Investigating the potential of hydroxyethylamino-di(methylene phosphonic acid) in inhibiting gypsum crystallization

Abstract

In different industries, the formation of unfavourable inorganic crystals or “mineral scaling”, which reduces the efficiency of the system, is prevented by using some chemicals known as “antiscalants”. However, the mechanisms by which antiscalants interact with growing crystals are still not well-understood. Therefore, herein, the effects of 10–70 ppm hydroxyethylamino-di(methylene phosphonic acid) (HEMPA) on the crystallization of gypsum as a common mineral scale were investigated. Various analytical techniques such as UV-vis, SEM, HR-TEM, EDS, XRD, XPS, and ICP-OES were employed. The results revealed that increasing the concentration of HEMPA, which enhanced the amount of additive molecules in the solution, increased the induction time and decreased the slope of the turbidity development graphs. This means that HEMPA effectively reduced the nucleation and growth kinetics of gypsum crystals. Moreover, raising the pH of the additive-containing solutions from ∼4 to ∼7 further deprotonated HEMPA and improved its ability to hinder gypsum formation. Hence, at pH ∼ 7, no gypsum crystals precipitated except in the 10 ppm HEMPA-dosed electrolyte. Furthermore, the solids formed in the blank system were thin, large, and elongated along the c direction compared to the tiny particles produced with 70 ppm HEMPA (at pH ∼ 4). The findings also indicated that at the appropriate HEMPA-concentration and solution pH, HEMPA significantly inhibits gypsum crystallization through various mechanisms including chelation of crystal-forming ions in the solution, surface adsorption, and structural incorporation.