The Modern Paper Mill

The modern paper mill is a highly complex industrial facility. Although the principles of papermaking have not fundamentally changed for many years, a papermaker from Imperial China or pre-industrial Europe would be hard pressed to recognize his craft amongst all the equipment of a modern mill. To explore how a present-day paper mill operates, let's follow the path of an individual wood fiber from its arrival at the mill to its departure.

Delivery and Preparation

Most of the mill's raw material arrives by truck or rail in the form of logs. The logs are soaked in water and tumbled in slatted metal drums to remove the bark. The debarked logs are then fed into a chipper, a device with a rotating steel blade that cuts the wood into pieces about 1/8" thick and 1/2" square. (In some cases, the wood may have been chipped, bark and all, when it was harvested.) The wood chips are stored in a pile outside the mill; as new chips are added to the top of the pile, others are withdrawn fro the bottom and carried by conveyor to the digester.

Pulping

Digesting is the process of removing lignin and other components of the wood from the cellulose fibers which will be used to make paper. Lignin is the "glue" which holds the wood together; it rapidly decomposes and discolors paper if it is left in the pulp (as in newsprint, which is usually made from groundwood pulp with little or no chemical treatment). Since this is a "kraft" mill, the lignin is removed by the action of sodium hydroxide ("caustic soda") and sodium sulfate under heat and pressure. The chips are fed into the top of a digester and mixed with the cooking chemicals, which are called "white liquor" at this point. As the chips and liquor move down through the digester, the lignin and other components are dissolved, and the cellulose fibers are released as pulp. At the bottom of the digester, the pulp is rinsed, and the spent chemicals (now known as "black liquor") are separated and recycled.

Bleaching and Refining

At this point, the "brownstock" pulp is free of lignin, but is too dark to use for most grades of paper. The next step is therefore to bleach the pulp by treating it with chlorine, chlorine dioxide, ozone, peroxide, or any of several other treatments. A typical mill uses multiple stages of bleaching, often with different treatments in each step, to produce a bright white pulp. Chlorine bleaching generally provides the best performance with the least damage to the fibers, but concerns about dioxins and other byproducts have led the industry to move towards more environmentally friendly alternatives.

At this point, the individual cellulose fibers are still fairly hollow and stiff, so they must be broken down somewhat to help them stick to one another in the paper web. This is accomplished by "beating" the pulp in the refiners, vessels with a series of rotating serrated metal disks. The pulp will be beaten for various lengths of time depending on its origin and the type of paper product that will be made from it. At the end of the process, the fibers will be flattened and frayed, ready to bond together in a sheet of paper.

Forming the Sheet

Once the pulp has been bleached and refined, it is rinsed and diluted with water, and fillers such as clay or talc may be added. This "furnish", containing 99% water or more, is pumped into the headbox of the paper machine. From the headbox, the furnish is dispensed through a long, narrow "slice" onto the "wire", a moving continuous belt of wire or plastic mesh. As it travels down the wire, much of the water drains away or is pulled away by suction from underneath. The cellulose fibers, trapped on the wire as the water drains away, adhere to one another to form the paper web. From the wire, the newly formed sheet of paper is transferred onto a cloth belt (or "felt") in the press section, where rollers squeeze out much of the remaining water.

Coating, Drying, and Calendering

After leaving the press section, the sheet encounters the drying cylinders. These are large hollow metal cylinders, heated internally with steam, which dry the paper as it passes over them. The sheet will be wound up and down over many cylinders in the drying process. Between dryer sections, the paper may be coated with pigments, latex mixtures, or many other substances to give it a higher gloss or to impart some other desirable characteristic. After another round of drying, the paper sheet is passed through a series of polished, close-stacked metal rollers known as a "calender" where it is pressed smooth. Finally, the sheet is collected on a take-up roll and removed from the paper machine. From the headbox, it may have traveled half a mile or more in less than a minute.

Cutting and Packaging

In many cases, the new paper roll is simply rewound on a new core, inspected, and shipped directly to the customer. Other paper grades, however, may be further smoothed by passing them through a "supercalender" where the sheet is polished by passing between steel and hard cotton rollers (much like ironing fabric), or they may be embossed with a decorative pattern. The paper may also be cut into sheets at the mill, often by automatic equipment which accepts a roll of paper at one end and delivers packages of cut sheets at the other, already boxed and wrapped for shipping.

Papermaking today is one of North America's most capital-intensive industries, devoting large sums of money to the development and construction of newer and more efficient equipment and processes. Although we ourselves might not recognize the paper mills of three hundred years from now, the same basic processes will almost certainly be in use to produce a product that will still be in demand far into the future.

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