Cyanidation of mercury-rich tailings in artisanal and small-scale gold mining: identifying strategies to manage environmental risks in Southern Ecuador

doi:10.1016/j.jclepro.2010.09.008

Journal of Cleaner Production

Volume 19, Issues 9–10, June–July 2011, Pages 1125-1133

https://doi.org/10.1016/j.jclepro.2010.09.008 Get rights and content

Abstract

In many countries, such as Brazil, Colombia, Ecuador, Indonesia, Venezuela and Zimbabwe, amalgamated tailings are leached with cyanide to recover remaining gold. This paper describes a recently completed study conducted at seven gold processing centers in Portovelo-Zaruma, Southern Ecuador, which involved consultation with local miners. The objective of the study was to understand the behaviour of mercury (with a focus on mercury loss) in artisanal gold mining operations through the evaluation of two cyanidation processes (Merrill-Crowe and Carbon-in-pulp), using a participatory approach. In order to assess the kinetics of mercury dissolution in cyanide, a bottle roll test was conducted in the laboratory. In the Merrill-Crowe cyanidation process, an average of 24.2% of metallic mercury was determined to be trapped at the bottom of the agitation tanks; 33.1% of mercury is lost in association with solid material and 11.7% is lost in solution. Approximately 31.0% of mercury in solution is sent to the zinc precipitation cells from which 27.8% is precipitated on the zinc shavings, with 3.23% remaining in solution. The mercury precipitated on to the zinc is lost to the atmosphere when the shavings are burned at 900 °C. In the Carbon-in-pulp (CIP) leaching system, 11.2% of the mercury is lost with the solid tailings; 31.6% of the mercury is associated with fine particles in suspension and 50.8% is likely dissolved. About 2.68% is trapped at the bottom of the tank and 3.72% is absorbed by the activated carbon. The bottle roll test revealed that mercury dissolution is directly proportional to cyanide concentration. At 10 g/ton of cyanide, approximately 42% of mercury was leached, whereas all gold was solubilized. During this study, miners recognized the risk associated with the cyanidation of mercury-rich tailings, and were aware of how much mercury is discharged to local streams and to the atmosphere. The active participation of miners in this study has led to the strengthening of their knowledge and awareness of mercury contamination, and has enhanced their understanding of the nature of the problem, as well as the weaknesses and strengths of the system they operate.

Section snippets

Introduction and background

Mecury amalgamation is carried out by 15 million artisanal and small-scale gold miners (ASGM) in more than 70 countries. This gold recovery method, however, has serious environmental and health consequences for miners, associated populations, and the global community (Veiga and Hinton, 2002, Sousa and Veiga, 2009). Annually, approximately estimated 1000 tonnes of mercury is released into the environment by ASGM due to poor mining practices (Swain et al., 2007).

In Ecuador, ASGM has been the main

The cyanidation process

Generally speaking, Chilean-mill Centers also have cyanidation facilities. However, in Portovelo, there are facilities present that consist of only leaching tanks for the processing of tailings, ores or gravity concentrates. In fact, there are approximately 375 cyanidation tanks in Portovelo, their capacity ranging from 14 to 40 m³ and which employ two types of cyanidation circuits: 1) Merrill-Crowe process (representing ∼40%); and 2) Carbon-in-Pulp (CIP) process (representing ∼60%). Many

Methodology

With the participation of local miners and owners of processing plants, this study was conducted in the Portovelo-Zaruma region and is based on field observations, and analysis of mercury in solid and liquid samples. While assessing mercury, informal consultations were used to characterize the cyanidation process and to explore the miners’ perceptions about mercury use and its loss. The participation of the miners was important for planning, communicating, and identifying the weaknesses and

Results

The technical characteristics of the Merrill-Crowe and the CIP processing systems are depicted in Table 1. In the Merrill-Crowe process, about 6000–7000 kg of tailings with grain size below 0.2 mm are added to cement agitated tanks of 14 m³. The amount of water added ranges from 8000 to 11,000 L. Overall, the pH values in the agitated tanks in Portovelo at the beginning of the cyanidation process were highly alkaline, with an average of 11.2 ± 0.5. At the end of the process, the pH was slightly

Mercury in Merrill-Crowe and CIP process

Miners have developed devices to trap mercury before administering tailings into cyanidation circuits. For instance, in the CIP process, they wash tailings with water in a small tank before introducing the material into the leaching tank. Miners believe they remove half of mercury from the pulp by this way, but in fact less than 3% of mercury is trapped at the bottom of this tank. In the Merril-Crowe process, the amount of mercury that is trapped in the tank and can be eventually recovered

Conclusions

Since all amalgamated tailings contain residual gold, the owners of the processing centers and the miners in Southern Ecuador leach the remaining gold using cyanide through either the Merrill-Crowe or the CIP system. This study has investigated the behaviour of mercury in these two circuits in ASGM operations. Findings indicate that an estimated 50% of the mercury entering the cyanidation tanks is dissolved forming cyanide–mercury complexes in both the Merril-Crowe and CIP processes. An

Acknowledgements

The authors would like to thank to APROPLASMIN (Asociacion de propietarios de plantas de beneficio mineral), in particular to owners and workers who participated and contributed with this study. This research was supported by the Canadian International Development Agency (CIDA) through the project “Sustainable management of environmental health risks in Ecuador”, and the Global Mercury project funded by United Nations Industrial Development Organization.

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Cyanidation of mercury-rich tailings in artisanal and small-scale gold mining: identifying strategies to manage environmental risks in Southern Ecuador

Abstract

Section snippets

Introduction and background

The cyanidation process

Methodology

Results

Mercury in Merrill-Crowe and CIP process

Conclusions

Acknowledgements

Sci. Total Environ.

Enzyme Microb. Technol.

Appl. Geochem.

Environ. Res.

Resour. Pol.

J. Geochem. Explor.

Environ. Res.

J. Hazard. Mater.

Toxicol. Appl. Pharmacol.

Ecol. Econ.

Sci. Total Environ.

Miner. Eng.

J. Cleaner Prod.

J. Cleaner Prod.

Speciation of mercury and mode of transport from placer gold mine tailings

Environ. Sci. Technol.

Microbial destruction of cyanide wastes in gold mining: process review

Biotechnol. Lett.

Incidents of cyanide spillage in Ghana

Trans. Inst. Min. Metall., Sect. C

Fluvial contamination associated with artisanal gold mining in the Ponce Enriquez, Portovelo-Zaruma and Nambija areas, Ecuador

Water Air Soil Pollut.

Fish contamination and human exposure to mercury in Tartarugalzinho River, Amapa State, Northern Amazon, Brazil. A screening approach

Water Air Soil Pollut.

Mercury cyanide contamination of groundwater from gold mining and prospects for removal

Neurophysiological anomalies in brainstem responses of mercury-exposed children of Andean gold miners

J. Occup. Environ. Med.

Neurocognitive screening of mercury-exposed of children of Andean gold miners

Int. J. Occup. Environ. Health

Elevated blood mercury and neuro-otological observations in children of the Ecuadorian gold mines

J. Toxicol. Environ. Health A

Mercury contaminations from gold mining using amalgamation technique in Xiaoqinling Region, Shanxi Province, PR China

J. Phys. IV

Environmental emission of mercury during gold mining by amalgamation process and its impact on soils of Gymple, Australia

Pure Appl. Geophys.