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desalination

not dehydration
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most so called desalination systems involve taking a volume of sea water and removing part or all of the water.

What follows are four vereations on the idea of how it might be possible to remove the salt from a volume of sea water, leaving behind potable water.

note
Most of us get get our drinking water from rivers. River water has an average of approximately 118.2 mg per litre of dissolved salts. Roughly 300 times less salty than sea water, but 16.6 times more salty than rain water, and a lot saltier than the ultra pure water that comes from most desalination systems.

Although sea water contains measurable amounts of over 80 of the known elements, and probably contain most of the other natural elements in lesser amounts. 99.9% of the salinity of sea water comes from.
Chloride 18,980 grams per litre 0.53 mol per litre
Sodium 10.556 grams per litre 0.475 mol per litre
Sulphate 2.649 grams per litre 0.028 mol per litre
Magnesium 1.272 grams per litre 0.054 mol per litre
Calcium 0.400 grams per litre
Potassium 0.380 grams per litre
Bromide 0.065 grams per litre
Borate 0.026 grams per litre
Strontium 0.013 grams per litre
Fluoride 0.001 grams per litre.

A commonly used demo in school chemistry lessons is the addition of silver nitrate solution to a sodium chloride solution, to give a sodium nitrate solution and a precipitate of silver chloride. Silver is expensive and sodium nitrate can be used to prevent unwanted erections. So this is not a practical process, and the only reason for mentioning it is to remind you of the types of chemical precipitations that will be needed to remove most of the salts from sea water.

one
First the calcium and magnesium are precipitated as hydroxides, by adding 2.7 grams per litre of sodium as sodium hydroxide. This used to be the way that magnesium was extracted from sea water commercially Then the chloride and sulphate are removed by adding lead fluoride. Giving a solution of sodium / potassium fluoride and precipitate of lead chloride / sulphate. This solution is then passed through a bed of alumina, the sodium fluoride reacts with the alumina to form cryolite, sodium aluminium fluoride, an insoluble mineral that will remove the fluorine and half of the sodium from the solution. The rest of sodium forms sodium aluminate, a soluble mineral, that is used as a water softener. The sodium aluminate becomes less soluble as the water is cooled, and can be used to regenerate the sodium hydroxide. Filtering trough chalk will remove any excess fluorine
the area of doubt here is with regards to the potassium, is there a potassium cryolite. And can enough of the aluminate without using to much energy for cooling

two
Remove the; calcium, magnesium, chloride and sulphate as above, this time using sodium followed by lead acetate. This will leave a sodium and potassium acetate solution. Sodium and potassium acetates are the food additives E262 and E261. The sodium and potassium can be reduced by passing it through an aerated bed containing; alumina, silica and an aerobic bacteria that will consume the acetate. The potassium and sodium will react with the alumina and silica to form insoluble zeolites and clay minerals. This water would need conventional treatment to remove the clay and bacteria
the area of doubt here is that the sodium can not be easily recycled.

three
Remove the; calcium, magnesium, chloride and sulphate as above, this time using metallic lead electrodes, and a low current AC supply. At the anode a coating of lead sulphate and lead chloride starts to form, but this flakes of when the current reverses. As the lead electrodes are eaten away, the solution becomes more caustic and the calcium and magnesium precipitate out. The sodium / potassium hydroxide solution that is left can be filtered through a mixed bed of low grade bauxite and sand.<be> draw backs here are the cost of electricity. and the fact that the magnesium and calcium will be mixed in with the lead salts, possibly making recovery of the lead more complex

four
for use on life boats etc.
part of the standard safety equipment would be two large foldaway water tanks, that can be filled with sea water as needed. And a box containing 100 pre measured doses of powders A & B. powder A is silver citrate and powder B is a finely ground mixture of alumina and silica.

j paul, Jul 29 2011

[link]






       In Canada most of us get our water from massive lakes full of water.
rcarty, Jul 29 2011
  

       // for use on life boats //   

       It was conclusively demonstrated (by D.M.W.D., q.v.) during WWII that the best system of provisioning lifeboats was to fill all available bunkerage with unleaded gasoline, and condense the water resulting from the combustion of this fuel via pipes run along the keel.   

       This meant that not only did motor-lifeboats have vastly extended ranges, but the occupants had a plentiful supply (after filtering though a special filter including activated carbon) of fresh water.   

       A precipitation process might provide fresh water, but not propulsion.
8th of 7, Jul 29 2011
  

       In Maine, most of us get our water from [rcarty].
Alterother, Jul 29 2011
  

       // lack of appropriate capitalization //   

       Well, if you find the right backers for the IPO, there's no reason that you couldn't raise a significant amount of R&D funding from the market for a concept like this.
8th of 7, Jul 30 2011
  

       Lead is, of course, vastly more toxic than sodium chloride; especially since the toxic effects of excess salt last hours (assuming fresh water is available), whereas lead in any amount will affect you for the rest of your life, give you brain-damaged offspring (if you're female), and still leave a toxic corpse.   

       Also, surely there's no need to remove the potassium, calcium, and other minor ions. Just removing most of the sodium and chloride should be enough.
spidermother, Jul 31 2011
  

       Lake water is just river water, that the fish have had more time to piss in.   

       I'm not saying that every one should drink desalinated water. The reason for making the point about dissolved solids in river water was to show that reducing the TDS of sea water from 35g to 0.1g, or less, will make it drinkable.   

       [21] I'm just a crap typist. At 300 words per hour I don't have time to worry to much about the niceties of grammar.   

       The Lead will mostly form insoluble ( Phosgenite ) Chloro-carbonate of Lead, and related minerals. If you are concerned that some Lead will inevitably leak through. Add a small amount of Hydrogen Sulphide, followed by a settling tank, followed be an aerator to remove the Hydrogen Sulphide.
j paul, Jul 31 2011
  

       Man I love technical expert talk. Bun for sincerity. Typos be damned, we need better desal.
white, Jul 31 2011
  

       Man I love technical expert talk. Bun for sincerity. Typos be damned, we need better desal.
white, Jul 31 2011
  

       ......and bad news for anyone called Sal...what is is people have against them?
not_morrison_rm, Aug 02 2011
  

       //Lake water is just river water, that the fish have had more time to piss in.//
I love it when they talk dirty.
AbsintheWithoutLeave, Aug 02 2011
  

       //the ultra pure water that comes from most desalination systems//   

       I'm pretty sure desalinated water (from reverse osmosis, the most common form) isn't all that pure. Specs I've read in the past had them leaving behind a fair ammount of salt in the output stream.   

       ...Anyhow, you talk about "insoluble" precipitates in absolute terms. Generally the vast majority, but not all of a low solubility substance will precipitate.   

       It just seems counterintuitive (and also counter to the concept of entropy) that you'd purify seawater by adding chemicals....
Custardguts, Aug 02 2011
  

       Adding metallic Potassium will "purify" seawater.   

       Admittedly into Potassium Hydroxide and Hydrogen, but it's fun to watch ...
8th of 7, Aug 02 2011
  

       Barium sulphate is one exceptionally insoluble substance; you can safely swallow it, despite barium's toxicity. But lead salts aren't like that. As far as I know, all salts of lead are toxic even in small amounts. They are converted to soluble forms by the stomach's acid.   

       I realised you were relying on their insolubility, but as [Custardguts] said, that's a relative term. Either way, having lead salts around is dangerous, and leaves you with a knotty disposal/recovery problem afterwards.   

       On the other hand, assuming you can get the spent chemicals to a lead refinery, magnesium and calcium contamination should be a trivial problem, as they are relatively easy to separate from lead.
spidermother, Aug 02 2011
  

       The desalination methods sound all well and good, but whats an unwanted erection?
bob, Aug 03 2011
  

       //what's an unwanted erection?// somebody else's.
FlyingToaster, Aug 03 2011
  

       Maybe he meant "unwonted".   

       I like the alchemical nature of this. It might be good for 16th century lifeboats. I very much like 8ths posting about recovering H20 produced thru combustion. That should be very pure. The folks in Dune had no such technique but anywhere that water is superscarce, oxygen and reduced carbon should get you a fair bit of water. I like the idea of catching a giant sandworm and burning it for the water.
bungston, Aug 03 2011
  

       Rain water contains all of elements that are present in sea water, just 5000 times less concentrated. So if sea water has 2*10^-6 parts per million of Lead, then rain water has 4*10^-10 parts per million. And that’s before it as ran over the ground. Average continental crust contain about 1.25*10^-3 of a present of Lead.   

       So we all drink traces of Lead, every day. My drinking water runs down off the North Pennines, there are both Lead deposits, and former Lead works up there. This means that there must be a legally expectable level for Lead in drinking water. The question that I have no way of answering is given the generally low solubility of most of the Lead compound involved, will water produced this way be able to meat those standards.
j paul, Aug 09 2011
  

       I have no idea if the chemistry and the economics are right, but I like this idea.
MaxwellBuchanan, Aug 09 2011
  

       The Lead may be replaceable if it bothers you so much. Bismuth forms an insoluble Oxychloride.   

       I think every one knows that it is possible to remove chloride ions from water, by ADDING a metal that will precipitate an insoluble Chloride.   

       For Custardguts and any one else who is troubled by the idea of adding something to a solution to precipitate Sodium or Magnesium.   

       Magnesium salts are the last to be deposited if sea water is evaporated, because Magnesium Sulphate ( Epsom salts) is very soluble However Magnesia used to be harvested from the sea. this was done by adding ( Wood ash liquor ) Potassium Hydroxide to sea water.
MgSO4 + 2KHO = K2SO4 + Mg(HO)2 Magnesium Hydroxide is insoluble To much Potassium Hydroxide would cause Calcium to come out of solution at the same time, not a problem for this idea.
  

       The version of the removal of Potassium and Sodium that I am going to look at in more detail is by the use of low grade Bauxite and sand. The grade of a Bauxite of cause depends upon how much alumina is present, however a more important figure is the amount of silica that it contains, 5% or more of silica would normally be considered to much. The reason that silica is so important that it reacts with the alumina and the Sodium Hydroxide that is normally be recycled in the Bayer process, to make an insoluble Sodium Aluminium silicate.
j paul, Aug 11 2011
  

       needless to say just because a salt is relatively insoluble does not mean that you will able to safely drink a solution containing it. If you under titer your solution then the final result will not be tonic and if you over titer the results would be toxic. I'm going to stick with known good solutions and leave toxic chemistry for non emergency measures. The risk of consuming even a small quantity of the evolved salts or the toxic solutes would give even a thirsty smart person cause to worry.
WcW, Aug 11 2011
  
      
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