6. OUTSTANDING ISSUES
6.2. Issues relating to the physical properties of tailings
The major root cause of catastrophic failure and ensuing dispersal of tailings material is the poor settling behaviour of tailings. Consequently, much research and technological development focuses on effective and cost efficient methods to dewater tailings either before or after impounding.
The physical properties are closely related to the chemical properties of tailings, as these typically constitute a finely dispersed, gel-like system.
(1) Research into the (site specific) dewatering behaviour of tailings with a view to improve dewatering is needed.
Table 6. A comparison of the advantages and disadvantages of the different approaches to uranium mill tailings containment [compiled by S. Needham].
Disposal option
Advantages Disadvantages Above ground > Can operate simultaneously with mining
> May be cheap to establish if tailings used in construction
> Valley fill sites may have low construction costs
> Whole tailings can be contained
> Tailings pond can also function as evaporative pan to assist in mine water management
> Most widely used
> Tailings easily accessed for reworking if required
> Authorities may regard this type as only temporary storage &
tailings may need to be relocated e.g. below ground level at end of mine life
> May require construction of associated structures to minimise risk of environmental impact in the case of failure, or to collect/treat seepage etc
> Seepage control essential
> Expensive if built as water containment structure
> Post close-out settlement may take a long time and lengthen period before operator can be released of responsibility
> May need long term maintenance
> Long term risk of tailings spill, increasing as structure weathers and erodes
> Increases land area impacted by mining
> Airborne and waterborne dispersal of contaminants possible following erosion etc
Below ground:
in pit
> Very long term containment possible
> Unlikely to ever require maintenance
> Whole tailings can be contained
> Pit preparation costs unlikely to be as high as above ground options
> Airborne dispersal of contaminants effectively impossible
> Structural failure of containment virtually impossible
> May need pervious-surround work to minimise ground water contamination risk
> Construction cost of impermeable containment could be high if suitable pit not available
> Not normally possible to operate simultaneously with mining at the same location
> Requires a suitable pit to be available pre-mining, or for all ore to extracted prior to milling (e.g. Nabarlek)
> May involve double-handing of tailings if no pit available at commencement
> Re-claiming of tailings if required for further treatment will be difficult owing to depth
Below ground:
underground mine workings
> Very long term containment possible
> Unlikely to ever require maintenance
> Can possibly incorporate whole tailings
> Can be operated simultaneously with mining
> Airborne dispersal of contaminants effectively impossible
> Structural failure of containment virtually impossible
> Slimes may need to be contained separately
> Need suitable groundwater conditions
> Mine waste water management system needs to be able to cope with evaporation requirements
> Tailings not available for reprocessing
Below ground:
> Very long term containment possible
> Unlikely to ever require maintenance
> Whole tailings can be contained
> Can be operated simultaneously with mining
> Airborne dispersal of contaminants effectively impossible
> Structural failure of containment virtually impossible
> Site can be selected in low-permeability country rock
> Benign rock available for unrestricted use in construction
> Construction required before milling commences
> Mine waste water management system needs to be able to cope with evaporation requirements
> Suitable site may be remote from mill and increase slurry/paste transport & infrastructure costs
> Paste stabilization normally necessary for underground and optional/preferable for pit.
Deep lake > Can operate simultaneously with mining
> Cheap to establish
> Whole tailings can be contained
> Very long term containment possible
> Unlikely to ever require maintenance
> Whole tailings can be contained
> Airborne dispersal of contaminants effectively impossible
> Structural failure of containment virtually impossible
> Authorities may not allow this approach to tailings disposal
> Requires nearby water body not otherwise used for social or economic benefit (i.e. fishery, water supply, recreation)
> Risk of water contamination and tailings redistribution from disturbance by major flood of changed climatic conditions
Table 7. A comparison of the advantages and disadvantages of methods for stabilizing and isolating uranium mill tailings [compiled by S. Needham].
Methods Advantages Disadvantages
Containment preparation:
Low-permeability membrane High short-medium term security against seepage; easily applied to floor and walls of surface containments.
High cost; prone to accidental damage; difficult to repair after tailings discharge commenced; unknown long term performance; application limited to above ground containments; cannot be retrofitted.
Clay seal Permeability decreases with overloading;
low cost if local material available.
High cost if local materials unavailable; application limited to above ground and in-pit containments; cannot be retrofitted.
Grout Targets known weak zones; ease of
application.
Misses unknown zones of weakness; grouting compound may degrade on interaction with tailings pore water, cannot be retrofitted; possible high drilling costs involving specialist equipment
Permeable surround Potential long term high security around entire containment against groundwater contamination; low maintenance.
Suitable high-permeability material needed; may clog;
application limited to below ground containments;
cannot be retrofitted. High cost (but distributed though operational phase)
Under-drain/basal filter bed Aids settlement. May clog; cannot be retrofitted; water disposal must be accounted for
Tailings preparation:
Neutralization Reduces acid producing potential and mobility of U and other heavy metals.
Availability and cost may affect viability Thickening Reduces water content of tailings; useful in
low evaporation regions to achieve water loss.
Increased pumping costs; or alternative placement techniques.
Paste Allows addition of compounds to
significantly improve chemical and physical stability of tailings pile and containment.
Present use limited to underground containments;
longevity unknown; tailings probably not recoverable.
[note: area of considerable development potential, research results discussed in Chapter 7].
Cycloning High stability product; provides sand-grade material for construction; removes main contaminants.
Slimes disposal
Tailings discharge and deposition
Slurry beaching Increase stability of embankment walls;
easier ponding and collection of decant water.
Not applicable underground
Thickened tails placement Improved immediate surface stability and access; improved control of placement (e.g.
layering)
Higher transport costs (pumping or trucking)
Dry cake placement High immediate surface stability. Higher transport costs (ie trucking).
Barrier/reactive layers High control over design and placement;
permits tailoring to address variations within tailings pile etc; potential wide range of treatments available.
Suits only paste or dry cake deposition of tailings; high placement costs (pumping or trucking), largely untested technology [note: area of considerable development potential, research results discussed in Chapter 7].
Tailings dewatering at discharge:
Thickened tailings Reduce water content problems, especially in low evaporation areas. Reduced use of clean water.
Higher transport costs (pumping or trucking) compared to slurry
Paste tailings Reduce water content problems, especially in low evaporation areas; allows addition of chemical and physical stabilisers.
Long term performance and effects on diagenesis etc unknown
Dry cake Removes water removal and stability
problems, significant reduction in leaching and groundwater contamination risks;
potential for seismically active, cold, arid regions. Reduced use of clean water.
Untested in uranium mill tailings
Tailings dewatering in-situ:
Under-drains Improves settlement density, reduces instability and chemical contamination.
Cannot be retrofitted; may clog.
Horizontal drains High improvement to settlement density, reduces instability and chemical contamination; design may be modified during operation.
Cannot be retrofitted; may clog.
Wells/jet pumps/electro-osmosis
Improves settlement density, reduces instability and chemical contamination; can be targeted to specific sites. Suitable for ex-post treatment.
May require closely spaced grid of wells with attendant higher costs (see also Table 4).
Methods Advantages Disadvantages Evapo-transpiration Some improvement to settlement density,
reduces instability, inexpensive.
Restricts access to tailings surface; of limited benefit once supernatant liquid evaporated unless used in conjunction with wells or wicks; no removal of contaminant load.
Sand drains/wicks Some improvement to settlement density, reduces instability, inexpensive and low maintenance. Suitable for ex-post treatment.
May take many years to reach dewatering capacity.
Surface loading speeds it up.
Tailings surface treatment
Water cover Limits radon flux, dust Negates dewatering as an option to improve settlement density. Must be removed and followed by
dewatering/settlement works prior to remediation.
Wetting Reduces dust and radon flux; assists in evaporative water loss.
Components need regular inspection, maintenance, and switching/moving.
Sealants Reduces dust, infiltration, improves access to tailings surface; good temporary protection between depositional or remedial stages.
Suitable only for short term protection.
Decant water treatment
Recirculation to mill Reduces clean water uptake, low cost, suits no-release criteria.
Possible build-up of agents may decrease mill circuit efficiency.
Treat and release Contains contaminants within the mine system.
Public perception of uncontrolled environmental harm;
probable environmental impacts in event of system failure.
Seepage control
Seepage detectors Provides early warning and allows early
intervention in event of significant seepage Costly; in themselves do not treat the problem; difficult and expensive to retrofit.
Collector wells Targets known confined seepage zones. Limited sphere of influence; unlikely to collect seepage from unknown/new seepage points; requires monitoring wells to validate performance.
Interception drains Minimises transfer of contaminants to groundwater or surface water systems; can extend around perimeter to provide high levels of assurance; essential backup for all operating tailings facilities.
Requires monitoring wells to validate performance.
Return to containment As for decant water treatment Essential Treat and release As for decant water treatment Essential Recirculate to mill As for decant water treatment Essential Groundwater monitoring Provides essential information for effective
water management during operational phase
Not practical for long term management/stewardship post closure
Capping
Engineered design High security short term (<1000 years);
well-known technology; extensive experience, guidelines, geotechnical expertise.
Unknown long term (1000–50000 years) performance;
probable high future maintenance costs; incompatible with natural processes, forces and systems; poor aesthetics.
Ecological design Caters for/conforms with natural processes;
enhances prospects for long term security against environmental harm; probable low future maintenance costs; good aesthetics
Unknown technology and costs; possible long lead time for research to deliver sufficient site specific knowledge to factor into design.