S. Chander

Physical and chemical interactions in coal flotation

Abstract Coal flotation is a complex process involving several phases (particles, oil droplets and air bubbles). These phases simultaneously interact with each other and with other species such as the molecules of a promoting reagent and dissolved ions in water. The physical and chemical interactions determine the outcome of the flotation process. Physical and chemical interactions between fine coal particles could lead to aggregation, especially for high rank coals. Non-selective particle aggregation could be said to be the main reason for the selectivity problems in coal flotation. It should be addressed by physical (conditioning) or chemical (promoters) pretreatment before or during flotation. Although the interactions between the oil droplets and coal particles are actually favored, stabilization of the oil droplets by small amounts of fine hydrophobic particles may lead to a decrease in selectivity and an increase in oil consumption. These problems could be remedied by use of promoters that modify the coal surface for suitable particle–particle, droplet–particle and particle–bubble contact while emulsifying the oil droplets. The role of promoters may be different for different types of coals, however. They could be employed as modifiers to increase the hydrophobicity of low rank coals whereas their main role might be emulsification and aggregation control for high rank coals. In this paper, a detailed description of the various phases in coal flotation, their physical and chemical interactions with each other in the flotation pulp, the major parameters that affect these interactions and how these interactions, in turn, influence the flotation process are discussed.

First-order flotation kinetics models and methods for estimation of the true distribution of flotation rate constants

Abstract To improve their versatility, many first-order flotation kinetics models with distributions of flotation rate constants were redefined so that they could all be represented by the same set of three model parameters. As a result, the width of the distribution become independent of its mean, and parameters of the model and the curve fitting errors, became virtually the same, independent of the chosen distribution function. For the modified three-parameter models, the curve fitting errors were much smaller and their robustness, measured by PRESS residuals, was much better when compared to the corresponding two-parameter models. Three different methods were compared to perform flotation kinetics analysis and estimate model parameters. In Method I, recovery vs. time data were used to obtain model parameters. No significant insight into the distribution of rate constants could be obtained because the distributions were presupposed. In Method II, the froth products were fractionated into several size fractions and the data for each fraction were fitted to a model. This task was easy to perform and the method could describe the flotation kinetics reasonably well. In method III, flotation products were fractionated into many size-specific gravity fractions. The procedure involved a large amount of time and effort and it generated relatively large errors. Based on the analysis presented in this article, it was concluded the smallest errors were obtained with Method II. The overall distribution of flotation rate constants could be obtained from a weighted average of the distributions of individual size fractions. The distributions so obtained were demonstrated to be less sensitive to the choice of the model used to represent the kinetics of individual size fractions, and therefore can be assumed to be “true” representation of the flotation rate distribution.

Electrochemistry of sulfide flotation: Growth characteristics of surface coatings and their properties, with special reference to chalcopyrite and pyrite

Abstract Characteristics of surface coatings which form when sulfide minerals react with aqueous solutions in the absence and presence of flotation reagents are discussed. The coatings form by electrochemical and chemical reactions, and dissolution, adsorption, surface reaction and precipitation processes. Of special interest to flotation is the proposed formation of a metal-deficient layer adjacent to the unreacted sulfide mineral and a precipitated layer of metal-hydroxide, oxide, sulfate, etc., adjacent to the solution phase. If the metal-oxide layer does not adhere to the metal-deficient layer collectorless flotation occurs, otherwise not. The characteristics of gangue minerals play a special role in detachment of the oxide under certain conditions. Since both kinetic and thermodynamic factors are involved, an interaction of hydrodynamic and chemical variables occurs. Selected results are discussed, particularly for the chalcopyrite/solution interface.

A brief review of pulp potentials in sulfide flotation

Abstract A state of the art review of pulp potentials in sulfide mineral flotation was presented. The measurement and interpretation of pulp potentials in sulfide mineral flotation dates back more than 30 years, yet the transfer of knowledge of pulp potential to industrial practice has been exceedingly slow. In this article, thermodynamic and kinetic issues were discussed to develop a rational scheme for interpretation of pulp potentials. Opportunities and challenges for using pulp potentials in industrial practice are discussed.

ELECTROSTATIC CHARGE ON SPRAY DROPLETS OF AQUEOUS SURFACTANT SOLUTIONS

Abstract Electrostatic charges on individual spray droplets were measured using a refined form of the Millikan oil drop method. The measurement system consisted of three main sections; a droplet generation cell, a settling column and a charge measurement chamber. The trajectories required for calculation of charge were determined using a high-speed motion analyzer coupled to a long-focal-length microscope. Charges on droplets were manipulated by the addition of surface-active agents into the spray solution. Droplet charge was a function of the type and concentration of the surfactant added. For ionic surfactants, it showed a maximum at low surfactant concentrations, decreased with further surfactant addition and was constant after the CMC. The charge on cationic surfactants was always more than that observed with the anionic surfactants. Nonionic surfactants displayed a steady increase in droplet charge with increasing concentration. The charges were lower compared to the ionic surfactants.

Kinetics of sulfidization of malachite in hydrosulfide and tetrasulfide solutions

Abstract The kinetics of sulfidization of malachite in sodium hydrosulfide and tetrasulfide as the sulfidizing agents was investigated in this study. The sulfidization reaction occurred in two stages. The first stage involved the formation of a primary sulfidized layer and the second involved two types of secondary reactions. One was the precipitation of copper ions which diffused through the primary layer, and the other was the oxidation of the sulfidizing agent to oxysulfide species. The influence of the type and concentration of the sulfidizing reagent and solution pH was determined. A significant difference in the surface morphology was observed for malachite treated with these two reagents. With sodium tetrasulfide, a uniform sulfidized layer formed on the particle surface, whereas for sodium hydrosulfide, the malachite surface was coated with loosely adherent precipitates, which might peel-off upon agitation. The results indicate that the extent of the reaction in the second stage was smaller in sodium tetrasulfide than in sodium sulfide.

An electrochemical characterization of pyrites from coal and ore sources

Abstract A preliminary investigation of the electrochemical behavior of pyrite samples from an ore and a coal source in alkaline borate solutions (pH 9.3) has been carried out by cyclic voltammetry, steady state polarization, and AC impedance measurements. Voltammetry and impedance results indicate that a product layer is initially formed on the ore pyrite during sample preparation. Such a layer is absent from the surface of the coal pyrite. Results of the steady state polarization and AC impedance techniques show that, under similar oxidizing conditions, coal pyrite begins to oxidize at potentials lower than the ore pyrite. The large difference in charge transfer resistance determined by the AC impedance measurements, shows that this technique is very sensitive for measuring reactivities of minerals.

Characterization of Airborne Particles and Droplets

Water sprays have been commonly used to suppress airborne dust. Water is doped with surface-active agents to enhance the dust capture efficiency through a reduction of surface tension. Nevertheless, dust collection efficiencies have been less than satisfactory historically.

Effect of sulfur dioxide on flotation of chalcopyrite

Abstract The mechanism of sulfur dioxide on collectorless and collector-induced flotation of chalcopyrite was studied by cyclic voltammetry on a rotating disc electrode, and by contact angle measurements. The first step in oxidation of chalcopyrite is the release of iron that upon hydrolysis may stay at the surface to render the surface hydrophilic. In stirred solutions, a significant amount of iron goes into solution to expose a copper–sulfur (Cu–S)-rich layer that leads to hydrophobicity. The Cu–S-rich layer reacts with the collector more readily than the original chalcopyrite or the iron-covered surface. In the absence of xanthate, the contact angles were larger when sulfur dioxide was present, indicating removal of some of the iron oxide at the surface. When sulfur dioxide and xanthate were added together, the contact angles were lower than those with xanthate alone. These results indicate that improved flotation may be obtained if treatment with sulfur dioxide precedes collector addition.

Coal Flotation Kinetics - Effect of Particle Size and Specific Gravity

To determine the flotation response for individual size-specific gravity fractions, the products of flotation were analyzed in detail for a high volatile A coal. Flotation rates for each size-specific gravity fraction were obtained using a classical first-order rate equation to fit the data. The results were evaluated to determine the effect of size and specific gravity of the coal particles on the flotation response.