Mercury in artisanal and small scale gold mining : identifying strategies to reduce environmental contamination in Southern Ecuador

• Main Author: Velásquez López, Patricio Colón
• Language: English
• Published: University of British Columbia 2010
• Online Access: http://hdl.handle.net/2429/27914

This investigation builds on research about mercury use in custom processing centers in Portovelo-Zaruma Southern Ecuador, where around 3000 people are directly involved to produce around 9 tonnes/annum of gold. The lack of understanding about mercury dynamics during gold processing reduces the development of appropriate solutions to mitigate the environmental contamination. The analysis of the amalgamation systems in 8 centers indicated that 12 to 40% of the total mercury used in the process has been evaporated when amalgams are burned, 40 to 60% of mercury has been recovered and 1 to 35% of mercury has been lost with the tailings. The amalgamation of the whole ore in barrels (“Chanchas”) contributes to the highest concentrations of mercury in tailings (350 ppm Hg). Around 1.5 tonnes/annum of mercury has been likely released to the environment in Portovelo from which 71% goes to the air and the remaining mercury enters the cyanidation process. As amalgamation does not extract all the gold present in the ore, the mercury rich-tailings are processed with cyanide preferentially through Merrill-Crowe or Carbon in Pulp (CIP) system. The analysis of 7 cyanidation processing plants revealed that 51% and 14% of mercury is released as dissolved mercury from CIP and the Merrill-Crowe process respectively. Approximately 27% of mercury is released to the atmosphere with the Merrill-Crowe when zinc shavings are burned. The CIP process releases 3.72% of mercury during carbon elution. A laboratory cyanidation test confirmed that mercury dissolution from tailings is much slower than gold dissolution. The established division of labor in place among miners and owners of processing centers forces the use of mercury in amalgamation combined with cyanide leaching. As miners do not understand the actual process, they accept low levels of gold recovery by amalgamation, leaving the rich tailings to the owners of processing centers. This generates dependency on mercury and reduces the opportunity to improve the system. One lesson gleaned from this study is that a participatory integrated assessment approach may contribute to learning, increasing awareness, and it can help identify practical and effective options for reducing mercury contamination in artisanal gold mining operations.