Toshiro
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Seawater
Hello my friends , I have a question please , i'd like to understand how to separate magnesium chloride , potassium chloride and magnesium sulfate
from seawater , i know that when you evaporate seawater between 100% and 50% first the calcium carbonate (CaCO3= limestone) precipitates out, Between
50% and 20%, gypsum precipitates out (CaSO4.2H2O . Between 20% and 1% sea salt precipitates (NaCl) but going further, how can i separate magnesium
chloride , potassium chloride and magnesium sulfate ?
Thanks
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JohnWW
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It is very difficult to separate the KCl and MgCl2 by differential crystallization, because of the similarity of their solubilities, which can result
in the formation of crystals of the double salt carnallite, KCl.MgCl2.6H2O, found in deposits as a mineral in some areas of ancient dried-up salt
lakes, although in association with potash, KCl, and chloromagnesite, MgCl2. They are much more soluble than common salt (halite), NaCl, and LiCl,
which would deposit out first. However, the solubility of MgSO4 is slightly less, and can come out as epsomite, MgSO4.6H2O, while the bromides and
iodides of alkali and alkaline earth metals are more soluble than the chlorides.
Also, before the sparingly soluble gypsum, CaSO4.2H2O (with Sr replacing some of the Ca, as SrSO4 is less soluble), precipitates out, there would be
the deposition from solution of the small amounts of the phosphates and fluorides of certain of the alkali and alkaline earth metals, noting that
CaF2/SrF2 (fluorite), LiF, and Li3PO4 are only very slightly soluble, and (Ca,Sr)3(PO4)2 and Mg3(PO4)2 almost totally insoluble.
[Edited on 28-3-10 by JohnWW]
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Formatik
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MgCl2 hydrated breaks down when heated giving off HCl and MgO (barely soluble), mechanism here. Since in sea water more Cl- than MgSO4 is present (chart below by Donald Garett, in terms of compounds see the same reference),
all of the Mg can be reacted by that route. Thus, you would be left with mostly alkali chloride and sulfate and MgO (difficultly soluble). After
filtration, mainly the former two. Much of the alkali sulfate should be separable by fractional crystallization by chilling. This will leave an impure
chloride behind (still containing some SO4, traces of MgO, Sr, BO3, Br, etc). You might want to hit the literature about some processes to see how
separation has been done.
[Edited on 27-3-2010 by Formatik]
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JohnWW
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That is a very incomplete analysis, particularly omitting N, Si, P, I, F, Li, Sr, Al, Fe, which are present in small but significant amounts, and many
other elements present in trace (ppm to ppb) amounts. I have a textbook on geochemistry somewhere which gives a very complete analysis
[Edited on 28-3-10 by JohnWW]
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Toshiro
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Thanks everyone , please post this analysis JohnWW and a process to separate all salts from seawater ...
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not_important
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You'll need to find some of the solubility charts worked out decades ago, as crystallisation of complex mixtures is ... complex, as JohnWW said.
The exact order of crystallisation, and the salts crystallising, depends on the temperature as well as the salinity. Potassium, magnesium, and sodium
form a number of mixed chlorides and sulfates, and the product at a given point of evaporation will often be a mixture of several salts such as
KMgCl3·6(H2O) (Carnallite) with K2SO4·MgSO4·MgCl2·6(H2O), K2SO4·MgSO4·4(H2O), and KCl.
And most of those mixed salts are only stable when crystallised from the starting mixture, recrystallising one from solution in plain water gives
different results; in fact this is how they often are purified. Carnallite as mined is dumped in a MgCl2 brine and heated, the KCl dissolves while
much of the NaCl and MgSO4 do not; cooling resulting in a mixture of mostly KCl with some magnesium and sodium as mixed halides and the remaining
magnesium-rich brine is used in another extraction from more carnallite.
See United States Patent 3852044 for a bit more, and the attached PDF discusses the fate of some of the trace elements
Attachment: 96-105_613_Amdouni.pdf (534kB)
This file has been downloaded 443 times
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Toshiro
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You answered my question , thanks a lot my friend
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Waffles SS
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If NaOH add to solution contain magnesium chloride or magnesium sulphate then magnesium hydrooxide will separate.
Here there is company that produce diffrent chemical from sea water(NaOH,NaClO,CaClO,HCl,MgSO4,KCl,KOH,MgCl2,..)
They boil water by sunlight(using mirror) and add certain chemical to it for separation.
[Edited on 30-3-2010 by Waffles SS]
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