Wednesday, 27 March 2013 10:15

Biological solutions for mines

Website: Acid mine drainage is a major problem with many hard-rock mines, including almost all mines where the metal ore is bound up with sulphur (metal sulphide mines).

Acid mine drainage (AMD) issues have reached critical levels in South Africa, with time running out and discussions in both public and private sectors dragging on. The most effective method in treating AMD is still a question that remains unanswered.

Acid mine drainage is generated through the oxidation of pyrite (FeS) and other sulphide minerals. When these minerals are exposed to oxygen and water, the result is the production of dissolved metals, sulphate and acidity through microbiological and chemical processes. AMD treatment systems can be roughly divided into two categories, namely abiotic and biological systems, with the former being widely implemented in South Africa. In recent years, a lot of emphasis has been placed on finding more economical and viable solutions.

AMD remediation options

Active abiotic systems
The most widespread system used to remediate acid mine drainage is the addition of a chemical-neutralising agent. The addition of such an agent will raise the pH, increase oxidation of ferrous iron, and the precipitation of metals as hydroxides and carbonates. The process involves dosing the AMD with the neutralising agent and allowing the heavy metals to precipitate over time.

The system usually consists of an agitator station, settling dams that allow for precipitation of the heavy metals and in some cases a wetland polishing station. The most widely used neutralising agents include lime, fly-ash and dolomite. Studies have shown that pre-treatment with fly-ash increases the effectiveness of the lime to precipitate the heavy metal ions.

A recent study found that coal fly-ash is a viable substitute for lime to lower input costs associated with acid mine drainage remediation. The drawback of an abiotic active system is that neutralising chemicals are expensive, and the system input cost is high and usually has a big footprint. The biggest drawback of this system is that a lot of heavy metal containing wet sludge is produced and additional infrastructure is required for the drying of the wet sludge before being disposed of.

Passive biological systems

The input and maintenance costs associated with active treatment systems have led to advances in recent years in the development of passive biological treatment systems. These systems’ popularity has increased due to the fact that they cost a fraction of the price to construct and maintain in respect to active abiotic systems.

Passive biological treatment systems rely on sulphate-reducing bacteria to reduce sulphate to sulphide – the reduction process generates alkalinity and the biogenic sulphide can precipitate dissolved metals as insoluble compounds. Natural and constructed wetlands have been shown in the past to be a viable alternative for treating AMD streams, given enough organic substrate is available for the micro-organisms to feed on and act as electron donor for the sulphate reduction process and the chemical composition (especially the heavy metal concentration) of the AMD received. A high concentration of heavy metals has shown to have an undesirable effect on some sulphate-reducing micro-organisms.

In recent years a lot of research has been done on the development of effective sulphate-reducing bioreactors (SRBR) to treat acid mine drainage. SRBR has the advantage of having a small footprint in regard to other treatment systems, and better control can be maintained over the system to maximise microbial efficiency and community health and creating a stable metal-absorbing substrate.

I-Cat Environmental Solutions is currently busy with field trials to assess the effectiveness of SRBR systems to remediate AMD at a prominent gold mine in South Africa.

Full thanks and acknowledgement are given to I-Cat Environmental Solutions for the information given to write this article.

GIL Africa 2017