Abstract
Crystallization fouling is one of the major problems in process industries using water. In the present work the precipitation of calcium carbonate (CaCO3), and of mixed calcium carbonate - calcium sulfate (CaSO4) deposits on the walls of a heated metal surface of stainless steel was investigated. All experiments were done at constant flow (2.3 ± 0.1 l/min), pressure and wall temperature (51± 1°C) using solutions either supersaturated with respect to calcium carbonate or both with respect to calcium carbonate and calcium sulfate. The kinetics of mineral deposition was monitored by the solution specific conductance change and from measurements of the calcium concentration as a function of time. The crystal growth of calcium carbonate followed a surface diffusion controlled mechanism. The dominant mineral formed was calcite, although aragonite and vaterite were found at lower proportions. Crystal growth kinetics as a function of supersaturation with respect to gypsum in solutions supersaturated with respect to both salts, suggested bulk diffusion control of the process. The solids deposited on the walls of the heated specimen, consisted of gypsum and aragonite. The morphology of the deposits suggested that gypsum crystals grow on the crystals of aragonite, which preceded in the sequence of deposition events.
