Kinetics of and preparation of nickel by Ni3S2−NiO reaction under reduced pressure

The reaction between Ni₃S₂ (liquid) and NiO (solid) resulting in the formation of Ni and SO₂ was investigated in the temperature range 800° to 1200°C under a reduced pressure of <0.1 mm Hg. From the kinetic studies in the temperature range 950° to 1150°C, the reaction was found to proceed in three stages: i) Up to about 25 pct reduction, the rate of reaction was high and followed approximately a cubic rate law. During this stage, the reaction is thought to be under mixed control. Activation energy for the first 10 pct reduction was found to be approximately 45 kcal per mole. ii) From about 25 to 90 pct reduction, it obeyed the parabolic rate law, with an activation energy of 86±6 kcal per mole. This value is in agreement with the activation energy reported in the literature for the diffusion of sulfur in nickel. iii) Beyond 90 pct reduction, the reaction was very sluggish owing to the poor availability of the reactants. Optimum conditions for preparing nickel sponge by the above reaction and its processing into thin strips have been standardized. Some of the properties of the metal thus produced have also been incorporated.     

Kinetics of dissolution of synthetic heazlewoodite (Ni3S2) in nitric acid solutions

The dissolution rate of heazlewoodite in nitric acid solution has been determined. The effects of nitric acid concentration, temperature, particle size, stirring intensity and addition of Cu²⁺ ions have been investigated. Solid residues after leaching were examined by SEM, X-ray diffraction and chemical analysis. In the solutions containing less than 2.0 M HNO₃, dissolution was observed to be completely inhibited after 30 min leaching time, and the rate of hydrogen sulphide production was faster than its oxidation to S⁰ and HSO₄^?. In 3 M HNO₃, an abrupt increase in dissolution rate of Ni₃S₂ was found. Two different regions of the dissolution of heazlewoodite were observed below and above 50°C. At temperatures below 50°C, the dissolution rate was very slow, even in 3.0 M HNO₃ solution, and H₂S gas was evolved. Above 50°C, the dissolution rate rapidly increased. Over the temperature interval 60–90°C in 3.0 M HNO₃ dissolution followed a linear rate law, and the activation energy was found to be 42.1 kJ mol^?1. Most of the oxidized sulphide ion was found in the solution as sulphate. The leaching rate was independent of stirring speed. The rate-controlling step of the Ni₃S₂ dissolution is the oxidation of hydrogen sulphide to elemental sulphur or sulphate ions on the Ni₃S₂ surface. Addition of small amounts of Cu²⁺ ions to the nitric acid acted as catalyst for the dissolution of Ni₃S₂. Bubbling air through the leach suspension increased the dissolution rate of Ni₃S₂ in solutions containing less than 2.0 M HNO₃.     

The catalytic action of cupric and ferric ions in nitric acid leaching of Ni3S2

The kinetics of dissolution of synthetic Ni₃S₂ in nitric acid solution containing ⩽ 2.0 M HNO₃ in the presence of cupric and ferric ions was investigated. The effects of stirring, particle size, temperature and concentration of cupric and ferric ions were examined. Solid residues at various levels of nickel extraction were examined by SEM, X-ray diffraction, electron microprobe and chemical analysis. The constant dissolution rate of Ni₃S₂ was attributed to the electrochemical reaction occurring on the surface of flat-plate type Ni₃S₂ particles. The reaction kinetics were found to be independent of both cupric and ferric ion concentrations up to 1 mM. Cupric ion did not act as a catalyst at temperatures below 60°C. At 80°C cupric and ferric ions show the same catalytic activity. The activation energy in the presence of ferric ions was 103.6 ± 4.2 kJ/mol. A mechanism for Ni₃S₂ dissolution in the presence of cupric and ferric ions is proposed.     

Reduction kinetics of Ni(OH)2 to nickel powder preparation under hydrothermal conditions

The reduction kinetics from Ni(OH)₂ to fine Ni metal powder under hydrothermal and H₂ pressure conditions using anthraquinon as an activator were investigated. The reduction ratio increased with temperature up to 225 °C. The H₂ molecule was activated by anthraquinone. This activated hydrogen acts as a reduction reagent from Ni²⁺ to Ni⁰. The process may proceed by heterogeneous reaction of Ni²⁺ ion and solid anthraquinon as follows: (1) anthraquinon is hydrated, (2) Ni²⁺ ion is absorbed on anthraquinon hydride, and (3) this complex decomposes to Ni⁰ powder, anthraquinon, and H₂O. The particle size of nickel powder obtained increased with increasing temperature and with decreasing pH. The average particle size was about 350 nm. The reduction kinetics were in good agreement with a core model equation with surface reaction, that is, 1 − (1 −x)^1/3 =kt, wherex is a reaction ratio andt is reaction time. Arrhenius plots showed two slopes with activation energies 65.9 kJ/mol at higher temperature and 377.2 kJ/mol at lower one. This result shows that the hydration of anthraquinon and adsorption of Ni²⁺ ion onto anthraquinon hydride and decomposition to Ni metal and anthraquinone proceed by consecutive reactions and rate determining steps change in each temperature range.     

Simultaneous sulfidation-oxidation of nickel at 603°c in argon-so2 atmospheres

The simultaneous oxidation-sulfidation rate of nickel has been measured as a function of SO₂ pressure (0.04 to 1 atm) in Ar-SO₂ gas mixtures at 603°C. The observed corrosion rates are about 10⁷ times faster than the oxidation rate of nickel in oxygen at 1 atm. The product scale consists of an inner Ni₃S₂ layer and an outer two-phase layer of NiO and Ni₃S₂. A linear rate law is observed during an initial time period, and the most probable rate-controlling step is dissociation of SO₂. An increase in the scale-gas interfacial area increases the corrosion rate during intermediate time periods. With increasing time, parabolic corrosion rates are measured for SO₂ pressures of 0.25 and 1 atm. Values of the nickel diffusivity in Ni₃S₂ calculated from our measured parabolic-rate constants are in good agreement with recently reported values. This agreement indicates that an interconnected Ni₃S₂ phase in the outer two-phase layer provides rapid transport paths for nickel diffusion through the scale. Formerly a Graduate Student in the Department of Metallurgy and Materials Science at the University of Pennsylvania, Philadelphia, PA     

Kinetics and mechanism of the reduction of molten nickel sulfide by hydrogen

The kinetics and mechanism of the reduction of M₃S₂ by hydrogen have been investigated between 1133° and 1300°C. When high flow rates of hydrogen and argon or helium bubbling through the melt are maintained the rate-determining step is a chemical process which can be expressed by a rate law of the form $$\begin{gathered} r_{H_2 S} = k_{expt} (N_S - \alpha )^2 p_{H_2 }^{1/2} \hfill \\ p_{H_2 } \geqslant 0.88atm \hfill \\ \end{gathered} $$ where k_expt = 85.1 atm^-1/2 min^-1, α = 0.17 at 1250°C. The experimental activation energy for this process is 20.1 ±3.0 kcal per mole. These results are discussed in terms of possible catalysis by nickel.     

Phase constitution of the Ni−Cr−S alloy system between 600° and 850°C

The diffusion couple method and electron probe microanalysis have been used to determine the ternary isotherms of Ni?Cr?S at 600°, 700°, 800°, and 850°C for sulfur compositions lying below 60 at. pct. It has been found that the solid and liquid phases Ni_3+x S₂ have a very shallow penetration into the diagram whereas the Ni_1?x S, Cr₃S₄, and Cr₂S₃ phases penetrate very deeply into the diagram. The phase compositions NiCr₂S₄ and NiCrS₃ have been detected near the limit of the latter two phase fields. The binary phases Cr₂S₃, Cr₃S₄, Cr₅S₆, Cr₇S₈, and CrS have all been identified and their existence ranges and free energies of formation have been established at 700°C. The phase diagram observed and proposed is consistent with the sequence of free energies of formation of nickel and chromium sulfides.     

Hydrogen reduction kinetics of nickel sulfide in the presence of calcium oxide

Recovery of pure nickel from nickel sulfide (Ni₃S₂) was studied by following to completion the hydrogen reduction reaction in the presence of calcium oxide. The effects of reaction temperature, molar ratio of calcium oxide to nickel sulfide, bed depth, and particle size of the nickel sulfide powder on the reaction were experimentally investigated. A simple empirical integrated rate equation describing the relationship among these variables over the temperature range 773 to 973 K was derived. The activation energy for the scavenged reaction was found to be 101.9 kJ from the experimental data. Over the range of experimental conditions, both globular and fibrous forms of metallic nickel were observed.     

Use of dithionite in the removal of nickel from ammoniacal solutions

An investigation has been carried out to study the feasibility of the removal of nickel from ammoniacal solutions using the reducing agent sodium dithionite. The results obtained indicate that dithionite can precipitate over 95% of nickel values in solution in the form of Ni₃S₂ and metallic nickel. A temperature of 45°C has been found to be optimal for the precipitation process.     

Thermodynamic properties of Ni-S melts between 700° and 1100°c

The vapor pressure of sulfur over Ni-S melts of various compositions was calculated from the equilibrium weight of the melt in gas streams of known H₂S-H₂ composition. The Gibbs-Duhem equation was used to calculate the activity of nickel and other thermodynamic properties. For the reaction: 3Ni(S) + S₂(g) ⇌ Ni₃S₂(l) the suggested free energy rslationship is: ΔG° = -57,910 + 15.89T (800° to 1100°C). The Calculations were extrapolated to predict that for the reaction: Ni(s) + 1/2S₂(g) ⇌ NiS(l), ΔG° = -26.730 + 10.5T (1000° to 1100°C)     

The precipitation of Ni3S2 from sulfate solutions

The formation conditions for the recovery of nickel from sulfate solutions as Ni3S2 have been investigated; this sulfide is more reactive than NiS in subsequent leaching operations. Hydrogen sulfide gas at atmospheric pressure is introduced into a NiSO4-Na2SO4-MgSO4-Al2(SO4)3 solution in the presence of reduced iron powder. Although the formation of Ni3S2 is compctitive with that of NiS, the nickel precipitation efficiency and the ratio of nickel as Ni3S2 to the total nickel precipitated reached 99.5 to 99.9 and 90 to 95 pct, respectively, under the following conditions: 363 K, Ph2s 31 kPa, Ni²⁺ 4.0 g · dm^-3, 3[Fe^o]/[Ni²⁺] 1.25 to 1.5, H2S flow rate 70 to 100 cm³ · min^-1, and 45 to 60 minutes retention time. Selective formation of Ni3S2 is achieved within 10 minutes, and a reaction on the surface of the iron is rate-determining during the early stages of precipitation. Since the iron is almost totally consumed after 1 to 2 hours of reaction, the precipitated Ni₃S₂ is gradually converted to NiS. Calculations considering the buffer action of sulfate ion and sulfate complex formation with polyvalent metal cations as well as with nickel ions confirmed that significant nickel precipitation as Ni₃S₂ should occur under the test conditions.     

Combustion characteristics of the Ni3Ti-TiB2 intermetallic matrix composites

Combustion synthesis (SHS) of Ni₃Ti-TiB₂ metal matrix composites (MMCs) was selected to investigate the effect of gravity in a reaction system that produced a light, solid ceramic particle (TiB₂) synthesized in situ in a large volume (>50 pct) of the liquid metallic matrix (Ni₃Ti). The effects of composition, green density of pellets, and nickel particle size on the combustion characteristics are presented. Combustion reaction temperature, wave velocity, and combustion behavior changed drastically with change in reaction parameters. Two types of density effects were observed when different nickel particle sizes were used. The structures of the combustion zones were characterized using temperature profile analysis. The combustion zone can be divided into preflame, reaction, and after-burning zones. The combustion mechanism was studied by quenching the combustion front. It was found that the combustion reactions proceeded in the following sequence: formation of liquid Ni-Ti eutectic at 940 °C → Ni₃Ti+NiTi phases → reduction of NiTi with B→TiB₂+Ni₃Ti.     

Reaction synthesis of Ni-Al-based particle composite coatings

Electrodeposited metal matrix/metal particle composite (EMMC) coatings were produced with a nickel matrix and aluminum particles. By optimizing the process parameters, coatings were deposited with 20 vol pct aluminum particles. Coating morphology and composition were characterized using light optical microscopy (LOM), scanning electron microscopy (SEM), and electron probe microanalysis (EPMA). Differential thermal analysis (DTA) was employed to study reactive phase formation. The effect of heat treatment on coating phase formation was studied in the temperature range 415 °C to 1000 °C. Long-time exposure at low temperature results in the formation of several intermetallic phases at the Ni matrix/Al particle interfaces and concentrically around the original Al particles. Upon heating to the 500 °C to 600 °C range, the aluminum particles react with the nickel matrix to form NiAl islands within the Ni matrix. When exposed to higher temperatures (600 °C to 1000 °C), diffusional reaction between NiAl and nickel produces (γ′)Ni₃Al. The final equilibrium microstructure consists of blocks of (γ′)Ni₃Al in a γ(Ni) solid solution matrix, with small pores also present. Pore formation is explained based on local density changes during intermetallic phase formation, and microstructural development is discussed with reference to reaction synthesis of bulk nickel aluminides.     

Reactions of molten Fe,Co, Cu,and Pb sulfides with hydrogen

The kinetics and mechanism of reactions of molten Fe, Co, Cu, and Pb sulfides with hydrogen have been investigated between 1080° and 1340°C. While maintaining high flow rates of hydrogen and argon to ensure chemical control, the rate of reduction of these sulfides was found to be second order in sulfur concentration and half-order in hydrogen pressure. These dependences are similar to those reported earlier for molten nickel sulfide reduction by hydrogen. The kinetics and mechanistic proposal suggest a metal catalyzed step. If the activation energies for the rate determining step of S₂ reacting with hydrogen in the melt for the various sulfides are compared with the values for the hydrogen-liquid sulfur or hydrogen-gaseous sulfur reactions, the order of catalytic activity seems to follow generally the order of the pure metals themselves as found for other processes involving activation of molecular hydrogen.     

Diffusion in the nickel-rich part of the Ni−Al system at 1000° to 1300°C; Ni3Al layer growth,diffusion coefficients,and interface concentrations

The formation of the Ni₃Al layer in NiAl (55 at. pct Ni)-pure Ni diffusion couples at temperatures above 1000°C has been found to be controlled almost completely by volume diffusion. At 1000°C and below, the relatively small grain size of the Ni₃Al compound in the layers caused such a large contribution from grain boundary diffusion, that the layer growth rates at 1000°C exceeded those at 1100°C and even those at 1200°C. In Ni₃Al (75at. pct Ni)-pure Ni diffusion couples the Ni₃Al compound rapidly converted into the solid solution of aluminum in nickel. Volume-diffusion coefficients calculated by the Boltzmann-Matano method yielded heats of activation of 55, 64, and 65 kcal·mol^?1 for NiAl, Ni₃Al and the solid solution of aluminum in nickel, respectively. In addition, eleven different types of diffusion couples were prepared from various Ni?Al alloys and annealed at 1000°C. Marker interface displacements and observations of porosity in these couples yielded a more detailed picture of the Kirkendall-effect than earlier work had done. The ratio of the intrinsic diffusion coefficients at the marker interface,D _NI/D _Al, is greater than one in the nickel-rich NiAl phase. For the Ni₃Al phase no statement can be made on the basis of this work. When the marker interface is located in the nickel solid solution,D _Ni/D _Al is smaller than one. The phase boundary concentrations in these couples did not show the expected deviation from the equilibrium concentrations in two-phase alloys; this finding is discussed with regard to the free-energycomposition diagram.     

Effect of Pd, Cu, and Ni additions on the kinetics of NiCl2 reduction by hydrogen

Differnetial thermal analysis (DTA) and thermal gravimetric analysis (TGA), at a heating rate of 10 °C/min, revealed a complete reduction of NiCl₂ by hydrogen in a temperature interval of 375 °C to 450 °C. However, addition of 0.1 mass pct of Pd, Cu, or Ni to the sample caused the reduction to occur at considerably lower temperatures, in the rather narrow range of 315 °C to 370 °C. The activation energy of NiCl₂ reduction by hydrogen (between 300 °C and 550 °C) without additives is 54 kJ/mol, and with Pd and Cu or Ni added, under isothermal conditions (from 260 °C to 380 °C), is 33 and 50 kJ/mol, respectively. These values confirm a positive effect of additives on the reduction kinetics. The positive effect of Pd is a consequence of the dissociation and spillover of hydrogen, whereas in the case of Cu and Ni(HCOO)₂, it is manifested in a decrease in bonds energy in the nickel lattice because of good Cu solubility, and in the formation of artificial nickel nuclei that intensify the reduction, respectively. Scanning electron microscopy (SEM) analysis of nickel powders obtained under isothermal conditions shows relatively rounded spherical particles (0.321 to 0.780 μm in size) of powder samples with additives, and particles of irregular shape (2.085 μm mean size) of the sample without additives. This illustrates the positive effect of Pd, Cu, or Ni added in the reduction process, in decreasing the size of nickel particles and in the production of a more uniform particle shape.     

Thermodynamics of the Fe-Ni-S system at 1250°C

The activity of sulfur in the Fe-S and Ni-S binary and the Fe-Ni-S ternary systems was investigated at 1250°C by equilibrating molten mattes with gaseous mixtures of H₂S and H₂. From the sulfur activity data the activities of iron and nickel in the Fe-S and Ni-S binary systems as well as the activities of FeS and Ni₃S₂ in the ternary system were calculated. Isoactivity diagrams are presented for S, FeS, and Ni₃S₂ in the Fe-Ni-S ternary system. Some comments are offered on the industrial problems of nickel slag loss and ferronickel separation.     

Thermodynamics of the solid Ni-S system

Published measurements of sulfur vapor pressure were used to determine thermodynamic properties of the solid Ni-S system above 800 K. They were Gibbs-Duhem integrated to estimate the formation properties of stoichiometric Ni₃S₂, NiS, and NiS₂. Statistical thermodynamics was applied to find partition functions, interaction energies, and free energies to find possible atom arrangements in Ni_3±x S₂ and Ni_1−x S. Theoretical calculations indicated that nickel vacancies may exist in quasichemical order in the former phase and randomly in the latter phase. A strong indication was that Ni_3±x S₂ should have a sulfur bcc structure with nickel atoms distributed in octahedral sites having a Ni₅S₄ configuration. The possible existence of sulfur vacancies was theoretically investigated.     

Kinetics of chloridization of nickel-bearing lateritic iron ore by hydrogen chloride gas

The selective chloridization of nickel in a lateritic iron ore by gaseous HCl is based on the principle of relative thermal stability of iron and nickel chlorides. This aspect has been discussed with differential thermal analysis (DTA) and thermogravimetric (TG) data of the hydrated chlorides of iron and nickel. The kinetics of chloridization of nickel in a lateritic nickel ore from Orissa, India, have been studied by using both pure HCl (g) and the HCl (g) + N₂ mixture. The sharp decrease in the rate of chloridization of nickel at temperatures above 250 °C is attributed to the rapid decomposition of molten ferric chloride hydrate (FeCl₃ · 3H₂O), which blocks the pores of the reactant solid. Therefore, kinetics of chloridization follow both the pore-blocking model (logarithmic rate law) and diffusion-controlled mechanisms. Very low values of apparent activation energy and effective diffusivity derived from the rate constants of the diffusion-controlled process suggest that diffusion of HCl (g) takes place either in a dissolved state in the molten ferric chloride (at 100 °C to 150 °C) or through cracks and fissures formed on the surface due to rapid decomposition of ferric chloride at 200 °C to 250 °C. Because of the complexity of the reaction system, the rate of chloridization of nickel is almost independent of grain size.     

Reaction behavior of Ni–Re alloys during direct current polarization in sulfuric acid solutions

The macrokinetic regularities of the reactivity of synthesized Ni–Re (20 and 60 wt %) alloys in a sulfuric acid solution (100 g/L, 25–40°C) during direct current polarization are studied using physicochemical methods. The phase composition of the synthesized alloys is determined by the formation of solid solutions as a function of the initial Ni/Re weight ratio. These are two types of nickel solid solutions (Ni₁₆Re_0.2 and Ni₁₄Re_0.9) and one rhenium solution (Ni_1.1Re). These solid solutions are anodically oxidized in the sequence of their structural rearrangement Ni₁₆Re_0.2 → Ni₁₄Re_0.9 → Ni_1.1Re with a combined transition of the metals into an electrolyte solution. These solid solutions provide the reduction of Ni³⁺ to Ni²⁺ due to the depolarization ability of rhenium, being their component.