Some questions regarding Nitric acid digestion

[yossarian]

So I have done a lot of reading both forum and hoke and decided to start experimenting. I wanted to attempt a a Nitric bath on some pins, so I got 1 gram of clean gold plated pins and put them in a 6 inch test tube, next I added about 1 ml of nitric acid 70%, to see how the reaction goes. I was amazed, I did not expect such a quick violent reaction and so much heavy fumes, of course that is why I started with such a small experiment. after a few washes and acid baths, all the base metal was used up and no more reactions happened. At this point I remembered reading in Hoke that after the metal is done reacting with the HNO3 you wash with water then HCL then again with water, I tried to find that place in hoke just to double check that that is really the procedure and I did not Imagine it but I could not find the spot,

so first question, am I correct that after washing with water you follow with a quick wash with HCL then again with water? If that is so, then why? ( I know this is one of those "go read hoke" questions but i have re-read the entire "commonest case" chapter and can't find where that is written, a simple yes or no answer would be great.

after I finished digesting the first gram of pins and washed them (water only) , I was left with what were clearly visible flakes of shiny yellow gold (actually had much more than i expected). Now I wanted to try the same experiment again, but this time I used a gram of different pins (from a different source) and diluted the HNO3 some while reacting. However this time I was not left with shiny yellow gold flakes, but a reddish-brown powder which confused me greatly.

why would one time I end up with gold flakes and another a brown powder? does that mean one of these samples of pins are not really gold plated?
I tried taking pictures but my crappy camera's macro setting can't get a clear picture so zoomed in.

I also have a question about procedure before moving on to aqua regia, Although I am not yet at that point. I have read on the forum that people suggest after the Nitric acid digestion that you should incinerate the remaining metal before putting it in AR. Do they literally mean placing the metal in a frying pan, adding fuel and setting it on fire? I know this is done with sweeps or filings that are linty or have a lot of organic debris, but why is this necessary after the Nitric acid digestion? In this case I am certain that Hoke does not instruct to do this.

I also have an apparatus question. will a pyrex test tube withstand the heat from the flame of an alcohol burner? assuming its close enough to boil?


P.S. I am quite new here so my lingo may be incorrect, for example I keep calling the dissolving of base metal in Nitric acid "nitric digestion" if this or anything is incorrect and confusing, I sincerely apologize.

 

[maynman1751]

So I have done a lot of reading both forum and hoke and decided to start experimenting. I wanted to attempt a a Nitric bath on some pins, so I got 1 gram of clean gold plated pins and put them in a 6 inch test tube, next I added about 1 ml of nitric acid 70%, to see how the reaction goes. I was amazed, I did not expect such a quick violent reaction and so much heavy fumes, of course that is why I started with such a small experiment. after a few washes and acid baths, all the base metal was used up and no more reactions happened. At this point I remembered reading in Hoke that after the metal is done reacting with the HNO3 you wash with water then HCL then again with water, I tried to find that place in hoke just to double check that that is really the procedure and I did not Imagine it but I could not find the spot,

so first question, am I correct that after washing with water you follow with a quick wash with HCL then again with water? If that is so, then why? ( I know this is one of those "go read hoke" questions but i have re-read the entire "commonest case" chapter and can't find where that is written, a simple yes or no answer would be great.

after I finished digesting the first gram of pins and washed them (water only) , I was left with what were clearly visible flakes of shiny yellow gold (actually had much more than i expected). Now I wanted to try the same experiment again, but this time I used a gram of different pins (from a different source) and diluted the HNO3 some while reacting. However this time I was not left with shiny yellow gold flakes, but a reddish-brown powder which confused me greatly.

why would one time I end up with gold flakes and another a brown powder? does that mean one of these samples of pins are not really gold plated?
I tried taking pictures but my crappy camera's macro setting can't get a clear picture so zoomed in.

I also have an apparatus question. will a pyrex test tube withstand the heat from the flame of an alcohol burner? assuming its close enough to boil?

First question...No! If you wash with HCl after nitric you can dissolve values because you just made AR from the left over nitric on the gold. Water does not completely eliminate all of the nitric. Either add HCl and a few drops of nitric to dissolve the gold or incinerate the gold before adding just HCl.
Second question... The second batch of pins may be kovar(iron) and that's why the brown powder. You have to make sure that all base metals have gone into solution. I am not absolutely sure, but I don't think that the iron should settle out if it was completely digested. Your gold may be cemented to the iron powder. Again, Not sure! You may want to get a confirmation on that from another member before proceeding. I have not dealt with nitric a lot yet.
Third... Yes, the test tube should hold up to flame, but heat very slowly or the liquid could just pop out all of a sudden when it reaches boiling.

 

[NobleMetalWorks]

So I have done a lot of reading both forum and hoke and decided to start experimenting. I wanted to attempt a a Nitric bath on some pins, so I got 1 gram of clean gold plated pins and put them in a 6 inch test tube, next I added about 1 ml of nitric acid 70%, to see how the reaction goes. I was amazed, I did not expect such a quick violent reaction and so much heavy fumes, of course that is why I started with such a small experiment. after a few washes and acid baths, all the base metal was used up and no more reactions happened. At this point I remembered reading in Hoke that after the metal is done reacting with the HNO3 you wash with water then HCL then again with water, I tried to find that place in hoke just to double check that that is really the procedure and I did not Imagine it but I could not find the spot,

A good way to get familiar with what you have read in Hoke is to do the acquaintance tests. MaynMan is right, you do not want to do an HCl bath right after HNO3. If there is any left over Nitric Acid, when you add the Hydrochloric you will have made Aqua Regia which as you know dissolves gold. Even just a tiny amount of nitric will do it.

so first question, am I correct that after washing with water you follow with a quick wash with HCL then again with water? If that is so, then why? ( I know this is one of those "go read hoke" questions but i have re-read the entire "commonest case" chapter and can't find where that is written, a simple yes or no answer would be great.

After your nitric acid digestion of base metals, you should filter, wash three times, and incinerate before digesting in AR.

EDITED: This should read "and incinerate before an HCl bath" not "and incinerate before digesting in AR.

after I finished digesting the first gram of pins and washed them (water only) , I was left with what were clearly visible flakes of shiny yellow gold (actually had much more than i expected). Now I wanted to try the same experiment again, but this time I used a gram of different pins (from a different source) and diluted the HNO3 some while reacting. However this time I was not left with shiny yellow gold flakes, but a reddish-brown powder which confused me greatly.

Have you tested with Stannous Chloride? Try this, take a small amount of the brown powder and digest it in AR, in a small test tube. Then test the resulting solution with Stannous. It will tell you clearly if there is any gold present. Dependent upon how the gold plating was applied, it may come off in different ways. MaynMan might be right about the pins being Kovar, but you should still go through the process of finding out if there are any values in what you digested.

why would one time I end up with gold flakes and another a brown powder? does that mean one of these samples of pins are not really gold plated?
I tried taking pictures but my crappy camera's macro setting can't get a clear picture so zoomed in.

The problem is that you cannot upload any pictures, someone might be able to tell you with more certainty what exactly is going on, but even at that it's still kind of a guessing game. You have to test for gold in solution yourself to be sure about anything. The reason the pins are different, is well, because they were made so. As gold became more and more expensive, industry had to figure new ways of plating gold on anything. It could be the pins were made totally different years, and each reflect the value of gold during that time by how much you are able to recover from them.

I also have a question about procedure before moving on to aqua regia, Although I am not yet at that point. I have read on the forum that people suggest after the Nitric acid digestion that you should incinerate the remaining metal before putting it in AR. Do they literally mean placing the metal in a frying pan, adding fuel and setting it on fire? I know this is done with sweeps or filings that are linty or have a lot of organic debris, but why is this necessary after the Nitric acid digestion? In this case I am certain that Hoke does not instruct to do this.

Do not incinerate by adding fuel to whatever it is you are incinerating. Also, remember when Hoke wrote her book that it was in the early part of the 1900s, and soon after, US law prohibited hoarding gold so there were really no revisions as technology and lab practices changed and became more sophisticated and clinical. She was dealing with equipment of her time, and wrote the book accordingly. So when she says to put something in a caste iron pan and put fuel on it then burn it, she is saying in today's terms to use a torch and proper equipment so you can incinerate your gold foils before digesting in AR. Incinerating does a lot more than just burning off organic debris. It also burns off the oil that came off your hands when you handled the material you recovered gold from. It might contain polymers, plastics, glues, whatever else might have been on the material you digested. It's extremely important to rid your material of anything that does not belong, BEFORE you move to AR. This step will help you avoid all kinds of problems later in your process. Please, do not skip this step.

I also have an apparatus question. will a pyrex test tube withstand the heat from the flame of an alcohol burner? assuming its close enough to boil?

You might want to go to a chemistry glass supply website to check and see if your test tube is meant to be placed over an alcohol burner. If you have a centrifuge test tube I am not sure it would withstand the head of direct flame. It's better to make sure before you do it.

P.S. I am quite new here so my lingo may be incorrect, for example I keep calling the dissolving of base metal in Nitric acid "nitric digestion" if this or anything is incorrect and confusing, I sincerely apologize.

I was able to understand you fairly clearly, with no confusion.

Scott

 

[maynman1751]

Thanks Steve for filling in my inadequacy's. That is why I point out that I am not sure. Don't want to mislead or give wrong info.

 

[NobleMetalWorks]

Thanks Steve for filling in my inadequacy's. That is why I point out that I am not sure. Don't want to mislead or give wrong info.

maryman,

You are not inadequate at all, I missed some things, and if someone else comes along and sees this post in a different way, they might have even more important information or suggestions. The beauty of this forum is such that each person contributes what they believe is important, then it is discussed, agreed on, and the thread marches on. So many different people see things in so many different ways, it's like having a think tank on your computer. Truly this forum is very special.

I didn't mean to make it seem as if you missed anything, I was only expanding on what you have said.

Oh yeah, and I'm Scott, not Steve, but I'll accept that as a compliment if it's alright with you. =)

Scott

 

[FrugalRefiner]

yossarain, I'll add a couple more thoughts:

I was amazed, I did not expect such a quick violent reaction and so much heavy fumes, of course that is why I started with such a small experiment.

Starting small is very wise. I hope you were also working in a fume hood or outdoors. It really doesn't take much NOx to damage your lungs as at least one of our members can attest.

I know this [incineration] is done with sweeps or filings that are linty or have a lot of organic debris, but why is this necessary after the Nitric acid digestion? In this case I am certain that Hoke does not instruct to do this.

I believe it has been Harold_V who has championed this practice, and if memory serves it was a practice he developed himself, not one he learned from Hoke. Incineration should be used any time you are changing from one acid to another. It will break down any remaining nitrate or chloride salts (even completely dry, salts can exist along with your gold) which will then create Aqua Regia when a different acid is used for further removal of base metals or washing. Following a nitric treatment, it will also oxidize problem metals like tin allowing them to be removed with an HCl wash.

I also have an apparatus question. will a pyrex test tube withstand the heat from the flame of an alcohol burner? assuming its close enough to boil?

Yes, you should be fine. That's how they're designed to be used. They'll do fine in the flame of a bunson burner as well, and that will generally be hotter. It's thermal shock that causes problems for glass in general, including pyrex. If you touch that same test tube to a red hot electric hotplate burner, it may shatter because the tiny portion of the tube that touches the burner heats up and expands much faster than the rest of the tube, causing stress and failure. An alcohol burner or gas flame provides a broader, more even heat. The test tube heats fairly equally all across its bottom, so it all expands at about the same rate, so less stress and the tube doesn't shatter. This is why asbestos heat spreaders are traditionally used with larger vessels like erlenmeyer flasks and beakers. They keep the heat from being concentrated in one tiny spot.

Dave

 

[maynman1751]

Oh yeah, and I'm Scott, not Steve

Sorry about that! My name is maynman not maryman!!!!! :lol: :lol: :lol:
I really don't have a lot under my belt yet but I still try to help when I'm reasonably sure I know what I'm talking about. If I'm not sure I either don't answer or state that I'm not sure of my facts. Like you said, it's nice to be able to bounce ideas off of each other. I guess that's the way we learn. Thanks Steve......Oh, I mean Scott! :mrgreen:

 

[NobleMetalWorks]

Oh yeah, and I'm Scott, not Steve

Sorry about that! My name is maynman not maryman!!!!! :lol: :lol: :lol:
I really don't have a lot under my belt yet but I still try to help when I'm reasonably sure I know what I'm talking about. If I'm not sure I either don't answer or state that I'm not sure of my facts. Like you said, it's nice to be able to bounce ideas off of each other. I guess that's the way we learn. Thanks Steve......Oh, I mean Scott! :mrgreen:

We are playing the wrong name game, sorry about that...

Scott

 

[Westerngs]

Maynman,

If after you dissolve the base metals in nitric acid, it is your intention to dissolve the gold in order to purify by dropping it out with sodium metabisulfite, I see no reason to incinerate after rinse and before HCl.

The purpose of incinerating something to remove the acid is if that acid will be harmful in the next step. In your case that is not true, because your next step is to dissolve the gold foils in aqua regia, which is a mixture of nitric and hydrochloric. Therefore no reason to incinerate to remove nitric acid traces.

If you suspect there is tin or some other metal that is best dissolved with hydrochloric, then you need to do the hydrochloric step first, rinse well, incinerate to remove hydrochloric traces then add nitric to remove remaining base metals. After base metals have all dissolved and been rinsed away, then dissolve foils in aqua regia.

 

[Harold_V]

Maynman,

If after you dissolve the base metals in nitric acid, it is your intention to dissolve the gold in order to purify by dropping it out with sodium metabisulfite, I see no reason to incinerate after rinse and before HCl.

That's correct. One incinerates only if the next operation is to digest with HCl, which has distinct advantages. It's a process I recommend highly, but only in the proper places. One of them is when processing a large volume of gold filled materials (such as eye glasses). The net result is to eliminate traces of materials that complicate handling. I struggled with such materials for a long time, then, one fine day, my head clicked in. HCl washes are very good at removing such things, so I incinerated all of the collected solids (including the filter) and did a wash with HCl. I was pleasantly surprised to discover the problems came to an end, and the quality of the recovered gold was improved.

I have long harped on readers reading Hoke. Yeah, I still do. The real message, however, is to UNDERSTAND what she teaches. That's really key to success. Had I not understood, I would have never made the decision to incinerate materials that had been recovered with nitric, nor would I have made the decision to do a digest in HCl. Thanks to a good understanding of what each operation was good for, the decision came easily. Remember, I had nothing but Hoke's book as a guide. I had no Gold Forum to which I could turn for guidance. There was no such a venue, nor would those who could help do so, in spite of my knowing one individual.

You guys are very fortunate to have this place to use for guidance. If you read the recommended texts, and listen to hard earned knowledge, you can find yourselves in a place that took me years to achieve, all in just a short period of time.

It is clear to me that Westerngs has a firm understanding of the processes that have been discussed here. My hat is off to you, sir. Making the right decision when called upon will be easy.

Harold

Edit:


It was brought to my attention that my post was not clear. Reread it, and I agree, it may not be, so I'm giving my thoughts once again.

If one does a digest with nitric, and then hopes to do a digest with HCl, BEFORE dissolving the values, yes, there should be an incineration process. If, at any time, one goes from a nitric digest to an HCl digest (or vice versa), with the idea in mind that the values are to not to be dissolved, (proper) incineration guarantees that there will be no loss of values via dissolution. I followed this practice for years with success.

By contrast, if one's purpose is to dissolve the values, there is no need to incinerate, assuming there has been an incineration process somewhere prior to the dissolution of base metals.
I used incineration regularly. It is one of the handiest of all operations, although it doesn't appear to have much value to many of the readers here. Why, I do not understand.

I discovered, long ago, that one can rinse multiple times and still have unwanted acid present. A good example of that was when I'd wash my cemented silver. It got rinsed multiple times, until the solution was clear, then it was filtered in a Buchner, where it was highly compacted and rinsed yet again. I dried all of my cement silver by heating. I stored it until I could melt, and didn't want acid fumes emanating from the dry silver. Using a large porcelain evaporating dish, I'd heat it to well above the boiling point of water. When the water was gone, it was common to see brown fumes being evacuated, in spite of the extensive rinsing the silver had undergone. This pattern was repeated time and again, convincing me that rinsing, alone, does not eliminate all of the nitric.

 

[Westerngs]

Thanks Harold,

In the few months I have been here I have learned to respect your opinion. It means a lot.

Also, what you have said about precious metal people not being very helpful in the past is true. When I first started many years ago I found the industry to be very secretive with their processes. I did find a few exceptions, especially one person from the now defunct Handy and Harman whose name I can't mention.

 

[maynman1751]

At this point I remembered reading in Hoke that after the metal is done reacting with the HNO3 you wash with water then HCL then again with water, I tried to find that place in hoke just to double check that that is really the procedure and I did not Imagine it but I could not find the spot,

Maynman,
If after you dissolve the base metals in nitric acid, it is your intention to dissolve the gold in order to purify by dropping it out with sodium metabisulfite, I see no reason to incinerate after rinse and before HCl.

The purpose of incinerating something to remove the acid is if that acid will be harmful in the next step. In your case that is not true, because your next step is to dissolve the gold foils in aqua regia, which is a mixture of nitric and hydrochloric. Therefore no reason to incinerate to remove nitric acid traces.

If you suspect there is tin or some other metal that is best dissolved with hydrochloric, then you need to do the hydrochloric step first, rinse well, incinerate to remove hydrochloric traces then add nitric to remove remaining base metals. After base metals have all dissolved and been rinsed away, then dissolve foils in aqua regia.

Westerngs and Harold, I understand exactly what you are saying. Yossarian was talking about a WASH with HCl prior to digesting. It is my understanding that if you WASH with nitric present, without incinerating, you will lose values. I don't think that I read that wrong! Right after the quote from yossarian he states that again.

so first question, am I correct that after washing with water you follow with a quick wash with HCL then again with water? If that is so, then why? ( I know this is one of those "go read hoke" questions but i have re-read the entire "commonest case" chapter and can't find where that is written, a simple yes or no answer would be great.

Please advise as I don't want to relay bad advice!

 

[Westerngs]

In all honesty, it has been many years since I read Hoke, so I don't remember where she would advice washing a material with hydrochloric after nitric acid digestion followed by rinses.

My advice still stands though.

At the starting point, if you think the material has tin, I would start with hydrochloric digestion to remove tin. If you start with nitric first, you will create metastannic acid which is a pain to deal with.

After the hydrochloric, rinse very well then incinerate.

Now nitric acid digestion to remove base metals. Rinse very well. If you are moving on to aqua regia at this point to dissolve the gold, no need to incinerate.

 

[maynman1751]

I totally agree with the HCl to remove solder(tin) before processing. I understand that. But it has been repeated here numerous times that incineration is needed after nitric digestion of base metals(or any contact with nitric) before HCl wash.....that's wash not dissolution. That's what I am saying. I'm not disagreeing with your statements. I'm only clarifying what I answered under the context that it was presented. Yossarian wanted to wash with HCl after nitric... not dissolve! Therefore making AR and a possible loss of values if this wash was simply discarded. And no, I see no need for HCl wash after nitric unless you would think that all base metals had not been thoroughly removed. It would be similar to the HCl boil that is needed after AP on ram fingers to remove remaining base metals before dissolution. Even then some individuals will incinerate first to oxidize any materials that they want to remove with the HCl wash. Do you understand what I am trying to convey?

 

[Night_rain]

I'm a newb and a nobody but this conversation has clarified several questions for me.
Thank you for this web site, I am learning so much.

 

[Harold_V]

For clarity, I edited my post, above. It may be of interest for those who still have questions about when one should incinerate.

Harold

 

[maynman1751]

I'm a newb and a nobody but this conversation has clarified several questions for me.
Thank you for this web site, I am learning so much.

I'm glad that you were able to benefit from our exchanges and never consider yourself a NOBODY! Everyone here has the ability to contribute to this forum in one way or another. John.

 

[maynman1751]

For clarity, I edited my post, above. It may be of interest for those who still have questions about when one should incinerate.

Harold

Thank You Harold for your contribution to this post and to the forum overall.

 

[yossarian]

I want to thank you all for your answers and replies. It amazes me how many people are willing to spend the time and effort to help complete strangers. I hope one day I am able to pay it forward and help new complete strangers with the knowledge I have mooched from all of you.

You explinations of when and why we incinrate makes complete sense, and like Harold said, Understanding why we do something makes knowing when to apply that something much easier. same goes with the HCL wash.

Of course all these answers ultimately lead to more questions. (hopefully I am not pushing it)

Do HCL and H2SO4 dissolve many of the same base metals that nitric acid does? such as copper? (If memory serves me HCL does not dissolve silver?) I noticed that H2SO4 instead of HNO3 is sometimes recommended by Hoke (she says if you are dealing with more than 100 oz use Sulphuric instead of nitric) But I am still unsure if you can really use them interchangeably?

what are some of the tougher metals to remove (besides lead and tin and Pt group) that may commonly be encountered in electronics refining? (if something is tricky to remove but is pretty much never encountered then don't worry about it)

for incineration will an ordinary blow torch (which I have) suffice? or do i need to get something better?


Thanks again guys for all your previous answers. if any of these questions are "go read hoke" questions.. by all means tell me

 

[Harold_V]

Do HCL and H2SO4 dissolve many of the same base metals that nitric acid does? such as copper? (If memory serves me HCL does not dissolve silver?)

No, they do not, or if the do, they don't do it well. You are correct, HCl does not dissolve silver.

I noticed that H2SO4 instead of HNO3 is sometimes recommended by Hoke (she says if you are dealing with more than 100 oz use Sulphuric instead of nitric) But I am still unsure if you can really use them interchangeably?

It would be my opinion that that is not the case. This may (it most likely will) require a better explanation from a chemist, but sulfuric acid is not well suited to dissolving silver, although it certainly can do so. In fact, it was common practice to use concentrated sulfuric acid to dissolve silver from slimes. It was performed in cast iron kettles, as concentrated sulfuric does not dissolve iron-----but it also is not capable of holding a great deal of silver in solution, very unlike nitric acid. It is also very hazardous to handle. I recommend you not work with heated concentrated sulfuric. You can dissolve, easily, 30 troy ounces of silver with a liter of 68% nitric, if memory serves.

for incineration will an ordinary blow torch (which I have) suffice? or do i need to get something better?

You may have to better describe what you call a blow torch. Folks from my generation understood that term to mean a torch fired on white gas, much like the heating system used by plumbers to melt lead for caulked joints, or for heating a soldering iron. More modern torches are propane fired. Bottom line is, any torch that can provide enough btu's to heat a pan to dull redness can be used, but it's really nice to use a torch along with oxygen, for the oxygen itself becomes an inspection tool. When the material has been well burned, remaining bits of carbon will glow intensely by introducing free oxygen to the pan. When the glowing bits are gone, it can be safely assumed that you have incinerated fully.

You need not use acetylene, however. It's costly and becoming ever more costly. There's nothing preventing you from using natural gas along with oxygen, or even propane and oxygen. So long as you're just heating and melting (your torch can serve a dual purpose), there's nothing wrong with making one of them your choice.

I recommend you not consider a propane torch without oxygen. It's too costly to use, and does not provide the necessary btu's to accomplish the task well.

Harold