Microstructural modification of Ni electrodeposit in an acidic NiCl2 solution
Introduction
Nickel presents excellent properties such as chemical stability, good plastic workability and weldability, so it has been extensively applied for protective and decorative coatings, electrical and electronic components, aerospace and military devices, etc. [[1], [2], [3], [4]]. In addition, high-purity Ni (more than 99.999 wt%) can be used to prepare targets for sputtering processes, as well as various catalysts for synthesis or cracking of organics in petroleum industry and [5,6].
Ni can be extracted from laterite ores through either a pyrometallurgical or hydrometallurgical route, involving several procedures such as calcining, electric furnace smelting, acid leaching and electrowinning [[7], [8], [9]]. Commonly, the purity of electrolytic nickel is about 99.8 wt% with the impurities of Co, Cu, Fe, Mg, Mn, Zn and Al, etc. [10,11]. These impurities have negative influence on the properties of Ni and will deteriorate the performance of devices [6]. Therefore, some purification processes should be employed to produce high-purity nickel. Firstly, Ni2+ ions in aqueous solutions was purified by an anion-exchange separation or extraction process, in which metallic ions (Co2+, Fe2+, Cu2+, etc.) can be rigorously removed. Subsequently, Ni metal was electrochemically deposited from the electrolyte [[12], [13], [14], [15]]. Finally, zone-melting can be performed with high-energy electron beam to remove the organic matters and no-metallic elements (N, O and C, etc.). The purity of Ni can be enhanced to higher than 99.999 wt% [11]. In general, electrowinning is a significant procedure to transform Ni ions to nickel metal in the production of both electrolytic Ni and high-purity Ni [9,[16], [17], [18]].
Electrowinning of nickel can be performed in sulfate or chloride solutions. Nickel sulfate solutions have attracted much attention because of the stability of NiSO4 and the avoidance of Cl2 gas on the anode. However, nickel chloride solutions also exhibit some superiorities, such as higher electrical conductivity and diffusion-limiting current density [9,[16], [17], [18]]. Till now, there are some challenges for the electrowinning processes in both sulfate and chloride solutions. For example, the structure, morphology and quality of Ni deposit are closely related to the operating conditions, such as current density, pH value, temperature, concentration of organic and inorganic impurities in the electrolyte [10,19].
Gas evolution is another issue that should be considered since hydrogen will be released in accompany with the deposition of Ni [9,20,21]. Hydrogen molecules will be absorbed and subsequently bond to Ni atoms, generating uniform morphology, high strain and pitting in the product. Various kinds of organic and inorganic additives have been added to the electrolyte to produce bright and commercially acceptable Ni metal, including sodium sulfate, boric acid and sodium lauryl sulfate, etc. [1,15,22]. However, these additives will aggregate in Ni deposit and increase the concentration of impurities. The quality of Ni metal is severely decreased, especially in the preparation of high-purity nickel. Therefore, well-crystallized nickel deposit will be desired to produce high-quality nickel.
In this work, electrowinning of nickel was performed in a NiCl2 solution without adding any additives. Microstructure and phase evolution of the nickel deposit were modified by introducing gas bubbles and magnetic agitation into the electrowinning process. The negative influence of hydrogen evolution on the deposit was also relieved. In addition, the nucleation and growth of nickel deposit in different processes were also discussed.
Section snippets
Experimental
NiCl2·6H2O (analytical grade, purchased from Sinopharm Chemical Reagent Co., Ltd.) was dissolved into de-ionized water to prepare the electrolyte for electrowinning (concentration of Ni2+ ions: 75 g·L−1). The pH value was adjusted to 0.1 with 0.5 mol·L−1 of hydrochloric acid. A 304 stainless steel sheet (1.5 cm × 2 cm × 0.1 cm) was firstly polished with abrasive papers, and then washed ultrasonically in ethanol to produce a smooth and clean surface. A rectangular Ni plate (5 cm × 3 cm × 0.5 cm)
Results and discussion
Fig. 1a shows typical CV curves in a 5.9 g·L−1 NiCl2 + 8 g·L−1 H3BO3 solution (pH = 3), which were reversed at different potentials during the negative scanning. The cathodic current rising at about −0.7 V (vs. SCE) corresponds to electrochemical reduction from Ni2+ to Ni (Eq. (1)) [23,24]. The current subsequently became fluctuant once the potentials were more negative than −1.0 V, which was related to the release of hydrogen gas (Eq. (2)) [24,25]. An anodic peak at around −0.1 V was
Conclusions
Electrowinning of Ni was carried out in an acidic NiCl2 solution by introducing magnetic agitation and upwards flowing bubbles to control the characteristics of Ni deposit. The employment of magnetic agitation and micron-sized flowing bubbles effectively accelerated the mass transfer occurred in the electrowinning process. The nucleation mode of Ni deposit was turned from the continuous nucleation mode to the instantaneous nucleation mode. These strengthened operations can transfer the grains
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors acknowledge National Key Research and Development Program of China (Grant No. 2017YFB0305401) and China Scholarship Council (grant No. 201906085006) for financial support.
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