mine closure
DESCRIPTION
Mine closureTRANSCRIPT
MINE CLOSURE:
GENERAL METHODS OF
OPEN-PIT MINE CLOSURE
ISABEL BARRERO BRAVO
MARÍA QUEVEDO ALVARÉ
MARÍA VICTORIA RIESGO GARCÍA
WHAT IS MINE CLOSURE?
Mine closure refers to the final stages of mining activity, after production
and processing have permanently ceased and any subsequent activities that are
directly related to shutdown of the mine.
A distinction must be made between such permanent cessation of mining and
temporary winding down of production, , when the site is placed on care and
maintenance, in the expectation that operations will resume.
Mine closure is thus defined as the process, where mine production operations
finally cease and the mine owner commences decommissioning the site
infrastructure and relinquishes rights to the mining concession.
OBJECTIVES AND PERFORMANCE CRITERIA
FOR THE CLOSURE PROCESS
In the following sections, closure criteria and procedures are outlined with
respect to general safety and enviromental management issues, as well as
social and economic considerations.
1) General safety criteria
2) Closure and remediation objectives in relation to environmental quality
3) Social and economic objectives in relation to mine closure
GENERAL SAFETY CRITERIA
Compliance with basic safety guidelines requires:
1) That all structures remaining at the site are physically and chemically stable
over the long-term.
2) That any other legacy of mining operations, which might pose a safety risk,
are either removed from the site, or rendered permanently inaccessible. Safety
criteria are defined so as to ensure that risk mitigation procedures are
sufficiently robust to cope with all conceivable geological and climatic
contingencies.
GENERAL SAFETY CRITERIA
Therefore, closure strategies need to anticipate the likelihood of:
-Extreme natural events (including intense rainfall, floods, storms, drought,
wildfires, landslides and avalanches, and in some countries, volcanic eruptions
and earthquakes).
-Potential hazards of geological origin, such as erosion, collapse and
subsidence and landslides.
-Environmental deterioration due to cumulative effects of successive floods or
erosion over time.
-Effects of climatic changes, such as variations in the frequency and intensity
of extreme precipitation events and floods.
-Progressive deterioration in quality and loss of integrity of materials used in
site management and remediation, including wooden, concrete and steel
structures as well as clay liners and membranes and rock fill, as a
consequence of weathering, or successive freezing.
GENERAL SAFETY CRITERIA
Planning and assessment of long-term stability measures is achieved
through the use of various indices based on likely risks associated with
particular features. Relevant risk categories for each specific structure or
procedure can be defined according to their potential impact on both humans
and the surrounding environment.
Examples of the kinds of indicator used for monitoring physical and
chemical stability and safety risk are as follows:
-Slope stability of permanent retention barriers and embankments and waste
dumps.
-Safety index for specific use in embankment stability calculations.
-Capacity of drainage systems.
-Peak flood capacity of dams and embankments, drainage channels, settling
ponds and water treatment facilities.
-Longevity and stability of building materials over time.
CLOSURE AND REMEDIATION OBJECTIVES IN
RELATION TO ENVIRONMENTAL QUALITY
The closure process is essentially aimed at restoring the surrounding environment to a state, resembling as closely as possible that which existed prior to the comencement of mining, as measured by both chemical and biological parameters.
An additional requirement is that the site is secure for any planned future land use. Therefore, it is necessary to ensure that neither discharge from the mine area, nor harmful substances remaining at the site soil, present any long-term risk to the environment or human health.
A further objective is to ensure that environmental restoration is adequate to allow the establishment of a diverse and functional ecosystem in the area.
Landscaping should also be carried out in a way that is appropriate for any future land use activities, planned for the area.
CLOSURE AND REMEDIATION OBJECTIVES IN
RELATION TO ENVIRONMENTAL QUALITY
After closure, the mine site is landscaped in an attempt to integrate it with the surrounding environment. The former tailings dump at the Zinkgruvan mine in Sweden, which is located adjacent to a lake, has been covered and landscaped to form a golf course and recreation area.
CLOSURE AND REMEDIATION OBJECTIVES IN
RELATION TO ENVIRONMENTAL QUALITY
Deterioration of surface waters and groundwater quality at mine sites can be
the result of contamination from tailings and waste rock areas and also from
heaps of concentrate, storage of chemicals on site and from accidental spillage
in the course of routine transport and maintenance at the mine.
To stabilize the site chemically, the first priority is to contain, or if possible,
remove the source of contamination.
In the case of contaminated soil or chemicals, the most cost-effective remedy
is to physically remove the affected material.
At sulphide mines however, where discharge of acidic and metal containing
seepage waters from tailings may occur, remediation measures are usually
more complicated, requiring either earthworks and drainage to channel
impacted waters to an appropriate site for further treatment.
CLOSURE AND REMEDIATION OBJECTIVES IN
RELATION TO ENVIRONMENTAL QUALITY
Assessment of quality of mine surface waters and groundwater is usually
based on measurement of metal abundances at discharge and outflow sites,
preferably before the water enters the surrounding watershed.
Monitoring of surrounding groundwater and surface waters at specifically
designated sites may also be carried out, including pH, electrical conductivity
and alkalinity measurements in addition to metal concentrations.
Possible contamination risks from tailings and waste rock areas can be
anticipated by measuring a number of parameters, including acid generation
potential, chemistry and mineralogy of the materials, and by conducting
leaching tests at the laboratory for selected metals.
CLOSURE AND REMEDIATION OBJECTIVES IN
RELATION TO ENVIRONMENTAL QUALITY
The ability to develop and maintain an ecologically functional environment
suited to the requirements of animal life as well as humans can be assessed
using indicators based on:
• Soil chemistry.
• Physical properties of the soil and regolith.
• Availability and quality of surficial and groundwaters.
• Efficiency of water supply and drainage systems.
The possibility of climatic changes or extreme weather events also needs to be
taken into consideration, as variations in physical and chemical parameters due
to changing climatic conditions could potentially lead to an increase in metal
solubility and transport in the mine environment.
CLOSURE AND REMEDIATION OBJECTIVES IN
RELATION TO ENVIRONMENTAL QUALITY
Desirable attributes of the landscaped environment following site remediation
include:
• General neatness.
•Landforms in harmony with surroundings and, where possible, pre-mining
landscape.
• Revegetation programs appropriate to future land use options.
• Aesthetically pleasing and appropriate site for designated land use purpose.
SOCIAL AND ECONOMIC
OBJECTIVES IN RELATION TO MINE CLOSURE
Specific and tangible social and economic objectives are more difficult to define within the context of mine closure than general safety and environmental issues.
This is principally because there are no binding regulations covering legal obligations to society on behalf of the mining operator.
However, the BAT reference document on management of mine tailings and waste rocks does make general reference to the socio-economic implications of mine closure, with respect to impact on employment and the ongoing viability of business and commerce in the adjacent area.
Therefore, minimization of potential adverse impacts on the local economy and consideration of long-term requirements of surrounding communities may be defined as general socio-economic objectives in relation to mine closure.
MINE CLOSURE STRATEGIES: OPEN-PIT WORKINGS
The legacy of open-cut mining is, depending on the nature and scale of operations, a pit of variable size and depth to width ratio.
As for underground workings, closure strategies for open-cut workings require that the area meets the legal requirements for general public safety.
This means in effect that public access to the open pit is restricted and unstable slopes surrounding the pit are landscaped and stabilized.
Steep or vertical pit walls should be made safe and they should be modified to gentler slopes where they remain above estimated final water level, or if they are susceptible to failure and slip.
Public access to the site is restricted by erecting a safety fence around the open pit, with clearly visible warning signs, in accordance with the Ministry of Trade and Industry decision.
MINE CLOSURE STRATEGIES: OPEN-PIT WORKINGS
Relevant factors to consider in relation to fencing include the location of mine, stability and quality of pit edges, equilibrium water levels and fluctuation range, and the suitability of the water for use by humans and animals.
Positioning of the fence is largely determined by identifying the limits of ground potentially subject to subsidence or collapse.
An additional measure for preventing public access is to close all roads leading to the mine.
However, possible use of the road network by other business activities in the region needs to be taken into consideration.
Landscape restoration around open pits should anticipate potential erosion and subsidence risk and likely variations in level of water table after mining and pumping has ceased.
MINE CLOSURE STRATEGIES: OPEN-PIT WORKINGS
Lahnaslampi open-cut talc mine, Sotkamo, Finland. Waste rock and magnesite
tailings from the new mine scheduled to open nearby will be transported to the
pit after mining operations have ceased.
MINE CLOSURE STRATEGIES: OPEN-PIT WORKINGS
In more vulnerable areas, erosion can be minimized by replanting appropriate vegetation, which has the additional advantage of enhancing aesthetic and ecological values at the same time.
In some cases, natural regeneration is an acceptable strategy. Another important aspect of landscaping is stabilization of pit edges and rock faces, to prevent collapse.
Waste rock may be used for landscape restoration and stabilization, as well as for backfilling the open pit, so long as it is environmentally non-reactive. In some cases, usage of waste rocks may require environmental permitting.
Similar to underground mining, closure strategies for open pits require the removal of any materials that could conceivably cause contamination of surface and groundwaters or soil, as defined under the Environmental Protection Act.
MINE CLOSURE STRATEGIES: OPEN-PIT WORKINGS
As long as dismantling does not cause any safety hazards, all steel structures,
pipes and hoses, and electrical and communications cables and wiring should
be removed from the site.
Concrete and wooden structures can normally be left untouched, if there is no
evident risk to the surrounding environment.
Any other materials stored around the open pit should also be removed.
EXAMPLE: Current practice in Finland is that open-cut mines and
surrounding areas affected by mining are designated as special areas within
municipal building code registers.
MINE CLOSURE STRATEGIES: OPEN-PIT WORKINGS
This means that in addition to the normal building application process through
local municipal authorities, approval needs to be sought from the current
holder of the mining rights to the area or, if mining rights have lapsed, from
the last operator recorded in the mining register.
A working group appointed to review the Mining Act has also recommended
that municipal government authorities consult with mining authorities
whenever construction or other activities are being planned in former mining
areas.
Delineation of the area, formally classified as affected by mining activities, is
usually accomplished during final technical inspections by the Safety
Technology Authority.
MINE CLOSURE STRATEGIES: OPEN-PIT WORKINGS
It is increasingly common for ownership of the mining lease to be retained by
the operator after formal mine closure, particularly if the land was actually
purchased during mining operations.
If it were necessary to pump during mining operations, then after closure, the
pit will normally fill with water, due to precipitation and surface runoff, as well
as groundwater infiltration.
Eventually the open pit may come to resemble a natural lake, especially after
revegetation.
Monitoring of water quality is however, advisable to ensure that there is no
ongoing risk of contamination.
MINE CLOSURE STRATEGIES: OPEN-PIT WORKINGS
For example, where mining of sulphide ores has occurred there is a greater
possibility for contamination of surrounding groundwaters or surface runoff
with acidic mine-influenced water containing elevated metal and sulphate
abundances.
Hydrogeological and geochemical modelling can be used to provide predictive
estimates of pit filling rate, and evolution of water chemistry of the final pit
lake.
If the results of such studies indicate that adverse effects on surrounding water
quality are likely, preventative measures, such as treatment of the pit water or
sealing structures for overflow, are required under the Environmental
Protection Act to ensure that there is no risk of overflow or leakage of water
from the mine site.
MINE CLOSURE STRATEGIES: OPEN-PIT WORKINGS
When mining ceases and pumping stops, water progressively fills the open pit.
Various options are available for water treatment, if necessary. At the former
Hammaslahti Cu-Zn open pit, shown here, water chemistry is modulated by
bacterial sulphate reduction, with addition of farm manure as one mechanism
for promoting bacterial activity.
BIBLIOGRAPHY
[1] ENVIRONMENTAL TECHNIQUES FOR THE EXTRACTIVE INDUSTRIES (P. M. Heikkinen (ed.), P. Noras (ed.), and R. Salminen (ed.), GTKU.-M. Mroueh…)
[2] MINING FOR CLOSURE: Sustainable Mine Practices, Rehabilitation and Integrated Mine Closure Planning (Benjamin Smith)
[3] IMPACTO AMBIENTAL MINERO. RESTAURACIÓN (Catedrático Javier Toraño Alvarez Prof. Ayud. Dr. Susana Torno Lougedo)
[4] INFOMINE E-BOOK: MINE CLOSURE (Dr. A. Robertson & S. Shaw)