causticizing

Causticizing is a chemical process used to convert soluble sodium carbonate into sodium hydroxide by reacting it with calcium hydroxide, commonly called lime. In the kraft pulping process, causticizing regenerates sodium hydroxide in white liquor from carbonate species present in green liquor, while producing calcium carbonate as a solid waste. The core reaction is Na2CO3 + Ca(OH)2 → 2 NaOH + CaCO3(s).

Industrially, lime is produced by calcining limestone to yield quicklime, which is then slaked in water to form a Ca(OH)2 slurry. The slaked lime is mixed with green liquor from the recausticizing cycle; the carbonate is converted to hydroxide and precipitates as calcium carbonate (lime mud). The resulting white liquor, containing NaOH and some sulfide for pulping, is clarified and returned to the cook liquor cycle. Lime mud is either sent to lime kilns to regen­erate lime or processed for disposal.

Causticizing efficiency depends on lime quality, reaction temperature, and mixing; incomplete conversion leaves carbonate species in white liquor and increases lime consumption. Typical concerns include handling of insoluble impurities, energy for lime preparation, and management of calcium carbonate sludge. The process is energy-intensive due to lime slaking and kiln operations but essential for maintaining alkaline conditions in cooking.