Speciation of Ca2+ ions in black liquors determined by a Ca-ion-selective electrode

There is a great need to understand calcium scaling in black liquor evaporators and to reduce the possibilty for scaling. However, it is still largely unknown how calcium is bound to organic compounds in black liquors. The main goal of this work has been to study the possibility to determine different Ca species in black liquor by using an all-solid-state Ca2+-ISE (Ion selective electrode). The Ca2+-ISE's where constructed by drop casting a Ca2+ selective cocktail on a conducting polymer film (PEDOT), which was electropolymerized on a glassy carbon rod in PVC body. These Ca2+-ISE's showed fast and reproducible response for free Ca2+ ions in diluted black liquors. The speciation studies was done by using Ca2+-ISE's to measure the free Ca2+ ions in black liquors during potentiometric titrations with a complexing agent (EDTA). The titration results were evaluated by using the method of linear titration curves. This method enabled simultaneous determination of both the free and bound Ca2+ ions in black liquors. The results showed that in the studied black liquor sample ca 50% (1.15.10(-2)M) of Ca2+ ions was in the free form and ca 50% (1.06.10(-2)M) was bound to organic compounds. The ultimate goal of this work is to measure Ca2+ ions on-line in a black liquor stream in order to get a more effective and environmentally friendly process by predicting and reducing the risk of calcium scaling in the evaporator equipments.