Abstract
Cobalt and nickel have become important strategic resources because they are widely used for renewable energy technologies and rechargeable battery production. Cementation, an electrochemical deposition of noble metal ions using a less noble metal as an electron donor, is an important option to recover Co and Ni from dilute aqueous solutions of these metal ions. In this study, cementation experiments for recovering Co2+ and Ni2+ from sulfate and chloride solutions (pH = 4) were conducted at 298 K using Al powder as electron donor, and the effects of additives such as activated carbon (AC), TiO2, and SiO2 powders on the cementation efficiency were investigated. Without additives, cementation efficiencies of Co2+ and Ni2+ were almost zero in both sulfate and chloride solutions, mainly because of the presence of an aluminum oxide layer (Al2O3) on an Al surface, which inhibits electron transfer from Al to the metal ions. Addition of nonconductor (SiO2) did not affect the cementation efficiencies of Co2+ and Ni2+ using Al as electron donor, while addition of (semi)conductors such as AC or TiO2 enhanced the cementation efficiencies significantly. The results of surface analysis (Auger electron spectroscopy) for the cementation products when using TiO2/Al mixture showed that Co and Ni were deposited on TiO2 particles attached on the Al surface. This result suggests that conductors such as TiO2 act as an electron pathway from Al to Co2+ and Ni2+, even when an Al oxide layer covered on an Al surface.
Highlights
Cementation, an electrochemical deposition of noble metal ions by a less noble metal as an electron donor, is usually applied to remove/recover metal ions from dilute aqueous solutions [1,2,3,4]
The results showed that cementation efficiencies of the metal ions were significantly enhanced by the addition of activated carbon (AC)
In the presence of Al, the efficiencies of Co2+ and Ni2+ recovery increased with increasing TiO2 dosage; without TiO2, the efficiencies were almost 0% for both Co and. Ni while they increased to 61% for Co2+ and 99.9% for Ni2+ when 0.4 g TiO2 was added. These results suggest clearly that addition of TiO2 enhanced the cementation of Co and Ni by using Al as an electron donor, and indicated that AC can be replaced with TiO2 even if its surface area is lower than AC [18,36,37]
Summary
Cementation, an electrochemical deposition of noble metal ions by a less noble metal as an electron donor, is usually applied to remove/recover metal ions from dilute aqueous solutions [1,2,3,4]. The advantages of cementation are (1) recovery of metals in zero-valent form, (2) simple methods, and (3) low-energy consumption [2,5]. In this method, the overall reaction of cementation is given by Equation (1) [6,7,8]: mN0 + nMm+ → mNn+ + nM0. The cementation reaction is divided into anodic (Equation (2)) and cathodic reactions (Equation (3)): Anodic mN0 → mNn+ + nme− (2). The noble metal ions (Mm+ ) are deposited on the surface of a less noble elemental metal (N0 ) spontaneously, and the driving force of this reaction is mainly determined by differences in the standard electrode potentials for Mn+ /M0 and Nn+ /N0 redox pairs, and it increases when the electrode potential of N0 is low
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