Closed-loop upgrading of pyrolytic carbon black from waste tires through integrated acid leaching and gravity...

Pyrolytic carbon black (CBp) from waste tires contains high ash content due to impurities introduced during pyrolysis and tire manufacturing, limiting its commercial use and making purification essential for resource recovery. In this work, hydrobromic acid (HBr) leaching was employed to remove metal impurities from CBp, and the influence of key operational factors on ash removal was systematically investigated. The experimental results demonstrate that under the optimal conditions with leaching temperature of 80°C, HBr dosage of 2 mol/L, leaching time of 30 min, and solid-to-liquid ratio of 100 g/L, the ash content in the clean CBp can be reduced from its original 20.46% to 10.58%, with a corresponding yield of 87.63%. In particular, the leaching results indicate that HBr can effectively dissolve zinc and calcium salts impurities from the CBp within an extremely short time frame of less than 10 min. The by-product ZnBr₂ can be formulated into a heavy medium and used to prepare an inorganic ZnBr₂-based heavy liquid for re-separation of the leached CBp. This closed-loop process enhances further purification and upgrading of the clean CBp. Under a dense medium density of 1.8 g/mL, the ash content of the leached CBp can be reduced from 10.58% to 7.54%, indicating that the gravity process effectively removes silica impurities from the CBp. This study establishes a closed-loop process by repurposing ZnBr₂ leachate as a heavy medium for gravity separation after HBr leaching. This approach enhances resource efficiency and enables high-value utilization of CBp-based products.