ASSESSMENT OF MERCURY LOSSES DURING THE THERMAL DECOMPOSITION OF SILVER AMALGAM USING RETORTS
The misuse of mercury in artisanal small-scale gold mining gained notoriety with the consolidation of the Minamata Convention (2013). There is a worldwide concern for improvements in the processes used for extracting gold in such places. One of the proposals is the use of retorts during the thermal decomposition of gold amalgam. Therefore, this study aimed at developing an occupational exposure assessment methodology for those operators before such equipment, and, at the same time, to evaluate the possibility of dragging precious metals during the pyrolysis process. According to preliminary results, the retort tested showed significant emissions of mercury to the atmosphere and so did precious metal. However, new trial tests shall be accomplished for validating those previously obtained results, taking into account, in these additional tests, the performance evaluation of different retort designs. Finally, as an attempt to validate the proper retorting temperature range for better decomposing the aforementioned amalgam a thermo-gravimetric analysis were run, which generated the right figures to use the proper thermo-decomposing temperature so as to avoid losing precious metals with the mercury vapor.
NANOGOLD ON POROUS GLASS AS A PASSIVE SAMPLER FOR MONITORING OF GASEOUS ELEMENTAL MERCURY IN ARTISANAL GOLD MINING
Mercury contamination is a major working hazardous in artisanal gold mines in developing Countries, particularly in Africa, Asia and South America. There are no portable and cheap methods able to alert workers of mercury exposure on site in remote locations.
In this work, a miniaturized mercury sampler prepared by the direct reduction of gold into the porous nanostructures of Vycor® glass (PVG) is introduced. Mercury retention on the PVG/Au sampler induces significant color changes, due to the formation of Au-Hg amalgam that affects the surface plasmon resonance characteristics of the material. The color change can be easily verified by eye and quantified based on the analysis of the RGB channels obtained from pictures of the sampler. Therefore, the sampler can be easily deployed in remote areas and the gold exposure can be evaluated using a somethings as simple as a cell phone camera.
The PVG/Au sampler was prepared in two steps: impregnation of PVG discs (0.6 cm Ø) with HAuCl solution, followed by the direct reduction in situ of AuCl4- to Au0. The presence of nanogold was readily detected by the red color of the PVG/Au disc. The PVG/Au was analyzed by high-resolution transmission electron microscopy (HRTEM). The amount of gold deposited in the PVG/Au discs was determined by ICP-MS. Mercury retained on the samplers, before and after exposure, was quantified by Direct Mercury Analyzer®.
The samplers were first exposed to Hg vapor (~2 ppmv) for different times (8 to 48 h) and these laboratory experiments showed the viability of using PVG/Au as passive sampler for monitoring of Hg0. The retention of mercury by PVG/Au samplers was monitored by Red-Blue-Green analysis of images of the samplers and by using ultraviolet-visible spectroscopy. PVG/Au samplers were also deployed in an Artisanal and Small Scale Gold Mining (ASGM) area in Burkina Faso and it was able to indicate mercury exposure. The amount of mercury quantified in the samplers for all miners was higher than the current personal exposure limit for Hg0 set by the US Occupational Safety & Health Administration (OSHA), which is 0.1 mg m-3 (~12 ppbv in air), indicating an unhealthy work environment. The nanogold sampler described here is therefore a potential tool to that can quickly and cheaply evaluate mercury exposure in work conditions of ASGM sites and other settings.
ASSESSING THE STATUS AND PERFORMANCE OF MERCURY CAPTURE DEVICES TO REDUCE GOLD SHOP EMISSIONS, RELEASES, AND ASSOCIATED EXPOSURES
Artisanal and small-scale gold mining (ASGM) is a major source of global mercury pollution. Miners often add mercury to ore to separate the gold, and then take this amalgam to a gold shop where the mercury is driven off at high temperatures. An estimated 50,000gold shops serve the ASGM community as a key step in the gold supply chain. In some cases, simple devices such as water-based box condensers and pipe retorts are used to capture the mercury during amalgam burns. If not captured, mercury can exceed 1,000mg/m3 inside gold shops, which is then released to ambient air. Chronic human exposures to mercury can harm the central and peripheral nervous systems and other organs and systems.
To help address this issue, the U.S.Environmental Protection Agency (EPA) has led efforts to develop and implement a mercury capture technology to reduce emissions in gold shops. In collaboration with Argonne National Laboratory, a simple mercury capture system (MCS) was designed that would be easy to make using local materials and capabilities. The MCS consists of a steel drum that houses a fan, which draws air from above the amalgam burn through slotted baffle plates on which the mercury condenses and then collects at the bottom of the drum for removal. The system was deployed in more than thirty gold shops across Peru and Brazil from 2005‑2010. Indoor test results indicated the MCS achieved mercury removal efficiencies of 80% and higher.
EPA, Argonne, and Mercer University are collaborating to assess the status of MCSs and other mercury capture devices in gold shops. The goal is to increase understanding and awareness of such devices as a measure that could substantially reduce emissions and releases from gold shops, thus reducing worker exposures and exposures of people in communities nearby. The collaboration will explore a number of factors around the presence and management of mercury capture devices in gold shops, including:
- Insights into uptake and operational factors;
- Design, fabrication, installation, and operation of such devices;
- Collection and disposition of the captured mercury;
- Costs, benefits, and incentives; and
- Supplemental information, such as additional design details and mercury measurements.
Possible future activities include exploring the potential role of citizen science, and of mercury capture technology as a strategy option for reducing mercury emissions as part of larger national efforts, with consideration as a technological strategy for addressing targets identified in National Action Plans for ASGM activities.
DEFORESTATION DURING ARTISANAL AND SMALL-SCALE GOLD MINING EXACERBATES MERCURY MOBILIZATION IN MADRE DE DIOS, PERU
Artisanal and small-scale gold mining (ASGM) is a significant contributor to mercury (Hg) contamination and deforestation, particularly in the Colorado River watershed in Madre de Dios, Peru. Within the Colorado River watershed, mining and deforestation have increased dramatically since the 1980s and have been largely concentrated in the Puquiri River, a tributary of the Colorado River. Field sampling in Feb 2015, Apr 2016, and Jun 2016 identified a strong correlation between Hg and suspended solids concentrations, with especially high suspended solids concentrations downstream of ASGM activity. This result suggests that Hg transport in this region was facilitated by soil mobilization and runoff. In order to understand how ASGM activity in the Puquiri affects sediment and Hg mobilization from the watershed over time, we employed a watershed-scale soil mobilization model using satellite imagery from 1986 to 2014. The model estimated that soil mobilization in the Colorado River watershed increased by 2.5 times during the time period, and increased by six times in the Puquiri subwatershed, leading to between 10 and 60 kg of Hg mobilized in 2014. If deforestation continues at its current exponential rate through 2030, soil and heavy metal mobilization may increase by five times. This research shows that deforestation associated with ASGM can exacerbate soil mobilization and Hg contamination.
FLUVIAL MOBILITY OF MERCURY, CYANIDE AND OTHER HEAVY METALS IN THE PUYANGO-TUMBES RIVER BASIN, ECUADOR-PERU
In Portovelo-Zaruma in southern Ecuador, 87 gold processing centers line the banks of the Puyango-Tumbes River, producing an estimated 6 tonnes of gold/a. Each of the plants processes gold using a combination of mercury amalgamation and/or cyanidation, resulting in an estimated 1.6 million tonnes/a of tailings enriched with 2040 tonnes/a of cyanide, 0.243 tonnes/a of mercury and other heavy metals being discharged into the Puyango-Tumbes River, that crosses from Ecuador to Peru and ends up in the Pacific Ocean. The results showed that the majority of the water and sediment samples collected from the Puyango-Tumbes River had elevated concentrations of total arsenic, cadmium, copper, lead and zinc, exceeding the CCME (Canadian Council of Ministers of the Environment) Long Term Effect thresholds for the Protection of Aquatic Life. At monitoring points immediately below the processing plants, total arsenic concentrations exceeded the threshold by 3000 times in solution in surface waters and 740 times in sediment. Even over 140 km downstream, arsenic concentrations in water and sediment were still well above the permissible limits, especially in the sediments. The highest total cadmium concentrations were 555 times over the guideline in surface waters and 120 times over the CCME standard in sediments, which raises concern about contamination of aquatic organisms. The highest copper concentrations in surface waters exceeded the threshold by 3567 times and in sediment by 85 times. The highest lead concentrations were 840 times over the guideline in surface waters and 27 times in sediment. Steps should be taken to reduce lead exposure, especially in regards to potential bioaccumulation in aquatic biota. Free cyanide concentrations were high in surface waters of the river for 50 km below the processing plants, with the highest concentration 13,560 times above the CCME standard of 5 μg/L and 1695 times above the 24-hour LC50 concentration (50 percent fatality) of 40 μg/L free cyanide for some fish species. The highest total mercury (THg) concentrations in sediments were found within a 40-km stretch downriver from the processing plants, with levels varying between <0.03-30.8 mg/kg during the dry season and 1.8-45.1 mg/kg during the wet season, with most concentrations above the CCME Probable Effect Level of 0.5 mg/kg. Data from mercury isotopic analyses support the conclusion that mercury use during gold processing in Portovelo-Zaruma is the source of Hg pollution found far downstream in the Tumbes Delta in Peru, 160 km away.
CO-EXISTENCE OF ARTISANAL AND CONVENTIONAL MINERS: A GOOD SOLUTION FOR FORMALIZATION
The formalization of artisanal gold miners in developing countries has been seen by governments and international agencies as the main way to eliminate bad practices, including mercury pollution that has reached 1600 tonnes/a. It is clear that formalization generates more control and tax revenue for the governments, while providing miners with a legal mining title and access to credit. However, in reality, these benefits are often not favorably viewed by the miners, as the complicated bureaucracy of banks and governments does not make it easy to access credit. In addition, it is observed in many countries that the main mercury polluters are the processing centers, which normally are allowed to operate freely. Without enforcement of legal and environmental regulations, formalization does not provide any solution. This study shows how the UBC Mining Engineering Department is currently working in Colombia with 3 conventional large-scale gold mining companies that have artisanal miners in their concessions, in order to establish initiatives for the implementation of a small mine and plant. This type of co-existence and joint venture between conventional mining companies and artisanal miners allows for the transformation from widespread atmospheric and fluvial contamination with mercury and cyanide to the adoption of Hg-free technologies. The co-existence model has a strong bottom-up component, where it is crucial to assess the needs of the miners, their motivations to change their rudimentary practices and their ability to understand and implement cleaner procedures. Once these factors are adequately assessed, a strong organizational and educational program is conducted with pre-selected leaders. In parallel, a small area of the companys gold deposit is decided upon for the artisanal miners to work using clean and safe practices, as well as analysis of an appropriate area for construction of a small plant (100-200 tonnes of ore/day). Concurrently, metallurgical testing is conducted in the lab to establish the processing parameters of the plant. A pre-feasibility study is then initiated to determine the economic viability of implementing the mine and plant using gravity and flotation systems, followed by intensive cyanidation or sale of concentrates to a third party. The various business models of operation of this new enterprise is discussed with the miners. In doing so, uneducated, subsistence miners living under the poverty line can improve their standard of living, reduce health impacts and contribute to more responsible environmental stewardship.
THE MINAMATA CONVENTION AND GOVERNING MERCURY USE IN ARTISANAL AND SMALL-SCALE MINING IN GHANA
ARTISANAL AND SMALL-SCALE GOLD MINING IN CÔTE D’IVOIRE: MERCURY ESTIMATES AND CONTRAST WITH LOCAL REGULATIONS
The artisanal and small-scale gold mining (ASGM) sector is the largest anthropogenic source of mercury to the environment with around 1,400 tons of mercury released annually. The Minamata Convention on Mercury, which opened for signature in 2013, contains an entire article dedicated to ASGM. This article requires ASGM countries to create National Action Plans to reduce, and where feasible, eliminate mercury use in ASGM. Under these National Action Plans, countries will be required to take technical measures to reduce or eliminate mercury use, and to create policy frameworks on formalization, trade, public health, multi-stakeholder engagement and education, to support the transition of ASGM to a mercury-free and more economically productive sector. Côte d’Ivoire, a West Africa country with 40% of the Economic Community of West Africa States’ (ECOWAS) economy, has signed the Minamata Convention on Mercury on October 11, 2013. In preparation of the ratification, enabling activities have been performed to assist the government in the process and have an early implementation. As part of that process, a national mercury inventory in ASGM was conducted based on field studies and research that includes socio-economic data, direct mercury use measurements, semi-structured interviews, household survey data, and the size of the secondary economy of involved communities. This research found that 11,275 of artisanal miners, coming mainly from all ECOWAS countries, are using 2.3 tons Hg/year and this mercury is being entirely released in the environment via open burning of the amalgamated concentrate gold ore. In many ECOWAS states, the national regulation and many sub-regional regulations prohibit mercury use in ASGM. Thus, it appears that the implementation of Minamata Convention in all ASGM - ECOWAS countries will be a challenging process that will require a deep reform of ASGM sector to comply with the Convention provisions.