Crystallization of calcium carbonate (CaCO_3) in a flowing system: influence of Cu~(2+) additives on induction time and crystalline phase transformation

摘要

Scaling of calcium carbonate (CaCO_3) is commonly found in piping systems in oil, gas, desalination and other chemical processes. The scale may create technical problems, leading to the reduction of heat transfer, increase of energy consumption and unscheduled equipment shutdown. This paper presents crystallization scaling experiments and evaluation of the effect of Cu~(2+) additives on the induction time and calcium carbonate transformation. The crystals precursors were prepared using equimolar of CaCl_2 and Na_2CO_3 resulted in concentrations of 3000 ppm Ca~(2+) in the solution. The Cu~(2+) in amounts of 0, 1 and 10 ppm was separately added in the solution. The flow rates (20, 35, and 60 mL/min) and elevated temperatures (27, 35 and 45°C) were selected in the study. The induction time for crystallization of CaCO_3 was observed by measuring the solution conductivity over time, while the phase transformation of calcium carbonate was examined by XRD method and SEM/EDX. It was found that the conductivity remained steady for a certain period reflecting to the induction time of crystal formation, and then decreased sharply afterwards,. The induction time was increased from 34 and 48 minutes in the presence of Cu additives (1 and 10 ppm), depending on the flow rates and temperature observed. In all the experiments, the Cu~(2+) addition leads to the reduction of mass of crystals. Apparently, the presence of Cu~(2+) could inhibit the CaCO_3 crystallization. In the absence of Cu~(2+) and at elevated temperature, the crystals obtained were a mixture of vaterite and calcite. In the presence of Cu~(2+) and at elevated temperature, the crystals formed were aragonite and calcite. Here, the presence of Cu~(2+) additives might have controlled the crystal transformation of CaCO_3.