Sustainable use of (cleaned) sulphidic mining waste in building ceramics

Waste materials should be seen as potential resources, which can help decreasing the pressure on the primary raw materials sector and the external market dependence within the European Union. A material’s passport, where the full characterisation and valorisation routes of (waste) materials can be given is a must to shift towards a more circular approach within the raw materials sector and boost more industrial symbiosis. After construction and demolition waste, mining and quarrying waste is considered the largest waste stream in the European Union. Sulphidic mine tailings and waste rocks, pose a large challenge due to their acid generation potential through acid mine drainage (AMD), which can cause significant environmental, health, and social hazards. Apart from the hazardous metal(loid)s, these mining waste materials contain base, precious, and critical metals that can be used in different applications of emergent technologies. Moreover, the mineral (bulk) residue fraction of these mining waste materials generally contains silica-rich minerals (e.g., quartz, clay minerals) that could be used in building ceramics.

The main goal of this research is to develop building ceramics as a high added-value application for the (cleaned) sulphidic mining waste materials from Belgium (Plombières Pb-Zn inactive mine), Germany (Freiberg Cu-Zn-Pb inactive mine), and Portugal (Neves Corvo Cu-Zn active mine). A detailed physical, mineralogical, chemical, thermal, and environmental characterisation of Plombières tailings (SUL_PL_62_I), Freiberg tailings (SUL_FR_01), Neves Corvo fresh (SUL_NC_01) and stored (SUL_NC_03) waste rock, and Neves Corvo cleaned fresh (SUL_NC_01_CL_FLOT) and stored (SUL_NC_03_CL_FLOT) waste rock materials, was performed. These mine waste materials were used to partially or totally replace primary raw materials (5-40 wt%), such as clay, sand, or loam, in company-specific ceramic blends for roof tiles, inner-wall blocks, pavers, and facing bricks. Test specimens were prepared in lab conditions by vacuum extrusion (roof tile and inner-wall block blends) and by hand-moulding (paver and facing brick blends). After lab or industrial drying, the test specimens were fired in electric lab kilns at 965°C (blocks), 985°C and 1000°C (roof tiles), 1060°C (facing bricks), and 1130°C (pavers). The test specimens were assessed for their shaping, drying and firing behaviour (production processes), and their technical, aesthetical, and chemical properties (product quality). Some of the test specimens were assessed for their compliance with environmental regulations (metal(loid)s leaching) for building ceramics during service life (diffusion leaching test), second-life (column leaching test), and for their end-of-life scenario (batch leaching test). Lastly, a cradle-to-gate life cycle assessment (LCA) was performed in order to compare the overall environmental impact of the best-performing waste-derived facing bricks with the standard facing bricks using primary raw materials.

Results showed that, from all the studied mining waste materials, Plombières tailings material (SUL_PL_62_I) can be used directly in ceramic blends, without any pre-treatment required. It can partly or totally replace primary raw materials, such as clay, sand, or loam, in a selected blend for roof tiles (5 wt%), blocks (10 wt%), pavers (10 and 20 wt%), and facing bricks (20 and 40 wt%). The test specimens containing Plombières tailings material not only showed overall satisfactory behaviour during shaping, drying, and firing, but also good product quality and environmental performance considering service life, second-life, and end-of-life scenarios. Moreover, a cradle-to-gate life cycle assessment (LCA) showed clear environmental gains of using up to 40 wt% of Plombières tailings material in a facing brick blend. However, when the incorporation of Plombières tailings material was increased in roof tile (10 and 20 wt%) and block (20 wt%) blends, technical (higher water absorption and lower E-modulus strength) and aesthetical (efflorescence after firing) problems emerged. The paver blends with Plombières tailings material (10 and 20 wt%) presented slightly better technical properties (lower water absorption and higher E-modulus strength) than the standard. The ceramic blends for roof tiles, blocks, and pavers with the highest incorporation of Plombières tailings material (20 wt%) proved to be suitable for a second-life scenario where shaped tailings-derived ceramics can be recycled as aggregates. For the facing brick blends with Plombières tailings material (20 and 40 wt%), similar aesthetical (no efflorescence and same firing colour), chemical (low soluble sulphates and cations) properties were achieved, and slightly better technical properties (lower water absorption and higher E-modulus strength) when compared to the standard. Facing bricks with Plombières tailings material (40 wt%) showed compliance for service life (diffusion test), second-life (column test), and end-of-life for landfill as inert waste (batch test), and presented clear environmental benefits, especially regarding natural resource usage, fine particulate matter emissions, human and eco-toxicity, and eutrophication, considering a cradle-to-gate life cycle assessment.

The untreated Freiberg tailings (SUL_FR_01), the crushed Neves Corvo fresh (SUL_NC_01), and stored (SUL_NC_03) waste rock materials were incorporated in roof tile blends (5 wt%) and block blends (10 wt%). Despite improving technical properties, such as lowering water absorption and increasing E-modulus strength, these alternative materials were not suitable to replace primary raw materials due to their high sulphidic content, which resulted in higher SOx emissions, efflorescence, and black core formation in the fired bodies. Therefore, a pre-treatment is needed. The Freiberg tailings material has been treated by bioleaching and froth flotation. However, in both cases, the sulphur and soluble sulphate content was still too high and, therefore, this material was not further assessed. The Neves Corvo waste rock materials were both treated by froth flotation, after grain size reduction, and a lower total sulphur and soluble sulphate content were obtained. Nevertheless, after incorporation of the cleaned fresh (SUL_NC_01_CL_FLOT) and stored (SUL_NC_03_CL_FLOT) waste rock materials in facing brick blends (20 and 40 wt%), aesthetical problems occurred, such as efflorescence after drying, which turned into white-staining marks in the fired bodies. Concerning environmental compliance during service life (diffusion test), the waste rock-containing (40 wt%) fired test specimens showed excessive leaching of As, meaning that such bricks cannot be commercialised under Flemish environmental regulations. Therefore, an optimised cleaning procedure is recommended. This research shows how alternative materials, such as mine waste, should be fully characterised and, when incorporated in building ceramics, what properties should be considered. Besides the production and quality aspects, compliance with regional or national environmental regulations is also important. Circular building ceramics can only reach the market when all aforementioned aspects are satisfying, thus comparable to, or even better than, the standard.