Mechanism of oxidation-sulfation reactions of coo in the presence of na2so4

Sulfation studies were conducted on CoO samples coated with a thin film of Na₂SO₄ and exposed at 600 to 900 °C in O₂-SO₂-SO₃ environments containing 0.02 to 2 pct (SO₂+SO₃). Within this range of conditions, Co₃O₄ (s), CoSO₄ (s), and liquid Na₂SO₄-CoSO₄ solutions were observed as reaction products. At lower concentrations of SO₃ where CoSO₄ (s) was unstable, the reaction product consisted of a thick Co₃O₄ layer close to the CoO, a layer of Na₂SO₄-CoSO₄ liquid, and large Co₃O₄ particles at the gas/salt interface. At higher concentrations of SO₃, the reaction procuct was a two phase mixture of solid CoSO₄ and an Na₂SO₄-CoSO₄ liquid. In gas mixtures containing 0.15 pct and 1 pct (SO₂+SO₃), the highest reaction rates were observed at about 750 to 800 °C. Some of these results are quite similar to those observed during low temperature hot corrosion of cobalt-base alloys. The overall reaction mechanism has been described in terms of two processes: oxidation of CoO to Co₃O₄ and sulfation of cobalt oxides.     

Low Temperature Hot Corrosion of Cobalt-Base Alloys: PartI. Morphology of the Reaction Product

Accelerated oxidation tests have been conducted on a number of Co-Cr, Co-Al, and Co-Cr-Al alloys coated with a thin film of Na₂SO₄ and exposed at 600 to 750 °C in O₂-SO₂-SO₃ environments containing 0.0095-5 pct (SO₂ + SO₃). Generally, Co-Cr and Co-Cr-Al alloys reacted nonuniformly, usually in the form of pits, and Co-Al alloys suffered broad frontal attack. The morphology of the reaction product was observed to be dependent on temperature andP _SO3 Under all conditions, a thin sulfur-rich band containing sulfides was observed at the alloy/scale interface, and cobalt dissolved near this interface and formed Co₃O₄ and/or CoSO₄(s) in different regions of the reaction product.     

Kinetics of dissolution of cobalt oxides in acidic media

The kinetics of dissolution of cobalt oxides Co₂O₃ and Co₃O₄ in aqueous solutions of acids (H₂SO₄, EDTA) is experimentally studied. Dissolution rate W increases with the temperature or the EDTA concentration. The reaction orders of dissolution for hydrogen ions in sulfuric acid and EDTA (dlogW/dpH = 0.5 ± 0.1) and for anions (dlogW/dlog[An ⁻] = 0.5 ± 0.1) are determined. A specific feature of the dissolution kinetics in EDTA is a maximum in the dissolution rate of the cobalt oxides at pH −1. The activation energy of the process E _a is 70 kJ/mol in H₂SO₄ and 60 kJ/mol in EDTA. The modeling of the process shows that the CoOH⁺ ion is a surface particle controlling the dissolution rate in mineral acids and the CoHY⁻ ion, in the complexone.     

Kinetics of the dissolution of cobalt, nickel, and iron oxides in sulfuric acid

The modeling of dissolution of cobalt, nickel, and iron oxides in sulfuric acid shows that the rate-determining step of the dissolution is the passage of oxide complexes formed on the surface of oxide particles to a solution. A system analysis of the dissolution curves (??-??) of the oxides is developed to calculate the kinetic parameters ( $n_{H^ + } $ ,E _a). Cobalt oxide Co₃O₄ dissolves in H₂SO₄ more rapidly than nickel and iron oxides (Ni₂O₃ and Fe₂O₃).     

Leaching of metallic silver with aqueous ozone

The recycling of silver from metallic scraps can be performed through O₃ leaching at an ambient temperature and low (∼0.1 M) H₂SO₄ concentration. The main by-product is O₂, which can be recycled to the O₃ generation or used as leaching agent in a pretreatment step. The stoichiometry and the effects of the stirring speed, ozone and acid concentration and temperature on the leaching of silver were investigated. Silver dissolved as Ag²⁺_(aq) in the range 10⁻³–1 M H₂SO₄, but for pH ≥4, insoluble Ag₂O₂ was the main reaction product. Kinetics appeared to be controlled by mass transfer of O_3(aq) to the solid–liquid interface, showing first order dependency with respect to [O₃]_aq and P_O3. Specific rates were only slightly dependent on the temperature in the interval 10–50 °C, but decreased at 60 °C due to the fall in O₃ solubility. The mass transfer coefficients showed an average activation energy of 17 kJ/mol. No significant effect of [H₂SO₄] on mass transfer coefficients was observed for 10⁻²–1 M. Leaching rate gradually diminished for pH >2, as a consequence of the influence of the [H⁺] in the transport control.     

Dissolution of bornite in sulfuric acid using oxygen as oxidant

Leaching of natural bornite in a sulfuric acid solution with oxygen as oxidant was investigated using the parameters: temperature, particle size, initial concentration of ferrous, ferric and cupric ions, and using microscopic, X-ray and electronprobe microanalysis to characterize the reaction products. Additionally, stirring rate, pH and P_O2 were varied. Dissolution curves for percent copper extracted as a function of time were sigmoidal in shape with three distinct periods of reaction: induction, autocatalytic and post-autocatalytic which levelled off at 28% dissolution of copper. The length of the induction period was not reproducible, causing the dissolution curves to be shifted with respect to time. The dissolution curves in the autocatalytic and post-autocatalytic regions were reproducible, and this property was utilized to treat much of the kinetic data. The iron dissolution curves had four dissolution regions. An initial small but rapid release of iron to solution preceded the three periods just given for copper dissolution. Aside from this initial iron release, the iron and copper dissolution curves were almost identical.Stirring rate had no effect on dissolution of copper above 400 min^?1 nor did oxygen flow rate in the range 20–40 cm³/min. Dissolution rate was slightly dependent on oxygen partial pressure for P_O2 < 0.67. Hydrogen ion concentration had no effect except that sufficient acid was required to prevent hydrolysis and precipitation of iron salts.The dissolution rate was directly dependent on the reciprocal of particle diameter indicating possible surface chemical reaction control, but the activation energy of 35.9 kJ/mol (8.58 kcal/mol) for the autocatalytic region of copper dissolution is slightly too small for that, though not unreasonable. Initial addition of Fe²⁺ had a rather complex effect and markedly enhanced dissolution of copper, as also did initial addition of Fe³⁺. Microscopic analysis showed nuclei of two new phases, covellite and Cu₃FeS₄, in the induction region. The new phases grow rapidly in the autocatalytic stage, which is controlled by nuclei formation and chemical reaction. The post-autocatalytic region is characterized by complete transformation of bornite into covellite on the particle surfaces and Cu₃FeS₄ as an internal product with an X-ray spectrum very similar to that of chalcopyrite. The post-autocatalytic region is controlled by autocatalytic growth of newly formed phases. Further reaction beyond the autocatalytic region (percent copper dissolution > 28%) occurs so slowly with oxygen as oxidant that it was not studied.The rate of copper dissolution appears to be controlled by the rate of iron dissolution. Using that and the other experimental evidence a mechanism for reaction is proposed in which iron-deficient bornite, Cu₅Fe_?S₄, is formed on the surface by initial preferential iron dissolution. Labile Cu⁺ diffuses into this from Cu₅Fe_?SO₄ and unreacted bornite to produce CuS on the surface. Depletion of labile Cu⁺ ions from Cu₅FeS₄ produces Cu₃FeS₄ in the interior of the mineral particles.     

Leaching of enargite in H2SO4–NaCl–O2 media

This paper discusses the leaching of enargite (Cu₃AsS₄) in sulfuric acid–sodium chloride solution using oxygen as oxidant at atmospheric pressure. The dissolution of arsenic from enargite in this medium proceeds with elemental sulfur as a reaction product according to:2Cu₃AsS₄ + 6H₂SO₄ + 5.5O₂ → 6CuSO₄ + 2H₃AsO₄ + 8S° + 3H₂OThe dissolution rate of arsenic was found to be very slow. About 6% of arsenic was dissolved in 7 h of leaching at 100 °C in a solution containing 0.25 M H₂SO₄ and 1.5 M NaCl under a flow of oxygen of 0.3 l/min.The kinetics of arsenic dissolution is well represented by a shrinking core model for spherical particles controlled by surface reaction. An apparent energy of activation of 65 kJ/mol was obtained for the temperature range 80 to 100 °C.     

The relationship between relative oxide ion content of Na2SO4, the presence of liquid metal oxides and sulfidation attack

According to the “oxide ion” theory, sulfidation attack does not occur until oxide ions present in the fused Na₂SO₄ melt react with the normally protective oxide scale. It has already been shown that chromia reacts with and decreases the oxide ion content of sodium sulfate and inhibits sulfidation attack. Based upon the results reported herein, the reduction of the oxide ion content of sodium sulfate is a necessary but not sufficient condition for sulfidation inhibition. It is shown that the oxides of molybdenum as well as vanadium react with and decrease the oxide ion content of Na₂SO₄. It is shown that the addition of either Mo or V to nickel imparts sulfidation resistance. However, it is also shown that whereas the addition of Cr₂O₃ to Na₂SO₄-coated nickel-base superalloys prevents or inhibits sulfidation attack, no beneficial effects are noted when either MoO₃ or V₂O₅ are codeposited with Na₂SO₄ onto nickel-base superalloy substrates. The reactions between V₂O₅ with metal oxides were also studied. V₂O₅ readily fluxes Al₂O₃ and slowly reacts with Na₂SO₄. The relationship between accelerated oxidation, oxide ion content of a fused melt and the fluxing of the normally protective oxide scale by liquid metal oxides is discussed.     

The formation of volatile corrosion products during the mixed oxidation-chlorination of cobalt at 650 °C

The reaction of cobalt with 1 pct Cl₂ in 1,10, and 50 pct O₂/Ar atmospheres has been studied at 650 °C with thermogravimetry and mass spectrometry. The principal vapor species appear to be CoCl₂ and CoCl₃. In all cases, CoCl₂ (s) forms at the oxide/metal interface and equilibration of the volatile chlorides with Co₃O₄ does not occur in the early stages of the reaction. In the 1 pct C1₂-1 pct O₂-Ar case, continuous volatilization occurs. In the 1 pct C1₂-10 pct O₂-Ar and 1 pct C1₂-50 pct O₂-Ar cases, volatilization occurs only in the first few minutes of reaction. Afterwards, the reaction is predominantly oxidation.     

Leaching of bornite in acidified ferric chloride solutions

In an acidified ferric chloride solution, bornite leaches in two stages of reaction with the first being relatively much more rapid than the second; the first terminates at 28 pct copper dissolution. The first-stage dissolution reaction is electrochemical and is mixed kinetics-controlled; ferric-ion transfer through the solution boundary layer and reduction on the surface to release Cu²⁺ into solution are both important in controlling the rate. The concentration of labile Cu⁺ in the bornite lattice governs the potential of the surface reaction, and, once Cu⁺ is depleted from the original bornite, stage-I reaction ceases. The solid reaction intermediate formed is Cu₃FeS₄. Minute subcrystallites formed at the latter part of stage I leach topochemically in stage II. This reaction which commences at 28 pct Cu dissolution is characterized by a change in mechanism at about 40 pct copper dissolution, though the overall chemical equation for reaction is unchanged in stage II; cupric and ferrous ions and sulfur as a solid residue are products of reaction. The region 28 to about 40 pct Cu dissolution is designated as a transition period to stage-II reaction. Reaction rate in this period is interpreted as being controlled by reduction of Fe³⁺ on active product sulfur surface sites, and hence the reaction rate is controlled by the rate of nucleation and growth of sulfur on the Cu₃FeS₄ intermediate surfaces. Strain in the Cu₃FeS₄ crystal lattice is released during this period by diffusion from the lattice of Cu⁺ remaining from the labile copper initially present in the bornite. After about 40 pct Cu dissolution the rate of reaction is controlled by diffusion through the fully formed sulfur layer in an equiaxial geometrically controlled reaction.     

Dissolution studies of mechanically activated Manavalakurichi ilmenite with HCl and H2SO4

The effect of the change in phase constitution, particle size distribution, surface area, crystallite size, strain and lattice parameters introduced by mechanical activation of the altered beach sand ilmenite from Manavalakurichi region, India on the dissolution kinetics of HCl and H₂SO₄ was investigated. The altered ilmenite showed different physico-chemical characteristics and was found to be more resistant to acid leaching than the less altered ilmenite from the Chatrapur beach sands, India investigated earlier. The dissolution behavior was also different in H₂SO₄ and HCl. For sulfuric acid leaching, the dissolution of Fe and Ti increased monotonically with time of milling and showed a continuous increase with time of leaching, whereas hydrolysis of titanium occurs in HCl medium, especially for the activated samples at lower acid concentration, lower solid to liquid ratio and higher temperature leading to lower solution recoveries. The dissolution kinetics in both H₂SO₄ and HCl prior to hydrolysis conforms initially to the reaction rate control model and for higher leaching times to the shrinking core model where diffusion through the product layer is rate controlling. It is postulated that the anatase formed by hydrolysis in milled samples impedes the further progress of leaching. The activation energies for the dissolution of Fe and Ti decreased with time of milling and were marginally lower in HCl than in H₂SO₄. An attempt has also been made to correlate the decrease in activation energy to the increase in the energy input to the material through high-energy milling. The relative contribution of the increase in surface area and structural disorder on the enhancement of the dissolution rates has been evaluated.     

Modification of Inclusions in Molten Steel by Mg-Ca Transfer from Top Slag: Experimental Confirmation of the ‘Refractory-Slag-Metal-Inclusion (ReSMI)’ Multiphase Reaction Model

High-temperature experiments and Refractory-Slag-Metal-Inclusion (ReSMI) multiphase reaction simulations were carried out to determine the effect of the ladle slag composition on the formation behavior of non-metallic inclusions in molten steel. Immediately after the slag-metal reaction, magnesium migrated to the molten steel and a MgAl₂O₄ spinel inclusion was formed due to a reaction between magnesium and alumina inclusions. However, the spinel inclusion changed entirely into a liquid oxide inclusion via the transfer of calcium from slag to metal in the final stage of the reaction. Calcium transfer from slag to metal was more enhanced for lower SiO₂ content in the slag. Consequently, the spinel inclusion was modified to form a liquid CaO-Al₂O₃-MgO-SiO₂ inclusion, which is harmless under steelmaking conditions. The modification reaction was more efficient as the SiO₂ content in the slag decreases.     

The function of sodium chloride in sulphation roasting

Abstract

The reactions that occur when Co₃O₄ is sulphated. By sulphur trioxide in the presence of sodium sulphate and/or sodium chloride have been studied by differential thermal analysis. Sodium chloride forms a eutectic with sodium sulphate, lowering its melting point to a temperature within the roasting range. This facilitates the dissolution of SO₃ in the melt and thus promotes the formation of sodium pyrosulphate the sulphation agent. In the absence of added sodium sulphate, sodium chloride reacts with sulphur trioxide to produce Na₂SO₄, which is, in turn, converted to sodium-pyrosulphate.

Résumé

Les réactions se déroulant lors de sulfatation du Co₃O₄ par l'anhydride sulfureux en présence de sulfate de sodium ou de chlorure de sodium ont été étudiees par analyse thermique différentielle. Le chlorure de sodium forme un eutectique avec le sulfate de sodium, abaissant son point de fusion jusqu'à une température située dans l'intervalle de température du grillage. Ceci facilite la dissolution du SO₃ dans ce liquide et ainsi favorise la formation de pyrosulfate de sodium, l'agent sulfatant. Sans sulfate de sodium, le chlorure de sodium réagit avec le SO₃ pour former du Na₂SO₄, qui est, à son tour, transformé en pyrosulfate de sodium.     

Pyrite oxidation with ozone: stoichiometry and kinetics

One of the most frequent causes of refractoriness in precious metals leaching is their occlusion or fine dissemination into a pyritic matrix. This study experimentally explores the acid leaching of pyrite with ozone, suggests the stoichiometry of the reaction, estimates its activation energy and defines the effect of the main variables on the leaching kinetics. The results of stoichiometry tests allow establishing that one mole of pyrite requires 7.7 moles of ozone to produce one mole of ferric ion and 2 moles of HSO₄? ions. A decrease in the particle size, solution pH and solids’ concentration of the leaching system increases pyrite dissolution. The type of acid (nitric, sulphuric and hydrochloric) does not affect pyrite dissolution rate. Up to 60% of pyrite is dissolved when the optimal experimental conditions are employed (1?g pyrite (?25?µm), 800?mL of 0.18?M of H₂SO₄, 800?rev?min^?1, 1.2?L?min^?1 gas stream O₂/O₃ with 0.079?g O₃?L^?1 and 25°C). The apparent activation energy of the pyrite-ozone reaction is 14.92?kJ?mol^?1, and the absence of a passive layer on the pyrite surface and the linearity of the dissolution profiles suggest that the dissolution kinetics is controlled by the chemical reaction.     

Refractory–Slag–Metal–Inclusion Multiphase Reactions Modeling Using Computational Thermodynamics: Kinetic Model for Prediction of Inclusion Evolution in Molten Steel

The refractory–slag–metal–inclusion multiphase reaction model was developed by integrating the refractory–slag, slag–metal, and metal–inclusion elementary reactions in order to predict the evolution of inclusions during the secondary refining processes. The mass transfer coefficient in the metal and slag phase, and the mass transfer coefficient of MgO in the slag were employed in the present multiphase reactions modeling. The “Effective Equilibrium Reaction Zone (EERZ) Model” was basically employed. In this model, the reaction zone volume per unit step for metal and slag phase, which is dependent on the ‘effective reaction zone depth’ in each phase, should be defined. Thus, we evaluated the effective reaction zone depth from the mass transfer coefficient in metal and slag phase at 1873 K (1600 °C) for the desulfurization reaction which was measured in the present study. Because the dissolution rate of MgO from the refractory to slag phase is one of the key factors affecting the slag composition, the mass transfer coefficient of MgO in the ladle slag was also experimentally determined. The calculated results for the variation of the composition of slag and molten steel as a function of reaction time were in good agreement with the experimental results. The MgAl₂O₄ spinel inclusion was observed at the early to middle stage of the reaction, whereas the liquid oxide inclusion was mainly observed at the final stage of the refining reaction. The content of CaO sharply increased, and the SiO₂ content increased mildly with the increasing reaction time, while the content of Al₂O₃ in the inclusion drastically decreased. Even though there is slight difference between the calculated and measured results, the refractory–slag–metal multiphase reaction model constructed in the present study exhibited a good predictability of the inclusion evolution during ladle refining process.     

Kinetics of evolution of SO2 from hot metallurgical slags

The kinetics of the evolution of SO₂ gas from a liquid synthetic blast furnace -type slag (CaO-Al₂O₃-SiO₂) in an atmosphere of O₂ + Ar gas at a total pressure of 1 atm for 0.003 ≤ Po₂ ≤ 1.00 atm has been studied in the range 1360 to 1460°C. The process has been followed by collecting and analyzing the SO₂ as it forms, and also by observing the change in weight of the slag sample with time. The effect of slag composition has also been studied. For partial pressures of oxygen less than about 0.1 atm, the rate is very rapid and is controlled by transport in the gas phase. At greater values of Po₂, the rate is much slower and is controlled by a chemical process. In the high Po₂ region, the process is half-order with respect to the concentration of sulfur in the slag. This half-order dependence on sulfur concentration in the slag may be explained by an initial fast irreversible reaction to form two intermediate species which then decompose at equal rates to give the final products. Additions to the slag of iron or manganese oxides greatly accelerate the rate of evolution of SO₂ at Po₂ = 1.00 atm. This is interpreted to mean that a charge transfer process, possibly involving S^2- and O^2- ions, is rate-controlling at Po₂ = 1 atm. It is also apparent that Fe²⁺ and Fe³⁺ (or Mn²⁺ and Mn³⁺) ions can act as charge carriers. Some measurements with actual industrial blast furnace slags are also reported. Formerly Visiting Scientist, Massachusetts Institute of Technology     

A study of the dissolution of Al2O3, MgO and MgAl2O4 particles in a CaO‐Al2O3‐SiO2 slag

The dissolution behaviour of alumina, MgO and MgAl₂O₄ particles in a 36%CaO‐21%Al₂O_3‐42%SiO₂ (mass contents in %) slag has been studied using a confocal scanning laser microscope in the temperature range of 1470 ‐ 1550 °C. The double hot thermocouple technique and metallographic examination was also used to characterize the reaction product between the particles and the slag. The total dissolution times for MgO and MgAl₂O₄ particles decreased with increased temperature and were almost identical (approximately 200 s for a 150 μm diameter particle at 1500 °C.). This phenomenon can be explained by the formation of MgAl₂O₄ layer on the surface of the MgO particle during dissolution. Alumina dissolution times were slightly longer (approximately 250 s for a 150 μm diameter particle at 1500 °C) than that of MgO and MgAl₂O₄ particles and no reaction layer was observed on the surface of the particles.     

Corrosion in atmospheric fluidized bed combustors—The reactions of CaSO4 with Cr,Ni, Co,Fe, and several alloys

When coal is burned in the presence of limestone in an atmospheric fluidized-bed combustor (AFBC), the sulfur emission can be kept below acceptable EPA levels. Calcining of the limestone produces CaO, which then forms solid CaSO₄ by a reaction with the SO₂ produced during coal combustion. The internal components (e.g., heat exchanger tubes) of the bed, however, become coated with a compact layer of CaSO₄, CaO, and ash during combustion. It has been suggested that the presence of the sulfate on these hot metal surfaces is the cause of observed instances of accelerated oxidation-sulfidation. This paper presents results which support the above suggestion. The reactions between Cr, Ni, Co, Fe, alloy 800, 2.25 Cr-1 Mo, 9 Cr-1 Mo steels, or 304 stainless steel with CaSO₄ were studied using differential thermal and thermogravimetric analyses. The reaction products were analyzed using X-ray diffraction, optical microscopy, and in some instances, X-ray energy dispersive analyses. The chromium-calcium sulfate reaction is the only case studied in which a sulfide is not formed. In that case, CaCr₂O₄ is the reaction product. In all other cases, the reactions are oxidation-sulfidation processes.     

The effect of salt-phase composition on the rate of soda-ash roasting of chromite ores

The formation of a liquid phase during the early stages of the roasting reaction is a common problem in the sodium chromate manufacturing process. The molten salt phase, which is primarily constituted of a binary mixture of Na₂CrO₄ and Na₂CO₃, creates major operational problems such as the granulation and blocking of the kilns. In addition to the operational problems, it was observed that the molten salt also affects the transport of oxygen toward the reaction interface. The mechanism of the soda-ash roasting reaction has been analyzed for improving the yield of sodium chromate. It was observed that the conversion efficiency of the roasting process changed dramatically, depending on the origin and the type of the chromite ores used. Thermal and scanning electron microscopic analyses of the products of roasting were carried out to establish the reaction mechanism. It was observed that the presence of silicates in the chromite ores interferes with the formation of sodium chromate involving the binary Na₂CO₃-Na₂CrO₄ liquid. The roasting reaction proceeds in a certain crystallographic direction in the chromite spinel in the presence of a nonsilicate molten salt, whereas a complete dissolution of chromite appears to take place in the binary liquid containing silicate phases present in the ore. This article is based on a presentation given in the Mills Symposium entitled “Metals, Slags, Glasses: High Temperature Properties & Phenomena,” which took place at The Institute of Materials in London, England, on August 22–23, 2002.     

The action of sulfur trioxide on chalcopyrite

Chalcopyrite reacts readily with SO₃ at about 100°C to form water-soluble sulfates; the reaction is approximately: 3CuFeS₂+26SO₃→3CuSO₄+FeSO₄+Fe₂(SO₄)₃+25SO₂ The presence of about 4 pct O₂ in the gas phase greatly accelerates the reaction presumably due to the complete transformation of ferrous into ferric sulfate in an extremely porous form: 2CuFeS₂+17SO₃+1/2O₂→2CuSO₄+Fe₂(SO₄)₃+16SO₂ A stoichiometric mixture of SO₂+1/2O₂ behaves towards chalcopyrite in nearly the same way as SO₃ although only in the temperature range 350° to 700°C.