The Effect of Citric Acid (C_6H_8O_7) and Flow Rate to the Growth and Transformation of Crystal Phase of CaCO_3

摘要

Scale formation, especially of calcium carbonate (CaCO_3) is a serious problem faced by many industrial processes, such as chemical, desalination and oil industries. This paper discusses crystallization scaling of CaCO_3 in pipes. The crystallizing solution was made by mixing equimolar solution of CaCl_2 and Na_2CO_3. The parameters studied were the solution flow rates 40mL/min, temperature (30, 40, 50°C) and citric acid (0.00, 5.00, 10.00 ppm) as additives for the scaling inhibition. The CaCO_3 scale formation process has monitored the change of conductivity the solution out of coupons. The scale formed was characterized by SEM, EDS and XRD. This conductivity decline period, known as induction time varied from 30 min to 46 min depending on the parameters studied. Higher flow rates and temperature resulted in more CaCO_3 scale mass, indicating that these two variables may promote CaCO_3 crystallization. The higher citric acid concentrations resulted in less mass of the scale and could reduce the mass down to 88%. Therefore, it can be postulated that citric acid may be an effective additive for CaCO_3 crystallization and scaling. SEM and EDS analysis of the scale indicates that the scale was essentially CaCO_3. In addition, an XRD test was applied to the scale and proved that the scale consisted of calcite, vaterite and aragonite phases. The scale formed under the influence of citric acid and temperature showed more aragonite phase, indicating that the acid could delay the transformation of CaCO_3 crystals into the ultimate stable phase, i.e. calcite.