Abstract
Co-precipitation of mineral-based salts during scaling remains poorly understood and thermodynamically undefined within the water industry. This study focuses on investigating calcium carbonate and calcium sulfate mixed precipitation in scaling. Scaling is often observed in the produced water supply as a result of treatment processes. Co-precipitation results were compared with experimental results of a single salt crystallization. Several parameters were carefully monitored, including the electrical conductivity, pH value, crystal morphology and crystal form. The existence of the calcium carbonate scale in the mixed system encourages the loose calcium sulfate scale to become more tightly packed. The mixed scale was firmly adhered to the beaker, and the adhesion of the co-deposition product was located between the pure calcium sulfate scale and the pure calcium carbonate scale. The crystalline form of calcium sulfate was gypsum in both pure material deposition and mixed deposition, while the calcium carbonate scale was stable in calcite form in the pure material deposition. In the co-deposition, apart from calcite form, some calcium carbonate scale crystals had metastable vaterite form. This indicated that the presence of SO42− ions reduced the energy barrier of the calcium carbonate scale and hindered its transformation from a vaterite form to a calcite one, and the increase in HCO3− content inhibited the formation of calcium sulfate scale.
Highlights
It is known that the scale formation process can be divided into three stages, namely, the scale induction, nucleation, and crystal growth periods [32]
With the continuous precipitation of scale crystals, the supersaturation of the solution decreased during the period from 34–64 min, and the rate of conductivity decline slowed down
The ion pair formed by sulfate and calcium was adsorbed on the surface of the formed scale crystal, and the formed scale crystals were complexed with each other to form a larger scale crystal, which is the crystal growth stage
Summary
It is known that the scale formation process can be divided into three stages, namely, the scale induction, nucleation, and crystal growth periods [32]. The scaling process is affected by many factors, and the scaling mechanism is very complex [35], but its main cause is the supersaturation of scaling ions in the solution [14,27]. The ion pair concentration increases and aggregates to form larger particles. In this process, the aggregation is in a dynamic equilibrium state of dissolution and aggregation in the solution [1]. The polyelectrolyte can be adsorbed on the surface of the molecular aggregation, affecting its growth and dissolution kinetics [17]
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