Sciencemadness Discussion Board - Solution leached from ashes

Solution leached from ashes

Levi - 14-3-2007 at 21:04

I did an experiment today with the intent of producing K2SO4 from hardwood ashes and MgSO4. The procedure was to leach hardwood ashes with boiling water and dissolve epsom salt in the obtained solution to produce insoluble MgCO3 and soluble K2SO4.

K2CO3 (aq) + MgSO4 (aq) --> MgCO3 (s) + K2SO4 (aq)

From this point I intended to decant and evaporate to dryness and continue to heat the product to convert KHSO4 to K2SO4 but when I added the epsom salt to the potash solution no precipitate formed :mad:

The solution I obtained from the ashes was a slippery liquid; translucent amber color after filtering the ash residue. Regardless of potassium content, the basic solution should have yielded Mg(OH)2 or MgCO3. Any ideas why I didn't get a precipitate?

benzbenz - 14-3-2007 at 21:49

From your post, i may think what you got is due to acid. In the procedure of experiment, there may be some acid appearing in your solution. And you mentioned KHSO4. KHSO4 is a strong acid, too.

Levi - 15-3-2007 at 00:40

Ah! That makes sense... The carbonate is being brought back into solution by the ever increasing acidity. I hadn't considered that. Is there some sort of buffer I can use to prevent this? My only source of K is potash and my only sulfate source is epsom salt, perhaps someone could suggest a possible route to K2SO4 from these?

If I first reacted epsom salt with NH3 (aq) I could obtain (NH4

2SO4 solution (with Mg(OH)2 precipitate [this works]) and then make a solution of NH4+ CO3-- HSO4- and K+ by adding the leached potash. Is there a way I can isolate the KHSO4 from the (NH4

2CO3? Maybe a solubility difference I could exploit? Temperature? Media? anyone?

not_important - 15-3-2007 at 00:50

Wood ashes are roughly one quarter to one third calcium, a tenth to a quarter potassium, about half as much magnesium as potasium, and about an eighth as much sodium as potassium. Hardwoods are higher in potassium and lower in calcium than softwoods, YMMV depending on the exact type or types of wood and the soils where they grew.

Depending on the conditions of burning and storage, the leachate may be a mix of hydroxide and carbonate, or a mix of carbonate with some bicarbonate. The more hydroxide the more 'slippery' the leachate the more hydroxide. Getting a good extraction can be a fairly slow process, the old fashion barrel arrangement does an OK job as it exposes most of the ash to a low concentration of dissolved salts at the tail end of the extraction, after much of the solubles have been removed.

Given the amount of ash you had and the amount of water used, you should be able to estimate the concentration of potassium in the leachate. Factor in the water used to dissolve the epsom salt. plus the water of crystallization in it. Hydrated magnesium carbonate has a solubility of a bit more than a tenth gram per 100 cc at room temperature.

Try boiling the stuff down a bit, see what happens.

You've lost me with the KHSO4, I've no idea where it entered the question from.

not_important - 15-3-2007 at 00:56

There wood ash leachate will be alkaline, unless your woodash is mostly charred sawdust (it should be nearly white).

You will not see any KHSO4 in these conditions. You may not get pure K2SO4, it can form mixed salts with magnesium and calcium, and there is the sodium in the ash as well. Expect to do some fractional leaching with boiling water, and much fractional crystallising.

You can't get agricultural grade K2SO4? Pottery grade pearl ash - K2CO3? KCl as a salt substitute, or as a soil additive?

Levi - 15-3-2007 at 01:04

Excellent information, not_important, thanks a bundle.

Quote:

Originally posted by not_important
You've lost me with the KHSO4, I've no idea where it entered the question from.

Probably my fault, I just assumed it would be present since sulfate has a -2 charge and this is happening in water. I have no idea if the HSO4 ion actually forms.

I did a little poking around and found that K2SO4 is the least soluble of the 4 possible products by far but I didn't know about the calcium content until your post.

If anyone is curious and thinking of trying this themselves, here is the info I found:

K2SO4 ::::::::::: 110g/L @ 20C
(NH4

2SO4 ::: 412g/L @ 25C
(NH4

2CO3 ::: 320g/L @ 20C
K2CO3 ::::::::::: 937g/L (no temp listed)

I expect the calcium will be even less soluble than potassium so that will mess things up considerably, good to know, though.

Levi - 15-3-2007 at 01:08

Quote:

Originally posted by not_important
You can't get agricultural grade K2SO4? Pottery grade pearl ash - K2CO3? KCl as a salt substitute, or as a soil additive?

Yes but I don't really have any great use for the product, I just wanted to see if it could be done. KCl is insanely pricey where I'm at--somewhere around $4/100g. If I have any luck with this project my next one will be to make my own KCl.

chemrox - 16-3-2007 at 19:22

"slippery" was the tipoff .. acid doesn't feel slippery ...which hardwood? what kind of soil? soil can make a difference

[Edited on 16-3-2007 by chemrox]

Pyridinium - 17-3-2007 at 03:19

Quote:

Originally posted by Levi

Quote:

Originally posted by not_important
You can't get agricultural grade K2SO4? Pottery grade pearl ash - K2CO3? KCl as a salt substitute, or as a soil additive?

If you want to see insanely pricey for 100g, try benzidine dihydrochloride

You threw me with that KHSO4 also, for a moment I was like, wait a second am I missing something here?

From what I understand, ash extracts contain a ratio of K2SO4 : K2CO3 ranging anywhere from 1:10 to 1:2, usually in between. You should be able to crystallize out the sulfate after you concentrate the soln. a little bit by boiling. Sulfate will crystallize out first. Since the concs. are different for various wood types, experiment with a small amount and see how much boiling you have to do for best results.

Whatever the case, don't try your MgSO4 treatment until you recrystallize a couple times. Too many unknowns in the preliminary leachate.

[Edited on by Pyridinium]

unionised - 17-3-2007 at 04:54

KCl as a salt substitute seems like a very much better bet pricewise.
(BTW, hasn't benzidine been banned as a carcinogen?)

not_important - 17-3-2007 at 06:40

The wood ash leachate is such a mix it's not worth attempting to crystalise from it if you're after K2SO4. Boil it down until it would be nearly saturated when cool, cool and decant/filter to remove lime, add the MgSO4 - but not too much, filter, and then evaporate for the K2SO4. Remember that there will be a fair amount of sodium as well, you won't be able to recover all the potassium.

http://www.mortonsalt.com/consumer/products/watersoftening/p...

http://www.mrwpca.org/html/potassium_chloride.html

http://cgi.ebay.com/Sulfate-of-Potash-0-0-50-Potassium-Sulfa...

http://www.kmag.com/general/questions.htm

http://www.cooperseeds.com/gardenchem.php3?cat=fertilizers

Pyridinium - 17-3-2007 at 18:39

Quote:

Originally posted by unionised
(BTW, hasn't benzidine been banned as a carcinogen?)

Not for analytical / lab use
https://www1.fishersci.com/Coupon?cid=261106&gid=3157715
$95 for 5 grams... wow

They just don't use it as a dye intermediate anymore.

.......

Now you guys have me wanting to do a wood ash leachate experiment. They used to crystallize K2SO4 directly from filtered leachates, this was how it was done 150 years ago. When I get the chance and the weather clears I'll try the experiment.