Application of Simultaneous Immobilization of Borate and Arsenate in Mine Drainages by Co-Precipitation with Hydroxyapatite in a Pilot Scale

Removal of borate in mine drainages by co-precipitation with hydroxyapatite (HAp) was developed from a laboratory scale to a pilot scale. Weakly acidic initial pH facilitated to enhance dissolution of Ca(OH)2 and decrease the dissolved carbonate concentration, leading to efficient immobilization of borate and arsenate. The NH4H2PO4 lowered best the equilibrium B concentration among different phosphate sources, avoiding the lattice strain of HAp. The added molar ratio of P/Ca significantly influenced the decreasing behavior of the B concentration, showing the optimal value of 0.3. In case of P/Ca larger than 0.30, the excess concentration of PO43- was probably adsorbed on Ca(OH)2 particles to prevent the dissolution, resulting in inhibiting the formation of HAp. In case of P/Ca smaller than 0.30, the production of HAp was limited, leading to less immobilized borate. All the optimized conditions as above were applied to the pilot scale with a 250 L reactor, where borate concentration was effectively reduced in also both batch and continuous tests.