Super Eco

Photo credit: Jan Tik, Flickr

Wood pulp is the raw material for modern industrially produced paper. In the pulping process, wood is first chipped into small pieces. Then water and heat, and sometimes chemicals, are added to separate the wood into individual fibers. The fiber is mixed with water to produce a pulp slurry. Paper mills use different methods to pulp paper.

The Kraft pulping process is a chemical pulping method that uses sodium sulfide and caustic sodium hydroxide in a water solution. The chemicals extract lignin from the wood fiber. Lignin, a natural component of plants, is undesirable in pulp and paper. Paper with too much lignin yellows and degrades quickly. The extracted lignin is processed to yield a fuel that provides heat necessary to recover the chemicals used in the pulping process. Bleaching chemically pulped slurry is a way to remove the last of the lignin, creating a much higher quality of paper.

For paper products that require less strength (such as newsprint), mechanical pulping is used. This is a process that mechanically separates the wood fibers using grinders and steam. The lignin remains in the pulp. The mechanical process produces almost twice as much pulp as the chemical process. Mechanical pulping usually requires bleaching to whiten the paper, but the whitening is temporary. The reason newspapers turn yellow with age is because the newsprint contains lignin.

Pulp slurry is sprayed onto a flat wire screen that moves very quickly through the paper machine. Water drains out, and the fibers bond together. The web of paper is pressed between rolls which squeeze out more water and press it to make a smooth surface. Heated rollers then dry the paper, and the paper is slit into smaller rolls, and sometimes into sheets, and removed from the paper machine.

Paper mills are almost always built on a freshwater lake or river where plenty of water is available. The pulping processes have an effect on water quality. There are many different bleaching methods, each with its own environmental impacts. Chlorine bleaching is the worst offender. The paper industry, responding to heightened environmental concerns, has shifted from chlorine bleaching to using chlorine dioxide. This has cut down on the organochlorine compounds and dioxin that were by-products of paper production until quite recently. The paper industry in Sweden and Finland has not yet converted from the use of elemental chlorine to chlorine dioxide.