Plastic Sorting Technologies: Methods, Performance, and Applications

1.1 Shredding/Granulation

Before any sorting, large plastic items (e.g., bottles, drums, automotive parts) are size‑reduced into flakes or granules (5–50 mm).

• Equipment: Single‑shaft or twin‑shaft shredders (7.5–75 kW motors) and granulators (15–132 kW).

• Benefits: Uniform feed size for downstream processes; improved sensor accuracy.

• Performance: A mid‑scale shredder with a 30 kW motor can process 1.2 t/h of PET bottles, achieving an average particle size of 20 mm.

1.2 Screening (Trommel & Vibratory)

Shredded material passes over perforated screens or trommel drums to remove fines (<10 mm) and classify by size.

• Trommels: 2–4 m diameter rotating drums, capacity 5–12 t/h.

• Vibratory Screens: Multi‑deck shaken decks, capacity 3–10 t/h.

• Outcome: Fines (dust, small contaminants) diverted to wash; oversize returned for re‑shredding.

2. Density‑Based Methods

2.1 Float‑Sink Tanks

Differentiates plastics by density in water or salt solutions:

• PE/PP (0.91–0.96 g/cm³) float

• PET/PVC (1.30–1.45 g/cm³) sink

• Setup: Single‑stage tank handles 3–8 t/h; multi‑stage lines isolate up to three polymer types.

• Example Data: A 5 t/h float‑sink line at a European MRF achieved 96.4% PE/PP recovery and 97.8% PET purity in a 2023 pilot.

2.2 Air Classification

High‑velocity air streams remove light films and foams from heavier flakes:

• Performance: 80–95% removal of floatable films at 4 t/h.

• Integration: Commonly installed upstream of optical sorters to reduce sensor load and air‑jet costs.

3. Sensor‑Based Sorting

3.1 Near‑Infrared (NIR) Spectroscopy

NIR sorters detect polymer types via their unique absorption spectra (1,000–2,500 nm range). Flakes on a fast conveyor (up to 3 m/s) are scanned and ejected by pneumatic jets.

• Throughput: 1.5–4 t/h per unit.

• Accuracy: 97–99% purity for PET, HDPE, PP lines.

• Case Study: A North American plant reduced PVC contamination in PET stream from 2.3% to 0.4% using dual‑head NIR, boosting food‑grade yield by 8%.

3.2 Optical Color and Shape Sorting

High‑resolution cameras (RGB/UV‑VIS) distinguish plastics by color, label presence, or shape:

• Color Sorting: Separates clear, green, and blue PET bottles with >98% accuracy.

• Shape Recognition: Rejects caps, labels, and multi‑layer sachets based on contour analysis.

• Machine Learning: Advanced systems retrain on‑the‑fly, improving recognition of new contaminants by 3–5%.

4. Electrostatic and Triboelectric Separation

4.1 Electrostatic Separators

Flakes pass through a corona discharge, acquiring charges based on surface resistivity. In an electric field:

• Positive/Negative flakes deflect to different electrodes.

• Capacity: 0.5–3 t/h.

• Application: PVC removal from PET/PE blends; post‑float purification.

4.2 Triboelectric Methods

Friction‑based charging in custom liners induces differential charges on PET vs. PE/PP flakes. Less energy‑intensive than corona units, suitable for lines targeting moderate purity gains (1–2%).

5. Mechanical/Ballistic Classification

5.1 Ballistic Separators

Oscillating paddles or inclined trays classify materials by shape and momentum:

1. Film‑Like flutter away.

2. Granule‑Like roll forward.

3. Block‑Like slide.

• Capacity: 2–6 t/h.

• Use Case: Film vs. rigid plastic separation ahead of NIR sorting to maintain 98% optical purity.

5.2 Magnetic & Eddy‑Current

Removes metals from mixed waste streams:

• Magnetic Rollers: Extract ferrous metals.

• Eddy‑Current Separators: Deflect non‑ferrous metals (aluminum, copper).

• Impact: Protects plastic sensors and downstream equipment.

6. Integrated Sorting Line Example

1. Feed Preparation: Manual pre‑sort → Shredder (30 kW) → Trommel (3 m, 6 t/h).

2. Wash & Dry: Friction washer → Hot caustic bath (65 °C, 1 kg NaOH/m³) → Centrifugal dryer to <1% moisture.

3. Primary Separation: Float‑sink (5 t/h) → Air classifier (4 t/h).

4. Optical Sorting: Dual‑head NIR (3.5 t/h) → Color/shape sorter (2 t/h).

5. Final Purification: Electrostatic unit (1.5 t/h) → PET purity 99.2%, PP purity 98.7%.

Key Metrics: Overall recovery 91%, energy use 35 kWh/t, uptime 94%.

7. Emerging Trends

• Hyperspectral Imaging: Extends beyond NIR (400–2,500 nm) to detect additives and multi‑layer films, improving purity by 2–3%.

• AI‑Enhanced Vision: Deep learning models recognize complex shapes and contamination patterns, boosting recovery by 4% without hardware changes.

• Raman Spectroscopy: Prototype units identifying PVC down to 0.1% contamination in high‑value PET lines.

8. Best Practices

1. Regular Calibration: Weekly NIR calibration with certified polymer standards maintains >97% accuracy.

2. Blade & Screen Maintenance: Replace shredder knives every 1,500 h; clean screens daily to prevent blinding.

3. Data‑Driven Optimization: IIoT dashboards track reject rates, energy use, and throughput for continuous line tuning.

4. Operator Training: Cross‑training reduces mis‑sorts by up to 15% and improves safety compliance.

Conclusion
Plastic sorting technologies—from coarse mechanical separation to advanced spectroscopy—form the backbone of a circular plastics economy. By combining size reduction, density and air separation, sensor‑based sorting, and electrostatic methods into an integrated line, facilities can achieve high throughput, exceptional purity, and strong economic returns. As AI, hyperspectral imaging, and hybrid approaches mature, next‑generation sorting lines will unlock even greater value from post‑consumer and industrial plastic waste.

Contact us to discuss customized sorting solutions for your recycling operations.