A new way to dispose of course coal rejects

To improve efficiency and environmental sustainability for a coal mine the Ausenco team developed an innovative method of disposing of coarse coal rejects.

To improve efficiency and environmental sustainability for a coal mine in New South Wales, Australia, the Ausenco team developed an innovative method of disposing of coarse coal rejects from the Coal Handling Processing Plant back into an operational goaf. 

The team successfully adapted the ‘co-disposal’ method of transporting coarse waste material by significantly increasing the solids concentration within the coal slurry. We designed and managed a series of tests to establish the slurry behaviour and simulate injection trials, with no impact on longwall production. We were able to develop the optimal slurry concentration and process for backfilling the goaf. 

The result is a system that is more cost effective and with a lower environmental impact than surface tailings dams and/or trucking rejects offsite. We believe that this is the first time the process of disposing coarse coal rejects underground has been proven. It has exciting positive ramifications for the ongoing sustainability of coal mining.
Some background

After undertaking a number of studies themselves and engaging other consultants to develop options for the disposal of coal rejects that would meet specific project site, environmental, community and project efficiency needs, the owner of an Australian coal mine contacted Ausenco for help. 

The Ausenco team’s solution, which involves injecting high density slurry back into the longwall goaf achieved the stated goals and has created a cost effective and environmentally friendly option for other underground coal mines to dispose of coal rejects. 

Longwall mining is a technique used in many types of underground mines, including coal.  A large cutting tool (shearing head) passes along the mineral seam face to cause the material to fall onto a conveyer belt, from where it is transported away for processing.  The roof above the shearing head and conveyer is supported by a line of hydraulic chocks – “C” shaped devices than can be lowered, moved forward and raised again.  With each pass of the shearing head, these chocks advance so the operation can follow 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.

Traditionally, fine coal tailings have been stored in surface tailings dams which eventually require extensive capping and rehabilitation or transported offsite. Tailings dams often reach full capacity before mine closure, requiring new dams, or dam extensions. Tailings dams have inherent environmental risks, including flooding or leakage and are expensive to develop, maintain and rehabilitate. 

Coarse coal rejects can be used as stable dry fill but occupy a substantial amount of space. With limited space on this site, both the fine and the coarse material was transported offsite to landfills. This was impacting the local community in terms of the level of dust, noise, visual amenity and heavy vehicle traffic along the disposal route. The client asked Ausenco to develop a solution to reduce offsite disposal by 60% in trials and ultimately eliminate offsite disposal. 
Ausenco’s solution is expected to enable all rejects to be retained onsite without tailings dams but in previously unused, but available space behind the goaf. It utilises the fine coal rejects to form a vehicle to transport coarse coal rejects. This method of “co-disposal” is common practice in coal mining, however we modified the slurry to a substantially higher solids concentration than conventional co-disposal methodology (55% to 65% versus 35% to 40% by weight) and determined the optimum process for injecting the slurry behind underground plugs into the operational goaf. 

Two of the environmental benefits of using high density slurry are that the higher solids concentration reduces both water consumption and volumetric throughput. Water consumption is important because collecting and returning the decanted water from lower density slurry to the surface would be problematic and costly. Additionally, with the underground injection point moving to follow the longwall, a dedicated collection pond is not practical. Furthermore, with underground coal mines typically over 300 metres below ground level, a large amount of energy would be required to bring water to the surface. The team determined that increasing the density of the slurry significantly reduced the need to bring it 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.

The team’s approach reduces community and environmental risks and concerns by ultimately eliminating the need for offsite landfills and onsite rejects tailings dams and provides a cost-effective method of underground disposal into otherwise unused space. We believe this is the first time this approach has been designed, developed and proven for coal rejects disposal and could be applied to other coal mines with similar challenges. 

We look forward to the opportunity to develop this approach further on the test site and other locations. To find out how we could help you, one of the Ausenco Experts.