Effect of Elemental Sulfur on the Reduction Process of Laterite Nickel Ore under the Action of Methane

Abstract

The contradiction between the supply and demand of nickel resources is intensifying, and it is urgent to develop new processing and smelting processes for laterite nickel ore. Gas based smelting with H₂ and CH₄ as reductants is of great significance to solve the “double carbon” problem faced by the traditional metallurgical industry. Taking low-grade laterite nickel ore as the research object, CH₄ as the reductant and elemental sulfur as the additive, under the conditions of different reduction temperature, reduction time, gas concentration and additive dosage, this paper discusses the reduction behavior of CH₄ and iron nickel oxide in laterite nickel ore. Combined with XRD, SEM-EDS, gas analysis and other characterization methods, the phase and morphology of laterite nickel ore and its reduction products were deeply analyzed. Results surface: under the conditions of reduction temperature 800°C, reduction time 60 min, CH₄ concentration 20%, elemental sulfur dosage 4%, the metallization rate of nickel and iron in the reduction product can reach 98.01% and 8.44%, respectively. Nickel oxide is almost completely reduced to nickel, and most of iron is reduced to low-price iron oxide. In the reduction process, the amorphous silicate recrystallizes into magnesium olivine phase in the reduction process. It hinders further reduction of nickel. According to SEM-EDS analysis, some metal iron and elemental sulfur generate FeS around the iron oxide region, which hinders the contact between reducing gas and FeO. Therefore, the reduction of iron is inhibited and the iron oxide content increases. It promoted the selective reduction of nickel.

1. Introduction

Nickel has excellent properties such as corrosion resistance, high melting point and strong magnetism. It is an important raw material for the production of various special steels, heat-resistant alloys, corrosion-resistant alloys, magnetic alloys and hard alloys. It is widely used in iron and steel, aerospace, machinery manufacturing, chemical industry and communication equipment. It is an important strategic resource.^1–4) Nickel ore is divided into nickel sulfide ore and nickel oxide ore (most of which are laterite nickel ore), of which nickel oxide ore accounts for about 70% and nickel sulfide ore accounts for about 30%.⁵⁾ With the gradual consumption of nickel sulfide ore and the continuous growth of nickel demand, the development and utilization of laterite nickel ore has received great attention.^6,7)

Since nickel in laterite nickel ore mainly occurs in the form of silicate minerals,^8,9) it is difficult to recover nickel in laterite nickel ore by traditional beneficiation process, which seriously restricts the effective utilization of laterite nickel ore. Moreover, the composition of laterite nickel ore is complex and the ore properties are variable. Many studies show that pyrotechnics is the most effective method to treat laterite nickel ore to produce ferronickel products. Among them, reduction roasting is one of the important methods for preparing ferronickel by pyrometallurgical methods.¹⁰⁾ And scholars at home and abroad have conducted extensive research on this. At present, the reducing agents used in the reduction roasting of laterite nickel ore are divided into solid based and gas based: (1) The solid base is mainly anthracite and coke, which is widely used in the pyrometallurgy of ferronickel. Its process is mature, but it is troubled by energy consumption and “double carbon” constraints;^11,12) (2) The gas base mainly uses H₂, CO, CH₄, coke oven gas, etc. as reductants. It has the advantages of mass transfer, heat transfer and low reaction temperature, and has become the current research hotspot. Based on the activity theory, Chen et al.¹³⁾ conducted a thermodynamic analysis on the CO₂/H₂ mixed gas reduction process of laterite nickel ore. They studied the effects of CO₂/H₂ ratio and temperature on the selective reduction of laterite. The results show that the metallization of Fe and Ni promotes each other due to the change of Fe and Ni activities, which is the inevitable reason of Fe metallization. When the reduction process is carried out under the conditions of reduction temperature of 800°C, reduction time of 1 h, CO₂/H₂ ratio of 9/1 and flow rate of 100 mL/min, the metallization rate of nickel can reach more than 95%. Hang et al.¹⁴⁾ studied CO reduction of laterite nickel ore. By controlling the CO₂–CO ratio, the nickel grade of 37.2% and the recovery rate of 95.89% can be obtained at the roasting temperature of 1200°C. Li Bo et al.¹⁵⁾ studied the kinetics of CO reduction of laterite nickel ore and found that the reduction reaction was mainly controlled by chemical reaction and gas diffusion. In addition, some scholars have studied the reduction of laterite nickel ore by CH₄. Ding Zhiguang et al.¹⁶⁾ used CH₄ as reducing agent, under the conditions of reduction temperature of 700°C, reaction time of 60 min, CH₄ concentration of 20 vol%, the metallization rates of nickel and iron in the reduced product were 91.17% and 23.67%, respectively. C. A. Pickles et al.¹⁷⁾ studied the thermodynamics of the reduction of laterite nickel ore by CH₄ and found that CH₄ has good reduction performance in the temperature range of 400∼800°C. In conclusion, gas-based reduction has certain advantages in improving the grade and recovery of concentrate and reducing energy consumption. At present, much research has been done on the reduction of laterite nickel ore by H₂ and CO. However, the lack of systematic research on the reduction of low carbon energy CH₄ laterite nickel ore limits its further application.

In addition, many studies in recent years have shown that additives can control the morphology and reduction degree of ferronickel products, greatly improve the nickel grade of concentrate and realize the selective reduction of nickel. Jiang et al.^18–20) believe that Na₂SO₄ can decompose to produce Na₂O, s and Na₂S, and then form Fe–FeS eutectic with low melting point, which promotes the selective reduction of laterite nickel ore. Iwan Setiawan et al.^21,22) found that in the selective reduction of laterite nickel ore by carbothermal, the addition of sulfur can not only improve the aggregation effect of nickel iron metal particles, but also inhibit the reduction of iron oxides. Sunging Pintowantoro et al.²³⁾ reduced laterite nickel ore with natural sulfur as an additive, and the reduction process was conducted at 1400°C for 6 h in a coal–limestone bed. Results showed that utilization of 10%wt natural sulfur resulted in a product with 13.62% Ni content and 97.91% recovery. Compared with the reduced ore without additives, the nickel content increased by 11.47%, while the iron content decreased. In conclusion, the sulfur component in the additive plays a key role in the selective reduction of laterite nickel ore and the grain growth of ferronickel alloy.

In this study, the solid-state reduction method was used to reduce ferronickel from low-grade laterite nickel ore. The clean energy methane was used as the gas reductant, and the solid additive elemental sulfur was used. The effects of reduction temperature, methane concentration and additive amount on the metallization rate of nickel and iron were studied, and the optimal test conditions were obtained.

2. Materials and Methods

2.1 Experimental raw materials and analysis

In this study, lateritic nickel ore samples obtained from Yunnan Province of China were used. Methane with purity of 99.99% was used as reductant, and elemental sulfur was used as solid additive. The additives used are pure analytical reagents produced by Tianjin Shentai Chemical Reagent Co., LTD. The sulfur content is not less than 99.5%. In the reduction experiment, methane concentration was adjusted by adjusting the flow of high-purity nitrogen (purity of 99.9%), which played the role of protective gas at the same time.

The chemical composition of laterite nickel ore used in this experiment is shown in Table 1. The mass fraction of nickel in the raw ore is 0.82%, and the mass fraction of iron is 9.67%. The main gangue minerals include MgO accounting for 31.49%, SiO₂ accounting for 37.37%, and Al₂O₃ accounting for 1.89%. From the results in the table, it can be seen that the laterite nickel ore used in the experiment is a typical low-grade silicon magnesium laterite nickel ore. The nickel ore resource of the experimental raw material comes from a certain place in Yunnan. The ore is finely ground in the prototype of the vibrating mill, and then sieved with a 100 mesh sieve to obtain a particle size less than 74 µm of mineral powder. Then put it into the drying oven for drying. In order to make the additive uniformly mixed with the ore sample, the additive is also finely ground to obtain a particle size less than 74 µm of sulfur powder. The element analysis of the laterite nickel ore is shown in Table 1. The content of nickel and iron in the ore is low, including 0.82% nickel, 9.67% iron, and high silicon and magnesium, including 37.37% silicon dioxide, 31.49% magnesium oxide, and a small amount of aluminum oxide.

Table 1 Chemical composition of the laterite ore sample.

The laterite nickel ore was analyzed by XRD and SEM. XRD analysis is shown in Fig. 1. The main mineral phases are lizardite (Mg₃Si₂O₅ (OH)₄), quartz (SiO₂) and iron oxide (Fe₂O₃), among which lizardite phase is distributed most. No goethite phase is found in the raw ore, and the nickel content in this laterite nickel ore is very small, only 0.82%, so the diffraction peak of nickel bearing minerals are not found in the XRD pattern.²⁴⁾

Fig. 1

X-Ray Diffraction (XRD) analysis of laterite nickel ore.

Figure 2 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.^25,26) The raw ore is seriously weathered under the action of nature, and there are more cracks. The distribution areas of magnesium, silicon and oxygen elements in the figure coincide, which are lizardite phase, and the iron and oxygen elements coincide, which proves that the main form of iron in the ore is iron oxide, which is consistent with the XRD analysis results, and the nickel element is less and evenly distributed in the whole ore. The phase without nickel in XRD is caused by the low nickel content in the ore.

Fig. 2

Scanning electron microscope of nickel laterite ore.

2.2 Sample preparation

The finely ground laterite nickel ore was mixed evenly with the additive elemental sulfur. Put the mixed sample into a cylindrical mold with an inner diameter of 1 cm and press it under 12 Mpa for 3 min to obtain a sheet press product. And then crushed into pellets with a particle size of 0.25–0.38 mm, so as to ensure its good permeability and reducibility.

2.3 Reduction process

The nickel in low-grade laterite nickel ore was reduced in a vertical tube furnace. The model of the tubular furnace is STGL-40-17, and the manufacturer is Henan Sante Furnace Industry Technology Co., Ltd. In the reduction process, methane is used as the gas reductant and high-purity nitrogen as the protective gas. Each time 3 g pellets are weighed, quartz cotton is first put into the quartz tube, and then the weighed pellets are put into the tubular furnace. Before heating, the pipe is flushed with nitrogen, and then the temperature is raised to the target temperature (700–1100°C), the mixture of methane and nitrogen is introduced, and the methane flow is controlled at 15 ml/min, Then the flow of nitrogen is controlled by the valve of the tubular furnace to adjust the concentration of methane. The reduction time is 60 min. After the reduction, the samples in the tubular furnace are naturally cooled to room temperature in nitrogen atmosphere to prevent reoxidation. The calcined microstructure was analyzed by scanning electron microscope, energy dispersive spectroscopy (SEM-EDS) and X-ray diffraction (XRD). The metallization degree of nickel and iron is determined by measuring the content of nickel and iron.

2.4 Analysis method

The phase composition of nickel minerals in laterite was determined by using Japanese D/Max-3B X-ray diffraction (XRD). The test conditions: Cu-Kα Ray source, voltage 40 kV, current 150 mA, scanning speed 5°/min, scanning range 2θ 10°∼90°. In addition, the reduced ore was analyzed by scanning electron microscope (SEM) to analyze its micro morphology.

Using bromine-methanol method to extract nickel metal from the reduction product of laterite nickel ore. After adding bromine methanol to the sample, the metal nickel is dissolved in methanol. The nickel content in the solution was determined by inductively coupled plasma atomic emission spectrometer after a series of steps including filtration and separation of insoluble residue, evaporation and drying of filtrate, dissolution of hydrochloric acid and dilution of fixed volume. And then through the calculation of metal nickel content.

Then, the metallization rates of nickel and iron were calculated using the equation:

  

\begin{equation} \gamma_{\text{Ni}}=\text{M}_{\text{Ni}}/\text{T}_{\text{Ni}} \end{equation} (1)

  

\begin{equation} \gamma_{\text{Fe}}=\text{M}_{\text{Fe}}/\text{T}_{\text{Fe}} \end{equation} (2)

where γ_Ni and γ_Fe is the metallization rate of nickel and iron respectively, T_Ni and T_Fe are the total nickel and total iron contents in the reduction pellet respectively, M_Ni and M_Fe are the metal nickel and metal iron contents in the reduction pellet respectively.

3. Results and Discussion

3.1 Analysis of reaction process of laterite nickel ore

The thermal properties of laterite nickel ore were studied by TG-DSC analysis. The temperature was increased to 1100°C at room temperature at the rate of 10°C/min. The experimental process was conducted in a high-purity nitrogen atmosphere. Figure 3 shows the TG-DSC curve. In the heating process, the weight loss rate of the raw material is about 13.41%. There are two endothermic peaks in the DSC curve. Free water will be removed during the heating process. The removal of free water is endothermic. In this process, the phase did not change and the weight loss rate of the sample changed little, so a weak endothermic peak appeared at 97°C. XRD analysis shows that the main phase in the raw ore is lizardite containing free water. When the temperature gradually increases to 607°C, most of the free water and structural water will be removed, and the lizardite will dehydroxyl, and a large amount of heat will be absorbed in this process. Thus a strong endothermic peak appears. As the temperature continues to rise, the lizardite phase changes at 810°C, changing from an amorphous material to a peridotite phase with good crystallinity, thus releasing a large amount of heat.

Fig. 3

TG-DSC curve of laterite nickel ore (nitrogen atmosphere).

Silica-magnesium laterite nickel ore can produce metal oxides of nickel and iron after initial reduction by reducing gas, and further reduction of nickel and iron metal oxides can produce low-price iron oxides or metals. Nickel oxide can be directly reduced to metallic nickel. For iron oxides, different iron oxides are produced at different reduction temperatures. In the whole reduction process, the reducing gas methane reacts directly with nickel oxide and iron oxide before cracking. According to thermodynamic calculation, methane is cracked at 550°C, and the hydrogen and carbon generated from the cracking react with the oxides of nickel and iron, and the CO generated during the reaction will also participate in the reduction process. The standard Gibbs free energy calculation results of possible reactions in the reduction process are shown in Fig. 4(a)–(d). Iron oxide is carried out step by step in the order of Fe₂O₃ → Fe₃O₄ → FeO → Fe at 600–900°C. The calculation results show that within the experimental temperature range, iron oxide is easier to be reduced by carbon, while nickel oxide is easier to be reduced by hydrogen. In addition, it is more difficult to reduce FeO to Fe than Fe₃O₄ to FeO in a shorter reduction time. Figure 4(a)–(b) shows that the reaction temperature of H₂ reducing FeO to generate metal iron needs to reach 1177°C, while CO can react with FeO below 667°C. Therefore, the generation of metal iron in the whole process mainly comes from the reduction of CO. To sum up, to realize the selective reduction of laterite nickel ore, it is necessary to choose a suitable reduction temperature.

Fig. 4

Gibbs free energy diagram of reaction of laterite nickel ore with different reducing components (a: CH₄; b: C; c: H₂; d: CO).

According to the chemical composition analysis of laterite nickel ore, the main components of laterite nickel ore are SiO₂, MgO and Fe₂O₃, accounting for about 80% of the total. However, Fe₂O₃ is mainly reduced to FeO in the reduction process, so the phase diagram of SiO₂–MgO–FeO ternary system is analyzed in this paper. As shown in Fig. 5, the phase diagram of SiO₂–MgO–FeO ternary system is divided into four regions: Region I is MgSiO₃ + SiO₂ phase region, region II is MgSiO₃ + Mg₂SiO₄ phase region, region III is Fe₂SiO₄ + SiO₂ phase region, and region IV is MgSiO₃ (Fe₂SiO₄) + SiO₂ + MgO phase region. According to the chemical analysis of laterite nickel ore, it can be seen that this ore belongs to area II, which has been marked (Area A) in the figure. It can be seen from the figure that the main phases after reduction roasting are pyroxene (MgSiO₃) and forsterite (MgSiO₄), which are consistent with the XRD results of the products after high temperature roasting. This makes the extraction of valuable metals (nickel, iron, etc.) in olivine more difficult. With the increase of iron content in laterite nickel ore, the phase moves from region II to Region IV, which is conducive to the reduction of metal nickel. Moreover, with the movement of the phase region, the generated MgO will make Ni and Fe have low-temperature catalytic activity and high-temperature stability, which also greatly increases the rate of methane cracking and improves the reaction efficiency.

Fig. 5

SiO₂–MgO–FeO ternary phase diagram.

3.2 Effect of reduction temperature on metallization rate of nickel and iron

Under the conditions of 60 min reduction time, 4% sulfur addition and 700°C∼1100°C reduction temperature, the effect of reduction temperature on the metallization rate of nickel and iron was investigated. The results are shown in the Fig. 6, from which we can see: As the reduction temperature increases, the metallization rate of nickel shows a trend of first increasing and then decreasing, while the metallization rate of iron shows a gradual increasing trend. When the reduction temperature increases from 700°C to 800°C, the metallization rate of nickel increases from 91.86% to 98.01%, and the metallization rate of iron increases from 5.24% to 8.44%. When the reduction temperature increases from 800°C to 1100°C, the metallization rate of iron increases from 8.44% to 10.38%, while the metallization rate of nickel decreases from 98.01% to 90.31%. This indicates that high temperature hinders the reduction of nickel oxide. This may be due to the formation of dense peridotite phase at 800°C, which subsequently inhibited the reduction of nickel.¹⁶⁾ In order to verify this point, the methane reduction products were analyzed by XRD at different temperatures.

Fig. 6

Effect of reduction temperature on metallization rate of nickel and iron in laterite nickel ore.

The results of the above XRD analysis are shown in the Fig. 7. It can be seen from the figure that the main phases of reducing minerals are (Mg, Fe)₂SiO₄, SiO₂ and Mg₂SiO₄. With the reduction temperature increasing from 700°C to 800°C, the diffraction peak of olivine is obviously enhanced. Its formation is not conducive to the reduction of metal oxides. This also verifies the above conjecture. However, no FeS phase was found in XRD, which may be because the content of FeS is too small to be measured.

Fig. 7

XRD analysis of laterite nickel ore at different temperatures.

3.3 Effect of CH₄ concentration on metallization rate of nickel and iron

Under the conditions of 60 min reduction time, 800°C reduction temperature, 4% sulfur addition and 15%∼35% methane concentration, the effect of methane concentration on the metallization rate of nickel and iron was investigated. The results are shown in the Fig. 8, from which we can see: When the concentration of methane is 15%, the metallization rate of nickel is only 78.12%, and that of iron is 2.3%. When the concentration of methane increased from 15% to 20%, the metallization of nickel and iron increased sharply, reaching 98.01% and 8.44% respectively. With the increase of methane concentration to 25%, the metallization rate of iron did not change significantly, while the metallization rate of nickel decreased to 88.76%. This may be because the pyrolysis of methane produces C and H₂, which leads to carbon deposition, thus affecting the reduction of nickel. When the concentration of methane exceeds 25%, the metallization rate of nickel basically changes little. And after analysis, the reduction gas used in the reaction process is obviously excessive. Therefore, the metallization rate of nickel and iron is not significantly affected by the increase of CH₄ concentration. Under comprehensive consideration, the optimal concentration of reducing gas methane is determined to be 20%.

Fig. 8

Effect of methane concentration on the metallization rate of nickel and iron in laterite nickel ore.

3.4 Effect of reduction time on metallization rate of nickel and iron

The effect of reduction time on the metallization rate of nickel and iron was investigated under the conditions of reduction temperature 800°C, methane concentration 20%, sulfur addition 4% and reduction time 30∼90 min. The results are shown in the Fig. 9. The experimental results show that: With the extension of reduction time, the metallization of nickel increased first and then decreased. When the reduction time is 60 min, the metallization rate of nickel can reach 98.01%, and the metallization rate of nickel in minerals has reached the maximum value at this temperature. When the reduction time continues to be extended, the carbon deposition level caused by methane cracking increases. Subsequently, the further reduction of nickel oxide was inhibited, and the metallization rate of nickel decreased. With the extension of reduction time, the metallization rate of iron tends to decrease, but the change is not obvious. Considering the serious waste of energy caused by too long reduction time, 60 min reduction time is selected as the optimal parameter.

Fig. 9

Effect of reduction time on metallization rate of nickel and iron in laterite nickel ore.

3.5 Effect of elemental sulfur addition on metallization rate of nickel and iron

Under the conditions of 60 min reduction time, 20% methane concentration and 800°C, the effect of sulfur addition on the metallization rate of nickel and iron was investigated. The results are shown in the Fig. 10, from which we can see: With the increase of sulfur content, the metallization rate of nickel gradually increases, and the metallization rate of iron first increases and then decreases. When the sulfur content is 4%, the metallization rate of nickel reaches the highest value of 98.01%, and the metallization rate of iron is 8.44%. When the amount of sulfur is increased from 0% to 2%, the metallization rate of iron increases. This is because the mineral with elemental sulfur has a loose structure, which is conducive to the reduction of iron. When the amount of sulfur additive continues to increase, the metallization rate of iron shows a downward trend. This is because when metallic iron increases, some metallic iron and sulfur form FeS, which leads to the reduction of iron metallization rate. Considering comprehensively, 4% sulfur additive is selected as the best parameter.

Fig. 10

Effect of the amount of additives on the metallization rate of nickel and iron in laterite nickel ore.

Adding appropriate sulfide can promote the growth and aggregation of metal particles. In general, the ore added with elemental sulfur has a loose structure.²⁷⁾ In the reduction process, elemental sulfur reacts with metallic iron to form FeS, and the formation of FeS will inhibit the reduction of metallic iron, which is conducive to improving the metallization rate of nickel.^28,29) And it is conducive to the separation of nickel iron particles and gangue minerals in the subsequent separation process. Some iron and sulfur form FeS into the slag phase, which promotes the reduction of iron grade in ferronickel and the improvement of nickel grade.

3.6 SEM-EDS analysis of reduced minerals

Figure 11(a) shows the electron microscope (SEM) of the reduced minerals under the conditions of reduction temperature 800°C, methane concentration 20%, reduction time 60 min, and no elemental sulfur additive. Many metal particles are observed in the reducing minerals without elemental sulfur. Most of the metal particles are dispersed, and a small amount of metal particles gather, but the aggregation effect is not obvious. After particle size statistics, the maximum particle size is about 16 um. Figure 11(b) shows the electron microscope (SEM) diagram of the reduced minerals under the conditions of reduction temperature 800°C, methane concentration 20%, sulfur addition 4%, and reduction time 60 min. compared with figure a, when elemental sulfur is added during the roasting process, the structure of the reduced minerals is loose. Although the number of metal particles decreases, the bright white area is significantly increased. The iron oxide area is also significantly increased, Energy spectrum (EDS) analysis was carried out on the edge of iron oxide region, and S element was determined. According to thermodynamic analysis, elemental Fe and elemental sulfur are very easy to react to form FeS. Therefore, FeS is formed around the iron oxide area. According to EDS analysis, the main elements around the iron oxide are Si, O, Mg, Fe, S, which cover the surface of the iron oxide, hindering the contact between the reducing gas (H₂ and CO) and FeO. According to thermodynamic analysis, it is known that the reduction of FeO requires high CO concentration, and the FeS film covered around the iron oxide will undoubtedly make the reaction more difficult, thus inhibiting the reduction of metal particles. This is one of the reasons why the addition of elemental s in the roasting process leads to a significant increase in the iron oxide area of the reduction product and a decrease in the number of metal particles.

Fig. 11

SEM-EDS analysis of roasted laterite nickel ore.

3.7 Reaction mechanism of CH₄ and laterite nickel ore

In order to verify the cracking reaction of CH₄, the tail gas produced in the reaction process was detected by a gas analyzer. The experiment was divided into two parts. First, a group of blank comparative tests were carried out under the conditions of 20% CH₄ concentration, reduction temperature of 900°C, reduction time of 60 min. the laterite nickel in the experiment was replaced by quartz sand with the same particle size. Before the experiment, zero and calibrate the gas analyzer, and then flush the pipeline with nitrogen. After the experiment, record the data at an interval of 1 s, and the test results are shown in Fig. 12(a). In addition, the laterite nickel ore was roasted under the same experimental conditions, and the experimental results are shown in Fig. 12(b).

Fig. 12

Relative content of gases in tail gas of blank test and roasting test at 800°C.

The experimental results show that CH₄ is the first to undergo the cracking reaction at 800°C, producing hydrogen and carbon black, which are both reductive and participate in the reduction reaction with laterite nickel ore. Among them, due to the high temperature required, carbothermal reduction rarely occurs within the temperature range studied in the experiment, and the content of CO and CO₂ in the tail gas accounts for a small proportion.

It can be seen from the relative ratio between the blank experiment and the roasting experiment that methane and hydrogen have carried out the cooperative reduction of laterite nickel ore. And from the change of the tail gas of the roasting experiment, it can be seen that when the methane concentration is stable, the methane concentration in Fig. 12(b) is significantly lower than that in Fig. 12(a). And when the reduction time reaches 30 min, the hydrogen content in the tail gas increases. These phenomena indicate that the reduced metal nickel and metal iron have a certain catalytic effect on methane.

The reaction process of CH₄ and laterite nickel ore is shown in Fig. 13. After the elemental sulfur is mixed with laterite nickel ore, lizardite decomposition occurs first in the process of heating up. The decomposition of lizardite will release nickel oxide. When the temperature rises to 800°C, methane is introduced. Methane splits at 800°C to produce H₂ and C. NiO and Fe₂O₃ are reduced under the cooperative action of CH₄ and H₂. NiO is almost completely reduced to nickel metal. Fe₂O₃ is partly reduced to metallic iron and the rest is mostly in the form of FeO. Part of the metal iron will form FeS with the additive elemental sulfur, and then inhibit the reduction of iron oxide.

Fig. 13

Mechanism diagram of the reaction between CH₄ and laterite nickel ore.

3.8 SEM-EDS analysis of fused ore

Figure 14(a) shows the electron microscope (SEM) diagram of fusing minerals under the condition of reduction temperature 800°C, methane concentration 20%, reduction time 60 min, without adding elemental sulfur additive. Figure 14(b) shows the electron microscope (SEM) diagram of fusing minerals under the condition of reduction temperature 800°C, methane concentration 20%, reduction time 60 min, adding 4% elemental sulfur additive. It can be seen from the figure that after adding elemental sulfur additive, The enrichment effect of nickel and iron in molten minerals is stronger than that without elemental sulfur additive.

Fig. 14

SEM/EDS analysis of laterite nickel ore under the best experimental conditions.

4. Conclusion

In this paper, the direct reduction experiment of methane at low temperature with silica-magnesium laterite nickel ore as raw material was carried out. The effects of reduction temperature, reduction time, methane concentration and elemental sulfur additives on the metallization rates of nickel and iron were investigated. The following conclusions are drawn:

1. (1)    The metallization rate of nickel first increases and then decreases with the increase of reduction temperature. Due to the recrystallization of magnesium silicate at 800°C, a dense olivine phase is formed, which inhibits the reduction of nickel. However, with the extension of reduction time, the metallization of nickel firstly increases and then decreases, and the carbon accumulation generated by methane cracking increases, which inhibits the further reduction of nickel, so the metallization rate of nickel decreases. With the increase of methane concentration from 10% to 20%, the metallization rate of nickel increases gradually. The optimum experimental conditions were determined as follows: reduction temperature 800°C, methane concentration 20%, reduction time 60 min, elemental sulfur additive content 4%. At this time, the metallization rate of nickel and iron can reach 98.01% and 8.44% respectively.
2. (2)    Different from the roasted ore without elemental sulfur additives, FeS appear in the roasting process after elemental sulfur is added. Nickel iron particles would have obvious aggregation effect, and FeS will be wrapped around the iron oxide. It would inhibit the contact between reducing gas and iron oxide. Therefore, the reduction of iron oxide is inhibited. It is beneficial to the reduction of iron grade and the improvement of nickel grade in Ni–Fe alloy after subsequent separation.

Acknowledgements

Financial support for this study was supplied from the Yunnan Provincial Key Research and Development Program - The National Natural Science Foundation of China (Project Nos. 52074140) and International Science and Technology Cooperation Special Project (Project Nos. 2018IA055).

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