PASTE & FILTERED TAILINGS
The downstream devastation of a burst tailings storage facility ( courtesy Wikimedia )
Filtered tailings progress and impacts
Dave Meadows , CTO at Bechtel Mining & Metals discusses the tailings way forward
Reflecting on the past few years the spotlight has continued to beam down on tailings dams for a variety of reasons , coupled together with the importance of Environmental , Social and Governance ( ESG ) factors .
From existing operating mines , closed mine sites and newly developing projects , the subject of tailings storage has become top of the list with virtually every mining company . Likewise , design requirements and permitting aspects have been stepped up to address the importance of the tailing ’ s storage facility and its safe longevity .
In many commodities ore grades have continued to decline , with the quantity of tailings generated from mining operations proportionally increasing . Robustness of design and consideration of longer terms impacts on the environment are vital to the success of new projects . The greater environmental and social impacts far outweigh some of the traditional technical challenges experienced in the project development phases . Figure No 1 and No 2 show the devastation from the past events in one of the iron ore mines in Brazil .
In the past five years some notable tailings dam failures include : n In 2014 , at Mount Polley Mine in British Colombia , Canada there was a breach of a tailings pond with an estimated 24 million cubic metres of mine waste passing into ecologically sensitive areas . n In 2015 a dam in Samarco ’ s Minas Gerais mine in Brazil collapsed , killing 19 people and releasing 50 million tons of waste into the Doce River . n On January 25 , 2019 , a tailings dam at Vale ’ s Corrego do Feijao mine tailings collapsed , spilling 12 million cubic meters of tailings resulting in over 250 deaths and several people unaccounted for . n In October 2019 , a dam collapsed at a Siberian gold mine killing more than 15 people . n In 2021 a breach in a spillway duct at the Catoca Mine in Angola lead to polluting of downstream rivers and water systems in and around the city of Lucapa . Images of some of these disasters have been circulated around the world in both mining forums and across many mainstream news channels with the messages being very clear . With safety at the forefront of our work , the loss of life in these situations is unacceptable .
Future approaches and the importance of filtered tailings
Several mining companies together with OEMs and technology suppliers are continuing to develop solutions and alternatives to address the challenge . The focus of these efforts has been to improve overall water recovery from the process as well as increase the geotechnical stability of the deposited tailings material . The implementation of filtered tailings technologies can take the freshwater usage down from around 0.4 to 0.7 m3 / t to levels of around 0.2 to 0.25 m3 / t resulting positively in freshwater usage reduction .
In the last decade a significant amount of effort has been made in advances to the thickener design with a view to improving underflow densities and performance in general . In conjunction with this continuing enhancement of polymer and reagent addition schemes have been made . Hi-density thickener performance has taken underflow densities to around 61 to 64 % solids and paste applications have taken underflow densities around 66 to 70 %.
However , there has been great interest in the appliance of filtered tailings in many commodities . Filtered tailings is already practised around the world in iron ore , gold , and
alumina but in general these are at smaller production rates ( typically less than 25,000 t / d ). For the large porphyry copper projects tailings processing rates can be up to 200,000 t / d in some cases thus changing the requirements .
As well as direct filtration of the processed tailings streams some alternatives are being investigated where a portion of waste stream is combined with the tailings to allow a more desirable final geostable product . This type of approach was previously named EcoTails™ and was investigated by GoldCorp at some of its past mining operations .
For the large scale copper and gold operations the OEMs are making advances . FLSmidth introduced the AFP-IV ® Colossal™ filter , a new large-scale filter press that may be able to match necessary tonnages at larger producers . The new 3 m x 5 m filter press takes this to a new level . At the end of 2021 , Diemme Filtration also launched its GHT5000F filter press for rapid filtration of tailing streams with the unit offering the largest size at 5 m by 5 m and have improved the level of autonomy on their latest machine designs . Metso Outotec developed its VPX™ filter as a solution for a range of dewatering applications . The filter can handle difficult-todewater tailings , because it has up to 25 bars operating pressure which enables up to 90 % water recovery .
With the larger filter units processing rates of over 25,000 t / d should be achievable resulting in more economic and compact solutions . Target product moisture contents of around 25 % should be achievable . Filtering tailings will not only increase the stability of tailings deposits by removing additional water , but that additional water can be recycled back into the concentrator / plant site .
Filtered tailings efforts will be integrated with bulk materials handing solutions in order to provide a complete solution to mining operations .
Bechtel Mining and Metals is in the unique position of being able to combine a number of its Center of Excellences with the Bulk Material Handling Center of Excellence to develop comprehensive sustainable solutions for its customers . To date , the combined group has completed several tradeoff studies and technology evaluations .
To conclude , the unfortunate repeated cases of tailings dam failures has continued to expose the huge risk associated with several existing designs as well as put the focus on the need for thorough due diligence of new tailings storage facilities . The target of no loss of life must be atthe forefront of any tailings facility design .
IM
P2 International Mining SUPPLEMENT | JULY 2022