# Evaporation versus boiling



## Harold_V (Feb 17, 2012)

I'm to the end of my rope with readers that insist on discussing evaporation by making reference to boiling. 
Readers are being fairly warned that it will not be tolerated any longer. 

http://goldrefiningforum.com/phpBB3/viewtopic.php?f=37&t=13091&p=131372#p131372

Harold


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## philddreamer (Feb 17, 2012)

Excellent Harold! 8) 

Phil


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## Barren Realms 007 (Feb 17, 2012)

Thank you!!!!!! 8)


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## Meh (Feb 17, 2012)

Sounds like someone's boiling mad.


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## jimmydolittle (Feb 17, 2012)

Meh said:


> Sounds like someone's boiling mad.
> Which rock did you crawl out from under?


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## Barren Realms 007 (Feb 17, 2012)

Meh said:


> Sounds like someone's boiling mad.



A good rule of thumb is when Harold put's his foot down because he has enough of something is not to make a joke about it. He dosen't take it to kindly.


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## rusty (Feb 17, 2012)

Page 53 from Hokes book, Refining Precious Metals. May explain some of the confusion boiling versus evaporation.

(5) REMOVING EXCESS NITRIC ACID
You were told to use no more aqua regia than was required. Beginners always use too much. 

Experienced workers often do the same thing, especially when in a hurry, because fresh acids work more rapidly than those that are partly exhausted.

Sooner or later you must get rid of the unused nitric acid. Most workers think it pays to do so right away, as follows:

Put the aqua regia solution into a big evaporating dish. It will contain some sediment, mostly silver chloride; this does no harm at this point, so do not filter it yet. 

Now add a little sulphuric acid, stirring all the time, and working slowly, as it might spatter. Use about an ounce or less of the sulphuric acid to each quart of liquid.

Now boil the solution down gently, to a syrup, being careful not to spatter. The purpose of the sulphuric acid will be explained later.

This evaporation is a tiresome process. It goes rapidly at first, but soon the liquid begins to spatter and you must lower the flame. 

Do not cover the dish, as that merely impedes evaporation. A sand bath or a steam bath is useful; these have already been described. When using a sand bath, start work by pushing the sand to the sides of the pan, so that the heat can quickly reach the evaporating


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## Geo (Feb 17, 2012)

some of these processes are forever morphing and changing. an amended version would be great. i remember a couple of processes that have been changed since I've been on the forum. i know from experience that boiling can be bad for someone that doesn't know how to do it properly and evaporation can be even worse. we give advice to read the book and do what it says but people should understand that the terminology of the time that the book was written has changed somewhat and that the book should be viewed as a guide but to still ask questions when moving from one process to another until they know what to expect.


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## butcher (Feb 17, 2012)

Notice in Hoke's book she used steam tables to heat the solutions, I do not think they use High pressure boilers, so then the temperatures at which she used to heat solutions would not quote "BOIL" aqua regia, the term boil is used in the wrong context here. when if we follow Harold's advice and use the term evaporation (which truly describes what we need to do), we use a better term to describe the process, and then the people trying to follow our advice or what we did, and do what we tell them, then They are not just boiling off their gold into vapors, and can use the proper technique. 

We have discussed this before, and if we use the proper terms many here will have better success, and understanding of these methods used in recovery and refining.

Evaporate Not Boil.


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## dtectr (Feb 17, 2012)

[kiss, kiss] whatever - Hoke recommended using a "casserole dish" for evaporation for a reason - greater surface area increases rate of evaporation per BTU - I use a corning pyroceram casserole dish for this function. 

An equal volume of identical liquids will evaporate faster from a casserole dish (of greater surface area) than from a cylinder of equal volume. 

"Read with understanding or not at all. "
Dos centavos ? ? 
Dtectr


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## Harold_V (Feb 18, 2012)

rusty said:


> Now boil the solution down gently, to a syrup, being careful not to spatter. The purpose of the sulphuric acid will be explained later.
> 
> This evaporation is a tiresome process. It goes rapidly at first, but soon the liquid begins to spatter and you must lower the flame.


She also suggests that one generously throw gasoline on waste material for incineration, but you and I both know that's a recipe for disaster. 

She screwed up. The false notion that you can, somehow, "boil gently" is misleading. If you achieve a boil, it's guaranteed, you will lose values. I had to learn that lesson the hard way, and it could have been avoided by the simple mention that once the boiling temperature is achieved, the bubbles burst on the surface with enough energy to carry off minute droplets of gold chloride. 

My baptism by fire came quickly. I watched the solution carefully, and restricted heat once it started boiling. It was not a heavy boil. How much I lost will never be known, but the afternoon I spent hand rubbing the paint on my travel trailer, those many years ago, were very instrumental in making me understand the significance of keeping the solution under a boil. My travel trailer was parked a few feet away from the window in which I had installed a fan. I had no fume hood at this point in time. You likely understand that I had one shortly thereafter. 

You can make determinations of occurring loss by placing a cold object in the vapors coming off when you're evaporating. If the condensate has any color, you're losing values. You likely understand the object should NOT be metallic. A porcelain spatula, or even a heavy glass stirring rod will work. 

While we're on this subject, there's one other thing that needs to be addressed. 
The idea that you can evaporate to a syrup three times and be assured that you have expelled all of the nitric *is not true*. You can evaporate to a syrup a half dozen times and not succeed. Key to understanding is what constitutes a syrup. It's difficult to define, because contaminants in the solution alter how it behaves. 

If you introduce HCl prematurely, nothing is accomplished. In such a case, you would most likely repeat the same mistake time and again, not understanding that the solution requires further evaporation before introducing the HCl. When it is introduced at the proper time, assuming the solution contains free nitric, the familiar brown fumes are expelled. 

The ultimate goal is to use no more nitric than is necessary. We all know that. Problem is, there are instances where a small excess is very desirable. It is an excellent indicator that all values have been dissolved when processing filing from the jeweler's bench, for example. In such a case, I discovered, long ago, that the addition of a button of pure to the solution, when evaporation commenced, guaranteed the consumption of the free nitric. It beats, hands down, attempting to evaporate, in particular when you introduce the HCl prematurely, and achieve no results. 

Harold


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## Harold_V (Feb 18, 2012)

dtectr said:


> Hoke recommended using a "casserole dish" for evaporation for a reason - greater surface area increases rate of evaporation per BTU -


Correct! And---if you explore chemical evaporating dishes, you'll understand that they are made to create large surface areas. Ceramic dishes are large in diameter and shallow, with a relatively large base, while Pyrex dishes are somewhat smaller in diameter, but with exceedingly small bases, but they flare quickly, to increase surface area. 

If one chooses to use a button (to consume instead of evaporate), it can be added to the vessel in which the values were dissolved. The objective, now, isn't to evaporate to a syrup, but to consume free nitric. That was my procedure, although I also allowed the solution to evaporate, although strictly to reduce volume. 

Harold


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## niteliteone (Feb 18, 2012)

STICKY :?: 

Tom C.


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## garage chemist (Feb 18, 2012)

What I find difficult to understand is that all of you seem to evaporate your excess HCl and HNO3 in the fume hood and into the atmosphere. In chemistry forums, you won't find many people doing that.
If I started evaporating acids openly in my fume hood, it would be corroded to hell in a few weeks (the blower in a few days at most), and the neighbors would call the police because of the choking white clouds constantly coming out the exhaust of my lab! 
How many here have a distillation setup to properly condense the acid fumes and prevent their release into the atmosphere? 
A simple glass distillation setup should be one of the first labware acquisitions of anyone who wants to evaporate acid solutions, much more important than a fume hood. Also, you can boil your solutions as vigorously as you like and there are no losses! 
The only fumes that a fume hood should have to vent outside are those from pouring acid from the bottle into the flasks, or the brown nitrogen oxides from dissolving things in HNO3 or AR. 
Digestion of alloys in AR should be done in a flask with reflux condenser, and boiling down in a glass still.


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## rusty (Feb 18, 2012)

garage chemist said:


> What I find difficult to understand is that all of you seem to evaporate your excess HCl and HNO3 in the fume hood and into the atmosphere. In chemistry forums, you won't find many people doing that.
> If I started evaporating acids openly in my fume hood, it would be corroded to hell in a few weeks (the blower in a few days at most), and the neighbors would call the police because of the choking white clouds constantly coming out the exhaust of my lab!
> How many here have a distillation setup to properly condense the acid fumes and prevent their release into the atmosphere?
> A simple glass distillation setup should be one of the first labware acquisitions of anyone who wants to evaporate acid solutions, much more important than a fume hood. Also, you can boil your solutions as vigorously as you like and there are no losses!
> ...



This is the direction I want to go, and I've been looking into distillation columns on ebay but the various types of ground glass connections are confusing the heck out of me.


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## its-all-a-lie (Feb 18, 2012)

garage chemist wrote: Also, you can boil your solutions as vigorously as you like and there are no losses! 


I am pretty sure the losses are not from the steam coming from the process but from the violent spatter of acids. That is also the reason the "B" word is taboo here because of the dangers to new members like myself who are likely to read your post and think it is ok to do this with no danger of losing values or harming themselves. I personally dont think boiling is safe in any of the processes used here but what do i know, i am just a beginner.


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## Geo (Feb 18, 2012)

garage chemist said:


> What I find difficult to understand is that all of you seem to evaporate your excess HCl and HNO3 in the fume hood and into the atmosphere. In chemistry forums, you won't find many people doing that.
> If I started evaporating acids openly in my fume hood, it would be corroded to hell in a few weeks (the blower in a few days at most), and the neighbors would call the police because of the choking white clouds constantly coming out the exhaust of my lab!
> How many here have a distillation setup to properly condense the acid fumes and prevent their release into the atmosphere?
> A simple glass distillation setup should be one of the first labware acquisitions of anyone who wants to evaporate acid solutions, much more important than a fume hood. Also, you can boil your solutions as vigorously as you like and there are no losses!
> ...



garage chemist,

not only are you by your own words saying that Harold is lying about the fact that you loose values by boiling, but you are doing the very thing that this thread is warning you not to do. is this your opinion or do you have proof that Harold is wrong. Harold has been refining for decades and all his advise comes from first hand knowledge, if he said C.M.Hokes entire book was bunk then i would consider it trash. perhaps all your years as a commercial refiner surpasses Harold's and you know some trick that he doesn't, so for the benefit of the entire forum please enlighten us why he's wrong and your right.


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## jimdoc (Feb 18, 2012)

Boiling in a distillation setup to properly condense the acid fumes and prevent their release into the atmosphere  is very different that just boiling a solution. If you capture everything that is coming off of the reaction properly then there can not be any losses as I see it.
Although I think that most people here aren't setup to do that, so just not boiling makes more sense for them.

Jim


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## Geo (Feb 18, 2012)

exactly my point, how many members do we have that come here with a chemistry education and a well stocked lab as apposed to someone who has never handled chemicals but wants to learn. in a perfect world no one would refine outside of a well supplied lab and foundry but this is the real world and Harold is trying to help the new guy with a small budget and a willingness to learn by teaching him to hold on to what he has in the best possible way. for someone to come on to thread he started to try and squelch the proliferation of bad advise say he is wrong isn't good. of coarse there are ways of doing things that are different but will achieve the same outcome but the simplest is to avoid the hazard.


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## garage chemist (Feb 18, 2012)

I am not saying that Harold is wrong! Harold was exclusively talking about openly evaporating solutions, in which case you will definately loose values when boiling due to spattering. However, if you boil in a distillation setup, the droplets from spattering cannot leave the flask and this way you don't have any losses. You can test the distillate if it contains any values, and generally it does not contain any. If it does, you can recover them sicne essentially nothing is leaving your apparatus. You can even work without a fume hood if the concentration of HCl in your values solution is below 20% and there's not much nitric in there.

Also, a distillation setup is absolutely not too expensive for the beginner.


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## butcher (Feb 18, 2012)

I use distillation for much of my work (not all), I also capture the red fumes as much as possible when dissolving metals in nitric acid, the nice thing as garage chemist stated is the temperature can be raised to boiling (I keep mine forming small bubbles not large bumping bubbles), and as long as you carefully watch for foam over or carry over, the metals stay in the heated reaction vessel (color of condensed liquids are also a clue you are getting carry-over), the fumes if condensed in very little water and H202 for NOx gas into the receiver bottle the acid can be reused as dilute HNO3, this saves me from wasting my acid, and from dumping NOx gases into the atmosphere that the EPA has tried so hard to eliminate (which creates acid rains), I am no tree hugger but I do care for our earth and the air we breath, even the HCl or aqua regia can be distilled (bubbled into water to make a weak solution, which can be reused with heating its action on metals and heat concentrating solution, it works well.

The small amount of gold added to aqua regia to use up excess nitric is nice when you do not want to evaporate just a little heat and let it react, very little fumes are released (normal amount for gold being dissolved), it is quick and easy, and you get your gold back.

What works for me also, is to calculate how much acid I need, calculating just under this amount needed to dissolve the gold, and measure my solutions,
(I can even use the 4 HCl : 1 HNO3 ratio if wanted, although normally just add HCL then adding in small additions), add a small portion of solution to metals with heating, add more as needed, the goal is to leave a small amount of metal un-dissolved with strong heat, and plenty reaction time (this portion of saturated solution is decanted from remaining metal), (the small amount of metal left is then dissolved, usually with just covered in HCl and the drops of nitric added as needed, and with heating (heat is an important part as more metal is reacted with acid with heat, and these hot solutions hold more metal salts, even if I over shoot on this last bit of metal I am dissolving, and use a bit more acid than necessary it is simpler to remove any excess nitric if I made a mistake.

I agree 100% with garage chemist, but I also think if we tell someone to distill their solution then we can explain that the solution can boil, actually when I distill I normally do not want large bubbles popping and splashing, but just want my temperature where lots of little tiny bubbles form (less carryover), also when distilling (or even evaporating in dish with watch glass) as the solutions concentration changes, from gases leaving the system, the temperature (or boiling point changes as this progresses), and if you are not closely monitoring the POT, so then keeping the temperature lower when using either of these methods is best, as you may get a boil over when you turn your back. 

Solutions will vapor of gases way below the boiling point of that solution so a lower temperature does no harm, it just takes more time.

I do have a brand new distilling glass lab set still in the box and never got it wet, but I do use my home made distilling rigs, the sand bath and pickle jar has distilled many gallons of solution, so I just keep using it, I also have a coffee pot Pyrex, it looks similar to a flask (wider mouth) with a handle, I made a Teflon cork and use a hose and distill with it sitting on an electric hotplate in a casserole dish (no sand used). 

That store bought distilling rig is just too pretty to use.


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## jimdoc (Feb 18, 2012)

butcher said:


> That store bought distilling rig is just too pretty to use.



Don't you hate when that happens? Sometimes you have to buy two, so you can use one and have one to display.

Jim


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## butcher (Feb 18, 2012)

It may also be a good idea to discuss some precautions if we do use distilling; I will note a few here.

As with any process in glass, the temperature of the glass should not be changed quickly, this can thermally shock the glass and crack or break it, also glass ware should be chosen for the heating method you are using, sand baths or oil baths can be used (I prefer sand bath as nitric concentrate and oil scares me), a sand bath is just a pot of sand surrounding the jar, to heat it indirectly and evenly (the pickle jar or canning jars were made to boil in water, and in a sand bath protected from direct heat and heated evenly these jars perform well as long as temperature is not changed fast).

Glass ware should be chosen to be able to withstand temperature and the pressures involved.

Tubing and fittings for distilling rigs bought, or homemade should be able to withstand the hot acids you are distilling, rubber is not a good choice for nitric acid nitric eats rubber. Teflon holds up well.

Also chemical resistant tubing can be used.


When distilling the hot glass vessel can build pressure, this is not a problem as long as this pressure has a place to escape.

On cooling a boiling vessel can go into a vacuum, when distilling this vacuum can suck cold liquids back into the hot glass breaking the glass and spilling hot acid everywhere, open the distilling rig to atmospheric air before lowering or turning off heat. 

Tubing and fittings for distilling rigs, store bought, or homemade should be able to withstand the hot acids you are distilling, rubber is not a good choice for nitric acid.
Nitric eats rubber. Teflon holds up well.

Also chemical resistant tubing can be used, I use a clear tubing (looks like PEX to me a type of HDPE, I really do not know for sure the type of tubing as I got it from the dumpster, but it holds up to chemicals and hot acids well), chemical resistance charts can help you select materials resistant to acids.


When distilling the hot glass vessel can build pressure, this is not a problem as long as this pressure has a place to escape.

On cooling a boiling vessel can go into a vacuum, when distilling this vacuum can suck cold liquids back into the hot glass breaking the glass and spilling hot acid everywhere, open the distilling rig to atmospheric air before lowering or turning off heat.


Here are a few links that will have some useful lab safety.


http://en.wikipedia.org/wiki/Distillation

http://www.radford.edu/fpc/Safety/ChemHyg/chp3.htm

http://web.princeton.edu/sites/ehs/labsafetymanual/sec7i.htm#glassware

http://wolfweb.unr.edu/~jacques/safety/glass_safety.html

http://www.ehs.indiana.edu/LabSafety/LCSP%20Part%2007%20-%203.0%20Standard%20Operating%20Procedures.pdf

http://www.des.umd.edu/ls/labguide/one.htm

Want more?


http://www.google.com/search?q=lab+safety++distilling&btnG=Search&hl=en&gbv=2


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## kurt (Feb 19, 2012)

Can &/or should you boil ? --- Yes – you can & at times you should (such as the case of “boiling” tungsten points to leach out the silver)

And I don’t think Harold is fed up with the word boil being used – when used in the right context.

It’s when boiling is suggested – when in fact the desired process goal is to evaporate – that Harold is fed up with - & rightfully so IMO

Another words – Harold is fed up with people being told to boil a solution - as the evaporation process – when doing so is the cause of lost values.

When the desired process goal is to evaporate – the words – “But Do Not Boil” – should be included - IMO 

Kurt


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## bswartzwelder (Feb 19, 2012)

Years ago, I graduated from Penn State University. I took the Nuclear Energy Technician course which included a lot of classes on thermodynamics and heat transfer. We were taught there are no less than seven types of boiling from the point where small bubbles start to form on the container to all out boiling where the steam bubble are huge and blow the liquid all over the place. I can see where Harold has problems when people use the term boiling when they should be saying evaporation and vice versa.


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## Harold_V (Feb 19, 2012)

kurt said:


> Can &/or should you boil ? --- Yes – you can & at times you should (such as the case of “boiling” tungsten points to leach out the silver)
> 
> And I don’t think Harold is fed up with the word boil being used – when used in the right context.



Correct. I used heavy boiling in many of my operations-----some where had I not, the process wouldn't have worked. An example would be the processing of polishing wastes, where the preliminary wash with HCl required heavy boiling in order to keep the solids in suspension. Even then, without care, I occasionally experienced areas that failed to suspend, resulting in superheated areas that lead to fractured vessels when they were stirred. 

There's more than a few examples of boiling being desirable. Evaporation just happens to *not* be one of them. The repeated *boiling down* that I see yields no end to problems, and it's clear to me that those who continue to use the terminology are perpetuating the problem. It's as if my words fall on deaf ears. 

Don't boil when evaporating, unless you have provisions for preventing the resulting spatters, or to capture the traces of values that are propelled from bursting small bubbles at the surface. Each of those circumstances lead to losses, and they're dead easy to prevent by simply keeping the solution below a boil. What the hell is so hard about that?

Harold


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## eeTHr (Feb 19, 2012)

1. Evaporation.
2. Boiling.
3. Distillation.

These are all different terms, with different meanings.

They have different purposes, plus different safety measures are necessary.

It's a very bad thing, for both financial and health reasons, to recklessly interchange them.

Thanks to Harold for continually pointing this out, and being adamant about it!


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