Desmond Tromans

Temperature and pressure dependent solubility of oxygen in water: a thermodynamic analysis

Abstract A thermodynamic analysis of the temperature dependent equilibrium between oxygen solubility (activity) in pure water and oxygen partial pressure PO2 was undertaken. Experimental oxygen solubility sources were used to determine a set of self consistent values for the chemical potential, entropy and partial molar heat capacity of dissolved oxygen. These were combined with established thermodynamic data for gaseous oxygen to develop a single thermodynamic equation describing the relationship between gaseous oxygen fugacity, dissolved oxygen activity, temperature and the equilibrium constant k. This equation was consistent with k values calculated from published experimental solubility data for temperatures from 273 to 616 K and pressures to 60 atm, encompassing the conditions encountered during oxygen leaching operations. Within these ranges, it was shown that the gas fugacity and dissolved oxygen activity coefficients were sufficiently close to unity that the k-equation may be used to relate the molal concentration caq of dissolved O2 for any combination of temperature T (K) and PO2 (atm), c aq =P O 2 0.046T 2 +203.357T ln (T/298)−(299.378+0.092T)(T−298)−20.591×10 3 (8.3144)T The significance of the difference in heat capacity between gaseous and dissolved oxygen was discussed, leading to suggestions for incorporating the effects of dissolved mineral solutes into the oxygen solubility equation.

Enhanced dissolution of minerals: stored energy, amorphism and mechanical activation

A theoretical evaluation of the origin and role of stored energy in the phenomenon of mechanically-activated dissolution of finely milled minerals has been undertaken. The changing mechanical behaviour of minerals with decreasing particle size is examined, leading to the treatment of stored energy in terms of the generation and retention of crystalline imperfections (dislocations) during extended milling. Development of amorphism, and polymorphism, is treated as a direct consequence of decreased periodicity in the crystal lattice that is associated with an increased accumulation of dislocations. Quantitative estimates of stored energy are presented and used to quantify changes in dissolution kinetics in terms of a decreased activation energy of dissolution and hence, an increase in the relative rate of dissolution.

Oxygen solubility modeling in inorganic solutions: concentration, temperature and pressure effects

A model has been developed to predict and estimate the PO2 and T-dependent molal solubility, (caq)I, of oxygen in aqueous solutions containing an inorganic solute, I. It is based on the concept that (caq)I may be obtained by multiplying the thermodynamics-based PO2 and T-dependent equation for the molal solubility of oxygen in pure water, caq, by a factor (φ-factor) that is <1 and which is functionally dependent only upon the molal concentration, CI, of I. The CI-dependent φ-functions of 21 different Is have been determined from an analysis of published solubility data. Subsequently, the (caq)I/PO2 ratios for several solutions, containing single and multiple I, were predicted for a wide range of T by combining the corresponding φ-function with the thermodynamics-based equation for pure water. The predicted ratios were compared with available published data relating to effects of T. Satisfactory agreement was found between the predicted and experimental behavior, thereby validating the general principles of the model. It is anticipated that the model will be provide a foundation for predicting and estimating oxygen solubility under widely different hydrometallurgy leaching conditions, ranging from bio-leaching processes to oxygen pressure leaching.

Aqueous Potential‐pH Equilibria in Copper‐Benzotriazole Systems

Thermodynamic data, together with equilibria involving 1-H-benzotriazole (BTAH) and cuprous benzotriazoie species (CuBTA), have been used to construct aqueous E-pH diagrams for Cu-BTAH-H 2 O and Cu-BTAH-Cl -H 2 O systems. The diagrams are the first of their kind for BTAH-containing solutions and show the domains of stability of the CuBTA salt. They are in reasonable agreement with the reported major effects of multilayer CuBTA polymer films on the corrosion inhibition of copper. Some kinetic considerations are discussed and the overall results provide a basis for understanding the combined effects of E, pH, Cl, and BTAH concentrations (activities) on corrosion inhibition of Cu.

Fracture toughness and surface energies of minerals: theoretical estimates for oxides, sulphides, silicates and halides

Theoretical estimates of the ideal fracture toughness and surface energies of 48 minerals have been modelled by treating them as ionic solids, using the Born model of bonding. Development of the toughness model required calculation of the crystal binding enthalpy from thermodynamic data and the use of published elastic constants for single crystals. The principal minerals studied were oxides, sulphides and silicates, plus a few halides and sulphates. The study showed grain boundary fracture is most likely in singlephase polycrystalline minerals. However, the fracture toughness for grain boundary cracking in pure minerals is not significantly lower than that for intragranular cracking. The computed critical stress intensity values for intragranular cracking, KIC, ranged from 0.131 to 2.774 MPa m 1=2 . The critical energy release rates for intragranular cracking, GIC, ranged from 0.676 to 20.75 J m � 2 . The results are discussed with relevance to mineral comminution, including energy considerations, particle impact efficiency, and lower limiting particle size. 2002 Elsevier Science Ltd. All rights reserved.

Caustic Stress Corrosion Cracking of Mild Steel

The stress corrosion cracking (SCC) behavior of cold worked mild steel in hot, aqueous, 33 pct NaOH solutions was studied with prefatigue cracked double cantilever beam specimens. SCC kinetics were studied under freely corroding potentials (Ecorr ≈ −1.00 VSHE) and potentiostatic potentials of −0.76 VSHE near the active-passive transition. The pH of the liquid within the crack was determined and fractography was studied by scanning electron microscopy. Cracking was transgranular atEcorr, intergranular at −0.76 VSHE, and produced no detectable change in crack liquid pH from that of the bulk solution. Crack rates were dependent upon temperature, potential, and stress intensity (K1). The apparent activation energy in Region II, where crack growth rate was independent ofK, was ∼ 24kJ/mol for both cracking modes. This was considered to be due to mixed rate control involving activation polarization and mass transport processes. The mechanism of cracking was entirely consistent with metal dissolution at –0.76 VSHE and may involve hydrogen embrittlement and/or dissolution effects atEcorr.

Pitting Corrosion of Passivated Zinc Monocrystals

Abstract The pitting and polarization characteristics of low-index sunfaces of zinc (Zn) monocrystals were studied in weakly alkaline (pH 9.2) solutions containing 0.1 M sodium chloride (NaCl), using single-cycle potentiodynamic polarization tests and a scan rate of 0.5 mV/s. Tests were conducted in the presence and absence of a bicarbonate (HCO3−) /carbonate (CO32−) buffer. Electrode surface orientations and crystallographic directions lying in these surfaces were determined using x-ray techniques. The surfaces were close to the (0001), (1010), and (1120) planes. Pit morphology was studied by scanning electron microscopy (SEM). Passivity was observed only in the buffered solution, and pitting of the passivated surface occurred near −0.85 VSCE. The (0001) basal plane appeared to have a slightly better pitting resistance and a lower passive current density (ip) than the other orientations. Basal plane surfaces prepared by cleavage exhibited superior resistance to anodic dissolution in the unbuffered soluti...

Anodic Behavior of Copper in Iodide Solutions Comparison with Chloride and Effect of Benzotriazole‐Type Inhibitors

The anodic polarization behavior of copper in 1 M NaI at ∼24°C was compared with calculations based on thermodynamic equilibria and shown to be consistent with the potential-pH diagram for the Cu-I - -H 2 O system. An active region was predicted and observed where cuprous complexes, CuI 2 - , are formed. The active region exhibited a Tafel slope of 60 mV, similar to that observed in 1 M NaCl where CuCl 2 - complexes are present. The active region occurred at less noble potentials in 1 M NaI than in 1 M NaCl and was consistent with an anodic dissolution model based on mass-transport control of the cuprous complexes. Polarization studies were conducted on copper in the presence of two related inhibitors, benzotriazole and tolytriazole. The inhibitors behaved in a similar manner to each other in 1 M NaI and 1 M NaCl and promoted passivation. Inhibitor-induced passivation commenced in the active region on oxide-free surfaces. The shape of the passive polarization curve in 1 M NaI differed from 1 M NaCl, suggesting structural differences between the inhibitor films in the two solutions. Possible differences are discussed in terms of the presence of copper oxides in the film, together with an assessment of the critical step in the initiation of passivation

Anodic Behavior of Copper in Weakly Alkaline Solutions

The anodic polarization behavior of copper was studied in the pH range 9.4 to 10.8, where pH was controlled (buffered) by additions of either borate, bicarbonate/carbonate, ammonia, or sodium hydroxide. Several solutions contained either 1M sulfate, 1M chloride, or 5M chloride. Passivation effects were observed in all solutions, except those containing 5M chloride, and were accompanied by two anodic current peaks involving formation of Cu 2 O films and films containing Cu(OH) 2 and CuO

A fracture mechanics study of stress corrosion cracking of type-316 austenitic steel

A new test specimen configuration, designated the T-notch double cantilever beam (TNDCB), was developed, calibrated and employed for a fracture mechanics study of stress corrosion cracking (SCC) of cold worked Type-316 austenitic stainless steel exposed to hot aqueous solutions of 44.7 wt pct MgCl2. The effects of stress intensity (KI), temperature (T) and electrochemical potential (E) upon the crack velocity (v) and fractography were investigated. The stress intensity (KISCC) below whichv became immeasurably small was ∼12 MN·m−3/2. Above this value, three regions of behavior were observed. Region I exhibitedKI dependent cracking followed by Region II which exhibitedKI independent cracking and an apparent activation energy of 63 to 67 kJ/mol, followed by Region III where cracking again became dependent uponKI. The relative proportions of intergranular and transgranular crack paths were markedly dependent upon bothKI andE, and less sensitive toT. Crack velocity was insensitive to small changes inE with respect to the free corrosion potentials (Ecorr), but could be terminated by an applied active potential of ∼−0.35 VSCE. The pH within the propagating crack was estimated to be <1.0 atEcorr, rising to ∼4.5 at −0.35 VSCE. The mechanism of SCC was discussed with respect to film rupture events caused by crack tip plastic deformation, adsorption controlled processes on the metal surface, and hydrogen diffusion in the metal lattice.