Clive Mitchell is an Industrial Minerals Geologist, and Tom Bide a Researcher in the mineral resource security and flows team at the British Geological Survey. Here, they give us an update on BGS’ Artisanal & Small-scale Gold Mining (ASGM) research in Kenya, and how we can extract essential, primary, raw materials in the most sustainable way, with the least harmful impact…
|East African Rift Valley|
In November 2019 we, alongside Dr Barbara Palumbo-Roe from BGS, and fellow researchers Dr Lydia Olaka, Jemimah Muluu & Cavince Odhiambo from the Department of Geology at the University of Nairobi, crossed the Great East African Rift Valley on our way to Migori County in SW Kenya for a field sampling visit.
|Jemimah Muluu, Cavince Odhiambo & Lydia Olaka from the Department of Geology at the University of Nairobi|
We were there as part of the BGS Official Development Assistance (ODA) integrated resources project ‘From source to sink: quantifying the local and downstream environmental impacts of ASGM’. One of our key concerns was the application of mercury, which is widely used in ASGM to recover gold.
|The BGS Team: Tom Bide, Barbara Palumbo-Roe & Clive Mitchell|
Artisanal and small-scale gold mining is a major industry worldwide, predominantly in developing countries, where individuals or small collectives of miners will extract, process and sell minerals. In many areas, like Migori County, it forms a major part of the economy and provides the livelihood for a large proportion of the local population who can earn more money from mining than from agriculture. AGSM tends to be poorly regulated and can cause severe environmental problems due to vegetation clearance and soil degradation in addition to the use of harmful chemicals, including mercury and cyanide.
In most large scale commercial mining operations, the recovery of gold can be a complex task based on a detailed characterisation of the geology and mineralogy of the ore to determine how best to liberate the gold. In Kenya the process is somewhat simpler.
|Queuing to go underground|
We visited many ASGM sites, all of which had an essentially similar set up. A deep vertical shaft, about 2 metres square and supported by wood, descends from fifteen to sixty metres down to a quartz reef. The reef is extracted with explosives from cramped horizontal tunnels with poor ventilation and inadequate mine dewatering. The ore is manually winched out of the mine and crushed using hammers. It is then milled using Tanzanian-designed ‘ball mills’, built locally by tiny engineering shops in Masara. After this, it is washed down sluices where the heavy gold particles settle in a bed of thick sacking, ready to be washed out, whilst the lighter rock particles are washed away into silt filled ponds. The concentrate from the mats is mixed with liquid mercury by hand, where the mercury will bond with any gold present. The gold-mercury amalgam is heated, boiling off the mercury as vapour, to leave a small ball of ‘sponge gold’ behind. All waste products are then collected and sent in six wheeled trucks to concrete lined tanks full of cyanide solution which will dissolve and collect any remaining gold.
|A typical gold mine|
Mercury is a very toxic chemical, which, if used improperly, can cause serious health problems and become concentrated in local soils and groundwater supplies. It doesn't break down once released into the environment and will remain in soils and watercourse for long periods.
|Gold panning with mercury|
Not only is mercury damaging to the environment, in humans mercury poisoning leads to the crippling and untreatable condition known as Minamata disease. Kenya is a signatory to the Minamata Convention which aims to reduce and eliminate the use of mercury. As part of this, countries with a significant ASGM community, such as Kenya, have agreed to implement plans to eliminate the use of mercury, formalise the ASGM sector, introduce good ASGM practice and protect the health of mining communities, particularly children and women of child-bearing age. Our research project is playing a part in effecting this important change. In the short term, we hope to reduce the amount of mercury used in the production of gold. In the medium to longer term, it will help to achieve the aims of the Minamata Convention by providing evidence to help improve ASGM practice and have great benefits for the environment and health of the communities in south-western Kenya.
|Sponge gold, after the removal of mercury|
Life is hard for the miners and mining communities. To affect any real change we need to be able to prove that by using more efficient gold recovery methods, miners can recover more gold whilst using less mercury. The local miners are well aware of the health risks involved in what they are doing but accept them, as using mercury is the quickest and easiest way to extract gold. We need to develop practical real world solutions to reduce the reliance on mercury in this process. Local mining communities in this part of Kenya are no strangers to innovation. This could be seen by the proliferation of new motorized 'ball mill' crushing machines, that have rapidly spread across the region in the last nine months, mechanising the crushing process by using a drum welded together filled with fist sized steel balls and lumps of gold ore. We believe that if we can develop and demonstrate better gold recovery methods, they will be taken up by the miners.
|A Tanzanian-designed 'ball mill'|
One of the issues that we think may be causing problems is over-grinding of the gold ore by the miners using the ball mills. If the ore is too fine the gold will not be captured by the sluice boxes. To understand the properties of the gold grains, we plan to study the ore samples we collected in detail, using laboratory techniques to understand where the gold is in the ore, the particle size of the gold grains, and whether they are trapped inside other minerals. If our theory is correct, we can then advise the miners to adjust the milling so that it does not over-mill the ore. This will enable the sluice boxes to recover more gold and potentially reduce the amount of mercury used.
|Tom and Clive, with colleagues from the University of Nairobi and local miners|
After returning to Nairobi, we applied for the necessary Mineral Export Permits and the samples were brought back to the UK. The next stage for us, after analysing the samples, is to work on good practice recommendations for the ASGM community in Migori. A workshop will be held in Migori Town in 2020 to share the findings of the research with the ASGM community and their stakeholders in the Migori County Government.