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 NH 4 H 2 PO 4 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 PO 43- 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.