Reductive behavior of nickel and iron metallization in magnesian siliceous nickel laterite ores under the action of sulfur-bearing natural gas

: This study is a process for the preparation of advanced nickel – iron alloys by selective reduction of nickel-poor laterite ores using a variety of reducing agents. The ﬁ rst part of the experiment was the reduction of nickel laterite ore using natural gas; the reduction yielded mostly nickel metal and a small amount of iron forming ﬁ ne nickel – iron particles. Fine nickel – iron particles are formed through the reduction of nickel and a small amount of iron oxides. These particles are dispersed and embedded within silicates. Additionally, H 2 S present in natural gas reacts with iron oxide, resulting in the formation of FeS. This, in turn, forms a low melting point eutectic with Fe, which reduces surface tension and promotes the growth and aggregation of nicke-l – iron particles. This study aimed to investigate the e ﬀ ects of various parameters such as roasting temperature, roasting time, natural gas concentration, and nickel laterite pellet on the formation and aggregation of ferronickel particles in low-grade nickel laterite ores. The results showed that the optimum reduction parameters were achieved at 900°C, 120 min, 40% natural gas concentration, and 40 – 60 mesh nickel laterite size, with roasting temperature being the most important factor followed by natural gas concentration, roasting time, and nickel laterite pellet. Using these parameters, the metallization rates of Ni and Fe were found to be as high as 95.3 and 8.5%, respectively.


Introduction
Nickel, a crucial non-ferrous metal with strategic importance, finds extensive applications in a range of sectors, including stainless steel manufacturing, battery materials production, electroplating, catalyst synthesis, and magnetic-sensitive materials development [1][2][3].It has earned the moniker of "industrial vitamin" in the field of metals and materials chemistry.The majority of nickel consumption, approximately 85%, is attributed to stainless steel production, while electroplating and batteries account for 6 and 5%, respectively.Due to the rapid growth of the stainless steel industry, the demand for nickel has significantly increased [4][5][6].Currently, globally proven nickel ores are primarily divided into two typesnickel sulfide ore and nickel laterite ore [7,8].Unfortunately, the over-exploitation of nickel sulfide resources in recent years has led to the gradual depletion of these resources.As a result, the demand for nickel has increased dramatically.As one of the most important sources of nickel, laterite nickel ore accounts for more than 70% of the world's nickel reserves, but nickel production only accounts for about 40% [9,10].The research on the electrometallurgical process of low-grade laterite nickel ore has, therefore, become a significant international metallurgical challenge at present.
Low-grade laterite nickel ore is characterized by high silica-magnesium content, low nickel grade, and complex ore phase, which makes the wet smelting process too costly to deal with.As a result, the pyrometallurgical process has emerged as a more economically efficient method for its production and utilization [11].In terms of energy conservation and carbon reduction, the traditional use of carbonbased reductants results in significant CO 2 emissions.
Meanwhile, the incorporation of hydrogen (H 2 ) and methane (CH 4 ) as selective reductants for low-grade laterite nickel ore can reduce CO 2 emissions in comparison to carbon-based reductants for the production of an equivalent amount of nickel metal.Furthermore, the use of CH 4 as a reductant can generate significant amounts of valuable gaseous by-products during the reduction process of H 2 and CO gas mixtures.H 2 is considered the most ideal reductant for these purposes [12].However, the reduction process can become expensive due to the difficulty of transporting and storing hydrogen, which is why methane becomes a potential alternative reductant.A significant amount of research has focused on gas-based reduction of laterite nickel ore [13,14].Studies on the reduction of low-grade nickel laterite ore by hydrogen under different conditions have shown that the nickel oxide in the ore can be almost completely reduced by low-temperature hydrogen reduction [15].Additionally, part of the iron in the ore can be reduced to metallic iron with nickel, forming a nickel-iron alloy, while the majority of the iron is reduced to FeO.
Natural gas is a clean and environmentally friendly energy source that produces minimal amounts of harmful substances and dust when burned [16], making it a highquality option for energy consumption.It also produces significantly fewer carbon dioxide and nitrogen oxide emissions compared to other fossil fuels.Additionally, developing natural gas reduction technology for nickel laterite processing offers several advantages, including improved reduction efficiency [17,18].Another benefit of natural gas as a reductant is that it typically contains trace amounts of sulfur (0.5-7%) [19], which can be utilized as an additive to enhance the reduction process.This is an important consideration that has been extensively researched.Sulfur additives are one of the most significant additives [20][21][22].Sulfur is known to enhance the reduction process in nickel laterite processing by promoting the aggregation and growth of ferronickel particles.Additionally, the formation of Fe-FeS eutectic during the reduction process contributes to the aggregation of ferronickel particles [23][24][25][26].Studies have shown that the utilization of additives, such as Na 2 S, FeO, and FeS, can effectively enhance the growth of ferronickel grains during nickel laterite processing.The addition of Na 2 S significantly increased the size of NiFe grains, relative to the other types of additives tested.This is attributed to the formation of the regional liquid phase, which facilitates the aggregation and growth of ferronickel grains.Moreover, the content of the liquid phase in the slag is modified by Na 2 S and FeO additives, while FeS affects the composition of the metal [27].Thermodynamic analysis has revealed that at high temperatures, FeS is a stable phase for sulfur.To investigate the microstructure evolution and phase transformation of sulfur on metallic iron growth, reduction experiments were conducted.The reduction process was observed to occur in three stages.During the first stage, liquid FeS was observed, and Fe particles were generated around the pores.In the second stage, Fe was wetted and covered by the liquid phase (Fe-FeS).Finally, during the third stage, Fe particles migrated towards uniformity in the liquid phase, and gradually transformed and aggregated into spherical shapes [28].Thus, it has been found that natural gas can effectively promote the grain growth of nickel-iron alloy without the need for additional accelerators during the reduction process of nickel laterite.
In view of the low grade and complex structure of nickel laterite-magnesium-silicon ore, the method of "preheating and drying of nickel laterite ore-multiple reductant selective reduction-high temperature preparation of nickel-iron alloy" was proposed.The new method of "high-temperature preparation of nickel-iron alloys" investigates the extraction of nickel and iron from low-grade laterite magnesium-silicon nickel ores.This new method takes advantage of the favorable reduction properties of natural gas and sulfur, and most of the nickel-iron in the reduction product is nickel metal and iron oxides, and then the iron oxide reduction is controlled to control the nickel-iron grade in the alloy.The objective of this reduction study is to further investigate the mechanism by which natural gas affects the aggregation and growth of nickel and iron particles during the reduction of laterite silica-magnesite.

Materials
The experimental raw material used was laterite nickel ore sourced from Yuanjiang, Yunnan Province, China.This particular deposit is a faceted nickel silicate weathering crust deposit, with a Ni grade of 0.82%, Fe at 9.67%, SiO 2 at 37.4%, and MgO at 31.5%.This is a typical silica-magnesium type lateritic nickel ore.The raw ore was ground into a fine powder using a vibrating mill and then sieved through a 100 mesh screen in order to obtain particles with a diameter of less than 0.15 mm.After drying, the sample was thoroughly mixed and subjected to chemical analysis, resulting in the main chemical composition as presented in Table 1.The table indicates that the sample composition is typical of low-grade Silica-Magnesium laterite nickel ore.The high percentages of MgO and SiO 2 , which constitute 68.9% of the total mass, suggest the presence of numerous impurities.This ore is not conducive to wet metallurgical smelting.The high levels of oxide impurities significantly increase acid consumption, making the cost exceed the benefit.Therefore, pyrometallurgy is a more appropriate way to smelt this type of laterite nickel ore.X-Ray diffraction (XRD) method (Cu-Kα ray source, voltage of 35 kV, current of 20 mA, scanning speed of 10°/min, the diffraction angle (2θ) was scanned from 10°to 90°) analysis was conducted to determine the main phase composition of the laterite nickel ore, and the results are presented in Figure 1.The analysis revealed that the predominant mineral composition comprises serpentine [Mg 3 Si 2 O 5 (OH) 4 ] and SiO 2 [29], which is consistent with the findings in Table 1.The metallic elements, namely Fe and Ni, it replaces magnesium in serpentine mainly in the form of homogeneity and is diffusely distributed in the mineral structure.Hence, the diffraction peaks of Fe and Ni could not be detected in the XRD pattern due to the poor crystallinity of the mineral.
The reducing agent used in this experiment is natural gas, whose chemical composition is shown in Table 2, and the main component is CH 4 .The source of natural gas is Sichuan industrial natural gas, and the experimental gas is formulated with main components 97% CH 4 , 1.5% H 2 S, and equilibrium gas 1.5% N 2 according to the actual natural gas composition.In the initial experiment, the flow rate of natural gas was 0.02 L•min −1 , and nitrogen was 0.04 L•min −1 .

Research method
Based on the mineralogical properties of laterite nickel ore, a new method called "preheated drying of silica-magnesium laterite nickel ore -natural gas reduction" has been proposed.The method involves several steps.(1) The laterite nickel ore is crushed and screened by passing it through a vibrating mill for 3 min and then through a 100 mesh sieve.
(2) The mixed nickel laterite powder is placed into a cylindrical mold with an inner diameter of 2 cm and pressed for 3 min at 18 MPa to obtain flake nickel laterite pressed products.(3) The flake nickel laterite is granulated and subjected to secondary screening by crushing it in a mortar and passing it through 20 mesh, 40 mesh, 60 mesh, 80 mesh, and 100 mesh sieves to obtain nickel laterite granules of different sizes.(4) Reduction roasting: The nickel laterite particles were placed into a column crucible and positioned in the heating zone of a vertical tube furnace (CHY-1700).Nitrogen gas was introduced to exhaust the air in the furnace, and natural gas was used to adjust the methane gas volume concentration through the nitrogen flow rate.(5) Analysis of nickel and iron metallization rates: The metallization rate of nickel-iron was determined by dioxime gravimetry (GB/T 223.and titration of titanium trichloride and potassium dichromate (GB/T 8638. .The trend change of nickel and iron metallization and the phase of products were analyzed using XRD and scanning electron microscopy (SEM).A schematic diagram of the specific experimental setup is shown in Figure 2. The experimental exhaust also contains toxic hydrogen sulfide gas, which is handled by introducing the exhaust into a certain concentration of copper sulfate solution.Hydrogen sulfide is used to reduce the emission of hydrogen sulfide gas by reacting with copper sulfate to produce a copper sulfide precipitate.

Calculation of metallization ratio
(1) The content of metallic nickel, metallic iron, all nickel, and all iron in the reduction product is calculated.Reduction of nickel and iron metallization in nickel laterite ore  3 (2) Nickel and iron metallization rate is calculated by the following formula: In the formula: γ Ni is the metallization rate of nickel.T Ni is the total nickel content, and M Ni is the metallic nickel content; γ Fe is the metallization rate of Fe.T Fe is the total iron content, and M Fe is the metallic iron content.
The metallization rate of nickel: The nickel content in Ni-Fe alloy was determined by the dioxime gravimetric method.The sample was heated and dissolved with saturated KClO 3 -HNO 3 solution and then added with sodium tartrate solution.Under constant agitation, the solution of dioxime was added to make Ni 2+ precipitate with dioxime, separate it from other elements, filter and wash it into a clean beater, add ammonium purpurate indicator, and titrate with EDTA standard titration solution until the solution changed from yellow to purple as the end point.The nickel content is then calculated.
The metallization rate of iron The content of iron in Ni-Fe alloy was determined by titration of titanium trichloride and potassium dichromate.In HCl medium, Fe 3 + was reduced to blue color by TiCl 3 with Na 2 WO 4 as an indicator and then titrated with K 2 Cr 2 O 7 , a standard solution with sodium diphenylamine sulfonate as an indicator, to calculate the iron content.

Results and discussion
In the selective reduction process of low-grade Yuanjiang laterite nickel ore, thermodynamic calculations of the resulting reactions are briefly discussed, the ranges of some parameters of the subsequent reactions are determined, and experiments on the effects of reduction temperature, reduction time, natural gas concentration and laterite nickel ore particle pellet on the metallic nickel and iron recovery and ore microstructure are carried out.The optimum reduction conditions for the selective reduction process of low-grade Yuanjiang laterite nickel ore were determined based on the best results of the nickel metallization rate in the concentrate.

Thermodynamic analysis
The mineral phase composition of clay nickel ore is complex and can usually be seen as consisting of oxides such as nickel oxide, iron oxide, and magnesium oxide.Methane and hydrogen sulfide gases in natural gas can react with mineral raw materials, as shown in Table 3.
The relationship between the standard Gibbs free energy and temperature for reactions 1-4 is shown in Figure 3.As can be seen from Figure 3, reaction 1 is the pyrolysis reaction of methane in natural gas, and the theoretical pyrolysis temperature is 549.16°C.It can be seen from reaction 2 and reaction 3 methane reduction of nickel- iron oxide that the Gibbs free energy of reaction 2 is smaller than that of reaction 3 at the same temperature.It shows that nickel oxide is more easily reduced than iron oxide in the same temperature range.Reaction 4 is the reaction of hydrogen sulfide and iron oxide in natural gas, and it can be seen from its Gibbs free energy that ΔG ＜0, and the reaction can be spontaneous.The ferrous sulfide produced by the reaction can form low-melting-point co-crystals with iron, thus promoting the polymerization and growth of nickel-iron.
Figure 4 shows the Gibbs free energy and temperature curves of the intermediates C, CO, and H 2 reacting with nickel-iron oxides.The oxidation-nickel reduction reactions of reactions 5, 9, and 13 are spontaneous over a range of reaction temperatures.For iron oxide reduction, the reaction conforms to the step-by-step reduction law of iron oxide: when the temperature is less than 570°C, the reduction is in the order of Fe 2 O 3 → Fe 3 O 4 → Fe.When the temperature is greater than 570°C, it is reduced in the order of Fe 2 O 3 → Fe 3 O 4 → FeO → Fe.However, in the reaction of 12 and 16, when CO reduces ferrous oxide at >840 K, ΔG >0, which is unfavorable to the reaction.The reduction of ferrous oxide by H 2 is also unfavorable at temperatures <1,450 K, where ΔG >0.Under thermodynamic conditions, the reduction of nickel-iron oxides by CO and H 2 may produce a large amount of ferrous oxide.

Effect of temperature on the metallization rate of nickel and iron in laterite nickel ore
The rate of reduction roasting of laterite nickel ore is primarily affected by the roasting temperature.High temperatures will cause the methane in natural gas to crack, producing carbon, which can hinder nickel reduction.For the reduction experiment, an initial natural gas concentration of 20%, a roasting time of 90 min, a laterite ore dosage t of 15 g, and a particle size range of 40-60 mesh were used.These experimental conditions yielded the results shown in Figure 5.
Figure 5 shows that the roasting temperature has a significant effect on the metallization rate of nickel.With the increase of roasting temperature, nickel metallization was the first to increase and then decrease, when the temperature increased from 700 to 900°C, the nickel metallization rate reached the highest 71.4%, and after 900°C, the nickel metallization rate showed a decreasing trend.This is because serpentine is decomposed into magnesium olivine [(Ni, Mg) 3 Si 2 O 5 (OH) 4 → (Mg, Ni) SiO 3 + (Mg, Ni) 2 SiO 4 + H 2 O], and its extensive formation at 1,000°C has a significant effect on the reduction of nickel-iron.The XRD analysis of the laterite nickel ore at various temperatures ranging from 700 to 1,100°C is illustrated in Figure 6.As  depicted in the figure, the peak of magnesia olivine in XRD at 1000°C exhibited a significantly higher intensity compared to that at 800°C and 900°C, indicating that the generation of magnesia olivine was greater at 1,000°C.Moreover, the Fe metallization rate exhibited an upward trend from 700°C to 1,100°C, reaching 5.3% at 1,100°C.These results suggest that selecting the appropriate roasting temperature is crucial in increasing the metallization rate of Ni.Consequently, the optimal roasting temperature was found to be 900°C.

Effect of time on the metallization rate of nickel and iron in laterite nickel ore
A series of experiments were conducted to investigate the effect of roasting time on the metallization rate of nickel    and iron.The experiments were conducted using a roasting temperature of 900°C, a natural gas concentration of 20%, a laterite nickel ore dosage of 15 g, and a laterite nickel ore particle size of 40-60 mesh.The results were plotted as shown in Figure 7.The results of the experiment showed that the metallization rate of nickel and iron increased initially and then leveled off with an increase in holding time.At a holding time of 30 min, the metallization rate of nickel was 41.6% and that of iron was 0.9%.As the holding time was extended, the metallization rate of nickel reached a maximum of 89.3% and iron 6.3% at a holding time of 120 min.
This phenomenon was attributed to the greater reaction of natural gas with nickel and iron oxides in laterite nickel ore as the holding time was extended.Based on the experimental findings, the optimum holding time of 120 min was determined for the maximum metallization rate of nickel and iron.This not only reduces energy wastage but also helps to improve the efficiency of the roasting process.It should be noted that the appropriate roasting time is a crucial factor to consider in the roasting process of laterite nickel ore.Reduction of nickel and iron metallization in nickel laterite ore  7 3.4 Effect of natural gas concentration on the metallization rate of nickel and iron in laterite nickel ore In order to investigate the effect of natural gas concentration on the metallization rate of nickel, a series of experiments were conducted under the following conditions: a laterite nickel ore dosage of 15 g, a roasting temperature of 900°C, a holding time of 120 min, and a laterite nickel ore particle size of 40-60 mesh.The results of the experiments are presented in Figure 8.
As shown in Figure 8, the metallization rate of nickel and iron initially increased and then decreased as the natural gas concentration was increased.The metallization rate of nickel increased from 39.6 to 88.7%, while the metallization rate of iron increased from 0.9 to 7.7% when the natural gas concentration was increased from 10 to 40%.
The maximum value of metallization rates for nickel and iron was obtained in this concentration range.However, when the natural gas concentration was increased to 50%, the metallization rate of nickel and iron showed a decreasing trend.The reason for this is that when the methane concentration is low, there is less reducing gas in the reaction chamber, and the hydrogen produced by cracking has a low concentration, which is not conducive to the reaction.Conversely, when the methane concentration is too high, it can lead to excessive carbon accumulation that covers the cracks on the surface of laterite nickel ore.This can hinder the diffusion and chemisorption of methane in nickel laterite particles, preventing the internal diffusion of methane and hindering the participation of nickel and iron oxides in the reduction reaction.Experimental results indicate that an optimum methane concentration of 40% yields the best results.To investigate the effect of laterite nickel ore particle size, the following experimental conditions were set: a roasting temperature of 900°C, a natural gas concentration of 40%, a laterite nickel ore dosage of 15 g, and a holding time of 120 min.The effect of laterite nickel ore particle size on nickel metallization rate was analyzed and the results are presented in Figure 9.According to the figure, the metallization rate of nickel demonstrates a pattern of initially increasing and then decreasing with the decrease in particle size of the nickel laterite ore.The metallization rate of nickel is only 88% when the particle size of laterite nickel ore is 20-40 mesh.However, as the particle size is reduced to 40-60 mesh, the metallization rate of nickel increases to its maximum value of 95.3%.When the particle size of laterite nickel ore was reduced to 80-100 mesh, the metallization rate of nickel in the ore decreased again to 90.7%.This is because when the particle size of laterite nickel ore is 20-40 mesh, the particles are larger, which hinders the internal diffusion of natural gas during the reduction reaction.As a result, there is insufficient reduction of nickel oxides inside the particles and the metallization rate of nickel is lower.On the other hand, when the particle size of laterite nickel ore is too small, although the specific surface area is larger, the surface energy of particles correspondingly increases.This makes the material layer less permeable, and natural gas in the reduction process is less likely to follow the standard direction.Part of the laterite nickel ore particles cannot come into contact with methane gas molecules, preventing chemical reactions from occurring, thereby reducing the nickel metallization rate.The iron metallization rate increased from 8.1 to 8.5% and then decreased to 7.1% with increased mesh size.The particle size of laterite nickel ore had minimal  impact on the experimental results.The optimum particle size was 40-60 mesh.
3.6 Study on the reaction mechanism of natural gas and laterite nickel ore The reaction processes involved in the reduction roasting of nickel laterite ore using natural gas are depicted in Figure 10.First, the serpentine present in the ore undergoes decomposition during the warming process, resulting in the release of nickel and iron oxides.As the temperature reaches 900°C, natural gas is introduced into the system.The high temperature causes the natural gas to undergo cracking, which in turn produces hydrogen and carbon.In the reducing atmosphere, the nickel and iron oxides undergo reduction.The NiO is almost completely reduced to metallic nickel, while Fe 2 O 3 is partially reduced to metallic iron.Most of the remaining Fe 2 O 3 exists in the form of FeO.However, a small portion of Fe 2 O 3 reacts with H 2 S to form FeS, which can inhibit the reduction of iron oxides.

Analysis of nickel laterite reduction results
SEM-EDS analysis was performed on ore samples reduced at 900°C with natural gas to study the aggregation patterns of nickel and iron in laterite nickel ores.Figure 11 shows the SEM of the ore samples reduced at a natural gas concentration of 40%, reduction time of 120 min, and original ore size of 40-60 mesh.The thermodynamic analysis showed that FeO is not easily reduced to metallic iron by hydrogen, and Fe 2 O 3 reacts with H 2 S to produce FeS.The formation of co-crystals of FeS and Fe can reduce surface tension and promote ferronickel particle growth and aggregation, but it also makes the reaction more difficult, hindering FeO reduction.Additionally, S aggregation was observed in regions where Fe was significantly aggregated, as shown in Figure 11.
Figure 12 shows an SEM-EDS plot of nickel laterite under optimal reduction conditions, which can be analyzed in conjunction with Figure 11.The region represented by point 1 has a high content of 93.32% Ni and Fe elements.
This indicates that the area corresponding to the red arrow in Figure 11 is a zone of Ni-Fe particle aggregation.The S content of the region represented by point 2 is 0.25%, while the S contents of point 1 and point 3 are 0.06 and 0.04%, respectively, and the high Fe content of point 2 suggests that a relatively large amount of FeS was formed around the Ni-Fe aggregation area.The major elements in the region represented by point 3 are Mg, Si, and O, which can be analyzed in combination with Figure 6 to be magnesium olivine.
Figure 13 shows the XRD analysis of the reduction roasting of nickel laterite ore under the optimal reduction conditions, from which we can find the presence of ferrous sulfide.Combined with Figure 12, it can be proved that ferrous sulfide is present around the nickel-iron particles.

Conclusion
The experiment focused on the reduction of roasted nickel laterite ore using natural gas.After a series of experiments, the following conclusions were obtained: (1) By conducting systematic experiments, the optimal process parameters have been determined.The calcination temperature is 900°C, and the calcination time is 120 min.The concentration of natural gas is 40%, and the particle size of the laterite nickel ore is 40-60 mesh.
Under the optimal experimental conditions, it was found that the metal conversion rates of nickel and iron are 95.3 and 8.5%, respectively.(2) The reduction process of iron oxide and hydrogen sulfide produces ferrous sulfide, which subsequently reacts with iron to produce FeS-Fe eutectic crystals.Its formation of a thin film around the nickel-iron particle region inhibits the deep reduction of Fe and selectively reduces Ni. (3) This new process enables the reduction of non-molten state metallization in Yuanjiang silica-magnesium-type nickel laterite ore and the effective separation and enrichment of nickel.

Figure 2 :
Figure 2: Schematic diagram of the experimental setup for natural gas reduction of nickel laterite.

Figure 3 :
Figure 3: Gibbs free energy for the direct reaction of natural gas with materials.

Figure 4 :
Figure 4: Gibbs free energy for indirect reactions of (a) C, (b) CO, and (c) H 2 with materials.

Figure 5 :
Figure 5: Effect of roasting temperature on the metallization rate of nickel and iron in laterite nickel ore.

Figure 6 :
Figure 6: XRD Analysis of laterite nickel ore at different temperatures.

Figure 7 :
Figure 7: Effect of roasting time on the metallization rate of nickel and iron in laterite nickel ore.

Figure 8 :
Figure 8: Effect of natural gas concentration on nickel and iron metallization rate of laterite nickel ore.

Figure 9 :
Figure 9: Effect of particle size on nickel and iron metallization rate of laterite nickel ore.

Figure 10 :
Figure 10: Reaction mechanism of natural gas and laterite nickel ore.

Figure 11 :
Figure 11: SEM-EDS Map of nickel laterite ore reduction roasting under optimal reduction conditions.

3. 5
Effect of particle size on the metallization rate of nickel and iron in nickel laterite ore

Figure 12 :
Figure 12: SEM-EDS Map of nickel laterite ore reduction roasting under optimal reduction conditions.

Figure 13 :
Figure 13: XRD Pattern of nickel laterite ore reduction roasting under optimal reduction conditions.

Table 1 :
Main chemical composition of nickel laterite raw ore

Table 2 :
Gas chemical composition