The Kraft process, also referred to as the sulfate process or as Kraft pulping, is a process used to produce paper pulp. The purpose of the process is to remove the lignin from the cellulose in the wood fibers that are placed in side larger pressure vessels that are called digesters. In order to accomplish this, the process utilizes sodium sulfide and caustic sodium hydroxide.
The Kraft process was developed in 1884 by Carl Dahl, with the name of the process coming from the German word kraft, meaning strong. The first kraft mill in the United States was not built until 1911 and was located in Pensacola, Florida. Today, the process is used in approximately 80% of paper production. Those companies using the Kraft process are easy to recognize by their strong, offensive smell. This is because one of the byproducts of the process is hydrogen sulfide gas and other sulfur gas compounds.
The Kraft Process is a cyclical, self-sustaining process. As a result of the process, a byproduct called black liquor is formed. It is a combination of the removed lignin, water, and chemicals used in the extraction process. The black liquor is concentrated through evaporation after it is produced and then burned in order to generate high-pressure steam for the mill processes used to make paper. The inorganic portion of the black liquor then regenerates the sodium sulphide and sodium hydroxide that is used for pulping.
When softwood, such as conifers, is used in pulping, a soaplike substance is collected after the process. This soap is then acidified and used to produce tall oil. Tall oil is a source of fatty acids, resin acids, and other chemicals. When broken down, certain components of tall oil called rosin are used in some rubbers, adhesives, inks, and emulsfiers. Other components, called the pitch, are used as an adhesive, as a binder in cement, and as an emulsifier for asphalt.
The sulfite process is another process sometimes used to remove the lignin from plant fibers. The two processes are different, however, because the Kraft process uses an alkaline solution. This is more desirable to paper manufacturers because it is less corrosive and damaging to the equipment. In addition, the Kraft process is more efficient at removing the lignin and results in a stronger fiber. On the other hand, the fiber created with the Kraft process is rougher and contains more impurities than that created with the sulfite process. This makes paper made with the Kraft process more difficult to bleach.
The Kraft process played a large roll at moving the United States to the forefront in the pulp and paper industry. First, the process was capable of pulping pine trees, which were and still are found in abundance in the United States. In addition, its cyclical nature made it possible to recover great amounts of energy. Without the invention of the Kraft process, the United States would most likely have never developed into the paper industry giant that it is today.