Acid mine drainage (AMD) and acid rock drainage (ARD) are problems that will be with us for a long time. Acid mine drainage is water with pH in the range of 1-3 that drains from abandoned mine sites. Acid rock drainage is acidic drainage or runoff from disturbed rock or soil that could be caused by road construction or other excavation work. Acid rock drainage can also result when affected rock is used as fill material.
The low pH and high concentration of metals are an environmental concern for several reasons:
Aquatic species don’t tolerate changes in pH.
In mine drainage, the low pH allows metals to dissolve into the water, and metals in high enough concentrations can be toxic.
The metals can cause discoloration to water and waterways as they precipitate out and stain the surrounding rocks and soil.
The metals need to be removed and the pH neutralized before the water can be discharged to the environment.
Sometimes through natural processes the drainage changes from acidic to neutral. This is commonly referred to as neutral mine drainage. It is typically in the pH 5-7 range and is also referred to as low TDS (total dissolved solids) acid mine drainage.
Neutral mine drainage is treated similarly to acid mine drainage, but a plant designed for acid mine drainage may require modification to effectively treat neutral mine drainage.
Neutral mine drainage typically contains dissolved iron in the 100-400 ppm range. The iron has to be removed before the water can be discharged. If the iron is dissolved, it will need to be changed to a precipitated form to be removed. The pH is raised to 7 or higher to ensure the iron is precipitated and the iron is oxidized from the ferrous to the ferric state. Ferric iron is rusty orange and doesn’t easily re-dissolve.
For neutral mine drainage, the low iron levels might make it cost effective to oxidize with a chemical oxidant, like hydrogen peroxide or sodium hypochlorite.
(Click on the image above to view the full flow sheet PDF for the acid mine drainage peroxide feed process.)
Sometimes minor pH adjustment is needed to convert the metals to their precipitated form. Discharge permits may also require a specific pH, usually around 6-8.
Special care should be taken when raising the pH. Raising the pH higher than needed adds the cost of the chemical used to raise the pH. Raising the pH too high could also result in precipitating metals that aren’t required to be removed, like calcium and magnesium. This could significantly add to the amount of chemical required to raise the pH and it would also increase the amount of sludge produced, possibly increasing the long-term operating cost.
Sedimentation or settling is the most cost effective method of removing precipitated solids. Precipitated solids settle slowly. The slower the solids settle, the more space they need to settle. (Photo: Settling test at an acid mine drainage project.)
Settling rates can be increased by flocculation. Flocculation is the addition of a long chain polyelectrolyte polymer to agglomerate or pull together the precipitated solids. Bigger and heavier agglomerated particles settle faster. Particles have to interact to flocculate. More solids improve flocculation. When there aren’t enough solids, you don’t get particle-particle interactions, and the solids don’t build quick settling particles. The amount of solids can be increased by recirculation.
A Solids CONTACT CLARIFIER™ is an example of a clarifier that recirculates large amounts of settled solids internally. The CONTRAFAST® High-Rate Thickener Clarifier is an example of a device that uses both internal and external solids recirculation in addition to thickening. The RapiSand™ is a ballasted flocculation clarifier that uses sand to enhance the flocculation and increase the settling rate.
Traditional clarifiers or high rate thickeners are used when there are enough solids for flocculation without any internal solids recirculation. They are easy to operate, require the lowest operator attention, and typically produce the thickest underflow. Another option, the SuperSettler™, is an example of a plate settling device. This can clarify just as well in a much smaller space, but won’t produce a thickened underflow.
The eye-catching color of acid mine drainage makes it a frequent target for media and environmental organizations. It’s often considered to be an unsolvable problem, a misconception that proactive operations can counter by installing treatment solutions such as those outlined here.