Struvite nucleation and crystal growth kinetics from cattle liquid manure

Abstract

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

Size-dependent growth kinetics of struvite crystals in wastewater with calcium ions

“…Corresponding process conditions -see Table 1. * according to [24] quality of the product manufactured from CLM may be, among others, the smallest value of linear crystal growth rate G ∞ : only 8.48•10 -9 m/s (CLM, Table 4). In solution, where inlet concentration of calcium ions was higher by 300 mg Ca 2+ /kg, while concentration of phosphate(V) ions was 1.0 mass % (0.34 mass % in liquid manure, CLM), linear crystal growth rate G ∞ was 1.47•10 -8 m/s (test No.…”

“…In general, quality of the product manufactured from liquid manure CLM should be regarded as unsatisfactory in respect to market demands [27,28]. Improvement of the product quality is, however, possible, among others, through elongation of residence time of struvite suspension in a crystallizer [15,16,24]. Tables 1 and 4) is presented in Figure 3, where G(L) determined for struvite produced from cattle liquid manure CLM is also added for comparison.…”

“…The results were also compared with kinetic parameters [11] estimated with the most simplified SIG (Size Independent Growth) MSMPR kinetic model [18]. Kinetic data from both models were also 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.…”

Recovery of phosphates(V) from wastewaters of different chemical composition

“…Experimental results comparing the effect of impurities present in real wastewater from the phosphorus mineral fertilizer industry, IWW (leachate from slag heap of phosphogypsum in chemical industrial plant Police S.A., Poland) [1] and impurities identified in real cattle liquid manure CLM (derived from cattle-breeding farm near Wroclaw, Poland) [2,18] on the quality of manufactured struvite are presented. Reaction crystallization of struvite ran continuously at magnesium ions excess in relation to phosphate(V) ions, in temperature 298 K, at pH 9 and for mean residence time of suspension in a crystallizer 3600 s, thus modeling the conditions recommended for industrial application [3,4].…”

“…Steering, control and multichannel recording of output data streams were organized and centrally synchronized by a PC computer unit, using original drivers. The SABW and CLM wastewaters were directed to a Mettler Toledo plant (iControl LabMax 4.0 drivers) [2,18,19], whereas SWWP and IWW wastewaters -to IKA Labortechnik experimental plant (IKA Labworldsoft and BioScadaLab drivers) [1,11]. Constructional details of the crystallizers integrated with experimental plants were presented elsewhere [1,2,11,19].…”