Application of a continuous FB MSZ type crystallizer with jet pump driven by compressed air for recovery of phosphate(V) ions from mineral fertilizer industry wastewater

Abstract

New original construction of continuous FB MSZ (Fluidized Bed with Mixed Suspension Zone) crystallizer with internal circulation of suspension driven by jet pump fed with compressed air was used for continuous reaction crystallization of struvite MgNH4PO4∙6H2O from phosphorus fertilizers industry wastewater by magnesium and ammonium ions addition. This wastewater of pH < 4 contained 0.445 mass % of phosphate(V) ions and impurities: aluminium, calcium, copper, iron, potassium, magnesium, titanium, zinc, fluosilicate, fluoride and sulphate(VI) ions. Tests were carried out in 298 K, in stoichiometric conditions and at 20% excess of magnesium ions. Crystals of mean size. 27 – 43 µm were produced. Struvite crystals of the largest sizes and acceptable homogeneity were produced at 20% excess of magnesium ions, at pH 9 and elongated mean residence time of suspension 3600 s. Corresponding crystal linear growth rate was 8.52⋅10–9 m/s according to SIG MSMPR (Size Independent Growth, Mixed Suspension Mixed Product Removal) kinetic model. Concentration of phosphate(V) ions decreased from 0.445 mass % in a feed to 9.3∙10–4 mass % in a postprocessed mother solution. In a product, besides main crystalline component – struvite, also all impurities from wastewater appeared in a form of hydroxides, phosphates(V) and other salts

Keywords

• Struvite, FB MSZ crystallizer, industrial wastewater, continuous reaction crystallization, product quality, kinetics, phosphorus recycling

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