New Process for Desalinization

Australia and China researchers appear to have teamed up to discover a new process for desalinization that requires less energy. It uses minerals found in common clay to assist the evaporation of the water. It does leave a toxic, saline brine behind, but I'm sure that could be handled safely (we handle salt and clay safely). I'm surprised that this works well enough to dramatically reduce the energy requirement. I know that calcium salts give off heat when mixed with water due to heat of mixing (people with a pool may have experienced this with CaCl2), so maybe a version of that concept is part of the program. It's not my branch of chemical engineering, however.

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Another unusual use for clay: carbon capture on the water's surface via zooplankton. If you coat the water surface at an algae bloom with clay dust at night, it will pick up carbon particulates and be eaten by zooplankton. When the sun rises, the zooplankton retreat to deeper water and defecate, where the carbon drops off to the bottom and remains there.

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Molten salt typically uses sodium, but not chloride. The formulations that I am familiar with include sodium (Na) and potassium (K) for the cat-ion (+) component, and nitrite (NO2) and nitrate (NO3) for the an-ion (-). You want a relatively low melting point so it isn't too difficult or dangerous to melt it, and this combination, in the right amounts, melts at around 300 F.

I've worked with a couple dozen molten salt reactors in my career (in the chemical industry, not in electricity generation). Most of them are a high-temperature grade of carbon steel. Any amount of chloride above 200 ppm has a good chance of rapidly corroding the reactor shell and destroying the reactor. I am familiar with one such incident.

Besides, the molten salt in such a reactor only needs to be changed out once every 20 years or so. Desalinization would crank out a tremendous amount of salt every day. About 99% of it would have to be disposed of even if it could be used. The salt could be stored underground, but it would be a lot.

My house apparently uses about 4200 gallons of water per month (seems high to me--I think my daughter's long showers are the culprit). If that water came from the sea, it would require the separation and disposal of over 7 tons a year of salt (4200 gal x 8.33 lb/gal x 0.035 salt weight fraction x 12 month/yr). Multiply that by the number of households in a typical city, and you can see it is a big number.

Many large-scale desalinization plants dump the brine back into the ocean, presumably away from their intake pipe. That's what the Saudi facility in Al-Jubail (one of the world's biggest) does.

 

I don't think sea brine would be preferable for road salts. Sodium chloride can only melt ice down to 20 F, while calcium chloride can melt ice down to -20 F. Also, calcium chloride is less damaging to concrete than sodium chloride. Plus, it would take more energy to evaporate the salt water down to a solid versus leaving it as a brine, plus more capital for solids handling versus pumping a liquid.

Calcium Chloride or Sodium Chloride: Which Is Better for Melting Ice?.