Sciencemadness Discussion Board

seperation of Cr2O3 and Al2O3

vinothan23 - 8-7-2011 at 00:31

I have a mixture of Cr2O3 and Al2O3, actually it's a catalyst which was prepared by co precipitation method. I need to seperate it out.

Can anyone suggest method or idea?

Cloner - 8-7-2011 at 01:18

Cr2O3 is acid soluble, Al2O3 only very sparsely.

woelen - 8-7-2011 at 01:29

Both Cr2O3 and Al2O3 are very inert when they are calcined. Cr2O3 and Al2O3 are absolutely insoluble in acids when they are calcined, they are more like SiO2.

Only freshly precipitated and non-calcined material can be dissolved in acid.

Separating the two will be very difficult. The only thing I can imagine is dissolving in strong base (or molten base if the material is calcined) and then oxidizing the chromium to the +6 oxidation state. Then by adding excess acid and then adding ammonia you can precipitate the Al as Al(OH)3 while the hexavalent chromium remains in solution.
Then you need to add acid and reduce the chromium to trivalent again and then add ammonia to precipitate the Cr as Cr(OH)3.

It's a long way and involves a lot of steps and other chemicals. Probably it's not worth the hassle when done on a small scale.

LanthanumK - 8-7-2011 at 02:52

You can find out whether it is calcined by placing it in HCl and seeing if the color of the acid changes. If it does not, then the material is calcined.

In woelen's method, hydrogen peroxide can be used as the oxidant in a non-molten base situation. (http://www.chemguide.co.uk/inorganic/transition/chromium.htm...)

blogfast25 - 8-7-2011 at 12:09

Try fusion with bisulphate or conc. H2SO4+solid (NH4)2SO4 to effect dissolution. Then what woelen said.

But it's worth wondering whether this [separation] is really worth doing: chromium salts aren't that expensive...

I also wonder if differences in solubility between their alums (potassium or ammonium double sulphates - (NH4,K)(Al, Cr)(SO4)2.12H2O) might be exploited here: fractionated crystallisation.



[Edited on 8-7-2011 by blogfast25]

vinothan23 - 28-7-2011 at 01:54

Quote: Originally posted by woelen  
Both Cr2O3 and Al2O3 are very inert when they are calcined. Cr2O3 and Al2O3 are absolutely insoluble in acids when they are calcined, they are more like SiO2.

Only freshly precipitated and non-calcined material can be dissolved in acid.

Separating the two will be very difficult. The only thing I can imagine is dissolving in strong base (or molten base if the material is calcined) and then oxidizing the chromium to the +6 oxidation state. Then by adding excess acid and then adding ammonia you can precipitate the Al as Al(OH)3 while the hexavalent chromium remains in solution.
Then you need to add acid and reduce the chromium to trivalent again and then add ammonia to precipitate the Cr as Cr(OH)3.

It's a long way and involves a lot of steps and other chemicals. Probably it's not worth the hassle when done on a small scale.

vinothan23 - 28-7-2011 at 02:15

Thanks woelen for your highly useful post. As you said it is calcined material only.

In one more mixture which is also a catalyst containing Cr2O3, Al2O3, ZnO. It is a calcined and fluorinated(may be in the form of Cr2O3, CrF3, CrOF, Al2O3, AlF3, AlOF or the mixture of all ) also contains 4-5% Zn as ZnO and/or ZnF2. For this separation also shall we follow the same above? how the interferance of F- and Zn would be? and how to tackle this?

Which strong base can we use, KOH, NaOH?

What are molden base can we use?

For oxidising the Cr(III) to Cr(VI) which oxidising agent can we use? 20% H2O2 okay?

I think if H2O2 is in excess the Cr(VI) will again oxidise that to O2 and it will reduce to Cr(III). How to avoid this? Or can you suggest some other Oxidising agent for this?

after oxidising the Cr(III) to Cr(VI) why should we add acid before adding Ammonia to precipitate Al as Al(OH)3?

Does the hexavalent chromium not react with ammonia?






[Edited on 28-7-2011 by vinothan23]

sternman318 - 28-7-2011 at 09:01

Is it necessary to oxidize the chromium? Adding excess ammonia will produce the insoluble Al(OH3) as you said, and the soluble [Cr(NH3)6)3+ ( lookie .

Vinthan23, I belive acid is added to dissolve the Cr2O3 into Cr3+

AJKOER - 28-7-2011 at 09:57

With respect to oxidizing the chromium, per "The principles of chemistry", Volume 2 By Dmitry Ivanovich Mendeleyev, pages 138 and 139:

"This oxidising property of the dioxide (referring to PbO2) is particularly striking in the presence of alkalis. Thus under these conditions it transforms chromium oxide into chromic acid, whilst lead chromate, PbCrO,, is formed, remaining, however, in solution, on account of its being soluble in caustic alkalis."

Also, with respect to preparation of PbO2 (same source): "The best method for preparing pure lead dioxide consists in mixing a hot solution of lead chloride with a solution of bleaching powder (Ferman)."

LINK:
http://books.google.com/books?id=5d0KAAAAIAAJ&pg=PA403&a...

There is also a discussion in Wikipedia (link: http://en.wikipedia.org/wiki/Lead_dioxide )

"Lead dioxide is well known for being a good oxidizing agent with example reaction listed below:[6]

2 MnSO4 + 5 PbO2 + 6 HNO3 → 2 HMnO4 + 2 PbSO4 + 3 Pb(NO3)2 + 2 H2O

2 Cr(OH)3 + 10 KOH + 3 PbO2 → 2 K2CrO4 + 3 K2PbO2 + 8 H2O"

There is more information and a link to a prior thread on PbO2 at my recent thread "Making NH4NO3 from Inexpensive Household Ingredients" at:

http://www.sciencemadness.org/talk/viewthread.php?tid=17058

woelen - 29-7-2011 at 09:56

If you go through molten base, then you have the option of oxidizing in the melt, e.g. with KNO3 or even with O2. You can also oxidize afterwards with H2O2 or peroxodisulfate. In the latter case use some silver salt as catalyst and avoid presence of chloride.

Hexavalent chromium does not react with ammonia. The reason for using ammonia instead of a strong base is that it precipitates chromium in oxidation state 3 while excess of this does not redissolve the resulting hydroxide. The same is true for aluminium. So, if you have a mix of hexavalent chromium and aluminium in solution, then you can use ammonia to separate them. Al precipitates, hexavalent chromium does not.

Zinc complicates matters, because this forms a complex with ammonia. I'll have to further think about that.

AJKOER - 31-7-2011 at 06:11

With respect to the comment "or peroxodisulfate. In the latter case use some silver salt as catalyst and avoid presence of chloride", it may be interesting (and important to some) to understand this catalytic process.

Per Gary Hodes in "Chemical solution deposition of semiconductor films", page 96 (link below) the author notes "Ag+ is a known catalyst for oxidations using S2O8- (it is oxidized by persulfate to Ag(III), which is then the active oxidation agent)." Link:
http://books.google.com/books?id=PjmQnVX4OmUC&dq=NH4OH+%2B+PbO2&q=n...#v=onepage&q=Ag(III)&f=false

From this I surmise that Ag(III) oxides and is then reduced back to Ag(I), which is further oxidized by available persulfate, etc., per its alluded to catalytic properties.

I may be correct in suggesting that increasing Ag+ concentration may marginally increase the reaction speed. Also, avoiding chlorides (as noted by Woelen) that impede the formation of Ag(III) oxide, as well as pH considerations as overly acidic conditions are not favorable to the stability or optimal formation of Ag(III) oxide (see, for example, "A New Economical Process for the Preparation of Silver(I,III) Oxide, AgO" by Dr. Sabine Kielhorn1, Dr. Dieter H. Buß1, Prof. Dr. Oskar Glemser1,*, Dr. Roland Gerner2, Dr. Gerald Jeske2, where the authors in an alkaline environment achieve 97% yield at pH 10.5. In addition, raising the temperature of the reaction to 90 C can significantly increase the speed of Ag(III) Oxide creation (see patent 4,213,889).

[Edited on 31-7-2011 by AJKOER]