Chemical pulp is made from wood fiber using chemicals and heat. It produces stronger paper than mechanically derived pulp and allows for chemical recovery. Two processes, sulfate and sulfite pulping, differ in chemicals used and paper quality. Both involve cooking wood chips in a chemical liquor, with the black liquor used for chemical recovery or energy production.
Pulp is the wood fiber that is typically used to make paper. Chemical pulp is created by a method that uses chemicals and heat to convert wood into pulp. This process can be, and is generally, used as an alternative to mechanical pulping, which involves obtaining wood fibers through a grinding process.
Chemical pulp tends to be more common than mechanically derived pulp. Chemical pulping generally leads to the production of paper with higher sheet strength than paper made from mechanical pulp. It is also a process that can be considered highly efficient due to the possibility of chemical recovery.
Chemical recovery is part of the process where chemicals used to treat wood are reused for another purpose. In some cases, this used liquid, often referred to as liquor, is used to make other chemicals. In other cases, chemicals are used to create energy.
There are two widely recognized processes for making chemical pulp. One is known as sulfate pulping. The other is known as sulfite pulping. The main differences between these are the chemicals used, the quality of paper produced, and the economics of chemical recovery.
Sulfate pulping is a process developed in Germany in 1879. It is sometimes called kraft pulping because kraft means strength in German and paper made from this chemical pulp is strong. It can be used to make paper bags, writing paper or diapers. Chemicals are typically always recovered in this process.
Sulfite pulp is a process that was developed in the United States in 1867. It typically results in a light chemical pulp that is easier to bleach and easier to refine. Paper made using this method can be used for newspaper, stationery, or cellophane. With this method, however, there is not always chemical recovery.
Both processes are usually performed in a similar way. Chemical pulping usually begins with debarking trees to create wood chips. Those chips are then cooked in a chemical concoction or liquor. The purpose of this is to dissolve the wood’s lignin, a natural component that binds fibers together, and to break down other elements in the wood. When this part of the process is complete, the liquor is usually black.
This black liquor is what is used in the chemical recovery process. It can be used in the production of another chemical, such as tall oil. It can also be burned to create heat or electricity.
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