NIR (NIR)

NIR stands for near-infrared spectroscopy, a sensor technology that identifies plastic polymer types by directing a beam of near-infrared light (wavelength 700-2,500 nanometres) onto a material surface and analysing the reflected spectrum. Different polymers absorb and reflect NIR wavelengths in characteristic patterns — a unique 'spectral fingerprint' — that the sensor's onboard processor matches against a reference library in real time, typically within 10-30 milliseconds per fragment.

Why NIR is the industry standard: NIR sensors can distinguish all the major commodity thermoplastics — PET, HDPE, LDPE, PP, PS, ABS, PC, PVC, and PA — at conveyor belt speeds of 2-4 metres per second, processing 5-15 tonnes per hour per metre of belt width. The sensor is non-contact, so it does not damage or contaminate the feed. NIR is fast, robust to dust, and unaffected by colour, making it dramatically superior to visual sorting. The technology has been the default polymer-identification step in European recycling plants since the late 1990s and is now standard in formal Indian plastic and e-waste recycling lines.

Operational details: A typical industrial NIR sorting line consists of a feed belt (delivering fragments in a single layer to ensure each fragment is individually scanned), the NIR scan head mounted above the belt, an onboard processor running the spectral matching algorithm, and a row of high-speed air-ejection nozzles positioned at the belt discharge that fire compressed-air bursts to deflect target fragments off the belt trajectory into a separate collection chute. Modern NIR sorters reach 90-95% purity in target fraction and 85-92% recovery yield. Capital cost runs Rs 80 lakh-3 crore for a 2-metre-wide industrial unit.

Limitations and trade-offs: NIR cannot identify black plastics reliably because the carbon black pigment absorbs across the entire NIR spectrum, suppressing the polymer fingerprint. Black plastics from e-waste (TV and monitor casings, computer chassis) therefore require an alternative — typically X-ray fluorescence (XRF) to detect chlorine in PVC or bromine in flame-retarded plastics, supplemented by density separation for the remaining black fraction. Multi-layer films and heavily contaminated fragments also produce ambiguous spectra. NIR works best on rigid, uncontaminated, single-layer polymer fragments larger than 5-10 mm.