Capture of Calcium Ion in Hard Water by Precipitation Reaction of Sodium Silicate-Sodium Aluminate System

Abstract

As to builder effect in synthetic detergent, the gelatinous precipitate was prepared by adding mixed solutions (Al/(Al+Si) molar ratio 0.17-0.83) of sodium silicate and sodium aluminate into hard waters (0.6×10-3-12.0×10-3 Ca2+ mol dm-3). The capturing and its mechanism of calcium ion in suspension (pH 7-11) of the gelatinous precipitate were investigated as against synthetic A-type zeolite (Na2(Al2Si2O8)⋅4.5H2O) used as a common builder, Characteristics of the gelatinous precipitate after capturing calcium ion were determined by X-ray diffraction, infrared spectroscopy, chemical analysis, dissolution test, measurements of specific surface area and ion-exchange capacity. The capturing capacity of calcium ion in suspension (pH 10-11) after adding the mixed solutions into hard water of 12.0×10-3 Ca2+ mol dm-3 increased remarkably against those of individual solutions. In a mixed solution having Al/(Al+Si) molar ratio of 0.5, the calcium ion in hard water was captured up to 95% in a shorter time as against the synthetic A-type zeolite. The composition of gelatinous precipitate after capturing calcium ion was formulated as Ca (Al1.4Si1.7O6.5)⋅5.2H2O. Also results of infrared spectrum and dissolution of calcium ion in water of the gelatinous precipitate showed that most of the calcium ion was strongly captured to the precipitate. Moreover most of the captured calcium ion as about 70% was exchangeable with sodium ion, That is, in the earlier stage of the reaction after adding mixed solution into hard water, a low crystalline precursor as familiar synthetic A-type zeolite was formed and changed finally to a low crystalline precursor like zeolite (gismondine) indicating as Ca(Al2Si2O8)⋅4H2O by ion exchanging of Na+→Ca2+. While, the effect of concentration of hard water and pH in suspension on capturing ability of calcium ion were investigated from a practical view. It was apparent that the capturing ability of calcium ion was almost equal to that of synthetic A-type zeolite in the case of the concentration of hard water (0.6×10-3 Ca2+ mol dm-3) as city water at pH 7-8.