The specifics of chemical sedimentation of calcium ions from diluted aqueous solutions

Abstract

Increasing the hardness of natural waters requires the introduction of mitigation systems in offices and private homes and apartments, where it is impossible to implement heating and settling of water, typical of traditional technologies. The efficiency of soda - lime softening of water in conditions of low temperatures and in the absence of settling has been studied very little. Therefore, the purpose of this work was to study the features of soda - lime softening under different conditions in terms of its use in household and office systems. The specifics of calcium carbonate formation process during diluted aqueous solutions softening for home or office were researched. At temperatures of 13 - 18 °C, the formation of calcium carbonate requires significant reagents overconsumption compared to stoichiometry, which is a negative factor for the application of this process in such conditions. In addition the reagents quantity cannot be comparable with the obtained effect. Increased 5 times soda dose reduces the total hardness of the treated water only twice. Even increased 10 times soda dose cannot reduce the residual hardness of the treated solution lower 0.6 mg-eq/dm3. Also there is no clear dependence of the settling intensity on the ratio of components. The pH increasing is less effective than temperature increasing. The process of formation sediment also depends on changing these parameters. It was found that in a neutral medium at temperature 30 °C and initial water hardness of 8 mg-eq/dm3 only single flakes of calcium carbonate were occurred in the solution. By further heating, the number of such individual flakes constantly increases and at 70 °C they were occurred even without treatment with sodium carbonate. Increasing pH up to 8.4 at 40 °C was accompanied by solution turbidity, and after treating by precipitant the instantaneous formation of a white fine precipitate was observed in all water volume. Increasing pH up to 10 reduces solution turbidity temperature to 30 °C and the instantaneous formation of calcium carbonate particles started at this temperature. It was also found that water softening is more effective with its big initial hardness. At the water temperature 13 °C calcium ions are sedimented quite intensively under certain conditions. The ratio between concentrations of carbonate ions and calcium ions K = [CO32-, mg-eq] / [Ca2+, mg-eq] is a determining factor. This is especially noticeable in the range of coefficient K from 0 to 15. Further increasing of carbonate ions dose did not provide a noticeable effect, regardless of the initial water hardness. However, even in such conditions, the use of such method at low temperatures and without settling has quite low efficiency and can’t be used for water softening in office or home conditions.