Interesting chemical engineering questions - 5: Why does gypsum in seawater not precipitate easily? Standard answer already exists
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This post was last edited by yjqin1 on 2015-8-24 at 21:38. As everyone knows, calcium sulfate (gypsum) is slightly soluble in water; at room temperature, its solubility in pure water is 0.205%, or 2050 mg/L. The calcium ion concentration in seawater is around 600 mg/L, while the sulfate ion concentration is around 2500 mg/L. Seawater can be concentrated from an initial concentration of 3.1% to a final concentration of 12%, which is a concentration increase of almost 4 times, yet no gypsum precipitation occurs. Even after being left for several days, no gypsum precipitate formed. In such concentrated seawater, the calcium ion concentration is about 2400 mg/L, while the sulfate ion concentration is 10,000 mg/L. Why is there no precipitation?
Personally, I think it has something to do with similarity and solubility; after all, seawater contains large amounts of chloride ions and sodium ions, and both calcium chloride and sodium sulfate are soluble in water
There must be other components that also have an impact, right?
This post was last edited by yjqin1 on 2015-8-24 21:40. Standard answer: The inorganic salts in seawater are mainly sodium chloride. When the concentration of sodium chloride is much higher than that of calcium sulfate, the solubility of calcium sulfate increases. The solubility of calcium sulfate reaches its maximum when the concentration of sodium chloride in an aqueous solution (such as concentrated seawater) is 12%. When ordinary seawater is concentrated from a salinity of 3.1% to around 12.5%, the solubility of calcium sulfate reaches its maximum value ； But coincidentally, the concentrated seawater can no longer hold calcium sulfate; at this point, with a slight concentration increase, calcium sulfate will precipitate out. In physicochemical terms, within a certain concentration range, an increase in sodium chloride concentration reduces the activity coefficient of calcium sulfate. Specific theories? People who work in chemical thermodynamics study them all day long.
When the salt content reaches 12.5%, adding more solid salt causes gypsum to precipitate
The solid salt here – do you mean sodium chloride? Or calcium sulfate (seed)?
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Adding sodium chloride is feasible, but the amount added is relatively large:D
In fact, according to the phase diagram, reducing the sodium chloride content is more effective than increasing it. It’s just that no technology can do this. From an application perspective, adding sodium chloride doesn’t make much sense. Yet, it is heaven that provides food to eat. If the concentration of sodium chloride does not affect the solubility of calcium sulfate, and the concentrations of calcium ions and sulfate ions in seawater remain as they are today, then there would be no such thing as a desalination industry. Why? If calcium sulfate in seawater is saturated at all times and everywhere, reverse osmosis membranes cannot be used, nor can multi-effect evaporation and multi-stage flash evaporation. It will soon be destroyed by calcium sulfate. Fortunately, calcium sulfate has the highest solubility in seawater that has been concentrated by a factor of about 3.8; therefore, multi-effect evaporation, multi-stage flash evaporation, and reverse osmosis can achieve a fresh water recovery rate of 40–60%. However, in any case, the presence of calcium sulfate makes it difficult to utilize the concentrated seawater. Because, starting from the precipitation of calcium sulfate in seawater with a specific gravity of around 12.5, it should be noted that once the concentration rises to around 25 degrees Baume and sodium chloride begins to precipitate, calcium sulfate (gypsum) continues to play a role throughout the process of seawater concentration, from start to finish.
Furthermore, I even doubt whether it is possible to use electrodialysis to separate the divalent ions calcium and sulfate from concentrated seawater. This involves the concept of ionic mobility; generally, monovalent ions such as sodium and chloride ions have high ionic mobility and move quickly under the influence of an electric field, whereas calcium and sulfate ions move more slowly. If seawater is concentrated nearly twice through reverse osmosis, and then sodium chloride is removed using electrodialysis, leaving behind sulfate ions and calcium ions, will calcium sulfate precipitate? Is this equivalent to removing sodium chloride? I also went to consult experts.