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Technology to encapsulate toxic mining waste in mine backfill cement

A study demonstrates the potential of encapsulation in cement to manage mining waste containing antimony, a toxic pollutant considered a priority by the European Union. The research has transformed a toxic waste into a non-toxic waste suitable for mine backfilling.

An antimony mine. Image: IRNASA/CSICA team of researchers led by the Instituto de Recursos Naturales y Agrobiología de Salamanca (IRNASA-CSIC), has demonstrated the potential of cement encapsulation to manage mining waste containing antimony, a toxic pollutant included in the Priority Pollutant List of the European Union as it can cause serious environmental and health problems. The University of Salamanca, the University of Extremadura and the University of Lorraine (France) have also participated in the research.

Antimony is a highly valuable chemical element used in the manufacture of a multitude of products, from mobile phones to glass. For this reason, its production, far from decreasing, will increase in the coming decades. Currently, the world's main producer is China.

"In Spain, although its exploitation took place between the 1940s and 1980s, the waste generated has not been properly managed and remains deposited in the environment," explains Esther Álvarez Ayuso, a scientist at IRNASA, who has lead the research.

There is a growing concern about the wide distribution of antimony in the environment. In fact, stibnite, which is the most important mineral and the main commercial source of antimony, oxidises very easily and can release antimony which, above certain concentrations, can contaminate soil and water. Antimony can cause toxic effects in plants, and from plants can move through the trophic chain to other organisms, threatening human and ecosystem health. Antimony is included in the Priority Pollutant List of the European Union.

To solve this problem, the research team is working to develop methods to prevent or minimise the dispersion of this toxic element. One of these methods is cement encapsulation. Although is not a new technique, it has hardly been explored for the treatment of waste containing antimony.

Cement encapsulation 'is an economical technique and has already been successfully applied for the treatment of other elements with similar geochemical behaviour. In addition, it allows different management options for the materials generated after the cementation process," explains the IRNASA researcher.

The team has treated the waste with different processes in the laboratory, using Portland cement and calcium hydroxide as a binder, in different proportions, determining the optimal conditions. "We have succeeded in transforming these wastes, which were characterised as toxic and were not even admissible in hazardous waste landfills due to their high antimony leaching, into a non-toxic waste and admissible in controlled landfills," says the researcher.

This method could be applied for transforming wastes with other pollutants into natural agregates for construction as long as they meet the corresponding leaching requirements

This scientific research on mining waste encapsulation has been carried out at laboratory scale. "Now, as a next step," adds Esther Álvarez Ayuso, "it is necessary to work at pilot scale to assess the encapsulated waste behaviour under environmental conditions and to find out the optimal formulations as a step prior to their application.”

This method can be applied by the mining companies themselves, which could carry out the encapsulation treatment or by the geological and mining engineering services they collaborate with.

The practice of backfilling mines is common and is a part of the mining process. One of the strengths of this research is that it proves that the resulting material meets the required compressive strength values.

But this is not the only alternative for the waste offered by this research. It has been observed that some of the mining wastes studied have a great potential to be used as substitutes for natural aggregates in construction materials.

Therefore, with this method "we would treat the waste, avoiding environmental pollution, and at the same time we would reuse it, and therefore preserving natural resources, which is very interesting from the point of view of the circular economy", emphasises Álvarez Ayuso.

Regarding its application as a substitute for natural aggregates in construction materials, it could be extended to mining waste with the presence of other toxic elements as long as the corresponding leaching requirements are met.

Scientific reference: 
E. Álvarez-Ayuso, A. Murciego, M.A. Rodríguez, R. Mosser-Ruck. Cement encapsulation processes to mitigate the risks posed by different types of antimony-bearing mine waste. Journal of Cleaner Production. DOI: 10.1016/j.jclepro.2022.133671