Study Case: Optimization of recausticizing plants in a Brazilian pulp mill, 2010 TAPPI/PAPTAC International Chemical Recovery Conference

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A control strategy was developed for recausticizing plants in a Brazilian pulp mill that had great variability on causticizing efficiency, presenting overliming and underliming problems. A control strategy was developed according to the principles of physic-chemical causticizing reaction. The strategy target is to achieve the maximum causticizing efficiency based on Goodwin Curve, with a safety margin previously set. The lime dosage, stoichiometric done, is based on sodium carbonate present in green liquor and it is recalculated according to the causticizing efficiency measured in the slaker outlet, whose updating frequency depends on the process retention time. The following gains were reached in 2008 compared to 2007: Reduction of more than 50% in the causticizing efficiency standard deviation, decrease of 4% in lime feeding, reduction of 3% in specific consumption of reburnt lime and 13% in acquisition of lime, decrease of 10% in the “dead load” (Na₂CO₃) on recovery circuit and significant reduction of overliming occurrence. Furthermore, it was noticed a better efficiency on lime mud filters, resulting in higher solids content lime mud and, consequently, greater thermal efficiency in lime kilns. There was an increase on white liquor stability and quality, resulting in lower suspended solids on white liquor, with perceptible reduction of incrustation in digesters, as well as higher content of effective alkali provided to them.