Continuous reaction crystallization of struvite from diluted aqueous solutions of phosphate(V) ions in DT MSMPR crystallizer

Experimental work concerning continuous reaction crystallization of struvite MgNH₄PO₄ · 6H₂O from diluted aqueous solutions of phosphates(V) (1.0 mass % of PO₄ ^3-) is presented. Laboratory tests were carried out in a DT MSMPR type crystallizer in 298 K assuming stoichiometric ratio of the reagents. Influence of pH (8.5 - 10) and mean residence time of suspension in a crystallizer (900 - 3600 s) on product crystal size distribution, their sizehomogeneity and process kinetics was identified. Struvite crystals of mean size from ca. 23 to ca. 86 μm, of diverse homogeneity (CV 62 - 90%) were produced. The largest crystal sizes of acceptable homogeneity corresponded to pH 8.5 and mean residence time elongated up to 3600 s. Under these conditions struvite nucleation rate did not exceed 2.9 · 10⁷ 1/(s m³) according to SIG MSMPR kinetic model. Linear growth rate of struvite crystals within the parameter ranges tested varied from 4.33 · 10^-9 to 1.75 · 10 ^-8 m/s.