Formation of pirssonite in green liquor handling systems

Abstract

The formation of pirssonite is commonly believed to be the cause of hard-scale buildup in green liquor handling systems in kraft pulp mills. The precipitation occurs when the concentration of sodium carbonate in the liquor exceeds the solubility of pirssonite. A laboratory study was conducted to systematically determine the causticizing conditions under which pirssonite precipitates. The results confirmed literature data showing that the solubility of pirssonite increases with temperature but decreases with increased concentrations of sodium carbonate and other sodium salts present in the green liquor. The solubility data obtained were used to create a database for pirssonite formation in OLI, a thermodynamic program for predicting phase stabilities of alkali salts in aqueous solutions. The OLI program, with the newly created database, was subsequently used to generate a series of pirssonite solubility curves in terms of saturated total titratable alkali that can be used as operational guidelines to prevent pirssonite precipitation and hard-scale formation in green liquor handling systems. A case study was performed using these solubility curves to explain the occurrence of the pirssonite deposition problem at a kraft pulp mill.

Design, optimization and modelling of a chemical recovery system for wet spinning of cellulose in sodium carbonate solutions

“…7 Solubility of pirssonite at 95°C in solutions containing Na 2 CO 3 -NaOH-CaCO 3 -H 2 O. Experimental data of: Frederick et al (1990), Zakir et al (2013), and model of Ulmgren et al (1999) higher than typical efficiencies (80-83%) obtained at green liquor causticizing plants in pulp mills (Frederick et al 1990). As the cellulose dissolution stage requires low content of residual CO 3 2-, the high causticizing efficiency is beneficial to the process since it reduces the need for further carbonate removal.…”

“…Precipitation of pirssonite is undesired because it leads to an effective loss of alkali in the causticized solution and high sodium content in the lime mud that affects recalcination efficiency (Frederick et al 1990). Solubility data published by Frederick et al (1990), Ulmgren et al (1999) and Zakir et al (2013) were used to estimate the safe ion concentration in the causticizing stage (Fig. 7).…”

“…10 based on the mass of Ca. Assuming that Ca is not dissolved or 2and OHduring causticizing at 95°C for the original solution ( ), and the solution diluted to 60% of its original concentration ( ), compared to the approximate solubility limit for pirssonite published by Zakir et al 2013 ( ) and Ulmgren et al 1999 ( ) Fig. 9 Experimental CE of synthetic solutions containing 24 wt% Na 2 CO 3 , 4 wt% NaOH, 0.4 wt% ZnO, causticized with industrial lime at 98°C, with and without pre-dilution and addition of 1.5 wt% additive and 1.5 wt% carbohydrates added by any other way during causticizing, this way of presenting the elemental composition of lime and lime muds allows a straightforward comparison of the changes in NPE content.…”