Thermodynamic modelling of the chemical precipitation synthesis of CoC2O4·2H2O from acidic solution and Co(OH)(CO3)0.5·nH2O from basic solution

Abstract

Cobalt, a strategically important metal, has become increasingly consumed in last two decades, mainly due to its use in lithium-ion batteries (LIBs). Thus, cobalt recovery from spent LIBs and other cobalt bearing secondary sources are critical. Since most of cobalt recovery processes are based on hydrometallurgical processes, it is important to understand the aqueous chemistry and precipitation behaviour of cobalt. In this paper, CoC₂O₄·2H₂O and Co(OH)(CO₃)_0.5·nH₂O were synthesized by chemical precipitation technique. These compounds were obtained by precipitation of Co(NO₃)₂·6H₂O solutions in the presence of anhydrous oxalic acid (H₂C₂O₄) and ammonium carbonate (AC) solutions, respectively. The precipitation mechanisms were elucidated by the thermodynamic models derived. Non-linear equations obtained by deriving the equilibrium constants and solubility product equations, the mass equivalence equations of the components and the charge equivalence equations of the dissolved ions were solved by WolframAlpha program and the modelling was carried out. The precipitated compounds were characterized by TG/DTA-MS, ICP-OES, XRD and FT-IR analytical techniques. When 50 % more than the stoichiometric amount of oxalic acid required to precipitate CoC₂O₄ is used, the solubility of Co²⁺ decreases due to the common ion effect as the $C_2{O}_4^{2‐}$ concentration in the solution increases and the formation efficiency of CoC₂O₄ precipitate increases from 93.12 % to 98.51 %. For dilute conditions, when the amount of oxalic acid used exceeded 50 % (140 % in this study), $\text{Co}{\left({\text{HC}}_2{O}_4\right)}^{+}$ was formed and passed into solution. Therefore, the dissolution of CoC₂O₄ and the formation of Co²⁺ in acidic medium negatively affected the formation efficiency of CoC₂O₄ precipitate (97.75 %). The precipitate was confirmed to be CoC₂O₄·2H₂O by TG-DTA analysis (theoretical 59.05 %, experimental 59.03 % weight loss for CoC₂O₄·2H₂O → CoO conversion) and XRD analysis. Co(OH)(CO₃)_0.5·0.5H₂O which has the lowest solubility was precipitated among the cobalt compounds that can precipitate at pH values between 8.404 and 8.814 (Co(OH)₂, CoCO₃, Co(OH)(CO₃)_0.5). The formation efficiency of Co(OH)(CO₃)_0.5·0.5H₂O precipitate was 99.9 %. The precipitate was confirmed to be Co(OH)(CO₃)_0.5·0.5H₂O by TG-DTA analysis (theoretical 34.80 %, experimental 35.28 % weight loss for Co(OH)(CO₃)_0.5·0.5H₂O → CoO conversion) and XRD analysis (amorphous structure). This study provides important information to increase the efficiency of the recovery processes of cobalt from aqueous solutions in acidic and basic environments.