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THE INFLUENCE OF PHOSPHATE ON CALCIUM CARBONATE FORMATION IN HARD WATER

Calcium carbonate fouling layers form when the concentration of scaling ions in solution exceeds their solubility limit. Although small amounts of phosphate ions (0.1-3.0 mg/L) are found in tap water in some part of Europe, the effect of these species on the kinetics and characteristics of calcium carbonate scale generated from a supersaturated aqueous salt solution containing phosphate is not reliably understood. In this study, a series of experiments using model solutions containing phosphate were conducted to study the chemical characteristics of the scale formed. The results suggest that the time constant of scale formation in solution at 65°C with 2.0 mg/L phosphate ions was about 17 times longer than that in solutions without phosphate. X-ray diffraction and SEM/EDX analysis of the scale layers confirmed that deposits formed in the presence of phosphate were calcite, containing trace amount of phosphorous, while those formed in the absence of phosphate were a mixture of aragonite and calcite. INTRODUCTION Calcium carbonate fouling layers, i.e. mineral scale, are often formed on heat exchanger surfaces in a direct water heating system where hard water is heated beyond 50°C (Hasson, Avriel et al. 1968; Augustin and Bohnet 1995). The CO2-H2O-CaCO3 system exists in three phases: (a) a vapour phase characterised by the partial pressure of CO2 in the atmosphere which is in contact with the aqueous phase (or that the liquid was exposed to before it was isolated from the vapour phase); (b) an aqueous phase in which CO2 absorbed from the gas phase and the carbonates dissolved from the solid phase lead to ionic equilibria between HCO3, CO3, H+, OH-, and Ca2+; (c) a solid phase of calcium carbonate being formed or dissolved. The distribution of the carbonate species changes markedly with the pH of solution (Pisigan and Singley 1985). The following three sets of solution equilibria need to be considered in calcium carbonate precipitation: (a) Carbonic species and water equilibria CO2 (aq) + H2O H2CO3 K0 (1) H2CO3 H+ (aq)+ HCO3(aq) KH2CO3 (2) HCO3 (aq) H+ (aq)+ CO3(aq) K2 (3) H+ (aq)+ OH(aq) H2O Kw (4) (b) Solid-liquid phase equilibrium Ca2+ (aq)+ CO3 (aq) CaCO3(s) Ksp (5) (c) Gas-liquid phase equilibrium CO2 (g) CO2 (aq) KH (6) The K values above are the thermodynamic equilibrium constants for the carbonic and calcium systems in pure water and are given by (Plummer and Busenberg 1982). The relative solubility, S, is a measure of supersaturation, usually calculated with respect to the equilibrium solubility of calcite, as it is the most stable calcium carbonate polymorph, via S = IAP / Ksp = [Ca2+] [CO3] • fD / Ksp (7) where IAP is the ionic solubility product and fD is the activity coefficient of divalent ions. The solution being super-saturated is not sufficient grounds for scale formation: nucleation must also occur. This is a random, kinetic process which is subject to a wide range of rate determining effects including adsorption of other species. When nucleation is a thermally activated process, the rate of formation of nuclei is expressed by the following Arrhenius-type equation (Davey and Garside 2000): ú û ù ê ë é = D = 2 3 3 2 3 ) (ln 3 16 exp ) / exp( S T k v A kT G A J C C pg (8) Here, T is the absolute temperature, k the Boltzman constant, nC the molecular volume of critical nuclei, g the interfacial tension and A is a kinetic pre-factor. Relationships such as (8) are often quoted in the crystallization fouling literature but the influence of trace species such as phosphate on the Proceedings of International Conference on Heat Exchanger Fouling and Cleaning 2013 (Peer-reviewed) June 09 14, 2013, Budapest, Hungary Editors: M.R. Malayeri, H. Müller-Steinhagen and A.P. Watkinson Published online www.heatexchanger-fouling.com