Delignification is a group of industrial processes where lignin, the natural cellular binding agent in wood, is removed during the production of wood pulp. The removal of lignin from wood pulps destined for the manufacture of paper is necessary to ensure brightness and clarity in the paper product. There are several delignification processes in general use, with the Kraft process and oxygen delignification being the most common, often even being used in tandem. The Kraft process is a high-temperature and pressure digestion system that breaks down the lignin in wood pulp chemically. Oxygen-based lignin removal processes utilize oxygen activation to remove the lignin and are often used as supplementary steps at the brownstock washing stage of the Kraft process.
Often referred to as “nature's glue,” lignin is an essential element in the cellular structure of wood and woody plants such as grasses. It serves as a cellular binder and hydration facilitator in the plants and also represent a useful commercial product when separated during wood processing. Lignin does, however, pose problems when wood fibers are used to produce pulps for the manufacture of paper. These include undesirable coloration that requires the raw wood to be subjected to a delignification process prior to pulping. The two primary processes used to remove lignin from wood pulp are the Kraft process and oxygen delignification.
The Kraft process involves running finely-chipped wood through a high-temperature, high-pressure chemical digestion system. The wood chips are introduced into a water-based solution of sodium sulfide and sodium hydroxide in a heated pressure digester vessel. This solution, also known as white liquor, along with the high pressure and temperature in the digester, chemically dissolve the lignin into the white liquor solution. This lignin-rich solution, or brownstock, is then pumped off and subjected to a multi-stage rehabilitation process that removes the lignin and recovers much of the original white liquor for re-use. Kraft delignification is highly effective, removing approximately 95% of the lignin in wood pulp.
Oxygen delignification is an oxidation process that relies on oxygen activation to remove the lignin from washed wood pulp. On its own, the oxygen process is not quite as effective as Kraft pulping, removing only about 55% of the lignin. It is, however, often used as a supplementary tandem process to the more traditional Kraft method. In these cases, the oxygenation process ties in during the Kraft brownstock washing phase.