A biological sulphate reduction process has shown potential to treat hypersaline brine ponds produced by coal mining operations. After successful bench-scale testing, pilot-scale work is now recommended, says State minerals and metallurgical research organisation Mintek.

Presenting the findings at the Coaltech Colloquium held in Benoni, in Gauteng, on May 28, Mintek principal engineer Dr Ryan Merckel explained that coal mining results in water containing high levels of sulphate and metals.

Mintek has developed cloSURE, a biological process to treat acidic, sulphate-laden mine water, which employs microbes to remove sulphate, neutralise the water and precipitate metals, followed by recovery of a biosulphur product. The process has been developed over about ten to 12 years and aims to produce water suitable for irrigation.

“Although reverse osmosis plants treat mine water to produce potable water, it produces the concentrated brine stream that is stored in large ponds, creating a long-term environmental liability that can remain for more than 30 years and delay mine closure.”

Mintek’s research project into brine treatment started during the commissioning of the cloSURE process demonstration facility at the eMalahleni water reclamation plant, where it was noted that sulphide was being produced in the nearby brine pond, showing naturally occurring microbiological activity.

The research team then realised the process could potentially be used for hypersaline brine treatment, Merckel added.

The team then examined whether biological sulphate reduction could be applied to hypersaline brines, with the potential to recover valuable products.

The project also aimed to develop a continuous bench-scale process, test the strength of the microbial community, generate the design information needed for pilot-scale work, and assess the technical and economic feasibility of scaling up the process for industrial use.

Merckel noted that biological sulphate reduction is not a new technology, as it has been studied for several decades, with a number of related biological treatment processes already operating locally.

The Mintek study investigated whether biological sulphate reduction could be maintained over long periods in a hypersaline environment while monitoring reaction rates, microbiology and factors that could affect performance.

After initially characterising the brine and its microbial population, researchers isolated and enriched microbial cultures from the brine pond before testing them in continuous bench-scale reactors. The results confirmed that biological sulphate reduction in hypersaline brines is achievable and sustainable over long periods.

However, managing hydrogen sulphide was critical, highlighted Merckel, as microbial inhibition occurs when dissolved hydrogen sulphide concentrations exceeded 1 000 mg/ℓ . To overcome this, the researchers added a stripping column to remove the gas.

The research project also identified the reactor designs and supporting unit operations needed for pilot-scale implementation, with researchers testing the process under different feed conditions and carbon loading to replicate industrial operating conditions.

The results showed that the biological process remained stable and reliable, despite the fluctuations expected at full scale.

“Mineralogical analysis showed that changes in mineral composition of the brine promoted the formation of co-minerals that could be easily precipitated and potentially recovered to improve the process economics,” he said.

Microbiological analysis confirmed that different microbial groups worked together throughout the treatment process, with heterotrophic microorganisms supporting sulphate-reducing bacteria by converting carbon into forms that could be more easily used.

Summarising the findings, Merckel said the process remained effective under hypersaline conditions, as the study confirmed changes in mineralogy and identified a microbial community unique to saline environments that complemented biological acid mine drainage treatment technologies already demonstrated.

Merckel also recommended moving to pilot-scale testing with a stronger focus on the process’s technoeconomic performance.