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Fouling by calcium oxalate in aqueous solution Lencar, Diana R.


Calcium oxalate, a normal solubility salt, gives rise to deposits in the human kidneys, and in process equipment in the food and pulp and paper industries. Although crystallization kinetics are available for medical conditions, and solubility data have recently been developed for bleach plant situations, no deposition studies under industrial processing conditions have been reported. In this work, the solubility-temperature relationship for calcium oxalate was first confirmed in beaker tests. A heat transfer loop originally designed for heating experiments was completely re-built to permit thermal fouling measurements under cooling conditions. Supersaturated solutions of calcium oxalate at controlled pH and 75°C formed by rnixing calcium nitrate and sodium oxalate solutions from a 175 L tank were recirculated over periods of about 6 days through a double pipe heat exchanger cooled with water at 5-10°C. The decline in heat transfer coefficient as the deposit formed was monitored with time. A continuous bleed of fresh chemicals was necessary to sustain fouling. The effects on fouling rate and deposition morphology of the pH, initial supersaturation, calcium/oxalate ratio and flow rate were explored. Interpretation of results was complicated by the presence of suspended crystals at high supersaturation. The fouling thermal resistance increased linearly with time so long as fresh oxalate species were added. Fouling ceased when the particles concentration in the solution decreased. In the 6-day runs, the overall heat transfer coefficient declined by 7-58% depending on operating conditions. The pH was varied between 2.2 and 6.3. The initial fouling rate was found to rapidly increase with pH reaching a maximum at pH = 3 and then rapidly decrease and remain constant for pH>4.5. The initial fouling rate also increased with Reynolds number from 8,000, reached a maximum at about 15,000, and then decreased for Re up to 22,000. The maximum initial fouling rate was obtained for a Ca/Ox ratio of 2/1. The initial fouling rate was found to decrease as the initial relative supersaturation increased from 1 to 6 and then remained fairly constant as the supersaturation was further increased to 14.5. Deposits were confirmed to be calcium oxalate monohydrate through thermogravimetric and wet chemical analyses.

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