Sodium hydroxide is used to help re-generate the chemicals used to pulp the wood chips into fibers in the chemical pulping process, which allows the pulp mill to re-use many of the chemicals and reduce the cost of producing pulp. It is also used in one of the stages of some common bleaching processes to increase the efficiency of the particular stage of the bleaching.
Sodium hydroxide is the most commonly used product for improving the alkalinity of process streams in the pulp and paper industry.