# Recycling of Low-Quality Carbon Black Produced by Tire Pyrolysis
## 3 Results and Discussion  
### Carbon Black  
The observed tendency toward agglomeration is presumably attributed to the migration of bituminous matter, waxes, and other adhesive compounds from the internal pores of ground particles to their surface layer. This process leads to the adherence of smaller particles, culminating in the formation of larger clumps (see Figure 2). These clumps are enveloped by a layer of very fine-grained amorphous carbon black. While the top surface of these agglomerates exhibits a minimal presence of contaminants, the disruption of these clumps allows for the extraction of up to 5 wt% of bituminous organic substances, including predominant polycyclic aromatic hydrocarbons, such as naphthalene, phenanthrene, fluorene, and pyrene, constituting up to 0.5 wt% of the total mass. Some samples also displayed an elevated moisture content. Alongside carbonized material, a noteworthy quantity of wire fragment scraps, ranging from 2 to 20 mm in length and originating from tire reinforcement, was identified in the carbon black pyrolysis samples, indicative of an insufficient separation process.  
Table 1 presents the concentrations of polycyclic aromatic hydrocarbons (PAHs) in the raw pyrolysis carbon black, expressed in milligrams per kilogram (mg/kg).  
\begin{table}
\begin{tabular}{c c} \hline
**PAH** & **mg/kg** \\ \hline Naphthalene & 24 \\ Fluorene & 13 \\ Phenanthrene & 13 \\ Acenaphthene & 12 \\ Pyrene & 11 \\ Fluoranthene & 6.7 \\ Anthracene & 5.7 \\ Benzo(g,h,i)perylene & 5 \\ Benzo(a)pyrene & 4.4 \\ Benzo(a)anthracene & 4.2 \\ Chrysene & 4.1 \\ Benzo(b)fluoranthene & 2.6 \\ \hline \end{tabular}
\end{table}
Table 1: Concentration of PAHs (mg/kg) found in raw carbon black.  
Figure 2: SEM microphotograph of agglomerated particles of carbon black.  
Table 2 displays the ultimate analysis of rCB, showcasing the elemental composition of the crystalline phases in weight percentages (wt%). Predominantly composed of carbon (74%), the material also contains significant quantities of silicon, sulfur, zinc, and iron. Additionally, minor components such as potassium and calcium are present. The determination of metals and silicon was conducted through XRD-XRF analyses (Bruker, Billerica, MA, USA). Elements susceptible to volatilization during incineration were computed based on incineration tests.  
Metals in the sample predominantly exist as sulfides, with elemental sulfur also present in the raw carbon black. The average ash content recorded in the sample was 19.7%. The heavy metal content was analyzed, suggesting that the pyrolysis process parameters might not have been optimal. Possible factors influencing this include a potentially low pyrolysis oven temperature, leading to the inadequate separation of heavier organics from the pyrolysis mass. Alternatively, insufficient mixing of the mass could yield similar results.  
The primary challenge in recycling rCB from tire pyrolysis lies in its structure and substantial organic content, hindering effective adhesion with other materials when used as a filler or pigment [30]. Moreover, the concentration of polycyclic aromatic compounds surpasses permissible limits (see Table 1). The limits depend on the nature of the final product and the ratio of rCB to total mass [31]. In most products, this must not exceed 0.2 mg/kg.  
While it is theoretically feasible to regenerate raw rCB into a form suitable for the rubber industry, this process is deemed impractical due to its high energy intensity, necessitating reheating to at least 600-700 \({}^{\circ}\)C and releasing significant sulfurous gases [32; 33]. Alternatively, rCB can be chemically modified using either acids or alkaline agents. Sulfur recovery needs further research; a sustainable means to achieve this purpose is application of biological methods [34]. Thus, the economically viable reprocessing of rCB involves transforming it into less energy-intensive products.