Reaction crystallization of struvite in a continuous DTM type crystallizer with a compressed air driven jet pump

Abstract

Experimental tests covering the production of struvite MgNH 4PO4•6H2O from water solutions containing 1.0 mass % of phosphate(V) ions using magnesium and ammonium ions in stoichiometric proportions were carried out in a crystallizer of 1.2 dm 3 working volume. The process temperature was 298 K. Struvite crystals of mean size Lm from ca. 14 to ca. 38 μm were produced depending on the process environment's pH (9-11) and the mean residence time of suspension in a crystallizer, τ (900-3600 s). In such defined process conditions the linear growth rate of struvite crystals changed from 1.45•10-8 m/s (pH 9, τ 900 s) to 2.06•10-9 m/s (pH 11, τ 3600 s) while the nucleation rate from 5.1•107 to 3.2•109 1/(sm3). Crystal product of the most advantageous granular characteristics was produced at pH 9 and the mean residence time 3600 s. Within this product population the largest sizes reached above 200 μm while the number of crystals smaller than 3 mm was kept below 6%.