Introduction

The pyrolysis of tires has been investigated by several research groups who have demonstrated that the char has potential as a carbon black or activated carbon. The solid product can also be used as reinforcing filler for rubber goods and possibly as a printing ink pigment after appropriate treatment [1]. Li et al. [2] studied carbon dioxide activation for the solid product to have a high surface area such as activated carbon. Rodriguez et al. [3] compared solid product with carbon black. They realized that tire-pyrolysis residues could partially replace semi reinforcing commercial blacks for the preparation of rubber parts, such as footwear, conveyor belts, dock fenders, etc., which are not so high quality demanding as automotive tires. Waste tires were pyrolyzed to obtain carbon black under optimum conditions by Barbooti et al. [4] and Zabaniotou et al. [5]. The potential of polyisoprene rubber pyrolysis was investigated by Roy et al. [6]. Also, adsorption capacity of activated carbon obtained from pyrolysis of TDF was studied by Banar et al. [7] and Gupta et al. [8, 9]. In this context, the literature was reviewed related to especially improvement of carbon black and demineralization and desulfurization process of coal which are summarized in Table 1 [10–20].

Table 1 Literature survey in related to improvement of carbon black and demineralization and desulfurization process of coal
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Tire pyrolysis is alternative technology for carbon black production, but the economics of the process strongly depends on the commercial value of the pyrolytic carbon black. To increase to quality of pyrolytic carbon black, some improvement studies should be realized.

Therefore, in this study, waste tire pyrolysis was evaluated from an environmental perspective with a focus on the commercial utilization of the solid product. TDF samples 1–4 mm in size were pyrolyzed at a 400 °C pyrolysis temperature with 5 °C/min heating rate. Improvements of the solid product (SP) were performed to obtain a final product that had comparable physical and chemical properties with commercial carbon blacks. High abrasion furnace black (HAF) and fast extrusion furnace black (FEF) are the most widely used commercial carbon blacks in industry. HAF is structurally hard and is used for the production of cable, conveyor, outer tire, etc. FEF is soft and is used for the production of laying cables, tube tire, etc. [21, 22]. Unlike other studies in the literature, waste tires were formed into TDF before the pyrolysis processes to ensure homogeneity. Furthermore, a new improvement process was developed to obtain carbon black from the pyrolytic solid product.

Experimental methodology

In the experimental studies, raw material (TDF) was characterized. Then, TDF was pyrolyzed and improvement studies were performed on the solid product. Finally, the products were characterized and compared with commercial products. Analysis methods are described in their own sections.

Characterization of raw material (TDF)

The TDF used for pyrolysis was prepared from the shredded mixture of cis-butadiene rubber (CBR) and styrene butadiene rubber (SBR) steel-free tire samples that were supplied from a tire recycling plant. This shredded mixture was crumbed and sieved to produce a size range of 1–4 mm. The moisture, ash, volatiles and fixed carbon content of the TDF sample were determined by ASTM D-3173-85, ASTM D-3174-12, ASTM D-3172-73 and ASTM D-3175-82, respectively. The elemental composition and higher heating value (HHV) of the TDF were determined using a CHNS-O Elementar Vario EL III and IKA C200 calorimeter (ASTM D-5865), respectively.

The comparative results of elemental analysis, proximate analysis and HHV of the TDF and different waste tires are listed in Table 2. The results of the TDF sample were similar to the values in the literature except for the ash value.

Table 2 Composition of TDF and different waste tires
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To identify the morphologic structure (crystalline/amorphous) of TDF, diffraction experiments were performed with a Rigaku Rint 2200 diffractometer (40 kV, 30 mA) using CuKα radiation (Fig. 1a). Diffractograms illustrated S (jcdp no. 00-020-1227), SiO2 (jcdp no. 00-043-0596) and ZnO (jcdp no. 00-036-1451) signals in the TDF. SiO2 in the TDF samples results from tire manufacturing while ZnO and S result from the vulcanization of the tire.

Fig. 1
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a XRD spectra of TDF. b The SEM images of TDF (50,000×)

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A ZEISS SUPRA 50 VP scanning electron microscope (SEM) was also used to obtain SEM micrographs of the TDF, which was prepared by gold coating to enhance electrical conduction. In Fig. 1b, the flat structure of the TDF can be observed.

TDF pyrolysis

Pyrolysis experiments were carried out under atmospheric pressure at a 400 °C pyrolysis temperature with 5 °C/min heating rate in a fixed-bed stainless steel (380 S) 240 cm3 reactor (well-swept with nitrogen). Pyrolysis conditions were determined based on the extensive experimental works conducted by Akyıldız [21]. The reactor was surrounded with a resistive oven (400 W). All experiments were performed in triplicate. In a typical run, a 10-g sample was placed into the reactor which was heated until it reached the set temperature, which was maintained for 1 h. Solid and liquid product yields were determined in each experiment by weighting. The gas yields were determined by the mass difference. All the yields were expressed on a dry basis and the average yields of at least three experiments were within an experimental error of less than ±1 wt.  %. Pyrolysis product yields for solid, liquid and gas were 34, 38.8 and 27.2, respectively at 400 °C with 5 °C/min heating rate.

Improvement of the solid product

After the solid product was obtained by pyrolysis, physical and chemical analyses were carried out to assess its quality as a carbon black (see “Characterization of products”). It was observed that the sulfur (1.71 %) and ash contents (12.14 %) of the solid product were higher than those of commercial carbon blacks. The ash values were similar to those reported by Zolezzi et al. [25]. However, these values were lower than those obtained by Helleur et al. [26] whereas higher than those reported by MuiEdward et al. [27] and Uçar et al. [28]. Therefore, the solid product obtained from tire pyrolysis needs to be demineralized. For that reason, a leaching procedure for demineralization and desulfurization was performed on the solid product with HCl (Merck, 37 %), HNO3 (Merck, 65 %) and HF (Sigma-Aldrich, 40 %) as shown in Fig. 2. First, HCl, HNO3 and mixture of HCl and HNO3 were experimented for these processes. In this trial, 0.5 grams of solid product (SP) was mixed with 10 ml of concentrated acid [(HCl, HNO3 and HCl + HNO3 (1:1, v:v)] solutions. Sample solutions were digested in a microwave oven (CEM MARS 5, 400 W). The temperature of each sample was raised to 175 °C in 6 min and remained there for 10 min before cooling down to room temperature over 15 min. At the end of the microwave program, the mixture was separated by filtration, and then washed with distilled water until the pH value of filtered water was neutral. Finally, the product was dried and weighed, and the ash and sulfur content were determined according to ASTM D-3174 and ASTM D-1619, respectively. Based on the results of this step, it was decided that a mixture of HCl and HNO3 solutions would give the highest removal efficiency. The SP was then digested with different concentrations of HCl + HNO3 mixture (90–10 % with 10 % intervals) in the microwave to determine the optimum mixture. Ash and sulfur analyses of the acid digested products were carried out and the optimum mixture was determined from the sulfur content. The product with sufficient sulfur was called the intermediate product (IP). Although, the IP reached commercial carbon black sulfur quality, ash was found to be rather difficult to remove and it was necessary to use HF extraction. Therefore, the IP was digested with different concentrations of HF solutions (100, 90, 80 and 70 %). For this procedure, 1 gram of IP was mixed with 10 ml of the HF solution to be tested, and the mixture was digested on a hotplate until a solid form was obtained. The solid forms were washed with distilled water until the pH value of the filtrate was neutral. Next, the washed fractions were dried, weighed and their ash contents were determined. In this manner, the optimum HF concentration that gave sufficient ash content was determined. The resulting product with a sufficient ash content formed by HF processing was called the final product (FP). Afterwards, the physical and chemical properties of the obtained solid, intermediate and final products were determined and compared with different commercial carbon blacks (HAF N330 and FEF N550) that are produced by the furnace technique.

Fig. 2
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a Demineralization and desulfurization yields of SP by acid treatment. b The effects of different acid ratios on ash and sulfur removal

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Characterization of products

To identify the physical structure of the products, the moisture, ash, volatiles and fixed carbon content, elemental composition and HHV of each product were determined using the same procedures for TDF characterization mentioned above. In addition to these analyses, the pH and density values were also determined according to ASTM D 1512 and ASTM D 1513, respectively.

XRD and SEM analyses were carried out similarly to TDF analysis. Surface morphology images for the SP, IP and FP were obtained using a Q-scope 250 atomic force microscopy (AFM) from Quesant instruments with standard silicon cantilevers (Quesant). Topography images were collected in noncontact mode at a scan rate of 4 Hz under ambient laboratory conditions.

The nitrogen adsorption measurements (BET, Brunauer–Emmett–Teller) for the SP, IP and FP were performed with a Quantochrome Autosorb 1-C. Before the adsorption measurements, the samples were outgassed for 17 h at 300 °C in vacuum.

The oil adsorption number (OAN) gives information about how hard or soft carbon black is. This metric is important for many applications of the carbon black. OAN analysis for the SP, IP and FP was performed according to ASTM D2414, and the results were compared with commercial carbon black values. The OAN value of the SP is not available in this analysis because of its large particle size.

The quantitative analysis of all products and commercial carbon blacks was conducted using an ICP-MS/MS (Agilent 8800) after extracting with a microwave according to Stals et al. [29].

Results and discussion

Results of the improvement studies

Results of the SP demineralization and desulfurization with 100 % HCl, HNO3 and HCl + HNO3 (1:1, v:v) are shown in Fig. 2a as a function of acid type. As shown as Fig. 2a, HNO3 and HCl + HNO3 treatments were very effective at sulfur removal (93.6 and 95.3 %, respectively). Although treatment with only HCl did not yield high removal efficiency (75.4 %), it provided sufficient reduction in sulfur content when compared with commercial carbon blacks (0.5 %). The content of sulfur in carbon black could reach more than 80 % of total sulfur in tire. Inorganic sulfides such as FeS2 and ZnS as well as elemental sulfur have been identified in the carbon black [28]. Kumar and Hari Shankar [30] indicated that pyrite (FeS2) can react with dilute nitric acid solution at moderate temperature and strong oxidizing conditions (such as high concentration) although it is almost unaffected by hot and concentrated HCl. Equation (1) explains the reaction between FeS2 and high concentrated HNO3 [30]. In addition to this, the results of this study show that also concentrated HNO3 is more effective than the concentrated HCl to oxide sulfur.

$$ 6{\text{FeS}}_{2} + 30 {\text{HNO}}_{3 } = 3{\text{Fe}}_{2} ({\text{SO}}_{4} )_{3} + 3{\text{H}}_{2} {\text{SO}}_{4} + 30{\text{NO}} + 12{\text{H}}_{2} {\text{O}} $$
(1)

For ash removal, extraction with HCl and HNO3 gave efficiencies of 48.8 and 45.9 %, respectively. The demineralization in HCl acid treatment is expected due to removal of soluble components of mineral matters. The removal of mineral matter by HCl acid brings an increase in surface area due to creation of new pores of small and intermediate size. On the other hand, as indicated by Kumar and Hari Shankar [30] an appreciable reduction in surface area of micropores and mesopores at higher concentration of HNO3 acid solution is expected, giving resistance to the penetration of HNO3 acid into the matrix, and thus contributes in impeding the extent of demineralization. Although reported individual reverse effects of HCl and HNO3 [30], in this study, the mixture of HCl and HNO3 gave the highest demineralization efficiency (60.5 %). It is thought that due to the new pores generation by HCl, HNO3 could penetrate into the matrix. In addition to this, HCl has better affinity for Zn more than that of HNO3. Consequently, it was decided that HCl and HNO3 mixture was the best solution for the demineralization and desulfurization process.

To determine the optimum concentration, the SP was digested in the microwave using different concentrations of the HCl + HNO3 mixture. The ash and sulfur contents of the acid digested products are given in Fig. 2b. As shown in Fig. 2b, 50 % HCl + HNO3 concentration was the optimum concentration because it resulted in sufficiently low sulfur content (0.22 %). Although this product, IP, had a good quality in terms of sulfur, it still had higher ash content (5.56 %) than commercially available carbon black (0.2 %).

The IP was digested with different concentrations of HF solutions. Table 3 shows the final ash content of the products according to various HF concentrations. As shown in this table, 100 % HF concentration reduced the ash content from 5.56 to 0.27 %, resulting in a removal efficiency of 95.1 %. The resulting product was designated the final product (FP). Although, HF aqueous solution treatment reduced mineral matter of samples, silicate based minerals, it had no effects on sulfur content as indicated in Vaccaro’s study [20]. Therefore, sulfur content of the IP product did not change after HF treatment.

Table 3 The effects of HF on the ash removal
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Characterization of the products

The properties of all products (SP, IP and FP) and commercial carbon blacks (HAF and FEF) are given in Table 4. The weight yield of the products according to TDF sample on dry ash free basis was determined as 34.00 % for SP, 31.76 % for IP and 29.97 % for FP after pyrolysis, HCl + HNO3 and HF treatments, respectively. As it can be seen from Table 4, ash and sulfur contents of the products are decreased after treatments while fixed carbon contents are increased. Volatiles are given in Table 4 as received basis. When the volatile matter content of the products was calculated on the dry ash free basis (daf), the results were found to be 18.18 %, 18.33 and 19.88 % for SP, IP and FP, respectively. As it can be seen from data HCl + HNO3 and HF treatments did not affect the volatile matter content, significantly. Ash removal in the improvement studies caused an increase in fixed carbon content and therefore HHV value of the IP and FP increased. The pH decreased with the acid extraction, and density increased with decreasing particle sizes. In addition, XRD, SEM, AFM, BET and OAN (ASTM D2414) analyses of the SP, IP, FP and commercial carbon blacks (HAF and FEF) were performed.

Table 4 Properties of the products
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XRD spectra of the solid product that show Si and ZnS content are given in Fig. 3a. It has been reported that the ZnO and sulfur present in tire react during pyrolysis to form ZnS [28] and reaction between sulfur and zinc combined with other minerals in tire rubber such as Ca, Fe and Si eventually increases the ash content in the final product [31, 32].

Fig. 3
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XRD spectra of the a solid product, b improved products, c commercial carbon blacks

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The XRD results of the improved products (IP and FP) and commercially available carbon blacks (HAF N330 and FEF N550) are given in Fig. 3b and c, respectively. As a result of the improvement process, the XRD patterns of the IP and FP are similar to commercially available carbon black because the removal of the Si and ZnS peaks. According to the XRD patterns of carbon blacks, it was observed that the commercial carbon blacks had an amorphous structure containing only carbon atoms (jcdp no. 00-026-1081).

The SEM images of the SP, IP, FP, HAF N330 and FEF N550 are given in Fig. 4, respectively. As shown in these figures, the SP has a sintered and porous structure resulting from pyrolysis while TDF has a flat structure (Fig. 4a). The SP particle size was between 40 and 100 nm. Figure 4 b and c shows that the particle size of the IP and FP were 60 and 50 nm, respectively, and their particle sizes are similar to the particle size of HAF N330 and FEF N550 (50 and 75 nm, respectively) (Fig. 4d, e). The smaller particle sizes of the IP and FP than the SP indicate that the improvement process also decreased the particle size. It was also observed that all product stages and commercial carbon blacks are similar to each other in terms of structure.

Fig. 4
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The SEM images (300,000×) for the a SP, b IP, c FP, d HAF N330 and e FEF N550 samples

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The AFM images for the SP, IP, FP, HAF N330 and FEF N550 are given in Fig. 5, respectively, in micrometer scale. As seen in Fig. 5a, the SP does not show a regular particle size distribution because of agglomeration. This heterogenic structure resulted from the tendency of nano-sized particles to agglomerate in the SP. The average height is between 100 and 200 nm and the roughness is 135.2 nm. The top points of the IP, FP, HAF N330 and FEF N550 were 517, 199.7, 383.5 and 513.4 nm as shown in Fig. 5b–e respectively. The particles of HAF N330, FEF N550 and the various stage products agglomerated quickly. According to these results, the particle size was decreased with HF application.

Fig. 5
figure 5

AFM images (5 × 5 μm) for the a SP, b IP, c FP and (40 × 40 μm) for the d HAF N330, e FEF N550 samples

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The BET and OAN results of products are given in Table 5. The BET surface area of the SP is 105 m2/g. Li et al. [2] and Zhang et al. [33] found that the BET surface area of the solid product of waste tire pyrolysis was 89 and 68 m2/g, respectively. The BET value in this study is higher than that observed in previous studies in the literature. When compared with HAF N330 (178 m2/g) and FEF N 550 (46 m2/g), the surface area of the SP was intermediate between these values. An increase was observed in BET values from the SP to the FP. This resulted from acid extraction, which decreased the particle size, resulting in an increase in the surface area. In addition, the BET value of the IP and FP (185 and 212 m2/g, respectively) were higher than commercial carbon black. It was not possible to carry out OAN analysis on the SP because of its large particle sizes. On the other hand, as shown in Table 5, the OAN values of the IP and FP are similar to each other and higher than those of the commercial carbon blacks. This result indicates that the IP and FP structures are softer than the HAF and FEF samples since an increasing OAN indicates a softening of the material and a decrease in particle size. BET and OAN are important parameters for the useable area of carbon black in many applications.

Table 5 The BET and OAN analysis results
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The quantitative analysis results for Al, Fe, Mn, Sr and Zn obtained from ICP-MS/MS are given in Table 6. These elements (with exception Zn) are considered contaminants for carbon black and they need to be controlled in industry in view of risks to change carbon black properties [34]. According to table, the concentrations of the elements in the products are decreased with improvement studies. Also, Zn concentration of the final products can be tolerated according to commercial carbon blacks while the values of the final products are lower than those of the commercial carbon blacks.

Table 6 ICP-MS/MS results of the products and commercial carbon blacks
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Conclusion

In Turkey, annually about 8 million tires are produced and an estimated 284,800 tons/year of tire waste must be disposed. The total capacity of the facilities for recovering waste tires as granulated material is 101,000 tons/year. Moreover, the total capacity of the cement plants licensed for using waste tires as alternative fuel for energy recovery is 130,000 tons/year [35]. Among these recovery applications, recently there is no plant that produce carbon black from waste tires in Turkey although the pyrolysis method is classified in the section of applicable recovery methods for waste tires titled “regulation for control of the tires which have completed their life-cycles (TCL)” that was published in the Official Gazette on 25 November 2006, number 26357 in Turkey. From this point of view, it was mainly focused on the quality improvement of solid product obtained from TDF pyrolysis to use it as a substitute for commercial carbon blacks. Experiments of the study have shown that:

  • Solid product of the pyrolysis should be improved in term of the sulfur and ash contents to reach the quality of commercial carbon black. To achieve this quality level, it was seen that the solid product should be treated with 50 % concentration of HCl and HNO3 mixture followed by the treatment with 100 % concentration of HF. After this processing, the final product had sufficiently low ash and sulfur contents (0.27 and 0.22 %, respectively), which were similar to commercial carbon black.

  • XRD patterns determined that ZnO and S which are built in TDF were converted to ZnS and Si during the pyrolysis. These patterns also showed that acid treatment of the solid product also provided ZnS and Si removal.

  • According to the SEM images, the solid product may be useful for applications which require a high surface area and porous structure (e.g., as an activated carbon) since its particle size is between 40 and 100 nm. Furthermore, SEM images show that acid treatment of the solid product decreased the particle size and provided similar particle size and structure to solid product with commercial carbon blacks have.

  • BET analysis showed that acid treatment increased the BET surface area of the solid product. It was also observed that surface area of the acid-treated solid product was higher than that of the commercial carbon blacks.

  • OAN values of the acid-treated solid products were found to be higher than those of the commercial carbon blacks. This result indicates that the acid-treated solid products have a softer structure and smaller particle size than commercial carbon blacks.

As a conclusion, in this study, a final product which has sufficiently similar properties to commercial carbon black was obtained through pyrolysis of TDF followed by a novel improvement process. This new and valuable product can be used as a raw material by many industries, including those involved in tire, plastic and paint manufacturing.