Gypsum Scale Control by Nitrilotrimethylenephosphonic Acid

In this paper the retardant effect of nitrilotrimethylenephosphonic acid (NTMP) on calcium sulfate dihydrate nucleation kinetics is evaluated by means of an optical technique able to accurately measure the induction period. A laboratory-scale experimental apparatus is used, in which sodium sulfate and calcium chloride are mixed to obtain a supersaturated calcium sulfate dihydrate solution; all experiments are carried out for various values of NTMP concentrations (0.005−0.05 g/L) in solution, while changing the saturation index in the range of 4.04 − 4.82; the temperature was varied in the interval 15−45 °C. Experimentally measured induction times are used to evaluate the interfacial tension (γ S ) between crystals and mother solution and to estimate the activation energy ( E att ) for precipitation in the presence of NTMP. The measured values of γ S and E att are compared with those found in the absence of any additive and in the presence of citric acid, showing that NTMP is a very strong retardant for gypsum nucleation.