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

摘要

Nanofiltration of saturated calcium sulfate solution of 0.02 mol/L calcium contnet,with CO_3~(2-)/SO_4~(2-) molar ratio ranging from 0.0500 to 2.8X10~(-3) was carried out in 6.5X10~(-4) m~2 active membrane area laboratory module at 1.2X10~6 Pa transmembrane pressure using the total re tentate recycle model.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 CaSO_4 solution,was also observed in the case of CO_3~(2-)/SO_4~(2-) molar ratio equal to 2.8X10~(-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 Ca_2(CO_3)(SO_4)·4H_2O (so-called repidcreekite) was also identified as a result of carbonate and sulfate co-precipitation under low carbonate content conditions.