# How thin a filter to filter particles out of salt water to make salt for human consumption?

I have gathered some salt in the Death Valley and for the fun of it I would like to clean it and make it edible. The process I plan to follow is simple:

1. Dissolve the salt in water.

2. Boil water for 30 min to kill bacteria.

3. Filter water to remove foreign particles.

4. Evaporate water to get clean salt.

At step 3, I'm planning to buy paper filters with pores of 3 µm to filter fine particles. Is it sufficient for the purpose of human consumption?

I'm mostly wondering if there is a key pore size threshold beyond which it is significantly less useful to filter, or if it's just a matter of degree, the smaller the better at an consistent size/benefit ratio.

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I wouldn't bother with step 2. Anything that's hyper-adapted to the extreme dry and saline environment will not survive in the wet and acid environment of your stomach. I'd be more worried about other minerals mixed in, e.g. borax. –  MSalters Jan 16 '12 at 11:47
Thanks MSalters - I was hoping that filtering would precisely help with sorting out other minerals. Are you saying it will not? I am worried about those, do you have an idea how to sort them out? –  Olivier Compagne Jan 16 '12 at 22:45
Well, borax dissolves too, so you can't actually filter it. A quick read shows that the conventional approach to that is multi-stage crystallization. NaCl dissolves quite well, so the first precipitate will be mostly other minerals. –  MSalters Jan 17 '12 at 9:42
Have a look at this: nature.nps.gov/geology/usgsnps/deva/devasalt.html . –  anna v Feb 15 '12 at 5:29

There are 3 kinds of mixtures in liquid...

1. True Solution
2. Colloids
3. Suspension

These three vary in between because of the size of the particle in them. see wiki. Now, the salt solution you were talking about comes under category "true solution" i.e. particle size less than 1 $nm$.

Now we don't have sieve to filter out this particles of this dimension. Even bacterias and microbes are orders of magnitude greater than this dimension.

As per classical textbooks, it's only suspension that can be filtered out using sieve or sedimentation. Even for colloids one needs ultrafiltration methods, with superfine pores in the filter.

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Thanks Vineet. If I understand you correctly, you're saying filtering with sieve is useless because such filtering method doesn't work with true solutions. If that's what you mean, you may have misunderstood my question. I want to pass the salt through the filter, precisely because it will let the salt through. I am hoping however to filter other particles that are in suspension. I would then evaporate the solution, so that (almost) only pure salt remains. So your explanation confirms my idea: sieve filtering is a good option. My question is: what pore size is most appropriate? –  Olivier Compagne Feb 15 '12 at 5:35
but the dissolved salt contains many other chemicals which you don not want to consume, as @MSalters pointed out Borax! The filtrate would contain borax in addition to salt! Have a look at this document(krebs-swiss.com/salex.pdf). Commercially chemical purifiers are added, but I guess you are just doing it for fun... –  Vineet Menon Feb 15 '12 at 5:50
Yes, just for fun. But it's a great resource you gave me, thanks. Let's cut the chase, I just want to clean this salt I gathered so as to be able to eat it without being sick or shortening my life span. So far, I boiled it, then filter it through a simple coffee filter, and evaporated it. I have some nice white salt (with barely visible particles of dirt, true, but it's okay). Based on your knowledge, would you eat 2 pounds of it? (over the length of time you usually eat 2 pounds of salt) –  Olivier Compagne Feb 15 '12 at 6:16
yup..i would! anyways I'm consuming more harmful things via air and water. A few pounds, I don't think will cause much trouble! :P –  Vineet Menon Feb 15 '12 at 6:59
Everything is relative, and I needed a relative answer. Thanks, that's what I wanted to know! Now I can brag with my Death Valley salt... –  Olivier Compagne Feb 15 '12 at 8:54

I wouldn't worry too hard since presumably you won't be consuming a huge amount, but 3um won't do much about bacteria or parasites. In biology a 0.2 micron filter is usually used when preparing sterile solutions. However, this still won't do anything about viruses or possible chemical contamination. Perhaps try asking at the biology beta stack exchange?

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The purpose of filtering is to filter out fine particles (other minerals, etc.), not bacteria or parasites. I'll try at the biology beta stack exchange - thanks! –  Olivier Compagne Jan 16 '12 at 22:42

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