J. Cent. South Univ. (2016) 23: 3072-3078

DOI: 10.1007/s11771-016-3371-y

Ferronickel preparation using Ni-Fe co-deposition process

WANG Fei(王飞), LI Lei(李磊), QIU Shi-wei(邱士伟), WANG Hua(王华)

State Key Laboratory of Complex Non-ferrous Metal Resources Clean Utilization,

Engineering Research Center of Metallurgical Energy Conservation and

Emission Reduction of Ministry of Education, Faculty of Metallurgical and Energy Engineering,

Kunming University of Science and Technology, Kunming 650093, China

Central South University Press and Springer-Verlag Berlin Heidelberg 2016

Abstract: Nucleation mechanism and technological process for Ni-Fe co-deposition with a relatively high Fe²⁺concentration surrounded were described, and the effects of Fe²⁺ concentration, solution pH, temperature, and sodium dodecyl sulfonate concentration were investigated. Electrochemical experiments demonstrate that iron’s electrodeposition plays a leading role in the Ni-Fe co-deposition process, and the co-deposition nucleation mechanism accords with a progressive nucleation. Temperature increase does favor in increasing nickel content in the ferronickel (Ni-Fe co-deposition products), while Fe²⁺ concentration increase does not. When solution pH is higher than 3.5, nickel content in the ferronickel decreases with pH because of the hydrolysis of Fe²⁺. With the current density of 180 A/m², Na₂SO₄ concentration of 100 g/L and Ni²⁺ concentration of 60 g/L, a smooth ferronickel deposit containing 96.21% Ni can be obtained under the conditions of temperature of 60 °C, Fe²⁺ concentration of 0.3 g/L, solution pH of 3 and sodium dodecyl sulfonate concentration of 40 mg/L.

Key words: Ni-Fe co-deposition; nucleation mechanism; high Fe²⁺ concentration surrounded; waste regeneration

1 Introduction

The eelectroplating sludge is an important raw material for nickel recovery [1], acidic leaching solution of which is used to produce Ni or NiSO₄·6H₂O. As a common cooperative element, Fe²⁺ has a negative influence on the nickel recovery. The Fe²⁺ concentration in the electrolyte is required to be lower than 4 mg/L in Ni electrodeposition process and it in NiSO₄·6H₂O production process should also be below 0.002% [2]. Strict iron removal from the electroplating sludge is inevitable, which increases process cost greatly and causes Ni loss. It is meaningful to recover nickel at a relatively high Fe²⁺concentration surrounded.

The co-deposition of Ni and Fe has been studied by many researchers. ROBOTIN et al [3] studied influences of different parameters on the Fe content in the Ni-Fe electrodeposit product. They found that Fe amount in the deposit reached a higher value at a low current density, but there was not a linear dependence of Fe content in the co-deposition product with current density, for the reason that deposition rate was controlled by the diffusion step as current density increased higher. Effects of magnetic field on compositions of co-deposited Ni-Fe products were studied by FRICOTEAUX and ROUSSE [4], and their results showed nickel content and current efficiency decreased with magnetic induction induced by the Lorentz force. VIRGINIA [5] found Ni-Fe deposits with higher iron contents could be obtained at a low pH. As mentioned above, almost all these studies were focused on the Ni-Fe electroplating process, and iron mass fractions of co-deposition products were commonly controlled between 30% and 40% for ensuring high hardness and good wear resistance of the plating [6]. It is infeasible in economic if using it to produce ferronickel (materials used for steelmaking of stainless steel), for the reason that high iron contained decreases the products value.

For recovering nickel effectively from electroplating sludge at a low cost, it is significant to make higher nickel content ferronickel in its acidic leaching solution with a relatively high Fe²⁺ concentration (0.1-0.5 g/L in this work) surrounded. Based on this, method of ferronickel preparation using Ni-Fe co-deposition process was proposed in this paper, and the study was mainly focused on the nucleation mechanism and technological process, using FeSO₄, NiSO₄ and mixture of these solution systems.

2 Experiments

2.1 Experimental equipment and methods

Linear potential scan (LSV) and chronoampero- metry (CA) measurements were performed with Princeton 2273 electrochemical workstation using a three-electrode system. In the electrode system, platinum rotating disk electrode was used as the working electrode, platinum electrode as the counter electrode, and saturated calomel electrode (SCE) as the reference electrode. Focusing on effects of Fe²⁺ concentration on Ni-Fe co-deposition process, the electrolyte was prepared with analytically pure FeSO₄·6H₂O and NiSO₄·7H₂O by a certain ratio. Electrochemical experiments were carried out at the temperature of 60℃. Electrolyte systems were respectively 0.1 mol/L NiSO₄·7H₂O solution, 0.1 mol/L FeSO₄·6H₂O solution, and the mixed solution of 0.05 mol/L NiSO₄·7H₂O and 0.05 mol/L FeSO₄·6H₂O. In addition, 0.7 mol/L Na₂SO₄ was added to each solution to increase the conductivity.

In order to avoid oxidation of Fe²⁺ by dissolved oxygen in the electrolyte, the Ni-Fe co-deposition process tests were carried out under Ar atmosphere in a cycle system consisting of a high slot, a low slot and an electrolytic cell (see Fig. 1). Electrolyte samples were taken through the sampling location using a pipette (see Fig. 1). The electrolyte was also prepared with FeSO₄·6H₂O and NiSO₄·7H₂O of analytically pure by a certain ratio. The electrolyte cycle time was 3 h, and solution temperature was controlled by a water bath during circulating. The anode was titanium plate, and the cathode was 316 L stainless steel plate, polar distance of which was 90 mm.

2.2 Characterization

Chemical composition of the deposit products, and Ni²⁺ and Fe²⁺ concentrations in the electrolyte were analyzed by chemical analysis. Surface morphology of the products was characterized by the scanning electron microscopy (SNE-1500X).

3 Results and discussion

3.1 Nucleation mechanisms determination

Determining reduction potentials of different cases, the LSV curves for 0.1 mol/L NiSO₄, 0.1 mol/L FeSO₄,0.05 mol/L NiSO₄+0.05 mol/L FeSO₄ recorded at a Pt electrode using a scan rate of 50 mV/s at pH 3 are presented as Fig. 2. The reduction peak for Ni is situated at approximately -1.071 V vs SCE; while for Fe, the reduction takes place at -1.198 V. As expected, the peak corresponding to Ni-Fe is situated between the ones for Ni and Fe, at around -1.135 V vs SCE. Therefore, we get the polarization curves obtained at different rotation speeds of a Pt rotating disk electrode (RDE) shown in Fig. 3.

By comparing the polarization curves obtained at different rotation speeds of a Pt RDE, it can be seen from Fig. 3 that the curves corresponding to NiSO₄-FeSO₄ are more similar to those of FeSO₄, with the reduction peak situated to be more negative than for NiSO₄.

Based on the above results discussed, it is meaningful to evaluate the nucleation mechanisms for these three different cases. Firstly, the potential step tests were performed (see Fig. 4). Based on the LSV data, a negative step was applied between 1.0 and 1.2, or -1 and -1.3 V. The maximum current is denoted as I_m. The time corresponding to I_m is denoted as t_m. The (I/I_m)²-t/t_m curves are obtained by nondimensionalizing data of each system near the peak potential and a nucleation model is chosen in order to fit the (I/I_m)²-t/t_m curves [7]. According to this model, the equations describing the nucleation are for instantaneous nucleation; and for progressive nucleation.

        (1)

       (2)

Fig. 1 Experimental device

Fig. 2 Linear polarization curves for NiSO₄, FeSO₄ and NiSO₄-FeSO₄

Fig. 3 Linear polarization curves for NiSO₄ (a), FeSO₄ (b) and NiSO₄–FeSO₄ (c) at different rotation speeds:

Fig. 4 Current transients for Ni deposition (a), Fe deposition (b) and Ni–Fe co-deposition (c) for different potential steps applied at pH=3:

The obtained plots are compared to the theoretical models in Fig. 5. The data are in accordance with an instantaneous nucleation in the case of NiSO₄. While for FeSO₄ and NiSO₄-FeSO₄, the obtained data points are closer to the model for a progressive nucleation. Differences in the nucleation mechanism for these three cases demonstrate that Fe electrodeposition plays a leading role in the electrodeposition of Ni and Fe.

3.2 Effects of main parameters on ferronickel preparation

Electrochemical experiments above demonstrate that iron’s electrodeposition plays a leading role in Ni-Fe co-deposition process, and the behavior of Ni-Fe co-deposition is heterogeneous. To improve Ni content in the deposit, the co-deposition should be carried out with a relatively high Ni²⁺ concentration surrounded [8]. The Ni²⁺ concentration was fixed at 60 g/L in all experiments.

Fig. 5 Comparison between theoretical curves for instantaneous and progressive nucleation and (I/I_m)² vs t/t_m plots obtained experimentally for Ni (a), Fe (b) and Ni–Fe (c) deposition from sulfate, at different potential steps:

Figure 6 shows that Fe content in the deposit increases from 1.19% to 3.8% while C_Fe increases from 0.1 to 0.5 g/L, the reason for which is that Ni–Fe co-deposition is anomalous under most experimental conditions [3]. The Fe is preferentially deposited compared to Ni, even if Fe is the less noble metal. In Fig. 6, J_d is current density, and is Na₂SO₄ concentration in the electrolyte. Referencing to traditional process of Ni electrowinning [9], the J_d was set to be 180 A/m², and C_Na2SO4 to be 100 g/L (equal to 0.7 mol/L). In addition, C_Ni is Ni²⁺ concentration, C_Fe Fe²⁺ concentration, C_SDS sodium dodecyl sulfonate concentration, and T_d deposition temperature.

Fig. 6 Mass fractions of Ni and Fe in deposits vs C_Fe (J_d=180 A/m², 100 g/L, T_d=60 °C, C_Ni=60 g/L, pH=3, C_SDS=40 mg/L)

Figure 7 shows the photo of ferronickel samples at different C_Fe taken by a camera. It shows that a smooth deposit is obtained when C_Fe is 0.3 g/L, and it has surface cracks when C_Fe is increased to 0.4 or 0.5 g/L caused by strong internal stress with the presence of Fe [10]. The surface cracks make the co-deposition process hard to be continued.

During the co-deposition process, some Fe²⁺ can be oxidized on the anode forming Fe³⁺ and then be reduced on the cathode forming Fe²⁺. As a result, current efficiency decreases with C_Fe (see Fig. 8), and it’s more obvious when C_Fe is high [11]. The higher acceptable Fe²⁺ concentration in this process, the less cost spent in removing iron when recovers nickel from the electroplating sludge. Considering aforementioned results, C_Fe is fixed at 0.3 g/L.

Figure 9 shows that current efficiency increases and Ni content decreases with solution pH when it is higher than 3.5. When solution pH is low, free hydrogen ion concentration is high, and hydrogen evolution causes cathodic current efficiency decrease. However, the Fe²⁺ is easy to be hydrolyzed when solution pH is higher than 4. It will suppress the diffusion of Ni²⁺ by attachment of hydrolysates on the cathode and not favor the deposition of Ni [12]. As a result, Ni content in the deposit is decreased (see Fig. 9). In addition, high pH decreases the wettability of cathodes, and it’s hard for hydrogen bubbles to be desorbed from cathodes, resulting in the appearance of much stomata at the surface of deposited ferronickel (see Fig. 10). To improve current efficiency and Ni content in the ferronickel products, the suitable pH is fixed at 3-3.5.

Fig. 7 Photographs of ferronickel deposited from electrolyte at different C_Fe (0.3, 0.4, 0.5 g/L) (J_d=180 A/m², 100 g/L, T_d=60 °C, C_Ni =60 g/L, pH=3, C_SDS=40 mg/L)

Fig. 8 Current efficiency vs C_Fe (J_d=180 A/m², 100 g/L, T_d=60 °C, C_Ni=60 g/L, pH=3, C_SDS=40 mg/L)

Temperature rising increases the diffusion and electrodeposition rates of Ni²⁺ and Fe²⁺, causing consumption of Fe²⁺ to be accelerated and suppression of Ni²⁺ diffusion to be weakened [13-14]. Besides, oxidation rate of Fe²⁺ is also speeded up [15], making C_Fe further decreased. Both above increase the cathodic polarization and decrease Fe deposition amounts [16], and as a result, iron contents in deposits are decreased (see Fig. 11). Figure 11 shows that iron contents decrease from 4.5% to 3.7% with temperature increasing from 15 °C to 60 °C. Besides, temperature rising decreases the solution viscosity and increases ions diffusion speed, and more active points can appear on the surface of cathode. As a result, the nucleation grain is refined (see Fig. 12), which does well in increasing mechanical properties of the obtained deposits. The temperature is fixed at 60 °C.

Fig. 9 Mass fraction of Ni in deposits and current efficiency vs pH (J_d=180 A/m², 100 g/L, T_d=60 °C, C_Ni=60 g/L, C_Fe=0.3 g/L, C_SDS=40 mg/L)

Fig. 10 Photographs of ferronickel deposited from electrolyte at different pH (J_d=180 A/m², 100 g/L, T_d=60 °C, C_Ni=60 g/L, C_Fe=0.3 g/L, C_SDS=40 mg/L)

Deposition potential of hydrogen is close to that of studyn time this paprecess osible in nickel, and hydrogen evolution is unavoidable. Attachment of hydrogen bubbles on the cathode results in appearance of stomatas at the deposited ferronickel surface [17]. Some researchers have found that sodium dodecyl sulfonate can stick on the surface of cathodes, help to decrease surface tension of the electrolyte, enhance wettability of the cathodes, and make hydrogen bubbles easy to be desorbed [18-19]. Photograph of ferronickel sample at different C_SDS (Fig. 13), taken by a camera, shows that a great deal of stomatas appears at the surface of ferronickel deposit without sodium dodecyl sulfonate addition, and numbers of stomatas decrease greatly when sodium dodecyl sulfonate concentration is increased to 20 mg/L. When its concentration is 40 mg/L, the stomatas almost disappears. The sodium dodecyl sulfonate concentration is fixed at 40 mg/L. At the same time, nickel content in the deposit reaches 96.21% (see Fig. 11).

Fig. 11 Mass fractions of Ni and Fe in deposits vs temperature (J_d=180 A/m², 100 g/L, C_Ni=60 g/L, C_Fe=0.3 g/L, pH=3, C_SDS=40 mg/L)

Fig. 12 SEM images for deposits at 45 °C and 60 °C (J_d=180 A/m², 100 g/L, C_Ni=60 g/L, C_Fe=0.3 g/L, pH=3, C_SDS=40 mg/L)

Fig. 13 Photographs of ferronickel deposited from electrolyte at different C_SDS (J_d=180 A/m², 100 g/L, T_d=60 °C, C_Ni=60 g/L, C_Fe=0.3 g/L, pH=3)

As mentioned above, a smooth high Ni content ferronickel plate is obtained through the Ni-Fe co-deposition under the conditions of Fe²⁺ concentration of 0.3 g/L, temperature of 60 °C, solution pH of 3 and sodium dodecyl sulfonate concentration of 40 mg/L with the presence of Ni²⁺ concentration of 60 g/L, current density of 180 A/m², and Na₂SO₄ concentration of 100 g/L. Compared with requirements of Fe²⁺ concentration in production process of Ni (≤4 mg/L) or NiSO₄·6H₂O (≤0.002%), the acceptable Fe²⁺ concentration is much higher in this method for recovering nickel from the electroplating sludge. This work gives a new thinking for recovering the nickel effectively from the electroplating sludge at a low cost.

4 Conclusions

1) Iron’s electrodeposition plays a leading role in the Ni-Fe co-deposition process. The nucleation mechanism of Ni-Fe co-deposition accords with a progressive nucleation, being similar to that of Fe.

2) Iron content in the deposit increases with C_Fe, while decrease with temperature. When C_Fe is over 0.3 g/L, surface cracks exist in the obtained deposit caused by strong internal stress with the presence of Fe. When solution pH is over 3.5 , Fe²⁺ hydrolysis does not favor the Ni deposition and Ni content in the deposit decreases.

3) With the current density being of 180 A/m² , Ni²⁺ concentration of 60 g/L, and Na₂SO₄ concentration of 100 g/L, a smooth ferronickel plate containing 96.21% Ni can be obtained under the conditions of temperature of 60 °C, Fe²⁺ concentration of 0.3 g/L, solution pH of 3 and sodium dodecyl sulfonate concentration of 40 mg/L. Compared with requirements of Fe²⁺ concentration in production process of Ni (≤4 mg/L) or NiSO₄ (≤0.002%), Fe²⁺ concentration is much higher in this method, and it will cost fewer for removing iron during recovering nickel from electroplating sludge.

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(Edited by YANG Bing)

Foundation item: Project(51574135) supported by the National Natural Science Foundation of China; Project(KKPT201563022) supported by Collaborative Innovation Center of Kunming University of Science and Technology, China

Received date: 2015-11-16; Accepted date: 2016-05-23

Corresponding author: LI Lei, Associate Professor, PhD; E-mail: tianxiametal1008@163.com