Thermochemical decomposition of hydrogen sulfide with nickel sulfide

In this research, the two-step thermochemical cycle shown below is proposed and experimental studies were made on the cycle. $$\frac{\begin{gathered} {\text{Ni}}_{\text{3}} {\text{S}}_{\text{2}} + {\text{H}}_{\text{2}} {\text{S}} = {\text{3NiS + H}}_{\text{2}} \hfill \\ {\text{3NiS = Ni}}_{\text{3}} {\text{S}}_{\text{2}} {\text{ + 0}}{\text{.5S}}_{\text{2}} {\text{(g)}} \hfill \\ \end{gathered} }{{{\text{H}}_{\text{2}} {\text{S = H}}_{\text{2}} {\text{ + 0}}{\text{.5S}}_{\text{2}} {\text{(g)}}}}$$ In the case where Ni₃S₂ alone was used without inert additions, nickel sulfide sintered or partly fused due to the melting point depression resulting from the thermal decomposition of formed NiS. Such sintering could be prevented by mixing the nickel sulfide powders with Al₂O₃ or MoS₂. The cyclic reactions were thereby shown to provide a stationary high decomposition rate of H₂S. Polysulfides, such as MS₂, have previously been employed in this kind of cycle. This research showed that the use of lower sulfides such as Ni₃S₂ may be regarded as rather promising based on the thermodynamic investigation of the respective reactions composing the cycle. The comparison between the sulfurization reactions of NiS to NiS₂ and of Ni₃S₂ to NiS further showed that the latter was superior to the former with respect to the kinetics and thermodynamical properties of the reaction.     

Sulfuric acid oxygen-pressure leaching of Ni3S2 prepared by a wet process

Ni₃S₂ prepared by a wet process was easily leached as nickel sulfate at 383 K, p_o2 1 MPa, and sulfuric acid concentration of 0.1–0.15 mol L⁻¹. The leaching reaction proceeds through the intermediate formation of NiS prior to complete dissolution. A constant leaching rate was observed for most of the duration of the reaction, and this has been attributed to an increase in the specific surface area of the sulfide particles. A thin sulfur layer was formed on the sulfide; the diffusion of oxygen through the sulfur layer was found to be rate-determining.     

Fluidized Bed Selective Oxidation-Sulfation Roasting of Nickel Sulfide Concentrate: Part II. Sulfation Roasting

The fluidized bed sulfation roasting process followed by water leaching was investigated as an alternative process to treat nickel sulfide concentrate for nickel production. The effects of several roasting parameters, such as the sulfation gas flow rate, roasting temperature, the addition of Na₂SO₄, and the roasting time, were studied. 79 pct Ni, 91 pct Cu, and 95 pct Co could be recovered with minimal dissolution of Fe of 4 pct by water leaching after two-stage oxidation-sulfation roasting under optimized conditions. The sulfation roasting mechanism was investigated, showing that the outermost layer of sulfate melt and the porous iron oxide layer create a favorable sulfation environment with high partial pressure of SO₃. Sulfation of the sulfide core was accompanied by the conversion of the sulfide from Ni_1?x S to Ni₇S₆ as well as inward diffusion of the sulfation gas.     

Oxidation of heavy nonferrous metal and iron sulfides via a chain mechanism

Gaseous sulfur monoxide (SO) is experimentally found to be a primary intermediate product in the oxidation of nonferrous metal and iron monosulfides. This finding is used to develop a theory for the chain oxidation of metal sulfides to form oxysulfide complexes MSO _x and gaseous SO. The main steps of the metal sulfide oxidation via a chain mechanism are thermodynamically characterized. The Gibbs energies of the oxidation reactions are shown to decrease in the absolute value relative to the type of sulfide as follows: FeS, Ni₃S₂, CoS, NiS, ZnS, PbS, CdS, CuS, and Cu₂S. The thermodynamic probability of all steps of the chain oxidation of the monosulfides with the evolution of gaseous atomic oxygen and sulfur is estimated.     

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.     

Solid state reactions between silicon carbide and (Fe,Ni, Cr)-alloys: reaction paths,kinetics and morphology

Solid state reactions between silicon carbide and various (Fe, Cr, Ni)-alloy compositions are studied. The different phases present in the reaction scale, their chemical composition and morphology are determined. The reaction path and the reaction kinetics are also measured. A typical composition distribution and morphology of the reaction scale is associated to each alloy element: iron yields broad α-iron zones with randomly distributed graphite precipitates; the presence of nickel results in typical reaction scales made of alternating bands of pure silicide and other bands of silicide containing a high density of carbon precipitates; chromium leads to large chromium carbide precipitates on the alloy side. The solid state reactions between SiC and (Fe, Cr, Ni)-alloys can be described as the dissolution of silicon carbide by iron and nickel, resulting in the formation of silicon-rich compounds (α-, τ-phase, Ni₅Si₂ or Ni₂Si) and in the precipitation of graphite flakes.     

Ti/TiO2/Ni2+ interface: unexpected protection of Ti by Ni2+ cations in hot sulphuric acid

Abstract

The processing of nickel ores involves: high temperatures (>100°C), high [H₂SO₄] and high [Cl^?]. Despite the severity of this service, pure titanium can be used because it presents very low corrosion rates. Looking for the fundamental causes of this behaviour, it was found that the potential of the couple Ti⁴⁺/Ti³⁺ shifted to more negative potentials than the couple Ni²⁺/Ni in these conditions. Thus, Ni²⁺ was reduced to Ni metal, oxidising Ti³⁺ to Ti⁴⁺. This increased the concentration of Ti⁴⁺ which was critical for the formation of the protective TiO₂ layer. In turn, the TiO₂ passivated the active titanium dissolution. Also, the deposited Ni catalysed the cathodic kinetics, producing a mixed potential in the passive region for titanium. This process involves Ti/TiO₂/Ni²⁺ and Ti/Ni²⁺ interface systems, and Ti/Ni²⁺ behaviour is somewhat like a titanium–nickel alloy.

Le traitement des minerais de nickel implique des températures élevées (>100°C) et une concentration élevée de [H₂SO₄] et de [Cl^?]. En dépit de la sévérité de ces conditions, on peut utiliser le titane pur parce que celui-ci présente de très faibles taux de corrosion. Examinant les raisons fondamentales de ce comportement, on a trouvé que le potentiel du couple Ti⁴⁺/Ti³⁺ se déplaçait vers des potentiels plus négatifs que celui du couple Ni²⁺/Ni sous ces conditions. Ainsi, le Ni²⁺ était réduit en métal Ni, oxydant le Ti³⁺ en Ti⁴⁺. Ceci augmentait la concentration de Ti⁴⁺, qui est critique pour la formation de la couche protectrice de TiO₂. À son tour, le TiO₂ rend passive la dissolution active du titane. Également, le Ni déposé catalyse la cinétique cathodique, produisant un potentiel mixte dans la région passive du titane. Ce procédé implique les systèmes de l’interface Ti/TiO₂/Ni²⁺ et Ti/Ni²⁺et le comportement du Ti/Ni²⁺ est plus ou moins comme un alliage de titane-nickel.     

Surface Effects in Sulfidation Reactions

Abstract

Pure iron and an Fe–41 wt% Ni alloy are reacted at temperatures in the range 793–1073 K with H₂–H₂S–N₂ atmospheres corresponding to equilibrium P _S2 levels from 6.5 × 10^?5 to 0.65 Pat The kinetics of iron sulfidation are intermediate in form to linear and parabolic rate laws. The instantaneous parabolic rate constant is found to increase with extent of reaction until a constant value is reached. For fixed equilibrium sulfur pressures, the instantaneous rate increases with hydrogen sulfide partial pressure; for fixed hydrogen sulfide partial pressure, the instantaneous rate decreases as the equilibrium sulfur pressure is increased. It is demonstrated that hydrogen sulfide is the reactant species. A Langmuir-Hinshelwood kinetic model based on the slow dissociation of adsorbed hydrogen sulfide accounts satisfactorily for the unusual gas-phase compositional effects, and also for the rate at which the reacting system approaches steady state. Similar effects are found for the Fe–41%Ni alloy, where nickel sulfide whisker formation results from localized catalysis of the hydrogen sulfide dissociation reaction.     

Acidity, valency and third-ion effects on the precipitation of scorodite from mixed sulfate solutions under atmospheric-pressure conditions

The present study is focused on the precipitation of scorodite from mixed sulfate media at 95 °C under atmospheric pressure. In particular, this study explores the effects of acidity (pH), valency [Fe(II)/Fe(III), As(III)/As(V)], and solution composition (third cation/anion) on the yield, crystallinity, and stability (leachability) of scorodite precipitates. Thus, it was found that the precipitation of crystalline scorodite can be achieved without stringent pH control once the precipitation has started. Nonetheless, the selection of the initial pH is critical to avoid the formation of an amorphous precipitate. A leachability as low as 0.5 mg/L As at pH 5 and 22 °C (TCLP-like test) is obtained when the initial molar ratio Fe(III):As(V) is increased to 3:1, but the precipitation yield is very low. When Fe(II) is used as excess iron, the precipitate solubility drops to 0.2 mg/L As with a yield exceeding 80 pct in 2.5 hours. The stability of the product is not measurably affected by the presence of Cu²⁺, Zn²⁺, Ni²⁺, Co²⁺, Mn²⁺, SO ₄ ²⁻ , and NO ₃ ⁻ . The presence of PO ₄ ³⁻ , however, leads to the formation of crystalline phosphate-containing scorodite precipitates of somewhat reduced stability. In most cases, the TCLP leachability of the precipitate was found to be between 1 and 3 mg/L As, and never exceeded the regulatory limit of 5 mg/L As.     

Distribution of nickel between copper-nickel and alumina saturated iron silicate slags

The solubility of nickel in slag was determined by equilibrating copper-nickel alloys with alumina-saturated iron silicate slags in an alumina crucible at 1573 K. The experiments were carried out under controlled oxygen partial pressures in the range of 10^-10 to 10^-8 atm by use of suitable CO-CO₂ gas mixtures, and at Fe/SiO₂ ratio 1.34. The results showed that nickel dissolves in slag both as Ni²⁺ (nickel oxide) and Ni‡ (nickel metal), and the relation obtained was: (Wt pct Ni in slag) = (ie33-01) The activity coefficient of nickel oxide (γ^dgNio) and distribution coefficient of nickel (A_Ni) is calculated to be 0.375 and 233.3, respectively. γ^dgNio and A_Ni are found to be independent of oxygen partial pressures. The presence of alumina increases the solubility of nickel in slags.     

Effect of oxide scale on carbon deposition on Fe-Ni alloys in carburizing gas

The behavior of carbon deposition on preoxidized Fe-Ni alloys containing 0 to 57.0 mass pct Ni in 10 pct CH₄-H₂ mixture at 1203 K was studied by metallography and thermogravimetry. Nickel retarded carburization and carbon deposition by lowering the solubility limit of graphite in austenite and by reducing catalytic activity for the pyrolytic reaction of CH₄. On oxidation in air, the addition of nickel to iron depressed the development of FeO and, thereby, caused a significant decrease in the thickness of the scale. The exposure of the alloys to 10 pct CH₄-H₂ mixture after the oxidation in air led to a sudden mass loss in the early stage and then a rapid mass gain. This mass change is primarily ascribed to mass loss by reduction of iron oxides and to mass gain by carbon deposition. The rapid mass gain by carbon deposition is probably due to the formation of active iron by reduction of iron oxides and to the increase in the reaction area by spallation of the scale; the active iron formed may promote filamentous carbon deposition through Fe₃C formation and decomposition. Carbon deposition on the alloys containing 27.2 mass pct Ni or more was considerably retarded because of the formation of a thin oxide scale which consists of α-Fe₂O₃ and spinel (Ni_xFe_3−xO₄) and the reduction of catalysis by enrichment of nickel in the subscale. However, the amounts of carbon deposition increased compared with those on the as-polished alloys, owing to the presence of reducible iron oxides.     

A solid-state emf study of ternary Ni-S-O,Fe-S-O,and quaternary Fe-Ni-S-O

Four two-phase equilibria in Ni-S-O, three two-phase equilibria in Fe-S-O, and one two-phase equilibrium and four three-phase equilibria in Fe-Ni-S-0 atP _{SO 2} ? 1 atm were studied using the following emf cell: $$Pt, O_2 (air) \left| {\begin{array}{*{20}c} {ZrO_2 \cdot CaO or} \\ {ZrO_2 \cdot Y_2 O_3 } \\ \end{array} } \right|\begin{array}{*{20}c} {mixture of phases, Au} \\ {SO_2 , S_2 , O_2 , SO_3 with P_T \simeq 1 atm} \\ \end{array} $$ The seven two-phase equilibria in Ni-S-O and Fe-S-O are NiS/Ni₃S₂, Ni₃S₂/Ni0, NiS/NiO, NiO/NiSO₄, Fe₂O₃/Fe₂(SO₄)₃, Fe₂O₃/FeSO₄, and FeSO₄/Fe₂(SO₄)₃, and the four three-phase equilibria and the one two-phase equilibrium in Fe-Ni-S-0 are sp + (Fe, Ni)S + (Fe, Ni)₄S₃, sp + (Fe,Ni)₄S₃ + (Fe, Ni)O, sp + (Fe, Ni)O + (Fe, Ni)SO₄, and Fe₂O₃ + sp + (Fe, Ni)SO₄ and sp + (Fe,Ni)S. The sp + (Fe, Ni)S two-phase equilibrium was measured by using mixtures containing a major portion of one phase and a minor portion of the other. The results are presented in terms of oxygen potential as a function of metal ratio,y _Ni =x _Ni/(x _Fe +x _Ni) at constant temperatures andP _{SO 2} = 1 atm. The calculated stability diagrams atP _{SO 2} = 1 atm from thermodynamic models are in agreement with experimental results. Several of the stability diagrams atP _{SO 2} = 0.1 and 0.01 are also calculated and presented.     

Surface and interfacial tension of the Ni-Fe-S,Ni-Cu-S,and fayalite slag systems

The surface tensions of the Ni-Fe-S system together with the Ni-Cu-S and fayalite-type slag systems were measured using the sessile drop technique. Drop images were obtained by X-ray radiography. For the Ni₃S₂-FeS pseudo-binary system, the surface tension of the matte increased with matte grade and showed a positive deviation from ideality with matte grade. This suggests that Ni₃S₂ dominates in the nickel matte system. The results also indicated that a fluctuation in the matte nickel content can alter the surface tension quite significantly. A ternary surface ten-sion plot of the Ni-Fe-S system showed that the addition of either Ni or S to the matte alters the surface tension. On the other hand, at a constant Ni:S ratio, the addition of iron has neg-ligible influence on the surface tension of the matte. The measured surface tension of the Ni-Cu-S system also showed a nonlinear relationship with the Cu₂S content in the matte. The surface tension decreased with Cu₂S content. The interfacial tension of the Ni-Fe-S and slag system was found to increase with increasing matte grade. The presence of MgO and A1₂O₃ in the slag did not affect the interfacial tension value.     

The performance,kinetics and microbiology of sulfidogenic fluidized-bed treatment of acidic metal- and sulfate-containing wastewater

A sulfidogenic fluidized-bed reactor (FBR) process was developed for treating acidic metal- and sulfate-containing wastewater. The process operating parameters were determined and the bacterial diversity of the FBR was described. The process was based on sulfate reduction by sulfate-reducing bacteria (SRB), precipitation of metals as sulfides with the biogenic H₂S and neutralization of the water with biologically produced bicarbonate alkalinity. The lactate- and ethanol-utilizing FBRs precipitated over 600 mg Zn l^− 1 day^− 1 and 300 mg Fe l^− 1 day^− 1 at a hydraulic retention time of 6.5 h. Metal precipitation was over 99.8% and effluent soluble Zn and Fe concentrations were below 0.1 mg l^− 1. Zinc and iron precipitated predominantly as ZnS, FeS₂ and FeS. The wastewater pH was increased from 2.5–3 to 7.5–8.5 during the treatment. Acetate oxidation was the rate-limiting step in ethanol oxidation. Ethanol oxidation was more affected by sulfide toxicity than was acetate oxidation. The FBR microbial communities contained SRB related to members of the genera Desulfovibrio, Desulfobulbus, Desulfotomaculum, Desulfobacca and Desulforhabdus, and also many species that do not reduce sulfate. The FBR communities contained many previously undescribed bacteria. This study demonstrated the feasibility of the sulfidogenic FBR for the concomitant removal of acidity, metals and sulfate from wastewaters.     

Structural aspects of the manufacture of semiproducts made from titanium nickelide-based alloys

The effects of the charge composition, casting method, and metal forming method on the structure and shape-memory-effect (SME) and superelasticity characteristics of titanium nickelide-based alloys are studied. The shape recovery temperatures of semiproducts are shown to depend substantially on the volume fraction of the Ti₂Ni intermetallic phase, whose formation is stimulated by the oxygen present in a charge or absorbed during casting. An increase in the volume fraction of Ti₂Ni in an alloy leads to nickel enrichment of the B2 phase and a decrease in the shape recovery temperatures. Subsequent metal forming at the stage of semiproduct manufacture only weakly affects the volume fraction of Ti₂Ni and favors the formation of its equiaxed shape and a more uniform Ti₂Ni distribution in the B2 matrix. In alloys where the B2 phase contains more than 56.5 wt % Ni, quenching from temperatures above 600°C and aging in the temperature range 400–500°C result in the dissolution and precipitation, respectively, of the nickel-rich Ti₃Ni₄ and Ti₂Ni₃ intermetallics. Therefore, the shape recovery temperatures of semiproducts and finished products can be controlled. Moreover, as the aging temperature changes, the volume fraction and size of nickel-rich intermetallic particles, the slip stresses, and the SME force characteristics change. For example, to increase the compression forces for osteosynthesis fixation devices, one has to use a titanium nickelide-based alloy with a high nickel content in the B2 phase and to perform aging at low temperatures (400–450°C).     

Phase equilibria in the metal-sulfur-oxygen system and selective reduction of metal oxides and sulfides: Part I. The carbothermic reduction and calcination of complex mineral sulfides

The difference in the standard Gibbs free energy for the formation of any two oxides or sulfides is the chemical potential for selective reduction of metals from complex minerals. The magnitude of the Gibbs free energy difference is shown by plotting the univariant relationships for relevant sulfides and oxides. In this investigation, three examples of mineral sulfides are considered, and the experimental results are compared with the predicted thermodynamic calculations. These examples include the reduction conditions for nickel and iron sulfides and pentlandite (Fe,Ni)₉S₈ and chalcopyrite (CuFeS₂) minerals. The reduction behavior of mineral sulfides, such as those of nickel, cobalt, iron, and copper, is illustrated by referring to both the sulfide and alloy phase equilibria. In particular, the solution thermodynamic properties of the metallic phase equilibria are featured for determining the physical chemistry of preferential or selective reduction of the metal oxides and sulfides. The mechanism for the reduction of the aforementioned sulfide minerals is explained with the aid of the governing phase equilibria for the calcination process. The results from the carbothermic reduction of sulfide minerals are also compared. The important roles of lime and calcium sulfate in controlling the emission of sulfurous gases during the reduction reaction are explained. A qualitative analysis of reduction reactions of nickel and iron sulfides is reviewed to provide a comparison of the mechanism for complex nickel-bearing minerals. The importance of these results in producing alloy and pure metallic phases is also examined.     

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.     

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.     

Improving the precipitation of arsenic trisulfide from washing waters of sulfuric-acid production of copper smelteries

Disadvantages of the sulfide method of purifying washing waters of the sulfuric-acid production are the formation of finely dispersed (particle size from 0.3 to 1.5 μm) difficult-to-filter precipitates of arsenic(III) sulfide and the danger of isolation of hydrogen sulfide into the atmosphere upon an overdose of sodium hydrosulfide. The coagulation of sols of arsenic sulfide is investigated in order to develop more efficient and rapid filtration technology of the precipitate. The filtration rate in various supply modes of sodium hydrosulfide and the dependence of settling rate and filtration on the presence of coagulants such as iron sulfate and aluminum sulfate are determined. It is established that the implementation of the distributed supply of sodium hydrosulfide during the precipitation of arsenic sulfide in combination with the application of such inorganic coagulant as iron(III) sulfate will make it possible to increase the particle size of the As₂S₃ phase by a factor of several times and increase the filtration and settling rates of pulps.     

Lanthanum Modified Ni/y-Al2O3 Catalysts for Partial Oxidation of Methane

La modified Ni/T-Al2O3 catalysts prepared by co-precipitation method using NaOH-Na2CO3 as a precipitator show high activity and selectivity for the partial oxidation of methane （POM）. Meanwhile, the addition of La is beneficial for the formation of an active component and stability of support. We investigated some factors including calcining temperature, nickel content, and space velocity, which turned out to have a strong influence on catalytic activity and selectivity. By XRD and TPR, it is concluded that Ni^0 reduced from amorphous NiAl2O4 is the major active component for POM.