Recausticizing

Recausticizing is a chemical process used in the kraft pulp industry to convert the soluble sodium carbonate in green liquor into sodium hydroxide, enabling the production of white liquor for cooking. It closes part of the chemical recovery loop by using lime derived from the lime cycle to regenerate the caustic components needed in pulping.

In kraft pulping, the recovery boiler produces green liquor, which contains dissolved Na2S and Na2CO3. Recausticizing introduces lime (CaO or Ca(OH)2) to this green liquor. The principal causticizing reaction is Na2CO3 + Ca(OH)2 → 2 NaOH + CaCO3(s). The calcium carbonate precipitates as lime mud, which is separated from the liquor. The clarified liquor becomes white liquor, containing NaOH and Na2S, suitable for use in the digesters.

The solid calcium carbonate produced is returned to the lime cycle, where it is calcined in a lime kiln to regenerate quicklime (CaO), which is then hydrated to Ca(OH)2 for reuse in recausticizing. This cycle reduces the need for fresh lime and improves chemical recovery efficiency. The recausticizing step also influences the quality of white liquor, affecting digesting performance and scale control.

Overall, recausticizing is a key link between the lime kiln and the chemical recovery process in kraft mills. It enables efficient regeneration of caustic chemicals, minimizes chemical inputs, and supports energy utilization within the recovery system. Potential operational considerations include control of solids, temperature, and impurities in green liquor.