Atmospheric Leaching Behavior and Kinetics of Nickel and Cobalt from Halmahera Limonite Ore

Halmahera Island in Indonesia holds great potential for nickel laterite due to its geological setting. As the product of chemical weathering, nickel laterite can be divided into saprolite and limonite. Despite containing invaluable elements like nickel and cobalt, limonite ore is seldom used in metal production. This study evaluates these two elements’ leaching behavior and kinetics from the Halmahera deposit. Limonite ore was leached using 50 mL of 0.5 M sulfuric acid at an S/L ratio of 10% and 20%. The leaching process was conducted at 30°C, 50°C, and 80°C. The experimental results showed that the highest recovery for cobalt and nickel was 100% in the adequate presence of H⁺ ions. The result shows that sulfuric acid is more selective to cobalt than a nickel. The shrinking core ash diffusion model represents the leaching kinetics of cobalt, while nickel recovery follows the Kröger-Ziegler kinetics model.

Cobalt; Kinetics; Leaching; Limonite; Nickel

2.1. Materials

    The limonitic laterite ore used in this study was from a mining area on Halmahera Island, Indonesia. A representative sample was obtained by coning and quartering. The dried sample was crushed, ground, ball milled, and sieved until the average size of the sample was < 74  The chemical components of the selected representative sample were determined using X-ray Fluorescence (XRF, Epsilon 3XLE PANalytical). X-ray Diffraction (XRD, X’Pert 3 Powder from PANalytical, Netherlands) was also used to identify the crystalline materials in limonite ore. The morphology and elemental map of the sample were analyzed using FE-SEM/EDS Thermo Scientific Quattro S. Pregnant leached solution in this study was prepared by diluting pro analytical sulfuric acid (CAS RN of 7664-93-9, Merck, Germany) in aquadest.

2.2. Menthods

Halmahera limonite ore was put into an Erlenmeyer, and 50 mL of 0.5 M sulfuric acid was added. The atmospheric leaching was done for 4 hours in an orbital shaker, as shown in Figure 1. The shaker speed was kept constant at 200 rpm, while the S/L ratio and leaching temperature varied from 30 to 80°C. The concentration of leached nickel and cobalt at any designated time (15 minutes, 30 minutes, an hour, 2 hours, and 4 hours) was determined using ICP-Optical Emission Spectrometer PQ 9000 from Analytik Jena AG, Germany.

Figure 1 The experimental apparatus for nickel and cobalt leaching: (1) orbital shaker; (2) Erlenmeyer flask

where C is the element concentration in the liquid sample (g/mL), V is the volume of sulfuric acid solution (50 mL), X is the element concentration in ore, and m is the mass of the limonite ore (g).

k_K is the leaching rate constant for the Kröger-Ziegler model, t is the leaching time (min),  is the recovery value of either nickel or cobalt, and kD is the leaching rate constant for the ash diffusion controls model.

3.1.   Ore Characterization

        The nickel content in the limonite ore from Halmahera Island, Indonesia, is 1.73%. The ore is considered middle grade, with the element content between 1.2% and 1.8% (Cao, Xue, and Duan, 2016). According to the diffractogram (Figure 2), nickel minerals are not present in the ore. Instead, limonite ore is composed mainly of goethite (-FeOOH), with minor lizardite (Mg₃Si₂O₅(OH)₄) and talc (Mg₃Si₄O₁₀(OH)₂). Nickel is most likely hosted in goethite, indicated by the coincidence of Ni and Fe in the elemental map (Figure 3). Generally, nickel is associated with goethite through surface adsorption or Fe substitution (Butt and Cluzel, 2013). The EDS results also project the mapping of Mg, the major constituent of talc and lizardite, proving the existence of both minerals in the ore (Ersoy et al., 2013; Carmignano et al., 2020).

Figure 3 Elemental Mapping of Limonite Ore: Cr (dark blue); O (light green); Mg (blue); Si (tosca); Fe (purple); Ni (yellow); Al (orange); Mn (green)

        XRF analysis shows that Halmahera limonite ore contains 0.12% of cobalt. This element is mainly carried by Mn-oxyhydroxides, like asbolane and lithiophorite-asbolane (Sagapoa, Imai, and Watanabe, 2011; Tupaz et al., 2020). These minerals have poor crystallinity and thus hardly appear on diffractograms (Burlet and Vanbrabant, 2015; Maci?g et al., 2019).

    However, the XRF result indicates the presence of manganese oxides, which undergo coprecipitation with cobalt and other elements, such as Ni (1.73 wt%), Co (0.12 wt%), Fe (34.12 wt%), Mn (0.99 wt%), Si (3.46 wt%), and Al (1.46 wt%). Aside from Mn-oxyhydroxides, cobalt is also hosted in lizardite and some in goethite (Tang and Valix, 2004; Sufriadin et al., 2020). FE-SEM/EDS analysis was also used to confirm the XRF data. FE-SEM/EDS result indicates the presence of Ni (1.72 wt%), Fe (26.17 wt%), Mg (10.80 wt%), Al (1.39 wt%), Cr (0.37 wt%), Mn (0.47 wt%), and Si (9.86 wt%) (Astuti et al., 2021).

3.2. Nickel and Cobalt Leaching

The leaching process of nickel and cobalt from Halmahera limonite ore by sulfuric acid was done at 30°C, 50°C, and 80°C. Along with the increase in temperature, the recovery of both elements also increases. Higher temperature provides greater kinetic energy, which allows rapid molecular movement. The fast movement accelerates molecular diffusion and surface reaction. This trend is the same at an S/L ratio of 10% (Figure 4) and 20% (Figure 5).
        Unlike temperature, the S/L ratio is inversely proportional to nickel and cobalt recovery. At 10% of the S/L ratio, sulfuric acid can recover 100% of cobalt after an hour. Meanwhile, the same value is achieved by nickel after 4 hours at 80°C. Complete recoveries are possible since there are adequate hydrogen ions (H⁺) to leach both elements from their carrier. However, a distinct case is found using a higher S/L ratio. Loading more solids into the system raises the number of leachable materials. However, the amount of H⁺ is insufficient to recover all available nickel and cobalt. At this point, leached solution selectively reacts with the host mineral that is easier to dissolve.