Kinetics of Re-Mineralization of Acidified Desalinated Water by Dissolution of Micronized Calcium Carbonate Particles

David Hasson, Chemical Engineering, Technion-IIT, Haifa, Israel
Hilla Shemer, Chemical Engineering, Technion-IIT, Haifa, Israel
Raphael Semiat, Chemical Engineering, Technion-IIT, Haifa, Israel

The common method of adding calcium and bicarbonate ions to desalinated water is by flow of the water acidified with CO2 or H2SO4 on a fixed bed of CaCO3 granules. A recently proposed process is based on dissolution of micronized CaCO3 particles in slurry flow with acidified desalinated water in the recycle system shown in Fig. 1.

Fig. 1. Schematic description of the micronized dissolution process

The powder dissolution concept has conspicuous advantages such as lower footprints and shorter retention times. It has a high potential for replacing the current fixed bed process.
Kinetic models were developed based on the assumption that dissolution of the particles by either CO2 or H2SO4 is mass transfer controlled, taking into account the particles residence time distribution in the recycle CSTR system. Dissolution parameters are displayed in Fig. 2.

Fig. 2. Dissolution of pulverized calcite calcium carbonate particles

The basic differential equation for calcite dissolution with CO2 is:

where [Ca]e and [CO2]e represent equilibrium concentrations. The outlet calcium ion concentration is given by:

Results of an extensive experimental study in which the effects of solution parameters and residence time were measured yielded excellent agreement with the theoretical models as exemplified in Figs. 3 and 4.

Fig. 3. Effect of inlet slurry concentration on outlet calcium ion concentration at constant inlet CO2 and retention time

As anticipated, dissolution with the strong H2SO4 acid was much more rapid and required lower acid concentrations for achieving the same outlet calcium ion concentration (Fig. 4).

Fig. 4. Outlet calcium ion concentration by dissolution
with identical CO2 and H2SO4 concentrations