Recovering Cobalt from Aqueous Solutions by Evaporative, Reactive, and Cooling Crystallization

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School of Chemical Technology | Doctoral thesis (article-based) | Defence date: 2024-03-22
Degree programme
75 + app. 69
Aalto University publication series DOCTORAL THESES, 58/2024
With the challenge of climate change, the restructuring of transportation and power sectors is crucial for achieving greenhouse gas neutrality. Cobalt, which plays a key role in the energy transition, has been recognized as one of the critical materials globally. The hydrometallurgical process has shown great potential in extracting cobalt from both primary and secondary resources during cobalt manufacture. Recovering cobalt from aqueous solutions is an essential step in the hydrometallurgy process of cobalt extraction. Crystallization is a separation and purification technology by forming solids from solutions. In this study, the recovery of cobalt salts from aqueous cobalt sulfate solution using vacuum evaporative crystallization, cooling crystallization, and carbonate precipitation was investigated. The thermodynamic data includes the saturation vapor pressure for cobalt sulfate solution and the cobalt sulfate solubility in aqueous solutions was determined. The effects of operational conditions on crystallization and final products were indicated by recovering cobalt sulfate through vacuum evaporative crystallization, batch, and continuous cooling crystallization. At temperatures below 40 °C, cobalt sulfate primarily crystallizes in the heptahydrate form, while at temperatures of 60 and 80 °C, it crystallizes in the hexahydrate form. The CoSO4∙7H2O is prone to dehydration during the drying process. Additionally, the primary nucleation kinetics of CoSO4 were determined by measuring the solubility, metastable zone width (MSZW), and induction time in batch cooling crystallization. The secondary nucleation dominated crystal kinetics for cobalt sulfate in a continuous cooling crystallization were also investigated based on the Mixed-Suspension, Mixed-Product-Removal (MSMPR) theory, and Population balance equations (PBEs). In addition, operation conditions like temperature, mixing speed, and impurities significantly affect the crystal size and crystallization kinetics.  In the cobalt carbonate precipitation, the precipitation mechanism was successfully investigated using inline Focused Beam Reflectance Measurement (FBRM) and pH monitoring and offline measurement (Scanning electron microscope, X-ray powder diffraction, Raman spectroscopy). With the pH decreasing, the cobalt initially precipitated as Co2CO3(OH)2 and continuously transferred to CoCO3.  Raman spectrometry has been found to a great potential in studying the crystallization of metal salts for both solid phase identification and ionic concentration quantification. Moreover, UV-Vis spectrophotometry is efficient for the quantitative analysis of cobalt and nickel concentrations in solutions.
Supervising professor
Louhi-Kultanen, Marjatta, Prof., Aalto University, Department of Chemical and Metallurgical Engineering, Finland
Thesis advisor
Louhi-Kultanen, Marjatta, Prof., Aalto University, Department of Chemical and Metallurgical Engineering, Finland
Lindberg, Daniel, Prof., Aalto University, Department of Chemical and Metallurgical Engineering, Finland
cobalt sulfate, cobalt carbonate, recovery, crystallization, nucleation kinetics, growth kinetics
Other note
  • [Publication 1]: Jianxin Zhang; Arshe Said; Bing Han; Marjatta Louhi-Kultanen. 2022. Semi-batch evaporative crystallization and drying of cobalt sulphate hydrates. Hydrometallurgy, Volume 208, 105821.
    DOI: 10.1016/j.hydromet.2022.105821 View at publisher
  • [Publication 2]: Jianxin Zhang; Rajaboopathi Mani; Marjatta Louhi-Kultanen. 2023. Process monitoring of cobalt carbonate precipitation by reactions between cobalt sulfate and sodium carbonate solutions to control product morphology and purity. Hydrometallurgy, Volume 224, 106232.
    DOI: 10.1016/j.hydromet.2023.106232 View at publisher
  • [Publication 3]: Jianxin Zhang; Marjatta Louhi-Kultanen. Determination of nucleation kinetics of cobalt sulfate by measuring metastable zone width and induction time in pure and sulfuric acid solution. Powder Technology, Volume 422, 118463.
    DOI: 10.1016/j.powtec.2023.118463 View at publisher
  • [Publication 4]: Jianxin Zhang; Marjatta Louhi-Kultanen. Continuous cooling crystallization of cobalt sulfate from pure aqueous, H2SO4, and NiSO4 solutions. Under review in Chemical Engineering Science