Anna Tornivaara, Geological Survey of Finland, P.O. Box 1237, FI-70211 FINLAND, e-mail: anna.tornivaara(at)gtk.fi
The addition of alkaline materials to control the pH of drainage from a waste facility is a relatively common approach to reduce the environmental impact of mining industry by mitigating acid rock drainage (ARD) from the waste facility. There are several different techniques for addition of additives. Layering is a method that involves placing acid producing and acid consuming (neutralizing) materials by turns to control or limit ARD. (INAP 2009)
Description of the method
In the layering method, acid producing and acid neutralizing materials are disposed together by turns in a waste facility. The effectiveness of the layering is governed by availability of materials, the general balance between acid producing and neutralizing materials, the type and reactivity of acid-consuming material, deposit geometry, weather conditions, the nature and flow of water through the deposit, and chemical armouring of alkaline materials (MEND 1998 & 2001; Miller et al. 2003 & 2006). Some layers can also be comprised of fine-grained, alkaline waste material or rock powder (paste layer), which retains water well and thereby attenuates or prevents the travel of oxygen into acid-generating waste rock layers.
The layering method in which waste rock and tailings are disposed of together is called co-disposal (INAP 2009). Co-disposal can be conducted in several ways, which vary depending on the method of mixing. Co-disposal in which layers of waste rock and tailings alternate is called layered co-mingling.
Layering can be a consequence of dust emission prevention, such as in cases where lime milk is applied to the flat surfaces of tailings. Lime milk is used particularly in dried areas where it forms a hard crust on the tailings, thereby preventing dust emissions from the waste. However, the lime milk crust does not endure mechanical strain, nor is it likely to remain intact over the winter, which means the treatment needs to be repeated annually in the spring, and results in layering in the tailings impoundment (Kauppila et al. 2013).
Waste rock layering has been used for example at the Samatosum mine site, B.C. Canada, as an ARD control measure.
Advantages (MEND 2008):
- Chemical stability
- Lower rate of ARD production
- Easy to implement and manage
Disadvantages (MEND 2008):
- Difficult to obtain mixing of alkaline material and acid leachate due to preferential flow
- Requires estimations of PAG/NAG ratio
- Requires detailed planning to schedule the turns and sequence thickness of the layers
- Cost and availability of reagents?
The capacity of layering depends on the amounts of the materials to be disposed of (waste rock/tailings/additive), the additive(s) used, the disposal impoundment (mine workings, open pit, constructed impoundment area), and the disposal method.
Changes in the source material can oblige maintenance and new calculations of neutralising potential (changes in PAG/NAG ratio).
Layering requires full scale waste characterisation of the materials to be disposed in order to calculate proper layering ratios.
Requirements for the materials and appliances
The layering method has to be effective and reduce ARD. It is important that the acid neutralizing material is not prone to leaching of metal ions. Alkaline material should be tested by a complete characterization analysis to evaluate the neutralization capacity, possible leaching of harmful substances and the effects on drainage quality (INAP 2009).
Minimisation / treatment of potential discharges
Key issues for the effectiveness of layering are the pathways of water movement through the system, the nature of the contact between acidic and alkaline materials and the proportion of the neutralizing material.
Monitoring / control needs
The layering method requires normal monitoring measures.
INAP 2009. The GARDGuide. The Global Acid Rock Drainage Guide. The International Network for Acid Prevention (INAP). http://www.gardguide.com/ modified 2014. Read 17.7.2014.
Kauppila, P., Räisänen, M.L. & Myllyoja, S. (Eds) 2013. Best environmental practices in metal mining operations. The Finnish Environment 29en/2011, Environmental Protection, Finnish Environment Institute (SYKE). The publication in English is available only on the internet: www.syke.fi/publications (available also in Finnish). 219 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 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 2001. Prevention and Control. Volume 4. Manual 5.4.2d. Mine Environment Neutral Drainage Program . Tremblay, G.A. & Hogan, C.M. (Eds.), CANMET.