5.2.2Gold and silver extraction with mercury-amalgamation processes (ASM) -
Mercury has been used in gold and silver mining since Roman times. Generally, this mining process involves the following: the wet ore (or mud or ore concentrate) is mixed with metallic (liquid) mercury; the mercury dissolves (algamates) the gold or silver in the mud; the remaining mud is washed away leaving a mercury-gold (-silver) amalgam; and the amalgam is then heated to release the mercury, with impure gold and/or silver remaining. Mercury released to the biosphere due to this ancient activity of gold and silver extraction with mercury amalgamation may have reached over 260,000 metric tons in the period from 1550 to 1930, after which known, easily exploitable gold and silver reserves were nearly exhausted, and the mercury amalgamation process was partly replaced by the more efficient large scale cyanidation process, enabling extraction of gold (and/or silver) from large deposits of low-concentration ores (UNEP, 2002).
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Increases in gold prices and the prevailing difficult socio-economic situation in the 1970’s, resulted in new rises of the gold rush, especially in the southern hemisphere, involving more than 10 million people on all continents. Presently, mercury amalgamation is used for gold extraction in many countries in South America, Asia and Africa. By way of example, in Brazil, mercury amalgamation was used for the production of 5.9 metric tons of gold in 1973. In 1988, this figure had increased to over 100 metric tons per year. During the 1990’s this figure decreased again due to falling gold prices and exhausted deposits (Uppsala University, as cited in UNEP, 2002); yet the activity persists in many countries throughout the developing world and is again on the rise with increasing gold prices.
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Based on studies by various researchers, it was estimated that somewhere between 350 and 1000 tons of mercury have been used globally per year in the 1990s for artisanal small-scale gold mining (ASGM) (UNEP, 2002). The latest estimate for the current mercury supply ("consumption") is some 990-2,200 metric tons in 2010 (Mercurywatch.org, 2012).
5.2.2.2Main factors determining releases and mercury outputs
Table 5 62 Main releases and receiving media from gold and silver extraction with the mercury-amalgamation process
Phase of life cycle
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Air
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Water
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Land
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Product
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General waste
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Sector specific treatment/
disposal
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Gold and silver extraction with the mercury-amalgamation process
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X
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X
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X
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|
|
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Notes: X - Release pathway expected to be predominant for the sub-category;
x - Additional release pathways to be considered, depending on specific source and national situation.
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Mercury is released directly from these mining activities to the air, water, sediments and soils. The mercury-gold amalgam from the extraction process releases mercury as vapour to the air when heated in one of the steps in the purification. The evaporation is often done with no retention of the evaporated mercury. Sometimes the evaporation step is done in "retorts", in which parts of the evaporated mercury are condensed and re-used. Mercury is also present in mine tailings, which can lead to future releases to land, water and air. Mercury is found at extraction sites, trading posts, and in soil, plants, sediments and waterways in the area of these operations. This gold extraction process is simple and cheap, but not very efficient either in terms of gold recovery or mercury retention. The process has lead to intense mercury pollution of the terrestrial, aquatic and atmospheric environment in large areas around these operations, and has also contributed significantly to the levels of mercury in the global environment (COWI, 2002).
5.2.2.3Discussion of mercury inputs
Table 5 63 Overview of activity rate data and mercury input factor types needed to estimate releases from gold and silver extraction with the mercury-amalgamation process
Potential Activity rate data
to be used to estimate releases
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Possible input factor
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Total amount of gold (or silver)
produced using such techniques
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Estimate of average ratio of mercury consumed per unit of gold or silver produced with feed materials and technology prevailing in the area investigated
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Or
Total annual mercury purchases (consumption) for ASGM
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(Factor is 1, as mercury consumption is the actual input)
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Note *1: The default mercury input calculations in the Inventory Level 2 spreadsheet are based on amount of gold produced, but in case total annual mercury purchase data are available, they can be
introduced directly in the calculations. For most countries with ASGM acivities, estimates of mercury
consumption can be found at www.mercurywatch.org.
Desk studies of mercury use and releases from artisanal and small-scale gold mining (ASGM) can only give very rough estimates, and whenever possible, field studies are recommended. Indicative estimates of potential mercury inputs to small-scale mining may be estimated from simple thumb's rules, but more factors can be involved when attempting to qualify mercury inputs and release estimates further in a country or region. These factors are discussed here. Note that many ASGM areas have been investigated by field experts, and assessments of mercury consumption may exist and should be included in the research made. Also, for most countries with ASGM acivities, estimates of mercury consumption can be found at www.mercurywatch.org, and these numbers can be used for a first estimate of the total mercury releases from ASGM.
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Main factors influencing mercury inputs and releases are:
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Total gold produced per miner using mercury
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Total number of miners
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Mercury consumed to gold produced ratio - this varies with practice with whole ore amalgamation consuming much more mercury than amalgamating a concentrate of the ore.
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Percent of mercury recycled (0% with no retorts, 75 to 95% with retorts or fume hoods).
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The amount of mercury used per unit gold (or silver) extracted varies according to methods and equipment used and other factors. For example, if mercury is used to extract gold from the whole ore, and no recovery devices are utilized, the ratio of amount of mercury used to amount of gold extracted (Hgused:Auextracted) is >3:1 (i.e., more than 3 kg of mercury used per 1 kg of gold obtained). If mercury is used on ore concentrates (instead of whole ores) the (Hgused:Auextracted) ratio is about 1.3:1. If ore concentrates and a retort are used, the amount of mercury used is much lower (ratio may be 0.1; range 0.05 - 0.2) (Telmer, 2012; UNIDO, 2003). Lacerda (1997) reviewed literature on estimated mercury amounts consumed per kg of gold produced with the amalgamation process and reported that while such input factors varied widely, most fell in the interval of 1-2 kg mercury consumed per 1 kg gold produced. But since then, other observations have shown mercury consumption rates of 20 Hg :1 Au when mercury is placed directly in the grinding circuit, and as high as 50 Hg :1 Au when ores also conatin significant concentrations of silver, which forms a poorer amalgam with higher mercury concentrations in it.
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In addition to the intended use of mercury, another - yet generally much smaller - source of mercury from small-scale gold mining is the mobilization of naturally occurring mercury impurities in gold ore (COWI, 2002).
5.2.2.4Examples of mercury in releases and wastes/residues -
The percent of mercury lost and the pathways of release vary with amalgamation method. If no controls are used and the amalgam is heated in open containers to evaporate the mercury, then all ofthe mercury in the amalgam is released to air and no mercury is recovered. On the other hand, if a retort is used, the atmospheric releases will be smaller, because some of the mercury is recovered in the retort and re-used. According to UNIDO (2003) mercury recovery from the process ranges from about 51 - 99%. Telmer (2012) states that average recovery is about 80-95%..
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According to Lacerda, an estimated 65-87% of the mercury inputs were deemed emitted to the atmosphere, and the rest was released to soil and aquatic environments (Lacerda, 1997, as cited in UNEP, 2002).
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According to newer estimates by Telmer and associates described in UNEP/AMAP (2012), it is estimated that, on average 45% of mercury used in ASGM is emitted to the atmosphere with the remainder released to land and water. In regions where concentrate amalgamation is practised, 75% of the mercury used is emitted to the atmosphere (although the absolute amount of mercury used is typically lower than in other practices such as whole ore amalgamation), whereas localities that practice whole ore amalgamation use much more mercury per unit gold produced, but release a much larger portion of the mercury to aquatic and terrestrial systems. Eventually, most of the mercury lost to water will ultimately be emitted to the atmosphere over the years due to secondary evaporation. Estimates from Australia and Canada suggest that a large proportion of the mercury used in historical gold mining operations in the 1800’s has been remobilised.
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Telmer (2012) adds that when whole ore amalgamation is practiced, much more mercury is used but a small percentage of the total use is emitted to air (25%), because much ends up in the tailings and other wastes. But the magnitude of what is released to air is still very large because the intensity of mercury use is so much higher. When concentrates and retorts are used, less mercury is used and a fraction - about 90% - of the otherwise emitted mercury is captured in the retort and re-used; thus only 10% of the 75%, a resulting 0.075%, is emitted to air (the 0.1 factor applied to the 0.75 factor). The distribution of the rest of the mercury releases between water and land will vary depending on local conditions and is difficult to say anything general about.
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With the use of retorts, mercury captured can be re-used after "re-activation", a process where impurities are cleaned out of the recyclec mercury to make it amalgamate better in the next use cycle. If a 5% mercury loss (to water and land) from each re-activation is assumed, it can be calculated that ultimately about 20% of the mercury used (for amalgamation of concentrates with retort application) will be emitted to the atmosphere and 80% will be released to land and water.
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In several countries, there are examples of programmes to promote less polluting mercury-based extraction equipment, raise awareness of hazardous qualities of mercury and provide other assistance and information regarding environmental, social and business aspects. Some projects are also assessing or attempting to enhance the possibilities and capabilities of authorities to enforce environmental regulations in small-scale gold mining areas (see examples at the web page of the Global Mercury Partnership on artisanal and small-scale gold mining at http://www.unep.org/hazardoussubstances/Mercury/GlobalMercuryPartnership/tabid/1253/Default.aspx).
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A useful reference on how mercury is used in ASGM is UNEP's Practical Guide: Reducing mercury use in artisanal and small-scale gold mining: http://www.unep.org/hazardoussubstances/Portals/9/Mercury/Documents/ASGM/Techdoc/UNEP%20Tech%20Doc%20APRIL%202012_120619%20with%20links_web.pdf.
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The update of MercuryWatch.org over the last 3 years has allowed the development of generalized output factors to air, water, and land of mercury released from artisanal and small scale gold mining. It is estimated that, due to varying practices, average emissions factors to air regionally vary from 0.25% to 0.75% of mercury used, and that globally 45.2% of ASGM mercury is emitted directly to the atmosphere (Telmer, 2012).
Recommendations for estimating mercury releases from mining operations
Telmer in UNEP/AMAP (2012) states the following about mercury release estimation from ASGM: "The total amount of mercury used in ASGM can be estimated using 4 main approaches: (1) direct measurements – using a balance to directly weigh amounts of mercury used; (2) applying a mercury/gold (Hg:Au) ratio to estimates of gold production based on the type of processing in use (whole ore amalgamation or concentrate amalgamation or the use of emission controls like retorts, etc.); the estimates of gold production can come from the number of miners actively mining and their average yearly gold production, or from other sources such as government reports on gold production or mining populations; (3) interviewing miners and gold merchants who buy or sell mercury; (4) using official trade data. The first three approaches involve directly working with miners and gold merchants. This information can then be used to constrain, through triangulation a more robust estimate of the amount of mercury used and released to the environment and the amount emitted to the atmosphere.
The most reliable results are rooted in field work and relationships with stakeholders. In order to do this, personnel making the estimation must be capable of understanding mining practices and gold trade. Mercury use practices and gold production are key pieces of information. Determining these requires combining information from field data, miners, mining communities, buyers, traders, geological surveys, ministries responsible for mining, mining commissions, the private sector, exploration company press releases, industry magazines, environmental ministries, and others. This information must be analysed to understand what is reasonable based on expert knowledge of geology, mining, ASGM practices, mining communities, and socio-economics. The results of the analysis should be discussed with stakeholders such as miners, concession holders, local governments, and national governments to obtain their input and help constrain the analysis.
The fundamental questions that need to be answered in order to make an annual estimate of mercury use and emissions are:
1. Is mercury used?
2. What are the practices in use ?(Consider: Whole ore amalgamation? Concentrate amalgamation? Mercury recycling/re-activation?)
3. How much mercury is consumed per unit gold? – grams of mercury lost per grams of gold produced? (Consider: Do miners discard used mercury?; Do the miners use retorts or recycle mercury?)
4. How much gold do miners produce per year, individually, collectively?
5. What is the total number of miners?
The format of the questions needs to be adapted to local conditions. For example, it is often necessary to convert the amount of gold produced per day into an annual number by taking into account further information about work habits throughout the year – for example, how work varies seasonally."
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Based on the information compiled above on inputs and outputs and major factors determining releases, the following generalised default input and distribution factors are suggested for use in cases where source specific data are not available. It is emphasized that the default factors suggested are based on a limited data base, and as such, they should be considered subject to revisions as the data base grows.
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The primary purpose of using these default factors is to get a first impression of whether the sub-category is a significant mercury release source in the country. Usually, release estimates would have to be refined further (after calculation with default factors) before any far reaching action is taken based on the release estimates.
a) Default mercury input factors -
As mentioned above, specific information on the gold extraction methods used will give the best mercury input estimates. If no specific information on the mercury inputs is available, the default input factor indicated below can give a rough indication of potential mercury inputs to this sector. If no information on whether whole ore or concentrates are extracted upon, and whether retorts are used, it is recommended to calculate an interval using the lowest and the highest input factors shown below to indicate the possible range of the inputs.
Table 5 64 Preliminary default input factors for mercury consumption for gold extraction with the mercury amalgamation process
Process
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Default input factors;
kg mercury per kg gold produced;
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Extraction from whole ore
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3
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Extraction from concentrate
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1.3
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Extraction from concentrate and with use of retorts and mercury recycling (see text above)
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0.1
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b) Default mercury output distribution factors -
Based on the description given above, the following default output distribution factors are suggested for gold (and silver) mining using the mercury amalagamation method.
Table 5 65 Preliminary default distribution factors suggested for gold (and silver) extraction with mercury-amalgamation.
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Air
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Water *1
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Land *1
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Products
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General waste
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Sector specific treatment/
disposal
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Extraction of gold with mercury amalgamation; combined average scenario
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0.45
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0.3
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0.25
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Extraction from whole ore (no retort use)
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0.25
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0.4
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0,35
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Extraction from concentrate (no retort use)
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0.75
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0.13
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0.12
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Extraction from concentrate and with use of retorts and mercury recycling
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0.2
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0.4
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0.4
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Notes: 1* The distribution here between water and land is an assumption made here. Actual distribution will vary depending on local conditions.
c) Links to other mercury sources estimation -
No links suggested.
5.2.2.6Source specific main data -
As mentioned, desk studies of mercury use and releases from artisanal and ASGM can only give very rough estimates, and whenever possible, field studies are recommended. Note that many ASGM areas have been investigated by field experts, and assessments of mercury consumption may exist and should be included as central parts in the inventory development.
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Note that for most countries with ASGM acivities, estimates of national mercury consumption for ASGM activities can be found at www.mercurywatch.org.
5.2.2.7Summary of general approach to estimate releases -
See discussions above.
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