Traditionally, fine coal rejects are stored in surface tailings dams which eventually require extensive capping and rehabilitation, or transportation offsite. Tailings dams are expensive to develop, maintain and rehabilitate. They often reach full capacity before mine closure, requiring capital investments for new dams, or dam extensions. Most importantly, tailings dams have inherent environmental and social risks, including flooding or leakage.
Coarse coal rejects can be used as stable dry fill, however, they occupy a substantial amount of space.
Mine sites with limited space often transport both fine and coarse material offsite to landfill sites. Not only does this impact the local community in terms of dust, noise, visual amenity and heavy vehicle traffic along the disposal route, it also presents an operational cost to mining companies.
To minimise impacts on the environment and the community whilst reducing costs, miners have been urged to search for better and safer ways to dispose of rejects.
Closing the loop
Longwall mining is a technique used in many types of underground mines, including coal. A large knife, called a shearing head, passes along the mineral seam and cuts the material onto a conveyer belt, from where it is transported for processing. The roof above the shearing head and conveyer is supported by a line of C-shaped hydraulic chocks which can be lowered, moved forward and raised again. With each pass of the shearing head, these chocks advance so the operation can proceed along the coal seam. As the longwall system advances through the seam, the unsupported roof behind the chocks caves in. This caved-in area is known as the “goaf” or the “gob” in the Americas. These goafs collapse under the weight of the overlying strata.
In 2013, our client set a challenge to reduce offsite disposal of coal reject by 60% in trials, with the ultimate goal of eliminating coal reject disposal by injecting it into the goaf. We were determined to bring our client’s idea to life and find a solution to meet their challenge.
‘Co-disposal’ is a common method in coal mining which uses fine coal rejects as a vehicle to transport coarse coal rejects. Using ‘co-disposal’ as a foundation, we developed an improved method by significantly increasing the solids concentration within the coal slurry by up to 25%. We designed and managed a series of tests to establish the slurry behaviour and simulate injection trials, without impacting longwall production. After several years of research and testing, we delivered the optimal slurry concentration and process for backfilling into the goaf.
An environmentally sustainable and cost-effective solution
Usually in longwall mining, collecting and returning decanted water from lower density slurry to the surface is problematic and costly. Additionally, as the underground injection point moves to follow the longwall, a dedicated collection pond is not practical. With underground coal mines typically over 300 metres below ground level, a lot of energy is required to bring water to the surface.
Through using high density slurry, the higher solids concentration reduces both water consumption and volumetric throughput. It also significantly reduces the need to bring water back to the surface and/or a water collection pond. The reduced volumetric throughput also minimises the amount of energy required to pump the slurry into the underground goaf, in turn making the process more cost-effective.
Additionally, the material deposited into the goaf originated onsite and no chemical rheology modifiers are added. It quickly dewaters naturally to an extent that removes liquefaction risks, should there be seismic activity. The removal of the liquefaction risk also effectively eliminated concerns about possible “in-rush” events. With the longwall constantly moving and leaving space behind it, the disposal method can continue until mine closure.
Our solution allows rejects to be retained onsite without tailings dams or landfills in previously unused, available space behind the goaf.
Not only did we help our client meet their goals, we also found a better way to dispose rejects and close the operation loop of longwall mining.
After three years, the mine has seen significant results. Our solution and design have proved to be a cost-effective and environmentally friendly option for other underground coal mines to dispose of coal rejects.
We believe this is the first time this approach has been designed, developed and proven for coal rejects disposal and can be applied to other coal mines with similar challenges.