Experiences in a full-scale cyclone plant treatment of Hg-contaminated port sediments.

The present work is based on a full-scale project for the treatment of port sediments with high Hg concentrations. The port area of this work is located in the Principality of Asturias (northern Spain). Preliminary studies for the implementation of a dredging activity on the sediments of the Llanes port showed Hg concentrations in the sediments ranging from 0.19 to 1.13 µg g^-1, with an average of 0.81 µg g^-1. This Hg average concentrations exceeds the legal threshold in Spanish territory for returning dredged sediment to the sea, so that according to current legislation it must be extracted from the coastal system and managed on land, generating a substantial loss of a significant volume of sediment in the system. In order to improve a circular economy and to be able to manage these Hg-contaminated materials to be dredged more efficiently, laboratory-scale tests verified that the Hg contamination was mostly found in the finets fraction <63µm. With these previous results and with regional governmental support, the company EXCADE S.L. in collaboration with the University of Oviedo designed and built a full-scale plant based on the treatment of contaminated sediments by cyclone for the efficient removal of the fine contaminated fraction.  Between June and August 2023, a total of 8,000 m³ of contaminated sediments from this port area were processed in a first phase through this treatment plant. The particle size and the Hg concentrations in the treated material were monitored every two working days, and after the treatment of all the material, four representative samples of the total volume treated were sent to an external laboratory to validate the results obtained during the procedure. The monitoring results showed that the treated material had fine material concentrations of less than 5%, and that the Hg concentration ranged from 0.05 to 0.13 µg g^-1, which resulted in a 7-fold reduction of the initial average Hg concentration in the sediments. The same occurred with the results from the external laboratory that showed that the treated material had fine material concentrations of less than 5%, and that the Hg concentration ranged from 0.13 to 0.17 µg g^-1. These Hg concentrations in the treated material were within the legal range in Spanish territory, which is why the treated material was authorized for deposit at sea. Throughout 2024, the same procedure will be carried out on 12,000 m³ of new contaminated by Hg material to be dredged to validate the results of this first real-scale experience for the treatment of sediments contaminated by high Hg concentrations.