Kinetic conditions of struvite continuous reaction crystallisation from wastewater in presence of aluminium(III) and iron(III) ions

Abstract

Kinetic parameters of the continuous struvite reaction crystallisation process from aqueous solutions containing phosphate(V) ions with impurities: aluminium(III) or/and iron(III) ions were determined. A simplified kinetic model of ideal mixed suspension mixed product removal (MSMPR) crystalliser - size independent growth (SIG) was adopted. Nuclei population densities, nucleation rates and crystal linear growth rates were determined from product crystal size distributions. It was concluded that within the assumed process parameter ranges, the linear growth rate of struvite crystals in presence of aluminium(III) ions varied from 4.53 · 10‒9 to 1.89 · 10‒8 m/s, whereas in presence of iron(III) ions it was within the 4.59 · 10‒9 ‒ 1.62 · 10‒8 m/s range. Nucleation rate varied within the 3.8 · 107 ‒ 1.6 · 109 1/(sm3) and 4.5 · 107 ‒ 1.2 · 109 1/(sm3) ranges, correspondingly. These large differences show significant influence of impurities and technological parameters on the struvite reaction crystallisation kinetics, thus on the product quality. It was experimentally demonstrated that aluminium(III) ions affect struvite reaction crystallisation more advantageously than iron(III) ions.