The Rule And Mathematic Model Of Scaling Ions Formation During Alkaline Pulping

Calcium scales of evaporator appear to be a serious problem in alkali recovery from black liquors, which restricts the normal operation of alkaline recovery system. Numbers of studies have been done on the control and reduction of calcium scales. However, most of them have focused on scale removal and reasons for scaling. There are few reports on the release or formation of oxalate, carbonate and dissolved calcium and even rarely known about the sources of oxalate and carbonate during the pulping processes. Therefore, the formation and sources of oxalate, carbonate and dissolved calcium were investigated in this work, in order to control these substances formation or release during the pulping processes. The result is significant to deepen the recognition of calcium scales of evaporator, control and reduce evaporator scaling, improve energy efficiency and ensure the normal operation of production.For these reasons, under typical alkaline pulping conditions, southern pine and acacia were taken as raw materials to reveal the oxalate formation rule, in combination with the novel headspace gas chromatographic method for determination of oxalate in pulping effluents. The results showed that acacia was found to have more oxalate than southern pine, and the oxalate content in acacia wood is 6~7 times as that of southern pine. Also the oxalate content in black liquor was increased during alkaline pulping processes. The trend rapidly increased during the early stages of the cooking. It was found that a higher hydroxide concentration led to a higher oxalate content in the black liquor. The pulping of acacia resulted in more oxalate being formed than that of southern pine under the same pulping conditions. The effect of pulping time, H-Factor, alkaline charge and sulfidity on the oxalate formation was also studied. Then a kinetic model of oxalate formation was developed based on the experimental results. The good regression coefficients (R2 = 0.981 and 0.963 for southern pine and acacia respectively) indicate that the models are justifiable for the oxalate prediction, and applicable to kraft and soda pulping black liquors. These models are reasonable expressions for the concentration of oxalate formed during alkaline pulping. Meanwhile, the oxalate source was revealed during alkaline pulping processes. Oxalate is formed in the pulping process from two main sources, i.e., oxalate and oxalic acid are existed in native wood, formed from lignin and carbohydrate degradation in the cooking.It was found that the carbonate content in black liquor was increased during alkaline pulping processes. At the same conditions, the pulping of acacia resulted in more carbonate being formed than that of southern pine. The relationships between the carbonate content and H-Factor, as well as the concentration of initial effective alkali were established. And then, a kinetic model of carbonate formation was developed based on the experimental results. The good regression coefficients indicate that the models are justifiable for the carbonate prediction, both for the kraft and soda pulping black liquors. Further investigation showed that carbonate was formed from lignin and carbonate degradation during pulping processA detailed investigation on the effects of the major compositions of black liquor on the soluble calcium formation was conducted. The studies showed that the content of the soluble calcium in black liquor is mainly affected by the amount of the dissolved lignin and, carbonate and oxalate. The results clearly indicate that dissolved calcium content is negatively correlated with the content of dissolved lignin. The formation constant of the dissolved calcium-lignin complex in this equation is 102. More soluble calcium was found in the kraft pulping spent liquors. The amount of the soluble calcium in cooking liquor can be effectively suppressed when a total concentration of oxalate and carbonate reaches a level of 0.3mol/L in the kraft pulping process.The rule of oxalate formation was revealed during oxygen delignification. The effects of process conditions of oxygen delignification on oxalate formation were also explored. Results showed that oxalate content in the effluent oxygen delignification increased with increasing time and the relationship between them was linear. The effect of temperature on oxalate formation was significance during oxygen delignification process. The oxalate content in the effluent of oxygen delignification increased with the temperature during oxygen delignification process. At the same conditions, the oxalate content increased when the alkali charge increased from 2.5% to 3.5%. Oxygen pressure had a little effect on oxalate formation. The relationship between the oxalate formation and dissolved lignin and Kappa number was investigated during oxygen delignification process. According to the relationship, a kinetic model of oxalate formation was developed based on the experimental results. There is a good agreement between the calculated and measured data, indicating that this model is reasonable expression for the concentration of oxalate during oxygen delignification. The regression coefficient between the predicted and the measured was 0.969. The empirical models can predict the oxalate content in the effluent of oxygen delignification.Furthermore, oxalate and carbonate formation rules were also revealed during hot-water and alkaline pre-extraction processes. The effects of hot-water and alkaline pre-extraction on the formation of oxalate and carbonate, characteristics of black liquor and pulp properties were analyzed. The investigation showed that the contents of oxalate and carbonate in black liquor of southern pine and acacia after hot-water and alkaline pre-extract were lower than those of corresponding control alkaline pulping. Hot-water and alkaline pre-extraction can improve action efficiency of effective alkali, increase the lignin solubility and reduce the methanol formation.This study will promote a new pulping approach of combining pulping with bio-refinery, control the formation of calcium scales and deepen the understanding of the evaporator scaling source, and provide a novel way to control evaporator scaling and increase energy efficiency.