Characterization of stainless steel spent pickling sludge and prospects for its valorization

Abstract

Fluorspar is considered a critical raw material for the European Union, due to its industrial uses and lack of sufficient extraction in European countries. It is a source for hydrofluoric acid manufacture, this latter chemical being employed, among other uses, in the pickling of stainless steels. From this latter activity, sludge is generated due to the need for used water treatment. In this article, we report a full characterization of this residue, obtained in an industrial plant in Vieira de Leiria, Portugal. Its chemical and mineralogical characteristics were determined, showing that it is mostly a mixture of calcium fluoride and calcium sulfate with some heavy metals content. Thermal behavior allowed us to determine that the material melts at around 950 ◦C. The influence of calcining operation on the residue was determined, especially concerned with the leachability of some elements. Taking into account the results of the characterization of this residue, some considerations are presented about the potential for the valorization of this industrial residue.