What is the magnetic separation process for rCB purification

Magnetic separation for rCB purification is a physical separation step that removes ferromagnetic iron and iron oxide contaminants from raw pyrolysis char. It works alongside pre-pyrolysis tire magnetic separation, but post-pyrolysis magnetic polishing targets fine iron particles formed when residual tire steel fragments oxidize inside the hot reactor. Reducing iron content protects downstream milling equipment, prevents oxidative ageing in rubber compounds, and enables tire-grade rCB production.

Core principle

Raw pyrolysis char is non-magnetic. Iron, steel fragments and iron oxides are ferromagnetic or paramagnetic. When char flows through a high-intensity magnetic field, iron-bearing particles are captured by magnetic elements, while rCB char continues through as the non-magnetic product stream.

Magnetic separation is only effective for iron-based contaminants. It cannot remove ash components such as ZnO, calcium carbonate, silicates or residual carbon coke; those require air classification or chemical demineralization.

Two stages of magnetic separation in the full rCB line

Stage 1: Pre-pyrolysis magnetic separation (tire pre-treatment)

Shredded rubber chips pass over drum magnets to remove large steel cords and bead wires before entering the pyrolysis reactor. This removes most bulk steel, but fine steel filaments trapped inside rubber chips often survive and enter the reactor. These micro steel particles oxidize at pyrolysis temperature and become embedded inside raw char, which creates the need for post-pyrolysis magnetic polishing.

Stage 2: Post-pyrolysis magnetic separation (char purification, the main rCB polishing step)

This operates on cooled raw pyrolysis char, before or after coarse crushing.

1. Char cooling: Hot char from the reactor must first be cooled to safe temperatures. Hot char carries fire and dust explosion risks inside magnetic and milling systems.
2. Feeding into magnetic separator: Char is fed in a controlled, thin, uniform layer via vibratory feeder or screw conveyor. Thin material layers ensure iron particles are exposed to the magnetic field and not buried under char.
3. Magnetic capture:
   • Drum magnetic separators: For larger steel fragments. Rotating drum contains permanent rare-earth magnets. Iron sticks to the drum surface, rotates out of the magnetic zone, and drops into a separate metal collection hopper.
   • High-gradient electromagnetic separators / drawer-type magnetic grids: For ultra-fine iron oxide fines (1–50 μm). These use high-strength magnetic matrices, ideal for polishing powder streams before fine grinding. Field strength typically ranges from 8000–19500 Gauss for fine iron removal.
4. Stream splitting: Two output streams are generated:
   • Non-magnetic stream: purified raw char, sent to grinding and air classification.
   • Magnetic reject stream: iron oxides and steel fines, collected as metal waste.
5. Recirculation: Low-efficiency fractions may be re-fed back into the magnetic separator for a second pass to lower residual iron.

Process sequence integration in rCB production

Reactor discharge → char cooling → coarse crushing → multi-stage magnetic separation → grinding → air classification → finished rCB

Key operating parameters affecting separation efficiency

1. Magnetic field strength: Higher field strength captures finer iron oxide particles. Standard drum magnets remove large steel; high-gradient electromagnetic separators target micro iron fines required for premium rCB.
2. Material feed rate and bed thickness: Overfeeding buries iron particles inside char and reduces capture efficiency.
3. Char particle size: Fine ground char exposes more embedded iron oxide, improving magnetic polishing after milling.
4. Moisture: Wet char causes agglomeration. Iron particles stick to char clumps and escape magnetic capture.

Impacts of residual iron if magnetic separation is insufficient

• Iron oxides act as pro-oxidants in rubber compounds, accelerating thermal ageing and reducing service life, which disqualifies rCB for tire applications.
• Hard iron fines cause severe abrasive wear on grinding rotors, classifier wheels and cyclone components.
• Metal particles create ignition sources inside powder handling systems, raising dust explosion risk.
• Iron speckles create colour defects in rubber and plastic end products.

Limitations of magnetic separation

• Cannot eliminate non-ferrous inorganic ash (ZnO, silicates, CaCO₃).
• Cannot remove coke or tar deposits on rCB agglomerates.
• Fully submicron iron oxide particles trapped inside char agglomerates are extremely hard to capture; multi-pass magnetic treatment can only reduce, not completely eliminate trace iron.

Magnetic separation for rCB purification is a physical iron-removal process deployed both before pyrolysis (bulk steel removal from tire chips) and after pyrolysis (polishing raw char to remove fine iron oxide). Post-pyrolysis high-gradient magnetic separators are critical to lower iron content to meet tire-grade rCB specifications. While it is highly effective for ferromagnetic contaminants, magnetic separation must be combined with grinding, air classification and optional ash reduction steps to achieve full rCB purification.