Paste deposition over an existing subaqueous slurry deposit of high sulphide content tailings – the Neves Corvo experience

Abstract

Neves Corvo is a world-class underground copper-zinc mine located in the south of Portugal. The mine has been in operation since 1988 by Somincor (Sociedade Mineira de Neves-Corvo SA), a wholly-owned subsidiary of Lundin Mining Corporation from Canada. The pyritic tailings produced have a very high acid generation potential and until 2010 had been placed subaqueously in the Barragem Cerro do Lobo (BCL), a large tailings pond (190 ha) created by a rockfill dam, 42 m high, across a natural water course. That disposal system provided safe and reliable storage of the reactive tailings. However, by 2001 Somincor realised that the remaining subaqueous deposition capacity was limited and that an alternative site or other deposition methods would be required at this site. Following nine years of studies and evaluation of various alternatives Somincor decided to extend the life of the existing facility, by switching in late 2010 to using subaerial paste/thickened tailings deposition over the existing subaqueous slurry deposit. This approach provides the required storage volume for Somincor's 2005 Life of Mine Plan, and allows maintaining the existing perimeter dam at its current elevation, and facilitates site closure. This paper presents the results of the first two years of operation of the paste production plant and depositional performance of the first two to four metres of paste/thickened tailings over the very loose subaqueous deposit. Routine monitoring data reported includes records of produced paste/thickened tailings densities, yield stress, particle size distribution (PSD), and average specific gravity (SG) of solids. Settlement, deposition beach slopes and piezometric levels in the paste and underlying subaqueous deposit are also routinely monitored. In addition, in 2012 a campaign of Cone Penetration Test Undrained (CPTu) testing and collection of piston samples was carried out to provide in situ strength and permeability data as well as evaluation of the state parameter of the deposited tailings. The 2012 CPTu data is being used for ongoing assessment of the dynamic stability of planned deposition for the next two years. This type of data and a planned special laboratory testing program will be used to calibrate a model of the evolution of the void ratio of the deposit as a function of time and of additional deposition loads. This model will guide the design of the final deposition landform. The environmental component of the deposition monitoring program includes the monitoring of the pH profile of the top one metre of tailings left exposed and allowed to dry as a result of the sequential rotation of tailings discharge.

Managing high-density tailings disposal – deposition, water management and closure considerations

“…The underground copper/zinc mine was commissioned in 1988. The tailings are acid generating and had been deposited subaqueously until 2010 when sub-aerial deposition of thickened tailings in cells began (Lopes et al 2013). The current deposition strategy is to store the life of mine tailings using sub-aerial…”

“… Tailings properties -The settling behaviour of tailings is strongly affected by the tailings rheology, which is function of the tailings material and the thickening and transportation processes. Various studies have attributed poor field performance to a lack of control on the tailings feed and inadequate plant operation (Wates et al 2014;Lopes et al 2013). While it is desirable to avoid variability of characteristics of the tailings feed, this is very difficult to achieve when the coarse tailings are used to backfill the mine (which is a batch process).…”

“…Laboratory and field testing of the deposited tailings have been performed periodically at all of the reference sites (Kam et al 2009(Kam et al , 2014Lopes et al 2013). Tailings properties that are measured have included grain size distribution, specific gravity, deposited dry density, in situ shear strength, water content, void ratio, settling, consolidation and drying behaviour.…”

Upstream stacking of thickened tailings at Neves Corvo

“…In late 2010 Somincor switched to using subaerial paste/thickened tailings deposition over the existing subaqueous slurry deposit. This approach accommodates the required 17.4 Mt storage capacity from 1 January 2010, as per Somincor's 2005 life-of-mine (LOM) plan, and allows maintaining the existing perimeter embankment at its current elevation and facilitates site closure (Lopes et al 2013).…”

“…The results of the first two years of operation of the paste production plant and depositional performance of the first 2 to 4 m of paste/thickened tailings over the very loose subaqueous deposit, were presented at Paste 2013 in Brazil (Lopes et al 2013).…”