1997 TMS Annual Meeting: Wednesday Session

AQUEOUS ELECTROTECHNOLOGIES: PROGRESS IN THEORY AND PRACTICE: Session V: New Processes and Products

Session Chairperson: D.B. Dreisinger, University of British Columbia, Department of Metals and Materials Engineering, 309-6350 Stores Road, Vancouver, B.C., Canada

8:30 am

INDUSTRIAL IN-PULP Co-Ni ALLOY ELECTRO-WINNING AT THE GECAMINES-SHITURU PLANT: K. Twite, J.-M. Dereydt, K. Mujinga, Gecamines Shituru Plant/Likasi, Bd. du Souverain 30, 32, B1170 Brussels, Belgium; P. Louis, Union Miniere, Allée de la Frènaie, B 1300 Wavre, Belgium

GECAMINES is a large mining concern and the world's leading cobalt producer having some of the richest cobalt deposits in the world. Cobalt is associated in the ore with copper as sulphides and oxides and in some locations with nickel too. Specific hydrometallurgical processes have been developed by GECAMINES which had done an important contribution to the development of the cobalt hydrometallurgy. In the Shituru plant, cobalt is obtained from an unique in-pulp electrolysis process, while in Luilu, cobalt is electrolyzed in a clear acidic solution giving a purer deposit. In the beginning of 1996 Shituru started the treatment of mixed Cu-Co-Ni Shinkolobwe hydrates obtained as by-product of an uranium solvent extraction plant. A typical analysis of this feedstock is: Cu: 2%, Co: 8%, Ni: 4%. The nickel sulfide depolarized cementation process developed previously by GECAMINES was inefficient to treat such a high level of Nickel contamination and it was decided to produce alloyed Cobalt-Nickel cathodes in the Shituru tankhouse. As anticipated from laboratory tests as previous studies and publications, a lower Ni/Co in the deposit was obtained compared to the Ni/Co ratio solution, despite the nickel reversible electrochemical potential is somewhat higher than the one of cobalt (-0.25 V vs -0.28 V). In this paper, the process flow-sheet and production data are given. Cobalt alloy with 5 - 20 % Ni was obtained at a production level of 300 t per month. The process developed is a first industrial realization in this way, bringing a new contribution to cobalt hydrometallurgy.

8:55 am

ELECTROLYTIC PROCESSING OF MANGANIFEROUS SILVER ORES IN ACIDIC NITRATE MEDIUM: O. Rutten, S. Van Sandwijk, G. Van Weert, Department of Raw Materials Technology, Delft University of Technology, Mijnbouwstraat 120, 2628 RX Delft, The Netherlands

Cathodic and anodic reactions in the acidic nitrate medium were studied in reference to the electrolytic production of manganese dioxide from pyrolusite (Mn02) ores by reductive leaching. Two USA ores were investigated. The reductant for Mn02 is produced cathodically and can either be nitrous acid or nitrogen oxide gas. In this work emphasis was placed on the nitrous acid leach. Manganese and silver are solubilized, iron is not. Cathodic regneration of nitrous acid was investigated in the range of 0.5 to 3.0 M nitric acid, 0.0 to 0.1 nitrous acid on graphite, platinum and platinized titanium cathodes at 20 to 80°C. It was established that the formation of the nitrosyl ion (N0+) is a prerequisite for the cathodic reduction of nitric acid. At cathode potentials < +700 mV (SHE), cathodic reduction of nitrous acid to nitric oxide takes place. Anodic deposition of manganese is as flaky -Mn02, similar to that produced in sulphate electrolytes. A conceptual flowsheet is presented and discussed.

9:20 am

A NOVEL ELECTROMETALLURGICAL PROCESS FOR THE TREATMENT OF REFRACTORY GOLD CONCENTRATES: N. de Jager, M.J. Nicol, University of the Witwatersrand, School of Process and Materials Engineering, Johannesburg, South Africa

In order to liberate gold from refractory gold ores and concentrates it is necessary to oxidize the surrounding sulphide minerals, typically pyrite and arsenopyrite. Traditional methods of oxidation include pressure leaching, roasting and bacterial oxidation. In recent years, bacterial oxidation has found increasing favour, although it is by no means completely satisfactory and is subject to large residence times. It is also extremely sensitive to operating conditions. Similarly, pressure leaching and roasting have their own particular disadvantages. A novel process has been developed as an alternative to the traditional methods whereby electro-generated ferrate [iron (VI)] ions in alkaline solution are used to oxidize a pyrite concentrate. This process has been observed to proceed fairly rapidly and could prove to be a more economically viable process, as well as being more environmentally acceptable. As an added benefit, operating in an alkaline environment is advantageous for downstream cyanidation processes. Iron (VI) is a relatively unknown species, due mainly to its instability in acidic solutions. However in alkaline solution it has been observed to be sufficiently stable for use as an oxidant when kept under the correct conditions. Iron (VI) is most conveniently manufactured electrolytically via the dissolution of high-carbon iron anodes in strongly alkaline solutions. Some literature has been published on the electrolytic generation of iron (VI) and relatively high efficiencies have been observed by both the authors and other researchers. This study utilizes both electrochemical techniques and leaching experiments to examine the kinetics and mechanism of the oxidation of pyrite by iron (VI). Cyclic voltammetry, rotating disk voltammetry and potential step experiments have been used to investigate the mechanism and kinetics of the oxidation process. Other potentiodynamic and poteniostatic techniques have also been employed. Leaching experiments have been used to provide a more detailed investigation into the kinetics of the process.

9:45 am

ELECTROLYTIC RECOVERY OF MERCURY FROM LOW CONCENTRATION BRINE SOLUTIONS: M. Rockandel, Universal Dynamics Ltd., Vancouver, Canada

Universal Dynamics Ltd. of Vancouver, Canada has developed, patented and commercialized the "REMERC" process for the treatment of mercury contaminated sludges and soils. Mercury is extracted into an acidified and oxidizing, sodium chloride brine solution. REMERC was initially developed to treat mercury contaminated EPA listed wastes (K106) generated by the chlor-alkali industry. More recent work has expanded the capability of REMERC to include remediation of mercury contaminated sites, equipment and building materials. In the process mercury is currently recovered from solution by cementation on iron powder in an agitated reactor. The process recovers high purity elemental mercury (99.9% purity). Cementation typically recovers about 90-95% of the mercury in 30 minutes. Higher recovery is not necessary because of the recirculation of the leach solution. When treating highly contaminated chloralkali wastes (5-13% mercury content), REMERC will reduce the mercury laden solutions from 400-1,000 mg/l mercury to 50-100 mg/l. In the treatment of less concentrated wastes (<1,000 mg/kg mercury) such as those encountered in site remediation, the treated solutions will generally contain 10-20 mg/l mercury. Cementation while being simple and able to achieve the required recovery has the undesirable properties of; requiring a solid-liquid separation and adding iron to solution which must be precipitated, ultimately increasing the weight of residue to landfill. The potential advantages of electrolysis were recognized early in the development of REMERC. Mercury electrolysis is well known in gold and chloralkali processing. Initial testwork utilized a liquid mercury cathode and a coated titanium anode both common to chloralkali producers. The mercury cathode appeared susceptible to polarization and solution impurities significantly affected performance. Agitation of the mercury pool improved performance but it was still not possible to achieve the reduction objectives. Current efficiencies were low, near 10%, and it was apparent that a relatively large cathode pool would be required to limit the current density. A number of electrode combinations with and without a membrane were then tested in a vertical electrode configuration but the desired performance was not obtained. In 1995 Universal Dynamics working with Dremco Ltd. of Arizona, USA began the development of a plate and frame style electrolytic cell. The objective of the electrolytic process was to achieve similar removals to those obtained by cementation, while producing high purity elemental mercury (>99.9% purity) and chlorine at the anode. Although current efficiency is a relatively minor concern the design objective was to achieve at least 50% CE. The current efficiency goal would require operation of the cell at current densities approaching the limiting current flow. The final cell current densities would therefore be in the range of 0.1 - 1.0 A/m2. Operation of a laboratory scale cell has demonstrated that the operating objectives can be achieved with a cell consisting of closely spaced shiny titanium cathodes and ruthenium oxide coated titanium anodes. Numerous variables were observed to be important including; inter electrode velocity, current density, pH and chlorine stripping. The cell has been tested on solutions generated by two REMERC operations, solution produced during site remediation pilot testing and on more highly concentrated solutions generated by oxidation of calomel generated at Cominco Metals, Norzink facility in Trail, B.C.

10:10 am BREAK

10:30 am

RECOVERY OF Cu2+ AND Cd2+ FROM DILUTE AQUEOUS SOLUTIONS BY ION FLOTATION AND ELECTROLYSIS: F.M. Doyle, University of California at Berkeley, Department of Materials Science and Mineral Engineering, Berkeley, CA 94720-1760; K. Sreenivasarao, Argonne National Laboratory, Energy Systems Division, Argonne, IL 60439

Copper and cadmium metal has been recovered from dilute (3 X 10-4 mol/dm3) chloride solutions by ion flotation with dodecysulfate, followed by electrolysis. This approach should facilitate treatment of effluents too dilute for effective direct electrolysis. The adsorption density of metal dodecylsulfate complexes on bubble surfaces was estimated from surface tension data, and compared well with experimentally-observed metal removal kinetics, and ultimate recoveries. Copper and cadmium were recovered by electrolyzing the foamate, using steel wool cathodes and a graphite anode. The stability constants of the copper and cadmium dodecysulfate complexes are estimated, and used to analyze the thermodynamics of electrolysis. Dodecysulfate was unaffected by the electrolysis process, and hence could be recycled to ion flotation. A conceptual flowsheet for an overall effluent-treatment process is presented.

10:55 am

NITROX METALS CORPORATION'S PROCESS FOR DIRECT LEACHING OF COPPER CONCENTRATES: R.N. O'Brien, E. Peters, Vancouver, B.C., V6N 2G1

It has been discovered that copper concentrates can be leached in strong (40-60%) sulfuric acid with air as the primary oxidant, if nitric acid is present as a catalyst. Chalcopyrite is rapidly decomposed at temperatures well below the boiling temperature of the lixiviant (150C.) and the resulting copper and ferric salts are precipitated as CuSO4 H2O or CuSO43 H2O and FeH(SO4)24H2O. Other metal sulfides such as zinc, lead, nickel, etc. are also converted to sulfates that are nearly insoluble. Sulfur is partially converted to elemental and partially to sulfuric acid. Reduction products of nitric acid can be recovered from the distillate with good recovery. Conversion of as-received copper concentrates is largely complete in about 1 hour. A number of process flow sheets are possible, and one that utilizes existing solvent extraction and electrowinning technology is considered economically feasible. This flow sheet involves (a) separation of metal sulfates and unleachable residues from excess acid by centrifuge, (b) water leaching of salts with partial neutralization (with limestone) of excess acid, to a tenor of 10 g/l Cu2+, (c) solvent extraction of copper using one of the LIX reagents, and (d) precipitating most of the ferric iron from raffinate with more limestone, so that it can be used as recycled water in step (a). Gold and silver are retained in a water insoluble small volume residue also containing any elemental sulfur formed, while minor elements like arsenic, antimony, etc. are retained by the strong sulfuric acid that is centrifuged from solids in step (a). These can be removed from this solution using methods previously developed for the purification of copper electrorefining electrolytes.

11:20 am

ACID RECOVERY AND PURIFICATION USING ABSORPTION RESIN TECHNOLOGY: M. Sheedy, Prosep Technologies Inc., Pickering, Ontario, Canada

Strong mineral acids, principally sulphuric, are widely used as electrolytes for the electrorefining and electrowinning of metals. Impurities in these electrolytes are controlled by continuously bleeding solution form the tank house. In addition to the contaminant these bleed streams contain high levels of sulphuric acid and the metal being recovered. Subsequent treatment of these bleed streams often requires neutralization which generates large volumes of solid waste. Absorption resin technology employing a novel ion exchange technique known as Recoflo allows the separation of the mineral acid from dissolved metal salts. The recovered acid is suitable for recycle back to the electrolyte circuit. The metal salt and other contaminants leave the process free of sulphuric acid. In this process, known commercially as the APU, ion exchange resin is used to sorb sulphuric acid while excluding the metal salts. The purified acid is then removed by washing the resin with water. The process has been extensively used for the recovery of waste pickling acids in the steel industry and anodizing solutions in the aluminum industry. In the mining industry the process has been successfully evaluated for the separation of excess sulphuric acid from copper electrolytes, the removal of magnesium and manganese contaminants from zinc electrolytes, and antimony, bismuth and nickel from copper electrolytes.

11:45 am

ELECTRODEPOSITION OF THIN MULTILAYER MAGNETIC MATERIALS: Z. Liu, K.C. Liddell, Washington State University, Department of Chemical Engineering, Pullman, WA 99164-2710

High-quality multilayers exhibiting giant magnetoresistance have been made by electrodeposition. The thickness of the individual layers was varied by changing the duration of the deposition pulses. Smooth and adherent layers as thin as 1A were made and characterized.