Experimental Study of Patterns in Hydrodynamically Deposited Dispersed Phase of CaCO3 on Surfaces of Straight Cylindrica Silica Tubing

Abstract

Deposition of dispersed phase from flowing dispersions onto a substrate surface is of utmost relevance for various industrial processes like fouling of sparingly soluble salts, such as CaCO3 and CaSO4 in heat exchangers and desalination evaporators which tend to form deposits on flow surfaces of tubes or pipelines conveying hard water and in water filtration and purification processes since concentration of CaCO3 in many natural water resources is equal to or greater than the saturation level. The study of deposition is also of intrinsic interest for biophysics and colloid science where vascular calcification i.e. precipitation and deposition of calcium phosphates (hydroxyapatites) in the muscular layer of the blood reduces arterial compliance and promotes congestive heart failure. Experiments were conducted on straight, circular cross section silica tubing of inner di-ameter (ranging from 0.88 mm to3.40 mm) to study the effect of electrostatic interaction and hydrodynamics on the deposition behavior of CaCO3 on silica surface when streams of aqueous solutions of Ca(NO3)2 and Na2CO3 with a concentration of 40.0 g l−1 and 25.9gl−1 respectively flowing at a volumetric flow rate of 1 l h−1 each is mixed to form CaCO3 dispersion which was then transported through silica tubing at a steady volumetric flow rate of 2lh−1. The in situ phenomenology of CaCO3 particles transport, deposition, and evolution of spatial and temporal patterns of the CaCO3 deposition on the silica surface along with the dendritic growth of CaCO3 during the flow was visually documented with the aid of a 100X optical microscope. The study discussed the deposition behavior of dispersed phase of CaCO3 from its aqueous dispersion on the silica tubing during flow and attributed charge inversion from negative to positive of silica surface, due to the adsorption of Na+ formed during precipitation reaction of CaCO3, as a plausible reason for the reversal of electrostatic interaction from attraction between initially negatively charged silica surface and positively charged CaCO3 particles which promoted deposition and subsequent evolution of patterns of CaCO3 deposition on the silica surface during the early stage of experiments to repulsion between finally net positively charged silica surface and positively charged CaCO3 particles which resulted in retarded deposition and subsequent emergence of sparsely adhered CaCO3 agglomerates as localized, limited patches of CaCO3 deposits on the silica surface during the later stage of the experiments