white liquor
A strongly alkaline caustic white liquid composed mainly of sodium hydroxide and sodium sulfide used in the production of wood pulp in papermaking.
white liquor: the pulping chemical that breaks down wood
White liquor is the cooking liquid used in the kraft pulping process to dissolve lignin and separate cellulose fibres from wood. It consists primarily of sodium hydroxide (NaOH) and sodium sulfide (Na2S), typically in a ratio of roughly 1:1 by weight, dissolved in water to create a strongly alkaline solution with a pH around 13-14. This chemical mixture is the active agent that transforms raw wood chips into usable pulp for papermaking.
The white liquor is heated to around 150-170 degrees Celsius and injected into large digesters along with wood chips. At this temperature and alkalinity, it breaks down the lignin that binds the cellulose fibres together, allowing the fibres to separate while remaining largely intact. The process typically takes 2-5 hours depending on wood type and desired pulp properties. Softwoods like pine and spruce dissolve faster than hardwoods, requiring adjustment of cooking time and chemical charge.
The name white liquor comes from its appearance: a clear to pale yellow liquid with a caustic smell. It must be distinguished from black liquor, which is the spent cooking liquid containing dissolved lignin and other organic compounds recovered after pulping. This black liquor is burned for energy recovery in the kraft mill's recovery boiler, and the inorganic chemicals are separated and recycled back into white liquor through the recovery process. This recycling loop is central to the economics and sustainability of modern kraft mills.
Fresh white liquor must be made up when chemical losses occur during the process and recovery steps. Sodium carbonate (soda ash) and lime are used to regenerate sodium hydroxide from the inorganic salt cake recovered from black liquor combustion. Maintaining correct caustic strength and sulfide concentration is critical: too weak and the cook will be incomplete; too strong and fibre quality suffers and caustic consumption rises. Mills monitor white liquor composition regularly using titration and ion chromatography.
White liquor handling demands respect for its caustic nature. Storage tanks are typically steel with internal linings, and exposure can cause severe chemical burns to skin and eyes. Spills can contaminate groundwater and require immediate neutralization. The smell, a combination of hydrogen sulfide and mercaptan compounds, serves as a warning signal but is not a reliable safety indicator at dangerous concentration levels.