J.F. Oliveira

Selective flotation of gold from pyrite under oxidizing conditions

Abstract The differential flotation of free gold from sulphide minerals can be interesting for both economic and environmental reasons. The use of hydrogen peroxide as an oxidizing agent in the selective flotation of gold from pyrite with potassium amyl xanthate was studied by contact angle and electrophoretic mobility measurements carried out under similar conditions. Hydrogen peroxide by itself rendered both gold and pyrite surfaces very hydrophilic. However, the subsequent adsorption of xanthate on gold produced a fairly hydrophobic surface (j ∼ 60°), whereas pyrite was maintained hydrophilic at pH 10 and higher, indicating a pH range where selective separation by flotation could be achieved. The electrophoretic measurements showed that the pyrite surface is completely oxidized by hydrogen peroxide, with an isoelectric point at pH 5.5. By contrast, the hydrophilic character of the gold surface is apparently enhanced simply by the leaching action of the hydrogen peroxide. The charge of the gold particles is pH-dependent and their isoelectric point in chlorine media is around pH 2. Bench-scale flotation tests confirmed the feasibility of selective separation of gold from pyrite by using hydrogen peroxide as an oxidizing agent and amyl xanthate as collector. At values of pH 8 depression of pyrite increased significantly, while the high recovery of gold remained practically unchanged.

INTERFACIAL STUDIES ON DISSOLVED GAS FLOTATION OF OIL DROPLETS FOR WATER PURIFICATION

Abstract The removal of emulsified residual oil from water is an important issue due to the increasing concern with the environment. In the present paper various surface chemistry aspects of dissolved gas flotation were studied aiming at a better understanding of the mechanisms of the process. These studies included surface and interfacial tension measurements of a model system composed of n-dodecane in the presence and absence of nonyl-phenol. Critical micelle concentration and spreading coefficients determinations were undertaken at different NaCl concentrations. The spreading coefficient proved to be a critical parameter in oil droplets flotation. After bubble–drop contact and rupture of the water film, oil drops have a tendency to spread over the bubble surfaces. The spreading coefficient increased with increasing salt concentration. An experimental setup was designed to measure the induction contact time between single n-dodecane droplets and air bubbles. Induction time decreased with increasing salt concentration. In the presence of the nonyl-phenol aqueous solution, induction time decreased with time of stabilization. On the other hand, induction time increased with time of stabilization when nonyl-phenol was added to the organic phase. The mechanisms involved are discussed and some results obtained in a dissolved air flotation batch unit in which bubbles size was measured and controlled are also presented.

Kinetics of pyrochlore flotation from Araxá mineral deposits

Abstract The niobium mineral deposits in Brazil account for more than 70% of the worlds known reserves. Currently flotation of pyrochlore is carried out in the Araxa plant using amines as collector at low pH, in the presence of NaF as an activator and with the addition of a modifier. In the present work bench scale flotation kinetic testwork was conducted, aimed at a better understanding of some aspects of the process used. A rate model based on equations for fast floating and slow floating particles was used for describing the cumulative recovery versus time. The results showed that the control of collector (amine acetate) at a minimum level was essential for effective action of the activator and modifiers studied. In the rougher stage, best results were obtained with the collector at 100g/t. The use of NaF at a dosage higher than 1000g/t significantly affected the cleaner concentrate grade.

Synergism in aqueous solutions of surfactant mixtures and its effect on the hydrophobicity of mineral surfaces

Abstract Surface tension and contact angle experiments were conducted aiming at studying the synergism of surfactant mixtures on the reduction of surface tension of aqueous solutions and on the hydrophobicity of mineral surfaces. The following systems were studied: 1) Potassium ethyl xanthate with sodium oleate mixtures for pyrite and gold, and 2) Sodium oleate with cetyl trimethyl ammonium bromide (CTAB) mixtures for fluorite and calcite. A synergistic effect of combined reagents was found to enhance the hydrophobicity and also to reduce the surface tension, depending mainly on the proportion of the mixtures.

Control of Particle Size in the Preparation of Colloidal Gold

ABSTRACT The preparation of colloidal gold is a subject of practical interest in many technological fields. It is generally accomplished by reduction of HAuCl4 solution with sodium citrate which is known to be influenced by several factors such as temperature and reactants concentration. In the course of the present investigation, the rate of addition of sodium citrate was observed to play a very important role on the particle size distribution. This aspect has not, to our knowledge, been investigated and reported previously. The studies undertaken including UV/visible absorption spectrophotometry, light scattering, and transmission electron microscopy (TEM) showed that the higher the rate of addition of sodium citrate, the smaller is the mean particle size and the narrower is the diameter of the gold particles produced.

STABILITY AND COLLAPSE OF MONOLAYERS OF STEARIC ACID AND THE EFFECT OF ELECTROLYTES IN THE SUBPHASE

Abstract The stability of monolayers at the liquid–gas interface was studied using stearic acid as a model surfactant. The influence of pH in the presence of different electrolytes in the subphase was investigated through experiments carried out using a Langmuir KSV-5000 film balance. These studies are particularly important with respect to the formation and homogeneity of the deposition of monolayers over solid substrates. The results are also useful in the control of surfactant adsorption on bubble surfaces as well as in the rate of film rupture and froth stability in the flotation process. Experiments were conducted for the area loss versus time for the monolayers of stearic acid spread over different electrolyte solutions (Ca2+ and Mg2+) as subphases as a function of pH. The stability of the monolayers increased significantly at high pH values due to ionization of the surfactant and the degree of stability was slightly different for the two electrolytes. The main cause of the instability of the stearic acid monolayer at low pH (5.7) both in the presence and absence of electrolytes is a collapse process which involves formation of three-dimensional nuclei on the monolayer surface. Some mathematical models have been proposed to predict the surface area changes as a function of time. The experimental results obtained in the present work were evaluated and discussed based on the Vollhardt model of collapse through nucleation and growth.

Surface properties of dental polymers: measurements of contact angles, roughness and fluoride release

Received: January 28, 2007; Revised: August 11, 2008Objective: Earlier studies on some dental materials measured roughness and/or contact angles or fluoride release separately. In the present study, five dental polymers were investigated to ascertain their contact angles, wettability, roughness, and fluoride release in dry or wet conditions. Methods: Samples for 5 materials were prepared and stored dry or wet in deionized water pH 6.8. Samples were submitted to finishing/polishing procedures, and the measurements in Goniometer, roughness (µm) and fluoride analysis Results and conclusions: Except for the Ariston pHc, all the materials displayed high contact angles when measured with water, showing hydrophobic characteristics. Roughness changed the contact angles, especially those of Ariston (α < 0.05). Fluoride did not modify the contact angles, but increased the roughness of the finished material.

Flocculation of colloidal silica with polyacrylamide and the effect of dodecylamine and aluminium chloride pre-conditioning

Abstract The stability of colloidal silica in the presence of polyacrylamide has been studied by electroacoustic potential measurements and sedimentation testwork. Prior conditioning with AlCl3·6H20 or dodecylamine resulted in more effective action of the polymer. Studies carried out by transmission electron microscopy (TEM) showed that the aggregates formed by AlCl3·6H20 addition were branched and larger than those formed by DDA, under similar experimental conditions. The polyacrylamide(PAM) adsorption on the two types of aggregates was induced by different mechanisms which affected the characteristics and resistance of the flocs.

Activated carbon adsorption of gold from thiourea solutions

Abstract Studies were carried out to evaluate the adsorption of gold and thiourea onto activated carbon. In the temperature range studied, 15 to 55°C, both thiourea and gold adsorption results showed good agreement with the Freundlich isotherm. However, while the thiourea adsorption is exothermic, the gold adsorption proved to be an endothermic process. An activation energy of 3.5 Kcal/mol was determined for gold adsorption, indicating that the controlling step in the kinetics of the process is the diffusion onto the porous structure of the activated carbon. A simple empirical model allowed the evaluation of the gold adsorption rate in bench scale batch tests.

Physico-Chemical Factors Affecting the Separation of Cassiterite and Fluorite by Froth Flotation

Abstract Cassiterite flotation is achieved industrially with some anionic collectors. However, the presence of fluorite in primary tin ores usually poses a problem in relation to the selectivity between the two minerals. In this work, a comparative study was undertaken involving adsorption, electrophoretic mobility and suspension stability measurements of both cassiterite and fluorite as a function of oleic acid concentration. These properties are discussed with reference to the calculated free energy of adsorption equation pending of oleic acid on the two minerals. Hallimond tube flotation tests conducted with the two minerals showed a distinctly higher floatability of fluorite at low oleic acid concentration. Micro-flotation studies using Procol CA-540. a sulphosuccinamate collector, presented similar results, indicating that fluorite can be floated selectively from cassiterite at low collector concentration and neutral pH.