Effect of pH on the efficiency of sodium hexametaphosphate as calcium carbonate scale inhibitor at high temperature and high pressure

Abstract Calcium carbonate (CaCO3) scale deposition causes serious issues in industries which deal with water, and its formation reaction is pH-depending. This study investigated the efficiency of sodium hexametaphosphate (SHMP) as scale inhibitor for calcium carbonate, as a function of pH and SHMP concentration, under drastic conditions of temperature, scaling ions concentration and pressure. The relationship between pH and CaCO3 formation parameters was evaluated by static and dynamic tube blocking tests and scanning electron microscopy (SEM) images. Quantum chemistry calculations were performed to evaluate thermodynamic values related to interactions between the inhibitor and CaCO3. At pH 6.5, excellent results were found with complete inhibition of CaCO3 deposition on capillary with just 2.5 mg L−1 of SHMP and deformed structures for CaCO3 crystals were observed by SEM images. Results also showed a reversal behavior of SHMP as inhibitor at pH neutral and slightly alkaline, with a decrease on inhibition efficiency even raising SHMP concentration, related to the fast precipitation of CaCO3 in the drastic conditions evaluated.