Separation of Struvite from Mineral Fertilizer Industry Wastewater.

Abstract: The research results of struvite reaction crystallization process from the phosphorus mineral fertilizers industry wastewater, carried out in DTM type continuous crystallizers with internal circulation of suspension driven by jet pump device fed with mother solution, are presented. The wastewater of pH < 4, contained 0.445 mass % of phosphate(V) ions and typical impurities: aluminium, calcium, copper, iron, potassium, magnesium, titanium, zinc, fluosilicate, fluoride and sulphate(VI) ions. Tests ran in three continuous jet pump DTM crystallizers of increasing working volumes: 1.2, 15 and 36 dm³, in temperature 298 K, assuming both stoichiometric conditions and 20% excess of magnesium ions in relation to phosphate(V) and ammonium ion contents. Influence of technological control parameters (pH, τ) on product crystal size distributions (CSDs) and crystals/mother liquor chemical composition were identified. Crystals of low size-homogeneity (coefficient of variation CV ca. 90%) which mean size varied from ca. 9 to ca. 33 μm were identified. Struvite crystals of the largest sizes were produced in a DTM crystallizer of working volume 36 dm3 at 20% excess of magnesium ions, at pH 9 and for elongated mean residence time 7200 s. Excess of magnesium ions in a process system influenced the struvite reaction crystallization process yield definitely advantageously. Concentration of phosphate(V) ions decreased from 0.445 mass % in a feed to 1.8 10^–3 mass % in a postprocessed mother solution, what can be regarded as a very good result of their removal process. In product crystals, besides main component – struvite, also all impurities originally present in wastewater appeared in a form of hydroxides, phosphates and other salts. Aluminium, copper, iron and zinc ions were the subject of practically total co- precipitation with struvite. [Copyright &y& Elsevier]