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Interaction of iron with wood pulp fibres Susilo, Robin


Considerable research has been done to reduce water consumption by recycling the process water. The goal is to achieve zero liquid discharge or "closed-cycle" mill operation. However, there are still some problems due to the presence of some materials that are not needed in the pulping process and which tend to accumulate in the system and create operational problems. These materials mostly are metal ions that are known as Non-Process Elements (NPEs). They originate mainly from wood, and also from process water and the makeup lime. Therefore, understanding the interaction of these harmful metal ions with the fiber is needed to manage these metals in a closed-cycle pulp mill. Fibers have many functional groups such as: carboxyl acid, phenolic hydroxyl, hexenuronic acid, polysaccharide acids and catechol groups. These groups create a negatively charged fibre when they dissociate in water. Hence, fibres can interact with the metal ions like iron, manganese and copper electrostatically, chemically or both. The focus of this work is the interaction of iron with wood fibres. Pulp samples from British Columbia interior and coastal mills were investigated. Metal ion concentration on the fiber at various pulping stages was determined. The sampling points at the pulp mill were taken after brown stock washer, after oxygen delignification, and at various stages of the bleaching processes. It was found that fibre from a coastal mill has a higher iron concentration compared with fibre from an interior mill. The water used in transporting the log might introduce iron into the wood so that pulp from coastal mill would have more iron. It was also found that iron concentration in the fiberline did not change much because iron form precipitates and trapped inside the fiber mat. Metal ion removal methods using a combination of acid washing and chelation in four-washing stages was developed. A chelating agent (DTPA) was able to increase the iron removal substantially. The fiber properties such as the water retention value (WRV) and the charge properties were determined for the metal ion partitioning prediction. The fiber saturation point was found to increase as the pH increase due to swelling for all samples. Two charged groups were able to represent the fiber charge on the fiber, one charge group dissociates at acidic condition and the other one dissociates at alkaline condition. The origin of these two functional groups was not identified. The acidic charge group might probably contain carboxyl acid bound to lignin (pK[sub A]»5-6) and uronic acid (pK[sub B]~3-4). The alkaline charge group might be the phenolic hydroxyl bound to lignin. It was also found that the charge content decreases down the fiberline as the lignin content decreases. Partitioning experiments were conducted whereby we measured the iron concentration on the fiber and in the surrounding liquor. The fibres were previously acid washed with a chelating agent to "free" them from the metals. The results indicated that iron strongly stayed on the fiber even at acidic conditions where the fibre did not contribute any charge to attract the metal ion. SEM & EDX analysis confirmed the presence of iron precipitates on the fiber and trapped inside the fiber mat so that these precipitates become the fibre phase belonging. Iron might probably form complexes with the anionic group in the water, especially the hydroxyl group. A partitioning model based on Donnan equilibrium was used to compare equilibrium concentration of iron in fibre and the surrounding liquor. The model calculated values were not found to be in agreement with experiment data probably due to iron-compound precipitation. It should be noted that the model was able to predict manganese and copper partitioning, although the predictions were slightly lower than the experimental data especially at higher pH (pH > 7). Manganese and copper containing precipitates were encountered at pH 7 but not at pH 5. Manganese and copper data were obtained in another study in our laboratory. It can be concluded that the iron interaction with the charge groups on the wood fibres is not a chemical binding interaction, but due to the iron-compound precipitates which are trapped inside the fibre web. Hence, a strong water soluble ligand like those chelating agents can bind the metal ions from the suspension so that the harmful metal ions can be removed from the suspension.

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