Trace elements distribution and enrichments in lime industry washing muds

Processing of carbonatic rocks by lime industry produces scraps, some of which are not yet valorised. An example are the washing muds, separated between extraction of the carbonate material and its processing in the kiln. The subject of this study is the characterisation of trace elements in the washing muds from different quarries, related to various geological settings and different processing, with particular focus on critical raw materials (CRM), and their potential relation with major elements, geological features and age of the exploited rocks. The project, run in collaboration with Unicalce, the major Italian producer of lime, and Carmeuse, one of the primary producers in Europe and partner of Unicalce, led to the collection of samples from six quarries in Italy and two in Belgium. 

Microfacies differences between quarries were determined through petrographic analysis on thin sections of the extracted rocks. The variability observed at microscale in the samples from a single quarry is partially lost due to the bulk blending  during crushing and milling operated to prepare the load for the kiln. As a consequence, the muds resulting from washing of the milled rock, represent a blend of the lithologies extracted in a relatively short lapse of time and sent to the comminution plant.   

Both rock and mud samples were collected for each site and processed into powder pellets for LA-ICP-MS analysis. Major and trace elements were measured, normalised and plotted accordingly. Rare earth elements were normalised to Post Archean Australian Shales (McLennan, 1989) and correlated with Ca, Mg and Al. Other traces were normalised to mean Upper Continental Crust (Rudnick & Gao, 2003), and concentration factors from rock to muds were calculated and plotted. Antimony is enriched in all studied muds, while Li, Cd, Sn and U have notable concentrations, considering the type of rock, with a wide variability range among the quarries. These concentrations are still only traces of few ppm, but the difference in trace element distribution between source rock and muds could serve as an important point of consideration in the future use of these materials and their potential recycling, since they constitute great volumes of unused material all throughout Europe.

McLennan S.M. (1989) Rare earth elements in sedimentary rocks: influence of provenance and sedimentary processes. Reviews in Mineralogy and Geochemistry, 21, 169-200

Rudnick R. L., Gao S. (2003) Composition of the continental crust. Treatise on Geochemistry, Volume 3, pp.659. ISBN 0-08-043751-6 Elsevier