Gold losses and mercury recovery in artisanal gold mining on the Madeira River, Brazil

For the last 40 years artisanal mining has been playing a key role in the economy of the Madeira River, State of Rondônia, in the Brazilian Amazon. The main process through which artisanal miners in the region recover gold is by river dredging. The dredges are floating platforms that excavate the bottom of the river and, after a sorting phase, separate the gold (Au) from the sediment by means of mercury (Hg) amalgamation. The objectives of this paper are to map the process, demonstrate the inability to completely recover the Au from the excavated sediments, and reveal the actual impact of Hg. Several dredges were visited and all the unit operations involved in the mining and processing circuit were analyzed. Significant losses of Au are associated with inefficient sorting techniques. The Au grade of the sediments rejected during the sorting phase ranges from 0.1 to 7.8 ppm, reaching values higher than the world average Au grade of the producing mines. Analyzing the amalgamation process, it was possible to quantify the Hg released into and recovered from the environment. The results show that, contrary to common belief, Hg is recovered from the bottom of the river instead of being released into it. The total Hg-mass balance is actually closed with a positive recovery and with an emission factor (Hg:Au ratio) lower than the average (0.1–0.2). Despite this result, Hg continues to be volatilized into the atmosphere in the form of vapors. In the region of the Madeira River a campaign must be developed to encourage the use of more efficient and Hg-free technologies for Au recovery. Responsible and Hg-free AGM operations can contribute effectively to rehabilitate the entire area.