Reuse of residual sludge from stone processing: differences and similarities between sludge coming from carbonate and silicate stones-Italian experiences

Residual sludge coming from dimension stone working activities represents a serious environmental and economic problem for both stone industry and community. Indeed, most of the time, residual sludge is landfilled because of the difficulties to recover it; such difficulties are mainly connected to local legislation and to a lack of proper protocols. In general, two different categories of sludge can be identified: residual sludge coming from carbonate rocks (CS) and those coming from silicate rocks (SS). Both of them are characterised by a very fine size distribution. CS is mainly made up of the same compounds of the processed stones (i.e. marble, limestone, travertine). On the contrary, SS is characterised by high heavy metal content, due to the composition of the tools employed during processing activities and to the original rock characteristics. Furthermore, total petroleum hydrocarbon content can often be recognised in residual sludge. In general, residual sludge, management of which in Italy is administered in accordance with ILD 152/06, can be used as waste for environmental restoration or for cement plants. Several researches investigate possible reuses of residual sludge, after a proper processing phase, as new products. Such new products should be certified not only on the basis of their technical and physical characteristics but also by means of appropriate chemical analyses to guarantee that the products are not polluted. The aim of this research was to evidence that, on the basis of a correct sludge characterisation, treatment and management, it is possible to produce secondary raw materials (filler, etc.) or new products (artificial soil, etc.), in order not to waste sludge resources. Some examples from Italian experimentations are here reported, focusing on the treatment and recovery of SS and CS.