Mine tailings may be produced in the thousands of tons per mine, but they are not all uniform in their characteristics. Depending on how the ore is processed before it becomes waste tailings, it may be mixed in with an equal amount of liquid, or more, by volume. Many mines go through extensive dewatering procedures to drain liquids like raffinates off of the tailings and reuse them elsewhere in the mine. Aside from reusing water, why go through the trouble to dry out tailings before storing them? Dry storing tailings has a number of benefits out weight storage, although it also comes with its own considerations and limitations.
Weight and Pressure
The biggest challenge of storing tailings in any wet form is the sheer weight of material. With pure water weighing around eight pounds per gallon, every extra gallon of liquid mixed in compounds the weight of the stored material. The increasing pressure on the dam or embankment holding the slurry back can lead to catastrophic collapses, even years after the mine is closed and forgotten about. However, dry stored tailings are not free from these issues either. Dry heaps can slide and shift at any time and may still knock out an embankment or dam. The pressure is just not pressing against the dam day after day with the same force as it does with wet storage.
Wind Loss
One of the greatest benefits of any wet storage method for tailings is the reduction of wind scouring. Whenever wind blows across a heap of tailings or other crushed and processed material, there is a good chance that the finest materials will blow away. This resulting dust can spread environmental damage far and wide since the dusts from mining can often be carried many miles away. It’s also hazardous to human health if inhaled by workers at the mine or anyone passing by as the wind blows. Mixing in water or raffinates helps trap the smallest particles within the slurry so they can’t blow away.
Dilution and Runoff from Rainfall
The type of impoundment chosen for storing the tailings will largely determine how much rainfall runs off into the storage area. Ring-dike impoundments built on flat surfaces only accumulate a small amount of water that falls directly onto the surface area of the tailings. In contrast, valley and pit impoundments tend to gather a lot more extra liquid due to the addition of this water. Building berms to redirect water away from the tailings storage area is usually worth the extra effort because they tend to cost less than extra excavation and lining to prevent flooding. If the tailings are expected to be wet most of the time, from the addition of a large amount of runoff from rainwater, they’re most likely best stored wet anyway rather than dewatered at any stage of the process.
Leftover Processing Chemicals
In addition to runoff and rainwater that becomes mixed in with the tailings while it is heaped or stored in open areas, consider the amount of processing chemicals already left over in the mix. This informs how much dewatering is needed. For only mildly wet tailings that are far from a consistent slurry, drying from wind and exposure alone is likely all that is necessary while the material is waiting for permanent storage. If the slurries will need extensive dewatering with high energy use, it is most likely better left wet and mixed with any liquids. The amount of demand for these liquids and water for reuse in the current mining operation also helps determine when dewatering is feasible or not.
Washout Risk
Consider what’s downstream of the storage site before selecting a wet storage method for tailings. Wet slurries travel much farther and can cause more damage than dry materials, especially if the dry tailings are compacted in lifts rather than piled high in steep-sided heaps. Any tailings that will be stored above or near residential or commercial areas should be stored dry, as long as dust can be controlled. Covering the dry storage area is usually well worth the chance to reduce flooding and washout risks compared to storing the materials wet if there’s any concern.
Only a consideration of all the characteristics of both the tailings and the site will determine whether dry or wet storage is a better choice. Many mines have started out with one method only to switch to another, then back again. Don’t fall for the misconception that only wet-stored tailings need lined impoundments. Even dry stacked tailings can cause runoff and leachate issues that contaminate the surrounding environment with heavy metals and more. Make sure that all impoundments built for containing tailings are lined with low permeability materials from BTL Liners. Our reinforced polyethylene geomembranes are ideal for dealing with challenging site conditions and harsh tailings materials that wear out lesser products.
