Mineralogical changes occurring during the ferric lon leaching of bornite

The reaction products formed during the leaching of bornite in either ferric chloride or ferric sulfate media depend on the leaching conditions as well as the particle size of the bornite. The extent of dissolution is always more vigorous in the ferric chloride system and increases with increasing temperature in either system. The reaction initially involves the rapid outward diffusion of copper to form slightly nonstoichiometric bornite (Cu_5-xFeS₄), chalcopyrite, and covellite. The non-stoichiometric bornite is progressively converted to a Cu₃FeS₄ phase, which varies considerably in its composition, and to covellite. Although the reaction at low temperature terminates at the Cu₃FeS₄ phase, leaching at higher temperatures results in further dissolution to elemental sulfur and soluble Cu²⁺ and Fe²⁺. The leaching ofmassive bornite illustrates the complexities of the leaching reaction more clearly than is observed for the finelypaniculate bornite. In leached massive bornite, a distinct covellite zone appears in the Cu₃FeS₄ phase; as well, chalcopyrite exsolution lamellae rimmed by a copper sulfide (possibly digenite) appear in the covellite zone, in the Cu₃FeS₄ phase, and in the nonstoichiometric bornite. The experimental leaching results, especially those involving massive bornite, are generally consistent with the mineralogical trends produced by supergene alteration of bornite ores, but a significant difference is that the Cu₃FeS₄ phase does not correspond closely to the mineral idaite.     

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.     

Oxidation of dense iron sulfide

Oxidation of stoichiometric iron sulfide was investigated. Rectangular plates of dense FeS were oxidized in an Ar-O₂ gas mixture at 1023 to 1123 K. Oxygen partial pressure was varied between 1.01 × 10³ and 2.03 × 10⁴ Pa. During the initial five minutes of oxidation, a magnetite layer of about 10 μm in thickness was formed on the surface without the evolution of SO₂ gas. Diffusion of iron from the interior of the sulfide to the sulfide/magnetite interface controlled the oxidation rate. Mass transfer through the gaseous boundary layer at the sample surface also affects the oxidation rate at lower oxygen partial pressures. Following this rapid formation of magnetite, the magnetite layer continued to grow for several hours in accordance with the parabolic rate law. Diffusion of iron through the magnetite layer controlled the oxidation rate during this stage. A thin layer of hematite was also formed on the outer surface of magnetite. When the composition of the inner sulfide core reached Fe_0.9S, the oxidation proceeded irregularly into the interior of the remaining sulfide. Porous oxide was formed and SO₂ gas was evolved. Former Graduate Student     

Oxidation of molten copper matte

An amount of 80 mg of molten copper matte of a pseudo-ternary Cu₂S-FeS-Fe system contained in a slender alumina sample tube was oxidized at 1503 and 1533 K in a mixed O₂-Ar gas stream and the oxidation path was followed, comparing the overall rate of oxidation with the gaseous diffusion in the sample tube. The following successive reactions were found to be controlled by gas diffusion. Initially, Fe was oxidized to form FeO. After the melt composition reached a pseudo-ternary Cu₂S-FeS-FeO system, FeS was oxidized to form FeO. As the amount of FeO increased, Fe₃O₄ was also formed and subsequently Cu was produced by the oxidation of Cu₂S. In the latter stage, the Cu was oxidized, and the final product under the condition of gas diffusion control was composed of Cu₂O, Fe₃O₄, and CuFeO₂. On the other hand, the rate of formation of Fe₂O₃, CuO, and CuFe₂O₄ was much slower and they were not formed during the oxidation duration where the overall rate of oxidation was controlled by gas diffusion.     

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.     

Oxidation of cobalt sulfide

Dense plate of cobalt sulfide was oxidized in a mixed O₂-N₂ gas stream at 873 to 1123 K. Atomic fraction of sulfur of the sulfide was between 0.505 and 0.525, and the partial pressure of oxygen in the gas stream was varied between 5.05 x 10₃ and 2.02 x 10₄ Pa. At lower temperature, cobalt diffused from the interior of sulfide to the surface due to the lower sulfur activity, and a dense oxide layer was formed without the evolution of SO₂ gas. The oxidation rate was controlled by the diffusion of cobalt in the sulfide in the initial few minutes, and it was controlled by the diffusion of cobalt through the oxide layer in the subsequent oxidation. At higher temperature, the oxidation of cobalt sulfide proceeded accompanying the evolution of SO₂ gas due to the higher sulfur activity, and a porous oxide was formed. The oxidation rate was determined by the mass transfer of oxygen through the gas boundary film and the oxide layer.     

The oxidation behaviour of amorphous alloy Fe40Ni40P14B6

A combination of thermogravimetry, optical microscopy, scanning and transmission electron microscopy, electron probe microanalysis and differential scanning calorimetry has been used to investigate the oxidation behaviour of amorphous Fe₄₀Ni₄₀P₁₄B₆. The oxide layer formed on amorphous Fe₄₀Ni₄₀uB₆ has a whisker-like α-Fe₂O3 structure, which grows very rapidly to build up a thick layer of oxide. Kinetic data indicate that the oxidation of amorphous Fe₄₀Ni₄₀P₁₄B₆ obeys a parabolic rate law as long as the alloy remains amorphous and the rate controlling process is diffusion of iron in the amorphous alloy matrix. However, the oxidation rate drops sharply if crystallization of amorphous Fe₄₀Ni₄₀P₁₄B₆ takes place during oxidation annealing. Crystalline Fe₄₀Ni₄₀P₁₄B₆ also obeys a parabolic rate law but with as much smaller rate constant than the amorphous alloy. The rate controlling process for oxidation of crystalline Fe₄₀Ni₄₀P₁₄B₆ is diffusion of iron and nickel in the multiphase oxide layer, which consists of a fine scale mixture of NiO, Fe₃O₄, Fe₂O₃ and NiFe₂O₄, crystals. The difference in oxidation behaviour between amorphous and crystallized Fe₄₀Ni₄₀P₁₄B₆ is caused by the different alloy microstructures.     

Surface characterization and reactivity of a nitrogen atomized 304L stainless steel powder

The oxide layer on the surface of the particles of a nitrogen atomized 304L stainless steel powder was quantitatively characterized using X-ray photoelectron spectroscopy (XPS), secondary-ion microprobe (IMP) analysis, selective reduction of surface iron oxide by hydrogen, and chemical analysis for total oxygen, all as a function of particle size. The composition and thickness of the oxide layer do not depend significantly on particle size. A 5.8-nm-thick outer layer made of MnO and Fe₂O₃ islands covers a 2.4-nm-thick inner layer of Cr₂O₃. The reaction of the powder with O₂ impurity in either H₂ or N₂ exhibits different kinetics and mechanism in both cases. In H₂, a selective oxidation of the alloying elements Mn, Cr, and Si takes place above 400 °C with a parabolic isothermal rate law. In N₂, iron also is oxidized, isolated Fe₂O₃-rich microcrystals form over a Cr₂O₃-rich uniform underlayer, and the isothermal kinetics are accelerated.     

Experimental investigation and three-dimensional computational fluid-dynamics modeling of the flash-converting furnace shaft: Part II. Formulation of three-dimensional computational fluid-dynamics model incorporating the particle-cloud description

A fluid-dynamics computer model of the flash-converting furnace shaft, which is based on basic principles, is presented. The model is fully three-dimensional and incorporates the transport of momentum, heat, and mass and the reaction kinetics between the gas and particles in a particle-laden turbulent gas jet. The k-ɛ model was used to describe gas-phase turbulence in an Eulerian framework. The particle-cloud model was used to track the particle phase in a Lagrangian framework. The coupling of gas and particle equations was performed through the source terms in the Eulerian gas-phase governing equations. Copper matte particles were represented as Cu₂S · yFeS _x . Based on experimental observation, the oxidation products were assumed to be Cu₂O, CuO, Fe₃O₄, and SO₂. A reaction mechanism involving the external mass transfer of oxygen from the gas to the particle surface and diffusion of the oxygen through the successive layers of Cu₂O-Fe₃O₄ and CuO-Fe₃O₄ was proposed. The predictions of the computer model were compared with the experimental data collected in a large laboratory furnace. Reasonable agreement between the model predictions and the measurements was obtained in terms of the fractional completion of the oxidation reactions and the sulfur remaining in the reacted particles. The relevance of the computational model for further analysis and optimization of an industrial flash-converting operation is discussed.     

Thermal Decomposition of the Froth Flotation Mineral Sulfide Concentrates Under Argon Atmosphere in the Presence and Absence of Carbon

Thermal decomposition of the mineral sulfide concentrates derived from froth flotation was studied in the temperature range between 1073 K and 1473 K. The experiments were carried out by heating the mineral sulfide concentrates under inert (argon) atmosphere and, in the presence and absence of carbon. The thermal decomposition of the mineral sulfide concentrates were analyzed by plotting the weight loss of the sample against the reaction time which were obtained from the thermogravimeric analysis (TGA) and characterizing the reacted samples by X-ray diffraction (XRD) and microscopic techniques. The extent and rate of thermal decomposition of the samples were temperature dependent. The high sulfur minerals (CuFeS₂ and FeS₂) were decomposed to low sulfur and stable phases (Cu_1.1Fe_1.1S₂, Cu₅FeS₄, and FeS). However, the thermal decomposition of the mineral sulfides were complex because of liquid phase formation at high temperature such that the low sulfur phases appear to have precipitated from the liquid phase during cooling of the sample. Metallic copper was produced in the mineral concentrates containing hydrated and carbonated compounds. It was concluded that the formation of copper cannot occur in the absence of internal oxidation in the sample, when the mineral concentrates are heated under inert atmosphere. The phases were compared with the thermodynamic predictions.     

The oxidation behaviour of amorphous and crystalline Fe78Si9B13

A combination of thermogravimetry, scanning and transmission electron microscopy, electron probe microanalysis and differential scanning calorimetry has been used to investigate the oxidation kinetics, and oxide morphology, structure and composition in amorphous and crystalline Fe₇₈Si₉B₁₃ alloys. Kinetic data indicate that the oxidation reactions of both amorphous and crystalline Fe₇₈Si₉B₁₃ obey a parabolic rate law over the temperature range 300 to 450°C with activation energies of 120 and 86 kJ/mol respectively, indicating that grain boundary diffusion is probably the rate controlling process. The parabolic rate constant for oxidation of crystalline Fe₇₈Si₉B₁₃ is consistently higher than for amorphous Fe₇₈Si₉B₁₃ over the temperature range 300–450°C, so that the amorphous alloy always shows a better oxidation resistance. Electron microscopy and electron probe microanalysis show that the oxide scales formed on both amorphous and crystalline Fe₇₈Si₉B₁₃ consist of SiO₂, Fe₃O₄ and Fe₂O₃, but the detailed microstructure and compositions are different. The oxide scale formed on amorphous Fe₇₈Si₉B₁₃ contains more SiO₂ and has a small particle size, while the oxide scale formed on crystalline Fe₇₈Si₉B₁₃ contains more Fe₃O₄ and consists of larger particles. The difference in oxide growth between amorphous and crystalline Fe₇₈Si₉B₁₃ is caused by the difference in alloy microstructure.     

Oxidation of Fe–18%Mn–0.6%C Steels in Air and a N2–CO2–O2 Mixed Gas Atmosphere at 1273–1473 K

Austenitic Fe–18 wt% Mn–0.6 wt% C steels were oxidized at 1273, 1373, and 1473 K for up to 2 h in either atmospheric air or an 85%N₂–10%CO₂–5%O₂ gas mixture. The alloys oxidized faster in air than in the mixed gas, but the morphology and composition of the oxide scale formed were similar in both atmospheres. The scales that consisted primarily of FeO, Fe₂O₃, and MnFe₂O₄ were highly susceptible to cracking and spallation due to the severe oxidation condition. Since Mn was consumed to form MnFe₂O₄, the original γ‐matrix changed to an α‐matrix in the subscale area, in which Mn‐rich internal oxide precipitates formed locally.     

A Microscopic study of the transformation of sphalerite particles during the roasting of zinc concentrate

The formation of zinc ferrite (ZnFe₂O₄) during the roasting of iron-bearing zinc concentrates requires substantial additional processing to recover the zinc from this compound by leaching and to eliminate the iron from the leachate. The phase changes that occur in the particles of a typical industrial zinc sulfide concentrate during roasting in a fluidized bed at 1223 K were investigated by the use of light microscopy, electron microprobe analysis, and SEM with EDS. The processes which the iron undergoes during its eventual transformation into ferrite have been clarified by examination of the phases and the morphology of partially roasted marmatitic sphalerite particles (Zn, Fe)S, and by reference to the known phase equilibria involved in the Zn-Fe-S-0 system. The oxidation of ironbearing sphalerite occurs in three stages. The first involves the selective diffusion of most of the iron to the particle surface resulting in the formation of an iron oxide shell enclosing a largely unreacted zinc sulfide kernel. In the second stage, this kernel is oxidized to form a solid solution of zinc oxide and iron oxide. The iron is initially present in the ferrous state but, with the disappearance of the sulfide kernel, is oxidized to ferric iron. In the final stage, this dissolved iron oxide and the iron oxide shell react with the surrounding zinc oxide to form the refractory spinel zinc ferrite.     

Cuprous hydrometallurgy Party VI. Activation of chalcopyrite by reduction with copper and solutions of copper(I) salts

Chalcopyrite is reduced by solutions of copper(I) sulfate and copper(I) chloride to chalcocite (Cu₂S) and bornite (Cu₅FeS₄) whilst the iron reports to the solution. Factors which affect the rate and efficiency of reduction are examined. The reaction is rapid on fresh surfaces of chalcopyrite but slows markedly as a film of chalcocite or bornite forms. The reduction in the presence of copper metal goes to completion and gives a material which is more readily leached by oxidising agents than is chalcopyrite. Thus 99% of the copper in the reduced chalcopyrite is leached when copper(II) sulfate in aqueous acetonitrile is the oxidising agent, whereas only 30% of the copper is leached from pure chalcopyrite under similar conditions. Concentrated solutions of copper(I) salts are less effective in reducing CuFeS₂ in a heterogeneous solid-liquid reaction than is copper metal in a “galvanic” solid-solid reaction. Solutions of copper(II) sulfate plus concentrated copper(I) sulfate in dilute acetonitrile (4 M) containing copper sheets are an effective reductant for chalcopyrite.     

The mechanism of oxidation of ferrous sulfide (FeS) powders in the range of 648 to 923 K

The oxidation of FeS powders in flowing dry air was investigated over the temperature range of 648 to 923 K. Thermodynamic calculations and experimental observations showed that the initial stages of oxidation are characterized by the formation of FeS₂ and Fe₃O₄ or Fe₂O₃. Sub-sequently, the oxidation process goes through a formation and eventual oxidation of Fe₂(SO₄)₃ to Fe₂O₃. The kinetics of oxidation of FeS are believed to be controlled by this last step, whose activation energy, 192 kJ-mol^-1, agrees reasonably well with a corresponding value measured for the oxidation of Fe₂(SO₄)3, 219 kJ.mol^-1. Thermodynamic calculations of phase stability diagrams in the Fe-O-S system are presented in support of the proposed oxidation mechanism.     

Investigation of the forms of occurrence of nonferrous metals in oxyhydroxides from the process of nitration of chloride solutions

Using the methods of X-ray photoelectron spectroscopy (XPES), X-ray phase analysis, gamma-resonance (Mössbauer) spectroscopy, thermogravimetry of solid samples and atomic absorption, and atomic emission spectroscopy for solutions, the forms of occurrence of nonferrous metals in oxyhydroxide sediments (nitration hydroxides, NH) from nitration of refining of rhodium, iridium, and ruthenium are investigated. According to the XPES data, nitration hydroxides involve oxidized and unoxidized (selenium) transition metals, namely, Fe(III), Sn(II), Te(IV), Cu(I), Cu(II), Ni(II), Pb(II), As(III), Se(0), and Se(IV), as well as nonmetals O, Cl, and C. All nitration hydroxides are formed mainly by an X-ray amorphous phase (>90%). During heating of nitration hydroxides up to t = 900°C in an inert atmosphere, dehydration of hydroxides and crystallization of certain phases take place, but the X-ray amorphous phase amounts to more than 50%. Purely oxide phases or mixed oxide phases Fe₃O₄, SnO₂, Cu^IFe^IIIO₂, and Pb₂O₃ (NH-1); Ni_0.4Fe_2.6O₄, Cu^IFe^IIIO₂, and CuNiSnO₄ (NH-2); and Fe₃O₄, Fe₂O₃, and SnO₂ (NH-3) are formed in this case.     

Natural convective mass transfer rates between solid magnetite and molten mattes

A twin balance apparatus capable of continuously weighing both the solid and liquid phase was constructed to measure the magnetite-matte reaction. The rate of magnetite consumption during the reaction with iron sulfide increased from 2.5 to 10.8 g Fe₃O₄/min with increasing temperature from 1473 to 1623 K. In the magnetite-(Fe-S-O) melt reactions, the dissolution rate decreased from 5.4 to 1.5 g Fe₃O₄/min with increasing oxygen content in the bulk liquid phase. The rate approached zero when the bulk phase contained approximately 32 at. pct O which was close to the magnetite saturation limit. The rate of reaction between magnetite and liquids of the FeS-Cu₂S binary system decreased from 5.4 to 0.2 g Fe₃O₄/min with increasing Cu₂S content from 0 to 60 mol pct Cu₂S. Examination of the reaction rate data of the Fe-S-O system in conjunction with a magnetite pellet profile study and oxygen analyses in the matte showed that the dissolution mechanism was one of diffusion enhanced by natural convection. The results of reacting magnetite with FeS-Cu₂S melts suggested that the same mechanism operated. The industrial significance of this investigation is discussed briefly in relation, to the problem of the presence of solid magnetite in copper smelting processes. Formerly with the Department of Metallurgy and Materials Science, University of Toronto     

Phase transformations and stress relaxation in γ′-Fe4N1-x surface layers during oxidation

Compound layers composed of γ′-Fe₄N_1-x on α-Fe substrates were oxidized in oxygen-containing gas atmospheres at 603 K. The microstructural changes were analyzed applying X-ray diffraction, dilatometry, and light, scanning electron, and transmission electron microscopy. Compositional changes were traced, in particular, with scanning Auger electron spectroscopy. Dual-phase, Fe₃O₄ (magnetite) and α-Fe₂O₃ (hematite), oxide layers formed at the compound layer surface. At the interface between the oxide and nitride layers, ε-Fe₂N_1=x nucleated because of a local nitrogen enrichment caused by the conversion of γ′ nitride into oxide. The volume of each of the three phases formed (hematite, magnetite, and ε nitride) increased with the square root of oxidation time, indicating solid-state diffusion-controlled layer growth. No evidence was obtained for the existence of γ′ or e oxynitrides. Within the γ′ layer, ferrite precipitated during oxidation as a consequence of an iron supersaturation of γ′ nitride due to a production temperature higher than the oxidation temperature. During this iron precipitation, stress relaxation occurred, as was concluded from X-ray diffractometric and dilatometric analyses. The stress relaxation was rate-controlled by nitrogen diffusion in γ′ nitride. The present findings were also used to explain microstructural changes during (commercial) oxidation of ε-Fe₂(N,C)_1-x compound layers. Formerly Graduate Student at the Laboratory of Metallurgy, Delft University of Technology     

Direct versus indirect bioleaching

The dissolution of metal sulfides is controlled by their solubility product and thus, the [H⁺] concentration of the solution, and further enhanced by several chemical mechanisms which lead to a disruption of sulfide chemical bonds. They include extraction of electrons and bond breaking by [Fe³⁺], extraction of sulfur by polysulfide and iron complexes forming reactants [Y⁺] and electrochemical dissolution by polarization of the sulfide [high Fe³⁺ concentration]. All these mechanisms have been exploited by sulfide and iron-oxidizing bacteria. Basically, the bacterial action is a catalytic one during which [H⁺], [Fe³⁺] and [Y⁺] are breaking chemical bonds and are recycled by the bacterial metabolism. While the cyclic bacterial oxidative action via [H⁺] and [Fe³⁺] can be called indirect, bacteria had difficulties harvesting chemical energy from an abundant sulfide such as FeS₂, the electron exchange properties of which are governed by coordination chemical mechanisms (extraction of electrons does not lead to a disruption of chemical bonds but to an increase of the oxidation state of interfacial iron). Here, bacteria have evolved alternative strategies which require an extracellular polymeric layer for appropriately conditioned contact with the sulfide. Thiobacillus ferrooxidans cycles [Y⁺] across such a layer to disrupt FeS₂ and Leptospirillum ferrooxidans accumulates [Fe³⁺] in it to depolarize FeS₂ to a potential where electrochemical oxidation to sulfate occurs. Corrosion pits and high resolution electron microscopy leave no doubt that these mechanisms are strictly localized and depend on specific conditions which bacteria create. Nevertheless, they cannot be called ‘direct’ because the definition would require an enzymatic interaction between the bacterial membrane and the cell. Therefore, the term ‘contact’ leaching is proposed for this situation. In practice, multiple patterns of bacterial leaching coexist, including indirect leaching, contact leaching and a recently discovered cooperative (symbiotic) leaching where ‘contact’ leaching bacteria are feeding so wastefully that soluble and particulate sulfide species are supplied to bacteria in the surrounding electrolyte.     

Microstructure of ultrafine carbonyl iron powder

Carbonyl iron ultrafine powder with sizes ranging from 5 to 30 nm was characterized by conventional transmission electron microscopy (CTEM), electron energy loss spectroscopy (EELS), high-resolution electron microscopy (HREM), and microdiffraction. It was found that the carbonyl iron ultrafine powder is composed of particles of a-Fe enclosed in about 5-nm-thick skins of polycrystalline Fe₃O₄. Some of the particles are completely oxidized into polycrystalline γ-Fe₂O₃ with grains 5 to 10 nm in size, which form about a 5-nm-thick shell with a hollow core in the center. It was observed that the oxidation of Fe₃O₄ can be enhanced by dispersion.