Effect of organic additives on manganese electrolysis

Electrolytic manganese is an important metal material that is widely employed in batteries, electronics, steel, and other fields. However, there are many issues regarding the production process of electrolytic manganese, such as low current efficiency, high-content toxic SeO₂ additives in the electrolyte, and difficulty in controlling the product morphology. These issues not only impact the production efficiency and quality of electrolytic manganese but also result in severe environmental pollution. To address these issues, a MnSO₄-(NH₄)₂SO₄ electrolyte system was prepared based on the electrolytic manganese industry standard. The effects of polyacrylic acid, glycine, ethylenediaminetetraacetic acid (EDTA), and gluconic acid were explored as auxiliary additives under neutral electrolysis conditions to lower the SeO₂ content, which is a highly toxic main additive. The effects of the different additives on the metal manganese morphology, crystal structure, and cathodic electrochemical polarization behavior were examined by field-emission scanning electron microscopy, X-ray diffraction, and constant current cathodic polarization curve tests. The findings reveal that the main orientation of the metal manganese crystal form deposited by the four auxiliary additives is the (330) crystal plane, and all of them can promote the formation of α-Mn, enhance the electrolysis efficiency, and lower the energy consumption. In particular, the optimized amount of polyacrylic acid of 0.08 g·L⁻¹ contributes to an energy consumption of 5735.34 kW·h·t⁻¹. Correspondingly, the addition of 10 g·L⁻¹ glycine is the most favorable amount, leading to an energy consumption of 5518.56 kW·h·t⁻¹. For EDTA, the lowest energy consumption of 5168.26 kW·h·t⁻¹ is measured at an added amount of 0.5 g·L⁻¹. Note that among the four investigated candidates, gluconic acid is the most favorable auxiliary additive, contributing to the increased cathode current density and reduced cathodic polarization as well as denser metal manganese products. Moreover, gluconic acid addition can lower the concentration of toxic SeO₂ from 0.03–0.06 to 0.015 g·L⁻¹, significantly increase the cathode current density, reduce the cathodic polarization, and result in a more dense and smooth product morphology, with the current efficiency being increased from approximately 70% to 89.73% and energy consumption being reduced from 6500 to 4990.58 kW·h·t⁻¹ at the same time. Gluconic acid, as the auxiliary additive, not only contributes to the best electrolysis indexes for electrolytic manganese but also facilitates the formation of metal manganese with the most desirable crystal structure. This work offers novel insights into the environmentally friendly production and cheap electrodeposition of metal manganese for the electrolytic manganese industry.