Journal of Hazardous Materials 197 (2011) 153–160 Contents lists available at SciVerse ScienceDirect Journal of Hazardous Materials jou rn al h om epage: www.elsevier.com/loc ate/jhazmat An engineered cover system for mine tailings using a hardpan layer: A solidification/stabilization method for layer and field performance evaluation Joo Sung Ahn a,∗ , Hocheol Song b , Gil-Jae Yim a , Sang Woo Ji a , Jae-Gon Kim a a Geologic Environment Division, Korea Institute of Geoscience and Mineral Resources, Daejeon 305-350, Republic of Korea b Department of Environment and Energy, Sejong University, Seoul 143-747, Republic of Korea a r t i c l e i n f o Article history: Received 9 June 2011 Received in revised form 7 September 2011 Accepted 17 September 2011 Available online 22 September 2011 Keywords: Tailings Hardpan Cover system Solidification/Stabilization a b s t r a c t A cover system for mine tailings with a solidified layer (called an engineered hardpan) was developed in this study to reduce water infiltration, acid generation and sulfide oxidation. Hydrated lime and water- glass were used to produce calcium silicate, which can serve as a binder when constructing a hardpan layer. The compressive strength of each solidified/stabilized material was found to be sufficient in the lab, and the amounts of heavy metals were significantly reduced in chemical leaching tests. Various characteristics of tailings may affect the layer’s mechanical strength early on, but a long curing period is capable of compensating for these effects. Heavy metals were stabilized as carbonate-bound phases and sulfide minerals were surrounded by calcium silicate matrix, thereby preventing further reaction. To evaluate the field performance of the system, a hardpan layer was installed on top of tailings on a pilot scale. Leachate with high salt content was generated in the tailings layer in the early stages of mon- itoring, but after approximately 6 months, the objective was achieved as the hardpan layer gradually stabilized. Notably, during the heavy rainfall season of the later monitoring stage, water infiltration was continuously prevented by the system. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Mine tailings are ground rock particles from which valuable minerals or metals have been extracted, and they are considered to be one of the most severe sources of acid drainage and heavy metal pollution in soil and water [1]. The management of acid- generating tailings is a primary concern for the reclamation of closed mines. Soil covers involving single or multiple layers are generally used to rehabilitate tailings disposal areas [2,3]. Vari- ous designs of cover systems with different materials have been suggested to prevent acid mine drainage and rainwater infiltration in tailings impoundments [4–7]. A previous study described the instrumentation system and site characterization procedures used to evaluate the performance of an engineered cover system placed on mine waste [3]. On the other hand, cemented layers within or on the surface of tailings deposits (so-called hardpans) form naturally and become zones of contaminant attenuation [8–10]. Sulfates, carbonates and secondary silicate minerals formed during the weathering process are the main cementing materials, and they reduce permeability and stabilize contaminants in hardpans. However, environmental ∗ Corresponding author. Tel.: +82 42 868 3227; fax: +82 42 868 3414. E-mail address: jsahn@kigam.re.kr (J.S. Ahn). contamination can occur because the hardpans do not form homo- geneously or cover the entire surface of tailings with a uniform thickness. In this study, we use a hardpan layer as one of the protective lay- ers in a mine tailings cover system. For this purpose, we developed a simple solidification/stabilization (S/S) method for the construction of hardpan on the tailings surface which should be easily applicable in the field, and a tailings cover system with hardpan was installed in the field to assess its performance for a relatively long period. Developing a solidification/stabilization method for tailings with binders other than ordinary Portland cement is another purpose of this study. 2. Engineered cover system with a hardpan layer The cover system for mine tailings impoundments suggested in this study is a model wherein a hardpan layer is constructed on top of the tailings that is homogenous both in its properties and thickness, as illustrated in Fig. 1. Also, a capillary break layer composed of a fairly coarse material, such as gravel, can be placed to prevent pore water in the tailings from moving upward until the hardpan layer is stabilized. The final top layer is covered with soil for revegetation in the model. A constructed hardpan layer can limit acid leachate, rain water infiltration and sulfide mineral oxi- dation, and it can also reduce the thickness of soil cover. Column 0304-3894/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2011.09.069