Abstract
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.
Highlights
Modern technologies designed for efficient phosphate(V) ions recovery from various wastewaters [1] or from liquid manure [2] are commonly based on fixing of these ions into sparingly soluble magnesium and ammonium salt MgNH4PO4∙6H2O – struvite [3]
If wastewater contains calcium ions, calcium phosphates co-precipitate during struvite reaction crystallization
The effect of the calcium ions presence on struvite reaction crystallization run and product properties was investigated by Le Corre [8] and Hutnik [9, 10]
Summary
Modern technologies designed for efficient phosphate(V) ions recovery from various wastewaters [1] or from liquid manure [2] are commonly based on fixing of these ions into sparingly soluble magnesium and ammonium salt MgNH4PO4∙6H2O – struvite (phosphorus recycling) [3]. Final phase and chemical composition of the product, as well as its crystal size distribution (CSD) depend mainly on liquid manure composition and on assumed parameters of continuous reaction crystallization [15, 17]. The results of experimental research and corresponding kinetic modeling data concerning continuous reaction crystallization process of sparingly soluble phosphates from aqueous solutions containing originally phosphate(V) and calcium ions are presented. The results were compared with kinetic parameters [11] estimated with the most simplified SIG (Size Independent Growth) MSMPR kinetic model [18] Kinetic data from both models were related to struvite nucleation and crystal growth observed in real cattle liquid manure (CLM) [24]. Knowledge of kinetic relations in struvite continuous reaction crystallization process can be practically utilized e.g. for elaboration of phosphates recovery technology based on aqueous solutions with high calcium content, including liquid manure
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