Making Recycled Pulp
Recyclers often use a combination of two processes to move ink from used paper: washing and flotation.
The paper moves by conveyor to a big vat called a pulper, which contains water and chemicals. The pulper chops the recovered paper into small pieces. Heating the mixture breaks the paper down more quickly into tiny strands of cellulose (organic plant material) called ﬁbers. Eventually, the old paper turns into a mushy mixture called pulp.
The pulp is forced through screens containing holes and slots of various shapes and sizes. The screens remove small contaminants such as bits of plastic and globs of glue. This process is called screening
Mills also clean pulp by spinning it around in large cone-shaped cylinders. Heavy contaminants like staples are thrown to the outside of the cone and fall through the bottom of the cylinder. Lighter contaminants collect in the center of the cone and are removed.
Sometimes the pulp must undergo a “pulp laundering” operation called deinking to remove printing ink and sticky materials like glue and adhesives. Papermakers often use a combination of two deinking processes. Small particles of ink are rinsed from the pulp with water in a process called washing. Larger particles and stickies are removed with air bubbles in another process called ﬂotation.
During ﬂotation deinking, pulp is fed into a large vat called a ﬂotation cell, where air and soaplike chemicals call surfactants are injected into the pulp. The surfactants cause ink and stickies to loosen from the pulp and stick to the air bubbles as they ﬂoat to the top of the mixture. The inky air bubbles create foam or froth which is removed from the top, leaving the clean pulp behind.
During reﬁning, the pulp is beaten to make the recycled ﬁbers swell, making them ideal for papermaking. If the pulp contains any large bundles of ﬁbers, reﬁning separates them into individual ﬁbers. If the recovered paper is colored, color stripping chemicals remove the dyes from the paper. Then, if white recycled paper is being made, the pulp may need to be bleached with hydrogen peroxide, chlorine dioxide, or oxygen to make it whiter and brighter. If brown recycled paper is being made, such as that used for industrial paper towels, the pulp does not need to be bleached.