Corrugated fiberboard, commonly known as cardboard, is one of the most frequently recycled materials, forming a closed-loop system that conserves virgin resources. This robust material, characterized by its fluted layer sandwiched between two liners, undergoes a precise industrial sequence to be reborn as new paperboard products. The process transforms used boxes into a clean, fibrous slurry, which is then reshaped and dried into a usable sheet.
Collection and Preparation for the Mill
The initial step involves the systematic gathering of used corrugated cardboard from various sources, which largely dictates the material’s quality. Residential curbside programs collect lower volumes, but commercial sources, such as warehouses and grocery stores, supply the vast majority of recyclable material. These high-volume generators often flatten and compact their cardboard on-site into large, dense rectangular bundles.
This compression process, known as baling, maximizes the weight-to-volume ratio of the material, making transport to the recycling mill efficient. Upon arrival at the mill or a material recovery facility (MRF), the bales undergo a preliminary sorting stage. Large contaminants like excessive plastic straps, non-cardboard packaging, and heavily soiled sections are removed to protect the machinery in the later stages.
Separating the Fibers and Removing Contaminants
The bales are broken apart and introduced into the hydrapulper, initiating the core transformation. Inside this tank, the cardboard is mixed with warm water and sometimes chemical aids, where a high-speed rotor agitates the mixture. This action breaks the bonds holding the wood fibers together, dissolving the cardboard into a thick, watery suspension called pulp or slurry.
Contaminants that persist through pulping, such as adhesives, ink particles, and small pieces of plastic or metal, must then be removed from the fiber slurry. The pulp flows through a series of screens with progressively smaller holes, which physically filter out larger debris like tape, staples, and bits of plastic film. This initial step is called coarse screening, separating the fiber from the macro-contaminants.
Further purification relies on differences in density and surface properties, utilizing centrifugal cleaners. In these devices, the slurry is rapidly spun in a cylindrical chamber, causing heavier contaminants, such as sand or metal staples, to be flung to the outside walls and removed. This separates the dense impurities from the cleaner, lighter fiber suspension.
To remove ink particles, a chemical process called flotation deinking is often employed, particularly for printed materials. Air is injected into the pulp, and specialized surfactants are added that cause the hydrophobic ink particles to attach to the rising air bubbles. These ink-laden bubbles then float to the surface, forming a foam that is skimmed off. This process converts the mixed solid waste into a purified fiber stock.
Forming and Finishing the New Cardboard
Once the fiber slurry is clean, it is refined, a process that mechanically treats the fibers to optimize their properties for strength and bonding. The refined stock, which is over 99% water, is pumped into the headbox of a papermaking machine.
The highly diluted pulp is uniformly distributed onto a wide, moving screen known as a forming fabric. As the fabric travels, gravity and vacuum suction boxes underneath pull the water out of the slurry. The cellulose fibers begin to interlock and mat together, forming a continuous, wet sheet called a web.
The wet web then moves into the press section, where it passes through a series of heavy rollers. These rollers physically squeeze out a significant amount of the remaining water, typically reducing the moisture content to below 50%. The sheet gains strength and density during this pressing phase.
Finally, the paper web travels through the dryer section, which consists of steam-heated cylinders. This process rapidly evaporates the remaining moisture, stabilizing the fiber structure and bringing the sheet’s moisture content to its final level, usually around 6-8%. The resulting continuous paperboard sheet is then wound onto massive rolls, completing the transformation from used box to renewed industrial material.