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Letter 8169
Nitric solution tank
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We are looking for a 24 inch wide by 24 inch long by 12 inch deep
tank/basin for leaching of glass and plastic parts in 20 percent
nitric acid solution. Have been unable to find anything that can
withstand this solution thus far.
Rick Trask
- Omaha, NE, USA
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Rick, Perhaps you haven't given us the whole story ... leaching
glass with ONLY nitric! Um! What, no HF?
There are or should be oodles of plastics that will take this
solution, assuming temperatures are not highish.
l. uPVC 2. CPVC 3. PVDF 4. ECTFE (ie. HALAR or any of the weldable
flurocarbons such as FEP, TEFZEL).
Materials I wouldn't use would be Pe, PP, ABS & fibreglass.
Structurally you have a piddly sized tank, hence # l, 2 & 3 by
themselves should be OK using l/4" thick material but # 4 should be
made of a dual laminate (max, available thicknesses is, I believe,
0.090") to achieve sufficient structural strength.
I hope that that helps. Mind you, the 'higher' the above number,
the higher the price!
Currently fumehoods are being made for Assay service for aqua
regia, nitric, sulphuric, HCL, HF and Perchloric (my favourite)...
with the sulphuric @ 93% concentration @ boiling away @ over 200
degr. C --- all PVC:FRP construction. And I have these in this
service, still working, since l980. Terrible repeat business as they
never seem to 'rust' away. Hence I feel that your chemicals are
pretty paltry and can be easily contained by any competent
fabricator.
Cheers !
Freeman Newton
40 odd years in the corrosion resistant plastics and also designing
low micron mist eliminators - White Rock, B.C. Canada
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PP will last less than a year. Most welded plastics will
eventually fail at the weld.
A very safe option for your tank is a low linear density or medium
density polyethylene ROTATIONALLY MOLDED tank. These happen to be
very inexpensive and are available from a large number of MFG's and
thousands of vendors. Nalgene and united states plastics come to mind
as veru good.
Use common sense, when the tank starts showing hints of stress
cracking in the corners or bottom corner, replace it. Normally
several years. Also put it into an adequate tray for secondary
containment.
James Watts
- Navarre, Florida
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Hi again,
What James Watt says about Polypropylene is probably true as in an
oxidizing environment, both PP and, yes, and, AND Polyethene are not
terribly good ESPECIALLY at welded areas where you can encounter
stress cracking.
If you can get a moulded (Engel Process ie. rotationally moulded)
Pe tank, go with it. But DON'T weld on any outlets. Use tank
adaptors!
Where James may err is to state that 'most plastics will fail at
the weld'. NOT so! But plastic welding is an ART and poor welding
will not last! Having run tensile tests and getting in the 'union
trained (hah!)' samples which were all done by an independant
Company, the best, BEST union weld was 30%. The best PVC weld (a real
exception, nevertheless) came to 99% or l00% but most highly trained
thermoplastic welders easily hit 85% of tensile. These tests were
done on l/2" uPVC for electrolytic cell tanks ...Further, butt fusion
(sheet) welding machines should achieve l00% weld strength.
One more thing about these rotationally moulded tanks. If the
manufacturer is local, ask them to make a tank but with a heavier
wall thickness. The max. wall they can do is 3/8", I believe, but a
small tank would only be around 3/l6".
This is NOT an expensive matter! All they have to do is to add
additional 'powder', But if you could increase the wall thickness for
an oxidizing environment, the life expectancy will zoom! I often did
this.
Cheers !
Freeman Newton
- White Rock, B.C. Canada
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Most tanks will fail - eventually--at the welds because welds
normally occur at the corners or edges. This is where the greatest
amount of stress is and is the place where virtually any tank will
fail. (short of being impaled by a fork lift)
Welds remove some of the plasticizers which will accelerate the
failure in that area.
Not all welders are good, few are very good, but they still weld.
PP is a bad choice for any oxidizing agent as it reacts with the
plasticizers and stress cracking results. About 3 to 6 months for a
welded tank of any significant size.
James Watts
- Navarre, Florida
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Regarding James Watts latest comments, Yes, PP (and I said that
right in the beginning!) is not too suitable for oxidizing
environments and problems occur at the welds, even when done by
experts.
However, the comment that most welded tanks will fail ain't (to
use colloquial English) right! When Robert Shaw Fulton Controls had a
plating plant in Toronto in the late 60's, they bought a number of
tanks, all 4-l/2 feet high x 2 ft square (I forget the exact size).
Unfortunately they ceased their operations l4 years later. I was
HIGHLY curious about the condition of those tanks as they were the
first ever true dual laminate tanks ever made in the world, ie. PVC
with fibreglass bonded to them.
They were never repaired. The welds lasted. But the top had
distorted due to excessive heat shortly after being installed. Yet l4
years later that distortion had not worsened nor had the welds
failed.
Moreover, the PVC used was somewhat experimental as it came from a
Company in Coburg, Ontario, that extruded sheet for (snow) fencing,
it was not a high quality pressed sheet like most of them are today.
Further, the colour was a royal blue (I had the rare choice as I'm
partially colour blind but love the deep blue) and everyone in the
shop cursed me as they were getting blinding headaches due to the
colour and had to weld using sun glasses! (Alcore Fabricating,
Richmond Hill, Ontario, Manfred Junkert (now the owner but then was
an assistant foreman to Canada's first German trained plastic
welder).
Lastly the comment about plasticizers being leached out.
The manufacturers of a high quality thermoplastic sheet will
produce/should produce a weld rod with identical properties and
certainly not impart undesireable effects by using any plasticizer. I
think that this really applies l000% to polyoelefins like PP an Pe.
In some cases (Trovidur) had a superb PVC but welding it with their
Trovidur rod was really difficult so I cheated and used their high
impact weld rod which DID have a plasticizer ... but allowed for easy
and good welding!
Lastly the comment about welds failing at corners and edges.
For a good job, a well designed tank will NOT have any welds right
in the corners. These are high stress areas. Hence the bottom corners
must be heat formed and then welded onto the already heat formed (ie.
bent) sides and the bases well rounded corners. This applies
particularly to larger tanks all duly FRP reinforced of PVC, PVDF or
fluorocarbon construction.
Finally, Pe can work with low concentrations of oxidizers such as
nitric .... IF .... moulded one-piece (Engel rotational stress free
method) tanks are used and if the wall thickness is OK ... AND no
welding is done!
Lastly, way back in the dark ages of l960, I.C.I. (the big U.K.
chemical giant) proved that whereas chlorine will easily attack
Polyethylene (and most plastics!!!) if a massive wall is used, the
attack mechanism peters out and an impervious boundry layer forms.
Hence, I always liked to think, the idea of a rotationally moulded
Poly tank with an extra wall thickness is an excellent, inexpensive
and a reasonably long lasting solution to minor oxidizers.
Here endeth the lesson.
Freeman Newton
- White Rock, B.C. Canada
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