Thermochemistry of aqueous sodium phosphate interactions with metal oxides in high temperature water

Quinlan, Sean Edward (1996) Thermochemistry of aqueous sodium phosphate interactions with metal oxides in high temperature water. Masters thesis, Memorial University of Newfoundland.

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Abstract

Most fossil-fired electrical power generating stations employ low concentrations of sodium phosphate to control boiler water pH. Sodium phosphate "hideout" has been identified as a cause of major corrosion problems in some stations. The objective of this work was to synthesize the principal iron reaction products, maricite and sodium iron(III) hydroxyphosphate, and to determine thermodynamic data pertinent to their formation in reactions of iron oxides with aqueous sodium phosphate under hydrothermal conditions. -- Sodium iron(III) hydroxy phosphate (SIHP), Na₄Fe(OH)(PO₄)₂•⅓NaOH, is believed to be the major iron(III) reaction product causing sodium phosphate hideout. The compound was first reported in the early 1980's, from pressure vessel studies on hideout, and its stoichiometry was inferred from elemental analyses on solid reaction products and hydrothermal solutions during hideout experiments. Maricite, NaFePO₄ is the major iron(II) reaction product, and has been found both as a natural mineral and in previous phosphate hideout studies. -- All syntheses of these solids were carried out at temperatures no higher than 250°C in 45 mL Parr 4744 Teflon-lined stainless steel reaction vessels. These vessels were modified to allow in situ filtration of reaction products at high temperature by simply turning the cells upside-down to drain the solution through a stainless steel mesh. Methods were successfully developed for synthesizing sodium iron(III) hydroxy phosphate from hematite, magnetite and iron phosphate, and maricite from iron powder at 95+% yield in these safe, rugged cells. A novel method for synthesizing maricite by thermally decomposing the complex of aqueous iron(III) nitrilotriacetic acid at 250°C has also been developed. -- The crystal structure of SIHP was determined and is consistent with observations on boiler corrosion product behavior. The crystal structure of maricite was also determined, and found to be identical to that of the natural mineral. -- Solubility studies for SIHP were carried out in a modified 450 mL Parr 4562 stirred reaction vessel fabricated from Hastelloy C. To avoid the need to control reduction potential, solubility experiments were carried out in the presence of hematite, according to the following reaction -- ½ Fe₂O₃(s) + 13/3Na⁺(aq) + 2HPO₄²⁻(aq) + ⅓OH⁻ (aq) ⇆ -- Na₃Fe(PO₄)•(Na₄/₃H₂/₃O)(s) + 5/6H₂O(1). -- Results were obtained up to 325°C at a sodium/phosphate ratio of 2.5, and up to 260°C at a ratio of 3.0 before severe corrosion reactions terminated the experiments. Kinetic experiments approaching equilibrium from unsaturated and supersaturated conditions at 225°C confirmed that equilibrium had been achieved. -- From this and previously reported data for the ionization of phosphoric acid, a thermodynamic database for the formation and release of the main iron(III) species under boiler conditions was developed.

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