Exploring the Potential for Utilization of Medium and Highly Sulfidic Mine Tailings in Construction Materials: A Review

Critical methods of geopolymer feedstocks activation for suitable industrial applications

As health and safety issues emanating from human activities on terrestrial environment is becoming ever challenging, the production of Ordinary Portland Cement is identified as a key contributor. This technology threatens environmental quality by emitting significant quantity of carbon dioxide (CO2) that threatens Net Zero delivery. Consequently, the development of cement alternatives with substantial CO2 reduction/sequestration during production has become imperative. Geopolymers obtained from industrial residues are poised as promising alternatives in managing environmental systems but selection of appropriate method of activation has limited their wider industrial applications. This article discusses four key activation methods and their combinations used in four main feedstocks to advise on their energy requirements, product compressive strength and environmental/industrial applications. Reviewing and characterising 302 published literatures with focus on most relevant and recent advances in the field, this review found that hybrid techniques combining mechanical activation method produces geopolymers with the highest compressive strength and thus the best method. Geopolymer made by mechano-chemical activation method of slag achieved the highest compressive strength while geopolymer produced by microwave assisted activation of clay and ultrasonic activation of fly ash cum slag are most economical in curing energy demand. Hybrid activation is the current development in the field and integration of this method with mechanical activation is poised as the future geopolymer activation technology as it demonstrates greatest efficiency potential.

Toward sustainable reprocessing and valorization of sulfidic copper tailings: Scenarios and prospective LCA

There has been increasing attention recently to reprocessing of mining waste, which aims to recover potentially valuable materials such as metals and other byproducts from untapped resources. Mining waste valorization may offer environmental advantages over traditional make-waste-dispose approaches. However, a quantitative environmental assessment for large-scale reprocessing, accounting for future trends and a broad set of environmental indicators, is still lacking. This article assesses the life cycle impacts and resource recovery potential associated with alternative waste management through mine tailings reprocessing at a regional scale. Sulfidic copper tailings in the EU were selected as a case study. We perform prospective life cycle assessments of future reprocessing scenarios by considering emerging resource recovery technologies, market supply & demand forecasts, and energy system changes. We find that some reprocessing and valorization technologies in future scenarios may have reduction potentials for multiple impact indicators. However, results for indicators such as climate change and energy-related impacts suggest that specific scenarios perform sub-optimally due to energy/resource-intensive processes. The environmental performance of reprocessing of tailings is influenced by technology routes, secondary material market penetration, and choices of displaced products. The trade-off between climate change and energy related impacts, on the one hand, and toxicity impacts, on the other hand, requires critical appraisal by decision makers when promoting alternative tailings reprocessing. Implementing value recovery strategies for building material production, can save up to 3 Mt. CO₂-eq in 2050 compared to business as usual, helping the copper sector mitigate climate impacts. Additional climate mitigation efforts in demand-side management are needed though to achieve the 1.5 °C climate target. This work provides a scientific basis for decision-making toward more sustainable reprocessing and valorization of sulfidic tailings.

Valorizing (cleaned) sulfidic mine waste as a resource for construction materials

Proper management and storage of mine waste, e.g., tailings and waste rock, is one of the main issues that mining industries face. Additionally, there is already an uncountable amount of existent historical mine waste, which may, even centuries after the closure of the mine, still be leaching contaminants into the environment. One solution to minimize the risks associated with the mine waste, with also potential economic benefits, is through the valorization of the waste. This can be done by first recovering valuable metals and removing hazardous contaminants. Then, the remaining residue can be valorized into green construction materials, such as geopolymers, ceramics or cement. For some mine waste materials, such as those with only trace levels of metals that are not economically viable to extract, the "waste" can be reused directly without this additional cleaning step. In the present study, mine waste originating from three different sites was characterized and compared with the cleaned mine waste (i.e., cleaned by bioleaching or flotation methods) and with different types of green construction materials containing 13-80 wt% (cleaned and uncleaned) mine waste. Particular emphasis was given to the mobilization of metal(loid)s from the mine waste and construction materials (i.e., ceramics, alkali-activated materials and cement) under different conditions, through a series of leaching tests (i.e., EN 12457-2, US EPA's Toxicity Characteristic Leaching Procedure, and a pH-dependent leaching test). The leaching tests were applied to either mimic current 'natural' conditions at the mining site, conditions in a landfill (end of life) or extreme conditions (i.e., extremely acidic or alkaline pH). Most of the original mine waste samples contain high levels of Pb (18-3160 mg/kg), Zn (66-10500 mg/kg), and As (10-4620 mg/kg). . The cleaning methods were not always efficient in removing the metal(loid)s and sulfur. In some cases, the cleaned mine waste samples even contained higher total metal(loid) and sulfur concentrations than the original mine waste samples. Based on the leaching studies, some alkali-activated materials, ceramics, and cement effectively immobilized certain metals (e.g., <0.5 mg/kg of Pb and <4 mg/kg of Zn). Also, longer curing times of the alkali-activated materials, in most cases, improved the immobilization of metal(loid)s. Additionally, for ceramics, the temperature at which the test pieces were fired (up to 1060 °C), also played a major role in decreasing the mobility of some metal(loid)s, while increasing others (e.g., As, potentially via the structural rearrangement of As and Fe). Overall, through this detailed characterization, the environmental impact from the mine waste to the downstream products was evaluated, determining which valorization methods are the most viable to close the circular economy loop.