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chemoleo
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Yellow as in Na2CrO4? Try and add some HCl, it'd form dichromate which is red.
Otherwise, it could be the hydrated form of Fe2O3, which is yellow too (albeit insoluble, but maybe it forms a very fine suspension).
Never Stop to Begin, and Never Begin to Stop...
Tolerance is good. But not with the intolerant! (Wilhelm Busch)
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S.C. Wack
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The electric yellow of chromate and sugar drink orange of dichromate are unmistakable but not indicative of the amount in solution. One could compare
with the weight before leaching.
edit: obviously this would require knowing the type of steel, to find the Cr %. Perhaps acid/formaldehyde -> Cr+3, would be a better test of Cr
content, though this would require one to have a little formaldehyde on hand.
Anyone interested should download/look under chromium in Thorpe's Dictionary of Applied Chemistry.
[Edited on 19-5-2005 by S.C. Wack]
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Pyridinium
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| Quote: | Originally posted by unionised
"Remember that Chromium is also amphotheric, like aluminium, and will react with alkalis (although not with the same vigour IIRC)"
That's news to me and to those who wrote the text books I have seen.
Granted, if you were to oxidise it to Cr(VI) it would be, but that's not really relevant to washing the metal with caustic.
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Merck Index indicates Cr metal is attacked by caustic alkali and even alkali carbonates.  It didn't say anything about them having to be molten, either.
So I could leave a piece of stainless in conc. NaOH for oh, about 6 months, and I might get some Cr salts even without electrolysis.
I have an old qual. analysis text here that says Cr(OH)3 is an amphoteric hydroxide.
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12AX7
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Picture
It looks a bit greener, say like dilute ferrous chloride, than the picture shows. Doesn't seem too concentrated to me, although I'm sure
it's strongly basic (it's kind of slightly viscous like that).
Dropping HCl didn't cause anything to precipitate or change color in the vicinity of the drop.
Other thing I was thinking was to heat it with a reducer (like some powdered charcoal) to get it down to powdered oxides and magnetically seperate the
Fe3O4. That would at least concentrate it to a brittle nichrome.
From what I've read, the stainless should be from 15 to 20% Cr and 8 to 15% Ni.
Tim
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Pyridinium
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| Quote: | Originally posted by 12AX7
Other thing I was thinking was to heat it with a reducer (like some powdered charcoal) to get it down to powdered oxides and magnetically seperate the
Fe3O4. That would at least concentrate it to a brittle nichrome.
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That sounds like a good idea; roast everything to dryness (first wash away all soluble alkali) to turn all hydrous Fe oxides to Fe2O3. Then roast
again, this time in contact with chunks of carbon (maybe not powder or it could go *poof* when it gets good and hot)... hot Fe2O3 will form Fe3O4 in
contact with carbon (I've done this).
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Jome
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Weird reaction?
Yesterday I tried to make elemental cromium from a stochiometric mix of Cr2O3 and Al.
7g initiated by 1,5g of KClO3+sugar burned in about 1,5 seconds, leaving a dark gray, partially molen powder which obviously had some Cr2O3 green in
it.
First I was not sure how to "prove" it was cromium, but I've read about this wonderful method of cromate-manufacture from the hive, so
I decided to give it a try. I added the dark grey powder to an ammount of 15% muriatic acid.
I didn't measure since this was only a 100ml beaker size experiment, not an actual manufacture. After a few hours the solution was very dark
dark green, with some solid pieces still laying in the bottom. Now about 10ml of this solution was added to 20ml of water.
After KOH flakes was added and the solutions stirred. First the excess acid was neutralised, then Cr(OH)3 precipated and at last the precipate
dissolved to form some sort of cromite.
To this cromite solution was then added 20ml of 30%hydrogen peroxide solution, which made it bubble about as much as a soda and coloured it dark
yellow.
Now I had potassium cromate! So far, nothing weird. The cromate actually looked just like soda, "forrest fruit fanta" to be exact. Oxygen
being evolved at a slow but steady rate. Guess I added to much H2O2....
But then I remembered a procedure from frogfots page to form red dicromate from glacial acetic acid and cromate. I decided to add some 25% to the
clear yellow solution in a test-tube.
I did not clean the 10ml measuring cylinder from cromate residue before measuring up the acetic acid, they were going to meet anyway I figured. So
about 0,1-0,2ml of yellow dicromate drops was still in there, but when the 2ml of acetic acid was added something weird happened: It turned blue. Not
red as It was supposed to.
About as blue as concentrated copper-sulphate soln. When 2ml of yellow chromate solution was added to 2ml of 25% acetic acid the solution got so blue
it was almost black.
Adding the dark-blue solution to 20 parts water made it dissolve in a weird way, leaving a very slight yellow tone.
I also discovered that the 0,1 cromate to 1ml of acetic quickly looses its strong blue color, after 20 minutes (writing this) the solution in the test
tube has lost all it's blue color, in fact the blue is very hard to see. What happened?
Only blue Cr-compound that I know of is perchromate, but could this be? I'd think that would be much harder than this to make.
Any idea, anyone?
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sparkgap
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From your description, the transient blue color does seem to be due to peroxychromate production. The peroxide added, with acidification are the
conditions, IIRC, for producing the transitory peroxychromate, which lasts only for a short period of time before the solution turns from a pale green
to a slight yellow. That much I remember from undergraduate qualitative analysis.
See also this.
sparky (~_~)
[Edited on 21-5-2005 by sparkgap]
"What's UTFSE? I keep hearing about it, but I can't be arsed to search for the answer..."
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Jome
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That explains it all! Addning it to water raises the pH so that the strong color gets more diluted than one would think it'd be. This seems like
viable way to retrieve cromate ifrom Cr2O3, now if only I could get the Al out....Al is also amphoteric forming aluminates floating around the
solution.
Btw how would the FFC cambridge process, that new method of making titanium from TiO2 work for chromium?
This page:
http://www.msm.cam.ac.uk/djf/FFC_Process.htm
States that Cr2O3 and lots of other metaloxides could also be reduced to its metal (even SiO2  ) but what I dont get is how it works, TiO2 is a semiconductor but Cr2O3 isn't.
CaCl2 melts at 782 C, not at all impossible to get.
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12AX7
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| Quote: | Originally posted by Jome
This seems like viable way to retrieve cromate ifrom Cr2O3, now if only I could get the Al out....Al is also amphoteric forming aluminates floating
around the solution. |
Could reduce it down to say CrCl3 + AlCl3 and try cementating with zinc or plating out. If you want metal, anyway.
| Quote: | Originally posted by Jome
This page:
http://www.msm.cam.ac.uk/djf/FFC_Process.htm
States that Cr2O3 and lots of other metaloxides could also be reduced to its metal (even SiO2 ) but what I dont get is how it works, TiO2 is a semiconductor but Cr2O3 isn't.
CaCl2 melts at 782 C, not at all impossible to get. |
An eutectic of NaCl, KCl, CaCl2 and/or MgCl2 (supposedly MgCl2 is quite corrosive so may be the best route, in terms of dissolving things) melts as
low or lower than red heat, though solubility may suffer from being cooler.
Anyway, the process described is the Hall process, with absolutely no details about how it is different and why it works where Hall doesn't. In
summary, the oxides are *dissolved* in the chloride solvent and electrolyzed, as electrowinning metal from an aqueous solution.
Edit: that was weird. I accidentially doubleposted, got the "only your second message was stopped" screen, but both posts appeared
anyway... so I deleted one, but that pulled both down! Good thing I copied to clipboard...
Tim
[Edited on 20-5-2005 by 12AX7]
[Edited on 20-5-2005 by 12AX7]
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Jome
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Great idea to use Zn.
However the FFC process does not seem to be similar to the Hall process except the use of elektricity.
http://www.corrosion-doctors.org/Electrowinning/Aluminum.htm
The Al2O3 is dissolved in cryolite and is reduced by a Carbon cathode. The molten Al sinks down to the bottom and is collected.
In the FFC process the carbon instead is the anode, and the oxide is the cathode, which is reduced to the pure metal (sponge) without ever being
dissolved. Quite different I´d say!
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12AX7
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| Quote: | Originally posted by Jome
However the FFC process does not seem to be similar to the Hall process except the use of elektricity.
http://www.corrosion-doctors.org/Electrowinning/Aluminum.htm
The Al2O3 is dissolved in cryolite and is reduced by a Carbon cathode. The molten Al sinks down to the bottom and is collected.
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Well no, the electrolysis directly reduces the aluminum to the cathode, the combustion reaction is incident. Would be great if aluminum didn't
form carbides. (I know, you can smelt aluminum (and silicon) just fine, but practically, it requires an arc furnace.)
| Quote: | | In the FFC process the carbon instead is the anode, and the oxide is the cathode, which is reduced to the pure metal (sponge) without ever being
dissolved. Quite different I´d say! |
Oops, right you are. I shouldn't read this early in the morning.
Tim
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Jome
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Seems like only half of the chromium pieces / powder really got dissolved in the hydrochloric.... Could it be that this reaction is usually slow but
proceeded faster because some of the Cr pieces has Al in it?
Im going to try purifying the Cr today, precipate cromium metal powder by addidion of zink, then rinse this. The thermite method could be a quite
useful route to small quantities of elemental cromium powder. Something like:
Cr2O3+Al---->(impure) Cr blobs
Impure Cr is dissolved in hydrochloric acid. The solution filtered and dried down to impure CrCl3 chrystalls, these are dissolved in water and solid
Zn is added which precipates Cr and not Al. Powder is rinsed and can then be used to whatever use Cr may have (Im trying to make ammonium dicromate
myself)
Just gotta find a good ratio of Cr2O3 to Al, the reaction with 25micron flake Al stoich mix was to fast, most of the powder flew away.
Edit: OR of course I could use NaOH as suggested earlier in this thread to remove Al and get fine Cr powder from the start. Only problem would be
Cr2O3 residue, this seems hard to dissolve in KOH at least (I've tried).
[Edited on 21-5-2005 by Jome]
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neutrino
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Or, Cr<sub>2</sub>O<sub>3</sub> + 6HCl -> 2CrCl<sub>3</sub> + 3 H<sub>2</sub>O.
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Jome
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That doesn't seem to occur, neither does KOH affect Cr2O3. I've had them sitting in test tubes now for over a week, and if Cr has been
dissolved its not much, in fact I can hardly see any green color. And since I just prepared CrCl3 I've seen how powerful that green color is.
I use 30% HCL and Cr2O3 from ceramic "chrome green".
I tried CuSO4 too but nothing has happened there either.
Aluminotermic reduction of Cr2O3 seems to be the only way if one starts from Cr2O3!
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chemoleo
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Well, there is always fusion of KNO3 with Cr2O3 to yield chromate (iirc). But to get Cr metal - I agree, thermite seems the easiest route from Cr2O3.
As to chrome oxide reacting with HCl - indeed this is NOT the case. Cr2O3 is very unreactive, and (anhydrous) CrCl3 is either made by direct
chlorination of Cr metal (forming red-violet needle crystals when sublimed at 945 deg C) or by reacting Cr2O3, coal at red heat with chlorine gas.
Aqueous solutions thereof are the hexahydrate, which is green, and crystallises nicely + is highly soluble (50g CrCl3 per 100g H2O).
Also, I am not sure if Zn is able to reduce Cr3+ to the metal, did you check the reduction potentials? A strongly acidic solution of Cr3+ is reduced
by zinc to Cr2+, i.e. the dichloride, which is blue.
CrCl2 is made by reacting HCl gas with Cr metal directly, it is colourless.
[Edited on 21-5-2005 by chemoleo]
Never Stop to Begin, and Never Begin to Stop...
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neutrino
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Oh, it's one of those oxides (like calcined MgO).
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Jome
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Zn+2 + 2e- ---> Zn is -0,76
Cr+3 + 3e- ---> Cr is -0,74
So in theory it should work. But perhaps it'd be better to use al (from foil) followed by rinsing the powder with NaOH-soln to remove Al(OH)3 and
other shit that could form.
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12AX7
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Hm, it is quite close. Worse comes to worse you could electroplate it out, possibly with a sulfuric electrolyte instead (being that electrolytically
chlorinated heay metal solutions may not be too fun  ). Might end up with a
sponge (not a problem if you can melt it), but at least it'll be more efficient than hexavalent plating.
Tim
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12AX7
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Orange soda! ...Don't drink it!
Maybe 50g (damn, how did I neglect to weigh it  ) of stainless steel, hydroxydized
electrolytically in sodium chloride solution. Suspension washed, dried and calcined to an orange/brown (Fe2O3 primarily). Powdered and added to a
Ca(OCl)2 solution (an uh, excessive amount...oops). Solution was warmed briefly, accelerating the already somewhat exothermic reaction. Let sit
overnight, yielding an orange solution decanted from brown sludge (which I might as well process further to yield the nickel, and iron oxide for
thermite or something). Sulfuric acid added, later precipitating calcium sulfate and forming chlorine and chloric acid (oops, remind me not to heat
and acidify hypochlorite solutions) as evidenced by the noxious ClO2 odor.
So, I'm basically ready to plate out chromium from this solution, eh?
Tim
P.S. Oh, and the subject? It DOES look like orange soda!
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Pyridinium
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| Quote: | Originally posted by 12AX7
Powdered and added to a Ca(OCl)2 solution (an uh, excessive amount...oops). Solution was warmed briefly, accelerating the already somewhat exothermic
reaction. Let sit overnight, yielding an orange solution decanted from brown sludge (which I might as well process further to yield the nickel, and
iron oxide for thermite or something). Sulfuric acid added, later precipitating calcium sulfate and forming chlorine and chloric acid (oops, remind
me not to heat and acidify hypochlorite solutions) as evidenced by the noxious ClO2 odor.
So, I'm basically ready to plate out chromium from this solution, eh?
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I'm not going to try coming up with a balanced equation for that first part with the Ca(OCl)2 but I'm guessing it first makes some calcium
chromate, chlorine, and HCl which then form some HOCl, ClO2, and dichromate, maybe with some residual chromate left over
...then the sulfuric acid really pushed it in the direction of dichromate, ClO2, etc.
but at least you got
2H(+) + 2CrO4(2-) <----> Cr2O7(2-) + H2O
I know you knew that I just felt like posting it :-P
So you're going to try plating from dichromate solution... super high current density I guess? Or can you plate out some mossy / granular Cr
even at low current?
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12AX7
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| Quote: | Originally posted by Pyridinium
but I'm guessing it first makes some calcium chromate
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Ya know, the funny thing about that- data states it is mildly soluble, yet I never saw any precipitate. It was an orange solution I poured off the
brown sludge before adding sulfuric acid.
| Quote: |
So you're going to try plating from dichromate solution... super high current density I guess? Or can you plate out some mossy / granular Cr
even at low current? |
I forget what they do to get the infamous shiny smooth deposit, I'll have to look it up. I could also add a reducing agent, precipitate Cr(OH)3,
cook to Cr2O3 and thermite it out.
Tim
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chloric1
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| Quote: |
You might try using a stainless steel as the anode in a cell with NaOH solution and see what you get. The iron may provide plenty of gunk to
complicate things. See what goes into solution.
Edit: linkified book and quoted relevant passage.
[Edited on 5-1-2005 by Polverone] |
Well, I had an accident about a year ago. I had made a 29% KOH solution that I was going to use for fuel cells and some how(Don't rember) I
mistook this solution for water and mixed some of it into another half filled water bottle. ARRG! I realized that if I boil it back down so it would
all fit in the 2 liter bottle I could approximate the 28-30% solution again.
Well, to make a long story short I used a stainless steel pale to due the boiling. To my dismay the alklali was eating the pan! I was floored that
it was doing this without an oxidizer and aqueous conditions none the less! Turned out the the attack was mild and superficial. And today I still
use the pail without incident. Unfortunately for my carefully prepared KOH electrolyte it has some hydrated ferric oxide contamination floating
around in it. Will have to filter it someday when I can commit an afternoon. Maybe
tommorrow?
Fellow molecular manipulator
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