Relations between Degree of Desalting and Brine Composition

Scale Formation in Concentrating Sea Water with Ion Exchange Membrane Method (I)

Abstract

In concentrating sea water with ion exchange membrane method, relations between degrees of desalting and composition of brine particularly concentrations of scale-forming components at concentrating interfaces of membranes were studied. Experiments were carried out in systems. consisting of divalent hardly permeable (“permselective”) membranes to only anion and to both anion and cation. The decreasing behaviors of ions in desalted solution started from sea water were shown graphically by ratios of corrected concentration of each ion in desalted solution to that in sea water (“residual ratios”). At a degree of desalting on individual curves in the graph, ratios of products of differences from unity and concentrations of the ions in sea water gave concentration ratios of the ions in brine, and ratios of products of inclines of tangents and concentrations of the ions in sea water gave-ratios of the ions migrated through the membrane. Both concentrations of calcium carbonate and calcium sulfate at concentrating interfaces of anion exchange membrane increased prominently with increasing degree of desalting. These were consistent with practical observation of scale deposition, and seemed to show that partial over-desalting might be. one of important causes of scale deposition at membrane surfaces. The other prominent in reasing tendency of concentrations of scale-forming components was recognized when “permselective” membranes were placed near each other in a concentrating compartment. This seemed to suggest important problems on scale deposition in future. Changes of composition of brine with changes of degree of desalting were delt well with improved expression of permselectivityin more than 0.2 in “residual ratio” of sodium ion or chloride ion. Then, composition of solutions at concentrating interfaces of membranes could be calculated in this range of degree of desalting. It seemed to be necessary hereafter to characterize membranes with manner of changing results with changing operating conditions in electrodialysis.