Ritva Muhlbauer and Nikki Fisher, Anglo American Coal.
Mining is a water intensive industry and coal is often found in regions of the world where water is scarce. Growing public concern about the scarcity of water resources poses a threat to the social license to operate for many mines. Restricted availability in water-limited regions requires mining companies to view water as a strategically important, valuable, non-renewable resource and to practice responsible and sustainable water management.
Reliable access to water is essential for coal mining operations. Limited water availability poses a significant risk to the industry, both in the short term, as well as for long-term sustainability, yet historically, the economic, social and environmental value of water was underestimated. Water supply shortages impact on the viability of current and future operations, adding to project and operational costs and risk. Consequently, the value of water must be integrated into business processes and decisions.
The largest costs associated with water are those associated with treating excess mine-affected water to meet discharge quality during operations, at closure and post-closure. Sound planning and water management practices ensure that the impacts on water quality are minimised and mines can secure water without compromising the water resource for other users.
Proactive water management
Anglo American is focusing on understanding the value of water to its business, both financially and in the broader sense. Water management cannot be limited to the boundaries of mining operations. Stakeholders and processes that occur outside of the direct control of the operations must be considered, as this can have significant implications on how a mine operates within the geographical, environmental, legal and social frameworks.
Anglo American believes that the optimal stewardship of water resources, improvement of water and energy efficiencies and management throughout the business framework must be seen as a strategic advantage for any mining company. A core focus of the Anglo American water strategy is to obtain water resilience in all of its operations.
The pillars of water resilience typically include combinations of the following: secure water supply, efficient use of water, prevention of water pollution and reduced post-closure liabilities, legal compliance, investing in water treatment and technology innovation, good stakeholder relationships and minimal risk of interruption at operations (excess or shortage).
Begin with the end in mind
Central to mine water management is the application of the Hierarchy of Water Management, the basic premise of which is to anticipate and prevent rather than assess and repair. The first priority should be to prevent or avoid impact on water through mine design and layout. For example, well-designed storm water management infrastructure with clean and impacted water separation will prevent the contamination of storm-water run-off, thereby reducing the mine water make. The greatest impact on mine water management can be made during the pre-feasibility stage by pro-actively including it in mine planning and design.
Geohydrological and geochemical studies during the project development stage should be sufficiently in-depth to inform mine designs. Geophysical, structural, geohydrological and geochemical studies can be used to establish initial conceptual site models to predict the early consequences of mining. Variability in climate, operations and mine designs need to be considered to ensure water impacts are mitigated.
Reducing liability costs
Governments internationally require financial assurance for mine closure to ensure that mines do not leave a negative legacy. Financial considerations need to be given for water treatment; therefore, minimisation of the impact on the water resource will ensure resource exploitation remains financially feasible. The costs associated with treating mine-affected water are high and a pre-requisite for obtaining government issued closure certificates is that sustainable and long-term control measures are in place to mitigate environmental contamination, which includes mine impacted water.
Appropriate mitigation of closure liability costs can be achieved through a number of other pro-active interventions, such as improved rehabilitation to reduce the volumes and load of impacted water and the application of innovative treatment methods.
The role of rehabilitation
Effective rehabilitation plays an important role in the reduction of rainfall recharge in mines. Recharge to rehabilitated spoils in an opencast mine in South Africa is estimated to be around 15 – 25% (compared to about 3% in undisturbed conditions). A pit that is badly rehabilitated, where all rainfall would run-off towards the spoils may expect a recharge of between 20% and 25%. A very well rehabilitated pit with free draining topography from the spoils can expect a recharge of 10%. Areas that have not been rehabilitated at all, i.e. backlog areas, typically have much higher recharge values, resulting in increased water ingress into the pits. This subsequently increases the risk of decant and water quality impacts, as well as increased pumping requirements. This leads to increased energy consumption and operations costs.
Capping and vegetation cover are important ingress control tools and are based on the engineered use of plants and their associated microorganisms for hydraulic control and environmental clean-up. Improved vegetation cover as a result of good quality rehabilitation will reduce groundwater recharge and migration through spoils material as a result of water uptake in the root zone and subsequent evapotranspiration. Passive treatment in the form of evapotranspiration is considered an effective recharge control mechanism and has gained acceptance particularly in the US and Europe over the last 10 – 15 yr as a cost-effective, non-invasive alternative or complementary technology for engineering-based remediation methods and is recommended by the US Environmental Protection Agency as such. The technology relies on the abstraction of water and contaminants as deep-rooted plants exert hydraulic control, thereby minimising the flux of contaminants in groundwater.
A number of benefits are associated with the implementation of phyto-technologies including: carbon sequestration, soil stabilisation, improvement of functional soil ecosystems and the possibility for the production of high-value wood, biofuel, fibre, gums and dyes and essential oils. The production of value-added products creates opportunities for setting up sustainable industries in association with communities. This will ensure that tree stands are managed appropriately, providing assurance that the water issue is managed effectively into the future.
Ensuring the rehabilitation backlog is minimised and physical rehabilitation or vegetation cover is maximised will reduce water ingress through mine impacted areas. Effective and timely rehabilitation is critical to sustainable water management.
Water quality changes over time
Water management post-closure is a long-term liability that could extend into perpetuity and is often underfunded and generally not well understood.
Stratification may play an important role in mine water management, as geochemical changes result from stratification within flooded underground mines. Mine-impacted water has a higher density and will thus form a layer beneath better quality water. Flooding of underground workings also restores reducing conditions and enhances microbial reactions, such as sulfate reduction, particularly if methane is present, resulting in changes in the water chemistry. Studies have shown that significant water quality improvement is often evident 20 – 40 yr post-closure as a result of natural attenuation resulting in little or no treatment being required.
Geohydrological and geochemical studies, which generate high confidence level water and salt balances that inform financial closure costing models, are therefore imperative to ensure financial assurance is adequate. The application of appropriate geochemical models that determine the evolutionary changes and improvement of the water quality over time will then allow the premise of ‘into perpetuity treatment’ of impacted water to be challenged. Other minimisation strategies include artificial flooding or allowing underground workings to flood naturally, creating oxygen reducing conditions. Such a reductive environment would be ideal to prevent the formation of acid mine drainage, and would be the ultimate goal for mine closure.
The need for innovation
Prevention of acid mine drainage formation at source should be the preferable option, but this is not always feasible and it becomes necessary to collect, treat and discharge water, which has significant financial implications. There is thus a substantial need for further investment in innovative R&D in the management of mine-impacted water.
Technology solutions should focus not only on the cost-effective sustainable treatment of mine water, but also on the minimisation of operational water consumption across the mining process. The sustainability of any water treatment process is a factor that is becoming increasingly critical in decision making, particularly with regards to waste generated and energy consumed.
There needs to be ongoing efforts to develop processes that are energy efficient, produce minimal waste and/or produce byproducts that generate revenue to offset treatment costs. Additionally, variability and changes in water chemistry require the development of alternate sustainable and cost-effective solutions.
Brine minimisation technology
Desalination is becoming increasingly important in maintaining water quality in production processes and in protecting the environment. One of the most significant issues associated with desalination processes utilising membrane technology is the generation of a highly-concentrated salt stream (brine). Brine management requires long-term handling and storage in brine ponds, which impacts considerably on lifecycle costs, while remaining an environmental liability with a substantial footprint that is not sustainable into the future.
It is on this basis that new brine minimisation or treatment technologies are needed to address the brine in a consumptive manner to extend the life of current and new brine ponds, as well as to eliminate the need for construction of additional brine storage facilities. A number of brine treatment or minimisation technologies, such as Eutectic Freeze Crystallisation, HybridIce and Ion Exchange technologies have been developed and tested on a laboratory/pilot scale under the auspices of Coaltech, a collaborative coal research association in South Africa.
Brine treatment, as opposed to brine minimisation technology, also has the potential to produce revenue from the pure salt byproducts generated; this mitigates some of the operating costs.
Passive water treatment
Recently, there has been a significant drive in the development of cost-effective passive treatment systems for the treatment of mine-impacted water or minimisation/prevention of acid mine damage (AMD) generation as long-term sustainable solutions that can be implemented. These technologies mitigate the requirement for major infrastructure or sophisticated levels of operational maintenance. They minimise waste generation and have reduced energy requirements, thereby reducing the carbon footprint of the process. Passive treatment systems utilise the chemical, biological and physical removal processes that occur naturally in the environment to modify the mine water characteristics.
A variety of passive treatment systems, particularly constructed wetlands, anoxic limestone drains, vertical flow systems, such as successive alkalinity producing systems and open limestone channels, have been developed that do not require continuous chemical inputs and that take advantage of naturally occurring physical, chemical and biological reactions to treat AMD in a controlled environment.
Frequently, more than one type of passive treatment or an integrated system of passive treatment technologies is employed to treat mine drainage in order to achieve the required discharge criteria. For example, ecologically engineered ecosystems, such as constructed wetlands, have been shown to remediate contaminated mine water but need to be properly planned, designed, constructed and monitored. These systems emulate the assimilative properties of natural wetland in an environment that can be controlled and manipulated to ensure treatment objectives are achieved. These systems are, however, mostly designed to deal with metal loads and pH rather than sulfate removal; therefore, research is required to address that particular aspect of treatment.
Mining has received negative publicity in recent years because of a legacy of mismanagement of contaminated mine water and AMD. As such, mining companies need to take a leadership role in water stewardship, preventing impacts where possible, mitigating impacts that cannot be prevented and being innovative about solutions to the issues that the industry has created
Ritva Muhlbauer and Nikki Fisher. Edited by Jonathan Rowland.
About the authors: Ritva Muhlbauer is Manager of Hydrogeology and Nikki Fisher is Coal Stewardship Manager at Anglo American Coal.
Read the article online at: https://www.worldcoal.com/special-reports/08062015/building-water-resilience-in-coal-mining-coal2388/