Continuous reaction crystallization of struvite from solution containing phosphate(V) and nitrate(V) ions

Abstract

Research results concerning continuous removal of phosphates(V) using magnesium and ammonium ions from solutions containing 1.0 or 0.20 mass % of PO43- ions and 0.0443 – 0.886 mass % of nitrate(V) ions are presented. Continuous struvite reaction crystallization process ran in stoichiometric conditions and at 20% excess of magnesium ions in DT MSMPR (Draft Tube, Mixed Suspension Mixed Product Removal) crystallizer with internal circulation of suspension in 298 K at pH 9 – 11 and mean residence time τ 900 – 3600 s. With the increase in nitrate(V) ions concentration in a feed solution mean crystal size Lm decreased by ca. 29%. Size-homogeneity within the crystal product population was not high. Increase in pH influenced product quality disadvantageously. Crystals of smaller sizes and lower homogeneity were manufactured. Elongation of mean residence time to 3600 s caused increase in crystals size even by 80% and improved size-homogeneity. Decrease of phosphate(V) ions concentration in a feed solution caused increase in mean size of struvite (by ca. 23%). Excess of magnesium ions influenced process results advantageously. Mean size of crystal products increased by more than 24%. Presence of nitrate(V) ions and process conditions in a crystallizer favored crystallization of struvite in a form of tubular crystals

Keywords

• Struvite, phosphate(V) ions, nitrate(V) ions, continuous reaction crystallization, product quality, phosphorus recycling