Tailings are the waste material left over after a portion of the valuable components have been removed from the ore. Comprised mainly of crushed and milled rock and water, they may also contain chemical reagents, such as mercury and/or cyanide, used in gold extraction. In artisanal and small-scale gold mining (ASGM), after operations cease or move to new locations, tailings are often left behind without proper stabilization or containment measures. This poses a risk to the environment and human health, especially if tailings contain residual mercury (or other toxic materials). Moreover, physical failure of ASGM tailings ponds and dams can greatly damage the surrounding environment and pose further risk to human health.
Mercury, cyanide and tailings don't mix
Due to inefficiencies in ore processing, some ASGM tailings contain significant amounts of unrecovered gold and may subsequently be reprocessed to recover it. Using cyanide to extract remaining gold from ASGM tailings to which mercury has been added without first removing the mercury is one of the worst practices as defined by Annex C of the Minamata Convention on Mercury. It leads to generation of mercury-cyanide complexes that are highly mobile in the environment and readily transformed into its bioavailable form - methylmercury.
Given the challenges regarding tailings management in ASGM context, COP-3 of the Minamata Convention requested the Secretariat, in cooperation with the UNEP Global Mercury Partnership, to improve the guidance on the preparation of national action plans for ASGM in regards to management of tailings from such mining. The draft revised guidance was submitted to COP-4, to be held on 21-25 March 2022, for adoption, and an in-depth technical document on mercury-containing tailings management in ASGM has been developed. The document provides an overview of the available knowledge and provides recommendations on the management of mercury-containing tailings generated from ongoing ASGM activities.
Towards sound management of mercury containing tailings
The best way to manage mercury-containing ASGM tailings is not to generate them in the first place, or generate much less of them. Utilizing mercury-free ore processing methods will ensure that tailings do not contain added mercury. If mercury is used, limiting amalgamation to concentrates (rather than the whole ore), combined with keeping the mercury-containing and non-mercury-containing tailings separate, will greatly reduce the quantity of mercury-contaminated tailings and make management easier. Applying cyanide to mercury-containing tailings for gold recovery is a worst practice and should not be permitted. Mercury must first be removed. Potential removal methods can be based on thermal, adsorption (e.g. metal plates), and gravimetric methods . Extreme care should be taken in any subsequent use of cyanide, best done by organized and trained miners that can comply with chemical management protocols.
Safe storage and disposal of mercury-containing tailings should be prioritized by ensuring that groundwater or surface water do not migrate from the tailings structure to the environment, that tailings ponds are structurally sound, and that dust emission is suppressed. Communities should be kept informed about the presence of mercury-contaminated tailings and the associated risk through signs or other means of communication. Local or national governments should make efforts to record the location of mercury-contaminated tailings for future risk reduction and remedial actions. Moreover, financial mechanisms and responsibilities must be allocated to guarantee sustainability of tailings management along the value chain, through mine closure, restoration, and monitoring.
ASGM Tailings In Colombia
planetGOLD Colombia and Pure Earth Colombia are advancing activities for the management of mercury-contaminated tailings. This includes review of existing information, analysis of primary information in the field, execution of pilot tests to identify ways to recover mercury and gold from ASGM tailings, and elaboration of technical guidelines to manage tailings in the main mining areas in Colombia. The efforts are currently under way with tailings characterization and mercury measurements in the field. A total of 90 tons of tailings are being loaded and transported to the pilot plant site located in the municipality of Yalí in the department of Antioquia.
As with any ASGM intervention, the local political, socio-economic, and environmental context, including formal status and organization, power dynamics, and roles of stakeholders along the value chain of tailings must be assessed. For example, the planetGOLD project in Burkina Faso has found that women involved in ASGM often receive tailings as form of payment for their work. With the new and improved mercury-free technology in sight, they fear that one of their sources of income will disappear. The planetGOLD project aims to anticipate gender differentiated impacts of the installation of the mercury-free gold processing system and identify opportunities to mitigate any adverse gender impacts. In Indonesia, women were found to collect the mercury-containing tailings dumped into rivers by small scale operations. They do so without the use of protective equipment. The planetGOLD programme is determined to raise awareness of the risks of mercury and introduce mercury-free technology that will result in generation of mercury-free tailings.
planetGOLD Philippines is planning to construct a brand new tailings facility in Sagada, Mountain Province. Currently, all mercury-free tailings are transported over 150 km away to Itogon, Benguet for secondary cyanide processing, where after the reprocessing the tailings are directly discharged into the environment. As such one of the goals of the planetGOLD Philippines is to establish a centralized processing plant with its own well-managed tailings management facility. The technical team is aiming to utilize a more benign leaching technology compared to traditional cyanidation in order to minimize effluent and solid waste management costs that are mainly linked to the cost of detoxification and destruction of residual cyanide in waste streams.