CALCIUM-CARBONATE SCALE FORMATION AND PREVENTION IN A FLOW-THROUGH SYSTEM AT VARIOUS TEMPERATURES

The deposition of calcium carbonate in lines carrying hot water or in pumping systems is an undesirable process in most cases. The mechanism of calcium carbonate scale formation is of key importance and has many practical implications. The deposition of calcium carbonate has been investigated in a closed, flow through circuit, in which heated specimens made of 316 stainless steel, aluminum and glass covered internally, 316 stainless steel, were used as points of initiation of the deposition. It was found, at plethostatic conditions, that in the temperature range between 50-80-degrees-C, vaterite is the initially forming phase converting to the thermodynamically more stable calcite. The deposition took place within the stationary layers in which the supersaturation gradient was established. The deposition of vaterite was done by a surface diffusion controlled process, as suggested from the apparent activation energy of 36 kJ/mol found. The deposition rates as a function of the solution supersaturation suggested a polynuclear mechanism for the formation of vaterite on the heated specimens. Pretreatment of the specimens with concentrations of EHDP varying from 0.1-mu-M to 1mM, increased the induction times significantly and resulted in drastic reduction of the rates of deposition, which were more pronounced for the 316 stainless steel specimens. From the kinetics of precipitation data, the surface energy of the forming vaterite was estimated and it was found to increase with temperature from 77 to 148 mJ m-2.