Introduction
Minerals and metals production could be certainly limited in the future by the increase of environmental restrictions, particularly those have to deal with the management of massive wastes, such as low grade ores, mill and concentration tailings and pyrometallurgical slags.
Massive wastes currently have been discarded only considering the final content of the main metal recovered during the primary process. However, other important components also remain in these wastes and could be recovered lately improving in some way the economical balance of the whole operation. Also, because of direct utilization, cost of transport and disposal will be lower than traditionally, avoiding problems of mechanical stability, contamination by heavy metals and so on.
In the case of copper extractive industry, with minerals containing around 1% of metal, the environmental impact associated to production of blister copper (98-99% Cu) is very clear. The situation could change if we consider wastes as new resources it self.
This article shows alternative propositions for re-utilization of valuable materials contained in massive mining and metallurgical wastes, with an especial attention to final copper slag produced during pyrometallurgical copper processing.
At present, Chile is the largest copper smelter producer in the world, through the seven copper smelters that produced 1,6 million tons of copper (13% of total world) in 2005, while around 3,5 million ton of slag are produced yearly. Its characterization, as well as different technical alternatives to treat this slag are analized and discussed in this paper. It is expected to make a contribution to the mining management by diminishing the environmental impact of their massive wastes, but also by recovering some valuable materials contained in it.
Chilean copper mining and its environmental impact
Chilean mining activity extends from the northern Atacama Desert - the world’s driest desert - to the central zone of the country with many different geographical environments. Large-scale mining is well established and coexists with medium, small, and artisan mining operations (Valenzuela and Sanchez, 2004).
Large-scale mining accounts 20 operations, representing 98% of the copper, gold and silver and molibdenum production. On the other hand, 20 medium and about 450 small-scale mining companies have an overall production of about 200,000 ton of fine copper annually. They have their own facilities or sell its ores to processing and trading to the state-owned company Empresa Nacional de Minería (ENAMI).
Several waste streams are produced in the current technology used for processing copper ores. These wastes are mainly flotation tailings, acid waste waters, acid mist, leached solid residues, gaseous emissions, slags, among others. On table I are shown the most important wastes and the present abatement technology applied in Chile.
Table I. Type of wastes and abatement methods for the two most important technological routes employed in the Chilean Mining Industry (Valenzuela and Sanchez, 2004).
The case of slag
Slag produced in Chile from copper smelters (properties and composition)
Copper sulphide concentrates are treated pyrometallurgically in copper smelters, trough a smelting-converting processes in order to obtain blister copper having 98-99% Cu. Also, slag is formed playing an important role during the process. At present, smelters use different reactors to smelt (Flash smelter, Teniente converter, Noranda reactor) and several Peirce – Smith converters to convert copper. Each reactor produce a different type of slag, therefore there are different possibility for reuse this slag. Today, some of these slags are recycled into the process; however, always a final slag is produced and discharged to the environment
A typical flowsheet showing a Teniente converter and Peirce – Smith converter schedule to treat copper concentrate in a Chilean copper smeters is shown in Figure 1 (Valenzuela, et al., 2006)
Figure 1. Typical flowsheet of a Chilean copper smelter.
It has been estimated that the production of one ton of blister copper generates 2.2 ton of slag, then if world smelter copper production was 12,4 million ton in 2004, about 27.3 million ton of slag were generated in the world (Demetrio, et al., 2000). In Chile, 2005 copper smelter production was 1,6 million ton, then about 3.5 million ton of slag were produced. It is estimated that over 30 million ton of slag have been stocked in the country in the last years.
Recently, data of slags produced in the different furnaces operating at the seven Chilean copper smelters were reported, and are summarized in Table II (Demetrio, 2000).
Table II. Chemical composition of Chilean copper slags. Year 2000
Final copper slags contain several metal oxides (Cu, Fe, Si, Mo and others), slag making oxides (silica, alumina, lime and others) and minor quantities of metallic copper. From the analysis shown in table III, it can be seen that final slag contains around 30–40 % iron, 30-40% silica, less than 10 % of alumina and calcium oxide and copper content is lower than 1%, depending on the slag cleaning process used. Minor amounts of molybdenum and noble metals such as gold and silver have been detected too. Since, final copper slag contains around 1% of copper, its recovery from the slag could be economically favourable if no expensive treatments are found.
Some experiences on recovery of species contained in slag have been developed in Chile, as well as worldwide. Nevertheless, as a primary copper producer, the main objectives of Chilean copper smelters have been focused only to the recovery of primary copper. In this part, different alternatives for processing copper slag are analyzed.
Proposal of recovery of valued products from copper slags
Today, only a small fraction of copper slag in Chile is retreated, mainly by flotation processes to produce copper concentrate, which is sent to the smelter. Also, some small amount of copper slag is utilized as abrasive materials, road construction materials and glass wool production
In this sense, it is important to calculate the potential economical value of the slag produced in Chile, in order to recover different final products, such as,metals of Cu, Mo, Ag, and Au, iron oxides and pure silica (Sánchez, 2006).
The chemical composition for the copper slag assumed for the calculation was Au – 0.05g/ton, Ag – 2.0g/ton, Cu – 0.8%, Mo – 0.3%, iron oxides (FeO and Fe2O3) – 40% and SiO2 – 10%. It is estimated a slag production of 1 million ton/year.
The estimated potential value for the project is US$ 40.7 / ton of slag, as it is shown in Figure 2.
Figure 2. Potential economical value of recovery of products from copper slags (Sánchez, 2006)
A pyrometallurgical and an hydrometallurgical way are proposed for the recovery of the different metals and materials contained in copper slags. The focus of this project is to study the two different processes proposed: one pyrometallurgical way and a second one by a hydrometalurgical way.
Regarding the hydrometallurgical process proposed, different steps were studied and analyzed, , which can be seen in Figure 3.
Figure 3. Proposed hydrometallurgical flowsheet.
The hydrometallurgical way proposed is basically a five-step operation: comminution, classification, leaching and solvent extraction, and electrowinning. In the comminution step, the cooling of the slag plays a relevant role, in the solidification process depending on the cooling rate the different structures are built up (crystalline and amorphous) and the different compounds are entrapped between each other. For this reason to achieve a proper liberation a size reduction is necessary. The cooling rate has also an influence on the hardness and brittleness of the slag, so it must be taken into account (Besnier, 2003; Waldron G., 2004; Waldron E., 2005).
The classification is based on size, density and magnetic properties, and the objective is to concentrate the valuable materials. With this concentrate a leaching step can be carried out with a further ion exchange process to concentrate the copper in solution. An electrowinning cell can be used to deposit the metal. The main metallic products of this hydrometallurgical process should be copper and molybdenum.
The pyrometallurgical process selected for treating copper slags is the direct reduction (see Figure 4). The idea is to inject carbon to the molten slag to produce an iron-copper-carbon alloy (pig iron) and a new slag suitable for cement production, without any solid or liquid residues. No heating will be required for melting, because the slag should be received in a liquid state from the slag cleaning furnace (González, 2005).
Figure 4. Proposed pyrometallurgical process for slag treatment (Sánchez, et al., 2004)
In Figure 5, it can be seen the predominance of phases in the Fe-Cu-Si-Mo-O-C system for slags in different processes, first in the Teniente converter, than in the slag cleaning process and finally in the pyrometallugical process proposed. It can be observed from the diagram that the ratio pCO2/pCO is very low for the direct reduction process, therefore it can be concluded that the partial pressure of oxygen in the process proposed must be very low too.
Figure 5. Predominance diagram for Cu-Fe-Mo-Si-O-C system (Sánchez, et al., 2004).
In experiences of direct reduction of copper slags performed at the Metallurgical Department of the Universidad de Concepción, it was observed that the slag formed after direct reduction is getting closer to the Portland cement composition (Sánchez, et al., 2004), as it can be seen in Figure 6.
Figure 6. Composition of slags and portland cement on the ternary diagram, CaO, Al2O3 and SiO2, after Imris (Sánchez, 2006).
Conclusions
It is estimated that Chile would increase its copper production in the next years. However, the cut off grade of its copper ores are lowering, which will implies to manage more massive wastes. The reuse or recycling of these wastes residues can be a possible way of solution to this environmental problem.
The reuse or recycling of wastes is environmentally favourable, as well as, in principle, economically attractive because a higher efficiency of the different processes can be obtained. However, research and development of new technologies for waste management in copper processing, are necessary.
In this way, Chilean slags have been analyzed and its characterization is shown on this work. It can be observed that some metals can be recovered from the final slags containing around 1% Cu, 40-45% l FeT, Mo, precious metals, and oxides (silica, MgO, alumina and others).
Two different alternatives for treating slags from copper smelters are analized, described and proposed in this paper. An hydrometallurgical way that includes five stages: comminution, classification, leaching, solvent extraction, and electrowinning, while a pyrometallurgical way used the direct reduction at high temperature.
Acknowledgements
Authors acknowledge CONICYT (Comisión Nacional de Investigación Científica y Tecnológica, of Chile), for its financial support given to the FONDEF Project No. D02I1159. Also Chilean smelter plants as well as Government institutions participating in this project are acknowledged.
References
BESNIER T. “Obtención por lixiviación de subproductos con valor comercial a partir de escorias pirometalúrgicas de la gran minería en Chile”. Chemical Engineer Thesis Supervisors: Professor F. Márquez and Professor F. Parada. Universidad de Concepción, Chile. December, 2003.
COCHILCO, 2006, “Anuario Estadísticas del Cobre y Otros Minerales, 1986-2005”, edited by the Chilean Copper Commission, www.cochilco.cl
DEMETRIO S., AHUMADA J., DURAN A., MAST E., ROJAS U., SANHUEZA J., REYES P. and MORALES E. “Slag Cleaning: The Chilean Copper Smelter Experience”. JOM, August 2000.
GONZALEZ C. “Direct Reduction of Caletones’ smelter copper slags”, Metallurgical Engineer Thesis Supervisors: Professor M. Sánchez. Universidad de Concepción, Chile, Sept 2004
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SANCHEZ M., PARRA R., IMRIS I. and GONZALEZ C., 2004, “Direct Reduction of Chilean Copper Slags: A Waste Treatment Alternative”. Proceedings of the VI International Conference on Clean Technologies for the Mining Industry. University of Concepción, Chile. 18th – 21st April, 2004.
Sánchez, M., 2006, FONDEF Project No. D02I1159 “Obtención de Subproductos con Valor Comercial a Partir de Escorias Pirometalúrgicas Provenientes de Fundiciones de Concentrados de Cobre de la Gran Minería en Chile”, CONICYT (Comisión Nacional de Investigación Científica y Tecnológica, in Chile), carried out by Dr Mario Sánchez, University of Concepción
VALENZUELA A., and SANCHEZ M., 2004, “Environmental Issues in the Mining and Metallurgical Industry. Part I. Copper Production and Environmental Management”. Universidad de Concepción, Chile, I.S.B.N. 956-8029-58-3.
VALENZUELA A., BALLADARES, E., SANCHEZ M. and CORDERO D., 2006, “Arsenic Maangement in the Metallurgical Industry: The Chilean Experience”, Sohn International Symposium, Advanced Processinf of Metals and Materials, Volume 9 – Legal, Management and Environmental Issues, F. Kongoli and R.G. Reddy (Editors), TMS (The Minerals, Metals & Materials Society), 2006, pp. 407-422.
WALDRON G. “Evaluación técnico económica de alternativas para el tratamiento de escorias finales de cobre”. Metallurgical Engineer Thesis . Supervisors: Professor M. Sánchez and Professor F. Parada. Universidad de Concepción, Chile. April 2004.
WALDRON E. "Obtención por tostación y lixiviación de subproductos con valor comercial a partir de escorias pirometalúrgicas de la gran minería en Chile” Chemical Engineer Thesis Supervisors: Professor F. Márquez and Professor F. Parada. Universidad de Concepción, Chile. Sept 2004.
Organization name: Departmento de Ingeniería Metalúrgica, Universidad de Concepción
Street address: Edmundo Larenas 285
City: Concepcion
Country: Chile
E-mail: msanchez@udec.cl
Web: http://www.udec.cl
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