# retention time calculation



## saadat68 (Dec 30, 2016)

Hi
I searched many in forum but couldn't understand how to calculate retention time ( Need formula )  

For example we have 3 columns for vacuum scrubber. 
columns diameter: 3.5 inch
columns height: 40 inch
packed columns height: 35 inch
vacuum pump: 3 CFM

Need another parameter ?


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## goldsilverpro (Dec 30, 2016)

saadat68 said:


> Hi
> I searched many in forum but couldn't understand how to calculate retention time ( Need formula )
> 
> For example we have 3 columns for vacuum scrubber.
> ...


You sure the column diameter is only .35"? Not 3.5"?


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## saadat68 (Dec 30, 2016)

Sorry. I work with metric system here
3.5 inch is correct

They are just example. I need to know how to calculate rention time. Couldn't find any reference in forum. Just an excel file from 4metal


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## aga (Dec 30, 2016)

Retention time of what ? 
What solvent (if any) ?
What is the column packed with ?

Sounds like a chromatography column to me.

If it is, then it would be better to ask on a Chemistry forum.


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## saadat68 (Dec 30, 2016)

aga said:


> Retention time of what ?
> What solvent (if any) ?
> What is the column packed with ?
> 
> ...


Hi
A vacuum scrubber for NOx
packed with bioball for example


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## Palladium (Dec 30, 2016)

http://goldrefiningforum.com/phpBB3/viewtopic.php?f=40&t=24788&p=262177&hilit=scrubber+seconds#p262177


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## goldsilverpro (Dec 30, 2016)

saadat68 said:


> Hi
> I searched many in forum but couldn't understand how to calculate retention time ( Need formula )
> 
> For example we have 3 columns for vacuum scrubber.
> ...


Disregarding the packing, the total volume of the columns is (3.5/2)2 X pi X 40 X 3 = 1155 in3
That is 1155/1728 = .67 ft3
The 3CFM would do 3/.67 = 4.48 turnovers per minute.
A turnover, the theoretical time it would take for a gaseous molecule to travel from one end to the other, entrance to exit, would be 60/4.48 = 13.4 sec. This, to me, is the retention time.

The packing will increase this somewhat and, at the same time, it will decrease the volume. There are lots of variables in the packing and I would shy away from putting a number on it.

Depending on who you talk to, the required retention time for an insoluble NO molecule to find an oxygen molecule, react with it, and produce the required soluble NO2 molecule is 8 or 9 sec. Therefore, 13.4 sec should be quite adequate.


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## saadat68 (Dec 30, 2016)

Understood 

Thank you

Can I add 4 or 5 columns instead of 3 columns ? :shock: 
I like to design smaller columns. it is safer


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## aga (Dec 30, 2016)

Uuum. Sorry. 

The use as an NOx scrubber was not immediately obvious (to me at least).

Bubbling NO/NO2/NO3 into an NaOH solution works, giving sodium nitrite/nitrate.
The NaNO2 oxidies in air so you end up with all NaNO3, given enough time.
Just evaporate off enough water, allow to cool and nice crystals form. 

Personally i feel that if a process produces a lot of something Nasty, better to capture it and make into a safe and useful by-product rather than just throw it away.

Better to not produce the Nasty in the first place, but heigh-ho. Needs must !

Edit:

By capturing the NOx and making sodium nitrate, it can be dried (sun or oven) then distilled with concentrated sulphuric acid to get near 100% pure nitric acid, which could be put back into the gold process, free of almost all contaminants that come with shop-bought nitric acid.

Final waste product of doing that is sodium sulphate, aka Glaubers' Salt : a laxative, amongst other things. 
Very safe environmentally and does not smell.


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## saadat68 (Dec 31, 2016)

aga
Thanks for information

Palladium
Thanks but 4metal does not explain complete in his post and I couldn't understand last section. Now I can

First want to make a scrubber with 5 columns 8) each one 20 inch


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## anachronism (Dec 31, 2016)

Aga

Welcome to the world of refining where the world of theoretical chemistry takes a few serious knocks mate. We're lucky to have some extremely good chemists on the site who also have a deep practical knowledge of refining, and some very good refiners who understand the chemistry they need to refine. 


Jon


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## Topher_osAUrus (Dec 31, 2016)

If making nitric was the end goal with scrubbing your fumes, why even go with NaOH, when you could just bubble the NOx through H2O2?

If it was from an AR reaction, you may want an empty vessel first to catch metal slimes that make their way through, but after the empty, you could have h2o2


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## aga (Dec 31, 2016)

anachronism said:


> Welcome to the world of refining where the world of theoretical chemistry takes a few serious knocks mate.


Chemistry is Chemistry. No magic at all no matter where it is used.

What works works. 

All depends on what the word 'works' means.
(please see below)



Topher_osAUrus said:


> you could just bubble the NOx through H2O2?


Maybe you could, i could not.

30% H2O2 is prohibited here, so it'd end up cheaper to go the NaOH to sodium nitrate & sulphuric acid route to recover nearly pure nitric acid than to use H2O2 in a scrubber.

With even 30% H2O2 and perfect scrubbing efficiency, there's still that 70% water in there, making very dilute nitric acid. Useless (for gold dissolution).

I appreciate that i know absolutely jack shit about the actual ins and outs of metal refinery, also that i will screw it up a Lot due to lack of experience in this Art, and will appreciate any help.

The Art is in Knowing what to do and when, which i certainly do not know, having no experience at actually Doing it.

Sadly, for me, i was not born knowing everything, so have to learn it all, usually the hard way.

All Great fun though !


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## anachronism (Dec 31, 2016)

aga said:


> anachronism said:
> 
> 
> > Welcome to the world of refining where the world of theoretical chemistry takes a few serious knocks mate.
> ...


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## Platdigger (Dec 31, 2016)

Well, 30 percent nitric is very useful. Both for silver and gold, and or base metals dissolution.
Of course by adding hcl for gold.


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## Topher_osAUrus (Dec 31, 2016)

Well, the 3% h2o2 will create stronger than 30% nitric, as evening bubbling no2 through water will create some nitric acid.


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## aga (Dec 31, 2016)

anachronism said:


> Oh you're going to have such an interesting ride on this train then mate.


Probably so.Looking forward to finding out !



Platdigger said:


> Well, 30 percent nitric is very useful. Both for silver and gold, and or base metals dissolution.
> Of course by adding hcl for gold.


Please expain that nonsense.



Topher_osAUrus said:


> Well, the 3% h2o2 will create stronger than 30% nitric, as evening bubbling no2 through water will create some nitric acid.


Please explain How 3% of one magically makes 30% of the other.

Are you a Banker ?

This is all starting to feel very very familar.

I just want to have some fun with trying to refine Gold, screw it up (inevitebly), then ask questions, try again and then maybe get better results next time.

As for Chemistry, i got sober for a couple of weeks 3 years ago, got interested and done/learnt a lot since then.
No PhD here, just an Amateur Chemist amongst many other interests, including precious metals, like Gold.

You know it ? The kinda yellow and Shiny one. The Nice one that shines. That one. Gold.


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## Topher_osAUrus (Dec 31, 2016)

I simply meant that 3% you can get at the pharmacy is perfectly fine, as the water itself gets partially converted to hno3 by the no2 gas. The h2o2 just aides the conversion some. But, i believe retention time of the gas plays a large factor for conversion too.

There are many threads here where people, including myself, have made nitric at home with this methodology.
Butcher has some of the bests posts on it.

And 30% is fine, because silver itself won't readily dissolve in azeotrope nitric, and it needs the h2o to help it kick off the dissolution. Something about the magical H+ ion...
When we dissolve sterling, we mix 50/50 nitric with distilled h2o. And when making ar, some choose to add water so silver chloride doesnt easily stay in solution and have to be diluted out later.. Or if we are separating gold from platinum, we can use cold dilute ar to only dissolve the gold.
And other reasons

Edit to finish


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## aga (Dec 31, 2016)

Thank you for a clear reply Topher_osAUrus.

Yes, how it all Actually works is still not perfectly understood by anybody.

Water does indeed have a mysterious role in many reactions.

The current thinking is that it's not lone protons (H+) but H3O+ thingies.
What difference that makes is also not really understood.

People Experienced with specific materials, such as yourself, have a very large advantage over those (like me) who think they Know something, never having had that specific experience.

I stand corrected and grateful.

Oh ! Happy New Year !


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## Topher_osAUrus (Dec 31, 2016)

Dont mention it. 

I learned from the great people at this forum, and without this site. I would know next to nothing.

Be sure to check out the library here, as it holds the best of the best threads. And download Miss Hoke's book, as it is the bible of refining. It was written for everybody, so someone with a chemistry background such as yourself will pick up on its information quite quickly.

Happy newyear to you as well!
Cant wait to see your first :G


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## anachronism (Dec 31, 2016)

aga said:


> Platdigger said:
> 
> 
> > Well, 30 percent nitric is very useful. Both for silver and gold, and or base metals dissolution.
> ...


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## Yggdrasil (Dec 31, 2016)

*Happy new year everyone.*  

Just to continue the last development in this thread.
Has anyone tried to pump the red fumes back into the reaction liquid by peristaltic pumps or the likes.
Do you need peroxide then as well or will it give some affect anyway?
How about need for fume hoods, will it decrease if you recycle the fumes?

I kind of like the idea if we can use our chemicals to the max.


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## anachronism (Dec 31, 2016)

Yggdrasil said:


> *Happy new year everyone.*
> 
> Just to continue the last development in this thread.
> Has anyone tried to pump the red fumes back into the reaction liquid by peristaltic pumps or the likes.
> ...



Hey Ygg

4metals posted a pic of a closed loop system a few years ago utilising a condenser to take a lot of the Nitric back into the reaction vessel prior to the scrubbing of the gases. I'll try to find it and pop a pic up for you. Happy New Year to you too. 

Jon


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## Yggdrasil (Dec 31, 2016)

Thanks Jon.
In my ideal world our operations would be without fumes and so on, but we have to be practical in the real world.
If we can find workarounds and techniques that minimize our foot print, it would be magnificent.

Like distilling the HCl after use to azeotrope after it has been spent or the same about nitric.
I have not looked about the economic part of this, but the environmental part looks shining  
Some day I will look into it ... I think :wink:


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## aga (Jan 1, 2017)

anachronism said:


> Not the most spectacularly helpful remark I've seen on here in recent weeks. When you understand why Platdigger said what he did, you'll actually know something. Then hopefully you'll acknowledge that.


That was stupid and arrogant of me.

Please accept my appologies Platdigger.


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## Platdigger (Jan 1, 2017)

no worries
apology accepted


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## g_axelsson (Jan 1, 2017)

It's really nice to see everyone playing nice and getting along on this new year! It makes moderating easy.  

H2O2 helps converting NO into NO2 but so does atmospheric oxygen too, so having a bleeder valve letting in some oxygen into the reaction vessel and then using refluxing condenser could stretch the amount of metal a certain amount of nitric could dissolve. In the simplest form a slow digestion in a large beaker with a watch glass or plate covering the top goes a long way.
For example the common knowledge is that it takes 1 ml nitric and 3 ml hydrochloric acid to dissolve one gram of gold. My best result so far is with an excess of hydrochloric acid and 28 ml nitric dissolved 53 grams of gold. That was with a beaker and a glass plate on low heat, not boiling. I haven't calculated how close I got to stochiometric values, maybe some day I'll do it for fun.  

I have bought a professional refluxing condenser but I haven't tested it yet.

Happy new year everyone! 

Göran


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## aga (Jan 1, 2017)

Thanks Platdigger and anachronism.

The problems with HNO3 are varied.

It's relatively high vapour pressure of 48 mmHg basically means it wants to be a gas and escape into the big wide world instead of doing the work you set for it. Sulphuric acid tends not to do that as it's vapour pressure is just 0.001 mmHg. A simple experient with that is to get two small pots, put a few ml of sulphuric on one, nitric in the other, weigh them both, leave them out for a few days, then weigh again to see how much of each acid escaped. (conc sulphuric will actually Gain weight as it takes water from the air)

A you all know well, nitric is very reactive, so any ducting of the NOx gasses tends to get attacked, meaning that it doesn't last long and has taken some of the gas you were trying to save.

Lastly, nitric decomposes very easily, hence the orange fumes which are not the NO3 originally required.

Increasing the pressure of the reaction might squeeze more 'bang for your buck' out of the acid.

Although that _might_ work, i have never tried that idea and it sounds like it could be quite dangerous !


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## Lou (Jan 1, 2017)

On commercial scale silver digestions, a mixture of dilute nitric acid (usually less than 10% w/v) and 35% w/v H2O2 is used in the first scrubber to get the NO and most of the NO2 into nitric acid. That scrubber solution gets used for the the next dissolve. 

Usually, most efficient to heat water and add fine silver granules or powder into the water, slurry and then pump in the nitric.


EDIT:


Keep in mind that in packed beds, your _actual_ retention time might be considerably longer than the estimated residence time of a gas molecule due to a variety of effects (turbulence, tortuous pathways, etc).

How important is that for a scrubber for NOx? Don't know. I'd say I'd rather assume worst case scenario and make sure the BFRC doesn't escape!


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## 4metals (Jan 1, 2017)

This is a difficult thread to give anything but the most basic of answers. There are too many variables and all we were given was the column area. So simply stated, the retention time is the amount of time the fume remains in the packing of the scrubber. And that is just a function of the volume of the fume in the scrubber and the CFM flow of gas through the scrubber. 

As Lou said, the type of packing effects how effective the scrubber actually is at reacting the fume. But for NOx in general a retention time of 8 seconds in the packed area of the scrubber is effective in a scrubber that recirculates the scrubber liquid. We do not know if the scrubbing liquid is being recirculated over the bed and if it is, what the flow rate is. And we do not know if the packing is one of the more popular tower packing types or if it is wiffle balls, marbles, or cut PVC pipe. 

If you can provide a retention time of 8 seconds or better, there is an effective combination of packing, flow rate, liquid distribution, and chemistry to effectively scrub the NOx generated.


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## saadat68 (Jan 2, 2017)

4metal
Thank you


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