Cattle liquid manure (CLM) in future may become excellent raw material for the solid mineral fertilizers production. Phosphates(V) recovery from CLM can be done, e.g. through reaction crystallization of sparingly soluble salts: magnesium and ammonium phosphate(V) hexahydrate MgNH 4 PO 4 •6H 2 O (MAP, struvite) or/and calcium phosphates(V). The original kinetic calculations based on experimental test results concerning continuous reaction crystallization of sparingly soluble phosphates(V) from cattle liquid manure in a continuous laboratory DT MSMPR (Draft Tube, Mixed Suspension Mixed Product Removal) crystallizer with internal circulation of suspension driven by propeller stirrer are presented. Two kinetic models of continuous mass crystallization in MSMPR crystallizer: SIG (Size Independent Growth) and RE SDG (Rojkowski Exponential Size Dependent Growth) were adopted. Advantageous effects of: low pH (8.5), elongated mean residence time of suspension in a crystallizer (3600 s) and magnesium ions excess in relation to phosphate(V) ions concentration (1.2:1) on the process results were observed. Under the considered process conditions linear crystal growth rates of struvite were: G from 4.11 × 10 −3 to 1.01 × 10 −2 µm s −1 and G ∞ from 5.30 × 10 −3 to 1.02 × 10 −2 µm s −1. Keywords Cattle liquid manure (CLM) • Struvite • Continuous reaction crystallization • SIG MSMPR kinetic model • SDG MSMPR kinetic model • Phosphorus recycling List of symbols Symbols a Parameter in Eq. (5) and (6) µm-1 ACP Amorphous calcium phosphate, Ca 3 (PO 4) 2 B Nucleation rate m-3 s-1 CLM Cattle liquid manure CV Coefficient of (crystal size) homogeneity % DCPD Brushite or dicalcium phosphate dehydrate, CaHPO 4 •2H 2 O DT Draft tube G Crystal linear growth rate µm s −1 G 0 Crystal linear growth rate for nuclei µm s −1

AbstractKinetic parameters describing continuous reaction crystallization of struvite from aqueous solutions containing also calcium ions (from 100 to 2000 mg Ca2+/kg) were estimated. Test results were compared with kinetic data of struvite manufactured from real cattle liquid manure. Kinetic model for ideal MSMPR (Mixed Suspension Mixed Product Removal) crystallizer was used assuming dependence of crystal growth rate G on its size L (size-dependent growth, SDG MSMPR model). Based on nonlinear regression and statistical analysis, one from within five considered G(L) models was selected – Rojkowski exponential (RE) model – rendering the experimental population density distributions the best. It was concluded, that calcium ions influenced all components of struvite manufacturing process disadvantageously. A rise of Ca2+ concentration in a feed from 100 to 2000 mg/kg increased nucleation rate ca. 160-time, whereas growth rate of nuclei up to macroscopic size G0 decreased more than 10-time. Linear (larger) crystal growth rate G¥ was nearly two-times smaller: 1.71·10–8 m/s (100 mg Ca2+/kg) – 9.10·10–9 m/s (2000 mg Ca2+/kg). Resulting in a product with deteriorated quality. Mean size of the crystals decreased nearly two-times (to 18.4 μm), non-homogeneity within product population enlarged and calcium fraction in the product increased. The product, beside struvite MgNH4PO4·6H2O, also contained hydrated amorphous calcium phosphate(V) Ca3(PO4)2·nH2O (ACP). It was observed, that 5-times smaller concentration of phosphate(V) ions in a feed and magnesium ions excess in relation to phosphate(V) and ammonium ions (1.2 : 1 : 1) influenced all kinetic parameters of continuous struvite reaction crystallization advantageously.

No abstract