Particle Size Distribution in ASGM

Many industries such as food, beauty and pharmaceuticals rely on a specific particle size in order for their product to excel. This is also true in mining; this blog will discuss the importance of particle size as it relates to artisanal and small scale gold mining (ASGM) and how the AGC is using particle size distribution to aid their work in Peru.

In the mining industry, large particles (rocks) are taken from the ground in the form of ore. The ore is then crushed by hand to a more manageable size before it is milled by a machine. Once the ore is milled to a fine dust, the gold is liberated. The size of the dust particles can have a great impact on how much gold a miner can process from their ore.

Machines like this crush the ore

The degree of liberation of gold is measured by particle size. Particle size is measured in ‘mesh’. Screens are stacked in order to measure the whole range of particles. Very coarse screens are measured in inches and smaller particles are measured in microns.

A set of sieves/screens (see image below) can be used to measure the particle size distribution in a sample. Ex: A Chilean mill (as used by the AGC in Burkina Faso) is 74mm (74 microns). This is important to:

·         Calculate the energy required to do any further grinding;

·         Determine if liberation size is sufficient;

·         Assess the need for an additional classification process to remove oversized particles.

Particle size distribution analysis in Cholito, Peru

In Cholito, Peru, at our mercury free processing and training centre, there is approximately 20 grams of gold exhumed from 1 tonne of ore (20 gr Au/t). This means that approximately 98-99% of the ore that is processed will be tailings/waste and the remaining 1% is a valuable gold concentrate.

At Cholito we use particle size distribution analysis to:

• ·         Separate oversized particles coming out of the secondary crushing circuit;
• ·         Prepare the grinding load at the ball mill to reduce incoming particles at a specific size;
• ·         Separate fine and coarse fractions at ball mill exit.

The ball mill exit separates the fine and coarse material

Particle size distribution in ASGM is an aspect of better practices that allows miners to be more efficient and reduce and plan additional classification circuits.

For additional information about particle size distribution and gold recovery, see our YouTube channel for videos on concentration, milling, and Sixto’s seminar series on the topic.

ASGM and Public Health

The AGC helps miners improve their productivity and helps them to comply with minimal health, safety and environmental standards; this can facilitate the formalisation of their livelihoods, allowing them to increase their income in a safer way for both them and their communities. One of the ways in which the AGC does this is through educating health workers and miners on the risks of certain aspects of ASGM and providing ways to mitigate these risks, while at the same time as training miners on how to process their gold in a safer way, largely by using less or no mercury.  

For the vast majority of the millions of people who work in ASGM worldwide, labor conditions are substandard. There are multiple health risk factors that affect these populations; for example, acute accidents and injuries are common and chronic conditions resulting from exposure to toxins are prevalent, although rarely quantified. Moreover, ASGM communities typically have little to no access to education, health care or sanitation, which further increase the likelihood of health problems.

Image Credit: Natural Resource Defense Council

Both occupational and community health hazards exist for ASGM miners and their families. Because of the working conditions and the nature of the work, miners are exposed to mild, moderate and even sometimes severe and fatal injuries and often there is no emergency response system available. Long term exposure to potential toxins such as mercury and silica result in a variety of health conditions that are the cause of a short life expectancy in ASGM miners.

A major health issue in ASGM is mercury exposure. Mercury evaporates at relatively low temperatures and is often emitted into the environment in different phases of the gold recovery process in ASGM. Multiple health effects have been documented from acute and chronic exposure to mercury vapors and it is recognized that mercury in its organic form is a potent neurotoxin. Pregnant women and children are particularly vulnerable to the effects of mercury.

Another important health risk is silica exposure. When people are exposed to dust repetitively, silica particles accumulate in the lungs causing inflammation and the person can develop a disease called silicosis, an irreversible disease that begins with fatigue and shortness of breath and can lead to progressive respiratory failure.

There are many other health risks such as the chronic musculoskeletal disorders as a result of long-term heavy and non-ergonomic labor practices. In many cases the long-term exposure to noise produced by improvised engines or generators can cause noise-induce hearing loss. Further, limited education, poor hygiene and sanitary conditions can precipitate transmissible infectious diseases, and the social environment in ASGM communities may factor into other health problems such as drug or alcohol abuse, sexually transmitted diseases, anxiety from conflicts and exploitation of women and children.

Better practices like these improve health standards for miners

The AGC provides training and education to prevent several of these health problems. In its work with miners and communities, the AGC offers training for the minimization of mercury use and the use of mercury-free technologies, the proper use of masks to reduce silica dust exposure and other forms of occupational health training. Practical educational material is also offered to raise awareness on other health risks factors identified in the specific communities with which the AGC works.

Some of the training materials we have developed can be found here: http://www.artisanalgold.org/publications/products/

               

        In addition, the AGC aims to achieve public heath sustainability through the engagement of local healthcare systems and local institutions involved in the health and environment sector. In this way, the AGC may help in the collaborative development of health curricula on ASGM to be used by local technical institutions, facilitating simple mercury prevention protocols or participating in the development of preventive programs as per the need of local communities.

Pages like these help to educate miners 

         Lastly, health activities carried out by the AGC have also the intention to serve as a channel to integrate ASGM communities to the formal society. AGC’s advocacy role help incorporate local health programs, such as Malaria or TB programs, in ASGM communities. In turn, occupational protocols that serve in pilot ASGM sites may also be incorporated in formal local health programs.

               

     Public health improvements in ASGM are challenging but the results of our work are rewarding to many.

Mercury Use in the Guinean ASGM Sector

The ASGM sector in Guinea is vibrant, growing, and ripe with opportunity. If properly managed, it could create a much needed West African example of best practices in ASGM.

The Artisanal Gold Council (AGC) is engaged in a United Stated Department of State (USDoS) funded project focused on reducing mercury use in Francophone West Africa. The project currently is active in Senegal, Burkina Faso, and Guinea. Here we will discuss the interesting and unique approach to mercury use in Guinea, which differs from common practices seen in surrounding West African countries.

Traditional gold concentration process using a calabash pan

While traditional gold mining has occurred for centuries in Guinea, there has been significant growth in modern ASGM in recent years, including the introduction of mercury to amalgamate gold. 

The Guinean ASGM sector is less developed (delayed) compared with the longer standing activities of surrounding countries (for example in Mali), and this is reflected in a simplified, less mechanized ASGM sector, and interestingly, less mercury intensive practices in Guinea. The young status of the country’s ASGM sector provides opportunities to prevent bad practices that have become more strongly entrenched in more developed ASGM sectors. 

In nearby countries (Senegal, Mali, Burkina Faso) mercury use is pervasive and present in almost all ASGM processing operations. The common West African process is to remove ore from the mines, crush, mill, concentrate using sluices, and amalgamate the sluice concentrate on site with mercury. The amalgamation and burning of amalgam is usually conducted on site by the processors or site bosses. This spreads mercury use widely to various ASGM sites, making controlling and reducing use difficult. Guinea differs. 

In Guinea, the initial process is similar (although commonly using less mechanised crushing and milling practices), but the resulting concentrate is further upgraded using pans and magnets (to remove magnetic minerals) to produce a very high grade, almost pure, gold dust. This high grade concentrate is brought to gold buyers in local mining hubs, where the buyers themselves provide mercury to the miners to amalgamate the concentrate on the buying site. In effect, this approach restricts mercury use to only the buying sites, rather than spreading further upstream in the production system to throughout the rural mining regions.

This difference in amalgamation approach provides easier opportunities to reduce and ultimately eliminate mercury use than in surrounding countries. The time for intervention is now, before mercury spreads upstream to the processing sites. Training and interventions provided now can ensure this does not occur. Interventions for mercury reduction with the gold buyers where the majority of mercury use occurs are also possible. Reduction options include:

• Training of gold buyers in direct smelting, to allow the smelting of high grade gold dust directly, rather than the amalgamation with mercury. This may not be feasible for all purchases as larger volumes of material are needed for direct smelting, and individual miners or small miner groups often bring small amounts of gold dust for sale at a time;
• Implementation of retort or fume hood programs on gold buying sites to further reduce mercury loss and human exposure during amalgamation and burning – see needed precautions in AGC’s retort guide on this page of our website;
• Early onset technical interventions on mine sites to demonstrate improved gravimetric processing workflows, and direct smelting, which will allow miners and processors to produce gold ingots themselves on site, before they are introduced to the often easier (although lower recovery) mercury intensive processes commonly practiced on site in other West African countries.

Celebration for the national day of the artisanal miner in Kankan, showing president Alpha Condé giving a speech (above) and the attending crowd (below)

The Guinean ASGM sector is large and unique. Miners are friendly and keen to learn and adopt improved practices to reduce potential negative health and environmental impacts associated with artisanal mining. Additionally, the Government of Guinea has made improving the governance and reducing the negative environmental impacts of ASGM a priority. This commitment is evidenced by an ongoing reform of existing policy and legislation pertaining to ASGM, culminating in the official opening of the country’s first four dedicated ASGM mining corridors in Siguiri on February 6, 2017 – suitably on the national day of the artisanal miner. Remarkably, this is a celebration of artisanal mining in Guinea. Something many other countries might consider doing in order to raise awareness of the positive aspects of artisanal and small scale gold mining. It was held in Kankan, and attended by over 1000 stakeholders, including government, business, NGOs, and countless miners and community members interested in learning how changes could improve or worsen their ASGM based livelihoods.

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Mercury Free Gold Processing

Using mercury amalgamation to separate gold particles from ore is not only dangerous to the health and environment of miners and their families but it is also inefficient. Recovery is an important aspect of efficiency in mining. It is a measurement of how much gold is split from the ore; how much gold goes to concentrate and how much goes to tailings.

In Cholito, Peru at our mercury free processing and training centre, the initial rate of recovery that miners were able to extract through mercury amalgamation was 30%-40%. This means almost half of their gold was lost to tailings! A waste of the resource at a great harm to the planet. (In comparison, most large scale gold mines maintain a recovery of over 98%.)

One of the most prominent roles of the AGC is to assist miners to improve their processing techniques and eliminate their use of mercury. The mercury free processing and training systems installed by the AGC are intuitive and easy to use since they are composed of technology the miners are already familiar with. They have improved crushing, milling, sluicing and a shaking table that separates gold from other minerals. And they can even improve their gold recovery by up to 40%.

AGC's mercury free processing and training system in Cholito, Peru

One of the ways in which the AGC promotes a safer, more economical and less harmful ASGM sector worldwide is through better practices, and a key aspect of better practices in ASGM can be a gravimetric circuit. There are many types of gravimetric technologies that can be used to more safely, cleanly and efficiently separate gold particles from the ore. But particle size matters for efficiency. Through gravimetric technologies the high/low density (high relative weight) of gold is taken advantage of.

The traditional processing workflow employed by artisanal miners usually has poor grain size control yielding inadequate gold recovery and is both mercury and dust intensive, causing associated environmental and health issues for not only the miners actively involved in the processing, but members of non-participating surrounding communities as well. To reduce these negative effects, the AGC has developed and improved practices by introducing an alternate processing system with the following components:

1.       Generator: For onsite production of electricity required to power the system.

2.       Scrubber: Removal of clay material from ore by the action of water inside a rotating drum.

3.       Jaw Crusher: Provides the initial grain size reduction of ore by impact energy between two steel plates, it usually delivers a gravel 4 to 8 times smaller than the size of the material fed into it.

4.       Wet Pan Mill or Ball Mill: Further grain size reduction to ensure complete liberation of gold.

5.       Screen: Grain-size control generally used after the milling stage. Its size selection depends on the grain size required for the best gold liberation.

6.       Sluice,shaking table,spirals, jigs: Primary concentration of gold

7.       Flotation: Secondary concentration of gold.

8.       Smelting and Refining Kit: Production of gold ingots.

9.       Tailings Ponds: Storage of mercury free tails and water recirculation.

A workflow option used by the AGC to process gold without the use of mercury.

This system workflow eliminates the need to use mercury in processing while increasing the gold recovery through better and more controlled milling and mineral separation practices. One of the most important improvements to the AGC system is the addition of process control at several stages of the workflow. Primary concentration after the milling process is a key to gold recovery. The grinding control allows for more complete liberation of gold from other minerals in the ore, resulting in improved gold recovery. 

In this general configuration the system is suitable for most ASM scenarios; however, the best workflow may vary slightly between sites depending on things such as mineralogy, size and distribution of gold particles in the ore, location of site, availability of water for processing, and technical skills of operators. Therefore, the adopted workflow is made site specific and optimized based on locally encountered conditions.

Health and Environmental Issues and Solutions

By advancing technology, business practices, governance and policy, the Artisanal Gold Council helps to improve the environment and health of the millions of people involved in ASGM worldwide.

The three pillar approach of the AGC

Mercury use, water consumption, and dust and wastewater management are the main environmental and health concerns associated with ASGM. At many artisanal gold mining sites, miners and their families are susceptible to inhaling both mercury vapour and silica dust, which can cause serious and often irreversible health problems. 

Although the environmental and health issues in ASGM are of serious concern, both can easily be mitigated through education, improved processes and best practices

A ball mill with wet processing keeps silica dust down and helps to recycle water used.

To alleviate health hazards, the AGC:

·               Provides seminars to local health professionals on how to identify and treat health issues that are specific to the ASGM sector, such as exposure to dust and mercury intoxication.

·               Promotes the use of wet milling techniques to suppress and mitigate dust exposure

·               Provides training to miners and processers on the health issues related to ASGM and measures that should be taken to protect their and their family’s health

·               Works with governments around the world to develop national public health strategies to address mercury exposure from ASGM activities

·               Promotes the adoption and use of personal protective equipment at mine sites

·               Has developed research guidelines and tools including surveys and questionnaires which can be used to:

o   Conduct rapid health assessments in mining communities

o   Assess the capacity of local and regional health institutions to deal with health issues related to ASGM

This shaking table separates gold from other minerals without the use of mercury.

To mitigate the environmental impacts, the AGC:

·               Provides access to simple and intuitive mercury-free processing equipment
.               Designs environmental management strategies that can be used by mining communities to reduce the impact of ASGM activities on the environemtn and monitor outcomes

·               Promotes the use of water recycling in processing techniques and workflows

·               Works with governments around the world to develop strategies to promote the reduction of emissions and releases to the environment of mercury, as part of their Minamata Convention commitments

Minamata and NAPs

The Minamata Convention on Mercury, adopted in 2013, is a world-wide treaty aimed at protecting human health and the environment from mercury’s harmful effects. As the artisanal and small-scale gold mining (ASGM) sector is a major anthropogenic source of mercury release and environmental pollution, a specific article targets the sector in the treaty; as per Article 7, countries that are part of the Convention and determine that ASGM and processing are “more than insignificant” in their territory shall develop a National Action Plan (NAP) to reduce, and where feasible, eliminate the use of mercury and mercury compounds in the sector.

The Artisanal Gold Council (AGC) is helping countries, such as Burkina Faso, Ecuador, Gabon, Laos PDR, Mali, Peru and Senegal, to elaborate their NAP, in collaboration with the national governments, the Global Environment Facility (GEF), the United Nations Environment (UN Environment) and the United Nations Industrial Development Organization (UNIDO). As part of the NAP elaboration, the AGC assists countries to develop a national overview of the ASGM sector, including a baseline estimates of mercury use and mining practices. Under this component, AGC provides training to national experts on the methodology used to prepare a mercury inventory for the ASGM sector. 

The national objectives and strategies are defined in consultation with local, regional and national governments, as well as other ASGM stakeholders such as miners, the civil society and academia, through workshops and meetings. During the elaboration of the NAP, the AGC also supports countries to improve the health of miners and ASGM communities by developing a public health strategy for the sector based on the assessment of the health situation of miners and their communities, as well as on the assessment of the institutional capacity of the health sector. Awareness is also raised during the project on the risks related to mercury use in ASGM to human health and the environment. 

The AGC has several NAP project on the go and many more pending. Click here for an updated review of the NAPs we are working on.

Spring Check Up – Looking Bright!

✅ 10 Countries;

✅ 5 Languages;

✅ Millions of rural poor positively impacted with better health, safety and economic conditions.

This February is shaping up to be one of the most influential months in AGC’s history. And we are rolling into the Spring with more momentum than ever.