Delignification is a group of industrial processes where lignin, the natural cell binding agent in wood, is removed during the production of wood pulp. The removal of lignin from wood pulp intended for papermaking is necessary to ensure brightness and clarity to the paper product. There are several delignification processes in general use, with the Kraft process and oxygen delignification being the most common, often being used together. The Kraft process is a high-temperature, high-pressure digestion system that chemically breaks down lignin in wood pulp. Oxygen-based lignin removal processes utilize oxygen activation to remove lignin and are often used as supplemental steps in the brown pulp washing stage of the Kraft process.
Often called “nature’s glue”, lignin is an essential element in the cellular structure of wood and woody plants such as grasses. It acts as a cellular binder and hydration facilitator in plants and also represents a useful commercial product when separated during wood processing. Lignin, however, presents problems when wood fibers are used to produce pulps for papermaking. This includes undesirable coloring that requires raw wood to undergo a delignification process prior to pulping. The two main processes used to remove lignin from wood pulp are the Kraft process and oxygen delignification.
The Kraft process involves passing finely chipped wood through a high-temperature, high-pressure chemical digestion system. Wood chips are introduced into a water-based solution of sodium sulfide and sodium hydroxide in a heated pressure digester tank. This solution, also known as white liquor, together with the high pressure and temperature in the digester, chemically dissolves the lignin in the white liquor solution. This lignin-rich solution, or brown paste, is then pumped through and subjected to a multi-stage rehabilitation process that removes the lignin and recovers much of the original white liquor for reuse. Kraft delignification is highly effective, removing approximately 95% of the lignin from the wood pulp.
Oxygen delignification is an oxidation process that relies on the activation of oxygen to remove lignin from washed wood pulp. By itself, the oxygen process is not as effective as Kraft pulping, removing only about 55% of the lignin. It is, however, often used as a supplemental tandem process to the more traditional Kraft method. In these cases, the oxygenation process takes place during the washing phase of the brown Kraft pulp.