Calcium carbonate scales on process equipment: A measure of the induction time for nucleation

Scale deposition on process equipment surfaces has several disadvantages: in particular, when scales crystallize on heat transfer surfaces, they offer a resistance to the heat flow and can accumulate in pipelines, orifices and other flow passages seriously impeding the process flow. Moreover, calcium carbonate scales, together with calcium sulfate scales, are the major cause of fouling in reverse osmosis membranes, resulting in a continuous decline in desalted water production thus reducing the overall efficiency and increasing operation and maintenance costs. Therefore, from an economic point of view, the formation of these mineral scales is an obstacle to the recovery of potable water from sea or brackish waters as well as to the industrial utilization of many natural waters. The aim of the paper is to measure the induction time for calcium carbonate precipitation, through a well-assessed laser light scattering technique previously devised. Experiments are carried in a batch thermostated reactor at room temperature, pH in the range 7-8, in a supersaturation range from 2 to 200. The induction time so measured could be used to estimate thermodynamic parameters such as the interfacial tension between crystals and mother suspension. The knowledge of interfacial tension is of major importance for guiding the choice of the most suitable additive for the inhibition of CaCO3 scale formation.