Fractional crystallization of salt solutions

Separation of salts by fractional crystallization of salt solutions

Fractional crystallization is a method for separating salts from solutions. The method is based on the fact that the solubility of each salt has a unique temperature dependency. In industry, this method is used to separate multi-component salt solutions into pure salts and water. Such salt solutions can be the effluents from solution mining, by-products, waste streams, or similar. Using this method, waste can be minimized and even turned into saleable products.

Bryant Fitch wrote in 1970: ‘Design of even moderately complicated fractional crystallization processes is recognized as beyond the ordinary skill of a chemical engineer’. This is still true today. The design of fractional crystallization processes is not a simple task. (B. Fitch, “How to design fractional crystallization processes” Industrial & Engineering Chemistry 62, no. 12 (1970): 6-33. https://doi.org/10.1021/ie50732a004). Today, thermodynamic models give us the ability to calculate phase diagrams at various temperatures. This can be of great help in designing such processes.

The design of fractional crystallization processes for ternary or quaternary salt solutions (solutions with two or three salts) is relatively simple. The reason is that the design can be based on phase diagrams. In the following example, it is shown how phase diagrams can help in the design of a fractional crystallization process.

Several possible schemes for fractional crystallization of salt solutions

Two different fractional crystallization schemes for the aqueous sodium chloride – sodium sulfate system are outlined below.

In the first scheme, an equimolar solution of the two salts is separated into glauber salt and sodium chloride. Glauber salt is crystallized at low temperature while sodium chloride is crystallized at high temperature by evaporation of water.

In the second scheme, sodium chloride is crystallized at low temperature while sodium sulfate is crystallized as anhydrous Na₂SO₄ at high temperature by evaporation.

A third scheme is also possible. Sodium chloride can be crystallized at 25-30 °C by evaporative cooling. The solution is then heated and anhydrous sodium sulfate crystallizes at for example 80 °C. This last scheme is not shown here.

Fractional crystallization of sodium chloride and glauber salt

To separate aqueous mixtures of sodium chloride and sodium sulfate, two crystallizers operating at two different temperatures are required. The question is: which of the two salts should be crystallized at the low temperature? This is where a phase diagram can provide some help. Ideally, a phase diagram like the one to the right is available. It describes phase equilibrium in mixtures of sodium chloride and sodium sulfate as a function of temperature.

The composition of the salt content is shown in the phase diagram. The water content however, is not shown. That would require a three-dimensional diagram. It is of more importance to know the phase diagram lines. They indicate brine compositions in equilibrium with two salts. These phase diagram lines were calculated with the Extended UNIQUAC model. Together with the experimental data, these lines divide the diagram in three regions:

Glauber salt (Na₂SO₄·10H₂O) precipitation region
Sodium sulfate (Na₂SO₄) precipitation region
Sodium chloride (NaCl) precipitation region

These three regions meet in one point which is called an invariant point: Three solid phases co-exist with a liquid phase and a vapor phase in a ternary system. The point is invariant because this can only happen at one temperature and one composition.

Process conditions

The temperature dependence of the phase diagram lines indicates that the glauber salt crystallizer should be run at a low temperature. The composition of the effluent from the glauber salt crystallizer will be near the phase diagram line. By heating this effluent, it moves into the NaCl precipitation area. It is therefore chosen to run the glauber salt crystallizer at 3 °C and the sodium chloride crystallizer at 45 °C.

The process path for a fractional crystallization process separating the two salts is shown in the phase diagram. It starts with an aqueous solution of equal amounts of NaCl and Na₂SO₄ which is fed to the process. Its temperature is 45 °C and the stream composition is marked with an “a” in the phase diagram.

The feed stream is mixed with the stream “e” to produce a stream with the composition “f”. Next, stream “f” is cooled to 3 °C along the path “fb”. At “b” this solution is supersaturated with glauber salt, Na₂SO₄·10H₂O. This salt will therefore precipitate. The precipitation stops at the composition “c”, which is close to the area where NaCl starts precipitating.

Mother liquor

The solution with composition “c” is called the mother liquor because it gave birth to the glauber salt crystals. This solution is now heated to 45 °C, point “d”. It is within the NaCl precipitation area but NaCl is not precipitating because the solution contains too much water. Therefore, two thirds of the water content is evaporated at 45 °C. Thereby, the solution becomes supersaturated and NaCl precipitates.

By precipitation of NaCl, the composition of the stream changes to “e”. This stream is the mother liquor from the NaCl crystallizer. It is saturated with NaCl and also near saturated with Na₂SO₄. Therefore it is recirculated by mixing it with the feed stream at “a”, thereby producing the stream with composition “f”.

To avoid the buildup of impurities, a small amount of the mother liquor “e” is purged and not mixed with the feed stream.

Key parameters

The amount of water evaporated in the NaCl crystallizer is a key parameter for designing this process. On one hand, evaporation of too much water leads to precipitation of sodium sulfate in the NaCl crystallizer. On the other hand, insufficient evaporation gives a brine that contains too much NaCl. As a result, NaCl will precipitate together with glauber salt in the low temperature crystallizer.

The glauber salt produced at 3 °C is stable at a relative humidity of 98 % (This value was calculated with the free demo program offered at this site). At lower relative humidity however, glauber salt will gradually loose its water of hydration. The water evaporates and the salt turns into anhydrous sodium sulfate.

Fractional crystallization of sodium chloride and anhydrous sodium sulfate

Production of anhydrous sodium sulfate from brine

The fractional crystallization process can be redesigned to yield anhydrous sodium sulfate and sodium chloride directly. To produce these two salts, it is an advantage to crystallize sodium chloride at low temperature and anhydrous sodium sulfate at a higher temperature. This conclusion can be reached by observing the shape of the phase diagram curve. The design is shown in the following section.

An equimolar aqueous solution of sodium chloride and sodium sulfate is fractionated into pure sodium chloride, pure anhydrous sodium sulfate, and steam. The operating lines of the process are shown in an enlarged section of the phase diagram. Sodium chloride precipitates at 30 °C along the short line from 1 to 2. After the sodium chloride crystallization, the brine is heated to 80 °C. The solution marked 3 is identical to the solution marked 2. Only the temperature is different.

From 3 to 4, 6.2 % of the water content is evaporated and anhydrous sodium sulfate precipitates. 0.5 % of the solution marked 4 is purged to avoid the build-up of impurities. The remaining part is mixed with feed stream, which results in the composition marked 1.

Design calculation performed with extended version of AQSOL software

The process was designed using an extended version of the software offered at this site. To ensure that the two salts produced are pure, some design constraints were made. These constraints were defined as limited saturation of secondary salts in each crystallizer. The saturation index of anhydrous sodium sulfate in the mother liquor from the sodium chloride crystallizer was not allowed to exceed 0.95. As a result, the saturation index of anhydrous sodium sulfate in the solution marked 2 is exactly 0.95.

At the same time, the saturation index of sodium chloride in the mother liquor from the sodium sulfate crystallization was not allowed to exceed 0.95. As a result of the design, the saturation index of NaCl in the solution marked 4 is exactly 0.95. The number 0.95 was chosen at random. The process can be designed with larger or smaller saturation indices based on practical experiences.

Intuitively, one would expect that the solution marked 2 is far from saturated with sodium sulfate because of the distance from the phase diagram line. At the same time, one would expect that the solution marked 4 is almost saturated with sodium chloride because the point is so close to the phase diagram line marking simultaneous saturation of sodium chloride and sodium sulfate. This shows that it is better to rely on the calculated values than on the visual impression.

Process flow sheet for fractional crystallization process

The compositions and sizes of all the streams in the fractional crystallization process are shown in the process diagram. All compositions are given in mass %. The four streams 1, 2, 3, and 4 are marked in the process diagram.

While the feed stream consists of 13.5 kg/s of brine, the recirculation stream is much larger, 169.4 kg/s. The amount of steam evaporated from the sodium sulfate crystallizer is 7.9 kg/s. The evaporation takes place at 80 °C and a pressure of 0.35 bar.

Further reading on the design, simulation, and optimization of fractional crystallization processes: Thomsen, Kaj, Peter Rasmussen, and Rafiqul Gani. “Simulation and optimization of fractional crystallization processes.” Chemical engineering science 53, no. 8 (1998): 1551-1564, https://doi.org/10.1016/S0009-2509(97)00447-8