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Session Chairperson: R.S. Kunter, Advanced Sciences Inc., 405 Urban Street, Suite 401, Lakewood, CO 80228
THE USE OF FRACTAL DIMENSION WITH AN AGGREGATION MODEL TO CHARACTERISE COPPER ELECTRODEPOSITION IN THE PRESENCE OF THIOUREA: S.P. Fox, S.A. Godorr, A.W. Bryson, School of Process and Materials Engineering, Private Bag 3, Wits 2050, South Africa
This work is an extension of a previous investigation on the development of surface roughness during electrodeposition of copper. A comprehensive model based on diffusion and migration in a force field together with surface diffusion and sticking probabilities is used to generate profiles which are compared to those produced experimentally. The roughness is characterized by using the technique of fractal geometry which is shown to describe both qualitatively and quantitatively the development of surface morphology during electrodeposition. The work is extended to the deposition of copper in the presence of various concentrations of thiourea. The development of roughness is observed experimentally on the edge of a thin copper disc mounted between two glass plates. This enables photographs to be taken at various stages during growth. The surface outline is digitized using an image analyzer which enables the fractal dimension to be determined as growth proceeds. As expected the presence of thiourea at low concentrations produces a fractal dimension which is closer to unity (i.e. the surface becomes smoother). The original model is modified to include diffusion and migration of thiourea to the surface. It is postulated that the presence of thiourea on a surface site changes the sticking probability of discharged copper ions at this site. This results in increased surface diffusion of the copper, which in turn produces a smoother deposit. Extensive experimental results are presented which show that the model adequately predicts the effect of changes in thiourea concentrations over a wide range of conditions including those that are of relevance in electrorefining practice.
DIRECT RECOVERY OF METALS FROM LOADED ORGANIC SOLVENTS: A REVIEW OF RECENT METHODOLOGIES: L.M. Abrantes, A.P. Paiva, Universidade de Lisboa, Faculdade de Ciências, Departamento de Quimica, Lisboa, Portugal
The application of hydrometallurgical methods to the recovery of metal values from ores and concentrates has been receiving a growing interest, either from researchers or from the mining industry. For the concentration/purification of the relatively low concentrated metallic solutions resulting form the leaching of those raw materials, a well established technique often used is solvent extraction (SX); the loaded organic solvent is then conventionally contacted with a suitable aqueous medium, the metal in its final form being usually recovered form this solution by electrowinning. Methods dealing with the direct stripping of metals from loaded organic extractants have recently been investigated, as an efficient stripping is sometimes difficult to achieve for some solvents. Amongst these innovating processes, the electroreductive stripping (2-phase electrolysis) and cementation (galvanic stripping) must be emphasized, the related research works being cautiously reviewed in the present paper. Particularly, the possibility of use of a 2 phase electrolysis to recover basic and precious metals is more extensively discussed. Examples are given for the recovery of copper and silver from suitable organic media, namely by direct application of an electrical potential to a mixture of a silver loaded organic phase containing Cyanex 471X - obtained by SX - and a stabilized sodium thiosulfate solution. The overall efficiencies of these one-step techniques are analyzed and compared with the results obtained by conventional stripping procedures. The purity of the cathodic deposits is also considered. The recovery of silver by galvanic stripping involving iron and zinc powders is also preliminarly evaluated, using a similar organic solution as described above. The influence of temperature on the efficiency of the process is also studied, either for concentrated or more diluted organic silver solutions. The literature data and the results reported here on the application of both methods for direct metal recovery from organic phases show promising advantages over the conventional procedures, encouraging a research effort to optimize and establish these alternative processes.
KINETICS MECHANISM OF GOLD ELECTROWINNING: J.A. Garcia, Universidad Nacional de San Juan, Argentina, C. Hecker, Universidad de Concepcion, Chile, E. Larenas, Concepcion, Chile
To determine the kinetic reaction mechanism, electrochemical experimental techniques of stationary polarization, linear voltammetry and faradaic impedance were used. Gold electrodeposition experiments were realized using an electrolysis cell, provided by a rotary disk of vitreous carbon and or platinum counter electrode of platinum mesh and reference electrode. Using an automatic measurement system voltammograms and Nyquist faradaic impedance spectra were obtained. In this paper, responses for different cases were analyzed through faradaic impedance criteria, considering several kinds of diagrams obtained. The importance diffusion influence of aurocyanide ions and adsorption as controlling steps previous charge transfer was verified. A model of proposed mechanism is discussed.
AN INVESTIGATION ON GOLD ELECTROWINNING FROM ACIDIC THIOUREA SOLUTIONS: C.M. Juarez, A.J.B. Dutra, Federa University of Rio de Janeiro, Metallurgical and Materials Engineering Program, Rio de Janeiro, Brazil
The use of thiourea as a gold leaching reagent has been extensively studied by many researchers, and it has proved to be promising for some cases as refractory gold ores and electronic gold scraps. Otherwise, the recovery of gold from these solutions did not receive so much attention. In this paper, the electrowinning of gold in acidic thiourea solutions was studied by voltammetry, chronopotentiometry and electrochemical impedance in a conventional three-electrode cell with a rotating disc electrode. These electrochemical techniques can provide valuable kinetic information to electrowinning practice. Results have shown that gold electrowinning should be carried out at low current densities in order to avoid cathode passivation and side reactions, as formamidine disulfide reduction and hydrogen evolution at higher overpotentials.
ELECTROCHEMICAL ASPECTS OF THE DISSOLUTION OF GOLD IN CYANIDE ELECTROLYTES CONTAINING LEAD: D. Mussatti, J. Mager , G.P. Martins, Colorado School of Mines, Department of Metallurgical and Materials Engineering, Golden, CO 80401
The role of lead (Pb), at low concentrations in high pH cyanide electrolytes, on the dissolution behaviour of gold has now been a topic of scientific and industrial concern for over 50 years. The recent paper by Kondos et al at CANMET (1995) attests to this continuing interest and apparent importance. Nevertheless, the mechanism(s) by which this (and other heavy metal) component influences the electrodics of this electrochemical system has still not been unequivocally demonstrated. Instead, several plausible hypothesis, by pioneering researchers in this field, have been advanced and reiterated over the ensuing years. While it is generally accepted, based on electrode potential calculations, that co-reduction of lead on the gold surface may occur due to cementation (contact reduction) the cemented lead has never been characterized. The subject matter is confounded further when sulfide (or hydrosulfide) species are also a consitutent of the electrolyte. The work to be presented re- examines the role of lead and sulfide in this system, by considering the species distribution of these components and by conducting potentiodynamic studies in conjunction with surface analyses of a gold (and lead) electrode.
STUDY ON WASTEWATER TREATMENT OF CHEMICAL PLATING SOLUTION USING RECOVERING PROCESSES: Y. Kumagai, National Institute of Material & Chemical Research, Laboratory of Organic Materials, Tsukuba, Japan; F.A. Souza, Federal University of Minas Gerais, Department of Metallurgical Engineering, Belo Horizonte, Brazil
Electroless plating processes or chemical deposition processes, have been used in many fields such as automobile and electronic industries. These solutions must be disposed or treated even when the components already have high performance and concentrations. Commonly, in these kind of solution among others components, metal ions are in very high concentrations. The cost of the plating process becomes higher and is very difficult to keep the environmental regulations. The main objective of this study was to investigate the ways to recover the bath components, among them, the metal ion, in order to decrease the cost of the process as well as to meet the environmental regulations. Specifically, in this study was investigated the way to recover, from the exhausted bath, the nickel ion, sulfate ion, phosphite ion and organic compounds. In this case, was investigated a solution for electroless for nickel deposition. The metal ion was recovered from the wastewater by chemical plating onto mica powder surface. The phosphite and sulfate ions were recovered from the wastewater by precipitation as calcium salts. The organic acids were recovered by solvent extraction. These recovery process, used for the metal ion, is particularly important because many studies are investigating the use of mica coated with metal in the development of new composite materials.
THE EVALUATION OF FOAMING BEHAVIOUR OF SURFACTANTS FOR ACID MIST CONTROL IN METAL ELECTROLYSIS PROCESSES: A.M. Alfantazi, D.B. Dreisinger, University of British Columbia, Department of Metals and Materials Engineering, Vancouver, B.C., V6T 1Z4, Canada; J. Synnott, M. Boissoneault, Falconbridge Ltd., Kidd Creek Division, P.O. Bag 2002, Timmins, Ontario, P4N 7K1 Canada
Acid Mist Generation is a major concern in zinc, copper and nickel electrowinning industry and there is a strong environmental drive to control its emission in the work environment. Foaming reagents are typically added to the electrolyte to alleviate this problem. In zinc electrowinning, licorice and saponin are examples of foaming reagents used for this purpose. In order for the foaming reagent to be effective, its foaming behaviour must be determined. In this work, a test procedure was developed to study the foaming characteristics of various surfactants for use as foaming agents to control acid mist in aqueous metal electrowinning. This new test was termed a "mini-flotation test" and it was used to study the foaming behaviour of a large number of reagents in an industrial zinc electrolyte under simulated zinc electrowinning conditions (gas sparging). In addition, the influence of important parameters such as current density, electrolyte temperature, reagent addition level and sulfuric acid concentration on the foaming characteristics of the electrolyte for each surfactant were investigated. This study has identified the most important factors that influence foam formation, growth and stability. This method could be used as a process control measure for reagent addition in the cellhouse by generating a calibration curve for the optimum electrolyte conditions.
ELECTROWINNING COBALT FROM SULPHATE SOLUTIONS: K.C. Lenthall, A.W. Bryson, Mintek, Private Bag X3015, Randburg, 2125, Republic of South Africa
The effects of operating parameters on cobalt electrowinning from sulphate solutions were investigated. Experiments were carried out in a divided five-litre cell holding two anodes and a single cathode (150 by 105 mm). The cell was operated in a continuous manner and the catholyte conditions (pH and temperature) were controlled. The following parameters were chosen as a base for the investigation: cobalt feed concentration 90 g/l, catholyte pH 2.5, temperature 65°C, current density 250 A/m2. The feed flow-rate was set to achieve a Co of 30 g/l. The cobalt concentration in the catholyte was varied from 25 to 65 g/l and the current efficiency was found to increase linearly from 90 to 96. Varying the catholyte pH from 2 to 2.75 caused efficiency to increase linearly from 91 to 98%, but above pH 2.75 the rate of increase slowed. Efficiency remained at 95% between 35 and 65°C. Cathodic current density was varied between 150 and 350 A/m2 and did not affect efficiency. Only temperature and catholyte H+ were found to affect the nature of the deposit. Below pH 2.5 many small pits occur. Above this pH, the deposit is shinier and has fewer, though larger, pits. The temperature of the catholyte has a marked effect on the deposit since pitting on the metal decreases dramatically below 50°C. In order to simulate realistic cell behaviour, control of catholyte pH was relaxed. The feed flow-rate was varied from 2.2 to 4.5 ml/min, corresponding to a Co of 30 to 60 g/l. The catholyte pH and cobalt concentration were both found to increase with increasing flow-rate, which caused an increase in efficiency from 82 to 99%. The cell is modeled by considering (i) the transport of H+ across the diaphragm, (ii) the effect of Co and H+ on efficiency using Wark's Rule, (iii) the overall mass balance across the cell.
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