Speciation and supersaturation model in papermaking streams

Environmental responsibility of papermaking industry leads to intensive reuse of water streams. As well known in chemical engineering, this closure of loops causes the accumulation of soluble species, including calcium ions and inorganic carbon. The result is increased scaling, mainly by calcium carbonate. Due to complex behaviour, predicting scaling in papermaking streams is not straightforward and process water disposal should be kept at a high level in order to prevent scaling. This is contradictory with the objective of preserving the environment.Calcium carbonate precipitation is considered as complex since:-speciation calculation must take into account several complexes and protonated species (CaCO30, CaHCO3+, CaOH+, HCO3−, H2CO3) in addition to Ca2+ and CO32−;-several polymorphs can precipitate;-the nucleation requires a rather high supersaturation to occur in homogeneous way.We combined a mass balance approach using the CTP proprietary software PS2000 (Ruiz et al., 2003), specially developed for the simulation of actual papermaking plants and the USGS software PHREEQC (Parkhurst and Appelo, 1999), devoted to the speciation in complex aqueous solutions, to predict the local value of the Ionic Activity Product (I.A.P.) in the nodes of paper plants streams.These data were analysed using the results and methodology by Elfil and Roques (2004), comparing the I.A.P. with the solubility products of the three relevant polymorphs (anhydrous calcium carbonate, monohydrate calcium carbonate, calcite) to predict the risk of scaling.A campaign of analysis on an industrial site permitted to benchmark the calculation.

Research highlights▶ Combination of iterative solution of heat and mass balance equations with use of a powerful software for speciation allows to accurately model the chemical behaviour of water streams in an industrial plant for the production of recycled cellulose fibres. ▶ Speciation in papermaking aqueous streams must take Volatile Fatty Acids (VFA's) into account. ▶ Prediction of scaling in papermaking circuits is possible and should be a useful tool in order to improve the closure of water circuit, a decisive challenge from the environmental point of view.