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Crystallization of mechanical pulp mill effluents through hydrate formation for the recovery of water Gaarder, Cathrine


This work presents data toward the development of a new zero-liquid discharge (ZLD) system, the aim of which is to concentrate a liquid effluent thereby recovering process water. The crystallization technology used in this work is clathrate hydrate crystal formation. A screening of clathrate hydrate formers was performed and three were chosen, propane, carbon dioxide, and a 30-70 mol% propane - carbon dioxide mixture. Experiments were carried out with these substances in a vessel that was immersed in a temperature controlled bath. The effluent samples used in this study were generated at four pulp mills, an unbleached and a bleached thermo-mechanical pulp mill (TMP1, TMP2), a bleached chemi-thermo-mechanical pulp mill (BCTMP), and a combined bleached BCTMP/TMP mill (CTMP). The aim of this work was two-fold. The prime objective was to study experimentally the formation of clathrate hydrate crystals in these effluents. Experiments were designed to measure: (1) The temperature and pressure conditions at which hydrates form in mechanical pulp mill effluent; (2) The induction time and the growth rate of crystal formation; and (3) The ability of this apparatus to concentrate the effluents in-situ. In addition, effluent characteristic tests were performed, and qualitative process characteristics were noted. A secondary objective was to perform a survey on the liquid effluents generated at TMP and CTMP mills. This survey focused on the type, concentration, source, and environmental impacts associated with the contaminants typically present in these effluents, on the current discharge regulations, and on the available treatment options. It was determined that hydrate crystals can form in these effluents at temperatures above the normal freezing point of water. The measurements were compared with similar hydrate formation data in pure water. It was also found that the presence of impurities did not cause any appreciable change in the hydrate crystal pressure and temperature formation conditions with the TMP and CTMP effluent samples, but that the BCTMP effluent, which had a high electrolyte and organic content, did affect these conditions. It was found that the induction period and growth rate of hydrate crystals are dependent on the driving force and the prior history of the effluent. And finally, that some level of in-situ concentration was attainable with our apparatus.

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