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

Experimental tests covering the production of struvite MgNH4PO4·6H2O 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 dm3 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%.

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