Endo - 14-10-2006 at 13:48
Hi all,
For one of my projects I was dissolving several lanthanated tunsten rods (1.5% La) in a solution of hydrogen peroxide and HCl.
Everthing was going well, and over about 2 wks using ~10ml HCl 37% with around 30ml of 35% H2O2 I had a heavy layer of pale yellow tungsten trioxide
precipitate in the bottom of the beaker and the rods are close to halfway consumed. I noticed that the reaction had slowed considerably so I added
around 200ml of 3% H2O2 (it is cheaper) to get the reaction going again. now I see that I have a perfectly clear solution except where I can visually
see a faint cloudy wreath that quickly dissolves in solution surrounding each rod.
I was surprised to see this happen as I thought that WO3 was pretty much insoluble.
I intended to filter it for the WO3 which after drying was to be used for additives to a thermite mix. I was also going to make an attempt at
reducing the La 3+ ions using ascorbic acid or possibly just saving some LaCl3 for later use.
Questions: Can I get WO3 to precipitate by evaporating the solution? Through control of the pH can I get WO3 to precipitate while leaving La 3+ in
solution? Is it possible to reduce La 3+ to La metal using ascorbic acid or something similar?
Thanks
Endo
Eclectic - 14-10-2006 at 19:38
Normally you would dissolve tungsten scrap in molten NaNO3. Sodium tungstate is very soluble.
You might have had enough phosphoric acid stabilizer in your H2O2 to allow phosphotungstic acid to form. Make alkaline with NaOH, filter off
Lanthinum Hydroxide (I'm guessing it's insoluble), reacidify with HCl to precipitate tungstic acid.
not_important - 14-10-2006 at 22:06
The RRE - lanthanum and kin - are going to laugh at your ascorbaic acid, a rather mild reducing agent, and at any attempt to reduce them chemically in
solution. The normal methods that I know of are electrolysis of the fused chloride, of the aqueous chloride with a mercury cathode, and reduction of
the halides with sodium or (better) potassium.
WO3 has two forms, a yellow nearly insoluble form and white or near-white more soluble form. Boiling will convert the white to the yellow.
WO3 also form pertungstates with hydrogen peroxide, and many of these are more soluble than WO3. Long gentle boiling, with replentishment of the
water, will decompose the peracids.
Assuming that your peroxide doesn't have phosphates, or only trace amounts:
I don't think that tungsten forms any heteropolyacids with the lanthanides, so boiling the solution until the peroxides decompose and making sure
there is a little free HCl should leave the La in solution with most of the W precipitating out as tungstic acid.
If that doesn't work then it is likely that there is a fair amount of phosphate in the peroxide. Treating the solution with base to a pH of 12 to 13
should leave La behind as a percipitate. After that stirring with calcium hydroxide should pull the phosphate out as caclium phosphate. After that
acidification and a gentle boil should give yellow tungstic acid.
I don't think that tungsten trioxide is that good of a thermite subject, unless it is being used as an alloying metal for the main metal being formed.
If you just wanted bulk tungsten you should have gone for pure tungsten welding rods.
Eclectic - 15-10-2006 at 14:46
I didn't know about the pertungstates. Thanks, not_inportant.
I found this and think it's more likely than my phosphotungstate idea:
Dissolving tungsten with H2O2
I was wondering how HCl was dissolving tungsten.
Endo - 16-10-2006 at 11:12
Thanks for the info on this.
I actually didn't buy the rods, I was given stub ends that were left over after the machinist shop that manufactures them cuts as many rods as they
can get from the stock.
I was planning on using this in a mixed thermite reaction as not_important said.
The pertunstates is something that I was also unaware of and it probably explains what I am observing.
Not_Important gave me the perfect answer and answered all of the questions I had concerning this experiment.
Thanks
Endo
Nick F - 14-12-2006 at 11:05
Tungsten is easy to dissolve by electrolysis. I did this once to get the thoria out of some 2% thoriated electrodes. A search should bring up the
thread if you're interested.