Continuous reaction crystallization of struvite from diluted aqueous solution of phosphate(V) ions in the presence of magnesium ions excess

Continuous reaction crystallization of struvite MgNH₄PO₄·6H₂O from diluted aqueous solution containing phosphate(V) ions of concentration 0.20 wt% PO₄³⁻ was investigated experimentally. The tests were carried out in a continuous DT MSMPR type crystallizer in temperature 298 K assuming 20% excess of magnesium ions at the inlet point in respect to struvite synthesis reaction stoichiometry. Influence of pH (8.5–10) and mean residence time of suspension in a crystallizer (900–3600 s) on the product crystals size distribution, their size-homogeneity and process kinetics were identified. Crystals of mean size from ca. 19 to ca. 73 μm, of diverse size-homogeneity (CV 60–87%) were produced. Struvite particles of the largest sizes and acceptable homogeneity were produced at pH 8.5 for prolonged mean residence time 3600 s. Under these conditions struvite nucleation rate did not exceed 5.3 × 10⁷ l/(s m³) according to SIG MSMPR model predictions. Crystal linear growth rate within the investigated process parameter values varied from 3.62 × 10⁻⁹ to 1.68 × 10⁻⁸ m/s. Magnesium ions excess in a process environment influenced yield of continuous reaction crystallization of struvite advantageously – contrary to product crystals quality. Concentration of phosphate(V) ions in mother solution decreased from inlet 0.20 wt% to 0.9 × 10⁻³ – 9.2 × 10⁻³ wt% (9–92 mg/kg) depending on pH and mean residence time of suspension in a crystallizer, what can be regarded as a very good result of their recovering from solution.