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A very dilute solution can be concentrated by reverse osmosis (RO). Technical advances in the field have allowed almost anything (high chemical oxygen demand (COD), suspended particles, etc.) to be concentrated till the limits of osmotic pressure. In most cases, the practical limit of osmotic pressure is reached at around 2-5 per cent concentration of dissolved solids in water. In order to concentrate this further, evaporation is normally carried out, which is costly.

Chemical dewatering (ChD) can take this stream as input and concentrate it to 25-40 per cent without evaporating water. In principle, ChD works like extraction, but instead of extracting the solute, it extracts the solvent (in this case water) from the aqueous solution. The raffinate can, therefore, be concentrated to its saturated concentration.

ChD case study

A waste stream with approximately 5 per cent caustic (with Ca2+ impurities) from nonferrous metallurgy industry was concentrated to approximately 42 per cent concentration. The lean stream can be recycled back to the process. The solvent used was ChD sol-1. Dewatering takes place at ambient conditions. Latent heat of solvent is < 100 kcal/kg.

Recovery of solvent can use waste heat (condensate). The operating cost (cost of energy and make up solvent) is 25 per cent of multi-effect evaporator (MEE) due to the use of waste heat. If waste heat is not available, the operating cost would be approximately 70 per cent of MEE (due to low latent heat of the solvent). The equipment can be made from less exotic material as high temperature is not involved. The overall capital cost is approximately 50 per cent of a three-stage MEE.

From Stream to Crystals

The process of ChD can be applied to any dilute aqueous stream to increase the concentration. The only limitation being that the impurities present should not react with the solvent used. Due to the availability of a large number of solvents for dewatering application, it is generally possible to choose a suitable solvent.

The concentrated stream thus produced can be fed to the crystallization system, or if needed, can be treated further by reactive extraction (ReX) to recover the component present in pure form.

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