Paper Recycling
Collection
Fig. 1 — Paper collection process flow
Paper recycling begins at the point of source separation, where consumers and businesses sort paper products from general waste. Collection occurs through two primary mechanisms: curbside collection programs and drop-off centers.
Municipal Recycling Facilities (MRFs) receive mixed recyclables and employ a series of mechanical and manual processes to separate paper grades. The quality of input material directly determines downstream processing efficiency — contamination rates above 15% typically render paper bales uneconomical for recycling mills.
Note:
Wet paper and food-soiled paper containers (such as pizza boxes) cannot enter the paper recycling stream due to fiber degradation from moisture and oil contamination. These should be composted rather than recycled.
Table 1: Paper Grade Recovery Rates by Collection Method
| Collection Method |
Recovery Rate |
Contamination % |
Cost/Tonne |
| Curbside (single-stream) |
78% |
8–12% |
$145–180 |
| Curbside (dual-stream) |
85% |
3–5% |
$120–155 |
| Drop-off centers |
91% |
1–3% |
$95–120 |
Sorting
Fig. 2 — Paper sorting into commodity grades
Paper sorting separates incoming material into commodity grades that meet specific quality standards for paper mills. The primary grades include Old Corrugated Containers (OCC), Old Newspapers (ONP), Sorted Office Paper (SOP), and Mixed Paper (MXP).
Modern MRFs employ a cascade of separation technologies: disk screens physically separate flat from three-dimensional items; optical sorters use near-infrared (NIR) spectroscopy to identify paper sub-types; and air classifiers separate lightweight paper from heavier materials.
Processing
Fig. 3 — Pulping and deinking process chain
At the paper mill, sorted paper enters a hydropulper — a large vessel filled with water where mechanical agitation breaks the paper down into individual cellulose fibers, creating a slurry called stock. The water-to-fiber ratio typically ranges from 4:1 to 8:1 by weight.
For grades destined to produce newsprint or writing paper, deinking is necessary. Flotation deinking introduces air bubbles and surfactants into the stock; ink particles attach to bubble surfaces and float to the top as a froth, which is then removed. The deinking yield typically achieves 85–92% fiber recovery.
Warning:
Fiber shortening occurs with each recycling cycle. Cellulose fibers lose approximately 10–15% of their length per cycle. After 5–7 recycling cycles, paper fibers are too short for further use and must be replaced with virgin fiber or diverted to composting.
Output Products
Fig. 4 — Paper fiber output product streams
Recovered paper fiber enters three main product streams based on fiber quality. Long-fiber grades (from office paper and corrugated) are suitable for packaging board and newsprint. Short-fiber grades (from mixed paper) produce tissue products, egg cartons, and molded fiber packaging. Heavily contaminated fiber may be used in roofing felt or insulation products.
Table 2: Paper Output Products by Input Grade
| Input Grade |
Fiber Length |
Primary Output |
Market Value |
| OCC (Corrugated) |
2.0–3.5 mm |
Containerboard, linerboard |
$85–130/t |
| ONP (Newsprint) |
1.2–2.0 mm |
Newsprint, tissue |
$40–70/t |
| SOP (Office Paper) |
1.8–2.8 mm |
Printing papers, tissue |
$110–175/t |
Plastic Recycling
Resin Types
Fig. 5 — Seven standard resin identification codes (RIC)
Plastics are categorized by the Resin Identification Code (RIC) system, a set of seven symbols identifying the type of plastic resin used. Understanding resin types is fundamental to recycling, as different polymers have incompatible melting points, densities, and chemical properties that prevent co-processing.
PET (#1) and HDPE (#2) represent the most commonly recycled resins due to established collection infrastructure and strong end markets. PVC (#3) requires special handling due to chlorine content — contamination of other plastic streams with PVC causes significant quality degradation.
Warning:
PVC contamination in PET recycling streams creates hydrogen chloride gas during reprocessing. Even trace quantities (<50 ppm) can render entire batches of rPET unsuitable for food-contact applications. NIR sorting must achieve contamination rates below this threshold.
Sorting Systems
Fig. 6 — Plastic sorting system with NIR optical separation
Near-infrared (NIR) spectroscopy has revolutionized plastic sorting. Each polymer absorbs infrared light at characteristic wavelengths — 1720 nm for PE, 1660 nm for PP, 1580 nm for PET — enabling automated identification at throughput rates exceeding 8 tonnes/hour. High-speed air jets (800 ms response time) eject identified materials onto separate conveyor streams.
Mechanical Processing
Fig. 7 — Mechanical plastic processing and pelletization
Sorted plastic bales undergo mechanical recycling, the dominant processing pathway. The sequence begins with shredding or granulation to reduce particle size to 8–25 mm flake. Flakes are then washed in hot caustic solution (80–95°C, 1–3% NaOH) to remove labels, adhesives, and surface contaminants.
After washing, a density separation float-sink tank separates by density: PET sinks (1.38 g/cm³) while HDPE floats (0.96 g/cm³). Dried and cleaned flakes are fed into extruders, melted, and formed into standardized pellets for sale to manufacturers.
End Products
Recycled plastics (identified by the rPET, rHDPE prefix notation) enter diverse manufacturing applications. rPET fiber is the dominant end market for bottle-grade PET — approximately 60% of rPET globally becomes polyester fiber for textiles. Food-grade rPET for bottle-to-bottle recycling requires additional decontamination using solid-state polycondensation (SSP) to restore molecular weight.
See also:
For chemical recycling pathways (pyrolysis, glycolysis, hydrolysis) that depolymerize plastics to monomer or feedstock, see Advanced Plastic Recovery. For biodegradable and compostable plastics handling, see Biopolymer Streams.
Metal Recycling
Ferrous vs Non-Ferrous
Fig. 8 — Primary ferrous/non-ferrous classification
Metal scrap divides into two fundamental categories based on iron content. Ferrous metals (iron and steel) are magnetic, enabling efficient magnetic separation. Non-ferrous metals (aluminum, copper, zinc, lead, tin, precious metals) command higher per-tonne prices and require more sophisticated sorting techniques.
Steel is the world's most recycled material by mass — global steel recycling rates exceed 85% for post-consumer scrap in developed economies. The steel industry sources approximately 40% of its iron units from scrap, rising to 70%+ for electric arc furnace (EAF) steelmakers.
Magnetic Separation
Fig. 9 — Metal separation system cascade
Magnetic separation is the first and most cost-effective stage of metal sorting. Overhead suspension magnets or drum magnets extract ferrous materials from conveyor streams with 99%+ purity. The residual non-ferrous stream then passes through an eddy current separator — a rapidly rotating magnetic drum that induces eddy currents in conductive non-ferrous metals, causing them to be repelled and ejected from the stream.
Smelting Process
Fig. 10 — Electric arc furnace steelmaking from scrap
Electric arc furnace (EAF) steelmaking uses high-powered graphite electrodes to melt scrap steel through electrical arcing at temperatures exceeding 1600°C. An EAF heat cycle typically takes 40–90 minutes and produces 80–350 tonnes of liquid steel per heat. Compared to basic oxygen furnace (BOF) steelmaking, EAF has approximately 75% lower CO₂ emissions per tonne of steel when using recycled scrap.
Steel & Aluminum Output
Recycled steel is functionally equivalent to virgin steel — unlike paper or plastic, metal recycling does not degrade material properties. Recycled aluminum requires only 5% of the energy needed for primary aluminum production from bauxite ore, making it one of the most energy-efficient recycling processes. A recycled aluminum can re-enters the supply chain in as little as 60 days from consumer disposal.
Table 3: Energy Savings from Metal Recycling vs Primary Production
| Metal |
Primary Energy |
Recycled Energy |
Saving |
| Aluminum |
211 MJ/kg |
11 MJ/kg |
95% |
| Copper |
93 MJ/kg |
16 MJ/kg |
83% |
| Steel |
25 MJ/kg |
9 MJ/kg |
64% |
Glass Recycling
Collection Methods
Fig. 11 — Glass collection and cullet processing
Glass collection typically employs bring-bank systems (drop-off igloos color-coded by glass color) or deposit-return schemes. Unlike paper or plastic, glass must be separated by color — clear, amber, and green — because mixing colors produces an off-color cullet suitable only for lower-value applications. Curbside collection of mixed glass often produces contaminated cullet that cannot be used for container manufacture.
Cullet Processing
Fig. 12 — Cullet processing and furnace charging
Processed glass fragments (cullet) provide significant benefits in container glass manufacture. Each 10% increase in cullet content reduces furnace energy consumption by approximately 3% and CO₂ emissions by 4–5%, as cullet melts at lower temperatures than virgin batch materials. Cullet also extends furnace refractory life by reducing peak temperatures.
Warning:
Heat-resistant glass (borosilicate/Pyrex), ceramic, and crystal glass are incompatible with container glass recycling. Even small quantities (<0.2% by weight) of ceramic contamination can cause cold spots in the glass melt, resulting in visible defects in the final containers. These materials have distinctly different melting points and must be excluded from cullet streams.
New Glass Products
High-purity, color-separated cullet enters the container glass furnace as a raw material substitute for silica sand, soda ash, and limestone. Depending on cullet quality, recycled content in new container glass can reach 90%. Lower-grade or mixed-color cullet is used in fiberglass insulation, construction aggregates, glasphalt (asphalt with glass aggregate), and filtration media.
Note:
Glass is theoretically infinitely recyclable without degradation of material properties, unlike paper (fiber shortening) or plastic (polymer chain degradation). The primary barrier to high recycling rates is contamination management and color segregation in collection systems.
Electronic Waste (E-Waste)
Hazardous Materials
Fig. 13 — Primary hazardous substances in electronic waste
Electronic equipment contains a complex mixture of valuable and hazardous materials. A single desktop computer may contain 1.5 kg of lead (primarily in older CRT solder and glass), 0.2 g of gold, and numerous brominated flame retardants (BFRs) in plastic housings. Improper disposal of e-waste in landfills or through uncontrolled burning releases these toxins into soil, groundwater, and air.
Warning:
E-waste is regulated as hazardous waste in most jurisdictions. Under the Basel Convention, transboundary shipments of e-waste to non-OECD countries for processing require Prior Informed Consent (PIC). Informal e-waste recycling — common in some regions — exposes workers to toxic metals and persistent organic pollutants without adequate protection.
Dismantling
Fig. 14 — E-waste processing and dismantling sequence
Formal e-waste processing begins with triage and data destruction, followed by manual dismantling to remove hazardous components before shredding. Batteries, particularly lithium-ion, require careful extraction — thermal runaway during shredding is a significant fire risk. LCD screens containing mercury lamps require specialized handling under capture ventilation.
Manual dismantling recovers high-value components (RAM, CPUs, hard drives for data destruction) and removes hazardous materials before shredding, which would otherwise contaminate downstream metal streams. Printed circuit boards (PCBs) are typically separated for precious metal recovery via smelting.
Precious Metal Recovery
Fig. 15 — Precious metal recovery from PCBs via copper smelting
Printed circuit boards contain concentrations of precious metals far exceeding ore grades — a tonne of mobile phones may contain 250–350 g of gold versus 1–5 g/tonne in gold ore. Integrated copper smelters process e-waste PCBs alongside copper ore concentrate, with the copper matrix acting as a collector for gold, silver, palladium, and platinum group metals (PGMs).
Electrolytic copper refining separates copper from precious metals, which collect in the anode slimes. The slimes are then further refined to recover individual precious metals through a series of hydrometallurgical steps including selective leaching with HNO₃, HCl, and precipitation.