Effects of Chloride, Potassium, and Sulfate on the Causticizing Reaction in the Kraft Recovery Process

We studied the effects of chloride (Cl), potassium (k), and sulfate (So4) on the equilibrium of the causticizing reaction over a wide range of causticizing conditions, using oli, an advanced thermodynamic program for equilibrium calculations in aqueous solutions of ionic salts. Cl, k, and So4 were shown to have little or no effect on the causticizing equilibrium. Under a given causticizing condition, coastal mills, due to their much higher naCl concentration, are predicted to have a causticizing efficiency 1%–2% lower than that of inland mills. Application: mills do not have to worry about the effect of chlorine, potassium, and sulfate on their causticizing plant performance. um in aqueous solutions of ionic salts, and is mainly used to examine complex chemical and electrochemical phenomena in aqueous and mixed solvent solutions [14, 15]. The program’s thermodynamic model is based on published experimental data. It uses data regression where applicable, estimation and extrapolation where required, and is capable of predicting composition and concentration for ionic salt mixtures in water over a wide range of temperature, total pressure, and ionic strength. The program has been successfully used to predict the solubility of recovery boiler precipitator ash in water under various conditions [16], and the solubility of PbSO4–NiSO4 mixtures in the metallurgical and mining industries [17]. In our study, the input parameters to the program were the amounts of different species, as well as temperature and pressure. The main species considered were water, Na2CO3, Ca(OH)2, Na2S, NaCl, KCl, K2CO3, Na2SO4, K2SO4 and KCl. In cases where simulated or predicted values were to be compared with experimental data, the input parameters also included the temperature and pressure of the liquor system and those under which experiments were carried out. The program outputs were the concentrations of Na+, K+, OH-, S-2, HS-, CO3, SO4, Cl-, etc. in the solution, and the amounts of precipitated CaCO3 as well as un-reacted Ca(OH)2. The concentrations were used to calculate the concentrations of the main species, causticizing efficiency (CE), according to Eq.1, total titratable alkali (TTA), and sulfidity of the liquor, which are defined in Eq. 3 and Eq. 4 as: ] CO Na [ ] S Na [ ] NaOH [ TTA 3 2 2 + + = (3) 100 TTA ] S Na [ Sulfidity 2 × = (4) where [NaOH], [Na2S] and [Na2CO3] are concentrations of NaOH, Na2S an,Na2CO3, in the solution, all expressed as g/L Na2O. To evaluate the suitability of the program in predicting CE, we used the program first to predict the causticizing equilibrium (or maximum CE) of pure systems that contain only Na2CO3, Na2S, Ca(OH2), and water, as a function of TTA and sulfidity. We compared the predicted results with experimental data obtained from the literature and with the data obtained from selected experiments we performed. We then used the tested program to examine the effects of Cl, K, and SO4 on CE. PROGRAM EVALUATION CE equilibrium for a pure Na2CO3 solution Figure 1 shows the CE predicted by the program (solid curve) for the pure Na2CO3 system at 90°C, with a Na2CO3 to Ca(OH)2 molar ratio of 1.25, as a function of TTA. The predicted CE is the maximum attainable CE for the system. The results are in reasonable agreement with experimental data obtained by various research groups that show the equilibrium CE decreases as TTA increases. We obtained the experimental data noted as open circles under well-controlled test conditions at 90°C for 1 h. The results were consistent with, but about 1%–3% lower than, the prediction results and literature data [1, 4, 5]. The lower experimental values we obtained suggest that the system may not have yet reached equilibrium. CE of Na2CO3-Na2S solutions The program also simulated the CE of liquors containing both Na2CO3 and Na2S. We performed the simulations at three sulfidity levels, 0%, 15%, and 30%, which have been frequently cited in textbooks [18, 19]. Figure 2 shows the predicted results 1. Effect of TTA on the maximum attainable CE at 90°C for pure Na2CO3 solutions with a Na2CO3/Ca(OH)2 molar ratio of 1.25. 22 TAPPI JOURNAL | march 2009 Inland mills Coastal mills Typical range Typical range Softwood Chloride, g/l as Cl 2.1 0.3 – 6.4 12 8 – 14 Potassium, g/l as k 4.5 2 – 7 4.5 2 – 7 Hardwood Chloride, g/l as Cl 2.1 0.3 – 6.4 12 8 – 14 Potassium, g/l as k 14 11 – 19 14 11 19 Inland mills Coastal mills Typical range Typical range Softwood Chloride, mole% Cl/(na+k) 1.4 0.2 – 4.2 8 5 – 10 Potassium, mole% k/(na+k) 2.8 1.3 – 4.5 2.8 1.3 – 4.5 Hardwood Chloride, mole% Cl/(na+k) 1.4 0.2 – 4.2 8 5 – 10 Potassium, mole% k/(na+k) 8.5 6 – 11 8.5 6 – 11 I. Concentrations of Cl and K in green and white kraft liquors.