Effect of reaction crystallization process parameters on struvite recovery from animal wastewater in presence of boron, cobalt, manganese and molybdenum

One of the sources of phosphates(V) is liquid manure where the typical content of phosphorus is 0.2– 0.4 mass %. It is usually accompanied by nitrogen (0.45–0.64 mass %), potassium (0.3–0.6 mass %), magnesium (0.1 mass %) and calcium (0.3 mass %). Recovery of phosphate(V) ions in a form of sparingly soluble salts is an extremely complex technological process, where competing for precipitation reactions of magnesium ammonium phosphate(V) hexahydrate (struvite, MgNH₄ PO₄·6H₂ O) and calcium phosphates co-run. The research results covering the effects of boron, cobalt, manganese and molybdenum in liquid manure on the recovery of phosphate(V) ions in a form of sparingly soluble salt, struvite, are demonstrated. The largest mean size of struvite crystals corresponded to the presence of boron, whereas the smallest one–the presence of cobalt. Boron and manganese influenced struvite habit (thinner and longer crystals). Depending on feed composition and work parameters of the crystallizer, crystal linear growth rate G of struvite varied from 4.79 × 10^–9 to 1.91 × 10^–8 m s^–1, whereas their nucleation rate B varied from 6.8 × 10⁶ to 4.6 × 10⁹ s^–1 m^–3. The largest mean struvite crystal size (L_m 75.5 µm) and acceptable homogeneity of population (CV 68.3%) corresponded to inlet 3– solution containing 0.20 mass % of PO₄ and 5.0 mg B kg^–1 3_–, with PO₄:Mg²⁺ set as 1:1.2, at pH 9 and for a mean residence time of suspension in a crystallizer τ 3,600 s.