K. Osseo-Asare

Effect of pH on the Anodic Behavior of Tungsten

Potentiodynamic and potentiostatic polarization, and the rotating disk electrode technique, were used to study the anodic behavior of tungsten (W) in a broad pH range (0.5-13.5) in H 3 PO 4 /KOH buffered solution. Surface oxides were found to play a prominent role in the anodic oxidation and dissolution of tungsten. Five distinct pH regimes and the corresponding reaction mechanisms were identified. Below pH I (region A) H + -assisted dissolution was the main dissolution pathway. As the pH increased, the role of H diminished and at pH 2.6 (region B), which was identified as the point of zero charge (pzc) of the surface tungsten oxide, dissolution was mainly H 2 O-assisted. The dissolution was observed to be OH -assisted above the pzc. The reaction order for OH was one between pH 4.5 and 6.5 (region C). The reaction order became zero at pH 8 (region D). This observation was attributed to the pH-independent dissolution of the hydrated oxide phase. Above pH 10, OH dependence of the anodic current commenced and at around pH 12.5 (region E) the reaction order for OH - became one.

Pyrite oxidation in alkaline solutions: nature of the product layer

Abstract The nature of the oxides formed during pyrite oxidation by molecular oxygen in alkaline solutions has been investigated with the aid of Eh–pH diagrams and direct analysis of the solid products. X-ray diffraction (XRD) and infrared analyses indicate that the products formed are determined by solution composition. In hydroxide medium, hematite, as the main phase, and small amounts of ferrihydrite are identified. In contrast, in carbonate medium, the main constituent is ferrihydrite, with some iron hydroxide carbonate phase also present. In calcium hydroxide medium, only calcium carbonate was detected on the surface of oxidized pyrite in an amount that increased when the system was opened to the atmosphere. Only by diffuse reflectance infrared spectroscopy (IR) was it possible to identify carbonate compounds among the products formed during pyrite oxidation in aqueous solutions. The morphology of the product layers was also affected by solution composition. In NaOH solutions, the particles are initially covered by a thin oxide layer that fractures after longer reaction times. Most of the oxide reports to the solution, where it remains as a stable suspension. Pyrite oxidation in Na 2 CO 3 /NaHCO 3 solutions results in particles that are initially covered by a discontinuous oxide coating that grows with reaction time, thus increasing the overall pyrite surface coverage. In this case, a precipitation confined to the solid/liquid interface is favored by the higher ionic strength of the sodium carbonate solutions.

Cerium Oxide Slurries in CMP. Electrophoretic Mobility and Adsorption Investigations of Ceria/Silicate Interaction

The interaction between ceria particles and silicate ions was studied in order to further understand the mechanism of chemical-mechanical polishing (CMP) of silicon-based materials. Electrophoretic mobility (zeta potential) and adsorption measurements were the techniques utilized. In the presence of silicate ions, the zeta potential of ceria particles changed from positive to negative; in addition, the isoelectric point IEP of the particles shifted to lower pH values with increasing silicate addition. The adsorption density of silicate ions on ceria was found to increase as the concentration of silicate ions in the system increased. Also, with increase in pH, silicate adsorption went through a maximum at about pH 9, then decreased rapidly at higher pH values. The adsorption and zeta potential results are rationalized by considering the solution chemistry of the dissolved silicate species, the protonation/deprotonation of ceria surface, and the accompanying surface complexation reactions. These experimental observations demonstrate a strong interaction between ceria particles and silicate ions. The implication is that adsorption of silicate ions on ceria may be directly involved in the removal mechanism of silicon-based materials during the CMP process.

Synergic extraction of nickel and cobalt by LIX63-dinonylnaphthalene sulfonic acid mixtures

Abstract The extraction of nickel and cobalt by mixtures of 5,8-diethyl-7-hydroxy-6-dodecanone oxime (H2Ox, active extractant in LIX63, a product of Henkel Corp.) and dinonylnaphthalene sulfonic acid (HDNNS or HD) has been studied by using the methods of slope analysis and continuous variation. On the basis of distribution and spectrophotometric data, the observed synergic extraction of nickel and cobalt by H2Ox-HDNNS mixtures is attributed to the formation of a mixed ligand complex with the stoichiometry: Me/H2Ox/HDNNS = 1/3/2. In the absence of H2Ox, log D versus log [HDNNS] plots give straight lines with slopes of unity for both nickel and cobalt extraction, thus indicating that the active extractant is the HDNNS micelle. In the presence of H2Ox the log D versus log [HDNNS] plots go through a maximum and a minimum. It is proposed that the active extractant under these conditions is a mixed HDNNS-H2Ox micelle of the form (HD)p·3H2Ox, and that the shape of the metal distribution curve, as a function of HDNNS concentration, follows the organic-phase concentration of this mixed micelle.

Semiconductor Electrochemistry of Particulate Pyrite Mechanisms and Products of Dissolution

Pyrite dissolution in acidic solution was found to occur via both electrochemical oxidation and chemical decomposition. The mechanism of chemical decomposition of pyrite in acidic solution may involve surface complexation of hydrogen ions. The anodic current of pyrite was observed to be of negligible magnitude in acetonitrile solution compared with that in aqueous solution, which indicated that direct reaction of the holes with S{sub 2}{sup 2{minus}} in the pyrite lattice was not significant and that the dissolution of pyrite required the presence of water. The anodic dissolution products of pyrite in acidic aqueous solution included elemental sulfur which was detected by x-ray diffraction.

Semiconductor electrochemistry and hydrometallurgical dissolution processes

Abstract The basic concepts of semiconductor electrochemistry are reviewed. In particular, the relationship between atomic orbitals, molecular orbitals and the band model of solids is highlighted. By using the fact that redox couples in aqueous solution can also be assigned energy levels, electrochemical reactions at the solid/aqueous interface can be viewed conveniently in terms of charge transfer between energy levels. Application of these concepts to selected hydrometallurgical dissolution systems then follows, including the role of lattice substitution and nonstoichiometry in the dissolution of oxides and sulfides, the role of surface states (i.e. localized energy levels at the solid/aqueous interface) on the reduction of oxidizing agents during sulfide dissolution, and the electronic structural basis of sulfur versus sulfate formation in sulfide dissolution reactions.

Reduction of As(V) to As(III) by commercial ZVI or As(0) with acid-treated ZVI

Zero-valent iron (ZVI) consists of an elemental iron core surrounded by a shell of corrosion products, especially magnetite. ZVI is used for in situ removal or immobilization of a variety of contaminants but the mechanisms for removal of arsenic remain controversial and the mobility of arsenic after reaction with ZVI is uncertain. These issues were addressed by separately studying reactions of As(V) with magnetite, commercial ZVI, and acid-treated ZVI. Strictly anoxic conditions were used. Adsorption of As(V) on magnetite was fast with pH dependence similar to previous reports using oxic conditions. As(V) was not reduced by magnetite and Fe(II) although the reaction is thermodynamically spontaneous. As(V) reactions with ZVI were also fast and no lag phase was observed which was contrary to previous reports. Commercial ZVI reduced As(V) to As(III) only when As(V) was adsorbed, i.e., for pH<7. As(III) was not released to solution. Acid-treated ZVI reduced As(V) to As(0), shown using wet chemical analyses and XANES/EXAFS. Comparisons were drawn between reactivity of acid-treated ZVI and nano-ZVI; if true then acid-treated ZVI could provide similar reactive benefits at lower cost.

Surface chemistry of carbonaceous gold ores I. Characterization of the carbonaceous matter and adsorption behavior in aurocyanide solution

Carbonaceous gold ore obtained from the Prestea Goldfield, Ghana, was fractionated into its organic components by means of NaOH leaching, benzene treatment, and hydrofluoric acid leaching. The initial ore as well as the extracted components were characterized by chemical analysis and infrared spectroscopy. The results indicate that the ore is dominated by silicates (∼53% SiO2) and contains about 5% total carbon. The hydrocarbon extract (0.27%) obtained from the benzene treatment analyzed 66.9% C, 18.2% S, 10.5% H and 1.7% O. The infrared spectrum of this material showed a predominance of CH3, CH2, CH and C = O groups suggesting the presence of long-chain hydrocarbons. No extraction of a humic acid fraction was achieved. The major organic component was the carbon extract (3.9%) which analyzed 59.4% C, 10.6% Al2O3 and 5.6% TiO2. The surface chemistry of the carbonaceous ore and the carbon extract was investigated by means of electrophoretic mobility and gold uptake measurements. Both the ore and the carbon extracts were found to be negatively charged in the pH range 3 to 11, with the negative charge increasing with pH. Gold adsorption from aqueous cyanide solution was found to be enhanced by increase in H+ and Ca2+ ion concentrations. The results are interpreted in terms of coulombic and specific chemical interactions.

Semiconductor Electrochemistry of Particulate Pyrite: Dissolution via Hole and Electron Pathways

Electrochemical and photoelectrochemical experiments were conducted to investigate the pyrite/aqueous interface reaction by using microparticles of synthetic pyrite as electrodes. The potential of the conduction bandedge of pyrite as a function of pH was estimated to be E{sub c} = 0.34--0.059 pH in volts vs. saturated calomel electrode (SCE). The open-circuit potential of pyrite electrode in 1 M HNO{sub 3} solution was 0.38 V{sub SCE}. Illumination of pyrite microelectrodes increased both the anodic current and the dissolution rate dramatically but had little effect on the cathodic current and the cathodic dissolution. These results indicate that pyrite, as an n-type semiconductor, dissolves anodically through a hole transfer (valence band) pathway, while cathodic dissolution only involves electron reaction (conduction band).

Solution chemical constraints in the chemical-mechanical polishing of copper: aqueous stability diagrams for the Cu-H 2 O and Cu-NH 3 -H 2 O systems

A general graphical approach to the solution chemistry of chemical mechanical polishing is presented with the aid of a variety of aqueous stability diagrams, such as Eh-pH (Pourbaix), log Metal-pH, and log Ligand-pH diagrams. The common thermodynamic origin of these diagrams is highlighted. The important role played by concentration gradients in effecting chemical mechanical polishing is stressed and illustrated with the aid of stability diagrams generated for the Cu-H2O and Cu-NH3-H2O model systems. It is demonstrated that chemical mechanical polishing is feasible when the following two conditions are satisfied simultaneously: (a) at the metal surface dissolved metal concentration is high and/or ligand concentration is low (this favors oxide film formation), and (b) in the bulk aqueous phase the metal concentration is low and/or the ligand concentration is high (this favors the dissolution of film fragments).