causticisering

Causticization is a chemical process used in the pulp and paper industry, most notably within the kraft process recovery cycle. It converts sodium carbonate, present in green liquor, into sodium hydroxide by reacting with calcium hydroxide, producing calcium carbonate as a solid. The principal reaction is Na2CO3 + Ca(OH)2 → 2 NaOH + CaCO3.

In kraft pulping, green liquor, which contains sodium carbonate and sodium sulfide, is treated with freshly prepared lime (Ca(OH)2) in a recausticizing reactor. The resulting white liquor mainly contains sodium hydroxide and sodium sulfide and is returned to the digester to continue cooking. The calcium carbonate formed precipitates as calcium carbonate (lime mud), which is filtered, washed, and typically calcined back into quicklime in a lime kiln to sustain the cycle.

Key equipment and steps include the lime kiln to produce CaO, a lime slaker to form Ca(OH)2, and a recausticizer where green liquor and lime slurry react. The process is designed to maximize the conversion of Na2CO3 to NaOH while producing manageable quantities of CaCO3. Efficiency is influenced by factors such as temperature, particle size and solution alkalinity, residence time, and impurities in the green liquor.

Causticization is primarily a recycling step, enabling the regeneration of caustic soda from carbonate species within the black liquor recovery loop. The resulting white liquor is used in the cooking processes, while the precipitated calcium carbonate is handled as a solid byproduct in the lime cycle.