Removal of antimony from concentrated solutions with focus on tripuhyite (FeSbO4) synthesis, characterization and stability

The precipitation of antimony (Sb) from acidic aqueous solutions (6 g·L− 1 Sb) as ferric antimonate, FeSbO4 (tripuhyite), its further crystallization by hydrothermal or calcination treatment and the determination of its degree of stability were investigated. The precipitation was studied over the pH range 1.5-5.3 at 95 °C via the oxidation of Sb(III) and Fe(II) by drop-wise H2O2 addition. Among all tests, oxidation at pH 5.3 over ≈ 3.5 h with 10 wt% H2O2 was retained as best precipitation protocol. The precipitate was characterized as poorly crystalline tripuhyite (possessing high specific surface area in the order of 160-200 m2·g− 1). Hydrothermal treatment in pH 1 water was done at 200 °C for 12 h, while calcination at 950 °C for 12 h to increase crystallinity, the latter being more effective. A separate highly crystalline tripuhyite product was synthesized at high temperature (950 °C) by solid-state reaction of goethite and antimony pentoxide in order to serve as reference. The stability/leachability tests indicated that pseudo-equilibrium was attained by all synthesized materials in ≈ 15 days. Between pH 6 and 8, the untreated precipitate released 100-170 mg·L− 1 Sb, hydrothermally treated precipitate released ≈ 0.5 mg·L− 1 Sb, calcined precipitate released 10-35 mg·L− 1 Sb, and the reference tripuhyite released ≈ 0.5 mg·L− 1 Sb. The results suggested that stability was mainly a function of tripuhyite crystallinity rather than pH (over the 6-8 range tested). The hydrothermally treated precipitate demonstrated the highest stability (aside from the synthetic reference material). The relatively higher Sb release recorded by the calcined precipitate was traced to its contamination with NaSbO3, a secondary high temperature phase. Lastly, tripuhyite produced via aqueous precipitation and hydrothermal treatment appears to offer a viable option in fixing antimony from metallurgical waste streams.