A theory of the nature of silica scale formation in evaporative cooling systems was presented at the 1976 CTI Annual Meeting. At that time, empirical observation and evidence from X-ray diffraction analysis and electron microscope examination had indicated that colloidal silica was bound in a crystal matrix, such as calcium carbonate, to form a tenacious scale. Since that presentation, experimental work has been conducted and is continuing in order to evaluate different approaches for preventing scale formation. Two methods which have been tested are chemical treatment with a phosphonate- polyacrylate blend and water softening. Both have been shown to control the tenacious calcium carbonate- silica scale. Amorphous silica alone, however, will form a deposit which is not as hard as the calcium carbonate - silica scale.
