Jan D. Miller

Reaction mechanism for the acid ferric sulfate leaching of chalcopyrite

The acid ferric sulfate leaching of chalcopyrite, CuFeS2 + 4Fe+3 = Cu+2 + 5Fe+2 + 2S0 was studied using monosize particles in a well stirred reactor at ambient pressure and dilute solid phase concentration in order to obtain fundamental details of the reaction kinetics. The principal rate limiting step for this electrochemical reaction appears to be a transport process through the elemental sulfur reaction product. This conclusion has been reached in other investigations and is supported by data from this investigation in which the reaction rate was found to have an inverse second order dependence on the initial particle diameter. Furthermore, the reaction kinetics were found to be independent of Fe+3, Fe+2, Cu+2 and H2SO4 in the range of additions studied. The unique aspect of this particular research effort is that data analysis, using the Wagner theory of oxidation, suggests that the rate limiting process may be the transport of electrons through the elemental sulfur layer. Predicted reaction rates calculated from first principles using the physicochemical properties of the system (conductivity of elemental sulfur and the free energy change for the reaction) agree satisfactorily with experimentally determined rates. Further evidence which supports this analysis includes an experimental activation energy of 20 kcal/mol (83.7 kJ/mol) which is approximately the same as the apparent activation energy for the transfer of electrons through elemental sulfur, 23 kcal/ mol (96.3 kJ/mol) calculated from both conductivity and electron mobility measurements reported in the literature.

Surface force measurements at the basal planes of ordered kaolinite particles

An experimental procedure is presented to order kaolinite particles on substrates for interrogation of the two basal plane surfaces by atomic force microscopy. Surface force measurements were performed between a silicon nitride tip and each of the two faces (silica tetrahedral face and alumina octahedral face) of kaolinite in 1 mM KCl solution at pH 4, 5, 6, 8 and 10, using atomic force microscopy. The colloidal force measurements reveal that the silica tetrahedral face of kaolinite is negatively charged at pH>4, whereas the alumina octahedral face of kaolinite is positively charged at pH<6, and negatively charged at pH>8. Such measurements have not been reported previously and the results suggest that the iso-electric point of the silica tetrahedral face is at pH<4, and that the iso-electric point of the alumina octahedral face lies between pH 6 and 8. These results contradict the generally accepted view that basal plane surfaces of kaolinite carry a permanent negative charge due to minor substitution of Al(3+) for Si(4+) in the silica tetrahedral layer, and suggest some surface charge dependency of the two faces with respect to solution pH. With this new information it may be possible to further explain the electrokinetic behavior of kaolinite particles, and their interactions in aqueous suspensions.

Particle interactions in kaolinite suspensions and corresponding aggregate structures

The surface charge densities of the silica face surface and the alumina face surface of kaolinite particles, recently determined from surface force measurements using atomic force microscopy, show a distinct dependence on the pH of the system. The silica face was found to be negatively charged at pH>4, whereas the alumina face surface was found to be positively charged at pH<6, and negatively charged at pH>8. The surface charge densities of the silica face and the alumina face were utilized in this study to determine the interaction energies between different surfaces of kaolinite particles. Results indicate that the silica face-alumina face interaction is dominant for kaolinite particle aggregation at low pH. This face-face association increases the stacking of kaolinite layers, and thereby promotes the edge-face (edge-silica face and edge-alumina face) and face-face (silica face-alumina face) associations with increasing pH, and hence the maximum shear-yield stress at pH 5-5.5. With further increase in pH, the face-face and edge-face association decreases due to increasing surface charge density on the silica face and the edge surfaces, and decreasing surface charge density on the alumina face. At high pH, all kaolinite surfaces become negatively charged, kaolinite particles are dispersed, and the suspension is stabilized. The face-face association at low pH has been confirmed from cryo-SEM images of kaolinite aggregates taken from suspension which show that the particles are mostly organized in a face-face and edge-face manner. At higher pH conditions, the cryo-SEM images of the kaolinite aggregates reveal a lower degree of consolidation and the edge-edge association is evident.

Wetting characteristics of liquid drops at heterogeneous surfaces

Abstract Well-defined heterogeneous surfaces consisting of hydrophobic and hydrophilic regions were prepared on gold (a 2000 A gold film supported on an Si/SiO2/Ti substrate) by patterning self-assembled monolayers (SAMs), using an elastomer stamp. One surface was composed of alternating and parallel hydrophobic (2.5 μm) and hydrophilic (3 μm) strips, and the second surface consisted of alternating hydrophilic squares (3 μm × 3 μm) separated by hydrophobic strips (2.5 μm). The wetting characteristics of these well-defined heterogeneous solid surfaces were examined by contact angle measurements. The contact angles for water drops, which varied in pH from 5.8 to 10.0, were measured with the strips both tangential to and normal to the three-phase contact line. The experimental contact angles are in good agreement with theory as calculated from the Cassie equation when the three-phase contact line is non-contorted (i.e. the three-phase contact line is situated along the hydrophobic strip). On the other hand, when the strips are normal to the drop edge, corrugation of the three-contact line affects the contact angle significantly. Contact angles, measured with the strips normal to the drop edge, were lower by 7–16° than those calculated from the Cassie equation. Analysis of these measurements, together with contact angle/drop size measurements for fully hydrophobic and hydrophilic surfaces, demonstrate the validity of a modified Cassie equation that includes a term describing the line tension contribution.

The chemistry of gold solvent extraction from cyanide solution using modified amines

Abstract The concept of amine basicity is discussed and used to explain the selective extraction of gold from alkaline cyanide solution. The apparent basicity of simple weak base amines with respect to Au(CN) 2 can be increased as much as 5 orders of magnitude by the addition of organic phosphorus oxide compounds (TBP, DBBP, etc.). It has been shown that the modified amines do not exhibit this effect to the same extent for other cyanoanions, such as Cu(CN) 3− 4 and Zn(CN) 2− 4 . The reasons for this selectivity as well as the stoichiometry of the gold extraction reaction and the nature of the extracted species are described from the results of slope analysis and infrared spectroscopy.

Reaction kinetics for the leaching of MnO2 by sulfur dioxide

Abstract The reaction kinetics for the leaching of MnO2 by sulfur dioxide have been studied using studied using monosize particles at dilute solid phase concentrations in a stirred reactor to determine the important chemical factors which govern the kinetic response of the system. The conclusion that the reaction rate is limited by a chemical reaction at the MnO2 surface is supported by: (1) an apparent activation energy of 35.9 kJ/mol (8.6 kcal/mol), (2) the inverse first-order relationship between the rate constant k and the initial particle diameter, (3) the independence of the reaction rate on stirring speed and, more importantly, the magnitude of the calculated reaction velocity constant (∼10−3 cm/s compared to predicted mass-transfer coefficients of 10−2 cm/s), and (4) the one-half order dependence of the reaction rate with respect to the SO2 concentration. The rate-limiting step is considered to be an electrochemical surface reaction, a conclusion which is substantiated by electrode half cell potential measurements. Using the Butler-Volmer equation, a theoretical analysis of the electrochemical reaction resulted in the following rate equation: d n MnO 2 d t =κ ∗ K 0.5 a A c [ SO 2 ] 0.5 total [ H + ] [[ H + ]+K a 0.5 which was consistent with the experimental results.

FTIR STUDY OF DEUTERATED MONTMORILLONITES: STRUCTURAL FEATURES RELEVANT TO PILLARED CLAY STABILITY

FTIR studies of six partially-deuterated montmorillonites (MS) reveal the presence of two O-D stretching bands, one between 2702–2728 cm-1 and another near 2680 cm-1. For homoionic (Li, Na, Mg, Ca, or La) Wyoming-type MS, the position of the higher frequency band, designated as (O-D)h, is between 2714–2728 cm-1, whereas for homoionic Cheto-type MS it is between 2702–2706 cm-1. The lower frequency band, designated as (O-D)1, is in the narrow range of 2674–2684 cm-1. Resolution of two corresponding O-H bands, appearing near 3670 and 3635 cm-1, was observed only after partial dehydroxylation of the smectites. The changes in the relative intensities of the two O-D stretching bands as a function of the smectite type and of the Lewis acidity (charge density) of the exchangeable ion were determined. For Wyoming-type MS, the intensity of the (O-D)h band is much lower than that of the (O-D)l band, whereas for Cheto-type MS, the intensity of the (O-D)h band is about equal or slightly higher than that of the (O-D)l band. The observed resolution can be ascribed tentatively to the presence of (at least) two types of octahedral OH groups in the smectites, the (O-D)h band being assigned to AlMgOH and the (O-D)1 band to AlAlOH groups. Pillaring of Cheto-type MS with hydroxy-Al13 oligocations resulted in products showing much higher thermal stability between 400–600°C compared to that of identically pillared Wyoming-type MS. Compositional and other factors, e.g., CEC values and mode of pillaring, may cause this difference in stability.

Dextrin adsorption by oxidized coal

Abstract Dextrin can be used as a depressant to varying degrees of success in the differential flotation of pyrite from coal. The objective of this investigation has been further identification of physico-chemical factors that control the adsorption of dextrin by coal. The research program involved oxidation of coal samples (both natural and demineralized), determination of oxygen functional groups (carboxylic and phenolic), characterization of coal samples in terms of their hydrophobic character and in terms of their electrokinetic behavior, and measurement of dextrin adsorption densities as related to the oxygen group content. Experimental results support previous suggestions that dextrin exhibits Langmuir adsorption behavior with hydrophobic bonding. The more hydrophobic demineralized coal was found to exhibit an adsorption density twice the adsorption density of natural coal. In both cases the extent of adsorption decreased with an increased level of oxidation but was largely independent of pH. Adsorption free energies of about −5.5 kcal/mol, independent of the extent of oxidation, were calculated.

Induction-time measurements at a particle bed

Abstract Induction times for a variety of natural hydrophobic minerals have been measured at a particle bed with an electronic induction timer. The experimental results clearly show a strong dependence on particle size. Detailed analysis suggests that the measured induction time at a particle bed should properly be defined as the bubble attachment time, which consists of the actual induction time (film-rupture time) and the film-displacement time. The relative significance of these two variables at a polished surface is quite different from that at particle surfaces.

Attraction between hydrophobic surfaces studied by atomic force microscopy

Abstract Attraction between hydrophobic surfaces, known as the hydrophobic force, is critically important for attachment of particles to air bubbles in flotation. However, the origins and models for this attractive force between hydrophobic surfaces have been a source of debate since the first direct measurements of this force in the early 1980s. Using an atomic force microscope (AFM) we studied the attraction between an AFM hydrophobic probe and a flat hydrophobic surface in water, in water–ethanol mixtures, and in water saturated by gases with different solubility. The strong attractive force with long-range jump-in attachment positions decreases with an increase in the ethanol content and disappears in pure ethanol. The size of steps on the force curves depends on the gas solubility. However, the measured forces do not depend on the gas solubility significantly. The influence of surface roughness and heterogeneity appear to be significant. Experimental results indicate the role of surface stabilized submicron-sized bubbles in the hydrophobic attraction. This is in line with recent direct and indirect evidences for the presence of gaseous bubbles at hydrophobic surfaces as well as with the early insights of flotation scientists.