Recovery of cobalt as mesoporous Co3O4 from ammonia leaching solution of spent lithium battery cathode material via reductive ammonia distillation and thermal decomposition

• A sustainable process for recovering Co 3+ from NH 3 –(NH 4 ) 2 CO 3 solution was studied. • More than 99% 97% of cobalt could be precipitated by reductive ammonia distillation. • The mesoporous Co 3 O 4 product was obtained at 300 °C for 3 h. • The phase transform during the thermal decomposition process was revealed. • The average particle size of Co 3 O 4 increased with rising roasting temperature. Ammoniacal leaching system is widely applied to cobalt extraction due to its selectivity and mild leaching conditions. However, in ammonia leach liquor, the complexation of Co 3+ and ammonia is stable ( K [Co(NH 3 ) 6 3+ ]=4.5×10 33 ), which brings difficulties to cobalt recovery. In this work, a clean and sustainable process for recovering Co 3+ as mesoporous Co 3 O 4 product was proposed from NH 3 –(NH 4 ) 2 CO 3 system, which included reductive ammonia distillation and thermal decomposition. Firstly, through ammonia distillation with simultaneous addition of hydrazine hydrate as a reducing agent, more than 97% of cobalt can be precipitated in the form of (NH 4 ) 1.89 Co 7.88 (CO 3 ) 6 (OH) 6 (H 2 O) 4.27 under the optimal conditions. Mechanism analysis shows that the addition of hydrazine hydrate can reduce part of Co 3+ to Co 2+ , which is beneficial to the precipitation of cobalt. Subsequently, the Co 3 O 4 products were prepared at 300 °C for 3 h via thermal decomposition of the precipitation. Meanwhile, the phase transformation and morphology changes of the product were investigated in-depth via relevant characterization methods. This process not only achieves efficient recovery of cobalt from the ammoniacal solutions but also the evaporated ammonia and residual solution can be returned to ammonia leaching.