Anna Tornivaara, Geological Survey of Finland, P.O. Box 1237, FI-70211 FINLAND, e-mail: anna.tornivaara(at)gtk.fi
Sulphide ore mining produces waste materials with different acid generation potential. The non acid producing materials produced can be utilized in the disposal of acid producing waste using encapsulation to prevent acid mine drainage (AMD). In the encapsulation method, acid producing mining wastes (typically waste rock) are placed inside the acid neutralizing materials, such as alkaline waste rock, soils or synthetic material. Synthetic materials, such as geomembrane and geosynthetic clay layers (GCL), can be used to encapsulate waste rocks. Encapsulation can be piled as a separate heap, mixed with benign waste, or it can be disposed into an open pit (Lottermoser 2007, INAP 2010).
Description of the technology
In the encapsulation method, acid producing mining wastes (typically waste rock) are placed inside acid consuming materials and are isolated from the surrounding environment to decrease AMD. The effectiveness of encapsulation of extractive waste depends on the availability of materials, the general balance between acid producing and acid neutralizing materials, the type and reactivity of acid-consuming material, deposit geometry, the nature and flow of water through the deposit, and chemical shielding of alkaline materials (e. g. limestone) (MEND 1998, MEND 2001, Miller et al. 2003, Miller et al. 2006). Before choosing the type and geometry of the encapsulation, comprehensive characterisation of the waste materials (both acidic and neutralizing) is needed. Characterisation should already be made during the mine planning stage and should particularly focus on the acid production and neutralization capacity of the waste materials (see Acid generation potential).
Typical strategy for the encapsulation is to use non-acid generating waste materials with a good neutralizing capacity in the basal structure and as a cover (and dam) material in the waste pile/facility. Potentially acid generating material is heaped inside or blended with non-acid generating waste if possible (Lottermoser 2007, INAP 2009). For example, encapsulation of acid producing tailings can be made by placing a layer containing carbonate minerals on the base of the facility and by covering the tailings with a similar carbonate rich material (Lottermoser 2007, INAP 2009).
Encapsulation of the tailings has been studied for example at the gold mine in the Timmins region, Ontario Canada, where historic acidic tailings required relocation due the expansion of an open pit. Sulphide rich tailings were placed onto a high-NP tailings facility and covered with a similar material (INAP 2009, MEND 2010). Case study is described in a more detail in the GardGuide (Pamour tailings; INAP 2009) and in the report by MEND (2010).
Encapsulation is suitable for both waste rocks and tailings.
Advantages (Lottermoser 2007, INAP 2009):
- Economical when neutralizing material is easily available
- Tolerably easy to implement and manage
- Versatile and allows localized treatment
- Can notably reduce AMD intensity
Disadvantages (INAP 2009):
- Site specific conditions, availability of eligible material -> cost. Transportation fees can limit the use of the method if suitable material can’t be found from or near the site.
- Time and release rate of alkalinity
- Alkaline materials are consumed by even pH neutral water
Capacity is limited by a combination of the characteristics of the available materials (e.g. AP/NP values) and their proportions (layer depths and amount of material).
AP/NP balance has to show that there is an adequate NP to consume all acidity in low-pH pore water as well as all AP that could be generated in the sulphide rich waste if all of the remaining sulphide reacted to form sulphuric acid.
Monitoring / control needs
Regular analysis of waste material (waste characterization) is ensuring the balance between AP/NP and the positive effect of encapsulation.
Water quality monitoring is needed to guarantee performance of the encapsulation method.
INAP 2010. GARD Guide. http://www.gardguide.com/index.php/Main_Page. Version number 0.8, updated 13.12.2010
Lottermoser, B.G. 2007. Mine wastes – Characterization, Treatment, Environmental Impacts 2nd ed. Springer. 304 p.
Miller, S., Rusdinar, Y., Smart, R., Andrina, J., & Richards, D. 2006. Design and Construction of Limestone Blended Waste Rock Dumps – Lessons Learned from a 10-Year Study at Grasberg. In: R.I. Barnhisel (Ed.), Proceedings of 7th International Conference on Acid Rock Drainage (ICARD), March 26-30, St. Louis, MO, American Society of Mining and Reclamation, Lexington, KY.
Miller, S., Smart, R., Andrina, J., Neale, A., & Richards, D. 2003. Evaluation of Limestone Covers and Blends for Long-Term Acid Rock Drainage Control at the Grasberg Mine, Papua Province, Indonesia. In: Proceedings of 6th International Conference on Acid Rock Drainage (ICARD), July 12-18, Cairns, QLD, Australia, AusIMM, 133-141.
MEND 2001. Prevention and Control. Volume 4. Manual 5.4.2d. Mine Environment Neutral Drainage Program . Tremblay, G.A. & Hogan, C.M. (Eds.), CANMET.
MEND 2008. Blending and Layering Waste Rock to Delay, Mitigate or Prevent Acid Rock Drainage and Metal Leaching: A Case Study Review. Mine Environment Neutral Drainage Program. Report 2.37.1, CANMET.
MEND 2010. Evaluation of the Water Quality Benefits from Encapsulation of Acid-Generating Tailings by Acid-Consuming Tailings. MEND report 2.46.1 http://mend-nedem.org/wp-content/uploads/2013/01/2.46.1.pdf