E-Waste Plastics: Density Separation & Flame-Retardant Removal

Electronic waste (e-waste) is one of the fastest-growing waste streams in India. E-waste plastics (primarily ABS, HIPS, Polycarbonate blends) represent about 30% of this weight. However, recycling e-waste plastics is difficult because they contain restricted brominated flame retardants (BFRs) and heavy metals. To recycle these materials, processing lines must separate polymer types and extract restricted substances. Understanding sink-float sorting, electrostatic separation, and RoHS screening is essential for e-waste recycling engineers.

• Map the composition of plastics in electronic waste (ABS, HIPS, PC/ABS).
• Explain the physics of density-based (sink-float) and electrostatic separation.
• Analyze the challenge of identifying and separating plastics containing restricted Brominated Flame Retardants (BFRs).
• Solve density-media sizing and polymer recovery yield calculations.
• Reference international e-waste recycling standards.

3.1 E-Waste Plastics Composition

Appliance and computer housings contain structural amorphous polymers:

• ABS (Acrylonitrile Butadiene Styrene): Density $\approx 1.04 - 1.07$ g/cm³.
• HIPS (High Impact Polystyrene): Density $\approx 1.03 - 1.05$ g/cm³.
• PC/ABS Blends: Density $\approx 1.12 - 1.15$ g/cm³.

3.2 Density Separation (Sink-Float)

Because polymer densities overlap, simple water flotation ($density = 1.00$ g/cm³) cannot separate ABS and HIPS from other materials. Recycling lines use salt-water solutions (media) of calibrated density:

• Water ($1.00$ g/cm³): PP/PE float; ABS/HIPS/PC sink.
• Salt/water medium ($1.08$ g/cm³): ABS and HIPS float (separated from sinking PC, PC/ABS, and PVC).

3.3 BFR Restrictions and Extraction

Many older e-waste plastics contain restricted flame retardants like OctaBDE or DecaBDE. Under RoHS, these must be kept $< 1000$ ppm. Recycling lines utilize sliding-spark spectroscopy or XRF scanners to sort out and incinerate BFR-containing plastics, reclaiming only HFFR plastic fractions.

Problem: An e-waste recycling line in Bengaluru processes a batch of shredded housing flakes containing HIPS (density $\rho_1 = 1.04$ g/cm³) and PC/ABS (density $\rho_2 = 1.14$ g/cm³). The operator prepares a water-salt solution to separate them. Calculate:

1. The required density ($\rho_m$) of the separation medium to achieve separation.
2. The mass of Sodium Chloride ($NaCl$) required to prepare $2000$ Liters of this medium (Assume density of water $\rho_w = 1.00$ kg/L, and volume does not change significantly on salt addition).
3. Identify which polymer floats and which sinks.

Solution:

1. Settle on the medium density $\rho_m$ as the average of the two polymer densities:

1. Calculate the required mass of $NaCl$ to prepare $2000$ Liters of the solution at 1.09 kg/L:

• Target mass of 2000L solution = $2000 \text{ L} \times 1.09 \text{ kg/L} = 2180 \text{ kg}$
• Mass of water (2000 L) = $2000 \text{ kg}$
• Required mass of $NaCl$ = $2180 - 2000 = \mathbf{180.0 kg}$

1. Floaters and Sinkers:

• HIPS ($\rho_1 = 1.04$ g/cm³) is less dense than the medium ($1.09$ g/cm³) and will float.
• PC/ABS ($\rho_2 = 1.14$ g/cm³) is denser than the medium ($1.09$ g/cm³) and will sink.

Interpretation: Dissolving 180 kg of $NaCl$ in 2000L of water yields a medium of density 1.09 g/cm³, separating HIPS (floats) from PC/ABS (sinks) with high purity.

Indian e-waste recyclers operate sorting facilities in hubs like Bengaluru and NCR. They run sink-float tanks and electrostatic separators to recover ABS/HIPS flakes. The recovered flakes are compounded with impact modifiers and sold to commercial packaging developers.

• Sink-Float: Density-separation method using fluid media to sort polymer flakes.
• BFR: Brominated Flame Retardants; restricted organic flame retardants found in older electronics.
• Electrostatic Separation: Sorting method separating polymers based on triboelectric charging characteristics.
• PC/ABS: High-impact engineering alloy common in laptop and monitor housings.
• XRF: X-Ray Fluorescence; analyzer used to detect bromine in e-waste flakes.

1. IS 14534 — Guidelines for recycling of plastics (BIS India standard)
2. European WEEE Directive guidelines for treatment of e-waste plastics

1. Explain the principle of triboelectric electrostatic separation of ABS and HIPS flakes. Focus on charge exchange mechanisms.
2. Why are brominated flame retardants restricted under RoHS? Discuss dioxin formation during crude recycling extrusion.
3. A batch of e-waste shred contains $5\%$ PVC contamination. Design a density-separation flow diagram to remove PVC (density $\approx 1.35$ g/cm³) and recover ABS.