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Paper mills use wood pulp and chemicals to create paper in a complex process. Logs are stripped, chipped, cooked, washed, bleached, and pressed into paper. Challenges include machinery corrosion and foul-smelling emissions.
Paper is a basic commodity that everyone is very familiar with. Its universal presence and simple nature belie the complicated processes involved in papermaking. Paper mills are factories that make paper from wood pulp and other ingredients. Some paper mills are integrated, which means that the pulp is produced on the same site as the finished product.
While paper has been around for thousands of years in one form or another, the first standardized process for making paper was invented in China in 105 BC Modern paper mills use large amounts of energy, water and wood pulp in one process highly complex and mechanized to produce a sheet of paper. A papermaking machine can be very large, up to 500 feet (152 m) in length. Rolls of fresh raw or uncut paper can be up to 33 m (10 ft) wide.
The basic raw material with which the integrated paper mills work is wood. At the beginning of the whole process, the wooden logs are passed through a machine which strips their bark and then into a chipper. The chipper cuts logs into square chips, smaller than the palm of a hand. Wood is composed of cellulose fibers bound together by a substance called lignin. To break down the wood chips into pulp, the lignin must be dissolved.
This is accomplished by adding heat, pressure and a mixture of chemicals to wood chips, in a vessel known as a digester. The wood chips are “cooked” over a period of several hours, which reduces the dough to a gray pulp with roughly the same consistency as oatmeal. The pulp is then removed from the digester using high pressure blowers and washed to separate the usable pulp from the lignin. Most paper mills also add a proprietary blend of non-chlorine bleach and other chemicals at this point, to lighten the color of the pulp.
After the pulp has been washed and bleached, lots of water is added and this mixture is placed on a wire mesh screen that circulates to help the fibers bind together into something more recognizable as paper. Most of the water is extracted from this process. The paper mat is then pressed between the water-absorbing fabrics and onto the drying cylinders. These cylinders are heated to remove the last of the water. This entire process step moves the paper very rapidly, at over 3,000 feet (0.9 km) per minute.
Finally, the paper is ironed to give it a smooth finish. When completely dry, it is wound onto large coils, which in turn transfer it into smaller rolls for cutting. However, despite all their advances, paper mills still have some constant challenges, such as machinery corrosion that results from the high humidity and heat of papermaking. Pulp milling is also slightly problematic for the surrounding communities due to the foul smelling emissions it produces.
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