Abstract
For more than a century, crystallization has remained a chief research topic. One of the most undesirable crystallization phenomena is the formation of calcium carbonate scale in drinking and industrial water systems. In this work, the influence of chemical additives on CaCO3 formation-in either nucleation, crystal growth, or inhibition processes-is investigated by using the CO2-degasification method. Chemical additives are foreign salts (MgCl2, Na2SO4 and MgSO4) to the calco-carbonic system and antiscalants (sodium polyacrylate 'RPI' and sodium-tripolyphosphate 'STPP'). The results show that additives affects both crystallization kinetics and the CaCO3 microstructure. Sulfate and magnesium ions, added separately at constant ionic strength, influence the nucleation step more than the growth of the formed crystallites. Added simultaneously, their effect was accentuated on both nucleation and the growth of CaCO3. Furthermore, antiscalants RPI and STPP affect the crystallization process by greatly delaying the precipitation time and largely increasing the supersaturation coefficient. It was also shown that the calco-carbonic system with additives prefers the heterogeneous nucleation to the homogeneous one. X-ray diffraction patterns show that additives promote the formation of a new crystal polymorph of calcium carbonate as aragonite, in addition to the initial polymorphs formed as calcite and vaterite.