The journey of cardboard, primarily known as Old Corrugated Containers (OCC), represents a significant cycle in resource management. OCC consists of long cellulose fibers that retain considerable value for reuse. Understanding the mechanical and chemical processes involved traces the path of this fiber from a collection bin back to a finished product. The recycling stream transforms the physical structure of the cardboard while preserving the fundamental fiber components.
Initial Steps: Sorting and Baling
The process begins after collection, when the used cardboard arrives at a Material Recovery Facility (MRF) for initial processing. Here, sophisticated machinery and manual sorting separate the cardboard from other recyclables, such as mixed paper, plastics, and metals. Effective separation is performed by disc screens, which are designed to allow smaller materials to fall through while the larger, flat cardboard moves along the conveyer belt.
Contamination poses the biggest challenge, as materials soiled with food residue, grease, or liquids are difficult to process and can compromise the quality of the batch. Wax-coated cardboard, often used for produce, must also be separated because it requires a specialized pulping process. Once sorted, the clean, flattened cardboard is compressed under high pressure into dense blocks known as bales. These bales are secured with wire and transported to a paper mill, where fiber recovery begins.
The Transformation: Pulping and Screening
At the paper mill, the baled cardboard is loaded into a large, agitated vat called a hydrapulper, which acts like a massive industrial blender. Inside the pulper, the cardboard is mixed with large volumes of water and agitated to physically break down the corrugated structure. This mechanical action separates the individual cellulose fibers, creating a watery, fibrous slurry known as pulp.
The hydrapulper’s primary function is defibrillation, the process of gently separating the fibers without damaging them. This initial wet process includes a coarse screening stage to capture and remove larger, non-fiber contaminants. Materials like staples, plastic tape, and heavy foreign objects that survived the MRF sorting are extracted before they can damage downstream equipment. The resulting raw pulp, which is still highly contaminated, is then pumped onward for further purification.
Refinement: Cleaning and De-Inking
Following the initial pulping, the fiber slurry undergoes multi-stage cleaning to remove smaller, denser impurities that remained suspended in the water. Centrifugal cleaners, which spin the pulp at high speeds, are used to force heavy particles like sand, grit, and metal fragments outward for removal. Lighter debris, such as small plastic film or Styrofoam pieces, are removed through fine screening techniques.
A significant purification step involves de-inking, performed to remove printing inks and dyes from the fiber surface. This process involves adding chemical agents, such as surfactants and dispersants, to help detach the hydrophobic ink particles from the cellulose. The most common technique is froth flotation, where air is injected into the slurry, causing the ink particles to cling to air bubbles. The ink-laden bubbles rise to the surface, forming a thick foam that is mechanically skimmed off and collected as waste sludge.
The Final Output: Manufacturing New Paper Products
The cleaned and refined pulp is ready to be reformed into new paper products. The slurry, which contains up to 99% water, is pumped onto a wide screen where the water rapidly drains away through gravity and vacuum suction. A wet sheet of interlocking fibers is left on the screen, which is then pressed between heavy rollers to squeeze out more water and consolidate the sheet.
The pressed sheet enters a drying section where it passes over steam-heated cylinders to reduce the moisture content to approximately 5–8%. The finished product is wound onto large rolls, typically destined to become new corrugated medium (the fluted layer) or linerboard (the flat outer layers of new cardboard boxes). Cellulose fibers shorten each time they are recycled due to the mechanical and chemical stress of the process. This fiber degradation limits reuse, generally cited as five to seven cycles, before the fibers become too weak to form a structurally sound new product.