Synthesis and solubility of nickel and iron hideout reaction products with aqueous sodium and ammonium phosphate under steam generator conditions

Harvey, Rosemarie Gail (2003) Synthesis and solubility of nickel and iron hideout reaction products with aqueous sodium and ammonium phosphate under steam generator conditions. Masters thesis, Memorial University of Newfoundland.

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Abstract

Low concentrations of sodium phosphate are added to the boiler water of electric power stations as a means of controlling pH. Hideout is the phenomenon by which sodium phosphate is observed to be retained in the boiler during conditions of high temperature and pressure, only to be released back into the water upon cooling. All-volatile amine treatment is an alternative boiler water pH control method, without the same adverse effects, but it is not known if problems will arise from a changeover from congruent phosphate control to all-volatile treatment. The objective of this research was (i) to develop improved synthetic methods for the known hideout reaction products, maricite, NaFeᴵᴵPO₄, and sodium iron hydroxyl phosphate (SIHP), Na₃Feᴵᴵᴵ (PO₄)₂•(Na₄/₃H₂/₃O), (ii) identify any ammonium-iron-phosphate reaction products that may form during the changeover, and (iii) measure the solubility of the hideout reaction product sodium-nickel-hydroxyl-phosphate (SNHP), Na₂Ni(OH)PO₄, so that a thermodynamic database can be derived. -- The syntheses of the solid reaction products were carried out in 45 mL Parr 4744 Teflon-lined stainless steel reaction vessels which allowed in situ filtration of the products fi-om solution by inversion of the vessel, allowing the remaining solution to drain through a stainless steel mesh. Maricite was synthesized using previous established methods, whereas new methods for synthesizing SIHP from thermal decomposition of iron(III) nitrilotriacetic acid, and chelates, iron oxalate and iron tartrate, have been developed. A new hydrothermal synthesis for (NH₄)FeᴵᴵFeᴵᴵᴵ (PO₄)₂ has been developed by thermally decomposing the chelate, iron tartrate. An additional ammonium reaction product, (NH₄)Feᴵᴵ(PO₄)•H₂O, was synthesized from the iron nitrilotriacetic acid complex. This provided a new synthetic route for this compound and proved that it formed under boiler conditions. The crystal structure of (NH₄)FeᴵᴵPO₄)•H₂O was also confirmed. The major sodium-nickel-phosphate reaction product Na₂Ni(OH)PO₄, was synthesized by two separate methods, fi-om nickel oxide and from the thermal decomposition of the nickel nitrilotriacetic acid complex. -- Solubility studies of Na₂Ni(OH)PO₄, SNHP, were carried out in a modified 450 mL Parr 4562 stirred zirconium reaction vessel, according to the following reaction: -- NiO(s) + 2 Na⁺(aq) + HPO₄²⁻ (aq) ⇋ Na₂Ni(OH)PO₄(s) -- Kinetic experiments were conducted at 250°C to ensure equilibrium had been reached and solubility data were collected over the temperature range 235-280°C at a sodium/phosphate mole ratio of 2.5 that had an initial phosphate concentration of 1.5 mol•kg⁻¹. The MULTEQ chemical equilibrium program was used to calculate the composition concentrations of relevant species at each temperature studied, and experimental equilibrium constants were calculated from the activity coefficient model used in MULTEQ. The results were used to create a thermodynamic model for SNHP, consistent with the Helgeson-Kirkham-Flowers model for the standard partial molar properties of aqueous phosphate species. -- In conclusion, the maricite synthesis reported by Quinlan (1996) was reproducible and SIHP was synthesized from similar conditions as the maricite synthesis and using the analogous chelate decomposition reaction. Ammonium-iron-phosphate reaction products can form under boiler conditions; those identified in this study were (NH₄)FeᴵᴵFeᴵᴵᴵ (PO₄)₂ and (NH₄)Feᴵᴵ(PO₄)•H₂O. The major sodium-nickel-phosphate reaction product synthesized was Na₂Ni(OH)PO₄ and the data from this study and that previously reported for this reaction product, were used to create a thermodynamic model for this system, consistent with the database for sodium-iron hideout reactions.

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