Dimension stone industry waste: a new source of industrial minerals and critical raw materials?

Dimension stones (marbles, granites, and stones) represent, at the EU level, a very important market. Some general characteristics must be faced to guarantee a profitable and sustainable quarrying activity: the variability of the lithotypes; the relatively limited productions of commercial blocks; and the low ratio between the “workable” and the gross volumes extracted, with a consequent huge production of extractive waste (irregular blocks, stone chips and residual sludge), whose management is still a matter of concern. The mining and quarrying industry represented the second most important sector in terms of waste quantities produced in the EU-27 (26.6% or 562 Mt), after Construction and Demolition Waste (36% or 769.32 Mt). The necessity to reduce the use of non-renewable natural resources and, at the same time, to minimize the negative impacts the on environment, has led to an increasingly high interest in recycling for different applications, such as aggregates, fillers, geopolymers and industrial minerals. To maximize profit and sustainability, it is crucial to value reuses with higher added value (e.g., industrial minerals) over those with lower value (e.g., aggregates), while at the same time seeking innovative and/or strategic uses. While it's relatively easy to find reuses for wastes of carbonatic composition (lime, cements, fillers), it is more complex to find applications for quartz-feldspathic materials (granites, gneisses, migmatites). The example proposed in this paper is related to the quarrying districts of Piedmont (northern Italy), which include a wide range of quartz-feldpathic rocks (granites and gneisses such as Serizzo, Beola, Luserna Stone). The recovery of granite waste has been successfully developed for years: after a series of crushing, comminution, and magnetic separation treatments (to remove biotite), a mix of quartz and feldspars, with a low FeO content, is produced from the waste and successfully used in the ceramic industry. The waste from gneisses, having a mineralogical composition comparable to that of granites, could find similar reuse: however, more difficulties are encountered, mainly due to the smaller grain size and metamorphic fabric, which make separation processes more complex (however feasible, apart from a few special and isolated cases). Extensive mineral-petrographic investigations have shown that these waste materials can also be useful in the extraction of critical raw materials, especially rare earth elements (REE). Granites contain monazite (one of the most important REE phosphates), which is concentrated in the predominantly biotitic magnetic waste fraction, while gneisses contain appreciable amounts of allanite (an epidote variety rich in REE). In both cases, it would be possible to concentrate these REE ore minerals by (relatively) simple processes of crushing, comminution, magnetic and gravitative separation, while supplementing and maintaining industrial mineral production (quartz and feldspars).  The feasibility of the processes should be tested in pilot plants, ideally side-by-side with those already existing and operating for the recovery of granite waste. It is imperative to change and update the view (and regulation) of mining and quarrying waste, which can provide important production of industrial minerals and even critical raw materials, fitting perfectly into the (often too abstract) concept of circular economy.