Chile is the world’s top copper producer (27% of global production) and exporter. In Chile, copper is mostly found in sulfide ores, especially in chalcopyrite (CuFeS₂), bornite (Cu₅FeS₄) and chalcocite (Cu₂S). The continual exploitation of ore reserves leads to the necessity of mining ores from deeper parts of deposits, often with increasing mineralogical complexity. Consequently, copper ores containing arsenic, such as tennantite (Cu₁₂As₄S₁₃) and enargite (Cu₃AsS₄), are increasingly being mined.

Since highly toxic arsenic poses a threat not only to human health but also to the entire ecosystem, some countries introduced arsenic limits for imported copper concentrates. In order to comply with those limits, arsenic-rich concentrates are currently being diluted with arsenic-poor concentrates. However, this strategy will not remain feasible in the long term due to steadily increasing arsenic levels, notes the German Federal Institute for Geosciences and Natural Resources (BGR).

Element distributions in enargite obtained with electron microprobe. Source: BGR

Conventional strategies for separating and landfilling arsenic are proving limited in this area. A German-Chilean project—ReAK, Reduction of Arsenic in Copper Concentrate, led on the German side by the Fraunhofer Research Institution for Materials Recycling and Resource Strategies IWKS, is working towards optimizing and advancing copper production processes so that Chile is able to tackle the problem of arsenic in the most effective, cost-efficient and environmentally friendly way.

Although copper is now readily recyclable, global demand is on the rise, thus calling for the continued mining of copper. In 2021 alone, Chile mined about 5.6 million metric tons of copper—equivalent to approximately one quarter of the world’s total mining volume. This makes the copper industry one of the country’s most important sources of income.

The copper in Chile is mined in open-pit copper mines in the Atacama Desert. Ore is blasted here from open-pits with diameters of up to two kilometers. This ore has a copper content of around 2%, which is extracted in several stages.

Ball mills used for the ore processing at a Chilean copper mine. © Juan Carlos Salas

The ore is first ground and then subjected to flotation, which involves the wet chemical foaming of the material, in order to separate the high-copper ore from the low-copper ore.

The thickening of flotation residues during copper production in northern Chile. © Juan Carlos Salas

This results in copper concentrate, which Chile also sells. The concentrate contains around 20% to 30% copper. The concentrate is then smelted in blast furnaces and finally purified in electrochemical electrolysis processes in order to obtain pure copper.

Raw copper smelting in a blast furnace. © Fraunhofer IWKS

As well as copper, the excavated ore layers often also contain arsenic—a highly toxic, inorganic pollutant that poses a threat not only to human health but to the entire ecosystem. Due to its toxicity, the arsenic needs to be extracted from the ore.

This takes place almost automatically when the ore is heated—the arsenic gasifies. In Chile, the current procedure is to capture the gas, dissolve it in sulfuric acid and then transform it into solid calcium arsenate or calcium arsenite in a process known as precipitation. These calcium compounds are then deposited in the Atacama Desert.

Previously, the fact that these compounds are water-soluble never presented any problems since the Atacama Desert is one of the driest regions in the world. However, climate change has caused an increase in rainfall in recent years, which now raises the issue of environmental contamination. At the same time, some mines are penetrating layers that are no longer oxygen-based but sulfur-based and thus contain more arsenic. As a result, the amount of arsenic to be deposited will increase in the future, particularly given that there are currently no industrial uses for arsenic.

These two developments are now making Chilean mine operators rethink the way in which they previously disposed of arsenic and adapt to these new circumstances without delay. We are investigating new ways of separating and depositing the arsenic as part of this project so that the environmental impact is minimized as much as possible.

—Anna-Lisa Bachmann, Fraunhofer Research Institution for Materials Recycling and Resource Strategies IWKS and coordinator of ReAK

Fraunhofer IWKS launched a first project in 2018, which brought together a consortium of numerous German and Chilean partners from both research and industry. The project officially began in September 2019 with funding from the German Federal Ministry of Education and Research.

Since then, several different potential process steps have been researched in more detail, starting with the further processing of arsenic-rich copper concentrate.

We are putting a range of possible options to the test, with the intention of identifying the most effective individual modules and the most successful sequence of steps.

—Anna-Lisa Bachmann

The consortium is exploring a number of processes, including arsenic-selective flotation, sulfation roasting, and microbial and sulfidic leaching.

Alternative oxidation processes resulting in more stable and less toxic arsenic⁵⁺ compounds instead of unstable and water-soluble arsenic³⁺ compounds are also being considered.

One option for this oxidation step could be the use of hydrogen peroxide, but this is very expensive. This is why we at Fraunhofer IWKS are investigating electrochemical oxidation with diamond electrodes as a viable alternative in the course of this project. They feature a particularly large electrochemical window, resulting in the formation of hydroxyl radicals in aqueous solutions. These then oxidize the dissolved arsenic efficiently and reliably without the need for additional chemicals.

—Anna-Lisa Bachmann

Other project partners are also evaluating UV- and ozone-assisted as well as microbial oxidation processes.

The first step of all work packages is to test whether the method in question actually works and whether it delivers the desired result. Fraunhofer IWKS will compare their cost-effectiveness and environmental impact in the context of life cycle assessment and life cycle costing analyses as soon as all of the individual results are available.

We will use this to develop a new concept for landfilling that will provide recommended actions to the Chilean government, considering both governmental requirements and the available financial resources.

—Anna-Lisa Bachmann

Much of Chile’s copper industry, including one of the mines participating in the project, is state-owned.

A number of these tests have already been completed, and others are expected to be completed soon. While reciprocal sample deliveries and in-person meetings have been delayed as a result of the COVID-19 pandemic, Anna-Lisa Bachmann remains confident that all partners will have jointly developed a promising strategy by summer 2023.