Mineralogy and mine waste31/08/17Environment
Speaking at Goldschmidt2017, Karen Hudson-Edwards, a professor of environmental geochemistry and mineralogy at Birkbeck, University of London, UK, discussed the effect of mine waste and tailing spills.
Professor Karen Hudson-Edwards is a researcher at Birkbeck, University of London’s Department of Environmental Geochemistry and Mineralogy. By training an economic geologist, she is the author of ‘Tackling mine wastes’ (Science, 2016) and has contributed to a book on minerals as well as several papers.
Pan European Networks attended the Goldschmidt2017 conference in Paris, France, on 13-18 August, where Hudson-Edwards discussed the effects of mining waste and tailing spills on geographic areas, human life, and animal habitats at several scales. She also discussed how such research was being carried out and identified areas for future improvement.
A sliding scale
“Mine wastes are, by definition, really large quantities of liquid and solid waste, of course gas waste as well,” Hudson-Edwards explained. A lot of mine waste has been generated historically; however, the issue remains a persistent factor today and is potentially damaging to the environment. “There are a number of different types of mine waste and we, as geochemists, mineralogists and environmental scientists, can study these properties,” she continued. “We’ve got waste rot, metallurgical wastes, water waste – we normally classify these by pH.” Tailings – a form of mine waste – take the physical manifestation of a residue which is left behind after the extraction of ore. Hudson-Edwards gave some current examples of mine waste tailings, one site being located in King Creek, Ontario, Canada, another is a red mud repository of bog site waste, and another a site containing soluble sulphate salts in Cyprus.
“Tailing is generally composed of both the waste sulphides and those sulphides that didn’t get extracted for ores – pyrite probably being the most reactive and leading to acid mine drainage ranging to arsenopyrite,” Hudson-Edwards said. She added: “In our mix we’ve got processing fluids from the extraction of the ore and what we call gang minerals. We’ve got the carbonates which are great because they will neutralise any acidity that’s around.”
The concern for residue in river systems is ever present, Hudson-Edwards said – “a lot of rivers in Wales fail regulatory guidelines for zinc concentrations”. Another waste site, the Río Pilcomayo, the Andes, is around 200km downstream of the famous mining area Potosí – a centre for silver mining. Following the Spanish invasion of the area, Spain extracted the silver and “dumped” the tailings in the river, leading to a legacy of waste within the river as well as mercury as a byproduct of silver ore extraction.
“Almost every country in the world has a mining industry now or has had a mining industry of some sort,” Hudson-Edwards added. Therefore, the issue of mine waste is a global legacy issue, and is not relegated to specific sites. In Calgary, Australia, she spoke of how the “footprint of the mine waste is as large as, or larger than, the actual town”. The dry season in Pilcomayo, South America, leaves behind only mine tailings in the river, whilst the acid river – the Rio Tinto – in Spain flows 80km down from the mine waste and mining area and maintains a pH of 2. Hudson-Edwards said: “We’ve got acid pit lakes like these left over from centuries ago, millennia mining in cycles.”
The role of geology
As an economic geologist, “we look at the minerals and we think about the minerals, what’s in them, and how they might react with liquids. What are these mine waste solids? If we’re going to understand their impacts we really need to go to the source and figure out what’s in them, how reactive they are, and so on,” Hudson-Edwards said.
She added: “I strongly think that it’s really important to understand all of the mineralogy in our system because even though we’re trying to understand the reactivity of our minerals than contain toxins, they’re going to react. We need to understand the whole package.”
The industry focuses on work on mine waste solid reactivity – looking at the reactions between mine waste minerals and fluids. Supplementary to this, researchers aim to figure out what the primary waste materials are and how they evolve in order to form a range of secondary minerals. “One of the things we’re most interested in is the contaminants of mine waste because the contaminants are arsenic, metalloids and metallic elements”, those byproducts which have potentially toxic effects on the environment, Hudson-Edwards said. “We’re interested in how these mine waste minerals weather, what they turn into and what [the] pathways of contaminants are as this happens.” She discussed how the process itself was complicated by and dependent on a number of factors including pH/Eh and bacteria.
Hudson-Edwards asked: “What are the micro-to-nanoscale properties of mine waste minerals?” Should there be a lack of understanding of minerals then researchers cannot predict what the potential impacts of contaminants could be. As economic geologists, researchers study families of minerals. Hudson-Edwards spoke about how bucket minerals, such as jarosite, can accept other substances which are put into such. For example, you can put calcium and sodium into acetate. Jarosite minerals, which Hudson-Edwards explained she first became aware of in the Rio Tinto, form under acidic pH conditions. As there is a lot of jarosite and other sulphate salts in the river, these control pathways of contaminants within the Rio Tinto.
“We study these minerals on a whole range of scales – we study them in the field, but we also work with chemists who do atomic modelling.” The professor also raised questions which must be asked: how stable are minerals, and what are the controls on contaminant release should an event take place? “We look at fluids, we can model using geochemical programmes, or we can do experiments.” However, Hudson-Edwards said that both inorganic and organic processes which happen and involve contaminant release from mine waste cannot be ignored.
A cycle of contaminants
Yellowknife in northwest Canada, Hudson-Edwards added, is “probably one of the, if not the, most arsenic contaminated site on Earth”. Within the site they mined gold – in arsenopyrite – roasting arsenopyrite to release the gold and consequently also released arsenic trioxide in the process. A colleague of the professor, Joanne Santini from the UK’s University College London, went underground to sample biofilms left behind on the walls of underground workings. From the original waste, arsenic was found to be in the 3+ form and, as a result, Joanna and her PhD student were able to isolate the bacteria, which they named GM1 – giant mine 1. The bacterium was arsenide oxidising, but performs the process at low temperatures. Hudson-Edwards explained: “This was one of the first low-temperature arsenic oxidisers to be found in mine environments.”
GM1 could be implemented as a method to remediate waste or to contain the waste, as researchers know that arsenic trioxide is highly toxic. For those working at Yellowknife, Hudson-Edwards spoke about how various methods to remediate and contain the waste are being explored. “The permafrost is all melting – I think they’re thinking of refrigerating the tailings at the moment. Bacterial remediation is still possible, but in the meantime, they’re trying to keep them cold to keep the reactions from happening,” she added.
“Colloids and nanoparticles are very, very fine on the nanometre range, and we’ve realised that nanoparticles probably control the transport of metals in river systems, mining-factored river systems and, actually, elsewhere as well.” Unfiltered metal concentrations accept fine particulates, whilst its filtered counterpart would absorb the majority of the dissolve, as well as, potentially, some nanoparticulate material. Hudson-Edwards said: “Another PhD student of mine is working in west Wales, and she’s looking at mining impacted rivers and she’d done some preliminary analysis of her data; zinc and manganese are mostly in the dissolved phase. We think they’re almost truly dissolved, whereas those which are non-conservative probably are transported as nanoparticles.” Recent research has proven the importance of transporting mine waste in rivers, in dust, and in other sediments as well.
Regulating the river systems
“River systems tend to be largely impacted by mining – especially in the past where they were seen as dumping grounds. That’s hopefully happening less and less, but we know in some places it still happens,” said Hudson-Edwards. Rivers are seen as being greatly affected by mining in some cases. “What are the relative effects of acute and chronic contamination events along rivers?” Chronic events are those occurrences in which they are constantly occurring, such as regular discharge of tailings into rivers. An acute shift on tailing failures is epitomised in the larger events which often reach media attention.
The largest recorded tailing failure, currently, is the 2015 dam spill which took place in Brazil and was one of the few in which researchers are aware that the tailings reached the Atlantic Ocean. Recent work produced in New Zealand in a gold mining site constituted the researchers coring into the floodplain and mapping the visible mine waste. Hudson-Edwards added that researchers mapped the waste “and we’ve linked it back to historic records – there was a big dam spill in 1906.”
The damaging effects of spills are widely known. “We know these things have happened for a long time, they can lead to loss of life, loss of property [and] contamination of systems,” Hudson-Edwards said. Tailing spills can be separated into two types – one is the type which has the ability to transform surrounding river systems. In Spain in 1998 a dam spill caused tailings to flood down river; however, it stopped at a wall north of the Doñana National Park. The park is a major migratory pit stop for birds journeying from northern Europe or Africa. In the clean-up effort the tailings were removed; however, in these efforts, they removed vegetation which stabilised sediment and consequently the system responded in a geomorphological way.
Responding to a legacy
The legacy of mine waste in rivers is something which needs to be considered, especially how it can be managed, Hudson-Edwards said. In the “Pilcomayo, we’ve done the study to show that vegetables are enriched with heavy metals”. A secondary study has also been carried out in Wales whereby a significant flood event led to new lead getting into the catchment and the chronic poisoning of cattle. “Tailing failures are terrible and we need to understand them and how they impact the entire environmental and ecological system,” Hudson-Edwards said.
“Thinking globally, we have a lot of information on mine waste now. Can we start to put it all together?” The amount of solid mine waste in 1981 was estimated to match the amount of materials moved by natural processes. In order to map the extent of mine waste, Hudson-Edwards took part in a study which harnessed “Google Earth and some data in the literature to try and map the aerial extent of mine waste sediments”. The professor expressed the need for a global effort to understand and manage the imprint of mine waste.
“We know that there are huge fluxes of zinc into the Atlantic Ocean; I’d like to get a PhD student working on this to try and get a global idea of what are the fluxes. How do they impact on natural bio-geochemical cycles? Is there something we can do and what are the impacts?” Hudson-Edwards added that a further, and larger, area of focus would be: “What are the acute and the chronic effects of mine waste contaminants on human health?” A clear health impact has been identified in relation to lead-rich dust produced as a byproduct of the mining of gold ores, as well as the remobilisation and formation of dust in drier areas where there are legacy tailings.
Hudson-Edwards said that the field is aware that mine waste is a global issue, causing potentially toxic effects. However, a lot of research and information has been accumulated already on the impacts of mining, as well as the possession of new technology for the analysis of nanoparticles and the approaches in which we can map mine waste globally. Despite this, she added that more still needs to be done. Modern mining has led to the extraction of critical metals, for example in the demand for batteries and, subsequently, lithium as well as nickel and other materials for solar panels. Though the volumes of mining for crucial metals are not high, there remains the threat of mine waste from such. “To my knowledge, we don’t really know a lot about how critical metal mine wastes behave, and I think that’s a big area that we can get into,” Hudson-Edwards added.
“Can we start to develop global bio-geochemical models for mine waste? Can we think about it in a bio-geochemical cycle where we know that mine waste can get into different spheres on the Earth?” Mine waste is able to process through the system, whereby it can get into the hydrosphere or the biosphere – where it is taken up by humans or plants – [into] dust in the atmosphere, and, eventually, into oceans. Hudson-Edwards concluded her talk by asking: “We have a reasonable handle on some of the forms of these metals, what we understand a lot of though is some of the fluxes – how much is going into the biosphere? How much is getting into river systems? What are the effects? Can we do this, and, of course, can we do it for most of the elements that are in mine waste? Can we put our heads together and think of looking at the real impacts of mining?”
This article will appear in Pan European Networks: Science & Technology 24, which will be published in September.