Sciencemadness Discussion Board

Making KOH from wood ashes why is it yellow ?

metalresearcher - 8-9-2010 at 12:28

I tried extracting KOH / K2CO3 from wood ashes by first filtering the ash to remove nails, charcoal and other coarse stuff, then boiled it and then filtered it. This results in a piss-colored liquid (see picture).
The wood I burned in my woodstove are pallets (hence the nails) and logs bought at the local gas station.
Is this color due to Fe3+ ? I let evaporate the water from a previous batch and the result turned rather white but with some brown crystals in it.
Bringing a MgO stick wetted with the solution in a flame results in apurple with some orange tinge in it so there is not much Na in it, otherwise the yellow color dominates.

Does somebody have experience in making KOH from wood ash ?

IMG_4108.JPG - 70kB

Ozone - 8-9-2010 at 16:02

The brown stuff is likely solublized phenolics from lignin (solublized in alkali) intrinsic to your wood...and probably some pentose alkaline degradation (reverse-aldol and re-condensation) products, primarily from hemicellulose (arabinoxylan, etc). Any iron (III) would have been filtered out as colloidal ferric hydroxide; it is insoluble at pH>4. A quick test would be to add FeCl3 and see if you get black/green stuff--which would the iron complex of 3,4-dihydroxyphenols (and others).

Cheers,

O3

blogfast25 - 9-9-2010 at 08:04

Decades ago, my potash (K2CO3) came out looking exactly the same. Recrystallising should get rid of much of the organic junk. It's not Fe3+ because in these alkaline conditions that would drop out as insoluble Fe(OH)3.n H2O, a reddish gel.

You can't make KOH directly from wood ash but a simple displacement reaction:

Ca(OH)2 + K2CO3 ---> 2 KOH + CaCO3

... can be used to make KOH solutions. Simply boil more or less stoichiometric amounts of slaked lime (Ca(OH)2) with potash for about 15 mins: insoluble CaCO3 drops out. Filter and Bob's your uncle...



[Edited on 9-9-2010 by blogfast25]

metalresearcher - 9-9-2010 at 08:32

Quote: Originally posted by blogfast25  
Recrystallising should get rid of much of the organic junk.

You mean redissolve the crystals ? Then everything dissolves including the organic junk.

blogfast25 - 9-9-2010 at 08:39

Recrystallising means redissolving and then crystallising only a part of the product, leaving some with the impurities in the liquid above. So, redissolve the crystals in just about the right amount of water, then carefully evaporate part of the water. Crystals of higher purity form because the impurities tend to amass in the saturated supernatant liquid...

Solubility data on K2CO3 are found in Wiki.

For K2CO3 the solubility at 0 Celsius is listed as 105 g / 100 g of water and at 100 Celsius 156 g / 100 g of water (Wiki). From a saturated solution of 256 g at 100 Celsius, about 51 g of K2CO3 will crystallise out on cooling to about 0 Celsius. That should be quite a good purity 1st crop. Decant off the supernatant (saturated) solution and gently boil it in until crystals start forming, always maintaining some liquid. A second crop can thus be obtained, slightly less pure probably.

[Edited on 9-9-2010 by blogfast25]

metalresearcher - 10-9-2010 at 06:23

Thanks, I'll try !

But when there is Na2CO3 in the solution that will precipitate first as it dissolves only 45g @100ml (according to http://en.wikipedia.org/wiki/Na2CO3).
But according to the flame color (which does not show much Na-yellow color) there is only a low Na content in the product.

[Edited on 2010-9-10 by metalresearcher]

Xenoid - 10-9-2010 at 15:24

Many years ago when I first moved into our present house, I discovered an ash pit below the fireplace. When emptied, I had several sacks of wood ash. I had a go at extracting the K2CO3 using buckets and cloth filters. I eventually gave up after realising how much "boiling down" would be required.

The colour of the liquid was an intense golden yellow, much stronger than shown in the image above. It was similar to a "dichromate" solution. At the time I wondered about this but came to no satisfactory explanation. The wood ashes contained nails and traces of green Cr2O3 (from wood preservative). I thought perhaps the colour was due to traces of ferrocyanide, maybe the previous owner had been burning food scraps. But potassium ferrocyanide solutions are a very pale canary yellow, not the intense orange-yellow I had.

I'll have to except that the colourant was some organic based compound, exactly what, I don't know. What I don't understand is how any organic compound could survive the temperatures involved, without decomposition!

watson.fawkes - 10-9-2010 at 16:13

Quote: Originally posted by Xenoid  
I'll have to except that the colourant was some organic based compound, exactly what, I don't know. What I don't understand is how any organic compound could survive the temperatures involved, without decomposition!
Most wood fires are at least a little oxygen-starved. High temperature in absence of oxygen does not imply decomposition; sometimes it's the opposite. Graphite production furnaces build C-C bonds, for example. My guess is that these colorants are high in polycyclic aromatics, which are, in a sense, very early graphite precursors.

Pomzazed - 11-9-2010 at 00:24

And those mentioned polyaromatics are insoluble in water, so I doubt this would be the case.

It's true that in the absence of oxidizing agent (O2), some of the organic compounds would survive.
(In an inorganic point of view, organic compounds are just "hydride of clustered carbon" and their derivatives, so w/o ox.agent it should stands to some point of heat!)

watson.fawkes - 11-9-2010 at 04:28

Quote: Originally posted by Pomzazed  
And those mentioned polyaromatics are insoluble in water, so I doubt this would be the case.
I doubt they're pure hydrocarbons, without some functional group somewhere. I was only thinking about the carbon skeleton.

Ozone - 11-9-2010 at 05:21

Lookup pyrolysis oil and/or pyrolytic lignin.

Put it under a cheap black-light. PAH will usually fluoresce, particularly when dilute. I'm thinking traces of wood creosote (or pyrolytic products thereof)--PAH and phenolic residues, maybe a trace of pyrazine/pyridine Maillard product (from high temperature reaction of wood-sugars with amino nitrogen) for extra smoky goodness (many are also yellow or brown at ppm concentrations).

Treat you liquor with granulated activated carbon, either in a column or applied as a slurry at 60-100°C, then filter.

O3

[Edited on 11-9-2010 by Ozone]

blogfast25 - 11-9-2010 at 07:53

How about trying to oxidise the stuff with H2O2?

blogfast25 - 11-9-2010 at 07:58

Quote: Originally posted by metalresearcher  
Thanks, I'll try !

But when there is Na2CO3 in the solution that will precipitate first as it dissolves only 45g @100ml (according to http://en.wikipedia.org/wiki/Na2CO3).
But according to the flame color (which does not show much Na-yellow color) there is only a low Na content in the product.

[Edited on 2010-9-10 by metalresearcher]


The colour of a flame is a poor indicator of Na, because the Na D-emission line is extraordinarily strong. Flame wise I still find sodium in carefully recrystallised samples of KCl, LiCl or CaCl2... The amount of Na in (real) potash is likely to be quite low, IMHO...

Random - 11-9-2010 at 12:02

Quote: Originally posted by blogfast25  
Quote: Originally posted by metalresearcher  
Thanks, I'll try !

But when there is Na2CO3 in the solution that will precipitate first as it dissolves only 45g @100ml (according to http://en.wikipedia.org/wiki/Na2CO3).
But according to the flame color (which does not show much Na-yellow color) there is only a low Na content in the product.

[Edited on 2010-9-10 by metalresearcher]


The colour of a flame is a poor indicator of Na, because the Na D-emission line is extraordinarily strong. Flame wise I still find sodium in carefully recrystallised samples of KCl, LiCl or CaCl2... The amount of Na in (real) potash is likely to be quite low, IMHO...


If I remember correctly, sodium is toxic for plants actually and plants don't need it.

Making KOH from wood ashes...

chipwit - 13-9-2010 at 12:07

From Wagner's Chemical Technology, 1872, on the manufacture of potash: "The liquor obtained by lixiviation is of a brown colour, owing to organic matter, humine or ulmine, which the potassium carbonate has dissolved from the small chips of imperfectly burnt charcoal." The product was then calcined to drive off water and burn off organic matter, leaving a white product.

ScienceSquirrel - 13-9-2010 at 14:24

Some plants like asparagus are far more salt tolerant than others and a light application of salt can be used to control weeds and some plant diseases.
The salt in the surface soil particularly will have a powerful antigerminative effect on a lot of common weed seeds. The asparagus will enjoy the salt and use its deep roots to take fresh water.

http://faq.gardenweb.com/faq/lists/tips/1998031825022979.htm...

bob800 - 17-9-2011 at 15:06

I just extracted the soluble components from garden soil (to do a crude qualitative analysis), and I noticed that the concentrated solution was almost identical in color to some potash solution I made awhile back (from ashes):




(The potash is on the right, soil is on the left)

Perhaps the same organic containment is present in both solutions? It would make sense, since ash comes from wood, wood obviously comes from trees, and trees obviously grow in soil.

Neil - 18-9-2011 at 01:19

A wood stove? Most of your K is likely gone long before you leached the ash.

check out wood fire fly ash studies; K volatilizes out of hot fires with incredible ease.

http://www.sciencedirect.com/science/article/pii/S0016236104...

http://www.iiasa.ac.at/Research/FOR/downloads/Kraxner/CD%20V...

There are some very good 70's era ones IIRC but I can't find them at the moment.

blogfast25 - 18-9-2011 at 04:55

Quote: Originally posted by Neil  
A wood stove? Most of your K is likely gone long before you leached the ash.

check out wood fire fly ash studies; K volatilizes out of hot fires with incredible ease.

http://www.sciencedirect.com/science/article/pii/S0016236104...

http://www.iiasa.ac.at/Research/FOR/downloads/Kraxner/CD%20V...

There are some very good 70's era ones IIRC but I can't find them at the moment.


Hmm...maybe so but potash is called so because lixixiating it from wood ash (in pots) was how it was once mainly produced. I've done it myself many moons ago. You don't get much potash from a kilo of ash, though... And it was slightly coloured too.

The colours are entirely coincidental: fresh soil anf burned wood are very different things. Even fresh wood and fresh soil are very different things...

[Edited on 18-9-2011 by blogfast25]

bbartlog - 18-9-2011 at 05:14

Quote:
K volatilizes out of hot fires with incredible ease.


One of your links doesn't say anything about volatilization (rather, it indicates that about 4% of fly ash from wood is potassium as K2O equivalent). The other makes mention of small ash particles (carried up the flue/exhaust) having alkali content, which is really not the same as volatilization. In both cases (incinerator and boiler), we are dealing with fires rather hotter than a home woodstove, and yet I don't see any suggestion of vaporized K or K compounds.
If you were correct, I would expect that potash would historically have been scraped from the inside of chimneys (perhaps built for the purpose), rather than leached from ashes. But I'm quite sure you're wrong.

blogfast25 - 18-9-2011 at 06:00

Good points bbartlog. The natural potassium content of fresh plant material is low and is what accounts for the low yields when making potash from wood ash, not volatilisation and not fly ash either. Remember also that the term 'alkali' derives from the arabic for 'from the ashes'.

In pursuit of some base chemicals (especially to fuel our favourite passtime: making war on each other!) in the past many woods have been chopped down (timber for fleets) or burned (e.g. the extraction of alum from mines, which required tonnes of hot water).

Neil - 18-9-2011 at 09:10

Quote: Originally posted by bbartlog  
Quote:
K volatilizes out of hot fires with incredible ease.


One of your links doesn't say anything about volatilization (rather, it indicates that about 4% of fly ash from wood is potassium as K2O equivalent). The other makes mention of small ash particles (carried up the flue/exhaust) having alkali content, which is really not the same as volatilization. In both cases (incinerator and boiler), we are dealing with fires rather hotter than a home woodstove, and yet I don't see any suggestion of vaporized K or K compounds.
If you were correct, I would expect that potash would historically have been scraped from the inside of chimneys (perhaps built for the purpose), rather than leached from ashes. But I'm quite sure you're wrong.




...yup :(

I had it my head that wood stoves ran a lot hotter then they do.


Here is the volitization data as well as percent weight compositions of different woods. Not the study I was thinking of but close enough.

http://www.fpl.fs.fed.us/documnts/pdf1993/misra93a.pdf

bbartlog - 18-9-2011 at 17:26

Fascinating. While 'incredible ease' is an overstatement, that's some interesting data. Home woodstoves don't reach 1300C (unless you're trying to destroy them), but I know that parts of my stove fire reach 900-1000C, as I inadvertently fused K2CO3 on my attempt to purify pearlash. And according to the graphs you show, that would be enough to reduce the potassium present. I suspect that, in practice, there is some recondensation of whatever potassium is evaporating, since the temperature drop down to 500-600C or so within the stove itself is rapid; but clearly it would be possible to reduce yields with too hot a fire under not-too-extreme circumstances, which I would not have guessed.

Neil - 19-9-2011 at 14:40

Context I suppose. The last time I had a wood fire that had no K streak in the flame was a long time ago. If you ash leaves and the like and hold the ash against a butane or candle flame you often get a really nice K colour. Fall is nearly here...

As to the fly ash, I get that fly ash is mainly bits of dislodged ash but it seems to me that any Ca/K going up in the smoke would freeze out into fly ash of sorts pretty fast. Anyone bored enough to try leaching a cinder block?

Neil - 16-10-2011 at 12:40

I am having a hard time finding cite worthy data on the running of a pot ash kiln, but there seems to be a consensus that they were designed and operated to burn their fuel slowly.

I tried to get a picture of a clear potassium streak in a ordinary wood fire in an open hearth but the digital camera was too hindered by its IR sensitivity, all the pictures looked like they where of a 10,000° inferno :(


In any case - a examination of the compounds found in wood fly ash (not exactly the same but as close as I have yet to get) http://marioloureiro.net/ciencia/ignicao_vegt/ragla91a.pdf
Properties of Wood for Combustion Analysis



http://www.mendeley.com/research/high-temperature-elemental-...
High temperature elemental losses and mineralogical changes in common biomass ashes

funnily though it contradicts the results of other papers they report;
"Losses during the initial ashing at 525 C were negligible, except for a K2O loss of 26% for wood..."

though they do not say what the loss mechanism was... so ptthhbbbbbb to them


http://www.vtwoodsmoke.org/pdf/Fine01.pdf
"Chemical Characterization of Fine Particle Emissions from Fireplace Combustion of Woods Grown in the Northeastern United States"

Which led me too finding this gem;

http://www.me.utexas.edu/~longoria/paynter/hmp/The_First_Pat...

"Hopkins's key advance lay in burning the raw ashes in a furnace before they were dissolved in water. This second burning resulted in much greater carbonate formation, apparently because the free carbon in raw ashes (which partly accounts for the black color) was more completely oxidized and because of exposure to concentrated carbon dioxide gas from the fire. Hopkins also increased yields by mixing the insoluble residue from one batch with the raw ashes of the next, instead of simply discarding it"


and this gem, as well;
Influence of metal ions on volatile products of pyrolysis of wood

Potassium in wood samples seems to have the effect of increasing the rate at which volatiles are driven off.

Perhaps smouldering a bunch of bark/grass/straw/twigs and then re-burning the ashes in the hotter cleaner wood stove would yield a cleaner ash with a greater K content..?

ScienceSquirrel - 17-10-2011 at 08:38

Looking at this experimental procedure I would guess that the yield is very low.

http://faculty.rmc.edu/jthoburn/ChemArt/EX03.Potash.pdf

Making respectable amounts is going to require bins full of water and ashes, big pans for boiling down and lots of fuel.
I suspect that if you had a big heap of wood to get rid of, lots of time and a lot of big gear you could make a kilo or so which would be worth less than a tenner on eBay!

Neil - 17-10-2011 at 09:02

Ack! I was relegated to the spam sandwich along with PHD :(


Original patent for improved production of potash

http://www.patentstation.com/mdm/firstpatent.htm


It seems that from an acre of old growth you would get about a tonne of potash.

ScienceSquirrel - 17-10-2011 at 09:30

Quote: Originally posted by Neil  
Ack! I was relegated to the spam sandwich along with PHD :(


Original patent for improved production of potash

http://www.patentstation.com/mdm/firstpatent.htm


It seems that from an acre of old growth you would get about a tonne of potash.


So a couple of trees or a whole heap of pallets for a kilo? :)

blogfast25 - 17-10-2011 at 12:13

I did it once when I was about 16. A whole heaped bucket from our open fire, not sure how much in weight. Went through the motions and sure enough I got a bit of potash, a few tablespoons full of the yellowish stuff. I remember foolishly converting it KCl, which is a pretty useless substance really.

Still, leaching it from wood ash was once a main source of K-alkali. If the yield was about 100 g per 1 kg of ash it probably still would be.

watson.fawkes - 17-10-2011 at 17:36

Quote: Originally posted by Neil  
I am having a hard time finding cite worthy data on the running of a pot ash kiln, but there seems to be a consensus that they were designed and operated to burn their fuel slowly.
This seems like an ideal use for a solar concentrator. For home use, it's set and forget for a while. The idea might be adaptable for generation of pyrolysis gas and making charcoal from biomass on a light industrial scale, although burning that charcoal for heat is at odds with potassium extraction.