Colin F. Jones

The role of surface sulfur species in the inhibition of pyrrhotite dissolution in acid conditions

Abstract Pyrrhotite, in anoxic acidic conditions, exhibits an induction period before rapid dissolution occurs. The length of the induction period is controlled by the amount of surface oxidation products on the mineral surface, acid strength, and temperature. During the induction period there is slow release of iron but little or no production of H 2 S. The induction period is best described as a period of inhibited dissolution, before the onset of H 2 S production and increased rate of iron release of at least 2 orders of magnitude. X-ray photoelectron spectroscopic (XPS) analysis of the acid-reacted surface shows the progress of the dissolution. Four stages of dissolution have been identified. (1) The immediate dissolution of an outermost layer of oxidised iron hydroxide/oxyhydroxide species and oxy-sulfur species. (2) Inhibited, diffusion limited dissolution during an induction period due to iron diffusion through the metal-deficient layer and oxidative dissolution of the polysulfide species. (3) Rapid, acid-consuming reaction of mono-sulfide species under nonoxidative or reductive conditions with production of H 2 S. (4) Inhibited dissolution due to reoxidation of the sulfide surface by oxidising solution species (i.e., Fe 3+ , residual oxygen) to produce polysulfide, elemental sulfur, and oxy-sulfur species. Dissolving synthetic pyrrhotite in similar, but aerated, acidic conditions, results in inhibited dissolution characterised by a lower rate of Fe release, minimal release of SO 4 2− and no release of H 2 S . The XPS sulfur (S2p) spectrum shows sulfate and a form of elemental sulfur on the reacted surface. Only the first two stages of dissolution occur. The second stage differs in this case in that there is a plentiful supply of oxidising species (O 2 ). Two reaction mechanisms are proposed for the dissolution of the iron sulfide lattice of pyrrhotite in acidic conditions. The mechanisms are oxidative and nonoxidative dissolution. Two distinct activation energies are associated with the two regimes. A lower activation energy corresponds to inhibited dissolution with no production of H 2 S. A t 1/2 rate law describes dissolution in air saturated solutions and supports diffusion controlled dissolution under these conditions. A higher activation energy corresponds to rapid dissolution with H 2 S production. The mechanism of dissolution is determined by the state of the surface, particularly the sulfur species.

Compositional and structural alteration of pyrrhotite surfaces in solution: XPS and XRD studies

Ground pyrrhotite (Fe1−xS) surfaces oxidised by exposure to (i) air, (ii) water and (iii) de-oxygenated perchloric acid solution (0.05–1M) were examined using X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). In air or water, the surfaces form amorphous layers containing carbonate species; sulfate species; iron(III) oxide/hydroxides; and an iron-deficient sulfide species with an S(2p) doublet shifted 1.0–1.8 eV to higher binding energy (BE). After acid reaction, the surface partly restructures to a crystalline, defective tetragonal Fe2S3 product in which linear chains of Sn atoms have a S-S distance similar to elemental sulfur (S8) but the S(2p) BE is still 0.2–0.6 eV less than S8. Initially, the acid-reacted surface may be partly hydrophobic, giving flotation separation, but, as oxidation proceeds, hydrophilic iron hydroxides deposit on the surface depressing flotation. The chemical forms of Fe and S in the surface layers are discussed in detail with changes in the proportion of the oxidised and iron-deficient sulfide products.

Surface area and the mechanism of hydroxylation of ionic oxide surfaces

The nearly perfect {100} surfaces of MgO smoke cubes formed in air do not show significant v(OH) absorption in infrared spectra from thin (10 mg cm–2), coherent films exposed to H2O vapour for several hours. It is shown that perfect, five-fold-coordinated sites are not protonated and that the proportion of protonated, low-coordination (i.e. less than five-fold) sites is < 5%. These results are in accord with theoretical predictions for H2 adsorption. In contrast, v(OH)[and v(CO3)] absorptions are observed in identical preparations subjected to prior abrasion. The increase in protonated sites is more than ten-fold. Electron microscopy shows that only minor initial alteration of surface structure at edges and corners is caused by abrasion but a major increase in the rate of surface roughening is observed when new, nucleating sites appear and develop in regions across the surface. This process results in a major, time-dependent increase in hydroxylation but it is substantially complete before infrared spectra can normally be obtained. B.E.T. studies, using N2 adsorption, do not measure the change in surface area produced by this surface roughening. Multilayer water adsorption followed by desorption more than doubles the B.E.T. surface area because of the formation of platelets decomposed from the Mg(OH)2 brucite surface layer. The particle-size distribution is altered and the B.E.T. method correctly measures this change.

Initial dissolution Kinetics of Ionic Oxides

It is shown th at factors previously recognized, but not regarded as critical, can dominate dissolution kinetics of ionic oxides. The use of the nearly perfect {100} MgO surfaces of smoke cubes to obtain very precise values of dissolution rates per unit surface area, in dilute HC1, HC1O4 and HNO3, has shown th at rates extrapolated to zero dissolution are almost independent of pH in the range 2.0- 3.5. Dissolution rates were measured by monitoring solution pH as a function of time. This revealed increasing rates with increasing pH up to about 5 % total dissolution, followed ultimately by a return to the linear relation between Ig(rate) and pH (slope ca. — 0.5) normally expected. The initial increase in rate is due to increasing [Mg2+] in solution and is observed with [Mg2+] as low as 1 % of the [H+]. A linear relation between lg(rate) and [Mg2+] is found during the early stages of dissolution. Other cations (Al3+, Na+) also increase the initial rate, to a similar extent. Electron-microscope observations of the cubes show alteration of the surfaces to a castellated structure (of {100}-based projections and intrusions) on wetting before dissolution, and the development of facets having an average (110}-natureduring dissolution. The results are in conflict with current theoretical models, and a qualitative account of the mechanism of the establishment of a ‘ stable ’ solution double layer is given.

Semiconducting oxides. The effect of prior annealing temperature on dissolution kinetics of nickel oxide

The paper describes studies of the dissolution rate of NiO (a p-type semiconductor) in nitric acid solution at 60°C as a function of prior annealing temperature in air. The dissolution rate per unit surface area decreased markedly on increasing the annealing temperature from 500 to 1450°C. This effect is not due to gross structural change or to major changes in dislocation density. The higher dissolution rates (i.e., > 10–12 mol cm–2 s–1) of oxide annealed at temperatures below 700°C is due to an excess concentration over thermodynamic equilibrium of point defects (nickel vacancies), introduced during decomposition of the hydroxide, and maintained as a consequence of limited diffusion. For annealing temperatures above 900°C, the defect concentration is roughly equal in all samples because of rapid equilibration in polycrystalline samples during cooling. The decreasing rates (i.e., < 4 × 10–13 mol cm–2 s–1) may be due to (i) limited conduction of charge due to changes in the space-charge region of the semiconductor and/or (ii) reduction of the density of kink sites on the surface of the more perfect crystallites.

Semiconducting oxides: effects of electronic and surface structure on dissolution kinetics of nickel oxide

The effect of decreasing dissolution rate of nickel oxide per unit surface area in acid solution with increasing prior annealing temperature (700–1450°C) has been shown to occur over a wide range of pH. The linear dependence of log (rate) on pH may be explained on a model of non-oxidative dissolution in which the pH variation changes the overpotential at the surface. Electron microscopy shows a different mode of attack at pH 200 fold) for all prior annealing temperatures but the oxide annealed at 1450°C is still the slowest to dissolve in the presence of cobaltic ions. This is believed to be because it has the lowest kink site density, the role of the Co3+ being hole injection into the p-type semiconductor at kink sites. The general conclusion of the work is that the supply of the majority carriers (the holes) may be rate limiting in the dissolution process.

Dissolution Mechanisms of Oxides and Titanate Ceramics — Electron Microscope and Surface Analytical Studies

The mechanisms which control the dissolution of simple oxides in aqueous solution are not fully understood, despite substantial research; those for ceramics are even less certain, and may involve “leaching” (preferential release of some species from the solid into solution) rather than “dissolution” (all species released at equal rates). In this paper we consider the reactivity in aqueous solutions of two materials — MgO and CaTiO3 — to illustrate current attempts to establish the detailed mechanisms and rate-determining steps. The importance of using surface analytical and electron microscopy techniques to characterize aspects of the surfaces before and after attack is emphasized. In the case of MgO, there is an initial stage of dissolution related to the restructuring of the initial oxide surface; the dissolution process is controlled by the second protonation reaction. For the perovskite CaTiO3, ion—exchange of the Ca2+ from surface sites is accompanied by base—catalyzed hydrolysis of the titanate lattice: there is no significant leaching of the Ca2+ from the intact solid unless hydroxylation has occurred. However, crystal defects due to deformation and microstructural characteristics of the material can have major effects on the reactivity of polycrystalline perovskite.

Dissolution kinetics of manganese oxides. Effects of preparation conditions, pH and oxidation/reduction from solution

The dissolution kinetics of the p-type semiconducting oxide MnO in dilute acid solutions have been investigated. For pure MnO, prepared by decomposition of the carbonate above 900 °C in a H2(5%)/N2 atmosphere, the specific dissolution rate at pH 2 and 30 °C is ca. 4 × 10–5 mol s–1 m–2. This is similar to the rate for ionic MgO, and two orders of magnitude faster than CoO. Like MgO and CoO, MnO exhibits an initial increase in rate with pH. Beyond this initial regime, the rate dependence on pH is consistent with rate control through the second protonation. The higher oxides, Mn3O4, Mn2O3 and MnO2, have been shown to dissolve about three orders of magnitude more slowly than MnO. The presence of these higher oxides as impurities in many preparations of MnO explains the apparently slower dissolution kinetics of such impure monoxide samples. Reduction from solution increases the dissolution rate of the higher oxides. It is concluded that the presence of Mn3+ associated with surface or bulk defects in MnO can be rate-controlling for the non-stoichiometric oxide.

The effect of irradiation on the dissolution rate of magnesium oxide

Abstract It has been shown by precision dissolution rate measurements that the kinetics of the dissolution of magnesium oxide in dilute acid solution are unaffected by high energy γ or fast neutron irradiation prior to dissolution. There is however an effect of irradiation during dissolution on the kinetics of the corrosion of MgO by water vapour.

Semiconducting oxides: infrared and rate studies of the effects of surface blocking by surfactants in dissolution kinetics

For nickel oxide annealed at different temperatures (after preparation from the hydroxide), surface blocking by adsorbed anionic surfactant reduces dissolution rates per unit surface area in acid solution. Cationic and non-ionic surfactants have little effect. Infrared studies indicate coulombic interaction of the anionic group with the positive surface charge, with little molecular structural modification. For NiO annealed at 700°C, there is a systematic decrease in rate with anionic surfactant concentration to a minimum rate at ≈ 20% of the rate without surfactants. The minimum rate is found close to monolayer coverage. For 1450°C annealed material, weaker attraction of adsorbed surfactant, due to lower surface charge on the less imperfect surfaces, produces less reduction of the rate and the minimum rate (≈ 35% of the rate without surfactant) occurs at concentrations corresponding to about six times monolayer coverage. Surface blocking by the anionic surfactant causes a reduction (to 10%) of the enhanced dissolution rate produced by Co3+ in solution; a cationic surfactant again has no significant blocking effect and oxidation by Co3+ gives a more than one hundred fold increase in rate.Rate changes are discussed in relation to changes in the oxide structure caused by pretreatment and in relation to the structure of the adsorbed surfactant layer.