Behavior of Zn-bearing phases in base metal slag from France and Poland: A mineralogical approach for environmental purposes

Abstract

Slag samples from three pyrometallurgical sites (two in France, one in Poland) were studied for their Zn-phase content, evolution and potential release of metals over time. Mineral assemblages were observed and analyzed using various complementary tools and approaches: chemical extractions, optical microscopy, cathodoluminescence, X-ray diffraction, Scanning Electron Microscopy, Electron Probe Micro-Analysis, and micro-Raman spectrometry. The primary assemblages are composed of analogs to willemite, hardystonite, zincite, wurtzite, petedunnite and franklinite. Some of these phases are sensitive to alteration (e.g., deuteric processes during cooling and by weathering) and, as a result, goslarite, smithsonite and hemimorphite have been identified as secondary products.In comparing these results to the geochemical conditions at each site in relation to mineralogical investigations, different steps of Zn-rich mineral destabilization could be identified. This procedure allows assessing potential environmental impacts due to a release of metals that may contain slag. The destabilization of zincite leads to the precipitation of both goslarite and smithsonite, whereas only smithsonite is formed once hardystonite has been weathered. The Ca released by hardystonite dissolution will indeed limit goslarite formation and moreover favor gypsum precipitation. The relative degree of hydration necessary to form goslarite is supplied by water released during primary phase dissolution and/or smithsonite precipitation. A hydrated form of willemite was observed here, despite being rarely reported in previous studies, corresponding to an intermediate alteration product. The hydrated willemite ultimately evolved towards hemimorphite, which has been identified at the very outer part of the weathered willemite by means of EPMA and confocal Raman spectroscopy. On the basis of these results, we are able to propose a sequence from the least to most stable phase with: hardystonite and zincite>hemimorphite>willemite. Our results are critical for decisions like whether Zn-rich slag may be reused as a material for geotechnical purposes and thus contribute to the sustainable management of similar industrial wastes.