Two-Step Hydrometallurgical Preparation of Ultrafine Cobalt Powders

A two-step hydrometallurgical method was carried out to prepare ultrafine cobalt powders for the first time. The products were characterized by scanning electron microscopy coupled with energy-dispersive spectrometry, X-ray diffraction, laser particle size analyzer, and inductive coupled plasma. The cobalt powders contained the hcp-cobalt phase and fcc-cobalt phase. The proportion of hydrazine hydrate used in the two steps was found to be essential for the size of cobalt powders. As the proportion of hydrazine hydrate changed from 1:4 to 4:1, an obvious mean size evolution was observed from 1000 to 300 nm. A transition state {[Co(NH3)6](OH)x}(x−2)– was proposed to explain the preparation process of ultrafine cobalt powders.


■ INTRODUCTION
Cobalt, known as "industrial vitamin", is widely used in cemented carbides, demand tools, magnetic materials, lithiumion batteries, and high-temperature alloys due to the great hardness, high melting point, stable chemical performance, and strong magnetism. 1,2 In particular, ultrafine cobalt powders are of deep interest over a long period of time for researchers from a wide range of fields. 3,4 For instance, spherical cobalt powders are mostly needed in the progressive cutting tool industries. 5,6 One-dimensional cobalt nanorods have applications in magnetic sensors and drug delivery. 7,8 It has been reported that nanosize cobalt powders could be prepared by hydrometallurgical methods. 9−13 Liquid-phase hydrogen reduction is one of the most typical process for cobalt synthesis due to the economic and environmental consideration. 9,10,14 However, this process requires high pressure and temperature. Kunda and Hitesman prepared CoSO 4 solution with the NH 3 /Co molar ratio ranging from 2.3 to 4.0 and reduced the solution at 200°C with hydrogen, which was controlled under 3 MPa. 9 The cobalt powders obtained possessed a large specific surface area of 12.4 m 2 /g. Other hydrometallurgical methods include reduction of cobalt salts by strong reductants, such as hydrazine hydrate, sodium borohydride, and potassium borohydrate. 11−13 Liu et al. added Co(NO 3 ) 2 /ethanol solution into the sodium hydroxide/ hydrazine hydrate mixture at 50°C and got cobalt powders with a size distribution between 500 nm and 4 μm. 12 It is obvious that such a simple method is valuable for producing nanosize cobalt powders as a consequence of the moderate reaction conditions. In this paper, a two-step hydrometallurgical route was developed to prepare ultrafine cobalt powders based on the previous report.
■ RESULTS AND DISCUSSION Figure 1 shows characterization results of cobalt powders of sample 4. In Figure 1a,b, detailed scanning electron microscopy (SEM) pictures are given at a large and small scale, respectively. It is obvious that the size of ultrafine cobalt powders ranges from 200 to 800 nm, as shown in Figure 1a. In Figure 1b, ultrafine cobalt powders are homogeneously distributed in the whole picture. In Figure 1c, the average particle size of cobalt powders is 300 nm and diameters of over 90% of cobalt powders are less than 650 nm based on laser particle size analyzer (LPSA) analysis. Figure 1d demonstrates the X-ray powder diffraction (XRD) pattern of ultrafine cobalt powders prepared. According to the reported data JCPDS no.: 05-0727 and 15-0806, phase compositions of cobalt powders include the hcp-cobalt phase and fcc-cobalt phase. In Table 1, similar phase percentages could be found between different cobalt samples. The average percentages of fcc-cobalt and hcpcobalt phases are 85 and 15%, respectively. Figure 1e presents the energy-dispersive spectrometry (EDS) spectrum of cobalt powders. The sample mainly consists of cobalt with a small amount of oxygen species. The small content of oxygen in cobalt powders may be due to reoxidation during the reaction.
The proportion of hydrazine hydrate used in the two steps plays an important role in the size of cobalt powders. An obvious mean size evolution from 1000 to 300 nm is observed. Cobalt powders with the size distribution of 500−1400 nm are obtained when the amount of hydrazine hydrate used in the
As the amount of hydrazine hydrate is up to 1.2 N 2 H 4 /Co molar ratio, the size of cobalt powders ranges from 400 to 1000 nm ( Figure 2b). When the amount of hydrazine hydrate is further increased to 1.8 and 2.4 N 2 H 4 /Co molar ratio, considerable size changes could be found. The size distributions of cobalt powders are reduced to 300−600 nm ( Figure 2c) and 200−800 nm (Figure 2d), respectively. It is evident that the proportion of hydrazine hydrate used is the crucial parameter to the size changes of cobalt powders. Figure 3 shows the formation process of ultrafine cobalt powder prepared by the two-step hydrometallurgical method.
The color of the mixture changes from cherry red to navy blue when hydrazine hydrate is added into cobalt salt solution. In this process, the existence form of cobalt varies from Co 2+ to a coordination compound [Co(N 2 H 4 ) 3 ] 2+ . However, hydrazine hydrate could decompose into ammonia as the reaction temperature rises, especially when sodium borohydride is added. 15 6 ]-(OH) x } (x−2)− provides a low energy state for the formation of ultrafine cobalt powder because the labilized hydrogen atoms could be more easily adsorbed. Finally, hcp-cobalt and fcc-cobalt are formed.
It is very interesting that the reaction could not occur once the addition order is changed. The color of the mixture changes from cherry red to pink when solid NaOH is added into the cobalt salt solution. Also, the color of the mixture remains pink until hydrazine hydrate blends and the reaction stops. Hydroxyl ions have more strong interactions with Co 2+ rather than hydrazine molecules. Therefore, cobalt hydroxide would be formed in priority and the transition state {[Co(NH 3 ) 6 ](OH) x } (x−2)− could not be obtained. Table 2 gives the chemical compositions of ultrafine cobalt powders. The content of oxygen in cobalt powders is 0.63%.
Other impurities, such as sulphur, are all less than 0.01%. Therefore, the purity of ultrafine cobalt powders is over 99.9% and could be a future material for the next generation of cutting tools.

■ CONCLUSIONS
Ultrafine cobalt powders are prepared by a two-step hydrometallurgical method. It is found that the proportion of hydrazine hydrate used in two steps plays a key role in the size changes of cobalt powders. A transition state {[Co(NH 3 ) 6 ]-(OH) x } (x−2)− is proposed to explain the preparation process. This work provides a novel perspective on the preparation method of cobalt powders.

■ EXPERIMENTAL SECTION
Ultrafine cobalt powders were synthesized through a two-step hydrometallurgical method. CoCl 2 ·6H 2 O was dissolved in deionized water to form 0.25 mol/L cobalt salt solution. Then, 50 wt % hydrazine hydrate was added into cobalt salt solution in two steps with a total amount of 3.0:1 N 2 H 4 /Co molar ratio. Table 3 gives the amount of hydrazine hydrate used in each step. In the first step, a certain amount of hydrazine hydrate was added into cobalt salt solution under vigorous stirring at 70°C. The color of the mixture varied from cherry red to navy blue. In the second step, the pH value of the mixture was adjusted to be 11 with solid NaOH and another amount of hydrazine hydrate was added into the mixture again to obtain a gray black slurry. The slurry was stirred continuously for 1 h until it became transparent. The precipitation was filtered, rinsed with deionized water and ethanol for several times, and dried in a nitrogen atmosphere at 60°C for 6 h. Finally, ultrafine cobalt powders were obtained. It was essential to add    a small quantity of sodium borohydride in every step to activate the reaction. Phase composition of cobalt powders was determined on XRD (AXS D8, Bruker, Germany). Morphology of cobalt powders was characterized by SEM (JEOL 6400, JEOL, Japan). Size distribution of cobalt powders was tested by a LPSA (BIC 90Plus, Brookhaven, USA). Chemical compositions of cobalt powders were measured by inductive coupled plasma (Optima 5300DV, PerkinElmer, USA) and EDS (JEOL 6400, JEOL, Japan).