Sulfur is an inherent trace component in recovered carbon black (rCB), inherited primarily from the sulfur-based vulcanization system used in original tire rubber formulations. Its content directly influences vulcanization kinetics, crosslink density and long-term aging performance in downstream rubber compounds, making precise measurement and consistent control critical for industrial-grade rCB quality. The global carbon black industry has established standardized testing methods for sulfur quantification, and rCB producers deploy a multi-stage control strategy spanning feedstock selection, pyrolysis optimization and post-refining purification to manage sulfur levels.
Standardized Measurement Methods for rCB Sulfur Content
Total sulfur content in rCB is tested following mature carbon black industry standards, with ASTM D1619 and ISO 1138:2022 being the most widely adopted specifications globally. Two analytical methods dominate industrial laboratory and quality control practice, each suited for different precision and throughput requirements.
1. High-Temperature Combustion with Infrared Absorption Detection
This is the reference and preferred method for formal quality certification and regulatory reporting, designated as Method A under ASTM D1619 and Method C under ISO 1138.
Working Principle & Procedure
A precisely weighed, homogenized rCB sample is fed into a high-temperature combustion furnace operating at 1350–1450°C in a pure oxygen atmosphere. All sulfur-bearing compounds in the sample are fully oxidized and converted to sulfur dioxide (SO₂) gas. The gas stream is then passed through an infrared detection cell, where the characteristic absorption of SO₂ at a specific infrared wavelength is measured quantitatively. The instrument calculates total sulfur content automatically based on the calibrated absorption intensity.
Key Advantages
- High accuracy and repeatability, suitable for formal product certification and third-party testing
- Covers the full 0.1–5 wt% sulfur range typical for commercial rCB grades
- Widely accepted across global supply chains for compliance and material balance calculations
2. X-Ray Fluorescence (XRF) Spectrometry
Designated as Method B under ASTM D1619, XRF is a rapid, non-destructive technique primarily used for in-process quality control and fast screening.
Working Principle & Procedure
The rCB sample is irradiated with a high-energy X-ray beam, which excites inner-shell electrons in sulfur atoms. As electrons return to ground state, they emit fluorescent X-rays with energies characteristic of sulfur. The intensity of this characteristic fluorescence is proportional to the mass fraction of sulfur in the sample, allowing quantitative calculation after calibration with certified reference materials.
Key Advantages
- Minimal sample preparation, with testing completed in minutes
- Non-destructive, enabling repeated testing of the same sample
- Ideal for at-line monitoring during production to catch content deviations early
Classical wet chemistry methods such as oxygen bomb combustion titration remain technically valid per older standard editions, but they are rarely used in modern production due to lower throughput and higher operational risk.
Origins and Typical Range of Sulfur in rCB
Before discussing control strategies, it is important to understand where sulfur resides in rCB. During tire pyrolysis, the elemental sulfur and sulfide accelerators from the original rubber vulcanization system do not fully volatilize. Approximately 60–70% of feedstock sulfur remains in the solid char, existing in two main forms:
- Inorganic sulfides: Sulfur bound to metal additives, primarily zinc sulfide (ZnS) and iron sulfides, formed by reaction between sulfur and zinc oxide / steel wire residues during pyrolysis.
- Organic / matrix-bound sulfur: Sulfur chemically bonded within the carbon black structure and residual carbonaceous deposits.
Commercial tire-derived rCB typically has a total sulfur content in the range of 1.2–3.6 wt%, which is noticeably higher than the 0.1–0.5 wt% typical of virgin furnace carbon black. This elevated sulfur level is one of the inherent compositional differences between circular rCB and synthetic virgin carbon black.
Strategies for Sulfur Content Control in rCB Production
Sulfur control in rCB is a systematic process implemented across three production stages, with different technologies delivering different levels of control depth.
1. Feedstock Pre-Selection and Source Stabilization
The most cost-effective sulfur control starts at the feedstock stage.
- Tire source classification: Tread compounds contain higher sulfur loading than sidewall and inner liner compounds. Truck and bus radial tires also have different sulfur profiles than passenger car tires. By sorting and batching feedstock by tire type and component, producers can narrow incoming sulfur variation before pyrolysis even begins.
- Impurity pre-removal: Pre-shredding and magnetic separation upstream of pyrolysis removes free steel wire, reducing the formation of iron sulfides during thermal processing.
2. Pyrolysis Process Optimization
Thermal process parameters influence how sulfur partitions between gas, oil and solid phases.
- Temperature control: Moderately elevated pyrolysis temperatures promote decomposition of labile organic sulfur compounds, releasing them as H₂S and COS into the gas phase and reducing residual sulfur in the solid char. However, sulfur bound as stable metal sulfides remains largely unaffected even at higher temperatures.
- In-situ sorbent addition: Calcium oxide, sodium carbonate or other alkaline sorbents can be introduced during pyrolysis to capture gaseous sulfur and fix it into disposable ash fractions. This approach primarily reduces sulfur in pyrolysis oil and off-gas, with secondary benefits for solid-phase sulfur when combined with downstream de-ashing.
3. Post-Refining Purification
Refining is where producers achieve the most consistent and targeted sulfur control. Technologies fall into two tiers.
Mechanical Refining: The Standard Industrial Approach
For mainstream commercial rCB, mechanical refining is the dominant method for stabilizing and moderately reducing sulfur content. This is the core processing route delivered by JACAN Powder Equipment’s integrated rCB grinding and classification systems:
- High-intensity magnetic separation removes iron-based sulfide particles and residual steel-derived sulfide agglomerates, eliminating the most abrasive and variable sulfur-bearing inorganic phases.
- Ultra-fine grinding breaks apart carbon-ash composite agglomerates, liberating encapsulated ZnS and other sulfide mineral grains from the carbon matrix.
- High-precision aerodynamic classification separates denser sulfide mineral particles from lighter carbon particles based on density and size differences, reducing total sulfur content alongside general ash reduction.
- Multi-parameter intelligent optimization maintains consistent refining intensity across variable feedstocks, ensuring stable batch-to-batch sulfur content rather than just lowering average levels.
Mechanical refining cannot eliminate sulfur that is chemically bound within the carbon matrix, but it reliably removes free and encapsulated sulfide impurities and tightens sulfur content variation, which is the primary requirement for most downstream rubber compounding applications.
Chemical & Thermal Deep Desulfurization: For High-Purity Specialty Grades
For applications with strict low-sulfur requirements, additional deep purification technologies are available, though at higher processing cost:
- Acid leaching: Hydrochloric acid, nitric acid or hypochlorite solutions dissolve metal sulfides and convert sulfur into soluble sulfate ions, which are then removed via filtration and washing. This can reduce sulfur content to below 1 wt% but generates wastewater that requires treatment.
- Molten salt treatment: Alkaline molten salt systems react with both inorganic sulfides and organic-bound sulfur at elevated temperatures, converting them into soluble salts for removal. Advanced formulations can achieve over 90% sulfur removal for high-purity rCB grades.
- Thermal upgrading: High-temperature heat treatment under inert or reducing atmosphere decomposes unstable sulfur compounds and drives off residual volatile sulfur species, while also lowering volatile matter content.
These deep desulfurization processes are reserved for specialty high-grade rCB, as they add significant cost and environmental processing overhead compared to standard mechanical refining.
Sulfur content in rCB is measured using well-standardized methods — high-temperature combustion infrared detection for formal certification and XRF for rapid in-process control — aligned with global carbon black testing norms.
Control of sulfur content is implemented across the full production chain: feedstock sorting reduces variation at the source, pyrolysis optimization adjusts sulfur partitioning, and post-refining delivers the final stable product specification. For mainstream industrial rCB, precision mechanical refining from suppliers like JACAN Powder Equipment provides the most practical and cost-effective solution, delivering consistent, specification-grade sulfur content through magnetic separation, size liberation and aerodynamic classification. For specialty low-sulfur applications, chemical and thermal deep desulfurization technologies are available to achieve higher purity at additional cost.
