The process of recycling cardboard transforms used corrugated containers and boxboard into a reusable fiber source, preventing substantial material from entering landfills. This mechanical and chemical transformation is a highly efficient industrial practice, making cardboard one of the most successfully recycled materials. In the United States, the recycling rate for cardboard, often referred to as Old Corrugated Containers (OCC), is consistently high, typically ranging from 69% to 74%. This recovery rate represents the recycling of over 33 million tons of cardboard annually, demonstrating its significant role in the circular economy.
Collection, Sorting, and Preparation
The recycling journey for cardboard begins with its collection from commercial, industrial, and residential sources. Businesses, which generate large volumes of corrugated cardboard for shipping, often utilize specialized collection services and compactors. Residential cardboard is typically collected through curbside programs, where it is often mixed with other recyclable materials.
Once collected, the mixed materials are delivered to a Material Recovery Facility (MRF) for separation. Machinery and human sorters isolate the cardboard, often separating thick corrugated cardboard from thinner boxboard. Contaminants that could compromise the final product quality, such as food residue, excessive tape, or plastic liners, are removed.
The clean, sorted cardboard is then prepared for transport to a paper mill. This preparation involves compressing the loose material into dense, standardized cubes called bales. Baling increases the material’s density, making transportation more efficient and cost-effective by maximizing the weight carried per truckload.
Fiber Breakdown Through Pulping
The first major industrial step occurs when the baled cardboard arrives at the paper mill and is introduced into a large vat known as a pulper or hydropulper. This machine acts like a massive, industrial-sized blender, using high-speed rotating blades to agitate the material. The cardboard is mixed with large volumes of water, and sometimes chemicals like sodium hydroxide or caustic soda, are added to assist the breakdown.
The mechanical action of the pulper, combined with the chemical agents, rapidly separates the cellulose fibers by breaking the hydrogen bonds that hold the cardboard structure together. Within minutes, the solid cardboard is disintegrated into a thick, wet mixture called a slurry or pulp, which contains about 99% water and 1% fiber. This process is necessary because the long, robust cellulose fibers must be fully suspended in water before they can be reformed into new material.
The resulting pulp is a suspension of individual paper fibers, which still contains various contaminants not removed during the initial sorting. Achieving a homogeneous slurry at this stage is important for the integrity of the final recycled product.
Purification and Contaminant Removal
After the initial breakdown in the pulper, the fiber slurry must undergo extensive cleaning to remove non-fiber materials. The first stage of purification is screening, where the pulp is passed through a series of filters with progressively smaller holes. These screens catch larger physical contaminants, such as remaining staples, plastic fragments, adhesives, and pieces of tape.
Following screening, the pulp often moves to centrifugal cleaning, which separates particles based on density. Devices called hydrocyclones spin the slurry at high speeds in a cone-shaped chamber. This rotational force pushes heavier contaminants, like sand, grit, and small metal pieces, to the outside wall where they are separated from the lighter cellulose fibers.
If the recovered material includes printed inks, a process called de-inking may also be employed to achieve a higher quality pulp. This involves using special chemicals called surfactants that bind to the ink particles. Air is then bubbled through the mixture in large flotation cells, causing the ink-surfactant complexes to float to the surface as a frothy scum that is skimmed away.
Creating New Cardboard Products
With the pulp now cleaned and refined, the process transitions to forming the new material on a large paper machine, often using a Fourdrinier setup. The diluted fiber slurry is pumped onto a wide, moving mesh screen known as the forming wire. As the pulp travels along the screen, gravity and vacuum boxes positioned beneath the wire draw the majority of the water out.
The fibers begin to interlock and mat together, forming a continuous, wet sheet of paperboard. This fragile, wet sheet then passes through a series of heavy press rollers. These cylinders apply pressure, compacting the fibers and squeezing out additional moisture.
Finally, the sheet moves through heated cylinders. This process rapidly dries the material, removing the final traces of water and setting the structure of the paperboard. The finished, dry paperboard is then wound into enormous, wide reels, which are later transported to conversion plants to be turned into new corrugated boxes, containerboard, or other paper products.