Related Experiment Video
Updated: May 17, 2026
08:01
Sulfate Separation by Selective Crystallization with a Bis-iminoguanidinium Ligand
Published on: September 8, 2016
Improved calcium sulfate recovery from a reverse osmosis retentate using eutectic freeze crystallization
D G Randall1, R Mohamed, J Nathoo
1Department of Chemical Engineering, University of Cape Town, Cape Town, South Africa. dyllon.randall@uct.ac.za
Summary
Eutectic freeze crystallization (EFC) effectively removes calcium sulfate from reverse osmosis streams. This novel process yields significantly more salt recovery at eutectic conditions compared to standard temperatures, enhancing water recovery.
Area of Science:
- Water treatment technologies
- Crystallization processes
- Chemical engineering
Background:
- Reverse osmosis (RO) is a key water desalination technology.
- Scaling in RO systems, particularly calcium sulfate, reduces efficiency and increases operational costs.
- Recycling RO reject streams is crucial for maximizing water recovery.
Purpose of the Study:
- To investigate the feasibility of eutectic freeze crystallization (EFC) for removing calcium sulfate from RO retentate.
- To compare the calcium sulfate yield and water recovery using EFC versus conventional temperatures (0 and 20 °C).
Main Methods:
- Eutectic freeze crystallization (EFC) was employed to treat RO retentate.
- Experiments were conducted at eutectic conditions and compared with operations at 0 °C and 20 °C.
- Calcium sulfate yield and ice recovery were measured.
Main Results:
- EFC operation under eutectic conditions yielded 48% calcium sulfate, significantly higher than 15% at 0 °C and 13% at 20 °C.
- Theoretical calcium recoveries were 75% at 0 °C and 70% at 20 °C, exceeding experimental results.
- EFC demonstrated efficient calcium sulfate removal while also producing water and salt.
Conclusions:
- Eutectic freeze crystallization is a highly effective method for removing calcium sulfate from RO reject streams.
- EFC offers superior salt recovery and potential for increased overall water recovery in desalination processes.
- The process provides a dual benefit of salt removal and water production.
Related Concept Videos
01:10Recrystallization: Solid–Solution Equilibria
Recrystallization is a purification technique used to separate impurities from solid compounds. In this technique, no chemical reactions occur. Instead, it exploits physical properties only, specifically, the solubility differences between the desired compound and impurities, either at a single temperature or at different temperatures, and under other selected conditions. The solid-solution equilibrium (solubility equilibrium) of each component in the solution represents a binary phase...
04:01Factors Affecting Solubility
Compared with pure water, the solubility of an ionic compound is less in aqueous solutions containing a common ion (one also produced by dissolution of the ionic compound). This is an example of a phenomenon known as the common ion effect, which is a consequence of the law of mass action that may be explained using Le Chȃtelier’s principle. Consider the dissolution of silver iodide:
01:17Precipitation and Co-precipitation
Precipitation and coprecipitation methods can be used to separate a mixture of ions in a solution. In qualitative inorganic analysis, ions that form sparingly soluble precipitates with the same reagent are separated based on the differences in solubility products. For example, consider the separation of Cu(II) and Fe(II) ions by precipitation as insoluble sulfides. First, copper(II) sulfide is precipitated by the addition of acidic H2S, where the dissociation of H2S is suppressed. Adding H2S...
01:12Precipitation Processes
The experimental conditions in a gravimetric analysis should be optimized to maximize the particle size and purity of the obtained precipitate. Ideally, the concentration of the precipitating reagent should be low with effective stirring to maintain low relative supersaturation for the growth of large crystals. In homogeneous precipitation, the precipitant is slowly generated by a chemical reaction in the solution to avoid local reagent excesses. For example, urea decomposes gradually to...
01:25Crystal Growth: Principles of Crystallization
Crystallization is a phase transformation process in which crystals are precipitated from a supersaturated solution or formed from other sources. During crystallization, atoms or molecules arrange themselves into a well-defined, rigid crystal lattice to minimize energy.
Initiating crystallization involves manipulating the concentration of the solute and the temperature of the solution. Since crystal growth occurs when the ratio of concentration and solubility of the solute in the solvent – the...
Initiating crystallization involves manipulating the concentration of the solute and the temperature of the solution. Since crystal growth occurs when the ratio of concentration and solubility of the solute in the solvent – the...
01:19Ionic Strength: Effects on Chemical Equilibria
The addition of an inert ionic compound increases the solubility of a sparingly soluble salt. For example, adding potassium nitrate to a saturated solution of calcium sulfate significantly enhances the solubility of calcium sulfate. Le Châtelier's principle cannot predict this shift in the equilibrium. Instead, this could be explained in terms of changes in the effective concentration of the ions in solution in the presence of added inert salt.
In this solution, the primary cation—the calcium...
In this solution, the primary cation—the calcium...