EFC Technology – How it works
The basis of the Eutectic Freeze Crystallization (EFC) technology 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.
On the right, the figure shows a typical phase diagram for a binary system (salt-water). In the (ideal) 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 ice and salt. However, in practice it is common that a solution has a salt concentration that is lower (ABE cooling path) or higher (CDE cooling path) than the eutectic concentration.
In the first case (ABE) ice will crystallize first (point B) 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 (E) is reached.
When the original salt concentration is higher than the eutectic concentration the opposite happens (Path CDE). First, salt is crystallizing (point D) till the salt concentration of the mother liquor decreases to the eutectic concentration, from that moment on also ice will be formed (E).
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 and concentration) for different systems.
The following graph is a typical cooling profile for a solution that is cooled to its eutectic point. Starting with an unsaturated solution cooling down will at some time lead to the crystallization of either ice or salt (depending on the initial concentration).
Crystallization will not initiate immediately when the equilibrium temperature is reached, but the solution will become supersaturated first. When nucleation occurs the energy that is released by the crystallization process leads to a jump in the temperature. This first jump is called either the salt or the ice jump (depending on the crystallizing component). The second jump in temperature marks the systems arrival at the eutectic point. After the jump, the temperature stays constant at the eutectic temperature of this specific system. These temperatures jumps (both for ice and salt) are not always observed very clearly.
The difference in density of the three phases present at the eutectic point (ice, salt and solution) makes it possible to separate these phases by gravity methods (i.e. settling).
The presence of additional components in a solution leads to a change in the eutectic concentration and temperature.