In most large-scale copper mining deposits, tailings represent around 97 to 99% of the total volume of rock processed in the plant, meaning that the volumes of waste material generated are close to the volumes mined. The pyrite content in these varies depending on the type of deposit, but ideally these bodies should be free of pyrite so that the final residue could eventually be used as aggregate material for construction or fill. The presence of pyrite in tailings prevents reuse in most cases.

According to data from Sernageomin’s Chile Tailings Deposit Registry 2024, there are 836 tailings deposits in Chile, of which 678 (81%) are inactive or abandoned.

The largest number of tailings deposits are in the Coquimbo Region, followed by the Atacama and Valparaíso regions.

Tailings represent an environmental liability with potential negative impacts due to the generation of acid drainage, which can contaminate groundwater and agricultural land, in addition to the threat of collapse in the current climate change scenario.

Pyrite: primarily responsible for the formation of acid water

Cobalt in tailings is associated with pyrite, an iron sulfide mineral whose crystalline structure accepts a variety of other elements, including heavy metals and metalloids. Pyrite is currently considered a gangue mineral, meaning that it is of no mining interest for exploitation, so it is discarded in tailings along with all other gangue minerals.

However, disposing of pyrite in tailings poses a risk and danger due to its potential environmental impacts and the stability of the tailings.

This is because, upon contact with air and water, pyrite undergoes oxidation and hydrolysis processes that lead to its dissolution into iron and sulfur, generating sulfuric acid, which can produce acid mine drainage. This drainage, which also carries other metals, can contaminate surface and groundwater by infiltrating aquifers and is toxic to varying degrees to human health, wildlife, and vegetation.

In addition to these environmental impacts, acid drainage can destabilize tailings dams and their walls by changing their permeability and hydraulic gradients, creating a high risk of collapse or spills, which in other countries has had fatal consequences.

A recent example is the failure of the tailings containment wall at the Las Cenizas mine in Cabildo, following rains that affected the Valparaíso region in June 2024, causing sediment runoff into the La Ligua river.

Removing pyrite from tailings is one of the main environmental benefits of the Cobalto Verde project. Reprocessing abandoned tailings in areas close to human settlements can improve the quality of life of their inhabitants, reduce the risk of potential environmental impacts, and eventually decrease the volumes of final waste materials, with the potential to make them suitable for use as aggregates, thus generating a circular economy process.