Figure 1. A typical phase diagram for a salt-water system showing the eutectic point. See description in text below.

Figure 1 shows a typical phase diagram for a binary system (salt-water). In the case that an aqueous solution has exactly the eutectic concentration cooling the solution down towards its eutectic temperature will lead to the simultaneous crystallization of both ice and salt. However, in practice it is common that a solution has a salt concentration that is lower or higher than the eutectic concentration. In the former case ice will crystallize first when the temperature is decreased. Due to the formation of ice the salt concentration in the remaining liquid (the mother liquor) increases, which leads to a decrease in freezing point and by continued cooling the ice line is followed till the eutectic point is reached. This is represented by path A in figure 1. When the original salt concentration is higher than the eutectic concentration the opposite happens; first salt is crystallizing till the salt concentration of the mother liquor decreases to the eutectic concentration, from that moment on also ice will be formed. The locations of the eutectic points in a water-salt mixture is dependent on the type of ions in solution and can vary over a broad range (both in temperature as well as concentrations) for different systems.

Eutectic Freeze Crystallization – The Process
The concept of eutectic freeze crystallization can be utilized as the working principle for a very efficient separation technology, the EFC-process.
In figure 2 a simplified representation of the continuous EFC-process is given. An aqueous waste stream containing dissolved salt(s) is fed to a special EFC crystallizer. In this step ice and salt are crystallizing simultaneously at the eutectic temperature of this specific system. Separation of the produced ice and salt from the crystallizing solution (or mother liquor) is done by utilizing the density difference between the three phases present (i.e. ice, salt and mother liquor). In one particular crystallizer design, called the hybrid crystallizer, this gravitational separation is taking place inside the crystallizer. In the hybrid crystallizer ice is leaving from the top section and a salt slurry is extracted from the bottom section of the crystallizer. Other process designs may utilize a separate three phase settling device to realize the desired level of separation.
The liquid in the product slurries is removed by mechanical operations (e.g. belt filtration, centrifuges, etc.) up to the desired level of product dryness. The extracted liquid is recycled back to the EFC process.
In some cases, a bleed stream is required to remove trace components from the system, however the volume of this stream is very small compared to the incoming waste stream.

Figure 2. Schematic block scheme of the EFC process. A bleed stream is not necessarily required but can be added for process optimization.

Fields of Application
The range of water streams to which the EFC can be applied is immense. In theory, it can treat any water stream containing dissolved salts and even organics, producing extremely clean water and a high purity product. Examples are:

• Recovery of valuable inorganics from a process or a wastewater stream.
• Reducing the salt content of a wastewater stream to improve water reuse strategies.
• Treatment of reverse osmosis retentates.
• Solution mining of salt deposits.
• Treatment of mine tailings.
  

The benefits of using the EFC process are in many cases twofold. In the first place the problem of having a wastewater stream (e.g. disposal costs, loss of water, etc.) is solved. Secondly, the user of the EFC process is able to recover (valuable) products from its wastewater. The latter characteristic makes that the EFC process is not only a wastewater treatment technology, it can be seen as a high-quality salt production process as well.
The companies and partners which have already shown serious interest in the EFC process are from the following sectors:

• Oil & Gas industry
• Waste incineration
• Various chemical industries
• Recycling industry
• Agriculture
• Mining companies
• Research institutes

Visit also www.efc.nl for more information.

SEIS Consult is engaged by EFC Separations BV as the first point of contact in the Asia-Pacific region.

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