Conceptual model for estimating water recovery in tailings impoundments

In most semi-arid and arid regions water is a scarce commodity and “reclaim water” from the tailings impoundment is commonly used in the milling process to minimize water consumption. A comprehensive study has been initiated to study the influence of tailings management on water losses and water recovery for a very large tailings impoundment (52 km) in the Atacama Desert in northern Chile. This paper develops the theoretical background and presents a novel approach for estimating water recovery. Water recovery is controlled by a complex interplay of various physical processes including tailings deposition and consolidation, evaporation, rewetting and seepage. The use of a deterministic water balance model, which accounts for all of these transient processes, was not feasible for this large tailings impoundment. Instead, a water recovery model was developed, which focuses only on water losses from the active tailings stream to estimate water recovery. The model assumes that only process water liberated during the initial settlement is available for water recovery. The major water losses in this model include (i) entrainment losses during initial settlement of the tailings, (ii) evaporation losses from flooded areas of the tailings beach and (iii) rewetting losses during discharge of fresh tailings onto older, desiccated tailings beaches. This water recovery model was applied to the Tranque de Talabre tailings impoundment to explain existing water losses and to predict water losses and recovery for alternative tailings discharge plans. Table 1. Water recovery at selected copper mines in Chile. the impoundment. Typically, any given discharge point is only used for a few weeks to months at a time. Those discharge points actively discharging tailings are referred to as active discharge points. Conversely, those discharge points temporarily not discharging tailings are referred to as inactive discharge points. During active discharge, an active (“wetted”) deposition area develops which typically has the shape of a fan. The areal extent of the active deposition area will grow over time as more and more tailings are discharged from the same discharge point and are deposited. Because of evaporation and seepage, only those areas continuously receiving fresh tailings (will remain saturated at the surface (so-called “flooded areas”). When tailings are first discharged from a new discharge point, the flooded area may represent almost all of the active beach area. However, as the active (“wetted”) beach area grows and the surface gradients increase, the tailings stream tends to migrate. As a result the flooded area in a large, mature deposition area is typically significantly smaller than the total active deposition area of that active discharge point. These flooded areas may grow and shrink over time depending on discharge volume, solids content of the slurry and local microtopography created by tailings deposition. As will be shown later, a good estimation of the size of flooded areas is critical for estimating water losses. Figure 1 also shows the potential pathways of water discharged with the tailings slurry. The key processes include surface runoff, evaporation and seepage. However, the magnitude of these processes varies depending on whether they occur on the flooded area, the active or inactive deposition areas or in the pond zone. Surface runoff predominantly occurs in the flooded area of the active deposition area where tailings are actively deposited. In arid regions, where surface run-on and precipitation are negligible, all surface runoff is supplied by process water, which is liberated from the tailings during the initial sedimentation and settlement. Surface runoff eventually reaches the recycle pond from where it can be pumped back to the mill for processing. The three processes contributing to water losses in the active deposition areas are (i) beach seepage (or “rewetting”) into deeper, previously deposited tailings layers and ultimately into the foundation soils/bedrock, (ii) beach evaporation and (iii) water entrainment in the pore spaces of the tailings. Because of flooded conditions (providing maximum downward gradient and an “infinite” supply of water), seepage rates within the flooded area are much higher than in other deposition areas, which are not flooded. For the same reason, evaporation will also be at a maximum (approaching potential evaporation) in the flooded areas. Entrainment losses depend mainly on the particle size of the tailings with fine clay tailings entraining much more water than freely draining coarse sand tailings. Water losses due to entrainment are expected to be relatively constant for tailings with a defined particle size distribution. Water losses due to seepage and evaporation will vary according to tailings discharge practices on the tailings dam. As soon as active deposition ceases (or the flooded area shifts to a different portion of the active, wetted deposition area) the rate of downward seepage and the rate of (actual) evaporation decline towards much lower residual values. Numerical modeling indicates that the decline in seepage and evaporation occurs very quickly (within days) in coarse-grained tailings but can be much more gradual (within weeks) in the fine-grained tailings near the pond (RGC, 2003). In the pond zone, the rate of seepage and evaporation remain nearly constant provided the pond level (hence the pond surface area) does not vary significantly. While the rate of pond evaporation is similar to that observed in the flooded areas, the rate of pond seepage tends to be much lower due to the preferential deposition of very fine tailings (slimes) in the pond zone with significantly lower permeability than the tailings deposited in the beach areas. Seepage losses from the pond may only constitute a significant factor in the water balance of a tailings impoundment if the pond water is contacting natural soils and/or bedrock with a high effective permeability). A review of daily records of tailings discharge and water recovery for the Tranque de Talabre indicated significant short-term variations in water losses (RGC, 2003). Since the grain size distribution of the tailings is relatively constant, these variations are a reflection of the dynamic and highly transient nature of tailings deposition, which controls water losses. The two main factors believed to control the high variability in water losses relate to (i) the (necMine Site Location Tailings Production (tpd) Solids Content