Delignification is the process of removing lignin from wood pulp to ensure paper brightness and clarity. The Kraft process and oxygen delignification are commonly used, with the former being more effective. Lignin is an essential element in wood structure but causes staining in papermaking.
Delignification is a group of industrial processes in which lignin, the natural cellular binder of wood, is removed during the production of wood pulp. The removal of lignin from wood pulps intended for papermaking is necessary to ensure the brightness and clarity of the paper product. There are several delignification processes in general use, with the Kraft process and oxygen delignification being the most common, often also used in tandem. The Kraft process is a high temperature and pressure digestion system that chemically breaks down lignin in wood pulp. Oxygen-based lignin removal processes use oxygen activation to remove lignin and are often used as supplementary steps in the brown stock washing step of the Kraft process.
Often referred to as the “glue of nature”, lignin is an essential element in the cellular structure of wood and woody plants such as herbs. It serves as a cell binder and hydration facilitator in plants and is also a useful commercial product when separated during woodworking. Lignin, however, poses problems when wood fibers are used to make pulp for papermaking. These include unwanted staining that requires raw wood to undergo a delignification process before 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 an aqueous solution of sodium sulfide and sodium hydroxide in a heated pressure digester. This solution, also known as white liquor, coupled with the high pressure and temperature in the digester, chemically dissolves the lignin in the white liquor solution. This lignin-rich solution, or brownstock, is then pumped out and undergoes a multi-step 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 in 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, only removing about 55% of the lignin. However, it is often used as a supplemental tandem process to the more traditional Kraft method. In these cases, the oxygenation process occurs during the washing phase of the Kraft brownstock.
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