Ash content is one of the most critical quality parameters that define the grade and application value of recovered carbon black (rCB). Unlike virgin carbon black, which typically has an ash content below 0.5 wt% due to refined fossil feedstocks and controlled furnace production, tire-derived rCB generally contains 10–20 wt% ash inherited from inorganic additives in tire formulations, including calcium carbonate, silica, zinc oxide, residual steel wires and mineral fillers. High ash content impairs the reinforcing efficiency, abrasion resistance and curing behavior of rCB, limiting its adoption in high-value rubber applications. Reducing ash content is therefore a core upgrading step to narrow the performance gap between rCB and medium-grade virgin carbon black.
Ash reduction in rCB relies on a layered technical system ranging from upstream feedstock control to physical separation and deep chemical demineralization. The optimal process route depends on the target ash specification, production scale and cost budget, with dry physical processing being the dominant industrial solution for large-scale commercial rCB production.
1. Upstream Control: Feedstock Selection & Pyrolysis Optimization
The most cost-effective ash reduction starts before pyrolysis, by controlling impurity input at the source.
Feedstock pre-selection
Ash content in finished rCB is directly determined by the inorganic composition of the feedstock tires. Truck tires typically have lower filler loading and lower ash content than passenger car tires. Producers can adjust the blend ratio of truck to passenger tire feedstock to set a baseline ash level. Industry data shows that a feedstock blend containing no less than 91 wt% truck tires can keep raw pyrolysis char ash content below 20 wt% without additional post-treatment upgrades.
Pre-processing steps such as debeading, shredding and preliminary magnetic separation remove steel bead wires and coarse metallic debris before pyrolysis, preventing large metal impurities from entering the char product and protecting downstream processing equipment.
Pyrolysis process tuning
Pyrolysis temperature directly affects the form of ash in raw char. Excessively high pyrolysis temperatures above 550 °C can cause inorganic fillers to melt, sinter and become embedded in the carbon matrix, making them much harder to remove in subsequent physical processing. Controlled moderate pyrolysis temperatures in the range of 400–500 °C preserve the discrete state of inorganic particles, enabling more efficient physical ash removal in downstream grinding and classification stages.
2. Dry Physical Separation: The Mainstream Industrial Ash Reduction Route
Physical purification processes remove free and loosely bound inorganic impurities without altering the chemical nature of rCB. They are environmentally friendly, low-cost and scalable, forming the standard processing backbone for commercial rCB production. As a leading provider of rCB grinding and classification systems, JACAN Powder Equipment integrates multiple physical purification stages into a continuous dry processing workflow for efficient ash reduction with reliable D90 < 10μm particle size control.
High-intensity magnetic separation
Magnetic separation is the first dedicated ash removal step in rCB processing, targeting iron-based impurities such as residual steel wires, fine iron filings and iron oxide particles originating from tire reinforcement cords.
JACAN’s optimized raw material pre-treatment line employs high-intensity magnetic separation adapted to various pyrolysis chars from tires, plastics and biomass. This stage removes residual steel wires and ferromagnetic impurities from the feedstock, not only protecting downstream grinding components but also eliminating the largest fraction of metallic ash. For tire-derived rCB, multi-stage magnetic separation can remove most iron-bearing inorganic solids, delivering a measurable reduction in total ash content.
Ultra-fine grinding and de-agglomeration
Raw pyrolysis char exists as hard, sintered granules in which inorganic ash particles are physically trapped inside carbon black agglomerates. Simply surface-cleaning these granules cannot remove encapsulated impurities.
Ultra-fine micron-level pulverization breaks apart sintered char agglomerates, releasing embedded inorganic particles and exposing them for subsequent separation. JACAN’s precision grinding process achieves D90 < 10μm fineness while preserving the native structural integrity of carbon black aggregates. By fully de-clumping the material, it liberates ash particles from the carbon matrix, which is a prerequisite for effective downstream classification-based ash removal.
High-precision aerodynamic classification
Air classification removes ash particles based on differences in particle size and density. Inorganic mineral fillers such as calcium carbonate and silica typically have higher density than carbon black aggregates. Under controlled centrifugal force and air velocity inside the classifier, denser oversized ash particles are separated and discharged as tailings, while purified carbon black fines pass through the classification wheel as finished product.
JACAN’s high-precision air classifiers deliver refined aerodynamic control to achieve an accurate cut-point, eliminating oversized mineral particles and narrowing particle size distribution simultaneously. This step not only ensures consistent fineness but also strips out a large proportion of non-magnetic inorganic ash, further elevating product purity.
Gravity and dense-medium separation
For higher ash removal requirements, gravity separation techniques can supplement air classification. Dense-medium separation uses suspensions of controlled density to float carbon black particles while sinking higher-density mineral ash. Research has demonstrated that zinc bromide heavy-medium separation, applied after acid leaching, can reduce rCB ash content from 10.58% to 7.54% through density-based secondary purification.
3. Chemical Demineralization: Deep Ash Removal for High-Grade rCB
Physical methods can effectively remove free and large-particle ash, but they cannot eliminate finely dispersed inorganic phases embedded in the carbon structure. For high-end applications requiring ash content below 5% or even below 3%, chemical demineralization is required to dissolve and remove bound inorganic impurities.
Acid leaching
Acid washing is the most widely used chemical demineralization method. Acid solutions dissolve carbonate, oxide and salt-based ash components such as calcium carbonate, zinc oxide and iron compounds.
- Hydrochloric acid (HCl) is the most common reagent. Under typical conditions of 4 M HCl at 60 °C for 1 hour, ash content can be reduced from 15.0 wt% to 4.9 wt%.
- For silica-bearing ash, hydrofluoric acid (HF) may be added to dissolve silicate minerals, though it requires strict safety and wastewater treatment protocols.
Acid-alkali combined treatment
For deeper ash removal, sequential acid and alkali treatment delivers superior results. After acid washing removes metallic impurities, sodium hydroxide (NaOH) treatment dissolves silica and acidic silicate minerals. Optimized dual treatment with 6 M HCl followed by 2 M NaOH at 70 °C for 60 minutes has been shown to reduce rCB ash content from 17.15% to as low as 2.25%, approaching the purity level of virgin carbon black.
Hydrothermal pre-washing
Milder hydrothermal washing with deionized water can remove soluble inorganic salts from rCB surfaces without chemical reagents. It serves as a low-impact pre-cleaning step that reduces the load on subsequent acid leaching and lowers reagent consumption.
4. Key Practical Considerations
When designing an ash reduction process, several trade-offs must be carefully balanced to ensure both product quality and production sustainability.
Balance between ash reduction and structural performance
Excessive grinding or overly aggressive chemical treatment can damage the primary aggregate structure of carbon black, reducing DBP absorption value and impairing reinforcing performance. The optimal process removes ash while preserving the native structural integrity of rCB. JACAN’s multi-parameter intelligent optimization system allows real-time adjustment of frequency, feed rate and air velocity to fine-tune purification intensity and maintain the desired balance between ash level and material properties.
Cost and environmental footprint
Dry physical processing has significantly lower operating costs and environmental impact than chemical treatment, making it the preferred choice for bulk commodity-grade rCB. Chemical demineralization achieves deeper ash reduction but adds reagent costs, wastewater treatment requirements and energy input. It is economically justified only for high-value, low-ash rCB grades targeting premium rubber and specialty applications.
Batch-to-batch consistency
Feedstock composition variation is the primary cause of ash content fluctuation in rCB production. Combining feedstock pre-selection with closed-loop intelligent process control ensures stable finished product ash levels even when raw pyrolysis char properties vary.
Reducing ash content is a fundamental step in upgrading pyrolysis char into commercially valuable recovered carbon black. For large-scale industrial production, the integrated dry process route — magnetic separation, ultra-fine grinding and high-precision air classification — represents the most balanced and sustainable solution, delivering effective ash reduction with preserved carbon black structure and performance. For specialized high-grade applications, chemical demineralization can further reduce ash to virgin-like levels.
As a trusted partner to over 35% of the world’s top-tier rCB processors, JACAN Powder Equipment’s end-to-end dry processing systems enable producers to achieve consistent, low-ash, application-grade rCB with D90 < 10μm particle size control, maximizing both product market value and production return on investment.
