Using blast furnace slag as an alkaline agent in remediating acid mine drainage from the Iberian Pyrite Belt and removal efficiencies for Rare Earth Elements

Blast Furnace Slag (BFS) is a waste product generated during smelting of iron ore in a blast furnace. It is composed of, among other oxides, CaO and MgO due to which BFS is capable of generating alkalinity above pH 8 thus has a high neutralizing potential. This characteristic feature makes BFS a potential remediation agent for acid mine drainage (AMD) generated from abandoned mines. Passive treatment systems (PTS) of AMD generally make use of alkaline materials such as limestone to neutralize the acidity and to precipitate metals. Moreover, the use of BFS as an alkaline material in remediating AMD can be considered as a way of achieving industrial symbiosis and hence sustainability. However, researches conducted to investigate suitability of BFS in remediating highly acid (pH≤3) and metal rich AMDs are absent in the literature despite the promising properties of BFS. In addition, Rare Earth Elements (REE), which are considered as technology-critical elements, are present in higher concentrations in such AMDs than those found in natural water bodies. Therefore, in addition to the AMD remediation, it would be worth to investigate the effectiveness of BFS to also retain REE in PTS. This study aimed at investigating the efficiency of BFS in remediating highly acidic and metal rich AMD generated at the Iberian Pyrite Belt (IPB) and especially in retaining REE. Three AMD samples having different chemical properties were collected from the Tharsis mining area situated in the IPB. Various amounts of BFS were added to aliquots of AMD samples in  such a way to collect solids and solution per each pH unit until neutralization (from pH 3-7). Suspensions were then kept in an orbital shaker for a week to ensure proper contact between the added BFS and AMD solution. Afterwards, the suspensions were centrifuged at 4000 rcf for 5 min to separate the supernatants from the precipitated solids and BFS. The supernatants were then analyzed by ICP-MS and the removal efficiencies for Al, Fe, trace metals and REE were calculated. BFS showed high removal efficiency for Al (97.8±2.1%), Fe (98.8±1.2%), Zn (93.2±2.7%), Cu (99.7±0.3%), Mn (92.2±0.1%), As (99.8±0.2%), Cr (98.2±1.5%), Cd (99.6±0.7%) and REE (98±0.8%) at pH 7. This result shows that BFS is also suitable to be used as an alkaline agent in treating highly acidic and metal rich AMDs and in retaining REE. Precipitated solids during alkalinization of AMD with BFS will also be characterized by XRD, SEM/EDX and LA-ICP-MS to identify the nature of the precipitated solids and the association of REE with the mineral phases contained in the precipitated solids. The fate of toxic metals and REE during alkalinization will be modeled based on the results from solid and solution characterization It is thereby intended to reach a better understanding of the AMD remediation and REE retention mechanisms using BFS as alkaline material in passive AMD remediation.

Keywords: Blast furnace slag, Acid mine drainage remediation, Passive treatment systems, Rare Earth Elements