Scaling analysis of nanofiltration systems fed with saturated calcium sulfate solutions in the presence of carbonate ions

2 2 Nanofiltration of saturated calcium sulfate solution of 0.02 mol/L calcium content, with CO3 !/SO4 ! molar ratio !3 !4 2 ranging from 0.0500 to 2.8×10 was carried out in 6.5×10 m active membrane area laboratory module at 1.2×106 Pa transmembrane pressure using the total retentate recycle mode. Permeate mass flow and retentate calcium concentration vs. time and concentration factor (CF) curves allowed identification of calcium sulfate crystallization mechanisms. Though both bulk and surface crystallization mechanisms were identified, they were, however, strongly affected by water quality. A non-fouling CF range up to 2, which is probably due to the existence of metastable supersaturated 2 2 CaSO4 solution, was also observed in the case of CO3 !/SO4 ! molar ratio equal to 2.8×10!3. Inorganic scales at the end of each experiment were removed from a NF module, dried at room temperature during 24 h and then examined using the X-ray diffraction method. Gypsum and aragonite were identified as the most common calcium sulfate and calcium carbonate precipitates, respectively. A mixed salt Ca2(CO3)(SO4)@4H2O (so-called rapidcreekite) was also identified as a result of carbonate and sulfate co-precipitation under low carbonate content conditions.

The scale formation mechanism as well as precipitate phase composition in the NF of saturated CaSO4 solutions with CO3 !/SO4 ! molar ratios in a range of 0.0500 to 2.8×10!3 were examined. Our experiments led us to following conclusions: C Most of the experiments showed that there are two pathways of CaSO4 scaling mechanism: heterogenous (surface) crystallization and homogenous (bulk) crystallization under high supersaturation conditions. C Under low carbonate content conditions (CO3 !/SO4 ! of 2.78×10!3) CaSO4 precipitates at the highest retentate concentration factor of about 2, which is probably due to the existence of metastable supersaturated calcium sulfate solution under these conditions.