Enhanced Cementation of Co²⁺ and Ni²⁺ from Sulfate and Chloride Solutions Using Aluminum as an Electron Donor and Conductive Particles as an Electron Pathway

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.

1. Introduction

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 + nM^m+ → mNⁿ⁺ + nM⁰

(1)

The cementation reaction is divided into anodic (Equation (2)) and cathodic reactions (Equation (3)):

Anodic mN⁰ → mNⁿ⁺ + nme⁻

(2)

Cathodic nM^m+ + nme⁻ → nM⁰

(3)

The noble metal ions (M^m+) 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.

Aluminum (Al) can be considered as a strong reductant (electron donor) used for cementation because of its extremely low standard electrode potential (i.e., E⁰_Al3+/Al = –1.67 V vs. standard hydrogen electrode (SHE)) [7,9,10,11]. The practical application of Al for cementation, however, is limited due to the presence of a dense Al oxide layer (Al₂O₃) on the Al surface, which inhibits electron transfer from Al⁰ to metal ions [9,12,13]. When the Al oxide layer is removed from the surface, Al can be used as an electron donor for cementation. To remove the Al oxide layer, however, high temperatures, acid/alkaline solutions, or high concentration of chloride ions are needed [2,5,9,14,15], and these extreme conditions make it difficult to use Al as an electron donor in the practical cementation processes.

Recently, the authors investigated the effects of activated carbon (AC) addition on the efficiency of cementation using Al as an electron donor for recovering gold ions from ammonium thiosulfate solution [16,17], and heavy metal ions (Co²⁺, Ni²⁺, Zn²⁺, and Cd²⁺) from acidic sulfate and chloride solutions. The results showed that cementation efficiencies of the metal ions were significantly enhanced by the addition of activated carbon (AC) even when an insulating Al oxide layer covered on the Al surface [16,17]. This “enhanced cementation using AC/Al-mixture” can be operated under mild conditions; i.e., it does not require extreme operating conditions such as high temperatures, and high concentrations of chemical reagent such as acid, base, and chloride ions. This new method may, therefore, provide a practical way to use Al, one of the strongest reductants (electron donor) for cementation to recover metal ions from dilute solutions.

Although the details of the mechanism of enhanced cementation using the AC/Al-mixture are not yet fully understood, the results of surface analysis for the cementation products have suggested that AC attached on the Al surface acted as an electron pathway from Al to noble metal ions, even in the presence of a surface Al oxide layer [17]. If this is the case and the essential role of AC is just as an electron pathway, enhanced cementation would occur even when AC is replaced by other (semi)conductors. On the other hand, as AC is a porous material and has a very large specific surface area [18], not only the electroconductivity but also large adsorption capacity of AC for metal ions may play an important role in the enhanced cementation using the AC/Al-mixture. If this is the case, replacing AC to another conductor with a low specific surface area cannot enhance the cementation using Al as an electron donor.

Cobalt (Co) and nickel (Ni) represent important strategic resources in the world market and their use is rapidly growing for renewable energy technologies and rechargeable battery productions, and the importance of the development of technologies for recovering and purifying Co and Ni is continuously increasing [19,20,21,22,23,24]. Therefore, this study aims to investigate whether the AC could be replaced with other (semi)conductors for recovery of Co and Ni from sulfate and chloride solutions. Titanium dioxide (TiO₂) was selected for a semiconductor because of its nontoxic, nonreactive, and high chemical stability, while silicon dioxide (SiO₂) was chosen for a nonconductor to clarify the mechanism(s) of the enhanced cementation using the mixture of conductor and Al [25,26].

In the present study, batch-type cementation experiments were conducted using Al as an electron donor to recover Co²⁺ and Ni²⁺ from sulfate and chloride solutions and the effects of the addition of AC, TiO₂, or SiO₂ on the recoveries of these metal ions were investigated. Surface analysis (Auger electron spectroscopy (AES)) for the cementation products were also conducted to elucidate the cementation mechanism.

2. Materials and Methods

2.1. Materials

As an electron donor, Al powder (99.99%, Wako Pure Chemical Industries, Ltd., Osaka, Japan) was used, and AC powder (99.99%, Wako Pure Chemical Industries, Ltd., Osaka, Japan), TiO₂ powder (99.0%, rutile form, Wako Pure Chemical Industries, Ltd., Osaka, Japan), and SiO₂ powder (99.0%, Wako pure Chemical Industries, Ltd., Osaka, Japan) were used as additives. Particle size distribution of these materials, measured by laser diffraction (Microtrac^® MT3300SX, Nikkiso Co. Ltd., Osaka, Japan), is shown in Figure 1. The median diameters (D₅₀) of Al, AC, TiO₂, and SiO₂ were 21.3, 38.1, 8.5, and 21.2 µm, respectively.

2.2. Recovery of Co²⁺ and Ni²⁺ from Sulfate and Chloride Solutions

2.2.1. Preparation of Co²⁺ and Ni²⁺ Solutions

The sulfate solutions containing 1 mM metal ions were prepared by dissolving CoSO₄·7H₂O (99.0%, Wako Pure Chemical Industries, Ltd., Osaka, Japan) or NiSO₄·6H₂O (99.0%, Wako Pure Chemical Industries, Ltd., Osaka, Japan) in deionized (DI) water (18 MΩ·cm, Mill-Q^® Integral Water Purification System, Merck Millipore, Burlington, Vermont, USA). For the preparation of 1 mM metal chloride solutions, CoCl₂·6H₂O (99.0%, Kishida Chemical Co., Ltd., Osaka, Japan), or NiCl₂·6H₂O (98.0%, Kishida Chemical Co., Ltd., Osaka, Japan) was dissolved in DI water. The initial pH of sulfate and chloride solutions was adjusted to 4.0 using 1 M H₂SO₄ and HCl (Wako Pure Chemical Industries, Ltd., Osaka, Japan), respectively. The total concentration of SO₄²⁻ and Cl⁻ were fixed to 0.1 M using Na₂SO₄ (99.0%, Wako Pure Chemical Industries, Ltd., Osaka, Japan) and NaCl (99.5%, Wako Pure Chemical Industries, Ltd., Osaka, Japan) to normalize their effects on experiments.

2.2.2. Cementation Tests

The cementation tests were carried out in a 50 mL Erlenmeyer flask using a thermostat water bath shaker (Cool bath shaker, ML-10F, Taitec Corporation, Saitama, Japan) with 40 mm of shaking amplitude and 120 min⁻¹ of shaking frequency at 25 °C for 24 h. (Note that these parameters were selected based on our preliminary experiments). Ten milliliters of the prepared solution were added to the flask, then ultrapure nitrogen gas (99.9%) was introduced for 15 min before experiments to maintain an oxygen-free environment. One-tenth gram of Al powder and/or a predetermined amount (0.01, 0.05, 0.1, 0.2, 0.4 g) of additive (i.e., AC, TiO₂, and SiO₂) were added to the solution. Ultrapure nitrogen gas (99.9%) was further introduced to the flask for 5 min, then the flask was tightly capped with a rubber cap and sealed with parafilm, and an experiment was conducted. After 24 h, the suspension was filtered using a syringe-driven membrane filter (pore size: 0.2 µm, LMS Co., Ltd., Tokyo, Japan); final pH of the filtrate was measured. The filtrate was diluted with 0.1 M HNO₃, and the concentrations of metal ions were analyzed by inductively coupled plasma atomic emission spectroscopy (ICP-AES, ICPE-9820, Shimadzu Corporation, Kyoto, Japan). The recovery efficiency of Co²⁺ and Ni²⁺ was calculated based on Equation (4):

$$\mathrm{Recovery}\mathrm{efficiency},R=\frac{C_i-C_f}{C_i}$$

(4)

where C_i and C_f are the initial and final concentrations of metal ions, respectively.

2.2.3. Surface Analysis

The solid products obtained by filtration were washed 5 times with DI water, dried in a vacuum oven at 40 °C for 24 h, and then analyzed by Auger electron spectroscopy (AES) using JAMP-9500F (JEOL Co., Ltd., Tokyo, Japan). The dried residue was mounted on an AES holder using conductive carbon tape. The analysis was conducted under the following conditions: ultrahigh vacuum condition, ~1 × 10⁻⁷ Pa; probe energy, 10 kV; and probe current, 19.7 nA. The spectra were analyzed by using Spectra Investigator AES software.

3. Results and Discussion

3.1. Recovery of Co²⁺ and Ni²⁺

3.1.1. Recovery of Co²⁺ and Ni²⁺ from Sulfate Solution

Cementation experiments for recovering Co²⁺ and Ni²⁺ from sulfate solutions (initial pH = 4) were conducted for 24 h using Al powder as an electron donor, and the effects of the dosage of additives (AC, TiO₂, and SiO₂) on the efficiency of Co and Ni recoveries were investigated. To access the adsorption of Co²⁺ and Ni²⁺ on the additives, experiments without Al were also conducted.

Figure 2a–c and Figure 3a–c show the Co and Ni recovery efficiencies and final pH as a function of SiO₂, AC, and TiO₂ dosages, respectively. In all experiments, final pH was in the range from 5.1 to 5.6, at which Co²⁺ and Ni²⁺ do not precipitate as their hydroxide (Figures S1 and S2).

As shown in Figure 2a and Figure 3a, without Al, the efficiencies of Co and Ni recovery were almost 0% at any dosage of SiO₂, suggesting that there was no adsorption of Co²⁺ and Ni²⁺ on the SiO₂ surface. Even with Al, the Co and Ni recovery efficiencies were also almost 0% regardless of SiO₂ dosage, suggesting that cementation of Co²⁺ and Ni²⁺ using Al as an electron donor did not occur. This may be due to the presence of an Al oxide layer covering the Al surface, which inhibits the electron transportation from Al to Co²⁺ and Ni²⁺ [2,27]. Because the cementation did not occur regardless of SiO₂ addition, the results also confirm that physical breakage of the Al oxide layer due to the collision of SiO₂ to Al powder in the shaking flask did not cause enhanced cementation.

As shown in Figure 2b and Figure 3b, even without Al, the recovery efficiency of Co²⁺ and Ni²⁺ increased with increasing AC dosage, suggesting that these metal ions adsorbed on the AC surface. It has been reported that there are functional groups such as carboxyl and carbonyl groups on the surface of the activated carbon and they act as adsorption sites to metal ions through the reaction described by Equation (5) [18,28,29]. Increase in final pH indicates that not only Co²⁺ and Ni²⁺, but also proton (H⁺) adsorbed on AC [30,31].

–C–COOH + M²⁺ → –C–COOM + 2H⁺ (M = Co or Ni)

(5)

In the range between 0.05 to 0.2 g AC dosage, recovery efficiency was much higher with Al than without Al; at 0.1 g AC dosage, the efficiency was 56% for Co and 61% for Ni with Al, while it was 31% for Co and 43% for Ni without Al. The difference of metal recovery efficiency between either with or without Al was 25% for Co and 18% for Ni, which cannot be ignored as an experimental error. This suggests that the addition of AC enhances Co and Ni cementation using Al as an electron donor (Equations (6) and (7)), even though the Al oxide layer remained on the Al surface.

3Co²⁺ + 2Al⁰ → 3Co⁰ + 2Al³⁺

(6)

3Ni²⁺ + 2Al⁰ → 3Ni⁰ + 2Al³⁺

(7)

Following these equations, it is expected that the stoichiometric amount of Al dissolves when cementation occurs; however, the dissolved Al concentration after cementation was less than 3 ppm (Tables S1 and S2), which means that most of the Al³⁺ was precipitated as Al-(oxy)hydroxide [7,32].

As shown in Figure 2c and Figure 3c, the recovery efficiency of Co²⁺ and Ni²⁺ without Al was almost 0% regardless of TiO₂ dosage, indicating that TiO₂ has no ability to adsorb Co²⁺ and Ni²⁺. When 0.1 g of Al was used together with TiO₂, the recovery efficiency continuously increased with increasing TiO₂ dosage and reached the maximum value of 52% for Co and 71% for Ni with 0.4 g TiO₂. As already discussed, Co²⁺ and Ni²⁺ do not precipitate as hydroxides at the pH ranges observed in this series of experiments; the enhanced recovery of Co²⁺ and Ni²⁺ with TiO₂ and Al suggests that the addition of TiO₂ enhanced the cementation of Co²⁺ and Ni²⁺ by Al (Equations (6) and (7)). It was also confirmed that the dissolved Ti concentrations were below detection limit, indicating that TiO₂ is stable enough to be used as an agent to enhance cementation of Co²⁺ and Ni²⁺ with Al in the sulfate solution (Tables S1 and S2).

3.1.2. Recovery of Co²⁺ and Ni²⁺ from Chloride Solution

Cementation experiments for recovering Co²⁺ and Ni²⁺ from chloride solutions (initial pH = 4) were conducted for 24 h using Al powder as an electron donor, and the effects of the dosage of additives (AC, TiO₂, and SiO₂) on the efficiency of Co and Ni recovery were investigated. To access the adsorption of Co²⁺ and Ni²⁺ on the additives, experiments without Al were also conducted. Figure 4a–c and Figure 5a–c show the Co and Ni recovery efficiencies and final pH as a function of AC, TiO₂, and SiO₂ dosages, respectively.

Similar to the sulfate system (Figure 2 and Figure 3), final pH values of the chloride solutions (Figure 4 and Figure 5) were less than 5.5 for Co and 6.1 for Ni (Tables S3 and S4), which means that removal of Co²⁺ and Ni²⁺ from the solutions by the formation of cobalt and nickel hydroxide precipitation does not need to be considered in this series of experiments (Figures S3 and S4).

It has been reported that in the presence of high concentrations of Cl⁻, the Al oxide layer was dissolved and removed from the Al surface [13,33,34,35]. If the Al oxide layer is dissolved, a high concentration of dissolved Al would be detected in the solutions, but the observed results (Tables S3 and S4) showed that concentrations of Al were less than 5 ppm under all conditions. This implies that removal of the Al oxide layer did not occur under the experimental condition used here.

As shown in Figure 4a and Figure 5a, when SiO₂ was used as an additive, the recovery efficiencies of Co²⁺ and Ni²⁺ both with and without Al were almost 0%. This indicates that in chloride solutions, Co²⁺ and Ni²⁺ were not adsorbed on SiO₂, and the cementation of Co and Ni with Al did not occur.

The results shown in Figure 4b and Figure 5b suggest that adsorption of Co²⁺ and Ni²⁺ on AC occurred in chloride solutions, because in the absence of Al, recovery efficiencies of these ions increased with increasing AC dosage. As in sulfate solutions, in the presence of AC, enhancement of metal ion recovery by Al addition was confirmed (Figure 4b and Figure 5b); e.g., at 0.1 g AC dosage, by adding Al, the efficiency increased from 57% to 70% for Co, and it increased from 57% to 70% for Ni. This suggests that enhanced cementation of these metal ions with AC occurred in chloride solutions.

Figure 4c and Figure 5c show that the efficiencies of Co²⁺ and Ni²⁺ recovery in the absence of Al were almost 0% at any dosage of TiO₂, suggesting that adsorption of these ions on TiO₂ can be ignored. 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].

3.2. Surface Analysis of Deposited Co and Ni

To investigate the elemental compositions of the deposited Co and Ni, residues obtained from the Co²⁺ and Ni²⁺ recovery experiment from chloride solutions using 0.4 g of TiO₂ and 0.1 g of Al were analyzed by AES. Figure 6 and Figure 7 show the AES photomicrographs (Figure 6a and Figure 7a) and scan results of Co (Figure 6b,c) and Ni (Figure 7b,c). In both AES photomicrographs, many small gray particles and light particles are attached together onto the surface of the dark particle. The wide AES spectra of the dark particle (point 1 in Figure 6b and Figure 7b) show strong signals of Al and O, indicating that these particles are assigned to Al powder. The small gray particles correspond to TiO₂ because of Ti and O signals observed at point 2 in Figure 6b and Figure 7b. Meanwhile, light particles are observed at point 3 in Figure 6b and Figure 7b are most likely the deposited Co and Ni, respectively.

To identify the elemental composition of the deposited Co and Ni, the narrow AES spectra in the range of 750–785 eV for Co and 830–858 eV for Ni were analyzed (Figure 6c and Figure 7c). These spectra were fitted using reference spectra of Co, CoO, and Co₃O₄ for Co composition, and Ni and NiO for Ni composition. Fitting results indicate that the deposited Co consisted of metallic Co (93.1%) and CoO (6.9%), while the deposited Ni was composed of metallic Ni (86.2%) and NiO (13.8%). The analysis range of Auger is 0.3–5 nm, which is a near-surface analysis [38], so it is speculated that only the outermost surfaces of deposited Co and Ni were oxidized due to the oxidation of metallic Co and Ni during the dry process.

These results suggest that Co and Ni were deposited on TiO₂ particles attached to the Al surface and TiO₂ can act as an electron pathway from Al to Co²⁺ and Ni²⁺, even if the Al oxide layer remains on the Al surface. These results showed that physical separation (i.e., ultrasonification) could be applied as the postcementation process for Co/Ni–TiO₂ particle and Al separation. Afterward, it is expected that only Co and Ni would be dissolved in aqueous solutions, while TiO₂ would not be dissolved because TiO₂ is more stable than Co and Ni.

4. Conclusions

This study investigated whether activated carbon (AC) could be replaced with other additives such as TiO₂ and SiO₂ for the enhanced cementation of Co²⁺ and Ni²⁺ using aluminum (Al) in sulfate and chloride solutions. In summary, the Co²⁺ and Ni²⁺ recovery efficiencies using Al in sulfate and chloride solutions were almost 0% because of the presence of an Al oxide layer on an Al surface. The adsorption of Co²⁺ and Ni²⁺ occurred when using only AC, while it did not occur when using only TiO₂ and SiO₂. When using an AC/Al-mixture or TiO₂/Al-mixture, the Co²⁺ and Ni²⁺ recovery efficiencies from sulfate and chloride solutions were enhanced compared to using Al, AC, TiO₂, and SiO₂/Al-mixture. From the results of AES analysis, Co and Ni were mostly deposited as zero-valent forms on TiO₂ attached to Al surface. This work establishes that using a conductor (AC) or a semiconductor (TiO₂) could enhance the recovery of Co²⁺ and Ni²⁺ by Al-based cementation even under mild conditions (e.g., low temperature, 25 °C; mild pH conditions, pH 4–5; no Cl⁻ or a low concentration). Moreover, it is expected that other conductive materials could also be used for the removal and/or recovery of metal ions using Al.