Daniel Fornasiero

Particle-bubble collision models--a review

A critical review of the various models existing in the literature for the calculation of the collision efficiency between particles and single, rising gas bubbles is presented. Although all of these collision models predict that the collision efficiency increases with particle size, their dependence on the latter is different because of the various assumptions and hydrodynamic conditions used in each model. Collision efficiencies of quartz particles with single bubbles have been obtained from experimental flotation experiments under conditions where the attachment and stability efficiencies were at, or near, unity. These collision efficiencies were then used to test various collision models. Good agreement between the experimental and calculated collision efficiencies was only obtained with the Generalised Sutherland Equation. The differences in collision efficiencies obtained between the various models were mainly explained in terms of, firstly, the degree of mobility of the bubble surface and, secondly, a consideration of the inertial forces acting on the particles.

Bubble particle heterocoagulation under turbulent conditions

An analytical model that enables the calculation of the flotation rate constant of particles as a function of particle size with, as input parameters, measurable particle, bubble, and hydrodynamic quantities has been derived. This model includes the frequency of collisions between particles and bubbles as well as their efficiencies of collision, attachment, and stability. The generalized Sutherland equation collision model and the modified Dobby-Finch attachment model developed previously for potential flow conditions were used to calculate the efficiencies of particle-bubble collision and attachment, respectively. The bubble-particle stability efficiency model includes the various forces acting between the bubble and the attached particle, and we demonstrate that it depends mainly on the relative magnitude of particle contact angle and turbulent dissipation energy. The flotation rate constants calculated with these models produced the characteristic shape of the flotation rate constant versus particle size curve, with a maximum appearing at intermediate particle size. The low flotation rate constants of fine and coarse particles result from their low efficiency of collision and low efficiencies of attachment and stability with gas bubbles, respectively. The flotation rate constants calculated with these models were compared with the experimental flotation rate constants of methylated quartz particles with diameters between 8 and 80 micro m interacting with gas bubbles under turbulent conditions in a Rushton flotation cell. Agreement between theory and experiment is satisfactory.

An electrokinetic study of pyrite oxidation

Abstract The electrokinetic properties of pyrite have been studied as a function of pH in various pretreatment conditions: conditioning in argon, nitrogen, air and oxygen atmospheres, at several conditioning pH values and for several conditioning times. The zeta potential of “virgin” pyrite is negative over the pH range from 3–10. Upon exposure to oxygen, a reversal of zeta potential from negative to positive occurs at low pH values. This reversal takes place more rapidly at lower conditioning pH values. The isoelectric point of “virgin” pyrite has been estimated to lie at a pH of 1.2 ± 0.4. A mechanism for dissolution of iron from the surface followed by electrostatic adsorption of positively charged iron hydroxide species onto negatively charged surface sites on pyrite has been proposed to account for the observed results. Good agreement has been found between the experimental and the calculated zeta potential data using electrical double layer theories.

Effect of oxidation on the collectorless flotation of chalcopyrite

Abstract The effects of conditioning gas and pH on the collectorless flotation of chalcopyrite have been studied. To explain the changes in flotation performance during mineral oxidation, a study of the oxidation species produced in solution and on the mineral surface has been performed. Results of mineral dissolution, zeta potential measurements and X-ray photoelectron spectroscopy have shown that iron and, to a lesser extent, copper dissolve from the chalcopyrite lattice leaving a metal-deficient sulfur-rich surface and readsorb or precipitate as hydroxide species on the chalcopyrite surface. The hydrophobicity of the surface and therefore the chalcopyrite flotation are then controlled by these two processes, i.e. the metal dissolution that produces a hydrophobic surface and the metal hydroxide precipitation that produces a hydrophilic surface. The kinetics of formation and precipitation of these metal hydroxide species may control the surface hydrophobicity.

Zeta potential study of the oxidation of copper sulfide minerals

Abstract The zeta potential of the copper sulfide minerals, chalcocite, covellite, chalcopyrite, bornite, enargite and tennantite was measured as a function of pH and oxidising conditions. The changes in zeta potential observed in this study are consistent with the presence of a copper hydroxide layer covering a metal-deficient sulfur-rich surface and with the extent of this copper hydroxide coverage increasing with oxidation conditions. The existence of these surface species and their percentage were also confirmed by X-ray photoelectron spectroscopy. Analysis of the zeta potential data revealed that during the acid titration of the minerals, dissolution of the surface copper hydroxide layer occurs at pH values less than 8 while during the base titration, precipitation of copper hydroxide on the mineral surface is observed at pH values higher than 6. Hysteresis between the zeta potential acid and base titration curves was only observed in oxidising conditions and is attributed to the dissolution of the minerals at acidic pH values. The following ranking for the oxidation of these minerals is obtained: chalcocite>tennantite>enargite>bornite>covellite>chalcopyrite.

Polymer depressants at the talc–water interface: adsorption isotherm, microflotation and electrokinetic studies

Abstract The behaviour of polymer depressants at the talc–water interface was investigated as a function of ionic strength and pH. Adsorption isotherms, microflotation and electrokinetic studies were used to examine the surface interactions involved. The polymers examined were carboxymethyl cellulose (CMC) and two synthetic polyacrylamides (PAM-A and PAM-N). The adsorption of the two anionic polymers, CMC and PAM-A, on talc, and hence, talc depression, is greatest when electrostatic repulsion is minimized. At high pH values and low ionic strength, the adsorption density of the anionic polymers on talc is low whilst at either high ionic strength or low pH, the adsorption density increases; talc depression is therefore largely influenced by variations in solution conditions. The adsorption of the nonionic polymers, PAM-N, on talc is not influenced by ionic strength or pH. The polymers exhibit Langmuir adsorption behaviour with adsorption occurring on the talc face surface and, possibly, the edge surface. The adsorbed polymer layer thicknesses, calculated from electrophoretic mobility measurements, corresponded to a monolayer indicating that adsorption of the polymers onto the talc surface occurs in a flat conformation.

Control of grinding conditions in the flotation of chalcopyrite and its separation from pyrite

Abstract A specially designed mill which allowed the control of pH throughout grinding was used to study the effect of grinding conditions on chalcopyrite flotation and chalcopyrite separation from pyrite. The mechanism of galvanic interaction between minerals and grinding media was investigated by ethylene diamine-tetra acetic acid disodium salt (EDTA) extraction and X-ray photoelectron spectroscopy (XPS) measurements. Chalcopyrite flotation was strongly dependent on both iron oxidation species and metal deficiency on the chalcopyrite surface. Iron oxidation species from grinding media played a dominant role in depressing chalcopyrite flotation, while metal deficiency from chalcopyrite oxidation improved chalcopyrite flotation. Therefore, chromium grinding medium produced a higher chalcopyrite recovery than mild steel grinding medium while gas purging during grinding had little effect on chalcopyrite flotation. Chalcopyrite separation from pyrite was affected by the activation of pyrite flotation by copper species dissolved from chalcopyrite. Grinding media had a large effect on the reduction of copper(II) to copper(I) on the pyrite surface. The reducing grinding condition generated by mild steel medium favoured formation of copper(I) sulphide phase, which resulted in high pyrite activation. Thus, chromium medium produced better chalcopyrite selectivity against pyrite than the mild steel medium.

THE EFFECT OF SURFACE MODIFICATION BY AN ORGANOSILANE ON THE ELECTROCHEMICAL PROPERTIES OF KAOLINITE

The electrochemical properties of kaolinite before and after modification with chlorodimethyl-octadecylsilane have been studied by electrophoretic mobility, surface charge titration, and extrapolated yield stress measurements as a function of pH and ionic strength. A heteropolar model of kaolinite, which views the particles as having a pH-independent permanent negative charge on the basal planes and a pH-dependent charge on the edges, has been used to model the data. The zeta potential and surface charge titration experimental data have been used simultaneously to calculate acid and ion complexation equilibrium constants using a surface complex model of the oxide-solution interface. The experimental data were modeled following subtraction of the basal plane constant negative charge, describing only the edge electrical double layer properties. Extrapolated yield stress measurements along with the electrochemical data were used to determine the edge isoelectric points for both the unmodified and modified kaolinite and were found to occur at pH values of 5.25 and 6.75, respectively. Acidity and ion complexation constants were calculated for both sets of data before and after surface modification. The acidity constants, pKa1 = 5.0 and pKa2 = 6.0, calculated for unmodified kaolinite, correlate closely with acidity constants determined by oxide studies for acidic sites on alumina and silica, respectively, and were, therefore, assigned to pH-dependent specific chemical surface hydroxyl groups on the edges of kaolinite. The parameters calculated for the modified kaolinite indicate that the silane has reacted with these pH-dependent hydroxyl groups causing both a change in their acidity and a concomitant decrease in their ionization capacity. Infrared data show that the long chain hydrocarbon silane is held by strong bonding to the kaolinite surface as it remains attached after washing with cyclohexane, heating, and dispersion in an aqueous environment.

Calculation of the flotation rate constant of chalcopyrite particles in an ore

Abstract The flotation recovery of chalcopyrite particles in a complex sulfide ore was measured in a Rushton turbine flotation cell as a function of particle size. The experimental flotation rate constants of these particles were compared to those calculated using a recently developed flotation model that incorporates contributions from the efficiencies of collision, attachment and stability between particles and bubbles, as well as their frequency of collision. For these calculations, the contact angle of the chalcopyrite particles in the ore was obtained independently using an approach based upon Time of Flight Secondary Ion Mass spectroscopy (ToF-SIMS). It was found that the calculated flotation rate constants were in good agreement with the experimental data and able to reproduce the characteristic maximum in flotation rate constant for particles of intermediate size. The values of bubble velocity and turbulent dissipation energy derived from these calculations are relatively low and may well correspond to mean values of these parameters inside the flotation cell.

The terminal rise velocity of 10–100 μm diameter bubbles in water

Single bubbles of very pure N2, He, air and CO2 were formed in a quiescent environment in ultra-clean water, with diameters ranging from 10 to 100 mum. Their terminal rise velocities were measured by high-speed video microscopy. For N2, He and air, excellent agreement with the Hadamard-Rybczynski (H-R) equation was observed, indicating that slip was occurring at the liquid-vapor interface. For CO2 bubbles with diameters less than 60 microm, the terminal rise velocities exceeded those predicted by the H-R equation. This effect was ascribed to the enhanced solubility of CO2 compared with the other gases examined. The presence of a diffusion boundary layer may be responsible for the increased terminal velocity of very small CO2 bubbles.