2018 Volume 59 Issue 7 Pages 1180-1185
This work focuses on the solid-state deoxidization of low-grade nickel laterite ore under various conditions using methane. The effects of the reduction temperature, reduction time, and methane concentration on the metallization rates of nickel and iron were investigated. The nickel metallization rate increased as the temperature increased to 600°C; it then decreased as the temperature was further increased. The iron metallization rate increased gradually as the temperature increased. The nickel metallization rate sharply increased to over 90% when the CH4 concentration was increased to 20 vol%. The reduction time, which ranged from 30–90 min, had a negligible effect on the reduction of nickel and iron. In the case of the reduced product, the nickel and iron metallization rates were 91.17% and 23.67%, respectively. The optimal conditions were determined to be a reduction temperature of 700°C, a reaction time of 60 min, and a CH4 concentration of 20 vol%. The nickel oxide was almost completely reduced to metallic nickel, and the majority of the iron was reduced to low-valence iron oxide. During the reduction process, a magnesium olivine phase (Mg2SiO4) was produced by the recrystallization of amorphous silicate in the reduction process, and hindered the reduction of nickel and iron.
Nickel is an important, strategic reserve metal and has an extremely important role with regard to economic development.1) The rapid development of China’s stainless-steel industry has resulted in great demand for nickel.2) Approximately 65% of the overall consumption of primary nickel is used for stainless-steel production.3,4) With regard to the high costs currently associated with stainless-steel production, the development and utilization of laterite nickel ore and ferronickel, obtained from low-cost raw materials, are increasingly important.5) High-grade nickel-containing sulfide ores are becoming depleted and increasingly difficult to mine. Consequently, there is increasing focus on oxide-containing laterite ores.6,7)
Nickel laterite ores are primarily processed using pyrometallurgical and hydrometallurgical extraction techniques.8) The hydrometallurgical process can be used to extract nickel and cobalt from the mineral to obtain electrolytic nickel, while ferronickel is the major product of the pyrometallurgical process. Ferronickel, a key nickel product, is used to produce stainless steel and other alloys.9) Low-grade nickel laterite ore has a low iron content, and high silicon and magnesium contents. It is suitable for reduction smelting within an electric furnace.10) Extensive studies have been conducted on the solid-state reducibility of nickeliferous laterite ores using gaseous reductants. Utigard et al.11) investigated the gaseous reduction of laterite ores using CO2/H2 mixed gas as a reducing agent. The rate of reduction was very low at 500°C, but then increased rapidly when the ore was heated to 600°C. The percentage of metallics increased as the H2:CO2 ratio of the reducing gas was increased. Caron12) reported that the maximum nickel metallization rate was achieved when 90% H2O+10% H2 or 75% CO2+25% CO was used during the reduction of nickel laterite ore at 900°C; here H2/H2O and CO/CO2 mixtures were used as the reductants. Itao et al.13) studied the reduction of nickeliferous laterite ores using liquefied petroleum gas (LPG). The reduction rate of the laterite ore increased as the reduction temperature increased. Olli et al.14) studied the reduction of nickel ore under gases with various H2 and CO contents. The results showed that pure nickel could not be obtained via selective reduction; however, ferronickel was obtained. Methane is the main component of natural gas. In the future, natural gas could be used for the reduction of nickel laterite ore. In addition, the use of sulfur in natural gas can promote the aggregation and growth of ferronickel particles.
The reduction temperature under methane is much lower than that of carbothermic reduction. The reduction temperature can be reduced from 1200 to 700°C, because methane is more reactive than coke. Therefore, the solid-state deoxidization of low-grade nickel laterite ore, using methane, at low temperature was investigated in this study. The reduction temperature, reduction time, and CH4 concentration were the operational parameters examined during the nickel-iron reduction process.
Nickel laterite ore samples, obtained from the Yunnan province in China, were utilized in this study. Methane, with a purity of 99.999%, was used as a reductant. During the reduction experiments, high-purity nitrogen (>99.999%) was also used to adjust the methane concentration and employed as a protective gas.
The chemical composition of the samples is presented in Table 1. The nickel laterite ore, which contains 0.82 mass% nickel, 9.67 mass% iron, 31.49 mass% MgO, and 37.37 mass% SiO2, is a low-grade nickel laterite ore. The ore was ground using a laboratory vibratory mill (XZM-100) so that 98% of the particles were smaller than 74 µm.
The nickel laterite ore was initially ground using a vibration grinding machine (XZM-100). The ground nickel laterite ore was screened using a sieve to obtain particles with sizes smaller than 0.25 mm. These were then formed into pellets with sizes of 0.25–0.38 mm to ensure good breathability and reducibility.
2.3 Reduction processThe nickel of the low-grade nickel laterite ore was reduced using a fixed-bed apparatus, as shown in Fig. 1. The reduction process employed a gaseous reductant, which contained CH4 and N2. This was metered into the reactor using a mass flow meter. The reduction procedure involved flushing the tube with nitrogen prior to heating. The reducing gases were then metered into the tube while it was heated to the required temperature. Following the completion of the reduction, the samples were cooled to room temperature within the tube furnace, under a nitrogen atmosphere to prevent re-oxidation. Approximately 5 g of the ore, which had been previously formed into pellets with a diameter of 0.25–0.38 mm, was used for the reduction. Nitrogen gas was used to regulate the concentration of the methane, and prevent oxidation of the reduced sample. The experiment was conducted under a total volumetric gas rate of 30 mL/min, with a reduction period of 30–90 min. The calcine microstructures were analyzed using scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS), and X-ray diffraction (XRD). The metallization of the nickel and iron was determined by measuring the contents of the metallic nickel and iron.
Schematic diagram of the reduction apparatus of nickel laterite ore.
The metallic nickel content of the reduction product of the nickel laterite ore was determined by a series of processes. These were performed following the extraction of bromine and methanol, separation of the filtrate via filtering, drying of the solution, and dissolution of the concentrated hydrochloric acid. In addition, inductively coupled plasma atomic emission spectrometry was performed. Conventional chemical analysis was used to analyze the metallic iron.
The XRD experiments were performed using a Japan Science D/Max-R diffractometer. Cu-Kα radiation (λ = 1.5406 Å) was used, under an operating voltage of 40 kV and a current of 40 mA. The diffraction angle (2θ) was scanned from 10° to 90°. In addition, the reduced ore was analyzed using a HITACHI-S3400N SEM, with a backscattered electron (BSE) resolution of 4.0 nm (30 kV). The EDS analysis was performed using EDAX-Octane Plus.
Subsequently, the metallization rates (γ) of the nickel and iron were calculated using eqs. (1) and (2):
| \begin{equation} \gamma_{\textit{Ni}} = \frac{M_{\textit{Ni}}}{T_{\textit{Ni}}} \times 100\% \end{equation} | (1) |
| \begin{equation} \gamma_{\textit{Fe}} = \frac{M_{\textit{Fe}}}{T_{\textit{Fe}}} \times 100\% \end{equation} | (2) |
An XRD pattern of the nickel laterite ore is presented in Fig. 2, which indicates that quartz [SiO2], lizardite [Mg3Si2O5(OH)4], and maghemite [γ-Fe2O3] are the main crystalline phases. No Ni-bearing mineral phase was observed; this may be because the nickel concentration of the raw ore is below the detection limit.15)
XRD pattern of nickel laterite ore.
Figure 3 shows a SEM image, as well as the distribution of various elements (magnesium, silicon, iron, nickel, oxygen) within the nickel laterite ore. These results are derived from the mineralogical analysis of nickel laterite ore that was performed in our previous studies.16,17) Magnesium, silicon, and oxygen occupy the majority of both the ore, and the aggregate with larger particles. Considering the XRD analysis (Fig. 1) results, it can be concluded that the magnesium, silicon, and oxygen primarily exist in the serpentine phase. Nickel is uniformly distributed throughout the ore and primarily substitutes magnesium [Mg] in lizardite.
SEM image and the distribution of different elements (Mg, Si, Fe, Ni, O) in nickel laterite ore.
The effect of the reduction temperature on the metallization rates of nickel and iron is shown in Fig. 4. As shown in Fig. 4, the metallization rate of nickel increases from 25.90% to 95.27% when the reduction temperature is increased from 500 to 600°C; it then decreases gradually. The metallization rate of iron increases gradually as the reduction temperature increases. Although the metallization rate of nickel is high at 600°C, the metallization rate of iron is only 10.79% at this temperature. In conclusion, a reduction temperature of 700°C was selected as the optimum temperature for the experiment. The methane reduction of nickel laterite ore at 700°C could be used to achieve nickel and iron metallization rates of 91.17% and 23.67%, respectively. The metallization rate of nickel decreases when the reduction temperature is increased above 600°C, which indicates that high temperatures hinder the reduction of nickel oxide. This may be owed to the initial recrystallization of magnesium silicate at 700°C, which results in the formation of a dense olivine phase that subsequently inhibits the reduction of nickel.18) To verify this, XRD analyses of the methane reduction products were performed at various temperatures.
Effect of reduction temperature on nickel and iron metallization rates. (Experimental conditions: reducing time of 60 min, total volumetric gas rate of 30 mL/min, CH4 concentration of 20 vol%).
The results of the aforementioned XRD analyses are shown in Fig. 5. The major phases of the raw ore are lizardite (Mg3Si2O5(OH)4), quartz (SiO2), and maghemite (γ-Fe2O3). As shown in Fig. 5, strong diffraction peaks, attributed to hematite, appear when the raw ore is reduced by methane at 500°C. This is owed to the phase transformation of the maghemite (γ-Fe2O3) phase in the ore, and the formation of hematite (Fe2O3). When the reduction temperature is increased to 600°C, the peak attributed to hematite completely disappears and a diffraction peak attributed to magnetite (Fe3O4) appears. Thus, the hematite was completely reduced to magnetite. Following the dehydroxylation of lizardite at this temperature, the crystal structure was destroyed, and was transformed into amorphous magnesium silicate.19,20) Therefore, diffraction peaks attributed to magnesium silicate were not detected. With a further increase in the temperature, to 700°C, the mineral phase of the nickel laterite ore changed significantly, and the amorphous magnesium silicate recrystallized to form a forsterite phase (Mg2SiO4) and enstatite phase (MgSiO3).21) The formation of the forsterite phase results in a decrease in the metallization rate of nickel. As the temperature increased, the nickel-iron nuclei continued to grow, while the peaks of α-Fe and Awaruite[FeNi3] gradually increased because of the presence of greater quantities of metal iron. The crystal phases taenite (20–65% nickel) and kamacite (5–10% nickel) exist in the nickel–iron alloy at varying percentages. The phase diagram of Ni–Fe alloy is shown in Fig. 6. Based on experimental temperature (500–1000°C) and Ni at% in Ni–Fe alloy (0–80%), the main phase areas of Ni–Fe alloy from reduction products are also α-Fe and FeNi3,22) so XRD result is consistent with phase diagram of Ni–Fe alloy.
XRD patterns of reduction products under various temperatures using methane.
The phase diagram of Fe–Ni alloy.
The effect of the reduction time on the nickel and iron metallization rates is shown in Fig. 7. The reduction time can be determined by kinetic analysis, and a sufficient reaction, as well as gas diffusion, can be achieved using a reduction gas with a flow of 30 mL/min. The experimental results showed that the metallization rate of nickel initially increases, and subsequently decreases as the reduction period is extended. A nickel metallization rate of 91.17% was achieved when the reduction period was 60 min. The metallization rate of the nickel within the reduction products peaks at a temperature of 700°C. As the reduction time is extended, the level of carbon deposition, due to methane cracking, increases, and subsequently, further reduction of the nickel oxide is inhibited. However, following reduction period of 60 min, the nickel metallization rate decreases; this may be because the metal nickel is oxidized, once more, by the oxygen existing in the ores. The iron metallization rate tends to increase gradually as the reduction time is extended; however, there is not an obvious overall change. The long reduction time results in significant energy waste; therefore, the optimal reduction time was determined to be 60 min.
Effect of reduction time on nickel and iron metallization rates. (Experimental conditions: reducing temperature of 700°C, total volumetric gas rate of 30 mL/min, CH4 concentration of 20 vol%).
The effect of the CH4 concentration on the metallization rates of nickel and iron is shown in Fig. 8. As shown in Fig. 8, when the CH4 concentration was increased from 10 vol% to 20 vol%, the nickel and iron metallization rates gradually increased. When the CH4 content was 10 vol%, the nickel metallization rate was below 55%. The nickel metallization rate increased sharply (>90%) when the CH4 concentration was increased to 20 vol%. When the CH4 concentration was over 20 vol%, the nickel and iron metallization rates were almost constant. This is because the nickel laterite ore used in the experiment is a complex mineral containing small quantities of iron and nickel oxides. When the CH4 concentration reached 20 vol%, the reducing gas within the mixed gas contained H2 and CO, which were produced by methane pyrolysis. Therefore, the balanced composition of the reducing gas used in the reaction process was deemed to be significantly excessive based on the thermodynamics analysis. Therefore, the nickel and iron metallization rates were not significantly influenced by the increase in the CH4 concentration. Considering economic efficiency, the optimum CH4 concentration of the reducing gas was determined to be 20%.
Effect of CH4 concentration on nickel and iron metallization rates. (Experimental conditions: reduction temperature of 700°C, reduction time of 60 min).
Figure 9 shows the SEM results obtained for the nickel laterite ore that was reduced for 60 min under a reduction temperature of 700°C, using a reducing gas with a CH4 concentration of 20%; the bright, white-colored substances are α-Fe and Awaruite[FeNi3]. After the sample was reduced, a portion of the iron oxides was reduced to metallic iron, forming a nickel–iron alloy along with reduced nickel. However, owing to the lower temperature, the nickel–iron alloy particles did not grow; in addition, the majority of the iron oxide was still present within the silicate materials, in the form of suboxides.
SEM images of the laterite nickel ore under the optimum reduction conditions using methane: (a) SEM images of roasted product, (b) pyroxene, (c) olivine, (d) wustite inclusion.
The pyroxene particles, shown in Fig. 9(b), exhibited a relatively porous structure, with relatively large pores and cracks existing on the surface and within the interior of the mineral. This may indicate that the nickel oxides from this phase can be relatively easily accessed by the gaseous reductant. Thus, the nickel and iron oxides that coat the particles are easily reduced. However, the compact structure of olivine (Fig. 9(c)) has a relatively low level of porosity; this means that is difficult to reduce the nickel and iron oxides of this mineral phase. Following the methane reduction under the aforementioned conditions, the morphology of the iron-oxide inclusions present in the silicate matrix did not seem to change significantly when they were reduced to wustite, as shown in Fig. 9(d).
In this study, the reduction of nickel and iron from low-grade nickel laterite ore was conducted under various conditions using methane. The effects of the reduction temperature, reduction time, and methane concentration on the nickel and iron metallization rates, respectively, were investigated. The following conclusions were drawn.
Financial support for this study was supplied from the National Natural Science Foundation of China (Project Nos. 51304091 and U1302274).
