The problem of alkaline scale formation from a study on Arabian Gulf water

An Experimental procedure is developed to study alakaline scale formation from Arabian Gulf water. The brine is refluxed at increasing temperatures under N 2 bubbling for five hours. After cooling to room temperature the water is analyzed for HCO − 3, CO 2− 3 in solution and for precipitated CaCO 3 and Mg(OH) 2. The results indicate the presence of appreciable quantities of soluble CO 2− 3 exceeding the limits of the solubility product of CaCO 3, and reveal the inadequacy of thermodynamic approach for studying scale formation. Solid CaCo 3 formation in measurable quantities starts above ca. 65°C, reaches a maximum at 80 °C and declines to a constant value at 90 °C. Mg (OH) 2 precipitation starts around 75 °C and increases steadily with rise in temperature. The OH − ions necessary for the precipitation of Mg(OH) 2 originate from the thermal decomposition of the CO 2− 3 moiety of the CaCO 3 supersaturating the brine. Experiments with water enriched with additional NaHCO 3 result in a marked increase in the deposition of CaCO 3. In the meantime the supersaturation with CaCO 3 and the formation of Mg(OH) 2 decrease to constant values. Factors leading to the formation and stabilization of CaCO 3−supersaturated solutions are considered to determine the nature and composition of alkaline scales.