Alluvial miners at work on Bougainville
ONE PNG | 15 January 2018
A recent mercury research study conducted at the small scale mining branch in Wau, Morobe Province is a collaborative work between the mining engineering department of Papua New Guinea’s University of Technology, the Mineral Resources Authority (MRA) through its small scale mining branch and the University of Kyoto-Japan through the leadership of Professor Takaiku Yamamoto, has released its findings.
The use of mercury has become very popular among artisanal and small scale miners because amalgamation is known to efficiently extract fine particles of gold from concentrates obtained by panning and sluicing operations. Gold alloys with mercury to form an amalgam from which the gold can subsequently be separated by evaporating the mercury.
The simplicity of the technique, low investment costs and its comparatively high gold recovery rate has made the mercury amalgam method an integral part of the artisanal and small scale gold mining operations.
In Papua New Guinea, most of the gold deposits worked by the artisanal and small scale gold miners are alluvial deposits in which the gold particles are liberated from gangue particles. It is customary to use riffled sluice-boxes to recover the liberated gold particles.
However, some of the gold particles, particularly the fine gold, does not settle in the riffle compartments but flows over to be discarded as tailings. In the hope of trapping these fine gold particles the artisanal miners frequently place some mercury in between the riffle compartments.
Then in recent years some semi-mechanised and mechanised alluvial mining operations used grinding mills or amalgam barrels for amalgamation of concentrates derived from their recovery systems before putting it through the knelson concentrators or shaking tables for cleaning.
Due to shear force between centrifugal force and drag force in knelson concentrators or the stratification action of the shaking tables, mercury is easily dislodged from the gold and is lost to the tailings. This is because the bonding mechanism holding gold and mercury together is weak and doesn’t require much force to sever the gold particles from the mercury, and because of size and density differences, mercury ends up in the tailings and eventually in the river systems.
However, by far the most dangerous practice adopted by the miners is the gold recovery process from the gold mercury amalgams. Gold is recovered by evaporating the mercury from the amalgam over an open fire
This process is commonly known as “cooking.” The mercury vapour, which includes fine globules, is partly inhaled while the remainder is released into the atmosphere, which returns as part of the “mercury cycle.”
Methods introduced to avoid the practice of releasing mercury into the atmosphere and which can reduce the mercury loss to less than 0.1 per cent are available but have not been so popular amongst miners due to the discolouring effect on the gold after distillation in a retort.
This discolouration is caused by the presence of iron and arsenic compounds and results in a lower price being offered by gold buyers for the product.
One such device is the “Mercury Retort” which evaporates the mercury in a closed cycle and recovers it by condensing the vapour with cooling water.
Mercury is toxic and an environmental pollutant which drew world attention in 1953 after it was reported that a large number of people living in the Minamata bay area in Japan developed symptoms of disease which affected their central nervous system after consuming fish.
The fish in the bay were contaminated with methyl-mercury as a result of mercury being released into the bay by the Chisso Corporation, a chemical company operating on the shores of the bay. The mercury poisoning was responsible for a variety of clinical symptoms which included speech impediments, failure of muscular coordination, and contraction of visual fields in the eye, disturbance in smooth eyeball movements, enteral hearing loss and unbalance of body. The disease is now commonly known as the “Minamata Disease.”
The recent study conducted at theMRA small scale mining branch in Wau was a collaborative work between the mining engineering department of Papua New Guinea’s University of Technology, the University of Kyoto-Japan and the small scale miners in Wau/Bulolo was to trial a an Amalgam retorting machine from Kyoto University-Japan.
The objective was to test run the Japanese mercury recovery kit, a prototype amalgam retorting machine for the recovery of mercury and critically assess the overall performance, its efficiency and ease of operation of the device.
The promotion and use of the retorts would be very beneficial in the long term as they are capable of reducing discharge of mercury vapour into the atmosphere and the environment. It can also recover bulk of the mercury for recycling which would be a potential economic gain for the small scale miners and the chances of them being poisoned can be minimized through the establishment of central facilities in alluvial mining active areas which will alleviate the more dangerous practice of ‘cooking” amalgams.
A batch of mercury gold amalgam samples were provided by the miners from around Wau/Bulolo mining areas for over a period of one week to conduct the research activity by retorting them in the furnace at four different temperatures (300-500 OC, 300-600 OC, 300-700 OC ,300-800 OC) and the mercury recovery results observed ,recorded and calculated.
From this activity, it is noted that mercury which was emitted during the process was mostly trapped in the condensers 1 and 2.
The carbon filter indicated zero mercury which concludes that the air released at the vacuum pump has zero mercury vapour.
From the results obtained, the research team concluded after careful assessment of the overall performance and efficiency of the mercury recovery kit that it is an appropriate technology and should be promoted and used in Papua New Guinea’s artisanal and small scale gold mining industry for mercury and recycling recovery.
MRA managing director, Philip Samar, who was instrumental in introducing the technology, said the purpose of this collaboration was to reduce and mitigate the increased use and disposal of mercury into the environment and increase alluvial gold production, resulting in the health of both the environment and people plus improving the wellbeing of ordinary PNG alluvial miners.
The MRA through its small scale mining branch in Wau would like to thank its research partners for the collaborative work undertaken.
This has set a milestone in being proactive in reducing and controlling mercury contamination to the environment and the users in the artisanal and small scale mining industry.