Basic Nickel Carbonate: Part I. Microstructure and Phase Changes during Oxidation and Reduction Processes
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- Rhamdhani, M., Jak, E. & Hayes, P. Metall and Materi Trans B (2008) 39: 218. doi:10.1007/s11663-007-9124-4
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A significant industrial problem associated with the production of nickel from basic nickel carbonate has been identified. Fundamental studies of the change of phase, product surface, and internal microstructures taking place during oxidation and reduction processes at temperatures between 110 °C and 900 °C have been carried out. The various elemental reactions and fundamental phenomena that contribute to the change of the physical and chemical characteristics of the samples during the processes taking place in Ni metal production through gas/solid-reduction processes have been identified and thoroughly investigated. The following phenomena affecting the final-product microstructure were identified as follows: (1) chemical changes, i.e., decomposition, reduction reactions, and oxidation reactions; (2) NiO and Ni recrystallization and grain growth; (3) NiO and Ni sintering and densification; and (4) agglomeration of the NiO and Ni particles.
This article describes a series of fundamental studies to identify the elemental reactions and phenomena taking place during oxidation and reduction processes used in production of solid Ni metal. A great deal of fundamental research has been undertaken on the factors influencing the gas/solid reduction processes; this includes thermal decomposition reactions and heterogeneous reactions involving gas/solid systems.[1, 2, 3, 4, 5, 6] These studies have formed the basis for the use of these reactions in metallurgical and materials production on an industrial scale. The evolution of materials characteristics during these processes is complex, since a number of elemental reactions and fundamental phenomena occur simultaneously; these may include, but are not limited to, solid-precursor decomposition, reduction reactions, oxidation reactions, gas- and solid-phase mass transfer, and sintering. Identifying and describing these fundamental phenomena is important for understanding the underlying science, as well as for improved control of technological applications. Differences in the relative contributions of these phenomena will lead to differences in the properties of the final product.
Despite the accumulated knowledge to date, the complexities of the processes mean that it is still necessary to characterize individual systems through systematic experimental studies. An important industrial example of the class of gas/solid-reaction processes is the production of nickel. Basic nickel carbonates (NiCO3·xH2O, NiCO3·xNi(OH)2·yH2O, NiCO3·xNiO·yH2O), or BNC, are used in the commercial production of nickel-metal powders and compacts through calcination and reduction with hydrogen.
In the case of nickel production from nickel oxide, residual oxygen and overall reduction rate are the key parameters determining the specification and value of the final Ni product and the production rates, respectively. The European Union’s new chemical policy regulations require that oxygen concentrations in the nickel product are less than 0.1 wt pct as nickel oxide. These restrictions are driven by the need to minimise the generation of residual NiO dust, which has been shown to be carcinogenic. From the point of view of workplace health and safety, and marketability of nickel products, it is important to characterize the form of any residual nickel oxides present, and to determine how they were formed, and therefore, how they might be further reduced. These processes, as they relate to industrial practice, have not been adequately described in studies to date.
The approach taken in the current study is as follows: (1) systematically investigate the modes of occurrences of residual oxygen (in the form of NiO) in the final Ni product; (2) carry out carefully planned and controlled laboratory experiments to investigate fundamental phenomena and reactions; and (3) relate results from these experiments to microstructures observed in industrial samples to identify fundamental processes occurring during production of Ni metal through gas/solid reactions.
type 1—trapped round particles surrounded by (1A) thick or (1B) thin dense Ni;
type 2—trapped blocky form surrounded by porous Ni;
type 3—surface layer of a NiO on a Ni particle (3A) with or (3B) without a fine layer of Ni on oxide surface;
type 4—bulky NiO (4A) with or (4B) without dense Ni inside; and
type 5—fine partially reduced NiO particles with individual particle size of one to five μm.
From Figure 1, it clearly follows that residual NiO is present in various complex structures, which resulted from different phenomena occurring during the Ni production process. Understanding of the phenomena resulting in the formation of these residual, nickel-oxide microstructures is important for the process control to obtain a high-quality nickel product.
In Part I of the series, a systematic investigation of changes in phases present, product surface, and internal microstructures occurring in starting material BNC during controlled oxidation and reduction conditions is provided. The principal aim of the study is to identify the various fundamental phenomena occurring and their effects on the processes and the final product. This is to provide a basis for future fundamental studies on each of the phenomena and also provide information in support of industrial operations. In Part II of the series, the analysis of the microstructure formation in intermediate and final products during industrial operations and the implications for plant practices are described.
Previous Studies on BNC Decomposition and NiO Reduction
The two decomposition steps are reported to apply to 2NiCO3·3Ni(OH)2·4H2O,[11,13] NiCO3·2Ni(OH)2·4H2O, and NiCO3·Ni(OH)2·2H2O. Only in the latter study have the microstructure changes occurring during the transformation from BNC to NiO particles been examined; however, this is a very cursory study of these phenomena.
In many cases, the starting point for these studies involved the use of finely divided NiO powder or compacts. Although reduction in hydrogen atmospheres was found by many investigators to occur readily, there are considerable differences in the reported kinetics of these processes due to the differing characteristics of the starting materials and the reaction geometries employed in these studies.[15, 16, 17, 18, 19, 20, 21] In relatively few studies, the reaction conditions have been well established, using either dense NiO sheets[16, 17, 18] or granules[20,21] to overcome gas-phase mass-transfer limitations. These studies show that the reaction rates increase with increasing temperatures up to 600 °C, and then a slowdown between 600 °C to 900 °C, before increasing again above 950 °C. There are no phase changes taking place in this range of temperatures, and no physical changes to the oxide or metallic phases were detected that might explain the slowdown of the reaction between 600 °C to 900 °C. A number of mechanisms have been proposed to explain the observed reduction behavior, but to date, the origins of these phenomena remain uncertain.
The majority of the studies on BNC are focused on the thermal decomposition in air and limited to low temperatures (<600 °C). On the other hand, although research has been carried out on NiO reduction at temperature ranges between 200 °C to 1000 °C, there are no published studies of microstructure evolution during reduction of BNC to NiO, and then to Ni.
Although the gaseous transformation of BNC to NiO, and then reduction to nickel by hydrogen appear to be simple reactions, the factors influencing the extent and rates of reduction, and its inter-relationship with the microstructure are not completely understood. The reduction of BNC and NiO appear to be strongly dependent on the characteristics of the starting materials, the process conditions, and thermal history. This is of particular concern for the accurate control of industrial processes used in the production of nickel oxide and nickel metal powders, in which variations in product quality and characteristics are undesirable.
The experimental plan in the current study was developed following careful evaluation of the modes of occurrences of residual nickel oxide in the materials (Figure 1), the thermal and atmosphere conditions in the actual process, and consideration of possible elementary reactions and phenomena involved in the gas/solid reaction processes.
Separate and combination heat treatments on BNC samples under specific experimental conditions were carried out to reproduce some of the residual nickel-oxide microstructures to allow the investigation of the underlying elementary reactions and fundamental phenomena occurring during the process responsible for the formation of these microstructures.
The heat treatments carried out in this study are as follows: (1) calcination/oxidation of BNC in air; (2) direct reduction of BNC in reducing (15 pct H2-N2 and 1.5 pct H2-N2) gas atmosphere; and (3) reduction in 15 pct H2-N2 gas atmosphere of preoxidized/calcined BNC.
The calcination/oxidation and reduction of the samples were carried out at temperatures between 110 °C and 900 °C. The samples were held at the peak temperatures for 30, 60, and 120 minutes to evaluate the effect of holding time at a temperature. The heating rate of 10 °C/min used in the study was selected to reflect the heating rate in the rotary kiln used in industrial production of nickel.
Approximately 5 g of sample were initially placed in the quartz tube. The samples were heated to the desired temperature in the desired atmosphere. After reaching the desired temperature, the samples were held at the temperature before they were quickly cooled by removing the quartz tube from the furnace and by passing a flow of inert gas (Ar or N2) through the system. The samples were then analyzed using materials-characterization techniques to study the phase changes and microstructure/surface-morphology evolution during the process. Selected repeated experiments were carried out to ensure the consistency and repeatability of the experiments.
Analysis Techniques and Sample Preparation
The microstructures of the samples were examined using scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) techniques, namely, FE-SEM JEOL1 6300/6400, variable pressure SEM JEOL 6460LA, and PHILIPS2 XL-30 with accelerating voltage of 15 and 20 kV. X-ray powder diffraction (XRD) analyses were carried out using PHILIPS PW 1130 X-ray diffractometer with a graphite monochromator using Cu Kαradiation.
Thermo-gravimetric analysis and DTA were carried out on the samples using the STA 409C/CD (NETZSCH GmbH, Bayern, Germany). Specific surface-area measurement of selected samples was carried out using the Tristar 3000 (Micromeritics Instrument Corp., Norcross, GA). Particle-size measurement of the samples was carried out using the Mastersizer 2000 (Malvern Instruments Ltd., Worchestershire, United Kingdom).
To enable the examination of the internal microstructures of the materials, the samples, after the experiments, were mounted in epoxy resin and cured in a vacuum chamber. Cross sections of the samples were then prepared by polishing, using SiC paper and diamond paste (down to a 0.25 μm size). For SEM surface-morphology observations, the powder samples after the experiments were directly placed on a carbon tab attached to an aluminium pin stub before being sputter coated using an Eiko IB-5 Sputter Coater (Eiko Co. Ltd., Hitachinaka, Japan) with platinum.
Results and Discussion
BNC Samples Calcined in Air
Effect of Maximum Calcination Temperature in Air on the Average NiO Crystallite Size, Calculated Using Debye–Scherrer Formula (BNC Heating Rate 10 °C/min; Holding Time at Temperature 30 Minutes)
Temperature of calcination (°C)
Average crystallite size, D (nm)
The concentric layer structure became more distinct upon heating up to 400 °C and 600 °C (Figure 6(d)). Some of the larger particles consist of two or more concentric layer structure cores, indicating the successive formation of layers on the particles’ surfaces during the precipitation reaction.
At 700 °C and 800 °C, there were signs that agglomeration had occurred during calcination; agglomerates in the range of 50 to more than 100 μm were observed at 800 °C. In some particles, a clear, physical separation between layers in the concentric layer structure had occurred by 800 °C. At temperatures in the range of 800 °C to 900 °C, random distributions of a very small (<0.1 μm) recrystallized NiO grains were observed throughout the particles, destroying the concentric layer structure.
Referring to the results in Figure 5, it can be suggested from a small DTA peak at 660 °C that the recrystallization of NiO occurred at that temperature. This peak can be clearly seen in the first derivative of differential thermal analysis (DDTA) curve. The newly recrystallized NiO grains were, however, indistinguishable at 700 °C and 800 °C, which was likely due to the very small size of the grains. From the XRD peaks in Figure 4, the calculated average crystallite sizes at 700 °C and 800 °C were 34 and 44 nm, respectively. Figures 6(g) through (h) show SEM images of the surface and sectioned particles at 900 °C. It can be clearly seen from Figure 6(h) that submicron grains, having diameter of approximately 0.05 to 0.1 μm, are observed on the surface and within the particle. This is consistent with the average crystallite size calculated using the Debye–Scherrer formula (0.052 μm).
BNC Sample Reduced in 15 Pct H2-N2 Gas Atmosphere
To study the behavior, fundamental reactions and microstructure change of BNC in reducing conditions, BNC samples were again heated to various temperatures using a 10 °C/min heating rate, but in this series of experiments, a 15 pct H2-N2 gas atmosphere was used.
At 100 °C to 200 °C, the surface morphology of the particles was similar to BNC particles oxidized at 110 °C to 200 °C in air. The majority of the 1- to 10-μm particles were spherical or ellipsoidal in shape, having smooth external surfaces. The sectioned microstructure of BNC particles at 200 °C appeared to be uniform, with concentric layer structure observed in some of the particles.
The SEM observations of the particles treated at 400 °C to 500 °C showed that the surfaces of most particles were covered with porous/spongy Ni and that extensive agglomeration of the particles had occurred. The size of the agglomerates can be more than 100 μm. By 500 °C, it appears that all of the particles had transformed to porous Ni crystals, confirming the XRD results.
At 700 °C, the spongy nickel particles consisted of 0.05- to 0.1-μm-diameter nickel grains (Figures 8(c) and (d). At 800 °C, grain growth of the subgrains had occurred, such that the mean grain size of the nickel was of the order 1 μm. Extensive agglomeration of the particles at 800 °C can be seen in Figures 8(e) and (f).
At 900 °C, the particles had formed agglomerates/lumps of more than 700 μm in diameter. The Ni grains had coarsened to more than 1 μm in size. There was clear evidence of sintering and densification of the Ni structure (Figures 8(g) and (h)). From the particle cross sections, there was no evidence of residual nickel-oxide trapped inside the dense nickel; the darker regions at the center of the particles in Figure 8(h) are isolated pores.
Preoxidized BNC Sample Reduced in 15 Pct H2-N2 Gas Atmosphere
During the BNC processing, for example, in a rotary kiln, the material moves from low-temperature oxidizing conditions at the feed input end to high-temperature reducing conditions at the discharge end. In order to simulate this, and to study the effect of preoxidation on the reduction behavior, BNC samples were first preoxidized in air, with a 10 °C/min heating rate to 900 °C, then reduced in a 15 pct H2-N2 gas atmosphere at various fixed temperatures for 30 minutes.
The morphology and microstructure of the particles after heating in 15 pct H2-N2 between 110 °C and 200 °C are similar to that of the starting preoxidized particles. The particles exhibited a structure that is composed of a random distribution of a very small (<0.1 μm) recrystallized nickel-oxide grains, rather than a clearly defined layer structure.
Figures 12(a) and (b) show the morphology of the surface and microstructure of sectioned particles at 340 °C. In this temperature range, the particles started to agglomerate as the nickel formed at the surface, provides bridges between particles. The size of the agglomerates can be more than 400 μm.
Between 500 °C to 600 °C, surface pores were observed on the nickel particles; the size can be up to 1 μm or more (Figure 12(c)). A concentric layer structure with distinct gaps between layers was observed in some of the particles (Figure 12(d)).
At 700 °C, quite dramatic changes in the nickel-product structure were initiated with the densification of the nickel metal on the surface of the particles, resulting in the formation of a dense, nickel layer on the particle surface (Figures 12(e) and (f)).
At 800 °C and 900 °C, the particles were characterized by the presence of thick, dense, metallic-Ni layer that encapsulates the residual NiO inside the particles (Figures 12(g) and (h)). This dense nickel consists of recrystallized nickel with large-grain size, and the particles were sintered together. The thick, dense Ni made further reduction of the NiO by direct access of H2 gas impossible.
Summary of the Key Changes Observed during the Processing of BNC; Heating Rate 10 °C/min; Holding Time 30 Minutes
BNC Calcined in Air
BNC Reduced in 15 Pct H2-N2
BNC Reduced in 1.5 Pct H2-N2
Preoxidized BNC (at 900 °C), Reduced in 15 Pct H2-N2
Preoxidized BNC (at 500 °C), Reduced in 15 Pct H2-N2
25 to 300
decomposition of amorphous BNC particles
decomposition of amorphous BNC particles
fine grained (0.05 to 0.5 μm) NiO crystalline
300 to 400
transformation from amorphous BNC (1 to 10 μm in diameter) to crystalline NiO
transformation of BNC to NiO; initial nucleation and growth of a porous Ni metal; some agglomeration of particles
initial nucleation and growth of porous Ni metal product; some agglomeration of particles
400 to 600
a concentric layered grain structure within each particle is formed; however, each particle remains physically intact
growth of porous Ni product; growth of Ni subgrains of 0.05 to 0.1 μm diameter
nucleation and growth of porous Ni; reduction of NiO is still occurring
reduction to Ni continues; agglomeration of particles
complete reduction of NiO forming porous multigrain Ni particles
600 to 800
recrystallization of NiO grains begins; sintering of NiO particles initiated
coarsening of Ni subgrains to 0.2 to 0.5 μm; evidence of sintering and agglomeration of Ni grains and particles
sintering and densification of Ni product on the surface of particles; evidence of NiO entrapped in thin dense Ni
substantial recrystallization of Ni subgrains and the formation of dense Ni layers on the surface of the particles; some residual NiO present inside particles
densification of porous nickel particles
800 to 900
formation of <0.1-μm-sized grains within the particles; further sintering and agglomeration of NiO
Subgrains form 0.5- to 2-μm dense grains, within fully recrystallized Ni particles; no evidence of NiO trapped inside Ni product
agglomeration of Ni particles; residual NiO remains, surrounded by thick dense recrystallized Ni
further agglomeration of dense Ni product grains; residual NiO remains in the interior of some of the particles surrounded by dense recrystallized Ni
more densification and agglomeration of Ni particles; no evidence of residual NiO in the interior of the particles
Summary of the Measured BET Specific Surface Areas from the Processing of BNC (Heating Rate 10 °C/min; Holding Time 30 Minutes)
BET Specific Surface Area (m2/g)
BNC Calcined in Air
BNC Reduced in 15 Pct H2-N2
Preoxidized BNC (at 900 °C), Reduced in 15 Pct H2-N2
In an oxidizing condition, i.e., in air, BNC decomposition is believed to occur in two stages. The first stage involves the removal of physically absorbed water. The formation of NiO occurs at the second stage at a temperature between 300 °C to 400 °C; this transformation occurs without significant change in the specific surface area, i.e., from 238 m2/g in the original BNC to 204 m2/g at 340 °C (Table III). At approximately 660 °C, the recrystallization of NiO commences; as the temperature increases, sintering and grain growth of the recrystallized NiO grains take place within the original particles. By 700 °C, these processes have resulted in a significant decrease of specific surface area to 6.6 m2/g; at 900 °C, fully recrystallized NiO particles are formed with specific surface area of 1.1 m2/g.
Direct gaseous reduction of BNC is suggested to take place in three stages. The first stage is the removal of physically entrained water. In the second stage, BNC decomposes to nickel oxide, accompanied by the removal of chemically bound H2O and CO2, producing particles having a porous, concentric-ring structure. In the third stage, nickel oxide is reduced to nickel metal, accompanied by the removal of oxygen. All of these changes can occur at relatively low temperatures, i.e., below 500 °C.
Similar to the case of calcination in air, at these low temperatures, the NiO particles produced from the decomposition of BNC are porous (Figure 8(b)); as a consequence, they still have a high, specific surface area, i.e., 207 m2/g at 340 °C. This high, specific surface area facilitates the delivery of fresh reducing gas to the interior of the particles, thus promoting the complete reduction of the nickel oxide at low temperatures, provided it has a high, chemical rate of NiO reduction, i.e., a high partial pressure of hydrogen (15 pct H2-N2). Further increases in temperature, i.e., above 500 °C, result in the densification of the nickel product, accompanied by a significant decrease in specific surface area, i.e., the specific surface areas at 700 °C and 900 °C are 1.1 and 0.3 m2/g, respectively.
In the case of the preoxidized BNC (at 900 °C) reduced in a 15 pct H2-N2 condition at 300 °C to 400 °C, the nickel metal nucleates on the surface of the recrystallized NiO grains. The reduction, however, proceeds at a slower rate compared to the directly reduced BNC. This is believed to be mainly due to the larger grain size, denser sintered structure, and higher crystallinity (low reducibility) of the recrystallized nickel oxide particles as a result of preoxidation. This observation is supported by the BET-specific surface-area measurements. Table III shows that the specific surface area of the preoxidized samples at 900 °C is 1.1 m2/g.
As the temperature is increased to 660 °C, it appears that recrystallization of the unreduced NiO occurs, as could be suggested from the small peak in the DDTA curve in Figure 13. As the temperature reaches 700 °C to 800 °C, recrystallization, grain growth, and sintering of the nickel-metal product take place. As a result, there is a closing of pores on the particles’ surfaces. Under the process conditions with the materials used in the current study, the rate of nickel densification (closing up of the pores) on the particle surface progresses quickly, relative to the overall reduction, resulting in the formation of a dense, nickel-metal product layer before the full reduction of the nickel oxide is complete. Once this dense, nickel microstructure has formed, further reduction of this trapped nickel oxide becomes progressively more difficult. There is no direct contact between the reducing gas and the nickel oxide at this stage; further removal of oxygen can only take place by solid-state diffusion through the dense, nickel layer formed, i.e., the reaction becomes mass transfer limited. As further oxygen is removed in this way, the layer thickness of the dense nickel becomes progressively greater, and the diffusion flux of oxygen through the layer is progressively reduced. Prolonged heating times at high temperatures (>800 °C) will also have a negative effect, as it promotes densification of overall agglomerated material. This is supported by the BET-specific, surface-area measurements; the specific surface area of the preoxidized (at 900 °C) samples reduced at 700 °C and 900 °C are 0.52 and 0.44 m2/g, respectively.
This analysis indicates that the strategy to approach a more complete reduction of nickel oxide to nickel metal is to control the relative rate of densification of nickel product to the overall NiO reduction rate. To demonstrate this point, additional sets of experiments were carried out.
at this temperature, the nickel oxide formed has lower crystallinity, higher porosity, and a higher surface/volume ratio, as compared to NiO preoxidized at 900 °C (Figure 4); and
500 °C is well below the Ni and NiO recrystallization temperatures of 600 °C to 700 °C (Table II) and below the temperature of 700 °C, at which the sintering and the densification of the porous nickel product is first observed (Table II).
Previous studies of the reduction of NiO with hydrogen gas[10, 11, 12, 13, 14, 15, 16] have shown that reduction to metallic nickel occurs readily with the formation of porous, nickel metal product. The porous product allows continuous access by gas-phase diffusion from the bulk gas to the NiO interface, at which a chemical reaction between gas and oxide occurs. The resultant H2O product gas is also able to diffuse through the porous product, thus enabling the reduction reaction to proceed to completion. It has been shown in the present study that under certain conditions, the morphology of the Ni-metal product microstructure can change, leading to the change in the rate-limiting reaction mechanism. The formation of a dense, product layer leads to a significant reduction in the overall rate of oxygen removal from the sample.
Utigard et al. reported that in temperature range of 700 °C to 900 °C, there is a decrease in the NiO reduction rates. Based on their extended experiments, they suggested that this is not due to physical changes of the oxide, nor the metallic phases. They speculated that the slowdown in the rate might be due to grain growth of the nickel formed or by surface segregation of sulfur. It should be noted that the characteristics, such as particle size, purity, etc., of the NiO and experimental technique used in their study are different than that used in the current study.
The current study shows that in the temperature range of 700 °C to 900 °C, there are several fundamental phenomena occurring simultaneously that may be responsible for the lowering of the NiO reduction kinetics. These include the NiO recrystallization and grain growth (600 °C to 900 °C); NiO sintering (>800 °C); Ni product recrystallization and grain growth (>700 °C); Ni product densification (>700 °C); and agglomeration of particles (>700 °C). It is also shown in the present study that the thermal history and original microstructure of the material have a significant effect on the microstructure evolution, and hence, on the final microstructure of the nickel product.
chemical changes, i.e., decomposition, reduction reactions, and oxidation reactions;
NiO and Ni recrystallization and grain growth; and
NiO and Ni sintering and densification.
maintaining a high-NiO, specific surface area to avoid NiO and Ni recrystallization and densification, which is achieved by carrying out reduction below 600 °C; and
maintaining a high chemical rate, which is favored by high H2 partial pressures and high H2/H2O ratios (i.e., high chemical driving force for reduction).
The authors thank the BHP Billiton Yabulu Refinery for supplying the basic nickel carbonate samples. The authors also acknowledge the financial support from the Australian Research Council and the BHP Billiton Yabulu Refinery as part of an ARC Linkage project. The authors further thank Mr. John Fittock and Dr. Joy Morgan (Yabulu Refinery) for valuable discussions. MAR also thanks Mr. Jiang Chen for carrying out the TGA/DTA measurements.