Co-disposal of waste rock with tailings

Henna Punkkinen, Markku Juvankoski, Tommi Kaartinen, Jutta Laine-Ylijoki, Elina Merta, Ulla-Maija Mroueh, Jarno Mäkinen, Emma Niemeläinen & Margareta Wahlström, VTT Technical Research Centre of Finland Ltd, P.O. Box 1000, FI-02044 VTT, Finland.

Introduction

Tailings and waste rocks are usually disposed of separately. In co-disposal, however, the tailings are combined with the waste rocks. The concept has been gaining lots of attention as it is said to offer many advantages over conventional methods. (Wickland & Wilson 2005, Wickland et al. 2006)

Description of the methodology

Co-disposal increases physical and chemical stability of the waste. Mixing/combining of coarse waste rock and fine-grained tailings increases the physical stability and the risk for liquefaction diminishes (Wickland et al. 2006, Gowan et al. 2010). In addition, moisture content of the tailings is usually higher and thus limits the ingress of oxygen and water (increases the chemical stability) compared to the conventional waste rock deposits, which can reduce the potential for acid generation (E.g. Wickland & Wilson 2005, Wickland et al. 2006, INAP 2009).

Several alternative ways to accomplish co-disposal exist. The methods can be divided in to different categories according to the degree of mixing and placement method. (Wickland & Wilson 2005, Wickland et al. 2006) (Figure 1). In addition to methods presented in Figure 1, also many other methods are described in the literature (e.g. Leduc & Smith 2003, Leduc et al. 2004, Habte & Bocking 2012).

Figure 1. Different co-disposal methods (Adapted from Wickland & Wilson 2005, Wickland et al. 2006).

Pumped co-disposal and paste rock methods can result in saturated conditions and are then capable to prevent acid mine drainage via reduced oxidation. The other methods described in the Figure 1 are heterogeneous in their nature and thus waste rock may remain unsaturated. (Wickland & Wilson 2005) However, according to INAP (2009), also layered co-mingling of waste rock and thickened tailings can limit the acid generation potential. Alkaline tailings can also be mixed with potentially acid generating waste rocks (Leduc el al. 2004), or tailings can be amended with alkaline material to increase chemical stability of co-disposal (INAP 2009).

Several case studies can be found from the literature. For example, case studies are presented by:

Habte & Bocking (2012)

Leduc & Smith (2003)

Leduc et al. (2004)

Appropriate applications

If waste rock is characterized as potentially acid generating, co-disposal may be an option worth considering. In the opposite situation where waste rock is non-acid producing (neutral or alkaline) and tailings have the potential to produce acid, layered co-deposition should not be used because the layers of waste rock may ease the oxygen ingress to the tailings. (Habte & Bocking 2012) In addition, according to Leduc et al. (2004), the co-disposal concept may not be suitable for sandy and clayey wastes due to their grain size distribution and available void ratios.

Some important advantages and disadvantages/challenges linked to the different co-disposal methods are summed up in Table 1.

Table 1. Advantages and disadvantages/challenges of co-disposal compared to the conventional (separate) disposal methods (e.g. Leduc & Smith 2003, Leduc et al. 2004, Wickland & Wilson 2005, Wickland et al. 2006, INAP 2009, Habte & Bocking 2012).

Advantages	Disadvantages/Challenges
Increases chemical stability → Reduces acid generation potential of waste rock	Mixing can be energy-consuming and expensive
Increases physical stability (shear strength) → Diminishes risk of liquefaction	Production schedule and sequencing of waste
Tailings dam may not be needed, minimization of footprint The dam construction and maintenance costs are avoided Saves space Reduces the size of the area needed for reclamation Reduces land disturbance Reduces closure costs, post-closure maintenance and environmental impacts	Defining and maintaining optimal mixing ratio/strip ratio of coarse and fine wastes
May have better public acceptance	Pumped co-disposal requires lot of maintenance, energy etc.
Retention embankments are not required → Removes the risk of breach and transportation of tailings outside the deposition area	Mixing and placement/spreading methods with respect to maximum particle size
Potential use as cover material	Restricts re-mining of tailings in the future
Formation of an elevated water table within the waste deposit	Ratio of potentially-acid-generating and non-acid generating wastes
Reduces dusting and erodability of tailings	Gap graded mixtures may be erodible
Reduces water consumption, possibly lower water loss, simplifies water management
Less impact on surrounding waters, agricultural uses and wildlife habitat
Forms better substrate for vegetation
Rock layers enhance consolidation of tailings and facilitate earlier closure

The behaviour and geotechnical properties of waste rock and tailings are very different mostly due to their different particle sizes. The mutual ratio of waste rock and tailings affects to the properties and behaviour of the mixture and its suitability for co-disposal (Table 2). The success of e.g. the co-disposal as homogenous mixtures (good physical and chemical stabilities) depends on the mixture design (Wickland et al. 2006).

Table 2. Suitability for co-disposal in relation to waste rock:tailings ratio (Leduc & Smith 2003).

Waste rock:tailings ratio	Suitability
More than 8:1	Probably suitable
Between 4:1 and 8:1	Significant testing and detailed analysis needed to determine suitability
Less than 4:1	Probably not suitable

If the ratio is less than 4:1, the strength characteristics change and tailings characteristics begin to affect to the behaviour of coarse waste rock (Leduc et al. 2004, Gowan et al. 2010). It also becomes more difficult to handle these types of mixtures (Gowan et al. 2010).

Performance

According to Leduc & Smith (2003) the estimated total costs (incl. capital and operating) for certain co-disposal methods presented in their paper vary between $0.15 and $0.5 per tonne of tailings. (Note: the methods are partly different than those presented in Figure 1).

Design requirements

The selection of the suitable co-disposal method depends on the following aspects (Habte & Bocking 2012):

Tailings and waste rock characteristics
Maximum waste rock size (coarse wastes are not suitable for pumping)
Mass ratio of waste rock to tailings
Topography of the mine site
Climate
Costs for the whole life cycle
Easiness of construction and operation
Potentiality of the method to minimize risks (erosion, stability, liquefaction, acid generation, etc.) (Habte & Bocking 2012)

The evaluation of tailings and waste rock characteristics includes the analysis of the following geotechnical and geochemical properties (Leduc & Smith 2003):

Different types of shear strength tests
Different types of permeability tests
Grain size distribution (waste rock)
Tailings gradation and consolidation
Chemical leaching tests
Whole rock chemistry
Acid-base accounting (ABA)
Blend ratios and drained moisture content (tailings)
Permeability (various simulated heap depths for different blend ratios)
Unconfined compressive strength (mix of tailings and cement/other binder). (Leduc & Smith 2003)

After these screening level tests, and additional and more complex tests are performed for more detailed design (Leduc & Smith 2003).

References

Gowan, M., Lee, M. & Williams, D.J. 2010. Co-disposal techniques that may mitigate risks associated with storage and management of potentially acid generating wastes. In: Fourie, A.B & Jewell, R.J. (Eds.), Mine Waste 2010. Australian Centre for Geomechanics, Perth, Australia. ISBN 978-0-9806154-2-5.

Habte, K. & Bocking, K. 2012. Co-disposal Practices in Mine Waste Management. Technical Memorandum, February 8, 2012.

INAP 2009. The GARD Guide. The Global Acid Rock Drainage Guide. The International Network for Acid Prevention (INAP). http://www.gardguide.com

Leduc, M. & Smith, M.E. 2003. Tailings Co-Disposal^TM. Innovations for Cost Savings and Liability Reduction. The Latin America Mining Record, July/Aug. 2003.

Leduc, M. Bachens, M, & Smith, M.E. 2004. Tailings Co-Disposal^TM & Sustainable Development. Proceedings of the annual meeting of SME, Denver, February, 2004.

Wickland, B. & Wilson, G.W. 2005. Research of Co-Disposal of Tailings and Waste Rock. Geotechnical News, September 2005, 35-38.

Wickland, B.E., Wilson, G.W., Wijewickreme, D. & Klein, B. 2006. Design and evaluation of mixtures of mine waste rock and tailings. Can. Geotech. J. 43, 928-945.