Preparation of ultra-pure ammonium metavanadate via heterogeneous self-assembly crystallization

Although the demand for ultra-pure V2O5 in industry is significant, the current technologies used to obtain the principal precursor, ammonium metavanadate, in high purity are costly and low-yielding. In this study, a cost-efficient and high-yielding heterogeneous self-assembly crystallization (HSC) process is proposed for obtaining large, high-purity NH4VO3 crystals. The method entails establishing a wide crystallization window, controlling the NH3 mass transfer rate and buffering the pH of the mother liquor with CO2 released during the pyrolysis of the ammonia source, (NH4)2CO3 to achieve the separation of chromium impurities, the most difficult-to-remove impurities in vanadium products, from the chromium-containing NaVO3 solution, and purification of NH4VO3 via reactive crystallization. Under the optimized crystallization conditions, namely an N/V mass ratio of 3.9, temperature of 60 °C, and pH of 12.5, the yield and purity of the obtained NH4VO3 in 20 g·L−1 V(V) simulated solution containing 0.05 g·L−1 Cr(VI) reached 93.09% and 99.99%, respectively, and the obtained NH4VO3 crystals were as large as 1.8 mm in size. In the case of a real leaching solution, the NH4VO3 yield and purity were higher than 80% and 99.99%, respectively, and the purity of final V2O5 reached 99.99%. The curbed pyrolysis rate of (NH4)2CO3 ensures heterogeneous nucleation and slow growth of NH4VO3 crystals to acquire large and pure crystals, and the excessive adsorption of Cr(VI) onto the crystal surface during self-assembly is avoided at the appropriate pH, thus guaranteeing efficient precipitation, crystallization, and purification. This study provides a reference strategy for the low-cost preparation to obtain high-purity V2O5.