Paper is a basic commodity which everyone is very familiar with. Its universal presence and simple nature belie the complicated processes involved in papermaking. Paper mills are factories which manufacture paper from wood pulp and other ingredients. Some paper mills are integrated, meaning that wood pulp is made 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 paper manufacture was invented in China in 105 BCE. Modern paper mills use large amounts of energy, water, and wood pulp in a highly complex and mechanized process to produce a sheet of paper. A papermaking machine can be very large — up to 500 feet (152 m) in length. Raw or uncut spools of fresh paper can be up to 33 feet (10m) in width.
The basic raw material which integrated paper mills work with is wood. At the beginning of the whole process, wooden logs are passed through a machine which strips off their bark, and then into a chipper. The chipper reduces the logs to square chips, smaller than the palm of one’s hand. Wood is composed of cellulose fibers bound together by a substance called lignin. In order to break down the wood chips into pulp, the lignin must be dissolved.
This is accomplished by the addition of heat, pressure, and a mix of chemicals to the wood chips, in a vessel known as a digester. The wood chips are “cooked” for a period of several hours, which reduces the mixture to a gray pulp with roughly the same consistency as oatmeal. The pulp is then removed from the digester by high-pressure blowers, and washed to separate out the usable pulp from the lignin. Most paper mills also add a proprietary mix 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, a great deal of water is added to it, 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 by this process. The mat of paper is then pressed in between fabrics which absorb water, and onto drying cylinders. These cylinders are heated to remove the last of the water. This whole stage of the process moves the paper very quickly, at over 3,000 feet (0.9 km) per minute.
Finally, the paper is ironed to give it a smooth finish. When it is completely dry, it is wound onto large spools, which in turn transfer it to smaller rolls for cutting. However, for all their advancement, paper mills still have a few constant challenges, such as the corrosion of machinery that results from the high humidity and heat of papermaking. The milling of pulp is also slightly problematic for surrounding communities because of the malodorous emissions it produces.