Abstract
The behavior of CaCO3 precipitation from solutions containing Ca2+ and HCO3- ions was examined using vessels with and without heat transfer surface. It was found for solutions at 25°C that the precipitation takes place whenever the ion product, Ksp=(Ca2+)(CO32-), exceeds 2×10-7. The situation does not change with stagnant and circulating solutions. These results were inconsistent with the expectation from the Langelier index because Ksp value of 10-9-10-8 is assumed in calculation of the latter. The amount of precipitates was found to increase with the rise of pH and temperature. This is expected because the fraction of CO32- increases with pH and the value of Ksp decreases with temperature. The behavior of scale formation on the heat transfer surface was shown to be explained on the same basis as described above. Deposition of CaCO3 is facilitated because the temperature of the surface is considerably higher than that of the bulk solution. It was indicated that the addition of mineral acid to the solution to keep the solution pH 6.5-7.0 is quite effective to avoid the scale formation.
