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

• Published in: Chemical engineering research & design Vol. 92; no. 3; pp. 481 - 490
• Main Authors: Kozik, Anna, Hutnik, Nina, Piotrowski, Krzysztof, Matynia, Andrzej
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.03.2014
• Subjects: Continuous DT MSMPR crystallizer; Crystal size distribution; Crystallization; Crystallizers; Crystals; Dilution; Inlets; Magnesium; Magnesium ions excess; Mathematical models; Reaction crystallization; SIG MSMPR kinetic model; Struvite; Continuous DT MSMPR crystallizer; SIG MSMPR kinetic model; Struvite; Reaction crystallization; Magnesium ions excess; Crystal size distribution
• ISSN: 0263-8762; 1744-3563
• DOI: 10.1016/j.cherd.2013.08.032

•Phosphates(V) were recovered as struvite from synthetic wastewater.•Changing pH and mean residence time can modify Lm within ca. 19–73μm.•Linear crystal growth rate varied from 3.62×10−9 to 1.68×10−8m/s.•Excess of magnesium ions increases process yield but not crystals quality. Continuous reaction crystallization of struvite MgNH4PO4·6H2O from diluted aqueous solution containing phosphate(V) ions of concentration 0.20wt% PO43− was investigated experimentally. The tests were carried out in a continuous DT MSMPR type crystallizer in temperature 298K 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 (900–3600s) on the product crystals size distribution, their size-homogeneity and process kinetics were identified. Crystals of mean size from ca. 19 to ca. 73μm, 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 3600s. Under these conditions struvite nucleation rate did not exceed 5.3×107l/(sm3) according to SIG MSMPR model predictions. Crystal linear growth rate within the investigated process parameter values varied from 3.62×10−9 to 1.68×10−8m/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 0.20wt% to 0.9×10−3 – 9.2×10−3wt% (9–92mg/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.