Strategies for remediation of very large deposits of mine waste the Aitik mine, Northern Sweden

Abstract: The Aitik mine, near Gällivare in Sweden, is the largest copper mine in Europe with an annual production exceeding 18 Mt of ore. After processing of the ore the residual waste material, so called tailings, are deposited in a tailings pond. In addition, 15 - 25 Mt of barren waste rock is mined annually in order to provide access to the ore. The waste rock is deposited in waste rock dumps located close to the mine. The production started 1968, and very large quantities of waste have been accumulated since. A comprehensive set of strategies for management and decommissioning of the waste deposits have been developed, based on studies, investigations and tests carried out by Boliden, as well as independent consultants and universities, since the first permit regulating reclamation issues came into force 1989. During operation of the mine, the emissions to the receiving environment are limited. During a normal year, the effluent contains less than 50 kg of copper. However the internal circulation of copper is orders of magnitude larger. This fact calls for suitable decommissioning measures to prevent an increase of the emissions after closure. Moreover, by developing and implementing strict waste management routines the potential environmental impacts can be minimized and closure costs drastically reduced. As a foundation for the test work a variety of site investigations were carried out to establish the hydrogeological and geochemical conditions. Material characterization results show that some but not all waste types and categories may cause acid rock drainage (ARD). In order to reach conclusive results extensive kinetic test periods, using column and humidity cell tests, were required in combination with geochemical modelling to provide reliable results. Results of long term weathering and leaching tests on mill tailings were compared with measured weathering rates in field. The results show that weathering in field is much slower than in lab, but that there is still a potential for generation of acid leachate in the upper section of the tailings storage facility after closure. An alternative strategy was therefore developed, in which the iron sulphides will be removed from the bulk volume of the tailings during the last years of operation. The tailings depleted of sulphides would hence constitute a cover on the underlying tailings, with water saturation of the untreated tailings with higher sulphide content. The enriched sulphide product needs to be deposited separately during the last years and decommissioned using other methods, e.g. engineered cover or water cover. This can be obtained either in a selected part of the existing tailings pond where water saturation can be guaranteed, or by depositing the high sulphide tailings in the bottom of the open pit which will fill up with water after closure. Various reclamation methods for the waste rock deposits have been evaluated. Tests show that only a fraction of the tonnage is reactive, in average 20% without taking into account mixing due to practical reasons,which will increase the tonnage of contaminated rock. Therefore, procedures for selective management and deposition have been introduced for each rock type present. Existing dumps of mixed rock, when completed, will be covered with 2'0,5 m of compacted till and a topsoil of till with a content of sewage sludge or similar additive as vegetation layer. The oxygen inflow is thereby limited to 1% of the case before covering. Old dumps containing marginal ore, which were found to be the main sources of metals at the site, have been removed and processed and hence eliminated as sources of contaminant release. Large quantities of non-reactive waste rock are managed in a separate mass flow, constituting a large future source for aggregates production for construction purpose. The Aitik decommissioning project constitutes an example of how scientific methods applied to real life problems can lead to reduced potential environmental impacts, reduced liabilities, reduced closure costs as well as improved resources use- all in line with the sustainable development philosophy.