Obtaining phosphorus concentrate from phosphate ore flotation tailings after Ca and Mg recovery using formic acid leaching

• Ca and Mg could be selectively leached by formic acid from POFTs. • Formic acid dissolved 96.43 % dolomite with tiny apatite in POFTs. • Content of P 2 O 5 in formic acid leaching residue achieved 28.90 %. • Ca was recovered as calcium sulfate whiskers by adding H 2 SO 4 . • Formic acid can be regenerated after recovering Ca and Mg. Phosphate ore flotation tailings (POFTs), principal by-products of phosphate ore flotation, are regarded as valuable secondary resources due to their high P 2 O 5 content (5–12 %) and potential for Ca and Mg recovery. To exploit these values in POFTs, the formic acid leaching-precipitation method was developed, starting from the selective dissolution of dolomite. The leaching efficiencies of Ca, Mg, and P were 67.62, 96.43, and 12.16 %, respectively, under the conditions of using 3.2 mol/L formic acid solution at 60 ℃ for 8 h with the liquid–solid ratio of 10 mL/g. The results showed that almost all the dolomite and tiny apatite were dissolved, while quartz was difficult to dissolve. Increasing temperature contributed to the dissolution of dolomite and suppressed apatite dissolution while increasing the liquid–solid ratio would facilitate formic acid hydrolysis resulting in higher P dissolution. The leaching residue comprised 28.90 % of P 2 O 5 , 3.66 times of the original POFTs (7.89 % of P 2 O 5 ), which can be reused as a phosphorus concentrate. In addition, the formic acid solution can be regenerated through fractional precipitation to obtain Ca and Mg products. Adding H 2 SO 4 at 60 ℃ to the formic acid solution precipitated calcium sulfate whiskers, and after filtration adding H 2 C 2 O 4 at 60 ℃ precipitated magnesium oxalate dihydrate. The formic acid leaching-precipitation method achieved low-cost and highly efficient utilization of POFTs.