Assessment of the Applicability of Steel Slag for Alkaline Addition to Acidic Streams in Southcentral Pennsylvania

Abstract

The presence of an existing steel slag material stockpiled from historic steel producing operations located in the Broad Top coalfield region of southcentral Pennsylvania sparked an interest in evaluating the material as an alkaline source which may be an alternative to limestone for remediating Acid Mine Drainage (AMD), which plagues local watersheds. Steel slag from active steel mills is currently used as an alkaline material in AMD treatment. This assessment project involved chemical and physical analyses of the stockpiled steel slag including metals analysis, x-ray diffraction (XRD), Toxicity Characteristic Leaching Procedure (TCLP), and ASTM Extraction. The steel slag material was subject to laboratory and field bench-scale water treatment tests using various water sources. Non-AMD impacted waters, including rainwater and local stream water above AMD impact zones and mildly acidic AMD discharge sources containing low and high dissolved carbon dioxide were tested using the steel slag. In addition to varying the water sources, raw and screened sizes of steel slag were used in container tests to evaluate alkalinity production and the importance of dissolved carbon dioxide. The results of the evaluation indicate that the steel slag has alkalinity-generating capacity due to the presence of calcite (CaCO3) and akermanite (Ca2MgSi2O7) minerals identified in the XRD analysis. Additionally, the leaching of potential contaminants in this steel slag was determined to be low based on the results of the TCLP analyses. In all conditions, steel slag increased pH and generated alkalinity within a few hours of contact. The analytical data provided a basis for determining innovative techniques for using the steel slag to add alkalinity within watersheds impacted by AMD.