Sciencemadness Discussion Board

Scrubber for low pressure mercury vapour ?

Sulaiman - 12-11-2019 at 18:54

I would like to distill mercury at low pressure,
my dual-stage oil-sealed rotary-vane (cheap-Chinese HVAC) vacuum pump was advertised as 0.5Pa minimum pressure.
In theory I could distill mercury at below 100oC (1.3Pa).
(the actual temperature will indicate the lowest practical working pressure)

I'd probably use an air condenser followed by an ice-water cooled condenser.
(I can't get dry ice at an acceptable cost)

I really do not want any mercury vapour to escape - especially not into the aluminium pump body.

What could I use as a scrubber of mercury vapour at these low pressures ?

Tdep - 12-11-2019 at 19:11

Have not looked into this, but sulfur is often used to clean up mercury. Passing the gas through some granulated sulfur has the advantage of not having any worries with the vacuum (its not some liquid that will also vapourise into the pump) but should still do a great job of stripping the metal out the air, forming inert sulfides.

Sulaiman - 12-11-2019 at 19:15

I'd considered powdered magnesium (I could buy powdered aluminium if that would be better).

Would mercury vapour react quickly enough with sulphur powder at room temperature ?

P.S. I'd probably put a bubbler under vacuum oil to guestimate the flow of gas to the pump whilst distilling.

[Edited on 13-11-2019 by Sulaiman]

wg48temp9 - 12-11-2019 at 19:38

Quote: Originally posted by Sulaiman  


P.S. I'd probably put a bubbler under vacuum oil to guestimate the flow of gas to the pump whilst distilling.

[Edited on 13-11-2019 by Sulaiman]


That would add about 100Pa to the pressure assuming a13mm oil head.

You can scrubb mercury with activated carbon see https://www.ncbi.nlm.nih.gov/pubmed/24645442
also activated silica gel.

But note after use you should flush your pump while it hot ie run the pump for several minutes after use to flush out any condensed vapour. That's what it says in some manuals.

[Edited on 11/13/2019 by wg48temp9]

Sulaiman - 12-11-2019 at 21:32

Quote: Originally posted by wg48temp9  
Quote: Originally posted by Sulaiman  


P.S. I'd probably put a bubbler under vacuum oil to guestimate the flow of gas to the pump whilst distilling.

[Edited on 13-11-2019 by Sulaiman]


That would add about 100Pa to the pressure assuming a13mm oil head.

You can scrubb mercury with activated carbon see https://www.ncbi.nlm.nih.gov/pubmed/24645442
also activated silica gel.

But note after use you should flush your pump while it hot ie run the pump for several minutes after use to flush out any condensed vapour. That's what it says in some manuals.

[Edited on 11/13/2019 by wg48temp9]


D'oh ! ... I did not consider the pressure required for a bubbler ... rookie mistake.

I like the activated carbon idea ... I'll research it ... along with sulphur reaction rates.


Thanks to you both.

wg48temp9 - 12-11-2019 at 22:09

Given the boiling point of sulphur is below that of mercury you may get more sulphur in the pump than mercury without the sulphur trap, not that either one would be significant but in the case of mercury I can see that an abundance of caution would not hurt if only to stop it getting in to the lab air.

B(a)P - 13-11-2019 at 02:53

I don't fully understand the process you are describing, but I have experience in the removal of mercury vapour from industrial off gas. In processes I have experience, generally in a wet scrubber is used to supply S2- ions, the additional of a chelating agent further increase effectiveness. There is a lot of info out there about this.
Not sure how this might be able to be integrated into your system though. Hope this helps.

Sulaiman - 13-11-2019 at 07:36

Quote: Originally posted by B(a)P  
... generally in a wet scrubber is used to supply S2- ions ...
I could use a wet scrubber after the vacuum pump as a safety measure,
but I really want to scrub mercury vapour before it gets to the vacuum pump at low pressure (for low boiling point)
such a low pressure that most liquids - certainly water / aqueous solutions - would boil - below room temperature.
(hence the consideration of powdered sulphur or metals that form an amalgam with mercury)
___________________________________________________________
Supplementary question;
what could I use to detect if any mercury vapour gets past the scrubber(s) ?
Any dry sensitive test for mercury vapour ?

(a complex / overkill possibility - maybe high voltage to cause a blue plus uv glow or a detectable current flow ?
not sure I want such complications, but may be interesting if I'm in the mood for it)
___________________________________________________________
@wg48temp9 : the sulphur powder and any mercury vapour would be after the condenser(s),
presumably at (or below) room temperature.

Solid sulphur has a vapour pressure of 10-5 torr (1.3 mPa) @39oC so should be ok ?
Attachment: Meyer-76.pdf (2.4MB)
This file has been downloaded 498 times

or am I over-thinking this potential problem ?

phlogiston - 13-11-2019 at 14:49

activated charcoal impregnated with iodine is also effective, but not sure what the charcoal and iodine will do to your vacuum.

We used to have a portable mercury sensor in the lab, and it relied on carbon/iodine cartridges to obtain mercury-free air as a blanc sample.

RedDwarf - 13-11-2019 at 15:33

While using a static head type filter (such as the bubbling tube) would reduce your attainable pressure, using a dynamic head filter (where pressure loss is only dependent on flow) wouldn't cause you any reduction in attainable pressure (at the levels of carry over from your two condensers the vapour flow would be negligible (for the purpose of pressure drop). I's therefore suggest using a traditional filter approach (the key is the convoluted path where individual mercury atoms path carries them onto the filter where they remain) or an electrostatic precipitator (again ideally with a convoluted path). For a normal filter if you could cool it in the freezer beforehand and then externally insulate it, that would lose little heat during a distillation as there would be almost no flow, combine that perhaps with a reactive surface such as a sulfur coating and that should avoid mercury atoms being able to leave the filter after impact.
I found this article on using electrostatic filters for collecting mercury vapour which might provide some food for thought:
NEWMAN, O. M. G., & PALMER, D. J. (1978). Collection of atomic mercury by electrostatic precipitators. Nature, 275(5680), 526–527. doi:10.1038/275526a0

BromicAcid - 13-11-2019 at 18:12

Mossy zinc would be my starting point

AJKOER - 16-11-2019 at 08:49

Perhaps this report (see https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/98JD... ) may be helpful:

"Kobayashi [1987] studied the oxidation of Hg by aqueous chlorine at various pH (1.2 -- 8.1). He bubbled Hg vapor (in an inert carrier gas) into solutions of HOCI/OCI-, and measured the dissolved Hg(II) in the solutions with respect to time. The initial rate of Hg dissolution/oxidation is reported to be extremely rapid and independent of the chlorine concentration in the solution [Kobayashi, 1987]. This is a strong indication that mass transfer limits the reaction in the system, "

I would try, based on the above, simple chlorine bleach (NaOCl) with some added acetic acid, or perhaps NaHCO3, to introduce some HOCl into the mix.

[Edited on 16-11-2019 by AJKOER]

Sulaiman - 16-11-2019 at 13:34

Thanks, but that would not be suitable for my requirements as the aqueous solutions would boil at my target operating pressure,
which will hopefully be in the low single digit Pa range,
and as pointed out to me above by wg48, ANY bubbler would cause a significant operating pressure increase.

@phlogiston: I checked the vapour pressure of iodine ... mmHg @RT so not viable.
For reference, a spreadsheet that I made based on data by Ramsay and Young, found in:
THE VAPOR PRESSURE OF IODINE. Gregory Paul Baxter, Charles Hendee Hickey, Walter Chapin Holmes
Attachment: Vapour Pressure.ods (16kB)
This file has been downloaded 380 times

It seems that granulated activated charcoal is my best bet, neither oxidised (by nitric acid) nor iodized.
I will heat the carbon under vacuum for some time to prepare it for use.
I shall not be able to do this experiment until next year so plenty of time for me to do a little more research.

[Edited on 16-11-2019 by Sulaiman]

AJKOER - 16-11-2019 at 15:23

Actually, my referenced chemistry could be also viewed from the perspective of a gas cloud consisting of moist chlorine!

The latter is chemically a mix of hydrogen chloride and HOCl (or Cl2O and water vapor).

Interestingly, this may further imply that all prep path to Cl2O may be of value. Cited examples include the action of dry chlorine on mercury(II) oxide or on even hydrated Na2CO3. Source, see https://en.wikipedia.org/wiki/Dichlorine_monoxide with cited reactions:

2 Cl2 + HgO → HgCl2 + Cl2O

2 Cl2 + 2 Na2CO3 + H2O → Cl2O + 2 NaHCO3 + 2 NaCl

I am assuming in the chlorine monoxide path that it is diluted in say dry air mixed with CO2 (as concentrated mixes are unstable, mildly explosive, as are mixtures in contact with organics and sunlight). Further, the postulated reaction, as chlorine monoxide per Wikipedia is 'a strong oxidiser and chlorinating agent':

Hg + Cl2O = HgO + Cl2
-------------------------------

More detail on Cl2O preparation paths, for those interested at https://pubchem.ncbi.nlm.nih.gov/compound/Chlorine-monoxide#... , to quote:

"Gaseous dichlorine monoxide is conveniently generated by passing chlorine gas through mercuric oxide in a packed tubular reactor. High yields are favored by cooling the reactor, diluting the chlorine with an inert gas, and mixing the dry HgO with an inert material, eg, sand. Use of excess HgO ensures complete conversion of chlorine. The spent HgO can be regenerated by treatment with aqueous caustic, filtering, washing with water, and drying at 110 °C. The relatively pure Cl2O in the exit gases can be passed through a trap cooled with dry ice and acetone to condense liquid Cl2O. The preparation can also be carried out under static conditions.
Wojtowicz JA; Dichlorine Monoxide, Hypochlorous Acid, and Hypochlorites. Kirk-Othmer Encyclopedia of Chemical Technology (1999-2018). John Wiley & Sons. Online Posting Date: April 16, 2004
from HSDB
Dichlorine monoxide is formed by passing chlorine diluted with an inert gas over the surface of a slowly stirred 50% NaOH solution at about 5 °C. The exit gas also contains some unreacted chlorine as well as HOCl and small amounts of water vapor. Passage through P2O5 absorbs the moisture and dehydrates the HOCl to Cl2O. Condensation of the Cl2O separates it from the inert gas and most of the chlorine. Fractionation removes the last traces of chlorine."

[Edited on 17-11-2019 by AJKOER]

SWIM - 16-11-2019 at 18:21

So how are you going to keep that most cloud in the apparatus with a pump trying to pull it out?

And how does pump oil do with chlorine dioxide contamination?

@Sulaiman: If you could use a Dewar condenser as a final additional condenser you could charge it with crushed ice/calcium chloride and get some 20 or 30 degrees below ice bath temperatures. (Be sure to insulate the outside of the condenser, and fabricate some sort of insulated lid for it for best results.)

Sulaiman - 16-11-2019 at 22:31

I would like to own and use a Dewar condenser / cold trap,
but I've not seen one at a price that I can justify to myself - yet.

If I get into more low pressure distillations I suppose that I may buy one,
but for now this is just a one-off forray into low pressure distillation,
partly to purify mercury for my U-tube barometer / manometer,
and partly to determine the low pressure capabilities of my pump, glassware, and sealing techniques.

My rotary vacuum pump has seen very little use so far,
and cost-wise it is more disposable than the mercury,
but I would like to protect the pump for cost reasons,
and to be a little more rigorous with my waste creation and disposal practices.

I could easily distill the mercury at atmospheric pressure,
ignore the tiny ammount of mercury vapour that would escape,
and get an acceptable result.
This planned experiment is more of a learning experience than a pragmatic one.

EDIT : After a little thought ... I like the idea of ice & calcium chloride, - thanks SWIM.
I may contrive something equivalent to a Dewar cold trap ... should be simple enough.

Wikipedia gives a 'recipe' of 1 : 0.8, w:w, calcium chloride : ice for -40oC... any comments ?


[Edited on 17-11-2019 by Sulaiman]

wg48temp9 - 17-11-2019 at 03:17

Quote: Originally posted by Sulaiman  


EDIT : After a little thought ... I like the idea of ice & calcium chloride, - thanks SWIM.
I may contrive something equivalent to a Dewar cold trap ... should be simple enough.

Wikipedia gives a 'recipe' of 1 : 0.8, w:w, calcium chloride : ice for -40oC... any comments ?


If I remember correctly the graph you linked to for the VP of sulphur and a VP graph for mercury indicates the VP of mercury at -40C will about less than the VP of sulphur at 38C ie between a good vacuum and space.

Sulaiman - 17-11-2019 at 03:47

I am considering an ice & calcium chloride trap instead of sulphur, carbon, or metals,
i.e. it would be the only trap, it would effectively be a third condenser
(after an air condenser and an ice water condenser)
so I should be able to capture and use all mercury boilled off... no waste generated.
The only waste should be anything that comes over before the mercury (?)
and whatever is left in the boiling pot.
My batch of mercury was bought several years ago as 'triple distilled mercury'
so I'm only expecting HgS and/or HgO and possibly amalgamated metals,
all of which should have a lower vapour pressure than mercury.

wg48temp9 - 17-11-2019 at 04:48

Quote: Originally posted by Sulaiman  
I am considering an ice & calcium chloride trap instead of sulphur, carbon, or metals,
i.e. it would be the only trap, it would effectively be a third condenser
(after an air condenser and an ice water condenser)
so I should be able to capture and use all mercury boilled off... no waste generated.
The only waste should be anything that comes over before the mercury (?)
and whatever is left in the boiling pot.
My batch of mercury was bought several years ago as 'triple distilled mercury'
so I'm only expecting HgS and/or HgO and possibly amalgamated metals,
all of which should have a lower vapour pressure than mercury.


But just incase exhaust any fumes to outside, not your lab.