Effects of iron-containing phases on transformation of sulfur-bearing ions in sodium aluminate solution

Sulfides in the high-sulfur bauxite lead to serious steel equipment corrosion and alumina product degradation via the Bayer process, owing to the reactions of sulfur and iron-containing phases in the sodium aluminate solution. The effects of iron-containing phases on the transformation of sulfur-bearing ions (S^2–, S₂O₃^2?, SO₃^2? and SO₄^2?) in sodium aluminate solution were investigated. Fe, Fe₂O₃ and Fe₃O₄ barely react with SO₃^2? and SO₄^2?, but all of them, particularly Fe, can promote the conversion of S₂O₃^2? to SO₃^2? and S^2– in sodium aluminate solution. Fe can convert to Fe(OH)₃^? in solution at elevated temperatures, and further react with S^2– to form FeS₂, but Fe₂O₃ and Fe₃O₄ have little influence on the reaction behavior of S^2– in sodium aluminate solution. Increasing temperature, duration, dosage of Fe, mole ratio of Na₂O_k to Al₂O₃ and caustic soda concentration are beneficial to the transformation of S₂O₃^2? to SO₃^2? and S^2–. The results may contribute to the development of technologies for alleviating the equipment corrosion and reducing caustic consumption during the high-sulfur bauxite treatment by the Bayer process.     

Das anodische und kathodische Verhalten des Eisens in sulfathaltigen Elektrolyten. Chaire d'Electrochimie,Faculté des Sciences,Strasbourg, France

The anodic and cathodic behaviour of iron in sulphate containing electrolytes The formation of Fe₂(SO₄)₃ on passive iron at p_H = 1 appears probable from a thermodynamical point of view. At high SO₄^2? concentrations the equilibrium system contains but low concentrations of Fe³⁺, and no Fe²⁺ ions, a fact showing the relatively elevated stability of the Fe₂(SO₄)₃ layer on passive iron. In slightly acid solution (p_H = 4) the passivity of the iron is determined by iron oxide layers. The formation of FeSO₄ from metallic iron and sulphate ions is restricted to the transpassive zone (p_H 4 to 7), in alkaline solutions even to the active zone. In the p_H region 2 to 14 the passive layer on iron has about the same composition in the systems Fe|H₂O + SO₄^2? and Fe|H₂O.     

Coprecipitation thermodynamics of iron(II-III) hydroxysulphate green rust from Fe(II) and Fe(III) salts

Iron(II-III) hydroxysulphate GR(SO₄²⁻) was prepared by precipitating a mixture of Fe(II) and Fe(III) sulphate solutions with NaOH, accompanied in most cases by iron(II) hydroxide, spinel iron oxide(s) or goethite. Its [Fe(II)]/[Fe(III)] ratio determined by transmission Mössbauer spectroscopy was 2±0.2, whatever the initial [Fe(II)]/[Fe(III)] ratio in solution. Proportion of Fe(OH)₂ increased when the initial [Fe(II)]/[Fe(III)] ratio increased, whereas proportion of α-FeOOH or spinel oxide(s) increased when this ratio decreased. GR(SO₄²⁻) is metastable vs. Fe₃O₄ except in a limited domain around neutral pH. Precipitation from solutions containing both Fe(II) and Fe(III) dissolved species seems to favour GRs formation with respect to stable systems involving iron (oxyhydr)oxides.     

A Refined Scheme of the Mechanism of Iron Anodic Dissolution in Acidic Sulfate Solutions

A refined scheme of iron anodic dissolution in acidic sulfate solution, in which Fe₀ is oxidized to Fe⁺ in stages, is proposed. The first stage consists in chemisorption of H₂O molecules with participation of SO₄ ^2– anions; the degree of the participation x 0.5 per a water molecule. This stage produces adsorbed OH groups (Fe(OH)_ads), which, as a result of their interaction with sulfate anions, form an adsorption complex Fe((OH)SO₄ ^2– _ads. The schemes of oxidation of Fe⁺ to Fe²⁺ do not differ from conventional ones. The scheme put forward agrees with the literature data on the kinetics of iron dissolution under steady-state conditions.     

Effects of sulfur pressure on the sulfidation behavior of 310 stainless steel

High-temperature sulfidation behavior of 310 stainless steel was studied over the temperature range of 700–900°C above a pure sulfur pool with the sulfurvapor range of 10^–4–10^–1 atm. The corrosion kinetics followed the parabolic rate law in all cases. The corrosion rates increased with increasing temperature and sulfur pressure. The scales formed on 310 stainless steel were complex and multilayered. The outer scale consisted of iron sulfide (with dissolved Cr), (Fe, Ni)₉S₈ and chromium sulfides (Cr₂S₃ and Cr₃S₄ with dissolved Fe), while the inner layer was a heterophasic mixture of Cr₂S₃, Cr₃S₄, NiCr₂S₄, and Fe₁–_xS. Platinum markers were found to be located at the interface between the inner and outer scales, suggesting that the outer scale grew by the outward transport of cations (Fe, Ni, and Cr), and the inner scale grew by the inward transport of sulfur. The formation of Cr₂S₃, Cr₃S₄, and NiCr₂S₄ partly blocked the transport of iron through the inner scale, resulting in a reduction of the corrosion rates as compared with the results in the literature.     

Behavior of S2O32- And SO32- In sulfur-bearing aqueous solution system for gold leaching

The concentrations of S₂O₃^2? and SO₃^2? were measured in gold leaching systems, including thiosulfate system, polysulfide system and the modified lime sulfur synthetic solution (ML) system in the process of chemical reaction. The interactions among S₂O₃^2?, SO₃^2? and S^2? were discussed. The behavior mechanism of sulfur-bearing reagents was proposed to describe the process reactions and their Gibbs free energy. The proper quantity oxygen and SO₃^2? reduce decomposition of S₂O₃^2? and react with sulfur derived from the decomposition of S_X^2?. So, SO₃^2? ions have action to stabilize sulfur-bearing system and are favorable to leach gold.     

Chemical studies of polythionic acid stress-corrosion cracking

Low temperature stress corrosion cracking of sensitized stainless steel, promoted by aqueous solutions of polythionates S_xO₆^2?, and related sulfur compounds, particularly thiosulfates, has been a long known problem in the petroleum industry and more recently has cropped up in the nuclear power industry. Membrane permeation results presented here show that S₄O₆^2? and S₂O₃^2? do not function simply as sources of sulfide to promote hydrogen permeation and embrittlement. They do promote anodic dissolution, also generating an unusual adsorption-related anodic spike in voltammetric scans. The oxidation of iron sulfide scales, said to be the source of the substances in petroleum refineries, has been shown to give rise mostly to Fe₂O₃ and elemental sulfur, with low yields of both S₄O₆^2? and S₂O₃^2?.     

Determination of the corrosion products of iron in sea water with and without sulphides

A method was developed to characterize and quantify iron corrosion products in clean and sulphide polluted sea water. This method is based upon a selective dissolution with suitable reagents (methanol, glycine, bromine-methanol, hydrochloric acid etc.) of the various compounds and the subsequent chemical analysis of the various dissolved elements. The information thus obtained is integrated by a diffractometric X-ray analysis. The following corrosion products were found:

• Fe(OH)₂ = Fe₃O₄ ? FeO(OH) ? Fe₂O₃ + unidentified compound Cr (probably oxychloride) in sea water with pH = 8.1 having a 7 ppm D.O. content.

FeOC1 ? Fe(OH)₂ ? Fe₃O₄ ? FeO.OH in partially deoxygenated sea water (D.O. = 3 ppm) at pH 8.1; Fe_0.95S ? Fe(OH)₂ in sea water at pH = 7 with 10 ppm of sulphides; Fe_0.95S ? FeS ? Fe_3.6 · Fe_0.9(O · OH · C1) and FeOC1 ? iron oxisulphide ? Fe₃O₄ ? FeO(OH) ? Fe₂O₃ · H₂O in sea water at pH = 7 and 10 ppm initial sulphides left to oxidate. Since the method of chemical analysis thus developed supplies quantitative information in iron distribution among the various anions and on the various oxidation forms, it is deemed a useful tool for investigation of the corrosion kinetics and mechanism.     

Influence of anions on the formation of artificial steel rust particles prepared from acidic aqueous Fe(III) solution

For simulation of atmospheric corrosion of steels, artificial steel rust particles were prepared in acidic aqueous solutions containing FeCl₃, Fe(NO₃)₃ and Fe₂(SO₄)₃. A single phase α-FeOOH was formed in only Fe(NO₃)₃ system. The β-FeOOH was formed by added Cl⁻ in FeCl₃–Fe(NO₃)₃ system. Adding SO₄²⁻ in Fe(NO₃)₃, FeCl₃ and a mixture of FeCl₃–Fe(NO₃)₃ solutions turned the products following as α- or β-FeOOH → Schwertmannite (Fe₈O₈(OH)₆(SO₄)·nH₂O). Further, increasing the added SO₄²⁻ suppressed the formation of steel rust particles. Accordingly, the influence of anions on the formation of steel rust particles was to be suggested in order of SO₄²⁻ ≫ Cl⁻ > NO₃⁻.     

Identification of corrosion products resulting from accelerated oxidation process

Abstract

Scanning electron microscopy analysis, X-ray powder diffraction and room temperature ⁵⁷Fe Mössbauer spectroscopy were used to identify the corrosion products of uncoated and coated low alloy steels (LAS) and low carbon steels (LCS) resulting from an accelerated steam oxidation test for 180 h at 660°C. From the Mössbauer spectral analysis, it was shown that in all cases, a series of iron compounds such as α-Fe₂O₃, Fe₃O₄, γ-Fe₂O₃, δ-FeOOH, α-FeOOH, Fe(OH)₂ and Fe(OH)₃ were formed, while XRD measurements revealed only the α-Fe₂O₃ and/or Fe₃O₄/γ-Fe₂O₃ phases. In the LAS uncoated sample, an amorphous phase with magnetic features is found. In the spectra of the borided samples and of the uncoated LCS, an additional doublet was observed, which reveals the presence of a superparamagnetic phase. From the relative areas of the subspectra, it is concluded that the boron aluminised sample underwent the lowest degradation. The mechanism proposed for corrosion products formation is based on the dissociation process.     

A new strain Acidithiobacillus albertensis BY-05 for bioleaching of metal sulfides ores

An acidophilic,rod-shaped Gram-negative sulfur oxidizing strain BY-05 was isolated from an acid mine drainage of copper ore in Baiyin area,Gansu Province,China.Ultrastructural studies show that the isolate has a tuft of polar flagella and possesses sulfur granules with clear membrane adhering to the cell innermembrane.Physiological study shows that this isolate grows autotrophically and aerobically by oxidizing S0and reduced inorganic sulfur compounds(SO, 2 23-SO, 2 24- S2 -and ZnS)with the optimum growth at pH 3.5-4.0 and at the temperature range of 25-30℃.The 16S rRNA gene sequence(DQ 423683)of strain BY-05 has 100%sequence similarity to that of Acidithiobacillus albertensis(DSM 14366).So it is identified and named as A. albertensis BY-05.Bioleaching experiments with this new strain show that it can play an important role in recovery of metals from chalcopyrite and sphalerite.     

High-temperature sulfidation of Fe-Nb alloys

The sulfidation behavior of Fe-Nb alloys containing up to 30 w/o Nb was studied over the range of 600–900°C in 0.01 aim. S₂ vapor. All alloys were two-phase, consisting of an Fe-rich solid solution and Fe₂Nb, and followed the parabolic rate law at all temperatures. Scales consisted of two layers-an outer layer of FeS and an inner, complex layer which contained some FeS, FeNb₂S₄ (possibly some FeNb₃S₆), NbS₂, and intermetallic particles which were either completely or only partially sulfidized. Platinum markers were located always at the interface between the two layers, which corresponded to the original metal surface. Activation energies were 18±3 kcal/mol in close agreement with the 19.8 reported for pure iron. The sulfidation rate decreased markedly with increasing Nb content of the alloys. The decrease is attributed to increasing amounts of Fe₂Nb with increasing Nb, the net effect being that the diffusion path for outward iron diffusion through the inner layer is reduced as the Nb content increases. An analysis of the structure of NbS₂ reveals that it is easily intercalated with Fe between loosely bonded layers of S-Nb-S. The S-Nb-S layers are covalently bonded which results in very low diffusivities of either S or Nb in pure NbS₂. Although intercalated Fe tends to change the Van der Waal's type bonding between layers to more ionic or covalent, Fe diffuses readily between the layers in NbS₂. Intercalation of Fe also increases the concentration of sulfur defects in NbS₂, which in turn increases the diffusivity of sulfur. Nb was observed to be immobile. Thus, it is thought that either outward iron diffusion or inward sulfur diffusion in the inner layer is the rate-controlling step, in spite of the close agreement of activation energies with that of the sulfidation of pure iron.     

Photocatalytic performance of nano-photocatalyst from TiO2 and Fe2O3 by mechanochemical synthesis

Nano-particles of homogeneous solid solution between TiO₂ and Fe₂O₃ (up to 10 mol%) have been prepared by mechanochemical milling of TiO₂ and yellow Fe₂O₃/red Fe₂O₃/precipitated Fe (OH)₃ using a planetary ball mill. Such novel solid solution cannot be prepared by conventional co-precipitation technique. A preliminary investigation of photocatalytic activity of mixed oxide (TiO₂/Fe₂O₃) on photo-oxidation of different organic dyes like Rhodamine B (RB), Methyl orange (MO), Thymol blue (TB) and Bromocresol green (BG) under visible light (300-W Xe lamp; λ > 420 nm) showed that TiO₂ having 5 mol% of Fe₂O₃ (YFT1) is 3-5 times higher photoactive than that of P25 TiO₂. The XRD result did not show the peaks assigned to the Fe components (for example Fe₂O₃, Fe₃O₄, FeO₃, and Fe metal) on the external surface of the anatase structure in the Fe₂O₃/TiO₂ attained through mechanochemical treatment. This meant that Fe components were well incorporated into the TiO₂ anatase structure. The average crystallite size and particle size of YFT1 were found to be 12 nm and 30 ± 5 nm respectively measured from XRD and TEM conforming to nanodimensions. Together with the Fe component, they absorbed wavelength of above 387 nm. The band slightly shifted to the right without tail broadness, which was the UV absorption of Fe oxide in the Fe₂O₃/TiO₂ particle attained through mechanochemical method. This meant that Fe components were well inserted into the framework of the TiO₂ anatase structure. EPR and magnetic susceptibility show that Fe³⁺ is in low spin state corresponding to μ_B = 1.8 BM. The temperature variation of μ_B shows that Fe³⁺ is well separated from each other and does not have any antiferromagnetic or ferromagnetic interaction. The evidence of Fe³⁺ in TiO₂/Fe₂O₃ alloy is also proved by a new method that is redox titration which is again support by the XPS spectrum.     

Effect of SO<Subscript>4</Subscript> <Superscript>2-</Superscript>, Cl<Superscript>–</Superscript> and NO<Subscript>3</Subscript> <Superscript>-</Superscript> anions on the formation of iron oxide nanoparticles via microwave synthesis

In the present work iron oxide nanoparticles have been prepared by microwave assisted synthesis with the influence of different precursor salts and synthesis of magnetite, hematite, Iron oxide hydroxide and maghemite nanoparticles. Synthesized iron oxide nanoparticles were characterized with Infrared Spectroscopy (FTIR), Field Emission Scanning Electron Microscopy (FESEM), and Energy-dispersive X-ray Spectroscopy (EDX). XRD measurements show that the peaks of diffractogram are in agreement with the theoretical data of magnetite, hematite, FeO(OH) (Iron oxide hydroxide) and maghemite. Crystallite size of the particles was found to be 33, 45, 36 and 43.5 nm for Fe₃O₄, α-Fe₂O₃, FeO(OH) and γ-Fe₂O₃. FESEM studies indicated that size of the particles is observed in the range of about 19.4 to 46.7 nm (Fig. 2a, average 32 nm), 29.1 to 67.6 nm (Fig. 2b average 45 nm), 29.1 to 40.8 (Fig. 2c average 36.6 nm), 29.1 to 80 nm (Fig. 2d average 43.5) for Fe₃O₄, α-Fe₂O₃, FeO(OH) and γ-Fe₂O₃ respectively. EDX spectral analysis reveals the presence of carbon, oxygen, iron in the synthesized nanoparticles. The FTIR graphs indicated absorption bands due to O–H stretching, C–O bending, C–H stretching and Fe–O stretching vibrations.     

Oxidation behavior of NixFe1−x(OH)2 in Cl-containing solution

The addition of Ni leads to the formation of protective rust layer on steel and subsequently high corrosion resistance of steel in Cl⁻-containing environment. α-FeOOH, β-FeOOH, γ-FeOOH and Fe₃O₄ are formed mainly on steels exposed to Cl⁻-containing atmosphere. It is expected that systematic investigation of the effect of Ni(II) on the formation process of each oxide in solution should lead to elucidation of the role of Ni in the formation of anticorrosive oxide layer. This study reports the oxidation behavior of Ni_xFe_1−x(OH)₂ in Cl⁻-containing solution at two different pH regions (condition I under which solution pH is allowed to decrease and condition II under which solution pH is maintained at 8) where γ-FeOOH and Fe₃O₄ are predominantly formed, respectively, upon the oxidation of Fe(OH)₂. In the presence of Ni(II) in the starting solution, the formation of Ni(II) doped β-FeOOH with very low crystalline was facilitated and the formation of γ-FeOOH was suppressed with increasing Ni(II) content and with increasing oxidation rate of Fe(II). Ni(II) was found to have Fe₃O₄-suppressing effect under condition II.     

The corrosion behavior of Fe-Nb-Al alloys in H2/H2O/H2S atmospheres

The corrosion behavior of eight Fe-Nb-Al ternary alloys was studied over the temperature range 700–980°C in H₂/H₂O/H₂S atmospheres. The corrosion kinetics followed the parabolic rate law for all alloys at all temperatures. The corrosion rates were reduced with increasing Nb content for Fe-x Nb -3Al alloys, the most pronounced reduction occurred as the Nb content increased from 30 to 40 wt.%. The corrosion rate of Fe-30Nb decreased by six orders of magnitude at 700°C and by five orders of magnitude at 800°C or above by the addition of 10 wt.% aluminum. The scales formed on low-Al alloys (3 wt.% Al) were duplex, consisting of an outer layer of iron sulfide (with Al dissolved near the outer-/inner-layer interface) and an inner complex layer of Fe_xNb₂S₄(FeNb₂S₄ or FeNb₃S₆), FeS, Nb₃S₄ (only detected for Nb contents of 30 wt.% or higher) and uncorroded Fe₂Nb. No oxides were detected on the low-Al alloys after corrosion at any temperature. Platinum markers were found to be located at the interface between the inner and outer scales for the low-Al alloys, suggesting that the outer scale grew by the outward transport of cations (Fe and Al) and the inner scale grew by the inward transport of sulfur. The scales formed on high-Al alloys (5 wt.% Al) were complex, consisting primarily of Nb₃S₄, Al₂O₃ and (Fe, Al)_xNb₂S₄, and minor amounts of (Fe, Al)S and uncorroded intermetallics (FeAl and Fe₂Nb). The formation of Nb₃S₄ and Al₂O₃ blocked the transport of iron through the inner scale, resulting in the significant reduction of the corrosion rates.     

Characteristics of iron corrosion scales established under blending of ground, surface, and saline waters and their impacts on iron release in the pipe distribution system

Interior scales on PVC, lined ductile iron (LDI), unlined cast iron (UCI) and galvanized steel (G) were analyzed by XRD, RMS, and XPS after contact with varying water quality for 1 year. FeCO₃, α-FeOOH, β-FeOOH, γ-Fe₂O₃, Fe₃O₄ were identified as primary UCI corrosion products. No FeCO₃ was found on G. The order of Fe release was UCI > G ? LDI > PVC. For UCI, Fe release decreased as % Fe₃O₄ increased and as % Fe₂O₃ decreased in scale. Soluble Fe and FeCO₃ transformation indicated FeCO₃ solid was controlling Fe release. FeCO₃ model and pilot data showed Fe increased as alkalinity and pH decreased.     

Corrosion resistance of Fe2O3-Cr2O3 artificial passive films formed with LP-MOCVD

Fe₂O₃-Cr₂O₃ artificial passive films were formed with a low pressure MOCVD technique using iron (III) acetylacetonate and chromium (III) acetylacetonate. The relationships between the crystal structure, the chemical state of the constituent elements, and the corrosion resistance of the films were examined in acid solutions. The films deposited above 300°C hardly dissolved in 1.0 M HCl and those deposited below 250°C, however, easily dissolved in the same solution. The dissolution rate of the films in solution increased with decreasing substrate temperature. When polarized cathodically in 1.0 M H₂SO₄, the films deposited below 250°C dissolved due to the reduction of the Fe₂O₃ component in the films. The reduction of the Fe₂O₃ component was, however, suppressed on the films deposited above 300°C. Therefore, with increasing crystallinity and the amount of M-O type chemical bonds, the corrosion resistance of the films increases in HCl and H₂SO₄ solutions.     

Hydrometallurgical recovery of zinc from industrial hot dipping top ash

Top ash from hot-dip galvanizing plant was investigated as a source of secondary zinc to be returned to galvanizing bath. The waste material contained 63% Zn as metallic, oxide and hydroxychloride phases. It was leached in H₂SO₄ solutions (20% and 25%) at various bath loadings (100−300 g/L). Leaching behaviors of zinc, manganese, iron and chloride ions were investigated. A few strategies of iron elimination from leaching liquors were examined. Flocculant addition was harmful for subsequent filtration of iron precipitates due to increased viscosity of solution, while a combination of zinc oxide and calcium carbonate for rising pH resulted in the formation of dense suspension unenforceable to separate from zinc sulphate solution. Zinc electrowinning was carried out at different pH (from −0.5 to 2.8) using a range of current densities (3−10 A/dm²). Optimal conditions for pure metal recovery were: leaching in 20% H₂SO₄ solution at zinc ash content 100−150 g/L, Fe₂O₃·xH₂O precipitation using H₂O₂ and CaCO₃, zinc electrowinning at pH of 0.1−1.0 at 3−6 A/dm². Correlations between pH and free H₂SO₄ concentration in electrolyte solutions were also discussed. pH−acid concentration dependence for zinc electrolyte was between experimental and calculated curves for pure H₂SO₄ solutions, while the curve was shifted towards lower pH if ferric ions were in the solution.     

Synthesis of novel rare earth—Iron oxide chalcogenides with the La2Fe2O3Se2 structure

Our searches for new oxide chalcogenides of rare earths and Fe, Co, Ni, and Zn resulted in preparation of two new compounds Ce₂Fe₂O₃S₂ and Pr₂Fe₂O₃S₂ which are isostructural to La₂Fe₂O₃Se₂ and Sm₂Ti₂O₃Sb₂. Crystal structures of the new compounds Ce₂Fe₂O₃S₂ and Pr₂Fe₂O₃S₂ were determined from powder X-ray diffraction data. Magnetic measurements were performed for Ce₂Fe₂O₃S₂ and revealed behavior very similar to that of isostructural oxide chalcogenides of iron and pnictides of titanium. In particular, no superconductivity was observed down to 4 K. Crystal chemical factors determining the stability of the La₂Fe₂O₃Se₂ structure type are discussed.