Separation and Resource Recovery Pathways for Steel Wire in Intermission Waste Tire Pyrolysis Plant

In the field of waste tire pyrolysis, batch pyrolysis equipment continues to hold a significant market share due to its operational flexibility and lower investment threshold. Unlike continuous pyrolysis systems, batch equipment requires cooling, slag discharge, and reloading phases after each processing cycle; consequently, the separation and processing of steel wire from the solid products present unique technical challenges. Although the steel wire—which serves as the tire's structural framework—does not undergo chemical cracking during pyrolysis, it is discharged mixed with carbon black in the form of tangled, knotted clumps. Efficiently separating this wire and realizing its resource recovery value is a critical aspect of designing batch pyrolysis production lines.

Morphological Changes of Steel Wire During Pyrolysis

Once waste tires enter the pyrolysis reactor, they are heated to a specific temperature range in an oxygen-deficient or low-oxygen environment; the rubber components decompose into oil, gas, and carbon black, while the steel wire framework retains its metallic form. However, during the operation of rotary pyrolysis reactors (typically running under slight negative pressure), the steel wire does not remain in a neat, linear state. Due to the dense structure of the steel wire beads (the part of the tire that mounts to the rim) and the tumbling motion of the material during pyrolysis, the discharged wire often becomes entangled into irregular clumps, with carbon black powder adhering tightly to the surface. This entanglement and clumping phenomenon is particularly pronounced in batch equipment, as each batch undergoes a complete heating, holding, and cooling cycle, giving the steel wire ample time to interlock while the material is static or rolling at low speeds.

Key Stages of the Separation Process

The mixture of carbon black and steel wire discharged from the batch pyrolysis reactor first enters a crushing unit. This unit typically consists of fork-like gears, intermediate crushing blades, and rotary crushing blades; it employs compression and shearing actions to break apart the wire clumps and create fissures in the adhering carbon black layer. The crushed material then enters a spiral wire-disentangling device, which serves as the critical piece of equipment for resolving the entanglement issue. The unit is equipped with internal spiral blades designed to unravel and separate the steel wire bundles; during transport, these blades exert continuous tearing and separating forces on the wire clumps. Simultaneously, a nitrogen injection port maintains an inert atmosphere to prevent the carbon black from oxidizing or spontaneously igniting at high temperatures.

The separated material then enters a spiral screening and separation unit. The core of this unit is a screw conveyor structure fitted with a screen—typically featuring a mesh size of approximately 6 mm—which allows carbon black and fine particles to pass through while retaining coarser steel wires on the screen surface for discharge via a dedicated outlet. The undersized material proceeds to a magnetic separator, where the ferromagnetic properties of the steel wire are utilized to completely separate it from the carbon black. Following magnetic separation, the carbon black meets purity standards for downstream processing, while the steel wire is sent to a baler for compaction.

Resource Recovery Pathways for Steel Wire

The steel wire recovered from pyrolysis consists primarily of high-quality carbon steel, with only minimal carbon black residue adhering to the surface. Once baled, the steel wire blocks can be sold directly to steel manufacturers as raw material; their recovery value constitutes a significant factor in the economic assessment of waste tire pyrolysis projects. For instance, in a project processing 27,000 tons of waste tires annually, the yearly output of steel wire can reach approximately 3,100 tons. Unlike steel wire generated during standard tire shredding, this wire is free of rubber coatings; its superior cleanliness ensures that no additional flue gas treatment burden is imposed during the steelmaking process.

From a policy perspective, the Department of Energy Conservation and Comprehensive Utilization under the Ministry of Industry and Information Technology (MIIT) recently emphasized at a symposium on waste tire utilization that efforts to "promote the application of advanced technologies and continuously improve the level of comprehensive waste tire utilization" would be intensified. As the final stage in the pyrolysis product processing chain, the technical maturity of the steel wire separation process directly impacts the continuous operational capability and economic efficiency of the entire production line. For batch-type pyrolysis equipment, the steel wire separation system's ability to withstand impact loads and resist wear is particularly critical, given the temperature fluctuations and intermittent discharge inherent to batch operations.

Equipment Maintenance and Safety Considerations

The steel wire processing unit in batch-type pyrolysis equipment faces two prominent maintenance challenges. The first is the wear of the spiral blades used for unraveling the wire. Due to the high hardness and irregular shape of the steel wire, the blades wear out rapidly during the continuous tearing process, necessitating regular inspection and replacement. Another key factor is the stability of the cooling system. Since the material discharged from the pyrolysis furnace is at a high temperature, both the steel wire separation unit and the screening unit require cooling jackets; circulating water is used to lower the temperatures of the equipment casings and the material, thereby protecting equipment seals and preventing the re-ignition of carbon black at high temperatures.

Regarding safety design, nitrogen blanketing is a crucial measure to prevent carbon black oxidation and dust explosions. Carbon black dust poses an explosion risk when it reaches a certain concentration in an enclosed space; nitrogen injection allows the oxygen content within the system to be maintained below safe thresholds. Additionally, dust collection systems should be installed in the steel wire baling area to prevent carbon black powder from dispersing and causing secondary pollution during the baling process.

Overall, the technical approach for processing steel wire in batch-type pyrolysis equipment has established a complete closed-loop process, ranging from crushing and separation to screening, magnetic separation, and baling for recovery. As industry standards are refined and advanced technologies and equipment are more widely adopted, there remains room for further improvement in the automation level and operational stability of the steel wire separation stage.