Novel two-stage oxidation/crystallization technology for high-purity calcium phosphates recovery from digester supernatant

Abstract

A novel hybrid technology consisting of oxidation and reactive crystallization was developed for recovery of high-purity CaP from digester supernatant. Initial phosphate concentration in digester supernatant was 10 mmol/L and initial calcium concentration was 0.2 mmol/L. H2O2 (20 ml of 30% ww H2O2/L) in combination with short ultrasound treatment (10 min, P = 50W, A = 94% and C = 70%) was used for the oxidation stage. Effect of pH at oxidation stage and oxidation time on the P-recovery rate and purity of the products was studied by performing the oxidation at 4 different pH values (2.0; 3.9; 7.8 and 10.0) for 24 or 48 h. Oxidation at pH 7.9 showed the best performance in terms of the properties of the crystallized products. Batch crystallization of calcium phosphates was done at the Ca/P ratio 2. Crystallization lasted for 2 h. Nucleation pH was determined and was chosen as operation pH. At the tested conditions, two compounds were recovered: dicalcium phosphate dihydrate (DCPD) and hydroxyapatite (HAp). Addition of DCPD seeds (0.2 g/L) promoted crystallization of pure DCPD. Oxidation at pH 7.9 for 24 h followed by crystallization without seeding resulted in the highest P-recovery (89%) with the production of DCPD as a major product plus minor amounts of HAp.