent at 450 C and 500 C as well as to the yield from thermogravimetric analysis (33.7%). The liquid yield increased from 300 Cto450 C but at 500 C there was a decrease due to stronger thermal pyrolysis. Therefore a carbon black sample was obtained from waste tyre pyrolysis in a batch reactor at 450 C. Surface area The surface area (A) of the carbon black obtained by

3 Waste Tyres into Carbon Black J. Chin. Chem. Soc., Vol. 53, No. 5, Fig. 1. TGA curve of waste tyre rubber at a heating rate program of 20 C/min. pyrolysis at 450 C was approximately 85 m 2 g -1 (Table 1). This value is close to the average value of surface areas of commercial carbon blacks used in passenger car tyres such Fig. 2. Effect of temperature on the yields of gas, liquid and carbon black from pyrolysis of used automotive tyres. as N330 (A = 82 m 2 g -1 ) and N339 (A = 90 m 2 g -1 ). Other authors also have reported the similar values. 13,18,19 This suggests that the carbon black structure changed only slightly during pyrolysis. To enhance the commercial value of carbon black from pyrolysis and to increase its potential usage as an activated carbon, further treatment was necessary. Hydrochloric acid and sulphuric acid were used for the demineralization of the pyrolytic carbon black, and the surface area increased after acid treatment, drying, crushing and sieving through 45 m. The surface areas were found to be 870 m 2 g -1 and 800 m 2 g -1, respectively. treated with hydrochloric acid and sulphuric acid were further activated at 900 C, and a substantial increase in the measured surface area was observed in both acid treated carbon black samples (Table 1). These values compare favorably with those of commercially available activated carbons (specific surface areas ranging between 800 and 1100

4 1088 J. Chin. Chem. Soc., Vol. 53, No. 5, 2006 Shah et al. Table 1. Surface area and elemental composition of pyrolytic carbon black and commercial activated carbon (wt%) Sample Surface area (m 2 g -1) C O Si S Zn Ca (not treated) (HCl treated) activated (HCl treated) (H 2 SO 4 ) activated (H 2 SO 4 treated) Activated carbon (commercial) m 2 g -1 ). Acid demineralization The elemental compositions of carbon black determined on the pre and post acid demineralization and activated samples are presented in Table 1. In the acid demineralization process the feed sample contained 83.1% carbon, 2.6% sulphur, 4.2% zinc and 2.4% calcium. After the application of acid treatment of samples, the carbon concentration was significantly increased, indicating the removal of non-carbon phases with acid. The highest carbon concentration achieved with hydrochloric acid treatment was 93%. It can be seen that elements such as calcium appear to be completely removed from the sample and about 85.7% of zinc was removed from the carbon black with hydrochloric acid. While with sulphuric acid demineralization, carbon concentration increased up to 87% and the concentration of other elements decreased to a lesser extent. This may indicate that the majority of the elements in the carbon are present in acid insoluble form. The results of hydrochloric acid demineralization show that the process has potential for the production of a high carbon concentrate. In commercial carbon black, in addition to carbon, only oxygen and sulfur were found (hydrogen can not be detected by EDX). Liquid-phase adsorption isotherms The adsorption of methylene blue from aqueous solution on commercial activated carbon, acid treated and activated carbon black were studied and compared. The commercial grade activated carbon was employed to adsorb methylene blue for the purpose of comparison. The adsorption isotherms of methylene blue onto the acid treated carbon black and commercial activated carbon are shown in Fig. 3, while acid treated activated carbon black adsorption isotherms of methylene blue are shown in Fig. 4. The adsorption isotherms are sharp and the plateaus are relatively horizontal for both types of carbon. The adsorption capac- Fig. 3. Comparison of adsorption isotherms of methylene blue on acid treated carbon black and commercial activated carbon (CAcarbon). Fig. 4. Comparison of adsorption isotherms of methylene blue on acid treated activated carbon black.

5 Waste Tyres into Carbon Black J. Chin. Chem. Soc., Vol. 53, No. 5, ity of methylene blue on hydrochloric acid and sulphuric treated carbon black are similar. The commercial activated carbon was employed to adsorb methylene blue for the purpose of comparison. It has been shown that commercial activated carbon has a larger surface area than the acid treated carbon black. This suggests a larger methylene blue capacity for commercial activated carbon because of the monolayer adsorption on the carbon surface, and the adsorption capacity was found to be greater than the acid treated carbon black. Therefore, the acid treated carbon blacks were activated and again the adsorption capacity of methylene blue was studied. As can be seen from Fig. 4, the amounts of methyelene blue adsorbed increased considerably with acid treated carbon black after activation. CONCLUSION An activated carbon was produced from waste tyre rubber through thermal pyrolysis and demineralized with acid prior to activation. Acid treatment is an efficient way to demineralize and increase the surface area of the carbon black after activation at 900 C. The liquid-phase adsorption results reveal that the acid treated activated carbon prepared from waste tyres is a suitable adsorbent, especially in systems which involve bulky organic molecules. The adsorption of methylene blue occurs as a monolayer. Received January 6, REFERENCES 1. Ariyadejwanich, P.; Tanthapanichakoon, W.; Nakagawa, K.; Mukai, S. R.; Tamon, H. Carbon 2003, 41, Benallal, B.; Roy, C.; Pakdel, H.; Chabot, S.; Poirier, M. A. Fuel 1995, 74, Chaala, A.; Darmstadt, H.; Roy, C. Fuel Proc. Technol. 1996, 46, Cunliffe, A. M.; Williams, P. T. Environm. Technol. 1998, 19, Kuhner, G.; Voll, M. In Manufacture of Carbon Black; Donnet, J. B., Bansal, R. C., Wang, M. J., Eds.; Carbon Black Science and Technology, 2 nd Ed., Marcel Dekker: New York, USA, 1993; Lin, Y.; Teng, H. Micro Meso. Mater 2002, 54, Mastral, A. M.; Murillo, R.; Callen, M. S.; Garcia, T. Resour. Conserv. Recycl. 2000, 29, Merchant, A. A.; Petrich, M. A. AlChE J. 1993, 39, Pantea, D.; Darmstadt, H.; Kaliaguine, S.; Roy, C. J. Anal. Appl. Pyrol. 2003, 67, Rodriguez, I. M.; Laresgoiti, M. F.; Cabrero, M. A.; Torres, A.; Chomon, M. J.; Caballero, B. Fuel Proc. Technol. 2001, 72, Roy, C. Recovery of Commercially Valuable Products from Scrap Tyres; US patent No. 5229, 1993, Roy, C.; Labrecque, B.; Caumia, B. Resour. Conserv. Recycl. 1990, 4, Teng, H.; Serious, M. A.; Wajtowicz, M. A.; Bassilakis, R.; Solomon, P. R. Ind. Eng. Chem. Res. 1995, 34, Waddell, W. H.; Bhakuni, R. S.; Barbin, W. W.; Sandstrom, P. H. In Pneumatic Tyre Compounds; Ohm, R. F., Ed.; The Vanderbilt Rubber Handbook, 13 th Ed., Vanderbilt Company: Norwalk, CT, USA, 1990, Williams, P. T.; Brindle, A. J. J. Anal. Appl. Pyrol. 2003, 67, Williams, P. T.; Brindle, A. J. Fuel 2003, 82, Zabanioton, A.; Lagoudakis, J.; Toumanidou, E.; Staropoulos, G. Energy Sources 2002, 24, Williams, P. T.; Besler, S.; Taylor, D. T. Fuel 1990, 69, Lin, S. H. AlChE J. 1993, 39,