Eutectic Freeze Crystallization – The Basics
The basis of the eutectic freeze crystallization (EFC) process is the existence of the eutectic point. The eutectic point is a characteristic point in the phase diagram of a salt-water mixture. At the eutectic point an equilibrium exists between ice, salt and a solution with a specific concentration. This specific concentration is called the eutectic concentration and the temperature at which this equilibrium is found is the eutectic temperature.
Figure 1. A typical phase diagram for salt-water system showing the eutectic point (3). 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 (point 2) 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 (3) 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 (Path B); first salt is crystallizing (2) till the salt concentration of the mother liquor decreases to the eutectic concentration, from that moment on also ice will be formed (3). 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.
In general three different process schemes can be made to treat a waste stream with EFC. In words these are:
- Single stage EFC: Crystallization of ice and salt(s) occur in one crystallizer operating at a constant temperature. For multicomponent mixtures this scheme might lead to incomplete removal of dissolved salts when the operating temperature is too high for all species to crystallize and a relative large amount of concentrated brine is produced in this case. However, when operating at the ultimate eutectic point of a multi-component system all ions and water will be removed as salt and ice. In that case no bleed stream is required, thus it becomes a Zero Liquid Discharge process. The salt product obtained from a process operated this way will be a mixture of all salts present in the feed stream.
- Multi-stage EFC: This is a combination of several EFC crystallizers each operating at a different temperature. Dissolved salts are removed in a sequential way by placing several EFC crystallizers in series. Each crystallizer is operating at different conditions, so each unit is crystallizing out a specific salt species. This will increase salt product purities and can eventually result in a Zero Liquid Discharge process.
- EFC combined with pre- and/or after-treatment: For some water streams it might be advantageous to use a pre- or after-treatment step in order to improve the EFC performance. Numerous pre- or after-treatments can be considered and their use is very dependent on the waste stream properties and product specifications. Examples are, reverse osmosis, chemical treatment, ion exchange and even evaporation.
In all these cases very clean water is produced in the form of ice. Besides the obvious advantage of having clean water, the ice slurry itself can also be employed as an excellent coolant in other processes.
Figure 3. Clean water and salt are recovered from an aqueous waste stream in our pilot facility.
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