Calcium carbonate scaling often is a critical problem for recycled board mills that have closed water circuits. The objective of this study was to determine local scaling risks throughout the production process. To predict scaling potential, we calculated several saturation indexes, based on speciation determined from detailed water analyses. Calculated scaling trends are in accordance with observed dissolution and precipitation of calcium carbonate in the process, when considering local aeration phenomena. The importance of volatile fatty acids (resulting from anaerobic bacterial activity) in calco-carbonic equilibriums is discussed, and taken into account in the speciation calculation. We also demonstrate the need to measure inorganic carbon instead of alkalinity in such conditions. This makes typical scaling indexes, such as the Ryznar Stability Index, irrelevant to predict scaling risk in closed circuit conditions; thus, it is necessary to use general speciation methods, as described in this paper.
