The journey of discarded paper into a new product is a complex industrial process, beginning the moment it leaves a residential bin or commercial facility. Recycling fundamentally aims to recover and reuse the cellulose fibers that make up paper. This involves several stages of mechanical and chemical transformation designed to separate the desirable fibers from contaminants like ink, plastics, and metals.
Sorting and Grading Discarded Paper
The first destination for collected paper is typically a Materials Recovery Facility (MRF), where it is separated from other recyclables and contaminants. Paper is separated from heavier materials like plastic bottles and metal cans using screens, air classifiers, and magnets.
The paper is then sorted and graded, which determines what kind of new product it can become. Different grades, such as Old Corrugated Containers (OCC), mixed paper, and high-grade office paper, require distinct processing methods. Automated optical scanners and manual checks isolate these grades and remove unwanted items like plastic film, sticky adhesives, and food residue. Once separated and cleaned, the paper is compressed into large, dense bales and shipped to a paper mill to begin fiber recovery.
Turning Paper into Clean Pulp
At the paper mill, the baled paper is introduced into a large vessel called a pulper. Here, the paper is mixed with large volumes of water, often heated, and various chemicals to break down the material. This mechanical and chemical action separates the paper into individual cellulose fibers, creating a watery mixture known as pulp or fiber slurry. The goal of pulping is to completely detach the fibers without causing them damage.
The slurry then undergoes screening and cleaning to remove physical contaminants missed during initial sorting. It is pumped through fine screens that catch larger debris like staples and plastic pieces. Specialized centrifugal cleaners, or hydrocyclones, use rotational force to separate small, dense particles, such as sand or metal fragments, from the lighter fibers.
The most complex step is de-inking, which removes ink and colorants so the fibers can be reused for brighter products. The common technique is flotation, where surfactants are added to the pulp. These chemicals help ink particles detach and become hydrophobic, meaning they repel water. Air is injected into the mixture, creating bubbles that latch onto the ink particles, lifting them to the surface in a foam that is skimmed away.
Preparing the Fiber Slurry for Manufacturing
The final stage focuses on quality control and standardization before the slurry enters the paper machine. If the new product requires a high degree of whiteness, the purified pulp may undergo a mild bleaching process. This uses compounds like hydrogen peroxide or sodium hydrosulfite to destroy remaining colorants and brighten the fiber’s appearance.
A crucial metric monitored is consistency, the precise ratio of fiber to water in the slurry. The pulp must be diluted until it reaches a very low consistency, often less than one percent fiber, ensuring the fibers spread evenly across the forming screen. The clean, diluted fiber slurry is temporarily held in large storage tanks, known as chests, where it is kept agitated to prevent settling. From these chests, the uniform stock is pumped directly to the head box of the paper machine, concluding preparation and beginning manufacturing.