Yuri Mikhlin

Effect of potential and ferric ions on lead sulfide dissolution in nitric acid

Abstract The dissolution of the rotating disk electrode (RDE) of natural lead sulfide (galena) and the ground mineral in nitric acid solutions has been studied as a function of electrode potential, HNO3 concentration and temperature. The rate of dissolution producing hydrogen sulfide slowly increases as the potential varies from +0.1 to −0.4 V (Ag/AgCl). The reaction order on nitric acid concentration has been found to be 1.2±0.15 at 0.2 V and 0.9 at −0.4 V (40 °C), and the apparent activation energy is 35 kJ mol−1 in 1 M HNO3 at 0 V, suggesting that the process is controlled by a chemical or electrochemical reaction. At higher biases the RDE of PbS dissolves for the most part anodically, showing the highest rate at ∼0.7 V, whereas the rate as a function of acid concentration is maximal in 1 M HNO3. The yield of sulfate increases with potential and is small for the leaching of both compact and ground galena, while it reaches 50% in the case of a flotation lead concentrate. Ferric ions catalyze the dissolution of compact and, especially, ground galena, with the peak rate at the potential of immersed platinum electrode of 0.4–0.5 V. The Fe3+/Fe2+ couple is concluded to act as an intermediator for the electron transfer between nitrate ions and the solid, indicating that the dissolution is electrochemical in nature.

Reactivity of pyrrhotite (Fe9S10) surfaces: Spectroscopic studies

Synthetic hexagonal pyrrhotite (Fe9S10) etched in hydrochloric acid solution and then dried in air has been studied using ex situ XPS, X-ray fluorescence, Mossbauer, solid-state NMR and EPR spectroscopies. The metal-deficient non-equilibrium, up to several micrometres thick, layer (NL) formed on pyrrhotite under non-oxidative conditions has been found to be composed predominantly of low-spin Fe2+, nearly equal quantities of di- and polysulfide sulphur (probably, chains of 3–5 atoms) and no or low oxygen. When pyrrhotite with the NL is kept in air, singlet ferrous iron converts into high-spin Fe2+ and Fe3+, oxygen is incorporated into the layer and the surface enrichment in sulfur over iron decreases. A Mossbauer signal with an isomer shift of 0.36 mm s−1 and negligible quadruple splitting has been detected for the etched sample, desiccation in air gives rise to a quadruple split of up to 0.65 mm s−1 and a minor decrease in the isomer shift. The application of variable X-ray tube accelerating voltage has made it possible to obtain depth-resolved Fe-Lα,β spectra of the NL and to find several alteration zones which include different forms of iron. Slow oxidative dissolution of the material in 1 M HCl+0.01 M FeCl3 electrolyte produces only a thin NL with mostly O-bonded Fe3+ and polysulfide prevailing over mono- and disulfide species. Subsequent air-drying of this sample results in an increase in the concentrations of oxygen, S-bonded Fe, and mono- and disulfide species, along with S0 formation. No unpaired electron spins have been registered in any of these NL.

Formation of bimetallic Au-Pd and Au-Pt nanoparticles under hydrothermal conditions and microwave irradiation.

The reduction of chlorocomplexes of gold(III) from muriatic solutions by nanocrystal powders of palladium and platinum at 110 and 130 °C under hydrothermal conditions and the action of microwave irradiation has been investigated. The structure and composition of the solid phase have been characterized by transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and chemical methods. Bimetallic particles with a core-shell structure have been revealed. The obtained particles are established to have a core of the metal reductant covered with a substitutional solid (Au, Pd) solution in case of palladium, and isolated by a gold layer in the case of platinum. The main reason for such a difference is the ratio between the rates of aggregation and reduction. It has been shown by the example of the Au-Pd system that the use of microwave irradiation allows us not only to accelerate the synthesis of particles but also to obtain more homogeneous materials in comparison with conventional heating.

Reactivity of pyrrhotite surfaces: An electrochemical study

The electrochemical and dissolution behaviour of natural pyrrhotite in 1 M HCl and 0.05 M Na2B4O7 have been studied using cyclic voltammetry and potentiostatic techniques. The formation of a massive non-equilibrium metal-deficient layer (NL) has been found to occur in 1 M HCl within the potential range − 0.08 to 0 V (s. Ag/AgCl), in which the dissolution rate changes by about two orders of magnitude. The cyclic voltammograms of pyrrhotite with the NL previously formed reveal an intense cathodic peak at − 0.2 V and, when the sweep is initiated in the positive-going direction, several anodic features, with the oxidation charge passed contributing to the reduction charge on the return sweep. The cathodic peak has been argued to correspond to the reductive split of the polysulfide with the production of monosulfide S02− centres which rapidly dissolve in acid with H2S release. The three anodic maxima have been attributed to the association of the S02− species with each other and with polysulfide clusters; the oxidation of the terminal S atoms (S1−), with the chains uniting to form larger ones; and the oxidation of residual S1−. The behaviour of the electrode with the NL created previously in hydrochloric acid and transferred into 0.05 M Na2B4O7 solution agrees with these assumptions. The reactions of unetched pyrrhotite involve the same centres in lesser concentrations. The reaction mechanisms have been supposed to include the transfer of electron couples ia the lone-pair S 3p-like orbitals and fast atomic rearrangement, similar to the reactions of amorphous chalcogens and chalcogenides.

Specific Features of Mandible Structure and Elemental Composition in the Polyphagous Amphipod Acanthogammarus grewingkii Endemic to Lake Baikal

Background In crustaceans, several mechanisms provide for the mechanical strength of the cuticular “tools” (dactyli, claws, jaws), which serve to catch and crush food objects. Studies on the mandibles of the endemic Baikal amphipod Acanthogammarus grewingkii by means of electron microscopy and elemental analysis have revealed specific structural features of these mouthparts. Methodology The fine structure of the mandible has been studied by means of SEM, TEM, and AFM; methods used to analyze its elemental and phase composition include XEPMA, XPS, SEM-EDS analysis, and XRD. Conclusion Functional adaptations of the mandible in A. grewingkii provide for the optimum combination of mechanical hardness and fracture resistance, which is achieved due to a complex structure and composition of its cutting parts. Teeth of the mandible are covered by a thin layer of silica (10–20 µm). Their epicuticle is characterized by a high density, consists of three layers, and increases in thickness toward the tooth apex. The epicuticle is enriched with Br, while the concentrations of Ca and P reach the peak values in the softer internal tissues of the teeth. These data broaden the view of the diversity of adaptation mechanisms providing for the strengthening of cuticular “tools” in crustaceans.

Preparation and characterization of colloidal copper xanthate nanoparticles

Despite the important role of metal xanthates in a number of industrial processes and emerging applications, no attempts have been made to prepare the metal xanthate nanoparticles and to study colloidal solutions of insoluble heavy metal xanthates. Here, we examined the formation of colloidal copper xanthate particles during the reactions of aqueous solutions of cupric sulfate and various potassium xanthates, which occur in flotation and water treatment slurries and can be used to manufacture nanoparticles for materials science (e.g., as precursors for copper sulfide nanoparticles and biomedicine). The products were characterized using UV-vis absorption, dynamic light scattering, zeta potential measurements, transmission electron microscopy (TEM), electron diffraction, Fourier transform infrared spectroscopy, thermogravimetry, X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy (XANES). Colloidal copper xanthates with compositions of ROCSSCu (R = ethyl, isopropyl, butyl, isobutyl, and amyl groups), disordered structures and average diameters of 20–80 nm easily formed and aggregated and were stable for at least several hours, especially if excessive xanthate was used. The hydrodynamic diameters of the nanoparticles were smaller at lower temperatures. Dixanthogens, which were produced in the reactions along with ROCSSCu, seemed to promote nanoparticle aggregation and precipitated with the copper xanthate, affecting their thermal decomposition. The TEM micrographs and S K- and Cu K-edge XANES spectra revealed core/shell particle morphologies, likely with Cu(I) bonded to four S atoms in the core and reduced copper coordination in the shell.

Ultrafine particles derived from mineral processing: A case study of the Pb-Zn sulfide ore with emphasis on lead-bearing colloids.

Although mining and mineral processing industry is a vast source of heavy metal pollutants, the formation and behavior of micrometer- and nanometer-sized particles and their aqueous colloids entered the environment from the technological media has received insufficient attention to date. Here, the yield and characteristics of ultrafine mineral entities produced by routine grinding of the Pb-Zn sulfide ore (Gorevskoe ore deposit, Russia) were studied using laser diffraction analysis (LDA), dynamic light scattering (DLS) and zeta potential measurement, microscopy, X-ray photoelectron spectroscopy, with most attention given to toxic lead species. It was revealed, in particular, that the fraction of particles less that 1 μm in the ground ore typical reaches 0.4 vol. %. The aquatic particles in supernatants were micrometer size aggregates with increased content of zinc, sulfur, calcium as compared with the bulk ore concentrations. The hydrodynamic diameter of the colloidal species decreased with time, with their zeta potentials remaining about -12 mV. The colloids produced from galena were composed of 20-50 nm PbS nanoparticles associated with lead sulfate and thiosulfate, while the surface oxidation products at precipitated galena were largely lead oxyhydroxides. The size and zeta potential of the lead-bearing colloids decreased with time down to about 100 nm and from -15 mV to -30 mV, respectively. And, conversely, lead sulfide nanoparticles were mobilized before the aggregates during redispersion of the precipitates in fresh portions of water. The potential environmental impact of the metal-bearing colloids, which is due to the large-scale production and relative stability, is discussed.

Specific Characteristics of Noble Metal Nanoparticles on Sulfide Minerals Observed by XPS and STS

Synchrotron radiation X-ray photoelectron spectroscopy, scanning tunneling microscopy and spectroscopy (STM/STS) have been used to characterize gold and silver species spontaneously deposited from HAuCl 4 or AgNO 3 solutions onto sulfide minerals at room temperature. The reacted substrates were studied too. It was found that Au 4f 7/2 binding energies increase up to 1 eV with decrease in the size of Au° nanoparticles in the range 3 to 30 nm. This is attributed to a temporal charging of the particles, lessening kinetic energies of the photoelectrons via interaction with final-state photoholes. STS revealed a positive correlation between the current magnitude and the diameter of Au particles, tentatively explained by Coulomb blockade effects. Silver deposited on the minerals exhibited neither shifts of Ag 5d bands, nor Coulomb blockade in STS. Possible mechanisms behind the suppression of electronic tunneling and its potential role for the charge transfer steps in reactions of gold nanoparticles were considered.

Electrochemical Aspects of the Nitric Acid Leaching of Lead Sulfide Concentrates: on the Way to a Highly Effective Method for Pb Recovery

Examination of the kinetics of hot nitric acid based leaching of a lead sulfide concentrate together with changes of potential of immersed Pt or PbS electrodes has revealed three stages of the leaching. The acidic decomposition of minerals that produces aqueous sulfide that is then oxidized by nitrate plays a major role in the first, slow leach stage. In the second stage, PbS oxidation is effectively accelerated by accumulated Fe 3+ /Fe 2+ ions, yielding largely elemental sulfur and making the process electrochemical in nature. The formation of predominant sulfate and the process impeding in the third stage seem to be due to oxidative action of nitrogenous species. It is proposed that leaching in practice is performed using cold ferric nitrate solution as this was found to be a surprisingly effective and selective oxidant for PbS. A process flowsheet for the treatment of the Gorevsky flotation lead concentrate was developed.