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Phosphate (V) ions were continuously removed from synthetic wastewater containing inorganic impurities using magnesium and ammonium ions. The product was magnesium ammonium phosphate (V) hexahydrate, struvite, MgNH 4 PO 4 × 6H 2 O. Research ran in stoichiometric conditions in DT MSMPR type crystallizer with internal circulation of suspension. Increase in process environment pH from 9 to 11 resulted in 3-time decrease of mean struvite crystals size (from 40.1 to 12.6 m). Elongation of mean residence time of suspension in a crystallizer up to 3600 s resulted in improvement of the product quality. Mean size of struvite crystals enlarged up to 50.2 m. Based on kinetic calculations results (SIG MSMPR model) it was concluded, that linear struvite crystal growth rate varied within 5.04 × 10-9-1.69 × 10-8 m/s range, whereas nucleation rate within 1.4 × 10 7-1.7 × 10 10 1/(s m 3) limits. In solid product, besides struvite, also all impurities present in wastewater were identified analytically as hydroxides, phosphates and other salts.

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Reaction crystallization of struvite in water solutions containing 0.20 mass% of phosphate(V) ions by magnesium and ammonium ions addition was investigated experimentally. Process was carried out in DTM type crystallizer with liquid jet pump device in 298 K assuming stoichiometric conditions. Struvite crystals of mean size L m 5.2 -23.0 m were produced depending on pH (9 -11) and mean residence time of suspension in a crystallizer  (900 -3600 s). Under these conditions linear growth rate of struvite crystals (SIG MSMPR kinetic model) decreased 2-time with the increase in pH and 3-time with the elongation of mean residence time of crystal suspension from 7.11×10 -9 m/s (pH 9,  900 s) to 1.65 × 10 -9 m/s (pH 11,  3600 s). Nucleation rate varied within the 7.9 × 10 8 -1.8 × 10 10 1/(sm 3 ) limits. Struvite product of maximal linear size exceeded 100 m with 10 vol% of <3 m fraction corresponded to pH 9 and  3600 s.

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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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type crystallizer with internal circulation of suspension driven by jet pump fed with compressed air was used. Effect of pH and mean residence time of suspension in a crystallizer on the product crystals quality was determined. Product crystals of mean size from ca. 23 to ca. 40 m were manufactured. The largest struvite crystals, of acceptable homogeneity, were produced at 20% excess of magnesium ions, pH 9 and for mean residence time elongated up to 3600 s. Concentration of phosphate(V) ions decreased from 0.445 mass % in a feed to 9.0•10 -4 mass % in a postprocessed mother liquor. In a product, besides main component -struvite, also amorphous calcium phosphate(V) (ACP) was detected, accompanied with hydroxides of some metal impurities present in wastewater.Index Terms-Struvite, industrial wastewater, continuous reaction crystallization, continuous DTM crystallizer, jet pump, compressed air, product quality, phosphorus recycling. I. INTRODUCTIONStruvite (MgNH 4 PO 4 · 6H 2 O, MAP) precipitates in ambient temperature from supersaturated aqueous solutions of magnesium phosphate(V) in presence of ammonium ions, within pH 7-11 (solubility product pK sp = 9.0 -13.26) [1]. Process course and results are mainly influenced by: pH, working supersaturation in mother solution, temperature, presence of impurities, mixing intensity and crystallizer construction [2]. In modern processes of phosphorus recycling struvite production results from contacting magnesium and ammonium ions (e.g. aqueous solution of magnesium chloride and some ammonium salt) with  Manuscript received March 27, 2015; revised May 24, 2015. This work was supported by the Ministry of Science and Higher Education of Poland within a frame of statutory activity grant realized in Faculty of Chemistry, Wroclaw University of Technology.N. Hutnik, A. Kozik, A. Mazienczuk, and A. Matynia are with Faculty of Chemistry, Wroclaw University of Technology, Wroclaw, Poland (e-mail: nina.hutnik@pwr.edu.pl, anna.kozik@pwr.edu.pl, agata.mazienczuk@pwr. edu.pl, andrzej.matynia@pwr.edu.pl).K. Piotrowski is with Department of Chemical Engineering and Process Design, Silesian University of Technology, Gliwice, Poland (e-mail: krzysztof.piotrowski@polsl.pl).preliminary purified municipal, industrial or agricultural (liquid manure) wastewaters containing phosphate(V) ions. Process runs under precisely determined and controlled hydrodynamic conditions, inside an appropriately designed crystallizer [3]. Besides crucial for environmental protection effect of wastewater purification (eutrophisation prevention), separated crystal product is regarded as an important, secondary phosphorus source. Recovered struvite can be further conversed or used directly as mineral fertilizer for agriculture [3], [4].Efficiencies of struvite nucleation and mass crystallization processes are strongly dependent on crystallizer, including: appropriate construction [5], spatial arrangement of reagent inlet places [6] and proper selection of its continuous work parameters [7], [8]. In most DTM (Draft...