How acid rock drainage management and humidity cell testing can mitigate environmental risk and ensure regulatory compliance

Rosalind Ma, Laboratory Manager, SGS Australia

06 Dec 2024 · 1100 words, 4 min read

Acid rock drainage (ARD) also known as acid mine drainage (AMD), forms when naturally occurring iron-sulphide minerals, mainly pyrite (FeS2) and pyrrhotite (FeS) are exposed to oxidising conditions in air and water.

This oxidation releases H+ ions and lowers the surrounding pH to acidic levels. Acidic rock drainage will subsequently leach additional metal ions from the adjacent rocks.

Any mining activity that exposes mined materials to air and water has the potential to generate ARD and contaminate water sources.

Predicting and managing the occurrence of ARD to minimise risks to human and environmental health is one of the most critical and challenging environmental issues confronting the mining industry.

What is the significance of acid rock drainage?

If not managed effectively, ARD/AMD can lead to significant environmental and financial impact to the mine operator.

Environmental

Australia is the earth’s driest inhabited continent. In fact, of all Earth’s continents, only Antarctica gets less precipitation than Australia.

Considering this, Australia has a water system under severe pressure to deliver a sustainable water future in a challenging environment – and a situation that has Australia seeking costly alternative water sources such as desalination.

Although mining in Australia accounts for a relatively small amount of total water (around two per cent of total water extracted from the environment, including all household and industry use – see this Bulletin article), the water the sector does use is often from high scarcity regional allocations, and mines can be the major water consumer in the area.

Despite this, the relatively scarce nature of water in Australia means that any activity that reduces the availability or quality of fresh water is likely to come under intense scrutiny (from investors, regulatory bodies, governments and the wider public) and risks long term, lasting reputational damage, as well as remediation costs.

In a recent ‘Future of Water’ survey conducted by the Australian Water Association, two-thirds of respondents agreed strongly that their physical and mental wellbeing is strongly tied to water, illustrating the significant role responsible water management has on the social license to operate in Australia. (AWA, Future of Water 2024).

Ongoing water pollution can damage aquatic ecosystems and degrade water quality for local communities and biodiversity. Unless correctly addressed, acid rock drainage will often lead to long term and costly remediation.

Water pollution from acid rock drainage may need to be managed for decades, if not centuries, after mine closure.

Financial

For the Australian industry, there is significant annual cost of managing potentially acid generating wastes at mine sites. Major remediation costs can arise late in the mine’s life or after mine closure if proper waste management strategies are not in place from the beginning. Ongoing control and remediation costs and lingering reputational damage can continue long after mine closure (Australian Government, 2016; Parbhakar-Fox and Lottermoser, 2015).

How can these risks be mitigated?

Successful management of ARD is vital to ensure that mining activities meet increasingly stringent environmental regulations and community expectations. Early identification and prediction of the likelihood of acid drainage can generate substantial, long-term cost and reputation savings.

The correct management of ARD supports:

cost savings
reputation as a sustainable mining operator
improved community engagement.

Leading practice in ARD management is continuously evolving, and mine operators need experienced and highly skilled experts to support development of the most appropriate monitoring and management strategy.

ARD testing/analysis services can help minimise environmental impact by predicting the production of acidic water from mining operations. This is done through analysing the acid producing potential of tailings, ores, and waste rock on site. A complete ARD testing service that predicts the production of acidic water from mining operations can provide preventative strategies and practical solutions to reduce the environmental impact of mining operations.

Testing, monitoring, and prevention

There are two common types of methods for assessing the acid generating potential of materials. These are:

Static ARD testing

Modified Acid Base Accounting (ABA)

Used to assist in determining the propensity of the tailings/waste rock to generate acid. The ABA test involves two main parts. The first part is to quantify the total sulphur, sulphide sulphur, and sulphate sulphur present in the sample which is then used to calculate the acid generation potential.

Next, the neutralisation potential (NP) is determined by reacting the sample with excess acid, and then back titration to pH 8.3 with NaOH. Together the two results provide the net acid producing potential, which is the sample’s capacity to generate acid and then its capacity to neutralise the acid it liberates.

Net Acid Generation (NAG)

This testing measures the acid remaining in the sample after the acid producing and acid consuming components of the tailings/waste rock samples have been depleted. The NAG results confirm the acid rock drainage characteristics after exposure.

Static tests are useful screening tools, but they provide no time related reactions.

Kinetic ARD testing

Humidity cell testing is the recommended kinetic test designed to model the geological processes of weathering and time at the laboratory scale. The test provides data on the rate of acid generation and variation over time in leachate water quality. They are often performed to confirm or reduce the uncertainty in the results of static prediction tests and to provide a preliminary assessment of acid rock drainage (ARD) control options.

This test is done by placing the sample in a cell and storing it in a controlled temperature like the natural environment. The cell is then subjected to three days of dry air and then three days of moist air to simulate precipitation cycles. At the end of each cycle, the sample is soaked for a specific length of time with deionised water, and the water that percolates through the sample is collected.

The leachate collected is analysed for several parameters including pH, sulphate, acidity, alkalinity, conductivity and metals (including Ca and Mg). The test design can be customised to better account for sample matrix effects by increasing or reducing the particle grain size or altering the shape of the cell and changing the flow rate.

Humidity cell tests typically run from 24 weeks to several years. Results are reviewed monthly and trended over the testing period to check progress and determine if extended leaching is required.