A paper mill is a type of factory that makes paper from wood pulp and other special ingredients. This is accomplished through a variety of special machines, including a tree chipper, a digester, and a Fourdrinier machine. Due to the Kraft process used to separate the lignin from the plant products used in the paper making process resulting in a sulfur byproduct, paper mills are associated with unpleasant smells.
The first known operating paper mill was in operation in 794 in Baghdad. From here, the technology spread to Europe. Today, paper mills use great amounts of water, energy, and wood and follow a complex process in order to produce paper. These modern machines are as much as 500 feet in length and move at speeds of over 100 mph, which makes them capable of producing sheets of paper as much as 400 inches wide.
A fully-integrated paper mill is one that receives forest logs or wood chips and processes them to the individual fiber level. The fully-integrated paper mill processes this fiber to a 4% pulp slurry, which is then made into a sheet of paper.
Non-integrated paper mills, on the other hand, purchase the pulp slurry after it has already been created at a pulp mill. When this is done, the pulp slurry is purchased and transported in a dried and baled form, which is called market pulp. These bales are rehydrated with into a 4% solution before processed into sheets of paper.
The undesirable smell associated with paper mills only occurs if the mill is also a pulp mill. In some cases, mills focus on only pulp processing or paper processing. All of the major mills, however, engage in both processes. The offensive smell is caused by the cooking process used to soften the pulp in order to form it into paper. Despite the bad smells they emit, the airborne particles are not harmful to a persons health.
Wood pulp is the most common ingredient used to make paper. The timber used to make wood pulp is generally called pulpwood and comes from softwood trees, such as fir, spruce, pine, hemlock, and larch. Hardwood tree such as birch and eucalyptus, however, are also used.
To make wood pulp, the bark must first be removed from the wood. This can be done with the help of water, called water stripping, though it is not always necessarily done this way. Usually, the stripped bark is used as fuel later in the paper making process.
The cellulose fibers that naturally occur in the wood are separated using one of six methods. One choice is to use crush the wood with grinders and to soak them in water in order produce what is called groundwood. This pulp is often used for products requiring less strength, such as paperboards and newsprint. The wood can also be crushed with refiners, which use high pressure steam and temperatures to created what is called thermomechanical pulp. Along with the refiners, some chemicals can also be added to help separate the cellulose from the wood. This produces what is called chemithermomechanical pulp. All three of these forms are considered to be types of mechanical pulp, which means they will turn yellow over time because the lignin is not completely removed.
When wood chips are combined with chemicals inside a digester in order to break down the cellulose, it produces what is known as a chemical pulp. The heat and chemicals inside the digester causes the lignin to dissolve without breaking down the wood fibers. The liquid byproduct containing the lignin is dried and used as a fuel and the paper product created from chemical pulp is used in materials needing to be stronger.
Recycled pulp can also be created from waster paperboard and paper. This pulp does not require as extensive of a process because the cellulose has already been removed. This pulp is usually used to make newsprint, paperboard, and sanitary paper.
Biological pulping is another form of pulping that is currently being researched. This process is similar to chemical pulping, but utilizes a fungi to break down the lignin rather than chemicals. The potential environmental benefits of this type of pulping are outstanding.
After the pulp is produced, it is bleached in order to create white paper. The chemicals that have been used in this process have caused environmental concerns. Therefore, the pulp industry has been using chlorine alternatives, such as hydrogen peroxide, oxygen, and ozone. After being bleached, the pulp is moved through the Fourdrinier machine where it is dried and shaped.
The paper mill industry is a multi-billion dollar industry with a large impact on the economy in the United States, yet most people are largely unaware paper mills and their role in our society.