Calcium Carbonate Scale Formation in Pipes: Effect of Flow Rates, Temperature, and Malic Acid as Additives on the Mass and Morphology of the Scale

It is well recognized that calcium carbonates (CaCO3) is one of the main components of scale that is commonly encountered in chemical and related industries. The calcium carbonate scale often grows extensively on equipment and parts, causing major operational difficulties. This paper presents experiments on calcium carbonate scale formation and control in a piping system where the scale-forming solution flowed in a laminar manner: 30, 40, and 50 mL/min, respectively. Other parameters evaluated were: solution temperature (25, 30 and 40 °C), and concentrations of malic acid (C4H6O5) added as impurities (3.00 and 5.00 ppm). The scale-forming solution was made by mixing equimolar solutions of CaCl2 and Na2CO3, respectively. The scale formation process was monitored by measuring the conductivity of the solution coming out of the piping system. It was found that in all experiments, conductivity decreased abruptly after a certain induction period, during which time the conductivity remained steady. The induction period varied from 17 min to 34 min, which means that the scale starts forming 17 min to 34 min after the mixing of the solution. Higher flow rates resulted in more calcium carbonate scale mass, which indicate that the fluid flow enhances the scale formation. Similarly, increasing the temperature of the solution (25, 30, and 40 °C) resulted in the increase of the scale mass. Overall, higher malic acid concentrations resulted in longer induction time and less scale mass. Depending on the temperature and the malic acid concentration tested, the reduction in scale mass could be ≥ 200%. This drastic reduction in scale mass suggests that malic acid could be an effective anti-scalant for calcium carbonate scale. SEM imaging and its associated EDS analysis confirmed that the scale formed corresponds to that of calcite (CaCO3). The X-ray diffraction analysis of the scale showed that the scale consisted of crystalline matter which corresponds to the powder diffraction data for calcium carbonate. The addition of malic acid in trace amounts (0.00 to 5.00 ppm) was able to alter the morphology of the scale crystals, indicating the preference adsorption of malic acid on specific crystal surface.