TAILINGS
with what sellers are used to receiving . As demand and markets for these minerals grow , new supply chains will result , and the market dynamics will change for the better . Transparency and better information are the natural outcomes from a larger market .”
Looking from the outset , when assessing tailings and waste piles , they say you should consider what has changed since the original mine plan or operation . Is there now demand for the by-products that didn ’ t exist when the waste was created ? Is there better technology allowing superior recovery ? Are there additional ESG benefits of recycling materials ?
Furey : “ Tailings and waste valorisation are attractive when the TSFs or the waste piles have valuable , easily liberated metals , minerals , and by-products . Closed mines are especially good candidates for valorisation projects because reprocessing the waste won ’ t disrupt the mine ’ s primary operation and selling the by-products can help offset closure costs . Processing , separation , and extraction methods would have also improved dramatically since the plant was likely built . Thus , extracting more than the original targeted minerals is potentially economically feasible . Even with the value-add of critical minerals , the most attractive waste valorisation projects are often those that focus on the mine ’ s primary metals . It ’ s not a secret that tailings and waste piles at some historic copper and gold mines offer higher grades than currently operating mines .”
Other projects have focused on waste rock and slag material which can be crushed and sold as an aggregate without the need to remove harmful chemicals .
The authors highlight the fact that valorisation projects that target minerals needed for the global energy transition are especially en-vogue . “ For example , in 2021 , Rio Tinto invested in adding a circuit to the Kennecott mine to recover tellurium ( the essential component of cadmium telluride , a semiconductor used to manufacture thin film photovoltaic solar panels ) from what would have been tailings . Because demand for tellurium is strong , the case for investment was obvious .”
On another project , Stantec assisted in the development of a proprietary technical process to produce several sources of revenue from bauxite residue . This work included scale up , preliminary layouts and capital costs estimates for a demonstration plant . The success of the outcomes of this work will result in a long-term liability ( bauxite residue ) becoming a valuable resource , contributing to the circular economy , and having an environmentally beneficial outcome .
Finally , rather than reprocessing , some projects have focused on monetising tailings as a potential carbon sink . These projects show real economic and environmental promise . An example is BHP ’ s Mt Keith mine .
Furey and Stine conclude : “ For mining , a truly circular economy would mean 100 % reuse of tailings and waste rock . It would mean that everything that is unearthed would have a preplanned destiny . Given efforts to make the industry more sustainable , creating a truly circular economy at mines could be viewed as a reward in and of itself .”
Paterson & Cooke on filtering coal fly ash
Jason Hamelehle and Casey Schmitt , both from Paterson & Cooke ’ s US offices , reviewed for IM an important filtration project for coal fly ash . Coal-fired power plants in the United States have come under stricter environmental regulations for their fly ash waste disposal , with some requiring the stored fly ash slurry to have no bleed water . Slurry with no bleed water is considered a paste by most definitions , and transporting paste can present significant technical problems . Instead , some coal-fired power plants have opted to filter fly ash and transport filter cake using conventional dry material handling methods such as trucking and conveying .
A power plant in the northwestern US has two active coal-fired generating units each around 740 MW output and producing approximately 2,800 t / d of fly ash . The previous fly ash dewatering system relied on thickening using high performance thickeners and pipeline transport with hydraulic piston pumps . An environmental agreement was passed to upgrade the fly ash dewatering system by 2022 to produce no bleed water . Paterson & Cooke teamed up with an EPCM firm to design a filter plant for truck transport and dry disposal of fly ash filter cake .
To meet environmental guidelines the fly ash
Four Diemme Filtration GHT2000 . P7 recessed chamber pressure filters for coal fly ash filtration ( two operating , two standby )
had to pass the EPA Test Method 9095B Paint Filter Liquids Test , defined as no liquid passing through a mesh number 60 filter in a five-minute period . At the passing concentration of 25 % moisture by mass the fly ash was a pumpable paste . To transport the material by truck , the transportable moisture limit of 18.5 % moisture by mass would need to be achieved ( moisture values defined as mass of liquid phase divided by total mass of mixture ). The dewatering test work completed by Paterson & Cooke using inhouse proprietary testing equipment was used to select recessed chamber pressure filtration technology to achieve the target cake moisture of 18.5 % moisture by mass .
One of the design challenges was the operator required the filter plant to have 100 % availability and 100 % redundancy despite the additional cost , to avoid any bottleneck or shutdown of the filter plant that could impact power plant operations . To meet this requirement , four Diemme GHT2000 . P7 recessed chamber pressure filters ( two operating , two standby ) were installed . When one filter is down for routine maintenance , one filter is still online standby , ready to come into service if one of the operating filters is unavailable .
The dedicated under-filter conveyors are reversible to be able to discharge to two independent take-away conveyors and loadout bins . If one of the take-away conveyors , or any equipment down stream of it , is down for maintenance , the under-filter conveyors change direction and deposit cake onto the standby take-away conveyor . This equipment ensures that abundant standby capacity is available to prevent long shutdowns but did not come without challenges . During commissioning and startup , excessively large piles would discharge in batches onto the take-away conveyor causing
60 International Mining | APRIL 2024