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What Is ML-ARD?
Metal leaching (ML) and acid rock drainage (ARD) are naturally occurring chemical processes that leach metals from rock. When minerals containing metals and sulphur (called sulphides) come in contact with oxygen and water, the sulphides oxidize producing acid. If acid enters streams, ponds or lakes, it is called acid rock drainage.
Acid can leach metals from surrounding rocks to produce drainage containing high amounts of dissolved metals, a process called metal leaching. Some mineral deposits actually owe their origin to prolonged metal leaching in the ground.
Sulphide-bearing rocks generate acid in varying degrees depending on the amount of neutralizing minerals (carbonates, silicates) they contain. If the capacity of minerals to neutral acid is equal to or greater than the capacity of sulphides to generate acid, then the rock will not create ML-ARD.
If there are more sulphide minerals than neutralizing species in a rock, the rock will produce ML-ARD once all the neutralizing minerals have dissolved. This can result in a time delay for the onset of ML-ARD, called the lag time. Once initiated, ML-ARD can persist for hundreds of years until the sulphides are completely oxidized and the acid and metals are leached from the rocks.
Mining can cause ML-ARD if the ore and waste rock contain high quantities of sulphides. Activities such as blasting, excavating, crushing and processing, greatly increase the surface area of sulphide-bearing rocks exposed to air and water. Other industries that disturb the ground, such as forestry, road construction, engineering and foundation excavation, can cause significant ML-ARD.
The potential for ML-ARD at a minesite depends on several factors including:
- the amount of metals in minesite drainage,
- the availability and amount of neutralizing materials at the minesite and downstream,
- the amount of surface water to provide dilution for acids, and
- the sensitivity of the receiving environment.
Significance of ML-ARD
High levels of metals or acid in the environment can be harmful or toxic to living organisms. Metals absorbed by plants and animals can be passed through the food chain.
The impact of ML-ARD and the influence of mining practices was not recognized until fairly recently. ML-ARD can be very expensive to manage. For example, in British Columbia water treatment plants to treat ML-ARD may cost more than $10 million to construct with operating costs reaching 15% per year. Other management strategies can also be very expensive.
Dealing with Acid Rock Drainage - Design Options and Mitigation Strategies
Planning, design and laboratory testing throughout all phases of exploration and development of mining projects will reduce the risks and costs associated with ML-ARD. Laboratory testing of rocks and overburden in the mining project area will predict whether ML-ARD is to be expected and aid in identifying the best practices for prevention or mitigation. There are several mitigation strategies that may be used depending on the deposit, minesite design and site conditions. Most strategies focus on reducing the amount of water or oxygen that contacts sulphide-bearing minerals. Because it can take centuries for ARD to dissolve the sulphides at a minesite, mitigation strategies must be designed to be effective for centuries, practically forever.
The main mitigation strategies currently advocated are:
(i) avoidance,
(ii) flooding,
(iii) covers,
(iv) blending, and
(v) water treatment.
Current policy in British Columbia is zero-tolerance towards ML-ARD.
 BC ML-ARD Guidelines
BC ML-ARD Policies
But There Are Alternative Strategies
Current policies of blending or interleaving sulphide-rich waste rock with non-acid generating rock results in the contamination of large volumes of benign rock. Back-filling of an 'mined-out' open pit may result in burial of low-grade ore, rendering it lost to future economic extraction.
Waste rock at one year might become low-grade ore in a different year with more favourable commodity prices and economic indicators. Submersion or burial of potentially acid-generating rocks renders any contained metals forever unrecovered.
Current bonding requirements for mining companies are designed such that it pays to put off and prolong treatment of ML-ARD-contaminated water for decades and even centuries.
Acceleration of ML-ARD
The idea behind acceleration of ML-ARD is to enhance the sulphidation of metals so the process exhausts quickly. Remaining metals in the waste rock and tailings are leached out as quickly as possible facilitated by smart engineering and design. Biological technology can be used to enhance the extraction of metals from waste.
Providing oxygen and flushing with water are the key ways to enhance ML-ARD:
- Aerate the waste rock dump enhancing the production of ARD
- Water the waste rock dump enhancing the leaching of metals
- Concentrate the contact water metals in the open pit by evaporation
- Extract metals from contact water using electrowinning or biosulphide reduction
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