Easy Guide to Mining Waste Characterisation
Waste characterisation is the study of the environmental impacts of mining waste during mining, processing and above ground storage of ore prior to treatment. Waste characterisation needs to consider whether the waste will cause:
- Acid & metalliferous drainage (AMD)
- Saline and/or sodic drainage (i.e. neutral mine drainage)
- Leaching and mobilisation of metals and toxic compounds
Once exploration resulting in the discovery and definition of an ore body has been completed, planning for managing potential mine wastes needs to commence. This has implications for:
- Mine water management
- Waste landform design & emplacement
- Tailings dam and heap leach pad emplacement & design
- Costing and planning for progressive rehabilitation, decommissioning and closure
Waste Rock Characterisation
Ore is divided into oxidised (above the water table); transitional (within the zone occasional inundated by groundwater); and primary (below the water table at all times). This is usually defined by geologists, as it impacts on metallurgy. Waste rock associated with mining ore for precious and base metals, coal, mineral sands and certain gems is often associated with sulfide minerals. Sulfide minerals consist of components of sulfur and metals such as arsenic, copper, lead, selenium etc. The most common sulfide mineral is pyrite. Most sulfide minerals when dug up and exposed to oxidising conditions, in the presence of water, degrade to produce sulfuric acid and dissolved metals.
If the waste rock produces acid at a faster rate than the inherent buffering capacity of the waste rock, it must be determined if the acid production rate is greater than that of buffering capacity available from other sources. Answering this question will decide whether the waste rock can be:
- randomly or selectively placed in the waste dump
- disposed underground (i.e. into stopes)
- or underwater (i.e. co-disposed with tailings or in pit lakes, or sub-aqueous workings)
Additionally, if there is the release of toxicants at harmful concentrations, selective placement will be required to minimise the release or adsorb that which has been released. If the waste rock is classified as a contaminating material, its use will be limited and will have certain statutory requirements.
Adverse Effects Produced by Waste Rock
Many coal deposits and associated overburden waste rock contain saline and sodic connate waters that are released during weathering, or washed out by rainfall to become a source of salinity on the neighbouring land.
In the special instances that the waste rock contains reactive clays or rock minerals that weather to produce reactive clays, and/or the neighbouring land contains these minerals, sodic producing waste needs to be carefully handled.
Sodic water or sodicity producing materials that contain reactive clays may result in tunnel erosion under rainfall and cause a high sediment load in runoff. If this is occurring, there are a number of questions you need to ask:
- Does your waste rock have, or is it capable of producing reactive clays?
- Will your waste rock produce saline or sodic drainage?
- If the drainage is sodic, does the surrounding land contain reactive clays?
- Will saline/sodic runoff or high sediment load in the runoff cause an adverse effect locally during and post mining?
- Will the release of sodicity, salinity and dispersion during weathering impede vegetative growth on the stockpiles?
Waste Characterisation of Tailings and Heap Leach Ore
The residues from processing of metalliferous ore or coal, and their long term impacts on the environment also need to be considered when assessing potential impacts from mining waste. The residue from processing of ores or coal is known as tailings. Tailings may be disposed of as a composite slurry or have sands and fines disposed separately. The coal industry has a gravel fraction known as chitters. Tailings are usually pumped into containment structures to settle and the water recovered for reuse or decanted to an evaporation pond.
Another substantial residue waste from ore processing is Spent Heap Leach Ore, produced mostly from gold and copper processing. The same questions that are addressed by characterisation of waste rock have to be also addressed on these waste materials: will the waste produce an adverse (acid, toxicant, salt, sodic, sediment) effect on the environment now or in the future, can it be contained, and can it be revegetated? In addition waste characterisation of these materials is expected to answer two additional questions:
- Does the ore contain reactive/dispersive clays?
Koalinite and iron minerals can absorb cyanide increasing processing costs. Similarly if the ore contains reactive clays, dispersed tailings do not settle readily and water recovery from the tailings is greatly decreased. Thus, cyanide concentration in the tailings is greater and less is recovered. This has both cost and environmental implications.
- Does the ore contain other metals, such as copper?
Many metals form complexes with cyanide that compete with gold and precious metals and consume cyanide. Many metals such as zinc and cadmium form weak cyanide complexes and decay rapidly; however some metals, such as copper, form moderately strong complexes and are favoured by cyanide which consequently take a long time to break down. Iron, cobalt and gold form insoluble cyanide complexes and are very stable in the absence of light (UV). The presence of copper and the speed at which decay occurs affects whether treatment is required prior to discharge of tailings and/or the size of tailings storage required.
Spent Heap Leach Ore
Heap leaching is most commonly associated with the extraction of gold, while certain copper, uranium and nickel ore bodies are amenable to heap leaching. The ore is crushed to typically less than 25 mm and placed on a pad and a leaching reagent is sprayed onto the ore.
The leachate is collected and processed. The leachant solution is normally a dilute sodium cyanide solution buffered with sodium hydroxide – clearly a sodic solution. Typically, oxide or transitional ores are heap leached. Oxide ores have the greatest potential to have clays prone to dispersion and tunnel erosion. Once dispersion of the ore occurs on the pad, this impedes the leaching process and reduces metal recovery and solution recycling. It also results in the drains being blocked with sediment and water and sediment flowing onto the adjoining land. Thus it is important to know whether the ore disperse, slake or could develop tunnel erosion.
Design of Waste Landforms – Integrating Waste Geochemistry
Our understanding of the behavior of mine waste landforms – specifically out-of-pit waste rock dumps and tailings dams – is constantly evolving. The design and placement of mine wastes is increasingly considering the impact of the waste geochemistry, as it impacts environmental performance during operation (particularly compliance), landform stability, and final rehabilitation and relinquishment.
By reconciling the types of wastes (i.e. potentially acid forming, acid consuming, barren, saline-sodic, etc) with the desired waste landform shape and dimensions, we can integrate the waste planning, scheduling and management with mine engineers, planners, geologists and environment staff to achieve the ultimate goal: a chemically and structurally stable waste landform that can be rehabilitated to meet regulatory requirements and fit with the surrounding landscapes.
Mine Waste and Water Monitoring Implications
Tailings storages, treatment ponds, waste rock stockpiles and stormwater runoff ponds, no matter how well constructed, will leak. If they contain or could solubilise toxicants, groundwater monitoring should be undertaken.
Groundwater monitoring will also be required as a licence condition. Internationally recognised standards for environmental management (i.e. ESMS and ISO 14000) require planning and practice for emergency and incident responses. Groundwater monitoring is a way of measuring an environmental incident in progress.
Thus, to be effective, groundwater monitoring needs to be based on a site specific conceptual model of formation of leachate and its migration through the vadose zone and aquifer. The model should predict the interaction of the leachate and natural environment, and provide early warning indicators and timing of leachate breakthrough. Constituents monitored should be those that provide early warning as well as those required by regulators. The nature and type of breakthrough should define the incident category and nature of response.
Questions that should be addressed include:
- What is the predicted leachate quality?
- What is the quality of the receiving groundwater?
- How will the leachate react with the soil/rock and groundwater?
- Are the monitoring bores correctly located and correctly constructed to detect a leachate front?
- Are the screen slots located entirely within one aquifer or one stratum?
- What chemical constituents are the precursors of the front?
- Have standpipes been located to detect potential perched water arising from seepage from the tailings storage etc?
- Is the monitoring program designed to confirm the conceptual model?
- What is the incident response program – is it achievable?