The vast scale of modern mining presents significant challenges for sustainable waste management, writes specialist environmental engineer, Dr Gavin M. Mudd.[hr] [insert_menu title=”Menu”] [link to=”Sustainability And Mining”]Sustainability And Mining: Beyond The Oxymoron[/link] [link to=”Major Mine Waste”]Major Mine Waste Trends Overview – Mineral Production[/link] [link to=”Ore Grade Trends”]Ore Grade Trends[/link] [link to=”Open Cut Mining Trends”]Open Cut Mining Trends[/link] [link to=”Waste Rock/Overburden Trends”]Waste Rock/Overburden Trends[/link] [link to=”Mine Closure and Rehabilitation”]Mine Closure and Rehabilitation[/link] [link to=”Economic Resources”]Economic Resources[/link] [link to=”Sustainability Reporting”]Sustainability Reporting & Mine Waste Issues Sustainability Reporting[/link] [link to=”Global Reporting Initiative (Gri)”]Global Reporting Initiative (Gri)[/link] [link to=”National Pollutant Inventory (NPI)”]National Pollutant Inventory (NPI)[/link] [link to=”Resource Intensity or Eco-Efficiency of Mineral Production”]Resource Intensity or Eco-Efficiency of Mineral Production[/link] [link to=”Conclusion”]Conclusion[/link] [link to=”References”]References[/link] [/insert_menu]
The issue of waste management is correctly perceived to be a major issue for municipal councils and the manufacturing, construction and chemicals industries. There is less recognition, however, of the vastly larger quantity of solid wastes produced by the mining industry. The reasons for this are most likely due to the perceived relatively benign nature of mine wastes, remoteness from population, apparent success in mine waste management, or other factors.
One of the most fundamental questions the mining industry is currently grappling with – on a global scale – is how sustainable is this substantive generation of solid wastes? Surprisingly, this question is extremely difficult to answer, and requires extensive data and other issues to be put into context.
The mining sector has been increasingly advocating and implementing sustainability across the industry, and many companies are now actively reporting on sustainability performance alongside corporate financial reporting. There is very little research, however, which seeks to link the traditional production side of mining and its solid wastes to the sustainability agenda, and very little examining mine wastes within sustainability frameworks and emerging sustainability reporting regimes.
[hr]”One of the most fundamental questions the mining industry is currently grappling with – on a global scale – is how sustainable is this substantive generation of solid wastes?”[hr]
Click On Images To Enlarge[hr] [title from=”Sustainability And Mining”]Sustainability And Mining: Beyond The Oxymoron[/title]
An individual mine or mineral deposit is commonly argued as ‘unsustainable’ since it’s perceived as a finite or non-renewable resource. Although this is perhaps obvious, the reality is that the cumulative sum of all mines and mineral deposits over time have not yet been depleted – we are producing more minerals and metals today than ever before, and commonly there is at least similar quantities or more known in remaining resources at operating mines and undeveloped mineral deposits. There is strong global evidence that ‘non-renewable’ mineral resources are a complex function of exploration, technology, supply-demand, economics as well as social and environmental constraints – the cumulative effect of which has historically ensured abundant supply of most minerals and metals.
A major gap in the sustainability debate for mineral resources is the ‘environmental cost’ – the pollution legacy, solid wastes, and so on. A major driver in this regard is the almost universal long-term declines in ore grades. For example, in Australia lead-zinc-silver mining at Broken Hill in 1900 had average ore grades of ~18% Pb, ~16% Zn and ~350 g/t Ag while in 2012 ore grades were 3.7% Pb, 5.0% Zn and 40 g/t Ag. Similarly, long-term declines in ore grades are now well recognised for global gold (Mudd, 2007b), copper (Mudd et al., 2013), uranium (Mudd, 2014) and nickel mining (Mudd & Jowitt, 2014). This means moving more ore for a given production – or with continually expanding production, an ever-increasing amount of ore. Following processing the residual solid waste becomes known as ‘tailings’. Concurrently over recent decades, due to the abundance of cheap diesel and associated earth-moving machinery, there has been a major expansion of the use of open cut mining techniques over underground mining. Thus, for every tonne of ore mined there is an equal or greater amount of overburden or waste rock which also needs to be mined – giving rise to the vast amounts of total solid wastes by modern mining.
A critical issue for tailings and waste rock is the potentially increasing scale of environmental liabilities associated with them. The safe storage of tailings, in large engineered storage dams, requires major design, construction, operation and decommissioning costs. A review of the literature shows that catastrophic failures of tailings dams are still occurring – sometimes leading to major environmental impacts and even human fatalities (eg. ICME et al., 1997; ICME and UNEP, 1998; Kumah, 2006). Additionally, the nature of many mineral deposits now being processed is that the associated waste rock may contain sulfidic minerals. When exposed to fluctuating cycles of infiltration and oxygen (ie. air), the sulfides oxidise and cause acid and/or metalliferous drainage (‘AMD’) (see Taylor and Pape, 2007). The escape of AMD into the surrounding environment often leads to extreme environmental impacts, sometimes for tens of kilometres downstream from a mine site. When this occurs, the large scale of environmental impacts can also have severe social and/or economic impacts. The effort required to plan, manage and rehabilitate tailings and waste rock is no simple feat and the global mining industry is expending significant effort in improving standards.
From a sustainability perspective it must be asked – how long can these relentless increases in the scale of mine wastes continue? This is, of course, a loaded question and very difficult to answer. As part of any attempted answer, it is clearlyimportant to understand the history of these issues, as this can give critical insights into future directions for the scale of mine wastes in Australia and globally.
Given the number of major legacy mines, combined with the introduction of more rigorous environmental legislation from the 1970’s, the mining industry has worked hard over the past three decades to improve environmental management, mostly successfully in industrialised countries.
Following the lead of the 1992 Earth Summit, a more comprehensive shift in thinking began in the mining industry – that of sustainability. From the mid-1990’s numerous mining companies began to release environmental reports, variably detailing their performance with respect to water, energy, greenhouse emissions, tailings, rehabilitation, etc. These reports were broadened to include social and economic aspects of mines and companies and are now commonly termed ‘sustainability’ reports.
With the recognition of the need for consistency in reporting, the ‘Global Reporting Initiative’ (GRI) was established by the United Nations in 1997, and the third edition of the GRI Sustainability Reporting protocol was released in October 2006 (GRI, 2006). A pilot mining sector supplement was released in February 2005 (GRI, 2005). Thus, when combined with traditional financial reporting, there is increasing publicly reported data with which to quantify the issue of mine wastes and associated sustainability issues (especially life-cycle costs and comparative waste ratios). [top][hr]
[title from=”Major Mine Waste”]Major Mine Waste Trends Overview – Mineral Production [/title]
As noted previously, the production of almost all minerals and metals is universally continuing to increase over time, some exponentially. This is a key driver – especially with the “super-cycle” global mining boom at present being driven by insatiable demand for metals and minerals from China (and to a lesser extent India). The historical data for metal and mineral production in Australia is shown in Figure 1, and ore commonly a reflection of global production. The critical issue is to note the general trend for all series shown – gradual growth followed by an exponential increase over recent decades. [top][hr] [title from=”Ore Grade Trends”]Ore Grade Trends [/title]
The available data for the trends in ore grades for select metallic and other minerals is presented in Figure 2. A general trend is indicated for all series included. Each series has its particular influences, such as initially rich oxidised ores being mined followed by lower grade sulfide ores (eg. Cu, Pb-Zn-Ag), or changes in the major mines operating (exhausted, opened, etc). Further details for each metal or mineral are given in (Mudd, 2009). For bulk minerals (bauxite, iron ore), impurities can often be as important as the ore grade though data is rarely available for both aspects (see Mudd, 2009). [top][hr] [title from=”Open Cut Mining Trends”]Open Cut Mining Trends [/title]
The major shift to large-scale open cut mining in the latter half of the twentieth century is the singular reason behind the extent of solid wastes now produced by the mining industry. Although data remains incomplete for some metals (eg. gold, copper, nickel), for others accurate statistics are reported (eg. coal, uranium), or it is known that no underground mines exist (eg. iron ore, bauxite, mineral sands). The available data for open cut mining, as the proportion of ore, is shown in Figure 3. In general, this is very similar to the contained metal, though as open cut mining is often lower in grade it is slightly lower if presented as the proportion of metal. As with ore grades, an individual series is often linked to major mines opening or closing. The introduction of economic large-scale open cut mining was led by the copper industry, with the great mines at Mt Lyell and Mt Morgan in the early twentieth century, and was progressively implemented at many other mines and sectors (further details in Mudd, 2009).[pullQuote]”The effort required to plan, manage and rehabilitate tailings and waste rock is no simple feat and the global mining industry is expending significant effort in improving standards.”[/pullQuote]
[top][hr] [title from=”Waste Rock/Overburden Trends”]Waste Rock/Overburden Trends[/title]
The extent of waste rock or overburden produced by mining is closely linked to the use and scale of open cut mining. The available data is summarised in Figure 4, though incomplete public reporting of data by many companies means that the data presented is a minimum for all commodities presented (though brown coal, uranium and diamonds are close to complete, some mines are still missing data). The reason behind the extra-ordinary growth in waste rock and overburden production is the combination of rapid metal / mineral production growth combined with the significantly increasing use and scale of open cut mining. For example, although the extent of open cut mining for copper was high in the 1930’s to mid-1950’s (when Mt Isa began underground copper mining on a large scale), the scale of the Mt Lyell and Mt Morgan mines during this period was minor in comparison to the modern generation of open cut mines such as Ernest Henry, Cadia Hill or Century.
Further to the extent of open cut mining, the ratio of waste rock to ore mined is also critical. The available waste rock or overburden data relative to ore milled is shown in Figure 5. When combined with increasing production, for many commodities the ratio of waste rock (or overburden in coal mining) to ore is gradually increasing over time. This is particularly the case for copper, gold and black coal. The ratio for brown coal is approximately stable over time (due to the unique nature of the Latrobe Valley brown coal field). The data for diamonds shows the initial development of the Argyle open cut mine, with high ratios declining gradually over time, followed by the expansion and extension of the Argyle pit in the late 1990’s (Argyle is now transitioning to an underground mine, with lower ratios to be expected in the future). For uranium, the high initial ratios show the development of the Rum Jungle project and the low quantity of ore processed in the early years, with the years after the late 1950’s being variable depending on the select mines in development and operation.[top][hr] [title from=”Mine Closure and Rehabilitation”]Mine Closure and Rehabilitation[/title]
A major sustainability aspect of the modern mining industry is the degree and extent of effort for rehabilitating and closing mine sites. In essence, this involves returning mined land to some type of functional purpose or land use, and ensuring that long-term potential pollution risks are minimised. The nature and extent of rehabilitation works are invariably site-specific, but commonly include placing engineered soil covers over tailings and waste rock deposits, possibly backfilling open cuts, sealing of underground mines, re-contouring for water resources, revegetation, ecosystem re-establishment, etc. There is a burgeoning literature on what constitutes ‘sustainable’ mine closure and rehabilitation, with the joint industry-government handbooks the most recent efforts (see Bell, 2006; Mulligan, 2006).
A major issue in mine closure and rehabilitation is the ‘legacy’ remaining. In effect, success should mean that there is no negative impact remaining, and ideally should move towards a positive residual legacy. This will, of course, be highly variable across numerous individual mines, but as an industry the net effect over space and time is cumulative. There is very little high quality data on the long-term success of rehabilitation works on formerly mined land, with evidence for both failure and success. In Queensland 73,586 ha has been disturbed by mining while only 20,313 ha had been rehabilitated by June 1997 (Anderson, 2002) – these numbers have increased substantially since. For Western Australia, it is estimated that a total of 165,040 ha has been disturbed by mining while only 36,952 ha has had preliminary rehabilitation to 2003 (Mudd, 2004). This gap is likely to be similar across Australia, let alone the question of the long-term success of engineered rehabilitation works. An example of an unsuccessful rehabilitation of waste rock is shown in Figure 6, while the need for rehabilitation at an abandoned mine is shown in Figure 7.
[hr]TABLE 1 Notes: M – mega or million; k – kilo or thousand; t – tonnes; conc. – beneficiated ore or concentrate; nd – no data; cum. – cumulative; WR – waste rock. aBased on ~2,400 million m3 of overburden; bBased on »26,700 million m3 of overburden; cBased on ~18 million m3 of overburden; dBased on ~640 million m3 of overburden. All overburden assumed to have a dry density of ~1.6 t/m3. Black and brown coal excludes ash wastes from combustion at the power station. Some data estimated or approximate (~). #Insufficient analysis of historical data, although cumulative ore processed is probably of the order of some 40 Mt.[hr] [title from=”Economic Resources”]Economic Resources[/title]
The extent of available mineral resources is often a key issue raised in the debate on sustainable mining. The major factors which have allowed Australia to continually expand production over recent decades are that new resources have been discovered, better technology allowing exploitation of lower grade deposits has been developed (especially for gold), as well as the relatively cheap cost of energy to facilitate open cut mining. The economic resources for many minerals over time are shown in Figure 7. In addition, there are commonly similar amounts known in sub-economic or inferred mineral resource categories.
In general, most economic mineral resources in Australia have grown either steadily (eg. lead) or experienced sudden increases (almost all, eg. iron ore, bauxite, nickel, gold) due to new provinces or mines being discovered or the advent of new technology (eg. carbon-in-pulp for gold and high pressure acid leaching for nickel). Based on data in Mudd (2009), future production from most resources will not increase average ore grades or quality. For some minerals (eg. coal, iron ore), the true extent of economically (or technologically) recoverable resources remains open to conjecture, though some resources appear to have stabilised. An often implicit aspect of the future viability of much of these resources is the extent of open cut mining, tailings and waste rock involved – though this is rarely discussed in a strategic sustainability context as outlined in this paper. [top][hr] [title from=”Sustainability Reporting”]Sustainability Reporting & Mine Waste Issues Sustainability Reporting[/title]
The global mining industry is moving to report on their sustainability performance alongside their financial performance. In the mid-1990’s this was primarily led by a select number of large mining companies but is now being undertaken by numerous mining companies. As discussed previously, there has been an evolution from environmental management through to sustainability to now include social and economic aspects of mines and the industry.
To meet the growing need for sustainability reporting and to ensure greater consistency between companies and different industry and government sectors, the United Nations established the ‘Global Reporting Initiative’ (GRI) in 1997 together with government, civil society and industry bodies. Under the GRI, a broad range of data and information is now reported under specific categories of social, economic, environmental, human rights and societal indicators. Though voluntary, the GRI continues to be adopted more broadly across the mining industry, and is expected to grow rapidly in the future.
Separately to the GRI, many countries now have statutory pollutant release reporting requirements in place. In Australia this is the ‘National Pollutant Inventory’ (NPI, 2001) while in the USA it’s the ‘Toxic Release Inventory’ (TRI). Similar systems are also in some European countries, and they are intended to underpin ‘State of the Environment’ style assessment of the health of the environment.
Overall, these and other emerging reporting regimes allow the public disclosure of various relevant data for mining. This includes greenhouse emissions, other gaseous pollutants (eg. sulfur dioxide), particulates, water usage, impacts on water resources, energy sources and consumption, amount and nature of solid wastes (eg. hazardous, putrescible, etc), as well as wealth of labour, economic and social data. The sections below will focus on the solid waste issues associated with such protocols. [top][hr] [title from=”Global Reporting Initiative (Gri)”]Global Reporting Initiative (Gri)[/title]
Under the current third edition of the GRI Protocol (GRI, 2006) and the additional mining sector supplement (GRI, 2005), the primary indicator for solid mine wastes is ‘EN22’, which is the “total weight of waste by type and disposal method”. It clearly includes wastes such as landfill (putrescible material), metal scraps, inert solids (eg. cement), construction waste, solid chemical wastes, used tyres, and the like. There is widespread inconsistency, however, in whether EN22 explicitly includes solid wastes such as tailings and waste rock.
The mining sector supplement goes on to state that “large volume wastes” – tailings and waste rock – should be reported after a site-specific risk assessment (pp 29, GRI, 2005). Therefore some companies who use the GRI as their sustainability reporting basis do not publicly disclose tailings and waste rock data under EN22 while some companies give variable levels of information. Two examples of the solid waste reporting under EN22 are shown in Figure 8, and highlight the variable way in which data is reported. In both cases the data does not distinguish between tailings or waste rock – which are fundamentally different in terms of their scale and nature with respect to long-term environmental risks. Curiously, some companies report tailings and waste rock data as part of financial performance while others do not. [top][hr] [title from=”National Pollutant Inventory (NPI)”]National Pollutant Inventory (NPI)[/title]
In Australia, the NPI only considers those emissions of pollutants which are effectively released to the environment and defines waste rock and tailings facilities as land transfers only (pp 30-31, NPI, 2001) – leaving waste rock and tailings data outside the scope of reportable NPI emissions (though any escape from a waste rock or tailings facility would still be reportable to the NPI). This is a critical weakness in the NPI accounts, as both tailings and waste rock have the potential to become major point sources of listed pollutants such as cyanide and various metals (eg. Sb, As, B, Cd, Cr, Co, Cu, Pb, Mn, Hg, Ni, Se, Zn). A simple search of the facilities in the databases via the NPI website (www.npi.gov.au) reveals that some major sites of acid mine drainage (eg. Mt Lyell, Tasmania) are included in the facilities reporting under the NPI, while others are not (eg. Mt Morgan, Queensland). Given the vast quantities of mine wastes now produced annually in Australia, there would be a very substantive quantity of listed NPI pollutants contained within tailings and waste rock yet they are excluded from, or least poorly addressed by, such accounting and reporting systems. [top][hr] [title from=”Resource Intensity or Eco-Efficiency of Mineral Production”]Resource Intensity or Eco-Efficiency of Mineral Production[/title]
An emerging area of sustainability research in mining is the application of life cycle analyses, especially with a view to estimating resource intensity or eco-efficiency of metals and mineral products. The increasing GRI-based or NPI data being reported provides an opportunity to quantify these aspects more accurately than has been possible in the past. The combined tailings and waste rock data is given by metal / mineral in Table 1. As can be seen, there are major gaps in quantifying the solid waste burden for numerous metals, such as Al, Fe, Ni, Pb, Zn and Ag. For bulk commodities such as bauxite and iron ore, sporadic data for existing mines suggests that waste rock is at least equal to ore mined, with beneficiation of raw ore also producing some tailings though only saleable product is reported (eg. most bauxite and iron ore projects include a beneficiation plant).
In addition to the solid waste burden, data reported under the GRI in particular allows the estimation of more accurate life cycle costs for metals and minerals, such as greenhouse emissions, energy, reagent and water consumption. A detailed analysis of these aspects for gold mines is given by Mudd (2007b, 2007c, 2010) and for uranium mines is given by Mudd (2014), with further research in progress for copper and other metals. A summary of key data is provided in Table 2. As demonstrated by this research, the ‘resource intensity’ (or ecological footprint) of these metals is clearly sensitive to the ore grade being processed, which, when combined with the long-term decline in global ore grades, points to a fundamental sustainability challenge to the modern mining industry : the resource intensity looks set to increase gradually over time. Examples are shown in Figure 9.[top][hr] [pullQuote]”…the GRI continues to be adopted more broadly across the mining industry, and is expected to grow rapidly in the future.”[/pullQuote] [title from=”Conclusion”]Conclusion[/title]
Moving from a production philosophy through improved environmental management to now embracing the ‘triple bottom line’ of sustainability – social, economic and environmental components – the debate and the performance of the modern mining industry, both in Australia and globally, has clearly made important progress over recent decades.
In terms of the major trends in modern mining, a number of fundamental aspects have been shown:
- Exponentially increasing production – almost all minerals and metals show strong growth over time, especially over the past three decades;
- Declining ore grades (or quality) – while early mines processed rich ores, average industry grades for most metals and minerals are now commonly lower, with known economic resources suggesting this decline in ore grades will continue. In addition, the quality (mineralogy) of mineral deposits are generally becoming more complex and difficult to process;
- Open cut mining – since the mid-twentieth century there has been a major shift in mining technique from underground to open cut mining, especially in some sectors such as coal, gold and nickel;
- Waste rock / overburden – combined with the increase in open cut mining, there has been an exponential increase in the waste rock or overburden excavated in modern mining. For most metals and minerals the quantity of waste rock / overburden excavated is significantly higher than the ore processed or product mined, and this ratio is increasing over time – presenting a major challenge in mine rehabilitation;
- Mine rehabilitation – the extent of mine rehabilitation still shows a major gap, mainly due to older legacy mines, though there remains concern over the long-term effectiveness of mine rehabilitation and closure approaches, especially as the scale of mine sites continues to grow;
- Economic resources – although often perceived as ‘non-renewable’, the extent of economic mineral and metal resources has often increased over time in Australia, though many appear to have stabilised. Growing production continues to exacerbate pressure on remaining resources;
- Sustainability reporting – the emergence of sustainability reporting protocols, such as the voluntary Global Reporting Initiative or the statutory National Pollutant Inventory, are helping to improve the transparency of modern mines, though there still remains clear reluctance to explicitly report all relevant data such as waste rock, tailings and other aspects;
- Resource intensity – the modern solid waste burden of metals and minerals is substantive, and continues to increase. Additionally, the resource intensity, in terms of inputs and outputs, is significant and very sensitive to ore grade, leading to the realisation that the resource intensity is likely to gradually increase in the future as mines shift to lower grade deposits. This makes more comprehensive sustainability reporting even more critical.[top][hr]
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