Posted 16th July 2013 byÂ Paleah Black Moher
The promotion of simple mercury recycling technologies called retorts to reduce human exposures and environmental contamination is one of the most widely recommended interventions in artisanal and small-scale gold mining (ASGM) communities. However if they arenâ€™t introduced correctly, retorts are often abandoned or misused, which wastes resources and develops cynicism from miners, prevents reduction of environmental contamination, and in some casesÂ actually increases mercury exposures.
Retorts are relatively simple devices that condense and collect the mercury vapour released from heating gold mercury amalgam. The collection and capture of mercury is important to reduce both the amount of mercury contamination released into the air and to reduce direct human exposure. There are many different types of retorts â€“ from simple kitchen bowl retorts made from a series of bowls purchased from the local market, to custom welded retorts with water-cooled condensers.
Retorts are relatively inexpensive to build and operate, they are simple and intuitive in construction and use, and they can significantly reduce environmental contamination and human exposure. However, in order for them to do this successfully, proper instruction with sufficient education is crucial. To illustrate this, here are three case studies and lessons from the AGCâ€™s field work (names are changed):
1. Financial incentives for using retorts can be misleading.
After a dayâ€™s work, Babakar adds mercury to his concentrated ore. After squeezing the excess mercury through a piece of t-shirt cloth, he has a half gram of amalgam, which is about 50% gold and 50% mercury. He then uses a propane blowtorch to heat the silver ball on a pan for about 3 minutes. He and his family like to watch the silver ball turn golden as the mercury vaporizes. He then walks over to the gold buyer and sells his sponge gold for $8.20. The value of the lost mercury is around 5-10 cents, or less than 1% of the value of the recovered gold. Babakar could use his neighbours retort but never does because it takes too much time and fuel â€“ about 20 minutes to heat the retort and using his own propane canister, which costs far more than the 5-10 cents he would save from the recovered mercury. However if Babakar knew that he was harming his health and the health of his family from mercury exposure, there is a higher chance that he would decide that it is worth the time and money to use a retort â€“ even for such a tiny bit of amalgam.
- Retorts donâ€™t always save money and in some cases actually incur a cost, especially for small amounts of amalgam.
- Promoting the financial benefits of retorts with out a proper cost benefit understanding is risky and can fail; but health benefits of proper use are universal
2. Field retorts always leak dangerous mercury vapour and need to be used outside.
When retorts are introduced into Mariamâ€™s community she is relieved to have a tool to protect herself and her family from mercury, which she had heard is dangerous and possibly illegal. Her husband now uses the new retort donated to him to burn the amalgam in their house. On a typical day, her husband burns about 3 grams of amalgam in a retort that is approximately 95% efficient at capturing mercury. This efficiency is typical of a well-designed field retort that is used correctly, but some only capture 80%. The 5% of the mercury vapour that leaks from the retort produces an indoor air concentration that isÂ 50 times higherÂ than the WHO occupational limitÂ . In addition to the acute exposure, burning indoors will also cause long-term chronic exposure to the family because mercury sticks to the household walls and surfaces and is re-emitted overtime into the air.
Burning amalgam indoors, even with a retort, causes acute and long term mercury exposure.
- If mercury is illegal, burning inside of houses or cars becomes more common.
- Retorts should never be used in areas that women and children frequent because they are much more sensitive to mercury poisoning.
3. Used retorts are highly contaminated and emit mercury vapour into any area where they are stored or transported.
Lemah is very happy when a pilot project was launched in his community and he was chosen to receive a retort. Only 10 retorts were given out and he was eager to use it. He and some friends use the retort outside where they apply the heat but then he stores it under his bed when he isnâ€™t using it for safe keeping. While it is stored under his bed, the retort releases residual mercury vapour into his bedroom exposing himself and his family to dangerous levels over long periods of time.
- Retorts should always stay outside or in a dedicated contamination zone with good ventilation.
- Retorts should not be transported inside the passenger cabin of a car.
Retorts can be an excellent tool in reducing mercury consumption, environmental contamination and human exposure if they are introduced with the proper training and education. However, without a carefully thought-out intervention, the intended benefits of retorts can fail and in some cases actually increase human exposure to mercury. This is particularly concerning when women and children are endangered through unintended secondary exposures, as they are more susceptible to mercury toxicity. With the creation of the Minamata Convention, mercury-reduction interventions in ASGM communities will certainly increase, and so too will the importance of proper retort training. When a retort intervention is being planned, consider these three key points:
- Retorts donâ€™t always save money so other incentives need to be explained and re-enforced such as the protection of health.
- Retorts leak mercury so they must be used outdoors and away from women and children.
- Used retorts are contaminated and continuously emit residual mercury, so they must be stored in a dedicated well-ventilated contamination zone (typically outside) and not be transported in a closed car.
 Calculation of value of 0.52 g of sponge gold:
Spot $ = $1400/toz (31.1 g/toz) = $45/g
Sponge gold is 50:50 Au:Hg, 0.26 g of gold
Miners in most regions get at least 70% of spot price in the field.
0.26 g Au = ($45/g * 0.26 g * 0.70) = $8.19
Field price of Hg = $200/kg = $0.2/g
0.26 g of Hg = 0.247 g of Hg = $0.05
$0.05/$8.19 * 100 = 0.6%
~ The relative cost of the lost Hg is < 1% of the value of the gold.
1. Inside a hut without a retort
Volume of a hut = volume of a cone + cylinder
Cone : height = 2 m; radius = 2 m; volume = 1/3piR2 = 4.189 m3
Cylinder: height = 2 m; radius= 2 m; volume = piR2H = 25.13 m3
Air volume of a hut = 4.189 + 25.13 = 30 m3
Air concentration if 1.5 g of Hg is burned (50% of amalgam is Hg, 50% is gold) = 1.5 g/30m3 = 1,500,000,000 ng/30m3 = 50,000,000 ng/m3
The WHO occupational limit for Hg concentrations in air = 50,000 ng/m3
50,000,000 ng/m3 /50,000 ng/m3 = 1000
~ The mercury air concentration in the hut would be 1000 X higher than the occupational limit.
2. Inside a hut with a retort
Retorts are ~95% efficient at capturing mercury (5% in the air of the hut)
Air concentration (from calculation 1. above) = 0.05 * 50,000,000 ng/m3 = 2,500,000 ng/m3
2,500,000 ng/ m3/50,000 ng/ m3= 50
~ Even with a retort, the mercury air concentration in the hut would be 50 X higher than the occupational limit.
3. Outside in an open bowl
949,000 ng/m3 = time-weighted average of breathing zone concentrations during two 8-hour work shifts for burners (Drake et al., 2001)
50,000,000 ng/m3/949,000 ng/m3 = 526.9
~ Burning amalgam outside rather than inside, even without a retort, reduces exposure by more than 500 times.However, this increases environmental contamination.
4. Outside in a retort
2000 ng/m3 = Air concentration during a burn with a retort at 1.5 m distance (personal communication, 2013)
949,000/2000 ng/m3 = 474.5
~ Using a retort reduces Hg air concentrations by 475 times when used outdoors, provided the retort is not opened until it is cool.