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A solvent-extraction approach for desalination of high-salinity brines

By Scott Jenkins |

Hypersaline industrial wastewater brines are difficult to treat. Current approaches for removing salt from such brines — such as evaporative distillation and reverse-osmosis (RO) — are problematic because evaporation is highly energy-intensive and RO cannot handle salt levels present in hypersaline brines, which can be several times that of seawater.

Now, a research team led by Ngai Yin Yip at Columbia University (New York, N.Y.; www.columbia.edu) has developed a solvent-extraction method they call temperature-swing solvent extraction (TSSE), that dramatically lowers the energy requirements for separating salts from water compared to evaporative distillation, but can also handle very high salt levels. The research could lead to an effective means of treating high-salinity wastewaters from oil and gas production (produced water), as well as landfill leachate, fluegas desulfurization wastewater and concentrated brine from brackish water desalination processes (for areas with no discharge options).

Using solvent-extraction principles, the process works by contacting the brine with a solvent mixture that extracts water from the brine, but that is also immiscible with water, so it can be easily separated. The Columbia team used three amines — diisopropylamine (DIPA), N-ethylcyclohexylamine (ECHA), and N,N-dimethylcyclohexylamine (DMCHA) — as solvents. “The solvents need to have sufficient hydrocarbon chains to make them separate easily from water, but also hydrophilic functional groups to effectively extract the water,” explains Yip.

Then, since the solvent’s ability to solubilize water is temperature-dependent (higher temperatures mean less water held), the temperature can be raised from ambient temperatures up to 60–70°C to release the water. The solvent can then be re-used in the process. If zero liquid discharge is the goal, the remaining dewatered raffinate can be distilled to yield solid salts. Since the volume is drastically reduced by the process, managing it with distillation is easier, Yip notes.

The team recently published its work at the laboratory-scale, but is in talks with partners looking to commercialize the technology. At industrial scale, the idea would be to use an industrial liquid-liquid extraction column to separate the salt from water, then use waste heat from another process, or from solar-energy collectors, to raise the solvent temperature and release the salt-free water, Yip says.

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