# Recycling of Low-Quality Carbon Black Produced by Tire Pyrolysis
## 1 Introduction  
Carbon black stands as a fundamental product within the chemical industry, playing a versatile role across various sectors of the material industry [1]. Defined as micro- or nano-sized carbon powder, carbon black is predominantly sourced from natural gas through combustion under oxygen-deficient conditions or derived from natural oil, such as through the process of cracking. This manufacturing process is energy-intensive, relying heavily on fossil fuels, which is environmentally undesirable amid the climate crises and escalating concerns about the depletion and rising costs of natural gas and oil resources [2]. To address these challenges, the scientific community seeks alternatives to virgin carbon black, turning their attention to recycled carbon black (rCB), primarily sourced from waste car tires. Some tire producers have already transitioned their entire production to incorporate rCB. In European Union countries, the landfill disposal of waste tires has nearly ceased, with almost 100% of these tires being recycled. Certain US states have also prohibited tire landfilling, resulting in the cleanup of many tire disposal sites since the early 2000s [3]. However, at a global scale, the recycling rate remains significantly lower, hovering at around 10% [4]. Many discarded tires are incinerated alongside hazardous waste, emitting substantial quantities of toxic gases (sulfur compounds, PAHs, etc.). Additionally, tire landfilling persists as a prevalent disposal method, with the worst-case scenario being abandoned tires littering the landscape [5]. This study introduces a pioneering approach--cavitational vortex milling--to valorize low-quality rCB produced through a low-temperature tire pyrolysis process. To the best of our knowledge, this innovative process has not been previously employed to enhance the quality of carbon black. Earlier, several methods were developed for the improvement ofthe properties of low-quality rCB, e.g., patent WO 2013/095145 Al [6]. This patent describes the use of additional high-temperature pyrolysis to remove impurities from low-quality carbon black. The primary objective of this project was to develop three potential products (hydrophobic and hydrophilic black pigments and solid fuel with elevated calorific value) and obtain their certifications. The certification process involves the data derived from laboratory experiments; however, it is acknowledged that certain indicators may vary in real industrial processes. Recycled carbon black (rCB) derived from waste tires holds significant potential as a raw material for various valuable products. To unlock this potential, it is imperative to devise straightforward, economically viable, and environmentally sustainable technologies that are capable of eliminating undesirable contaminants from the recycled carbon black powder. The composition of rCB differs from that of virgin carbon black due to the substantial presence of additives used in rubber production. The high impurity content in rCB precludes its direct reuse in the rubber industry, as these impurities significantly diminish the specific surface area of carbon black. Key additives include sulfur (for vulcanization), and ZnO, SiO\({}_{2}\), and TiO\({}_{2}\) (for enhancing rubber's mechanical properties). For instance, the addition of TiO\({}_{2}\), even in small concentrations, enhances the resistance of tire mixtures to solar UV radiation, O\({}_{3}\), and other oxidizers [7]. During the pyrolysis process, metal oxides react with sulfur, forming sulfides. Similarly, steel wire in tires reacts with sulfur, resulting in FeS. Optimal pyrolysis conditions lead to a solid residue with minimal organic impurities, while inadequate temperatures result in rCB with elevated bituminous substances and undesirable organic impurities, such as PAHs, causing particle agglomeration and reduced specific surface area [8]. The global management of waste tires has become a pressing issue, given the exponential increase in tire production each year.