Mine waste produces fuel
Power generation from acid mine drainage (AMD) is the subject of research being conducted at Pennsylvania State University, USA. The team is investigating treatment of toxic mine waste using a specially developed fuel cell that generates electricity while recovering ferric iron for use in pigments.
Acid mine drainage is the outflow of metal sulphides (often pyrite – iron sulphide) from abandoned or existing metal and coal mines. Oxidising on exposure to air and water, the waste stains the surrounding area and produces a sulphuric acid mix that kills fish and other living organisms, and blocks streams and sewers.
‘Both active and abandoned sites are good candidates for our fuel cell strategy,’ says Brian Dempsey, Professor of Environmental Engineering at the University.
Professor Bruce Logan and researcher Shaoan Cheng of the same department are developing microbial fuel cells (MFCs) to generate electricity from organic matter, waste water or inorganic matter such as sulphides, rather than from liquid fuels such as hydrogen and methanol. This research has been applied to AMD, and feasibility tested using a synthetic AMD solution.
The laboratory-scale fuel cell was constructed from two plastic containers separated by an anion exchange membrane, with carbon cloth anodes and cathodes at either end of the chamber. The catholyte contains sodium chloride to increase solution conductivity, and sodium bicarbonate to reduce acidity, while the anolyte also contains iron sulphate (FeSO4).
Acid mine drainage is treated by oxidising the ferrous iron to usable and insoluble ferric iron at the anode and precipitating it, while oxygen is reduced to water at the cathode. Dempsey says, ‘Whenever ferrous iron is oxidised using a solid phase as a catalyst, we [can] produce high quality, high-density solids [as by-products for pigments]. We produce ferric oxide on the electrode and then subsequent oxidation occurs on the ready-formed precipitate. The process releases energy because the chemical reactions are spontaneous [and thermodynamically favourable]’.
The technology removes all ferrous iron and manages the pH level to generate up to 290mW/m2 of energy – about 48% of that achieved by using acetate as a substrate for bacteria.
The team believes the energy could be applied for small operations, such as operating pumps, particularly in remote areas. The oxide layer on the anode remains conductive to ensure a sustainable process – the fuel cell was batch fed over 42 days without a reduction in power generation.
The active and the passive
Dempsey argues that this technique has not only the added benefit of generating energy, but also improves upon conventional active and passive routes for AMD treatment and metal recovery. ‘Abandoned sites are usually treated using passive techniques, [namely passing the waste over] limestone, aerobic ponds or channels, and wetlands. These systems require a lot of land for even small discharges, so the strategy is not economical,’ he explains.
‘Active sites and some high flow abandoned sites are usually treated using lime [to neutralise the acid], aeration lagoons, and settling ponds or tanks. This requires a lot of chemicals and energy, and results in high volumes of sludge with little solids content, at a higher pH, which are useless for pigments. Therefore there is the secondary problem of what to do with the sludge?
‘Since [our solids] are produced in a contained system, they are cleaner and easier to harvest than in passive treatment systems.’
The next step for the team is to attempt field-scale trials, but first, elements of the AMD fuel cell must be refined. The team is liaising with materials scientists to develop electrodes ‘that, like a hydrogen fuel cell, can process a high current density without getting fouled’, explains Dempsey.
This research could be applied to AMD for more scaleable systems.