Variation of Fe3+/Fe2+ and Cu2+/Cu+ equilibrium with basicity of oxide melts

Oxidation-reduction equilibrium experiments were conducted with Li₂O-CaO-Al₂O₃-ZnO-SiO₂-B₂O₃ melts containing small concentrations of either copper oxide or iron oxide. The results indicated that, for oxide melts containing relatively large proportions of amphoteric oxides, i.e., Al₂O₃ and ZnO, the ratios lcub;(Cu²⁺)/(Cu⁺)P _{O 2} ^1/4rcub; and lcub;(Fe³⁺)/(Fe²⁺)P _{O 2} ^1/4rcub; decreased with an increase in basicity. With further increase in basicity, however, these ratios became independent of basicity. For very basic melts, the ratios increased with an increase in basicity. These variations of lcub;(Cu²⁺)/(Cu⁺)P _{O 2} ^1/4rcub; and lcub;(Fe³⁺)/(Fe²⁺)P _{O 2} ^1/4rcub; can be interpreted in terms of the optical basicity of the oxide melts and are consistent with the following general expressions for the red-ox equilibria within acidic and relatively basic melts, respectively: M ⁿ⁺+(1/4)O₂=M⁽ⁿ⁺¹⁾⁺+(1/2)O²⁻ M ⁿ⁺+(1/4)O₂+(3/2)O²⁻=MO₂ ⁽³⁻ⁿ⁾ and M ⁿ⁺+(1/4)O₂=(3/4)M⁽ⁿ⁺¹⁾⁺+(1/4)MO₂ ⁽³⁻ⁿ⁾     

Study of the mechanism of anodic dissolution of Cu2S

The anodic dissolution of Cu₂S in sulfuric acid solutions was studied under galvanostatic and potentiostatic conditions. The anodic products were studied by mineralogical and X-ray diffraction methods. In every case, the formation of a digenite Cu_1-8S layer is observed at the surface of Cu₂S according to 5Cu₂S → 5Cu_1.8S + Cu⁺⁺ + 2e A copper concentration gradient appears through the digenite layer whose thickness remains constant as soon as a Cu_1.1S layer appears at its own surface according to 3Cu_1.8S → 4Cu_1.1S + Cu++ + 2e If the electrolysis conditions are such that the anodic potential remains low, the next reaction to occur is 10Cu_1.1S → HCu⁺⁺ + 10S + 22e But if under galvanostatic conditions, the current density is high enough at a given temperature to reach the sharp rise in anodic potential, or if under potentiostatic conditions the potential is kept high, two other reactions are possible: 10Cu_1.1S → 10CuS + Cu⁺⁺ + 2e followed by CuS → Cu⁺⁺ + S + 2e Moreover, at high anodic potential, the following reaction occurs also to some extent CuS + 4H₂O ? Cu⁺⁺ + SO₄ ⁼ + 8H⁺ +8e resulting in a decrease in anodic current efficiency for the copper dissolution. From a more practical point of view, it was shown that it is possible to deplete virtually completely the copper content of the anode (residue at less than 0.5 pct Cu)keepingthe electrode potential at a low value (less than +650 mV/ENH). Providing the temperature is high enough (75°C at least), the mean current density remains near to 2 A/dm², a suitable value to obtain good cathodic deposits.     

The kinetics of the dissolution of chalcocite in alkaline cyanide solution

A kinetic study of the dissolution of chalcocite in an alkaline cyanide solution indicates that the reaction is first order with respect to surface area and free cyanide ion concentration, and inversely proportional to the sulfide ion concentration to approximately the 0.1 power. The experimental rate constant is approximately 7.5 × 10⁻⁵ (mole Cu¹⁺/ min) ([S⁻²]^0.1/[CN¹⁻]/cm²/l at 25°C. The activation energy of 2.5 kcal/mole indicates rate control by diffusion through a limiting boundary layer. The concentrations of the important ions in the cyanide solutions are obtained by solving the ionic equilibria and mass balance equations for the system.     

Activities in Cu2S-FeS-PbS melts at 1200 °C

The dew-point method was used to determine the vapor pressures of PbS over liquid sulfides of the system Cu₂S-FeS-PbS at 1200 °C. From the PbS activity data, activities of Cu₂S and FeS were evaluated both in binary and ternary melts by Gibbs-Duhem calculations. The systems Cu₂S-PbS and Cu₂S-FeS exhibit negative departures from ideal behavior, while the FeS-PbS melts are ideal solutions at 1200 °C.     

Activities in Cu2S-FeS-SnS melts at 1200 °C

The dew-point technique was used to measure the vapor pressures of SnS over liquid sulfides of the system Cu₂S-FeS-SnS at 1200 °C. Activities of SnS were generated from the measured vapor pressures of SnS. Activities of Cu₂S and FeS were evaluated both in binary and ternary melts by Gibbs-Duhem calculations from the known SnS activity data. The systems Cu₂S-SnS and Cu₂S-FeS exhibit negative departures from ideal behavior, while FeS-SnS melts exhibit positive deviations.     

Fabrication of wide temperature lanthanum and cerium doped Cu/TNU-9 catalyst with excellent NH3-SCR performance and outstanding SO2+H2O tolerance

The effects of La on the catalytic performance,SO₂ and H2O resistance of Cu-Ce/TNU-9 catalyst were studied in the selective catalytic reduction of NO_x via ammonia(NH₃-SCR).The results show that the La doped Ce-Cu/TNU-9(CCL/T9) catalyst exhibits better SCR performance than Ce-Cu/TNU-9(CC/T9) and Cu/TNU-9(C/T9) in the wide temperature window(200-450 ℃) due to La benefiting from enhancing Cu⁺+Ce⁴⁺?Cu²⁺-+Ce³⁺ to facilitate ...     

A partial equilibrium chemical model for the dump leaching of chalcopyrite

The chemistry of the dump leaching of chalcopyrite has been studied by means of a chemically based model that makes it possible to calculate concentration changes for solution species during mineral dissolution. In a dump, chalcopyrite can dissolve in two ways: CuFeS₂ + 4Fe³⁺ → Cu²⁺ + 5Fe²⁺ + 2S° CuFeS₂ + O₂ + 4H⁺ → Cu²⁺ + Fe²⁺ + 2S° + 2H₂O CuFeS₂ dissolution is not at equilibrium in a leach dump. However, there are a large number of homogeneous reactions taking place in the leach liquor. These may be treated as rapid equilibrium reactions in this “partial equilibrium” model. The model equations are linear and consist of an equation for each aqueous phase reaction, mass and charge balances, and a kinetic equation. Dissolution by O₂ and acid should be prevented if possible. The acid consumed is partly replaced by shifts in the solution equilibria. However, the net pH increase that occurs leads to precipitation of ferric ion. The total amount of copper dissolved without precipitation is highest if high Fe(III) concentrations are used and oxygen is excluded. High H₂SO₄ concentrations are beneficial and high FeSO₄ concentrations deleterious because of their influences on the precipitation equilibria. KNONA C. LIDDELL, formerly Graduate Assistant, Ames Laboratory USDOE and Department of Chemical Engineering, Iowa State University, Ames, IA 50011     

Leaching of Cu2O with aqueous solution of sulfur dioxide

Leaching of Cu₂O with aqueous SO₂ solution is significant, since during the dissolution process, precipitation of copper as Chevreul’s salt also takes place under appropriate conditions. The dissolution is controlled by surface reactions and proceeds through both aqueous SO₂ and acid dissolution paths. An overall rate equation based on the above premise has been found to agree well with the experimental data. At pH values higher than 1.8, precipitation of copper as Chevreul’s salt takes place after about 10 minutes of leaching. The extent of the precipitation depends upon the pH, SO₂ concentration, initial Cu²⁺ concentration, and sulfate concentration in the leaching solution.     

The effect of electrolyte composition on the cathodic reduction of CuFeS2

The electrochemical reduction of CuFeS₂ mineral electrodes has been investigated by performing cathodic polarization curves, constant potential experiments, and cyclic polarization curves in various electrolytes. The effects of H₂SO₄, Fe²⁺ and Cu²⁺ concentrations have been examined as well as the effects of various dissolved gases, air, O₂, H₂S and N₂. Decreasing H₂SO₄ only shifts the curve to more negative potentials but initial concentrations of 0.36 M Fe²⁺ cause up to a ten-fold enhancement in the observed current. The presence of oxygen or Cu²⁺ also leads to an increase in the reduction current but in either case further increases in current are observed when Fe²⁺ is added to the electrolyte. Chalcocite (Cu₂S) or djurleite (Cu_1.96S) have been identified as products of reaction, although it is possible that a thin layer of a different copper-iron sulfide may form as an intermediate based on the color changes which appear after cathodic excursions. In addition, scanning Auger microprobe analyses of these surfaces show an increasing Cu:Fe ratio as the time at potential is increased. The results also indicate that the solid product layer is porous. A mechanism which accounts for the observed current increase in the presence of Fe²⁺ is proposed which involves a redox reaction between dissolved Fe²⁺ and cupric ion in the copper sulfide product layer.     

A solid-state emf study of ternary Ni-S-O,Fe-S-O,and quaternary Fe-Ni-S-O

Four two-phase equilibria in Ni-S-O, three two-phase equilibria in Fe-S-O, and one two-phase equilibrium and four three-phase equilibria in Fe-Ni-S-0 atP _{SO 2} ? 1 atm were studied using the following emf cell: $$Pt, O_2 (air) \left| {\begin{array}{*{20}c} {ZrO_2 \cdot CaO or} \\ {ZrO_2 \cdot Y_2 O_3 } \\ \end{array} } \right|\begin{array}{*{20}c} {mixture of phases, Au} \\ {SO_2 , S_2 , O_2 , SO_3 with P_T \simeq 1 atm} \\ \end{array} $$ The seven two-phase equilibria in Ni-S-O and Fe-S-O are NiS/Ni₃S₂, Ni₃S₂/Ni0, NiS/NiO, NiO/NiSO₄, Fe₂O₃/Fe₂(SO₄)₃, Fe₂O₃/FeSO₄, and FeSO₄/Fe₂(SO₄)₃, and the four three-phase equilibria and the one two-phase equilibrium in Fe-Ni-S-0 are sp + (Fe, Ni)S + (Fe, Ni)₄S₃, sp + (Fe,Ni)₄S₃ + (Fe, Ni)O, sp + (Fe, Ni)O + (Fe, Ni)SO₄, and Fe₂O₃ + sp + (Fe, Ni)SO₄ and sp + (Fe,Ni)S. The sp + (Fe, Ni)S two-phase equilibrium was measured by using mixtures containing a major portion of one phase and a minor portion of the other. The results are presented in terms of oxygen potential as a function of metal ratio,y _Ni =x _Ni/(x _Fe +x _Ni) at constant temperatures andP _{SO 2} = 1 atm. The calculated stability diagrams atP _{SO 2} = 1 atm from thermodynamic models are in agreement with experimental results. Several of the stability diagrams atP _{SO 2} = 0.1 and 0.01 are also calculated and presented.     

A thermodynamic study of digenite solid solution (Cu2-δS) at 600 to 1000°C and a statistical thermodynamic critique on general nonstoichiometry

A very accurate experimental method to determine the composition of metal-saturated sulfides has been developed and applied to Cu_2-δS. The technique consists of using a high-sensitivity thermobalance which operates in a controlled atmosphere of H₂-H₂S gas. By this method it was shown that Cu-saturated Cu_xS is stoichiometric(x= 2.0000 ± 0.0002) between 700 and 1000°C. The free energy of formation for Cu₂S was found to be: 2Cu(s) + 1/2 S₂(g) = Cu_2.0000S(s), ΔG⁰ = -30,610 + 6.80T (cal/mole). The sulfur partial pressure was determined over nonstoichiometric Cu_2-δS for sulfur contents up to 21 pct. From the result, thermodynamic functions such as activity of copper, heats and entropies of solution were calculated as a function of nonstoichiometric composition. It was thermo-dynamically demonstrated that previous models dealing with nonstoichiometric oxides (or sulfides) based on the law of mass action,i.e., δ = const ie 67 01 (or ie 67 02), are inconsis-tent as they fail to satisfy the Gibbs-Duhem relation and also fail to account for the dis-sociation pressure over the stoichiometric composition. To resolve this dilemma, a sta-tistical thermodynamic method of constructing the grand partition function was introduced. Stoichiometry and nonstoichiometry of the Cu_2-δS were then explained by postulating an ionic crystal consisting of Cu⁺, Cu²⁺, Cu⁰ and neutral vacancies in the copper sublattice and S²⁻ and neutral vacancies in the sulfur sublattice. Formerly Postdoctorate Fellow, National Research Council of Canada, Ottawa, Ontario     

Oxidation‐reduction equilibria of ferrous/ferric ions in oxide melts

Oxidation‐reduction equilibrium experiments were conducted with oxide melts containing CaO, Li₂O, Al₂O₃, ZnO, B₂O₃, SiO₂ and small concentrations of iron oxide. The results indicated that for compositions within the acidic regime, the ratio {(Fe³⁺)/(Fe²⁺) P_O2^1/4} decreased with an increase in basicity while for relatively basic melts, the ratio increased with an increase in basicity. This variation of {(Fe³⁺)/(Fe²⁺) P_O2^1/4} could be interpreted in terms of the optical basicity of the glass melts and is consistent with the following expressions for the red‐ox equilibria within acidic and relatively basic melts, respectively: In an acidic melt, the temperature dependence of the reaction yielded an enthalpy value which was in excellent agreement with the enthalpy change for the reaction: For a relatively basic melt, the enthalpy value for the corresponding reaction was about half of that found for the acidic melt.     

Effect of solution composition on the optimum redox potential for chalcopyrite leaching in sulfuric acid solutions

The leaching rate of chalcopyrite (CuFeS₂) by Fe³⁺ in H₂SO₄ solutions depends on the redox potential determined by the Fe³⁺/Fe²⁺ concentration ratio, and there is a maximum leaching rate at an optimum redox potential. The present study investigated the effects of solution composition on the optimum redox potential by electrochemical measurements using a CuFeS₂ electrode and electrolyte solutions containing 0.01–1 kmol m^− 3 of H₂SO₄, Fe²⁺, and Cu²⁺ at 298 K in nitrogen.Anodic-polarization curves of the CuFeS₂ electrode showed that there was a current peak on the curves in the presence of Cu²⁺ and Fe²⁺, corresponding to the maximum leaching rate. The redox potential of the peak increased markedly with increasing Cu²⁺ concentration, while it was little affected by the H₂SO₄ and Fe²⁺ concentrations. These results agree with the results of leaching experiments reported previously, and indicate that the optimum redox potential for chalcopyrite leaching is a function of the Cu²⁺ concentration. An empirical equation for the optimum redox potential for CuFeS₂ leaching is proposed.     

Kinetics and mechanism of the conversion of covellite (CuS) to digenite (Cu1.8S)

Rates of conversion of covellite (CuS) to digenite (Cu_1.*S) in N₂ and various partial pressures of O₂ have been studied in the temperature range 260° to 400°C. The reaction is first order with respect to O₂ concentration in N₂?O₂ mixtures at 1 atm. The experimental activation energy for the conversion of CuS to Cu_1.8S is 23±3 kcal. In a nitrogen atmosphere, the conversion proceeds at a slower rate but with the same activation energy and leads to the formation of elemental sulfur instead of SO₂. An externally added small amount of iron powder (0.6 pct) increased the rate of conversion, but had no effect on the apparent activation energy. The rate-controlling reaction in the conversion mechanism is postulated to be Cu_1.8S→1.8Cu⁺+1.8e ^?+S at the Cu_1.8S/gas interface. Reduction of CuS by copper(I) ions and electrons occurs by a rapid reaction, 0.8 Cu⁺+0.8e ^?+CuS→Cu_1.8S at the CuS/Cu_1.8S interface, which steadily contracts with time.     

Removal and recovery of gallium from aqueous solutions by complexation with sodium di-(n-octyl) phosphinate

A new precipitation process is reported for the removal and recovery of gallium (III) from aqueous solutions using sodium di-(n-octyl) phosphinate. The complexes formed have low solubility in water and the ligand is easily regenerated, as previously reported for the precipitation of lead and zinc. The effects of different ligand-to-gallium mole ratios, pH and temperature are studied. Essentially complete removal of gallium was obtained at 22 °C using a ligand-to-gallium mole ratio of 3 and pH between 2 and 7.5. The effects of competing ions, such as SO₄²⁻, Cl⁻, Na⁺ and Ca²⁺, are documented. While there is a clear selectivity of gallium over Ca²⁺, the presence of Na⁺ cation and anions such as SO₄²⁻ and Cl⁻, instead of NO₃⁻, decreases the efficiency of the process. Gallium is recovered from the insoluble complexes using diethyl ether or chloroform in contact with an aqueous solution at different values of pH. While gallium is solubilized in the aqueous phase, the ligand is extracted into the organic phase. The ligand is recovered by evaporating the organic solvent.     

Speciation of the Fe(II)–Fe(III)–H2SO4–H2O system at 25 and 50 °C

This work presents theoretical and experimental results on the speciation of the Fe(II)–Fe(III)–H₂SO₄–H₂O system in concentrated solutions (up to 2.2 m H₂SO₄ and 1.3 m Fe). The aim was to study the chemical equilibria of iron at 25 and 50 °C in synthetic aqueous sulphuric acid solutions that contain dissolved ferric and ferrous ion species. Raman spectroscopy, volumetric titration and conductivity measurements have been carried out in order to study the presence of specific ions and to characterize the ionic equilibrium. A thermochemical equilibrium model incorporating an extended Debye–Hückel relationship was used to calculate the activities of ionic species in solution. Model calculations were compared with experimental results. Model simulations indicate that anions, cations and neutral complexes coexisted in the studied system, where the dominant species were HSO₄⁻, H⁺, Fe²⁺ and FeH(SO₄)₂⁰. This indicated that these solutions showed a high buffer capacity due to the existence of bisulphate ions (HSO₄⁻), which presented the highest concentration. A decrease in the concentration of H⁺ and Fe³⁺ took place with increasing temperature due to the formation of complex species. Standard equilibrium constants for the formation of FeH(SO₄)₂⁰ were obtained in this work: log K_f⁰ = 8.1 ± 0.3 at 25 °C and 10.0 ± 0.3 at 50 °C.     

Unique structural and magnetic traits of Nd3+-substituted Co–Zn nanoferrites

Because of the technological potential of magnetic spinel nanoferrites, we prepared neodymium ion (Nd³⁺)-substituted cobalt-zinc ferrites (CZFs) with the form Co_0.5Zn_0.5Nd_xFe_2–xO₄ (0.0 ≤ x ≤ 0.05) via a hydrothermal method. The as-prepared samples were thoroughly characterized using various analytical techniques. XRD, FTIR and FESEM analyses confirm the formation of a cubic spinel phase of the CZFNPs (CZF nanoparticles). A decrease in the lattice parameter due to the substitution of Fe³⁺ by Nd³⁺ in the lattice structures is manifested in the XRD refinement data. The magnetic properties of the proposed CZFNPs were evaluated in terms of the saturation magnetization, remanence, coercivity, squareness ratio and magnetic moment. These CZFNPs exhibit superparamagnetic behaviors at room temperature. Moreover, the Nd³⁺ inclusion does not significantly affect the measured magnetizations and coercivities of the CZFNPs. Samples containing 0.01 and 0.03 Nd³⁺ exhibit lower saturation magnetizations than that of the pristine product. The squareness ratios much less than 0.53 are ascribed to surface spin disordering. The unique magnetic traits of the synthesized CZFNPs are primarily attributed to the substitution of Fe³⁺ ions, with smaller ionic radii, by Nd³⁺ ions, with larger ionic radii. The proposed CZFNPs may be useful for diverse magneto-optic applications.     

A thermodynamic study on cobalt containing calcium ferrite and calcium iron silicate slags at 1573 K

Redox equilibria, activities of cobalt, iron and their oxides in calcium ferrite and calcium ironsilicate slags, were measured through metal-slag-gas equilibrium experiments under controlled oxygen potentials (10⁻⁷ to 3 × 10⁻⁷ atm) at 1573 K. Results on the redox equilibria show that addition of CoO to calcium ferrite slag increases the equilibrium Fe³⁺/Fe²⁺ ratio in these melts. Measured activities of CoO and FeO showed positive deviations from ideal behavior, while that of Fe₂O₃ showed negative deviation. Partial substitution of CaO by SiO₂, by up to 4 wt pct SiO₂ in the calcium ferrite based melts, resulted in increases in the activity coefficients of CoO and Fe₂O₃. Phase equilibria studies on the cobalt containing CaO-FeO-Fe₂O₃-SiO₂ slags were also carried out using the drop-quench technique. Good agreement between the activity data and the liquidus temperature with respect to magnetite solid solution containing CoO was observed.     

Novel Application of Alkali Oxides in Basic Tundish Fluxes for Enhancing Inclusion Removal in 321 Stainless Steels

Fundamental work on the effect of alkali oxides including Li₂O, Na₂O, and K₂O on the absorption ability of inclusions in a typical basic tundish flux for 321 stainless steels has been studied. The effects on the absorption ability are dependent on the type of alkali oxides and the amount composed within the tundish flux. Results from kinetics studies using an induction furnace at 1823 K (1550 °C) on the reaction of tundish fluxes containing alkali oxides with 321 stainless steels suggest minimal improvement with Li₂O and Na₂O additions and in some cases hindered inclusion removal, but K₂O additions seems to significantly improve the cleanliness in the as-quenched 321 stainless steel melts compared to preexisting tundish flux compositions. Both Li₂O and Na₂O significantly lower the viscosity of the melt, while K₂O increases the viscosity. Although alkali oxides have a propensity to enhance the cohesion of aluminate melts due to the ionic compensation effect in [AlO₄]^5?-tetrahedral structural units, this effect was not pronounced for Li₂O and Na₂O compared to K₂O additions, which may be due to the large ionic radius size of potassium. An automated SEM–EDS analysis was utilized to identify the inclusion morphology and abundance within the steel. Fourier transform infrared spectroscopy was used to identify the effects of alkali oxides on the tundish flux structure for improved absorption capability and described the characteristic effect of K₂O on increasing the asymmetric stretching vibrations of [AlO₄]^5?-tetrahedral structural units and Si-O-Al bonding within the flux, thus polymerizing the flux and selectively absorbing inclusions.     

Solvent extraction of metals by carboxylic acids

The solvent extraction behaviour of solutions of Versatic 10, naphthenic, 2-bromodecanoic and 3,5-diisopropylsalicylic acids in xylene towards a large number of metal cations is described. The effect of the structure of the carboxylic acids upon their extraction behaviour is discussed, and relationships between the experimental pH_0.5 values and characteristic properties of the metal cations, such as the stability constants of carboxylate complexes and ionic radii, are examined.For some metals (Cu²⁺, Zn²⁺, Cd²⁺, Hg²⁺, Pd²⁺, Fe³⁺, AI³⁺, Ga₃₊, In³⁺, and Tl³⁺), a comparison of their extraction from nitrate and, chloride media has been made, and the observed differences have been quantitatively related to the formation constants of the chloro-complexes of the respective metals.