Abstract

The inhibition of calcium carbonate (CaCO3) precipitation by phosphonates containing different functional groups has been investigated in aqueous solution. It has been found that the performance of phosphonate as CaCO3 inhibitor depends upon its architecture. The influence of iron oxide (Fe2O3, rust) on the performance of CaCO3 inhibitors has also been examined. Results show that addition of small amount (10 mg to100 mg) of Fe2O3 to the CaCO3 supersaturated solution exhibits marked antagonistic effect on inhibitor performance. Various mechanisms have been proposed to explain the negative influence of Fe2O3 on inhibitor performance. The impact of inhibitors on CaCO3 crystal morphology as investigated by scanning electron microscopy and XRD show that various polymorphs are stabilized in the presence of inhibitors.

Introduction

Carbonates and sulfates scaling of alkaline earth metal ions in many industrial processes presents serious operational challenge because these salts, as a rule show inverse solubility, i.e., their solubility decreases with increasing temperature. Moreover, in the case of calcium carbonate polymorphism is a complicating factor. The three polymorphic phases, vaterite, aragonite, and calcite, in order of increasing thermodynamic stability have different solubility, morphological and crystallographic characteristics.1 Depending on the fluid conditions (composition, temperature, fluid dynamics, substrate) less stable polymorphs or hydrated phases may be stabilized and/or converted in to the thermodynamic most stable calcite.

Scale deposits, according to their mode of formation, may be distinguished in two categories: 1) salts depositing more or less selectively onto the surface of equipment in contact with the aqueous fluids (usually at elevated temperature) and 2) precipitates accumulating because of sedimentation or transport by fluid flow. As a rule, in this latter case the deposits are formed in the bulk spontaneously due to the increase of the solution supersaturation, or they form as a corrosion by-products which at second stage sediment out.

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