Topic 5: Environmental and Social Concerns From a series of 5 lectures on Metals, minerals, mining and (some of) its problems prepared for London Mining Network by Mark Muller firstname.lastname@example.org 24 April 2009
Outline of Topic 5:• Focus on acid mine drainage (AMD) Production of acid waters by oxidation of sulphide minerals Factors influencing acid development Impacts of AMD Control of AMD during mining Control and remediation of AMD after mining• Control and remediation of uranium bearing wastes• Mine rehabilitation case study• Spontaneous combustion of coal• “Sustainability” and mining – heap leaching case
Acid Mine Drainage:The most serious and pervasive environmental problem related to mine waste management is arguably acid mine drainage (AMD).AMD is an oxidation process which takes place wherever sulphide minerals (e.g., pyrite) are in contact with both oxygen and water, wherever they are present on the mine.Metallic sulphide minerals (e.g., pyrite) oxidise in the presence of water and oxygen to:• produce acids and• release dissolved metals into water.
Acid mine drainage Water (H2O) Atmospheric oxygen (O2) Pyrite (FeS2) + other sulphides + bacteria Sulphuric acid (H2SO4) + Iron (Fe3+) dissolved in water Iron-hydroxide Fe(OH)3 precipitated Mine dump, - water becomes more acidic St. Kevin Gulch, Colorado, USA http://toxics.usgs.gov/photo_gallery/photos/upper_ark/mine_dump_lg.jpgFeS2 + 15/4 O2 + 7/2 H2O Fe(OH)3 + 2 H2SO4 + energyPyrite Oxygen Water Iron-hydroxide Sulphuric acid heat(solid) (dissolved) (liquid) (dissolved) (dissolved)
Acid mine drainage:Sulphide oxidation is a positive feedback reaction - as the reaction proceeds, the fluid becomes more acidic and more heat is generated, which in turn speeds up the oxidation reaction, which produces a more acidic fluid and more heat. The reaction will continue at an ever increasing rate until either the sulphide or oxygen source is exhausted.
Acid mine drainage - example AMD seep into Slickrock Creek at Iron Mountain Mine. Location: Redding, Shasta County, California, USA Photo Date: November 17, 1994 Iron Mountain Mine (IMM) has been a source of acid mine drainage resulting from over one hundred years of mining activity. Though mining operations were discontinued in 1963, underground mine workings, waste rock dumps, piles of mine tailings, and an open mine pit still remain at the site. Iron-hydroxide precipitatesNOAA Restoration Center & Damage Assessment and Restoration Programhttp://www.photolib.noaa.gov/htmls/r00immb7.htm
Acid mine drainage - example Iron-hydroxide precipitatesAMD seep into Slickrock Creek at Iron Iron Mountain Mine.Location: Redding, Shasta County, California, USAPhoto Date: November 17, 1994Credit: NOAA Restoration Center & Damage Assessment and Restoration Programhttp://www.photolib.noaa.gov/htmls/r00immc1.htm
Acid mine drainage – oxidation rates of different sulphide minerals:Different sulphide minerals are more (or less) less reactive in oxygen rich environments (Lottermoser, 2007). Sulphides which do not contain iron do have a significantly reduced capacity to generate significant amounts of acid (Plumlee, 1999).Pyrite (Iron-sulphide) FeS2 High reactivity High acidityMarcasite (Iron-sulphide) FeS2Pyrrhotite (Iron-sulphide) FeSMakinawite (Iron-nickel-sulphide) (Fe, Ni)9S8Covellite (Copper-sulphide) CuSMillerite (Nickel-sulphide) NiSGalena (Lead-sulphide) PbSCinnabar (Mercury-sulphide) HgSMolybdenite (Molybdenum-sulphide) MoS2 Low reactivity No acidity
Acid mine drainage – different sulphide minerals in contact with each other affects oxidation rate:Pyrite in direct contact with other sulphide minerals does not oxidise as vigorously as it does in isolation (Cruz et al., 2001), and the oxidation of pyrite can be delayed while other sulphides are preferentially oxidised (Kwong et al., 2003). (The industrial galvanizing of iron with zinc, to prevent rusting of iron, takes advantage of the same electro-chemical principle) Slower oxidation when in contact with High electro-conductivity less electro-conductive mineralsPyrite (FeS2)Galena (PbS)Sphalerite (ZnS) Faster oxidation Low electro-conductivity when in contact with more electro-conductive minerals
Acid mine drainage – acid buffering by non-sulphide gangue minerals:Gangue minerals (mostly silicates and carbonates) have the capacity to buffer acid.Whether the gangue minerals react with the acid depends on the pH of the solution – different minerals react at different pH values.Thus depending on the abundance and types of both gangue minerals and sulphide minerals, a sulphide waste pile may, or may not, produce acidic leachates (Lottermoser, 2007). The production of acidic leachates is a far more common situation though.Given all the variables (e.g., variations in the types and amounts of sulphides and gangue minerals, oxygen and water supply, grain size and porosity, and bacterial population) it is difficult to predict reliably the acidity of potential drainage from waste piles.It is more difficult to say with any certainty that the drainage will not be acidic.
Acid mine drainage – environmental impacts:Acid mine drainage may be released into the environment from mine sites in two ways:• Water drainage through sulphide-rich oxidising waste dumps, leach heaps and tailings dams.• Release of uncontrolled or improperly treated process-waters that into surface drainage systems.Erosion of waste dumps and tailings dams can cause sulphide minerals to be transported directly into soils and streams, and therefore dispersed away from the mine site.Acidic, metal-bearing waters can migrate for large distances away from the immediate mine site. The environmental impacts of such waters are numerous:(i) Surface water contamination. AMD waters have high metal and salt concentrations that impacts on the use of waterways downstream for fishing, irrigation, stock watering and drinking water supply.
Acid mine drainage – environmental impacts:(ii) Aquatic life. The acidity of AMD can destroy the natural bicarbonate buffer system which keeps the pH of natural waters within its normal range. The loss of bicarbonate also affects photosynthetic aquatic organisms that rely on bicarbonate as a non-organic source of carbon.(iii) Heavy metals and metalloids at elevated and bioavailable concentrations are lethal to aquatic life and are of concern to animals and humans. Loss in biodiversity, depletion in the numbers of sensitive species and fish kills can occur.(iv) Groundwater contamination. AMD impacts more frequently on the quality of subsurface waters than on surface drainage. Water seeping from below uncapped and unlined waste repositories, or ones with ruptured liners, form plumes of contaminated subsurface water that allows sulphates and metals to migrate in aquifers, and subsequently down the hydrographic gradient within the acquifer.
Acid mine drainage – environmental impacts:(v) Sediment contamination. Precipitation of dissolved constituents in AMD can cause soils, flood plain sediments and stream sediments to become contaminated with metals, metalloids and salts. Rum Jungle uranium mine, Australia. Stream channel impacted by AMD is devoid of plant life and encrusted with white “effloresences” (precipitated minerals) Figure from Lottermoser, 2007.
Acid mine drainage – control and remediation:Sulphidic rock dumps are the major source of on-mine AMD generation due to the fact that they are generally unlined, and consisting of coarse rock material, are highly porous and permeable to water and oxygen.Tailings dams are associated with a somewhat lower risk of generation and release of AMD due to the presence of liners, and the tailings being fine-grained and less permeable. While the tailings dams are wet (i.e., at all times during operation) the risk of AMD seepage is higher than after decommissioning, when the tailings dams dry out, provided surface erosion of the dams is prevented.Sulphide oxidation of rock dumps can (potentially) be controlled during mining, and remediated after mining, by the exclusion of one or more of the factors that cause oxidation (water, oxygen) or enhance oxidation (bacteria), or by the introduction of a buffering agent.
Acid mine drainage – rock-dump control during mining:Because mines operate for long period of time (decades), control strategies that attempt to minimise AMD generation from rock dumps during the life-of-mine should be implemented.(i) Mixing and encapsulation. Acid generating rock Encapsulation material can be encapsulated or mixed with benign rock waste (e.g., oxide waste) or neutralising (e.g., limestone) rock waste.(ii) Co-disposal or blending. Co-disposal refers to the mixing of rock waste with fine grained tailings Mixing so as to reduce the overall porosity of the dumps, and minimise water and oxygen ingress. If an alkaline material (e.g., lime) is added to the tailings beforehand, the process is called blending. Figure from Lottermoser, 2007(iii) Bactericides may be applied to rock dumps to inhibit growth of bacteria that might otherwise enhance the oxidation process (e.g., Kleinmann, 1999). Repeated treatments are necessary as the chemicals are washed away by rain percolation. The applied chemicals may cause toxicity to other organisms.
Acid mine drainage – rock-dump remediation after mining:Dry covers. Capping sulphidic wastes with a thick layer of solid material, called a “dry cover”, is the most widely used approach to countering acid generation (by reducing water and oxygen flux into the waste rock).Placing a layer of neutralising materials on the surface of rock dumps, to establish a source of alkali water percolating into the dump, has not been successful in countering AMD generation (Smith and Brady, 1999). Acid buffering or neutralising materials are most effective when mixed in with the sulphidic waste.Similar dry covers are also used to rehabilitate tailings dams and spent leach-heaps.
Rock dump remediation with dry coversUnsaturated covers are designed for semi-arid to arid areas, to maximise rainfall run-off and minimise water infiltration and oxygen diffusion into the waste.Saturated covers are designed for sites with a wet climate. The outermost “sandy-clay” layer remains wet permanently, providing a Fig 2.15 very good barrier against oxygen diffusion. Lottermoser 2007Sponge covers are designed for climates with distinctly seasonal rainfall. They aim to capture large volumes of infiltrating rainwater for short periods of time, and then allow the water to drain away during dry seasons.Problems with dry covers:(i) The clays layers in dry covers may crack if they dry out too rapidly.(ii) Covers are prone to erosion on the steep slopes of rock dumps. Figure from Lottermoser, 2007
Monitoring of sulphidic rock dumps for oxidation and acid generationSulphidic waste rock dumps and tailings dams need monitoring during operation to detect at the earliest time whether waste material is “turning acid”.Rehabilitated waste repositories also need monitoring to establish the effectiveness of the control measures used to curtail oxidation. Temperature profiles Pore gas sampling to using electrical determine oxygen probes. Increasing concentration. Decreasing Monitoring of water temperatures indicate concentrations indicate quality in aquifers heat generation by consumption of oxygen by using boreholes. oxidation reactions. oxidation reactions. Water analysis to monitor acid and metallic ion buildup in drainage channels and DUMP surface water. DRY COVER ACQUIFER
Management of uranium-bearing wastes:The problems associated with uranium rock and tailings wastes are identical to those present in the case of sulphidic wastes (acid water generation and mobilisation of metals and metalloids into the water system) with the additional impact of mobilisation of both uranium and radium radionuclides into waters and the release of radon gas.Control and remediation strategies focuses primarily on the exclusion of both oxygen and water from waste rock dumps and tailings dams (to prevent oxidation of both sulphide and uranium bearing minerals), using the same array of techniques discussed previously for sulphidic wastes.Uranium tailings should be covered during operation in order to reduce radon-222 gas emanation. A permanent water cover will reduce the radon flux to 1% of that from dry tailings (Davy and Levins, 1984) (but remember that a tailings dam with an overfull decant pond that encroaches on the dam wall increases the risk of dam failure).Other on-mine radiation hazard mitigation measures include: dust suppression, appropriate ventilation, use of protective clothing, strict hygiene standards, radiation dose measurements.
Rehabilitation case study – Sherwood Uranium Mine, Washington, USA:http://ecorestoration.montana.edu/mineland/histories/minerals/sherwood/default.htm#The open-pit mine operated from 1976 to 1985.Construction of the mill to process the ore was completed in 1978, and operated until 1984. Nominal milling capacity was 2,100 tons of ore per day, with an average design ore grade of 0.088% U3O8 (0.88 kg U3O8 per ton of ore). Approximately 2.9 million tons of acid-leached tailings were neutralised with lime prior to placement in a synthetically-lined tailings impoundment. The estimated radium-226 activity in the impoundment is 470 curies (17.39 TBq).Mill decommissioning began in 1992 and was completed in 1995. Approximately 350,000 cubic yards of contaminated mill-site soils, building equipment and debris were excavated from the processing-site and placed in the tailings impoundment.Areas disturbed were approximately 2 km2 by mining and an additional 0.8 km2 by the processing and tailings area. At end of mining, and prior to reclamation, the pit seasonally contained surface water.
Sherwood Uranium Mine (1976 – 1985): mine development timelinehttp://ecorestoration.montana.edu/mineland/histories/minerals/sherwood/supp1.htm Late 1960s FINAL HIGHWALL POSITION
Rehabilitation case study – Sherwood Uranium Minehttp://ecorestoration.montana.edu/mineland/histories/minerals/sherwood/default.htm#Mine Closure and Reclamation Objectives:• Maximize the potential for future retrieval of remaining ore in the deposit.• Return the mine to a condition that will not pose a hazard to public health and safety.• Return the site to a condition that will support wildlife habitat.• Create a self-sustaining vegetation community.• Enhance the visual appearance of the area.• Use reclamation methods that are technically effective, cost efficient and employ tested engineering practices.Specific Closure and Reclamation Activities:• Remove all mine-related facilities.• Re-grade the overburden materials and mine the benches to create surfaces that promote drainage and minimize potential for ponding of water and erosion.• Establish stable slopes.• Replace the topsoil and growth media and revegetate with native plant species.• Monitor the performance during and after reclamation to ensure objectives are achieved.
Rehabilitation – Sherwood Uranium Mine: establishment of stable slopeshttp://ecorestoration.montana.edu/mineland/histories/minerals/sherwood/default.htm# Slope modification during rehabilitation. Shallower gradients provide greater slope stability (less risk of collapse or slumping) and reduce the effects of surface erosion. Reclaimed mine-site calculated to be erosionally stable with respect to a 100 year storm event (2.5 inches of water over a 24 hour period). During mining After slope modification
Rehabilitation – Sherwood Uranium Mine: post rehabilitation viewhttp://ecorestoration.montana.edu/mineland/histories/minerals/sherwood/default.htm#On-line documentation doesnot record the nature of thecovers (if any) placed overthe rock-dumps and tailingsdams, and particularlywhether the covers aredesigned to be impermeablebarriers or not. TAILINGS ROCK
South African mining-industry fatality and injury rate (all mining) From: Mine Health and Safety Inspectorate annual report, http://www.dme.gov.za/mhs/documents.stm#3
Mining strategy can make a difference to rockburst activity and fatalities:Decreasing fatality rate due to increasing use of:• Backfill – to provide support in panels after completion of mining to reduce stress buildup• Preconditioning – mining small portions of future mining areas in advance to allow stress changes to occur more gradually• Bracket and stabilising pillars• Seismic monitoring – to provide early warning of seismically active panels• Education – to increase consciousness of safety Rockburst fatality rates for SA gold mines (1984 – 2002) Year
Is a zero fatality rate possible in the South Africa mining industry?I presented the above question to two people closely involved in the South African mining industry, one a leader in the field of mine-seismicity and rockbursts, and the other an experienced mining engineer, having worked on both coal and gold mines. (Their responses are included with the course material as anonymous submissions).The prospects for achieving zero fatalities in deep gold and platinum mines do not look good currently, at least not until underground operations are fully mechanised. Regardless of the mining method used (human manpower or machine), the creation of a cavities underground induces huge stresses we currently have no way of dissipating harmlessly (and nature takes its course in the form of rockbursts).However their responses (obviously expressing very personal views) make interesting reading in illuminating a culture within mining (within their experience) that is not conducive to the safest mining possible.
Is a zero fatality rate possible in the South Africa mining industry?The submissions indicate:(i) A culture in which (financial) reward is heavily weighted towards meeting production targets.(ii) Senior management that may turn a blind eye to potentially risky situations, in favour of retaining high production rates.(iii) Lack of legal accountability of senior management in the light of decisions made in the leadup to fatalities.(iv) Rock mechanics and health-and-safety officers who have no power to halt operations in dangerous situations – deferring to more senior management.(v) Incentive (or disincentive) schemes around safety that are counter productive – leading to situations where miners will not report injuries, or avoid going for treatment, in order to avoid penalties.(vi) Resistance to implement or test changes in procedures and methods that might improve mining safety.(vii)The better safety record of more experienced crews indicates that overall better statistics could be obtained if all crews operated to the same standards.
Heap leaching and sustainability:It has been argued that heap leaching offers a number of environmental and social benefits. Smith (2004) claims that heap leaching meets the seven criteria for sustainability established by the North American MMSD (Mining, Minerals and Sustainable Development) project (make your own judgment on MMSD and on “sustainability” of heap leaching!)(i) Engagement. Heap-leaching is the “low-technology” solution for low-grade ores. Construction and operation technologies of heap leaching are sufficiently “accessible” to provide local contractors with engagement opportunities.(ii) People. Heap-leaching is a more “hands-on” process than milling, providing opportunity for skills transfer to local people in the areas of pipe laying, irrigation, operation and maintenance of pumps, surveying, earthworks, liner construction, slope and erosion control, reclamation and revegetation, and other aspects of civil construction.
Heap leaching and sustainability (continued):(iii) Environment. Heap leach facilities have far fewer serious Acid Rock Drainage problems than conventional mill operations because use of the leaching approach lowers the cut-off grade and therefore reduces the size of rock dumps, as well as reducing the overall sulphide content in waste from copper projects. Spent heap leach ore from gold operations is strongly alkaline and mixing waste types can compensate for acid waste rock. Self-draining characteristics of spent leach heaps make them more easy to reclaim than old tailings deposits. No history of catastrophic failure of leach heaps and dumps, in comparison to tailings dams. There is overall a reduced reliance on conventional tailings disposal.(iv) Economy. Allows more ore to be processed at lower cutoff grades, allowing a longer life or larger mine operation, therefore increases employment. Projects are less capital intensive and thus less sensitive to commodity price fluctuations. A lower risk investment in general. Because capital and operational costs are lower, there is greater potential for investment into the projects by local people.
Heap leaching and sustainability (continued):(v) Traditional and non-market activities. By expanding employment in occupational areas with transferable skills, a more sustainable workforce results. The tools and activities of heap leaching are more directly applicable to traditional activities.(vi) Institutional arrangements and governance. Types of problems inherent to heap leaching projects tend to be more manageable at a local level.(vii) Synthesis and continuous learning. It uses technologies that are both locally available and have more applications outside mining. Because the projects are less capital intensive and typically subject to expansions or revisions in the leach pad and stacking operations annually or bi-annually, project reevaluation is a deeply engrained part of the heap leach process, and expanding this [culture] to include the local community should be an easy step. (!?!!)
Spontaneous coal combustion:Coal seams contain large amounts of disseminated sulphide minerals.Both coal (carbon) and sulphide minerals oxidise when exposed to oxygen, and generate heat at the same time.If the heat is not allowed to dissipate (for example in the interior of a coal pile, or inside a mine), temperatures will start risingAt about 70 – 150°C, coal begins to give off small, but measurable, quantities of gas – aerosols, hydrogen, and CO and CO2 – which are the precursors of combustion.As the temperature increases further – at about 315 – 370 °C – relatively large, visible (coal) particulates are emitted.At temperatures of about 400 – 430°C, incipient combustion, and self- ignition and flame, will occur. (http://www.saftek.net/worksafe/bull94.txt).
Spontaneous coal combustion:Sulphur and hydrogen (and methane) present in the coal contribute to the combustion as well. Sulfur dioxide (SO2) is a major pollutant. Carbon monoxide (CO) is poisonous and a major threat to life when burning occurs underground.Once burning is underway, hydrogen will be exhausted first, followed by sulphur, and finally coal (which although has the lowest ignition temperature, burns the slowest). From: http://www.eas.asu.edu/~holbert/eee463/FOSSIL.HTML
Possibilities for recycling of mine waste:Some reported secondary uses of mine wastes include (Lottermoser, 2007):- Slag from mineral smelting is commonly used in road construction.- Manganese tailings may be used in agro-forestry, building and construction materials, coatings, resin cast products, glass, ceramics, glazes.- Fertiliser for golf courses.- Clay rich wastes can improve sandy soils or provide raw material for bricks.- Mine water can be purified into drinking water (e.g., in arid areas)- Mine water can be used for heating or cooling purposes.- Mine drainage sludges can provide a resource for pigment.- Pyritic waste rock can be a good amendment to neutralise alkali agricultural soils.(There is currently very limited demand for, and use of, mining waste products).