Anna Tornivaara, Geological Survey of Finland, P.O. Box 1237, FI-70211 FINLAND, e-mail: anna.tornivaara(at)gtk.fi
Blending is one of the specialized handling techniques for mine waste to prevent acid formation by improvement of neutralising capacity using lime or other alkaline material. In this method waste rock or tailings with varying acid generation and neutralizing potential are mixed together to create a waste deposit that generates a discharge of acceptable quality. For example, carbonate-containing tailings with neutralising capacity may be mixed with the acid-forming waste rock (Kauppila et al. 2013). This technique can be implemented, for instance, for backfilling open pit or underground mine workings.
Description of the method
Acid generation can be reduced by increasing the neutralising capacity of the waste. The availability of neutralizing material often limits the use of the method and the effectiveness depends on the general stoichiometric balance between acid neutralizing and acid producing materials, geochemical properties and material reactivity, flow pathways created within the deposit, and the extent of mixing in the blending process (INAP 2014).
From INAP 2009: “Operational experience indicates that, for effective blending of PAG rock with limestone, it is essential that all size fractions within the blend be at least acid-base neutral (i.e., NPR (ANC/MPA) of at least 1). Since the run of mine particle size of limestone is generally coarser than PAG rock, the acid base balance or NPR of the bulk waste rock needs to be greater than one. Experience with rock types at Freeport (Indonesia) and Ok Tedi (Papua New Guinea) indicates that well-mixed PAG and limestone rock needs to have a bulk NNP of more than 150 kg CaCO3/t (or net acid production potential (NAPP) of less than 150 kg H2SO4/t). The actual blend will depend on site factors and particle size distribution for each rock type”.
Reclaimed Flambeau Mine (Wisconsin department of natural resources 2013): The reclamation plan and mining permit of Flambeau Mine specified backfilling of the open-pit. The backfilling process involved blending the stockpiled waste rock with a prescribed amount of limestone, which was chosen because of its neutralization capacity to minimize the potential for the development of acid conditions prior to reflooding. The threat of acidification was largely eliminated after the pit was reflooded, indicating the intended effects of the blending were achieved.
Advantages (INAP 2009):
- Excellent pH control
- Limestone and lime usually fairly cheap, easy to get, and both safe and easy to handle
- Proven to work
Disadvantages (INAP 2009):
- Achieving effective neutralization is often a difficult task, as adequate mixing requires a large effort
- Cost of mixing and blending can be high
- Availability of materials can be limited
- Widely used limestone doesn’t have cementing properties and cannot be used as a barrier or low-permeability material by itself
Good results of blending have been gained by mixing waste rock with limestone using a conveyor system and stacker. Haul trucks have been used with limited success (Miller et al. 2006). To achieve maximum benefit the method requires homogeneous and thorough mixing (MEND 1998, MEND 2001).
Changes in material or in mixing ratio can oblige maintenance.
Broad waste characterisation is essential to constitute the demands of the blending proportions.
Requirements for the materials and appliances
Common additives at metal mines are limestone (CaCO3) and lime (CaO or Ca(OH)2). Liquid forms are considerably diluted relative to solid forms and have limited longevity, but may provide better penetration of the acid generating mine waste. Other neutralising alkaline materials are kiln dust, steel slags and different ashes. It is important that the alkaline material is not prone to leaching metal ions. Potential alkaline material should undergo thorough analysis prior to application to evaluate possible leaching and the resultant effects on drainage quality (INAP 2009).
Minimisation / treatment of potential discharges
Key issues for the effectiveness of blending are the pathways of water movement through the system, degree of mixing, and the nature of the contact between acidic rock and the alkaline materials. Also the type, purity, reactivity, availability, and proportion of the alkaline material are important considerations. Acid generation may occur if the mixture is not homogenous and/or there is not sufficient contact between acid and neutralizing materials (INAP 2009).
Monitoring / control needs
Ongoing or regular monitoring is needed to ensure the mixing method works and the proportions are as required.
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.
MEND 1998. Design Guide for the Subaqueous Disposal of Reactive Tailings in Constructed Impoundments. Mine Environment Neutral Drainage Program, Report 2.11.9, CANMET.
MEND 2001. Prevention and Control. Volume 4. Manual 5.4.2d. Tremblay G.A. & Hogan C.M. (Eds.), Mine Environment Neutral Drainage Program, CANMET.
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.
Wisconsin department of natural resources 2013. Reclaimed Flambeau Mine. http://dnr.wi.gov/topic/mines/flambeau.html Revised 12.11.2013. Read 17.7.2014.