Indonesian Gold - First Try, First Step(s)

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JoeyJoystick

Well-known member
Joined
May 25, 2019
Messages
53
Location
Ban Chang, Rayong, Thailand
Hi All,

So I spend a year reading Hoke's, GRF and a whole bunch of documents I found on-line. But similar to getting your drivers licence, you only learn until you practice and practice and practice. Or so I figured. I felt confident I could do it. But I was equally confident that Murphy was just around the corner and waiting for a chance to screw things up for me. lol.

I had ordered some kit. Some glassware, chemicals, hotplate, filter set-up and a whole bunch of other things. Not quite everything yet, but enough to get me started I figured.

I had some old (late 1800s) jewellery made from Indonesian gold. I knew from a jeweller, who told me that it was beyond repair, that this gold contained a lot of copper.

Anyways, I decided to remove the base metals with Nitric Acid 68%. I buy a liter for 2.5USD so I do not feel the need to be careful using this for monetary considerations. I added the gold into a beaker and covered it with Nitric Acid. I think this was too much acid though, but I do not think that this would cause problems down the line. No major issues anyway. I was impressed at how quickly the base metals dissolved and the colour changed from clear to a very dark greenish liquid. Kind of clear, but still dark-green. The reaction must have stopped within 30 minutes but I left it for 2 days standing. (I have plenty of time...) Surprise! The next day it was clear blue. Almost like aqua-marine-blue with very slight haze of green over it. I believe I have read about this in the past somewhere, but now that I am looking for it, I can obviously not find this. (Any Ideas?)

So now I pulled out my newly purchased vacuum pump. Set up my filter unit consisting of a 1l Erlenmeyer flask and a glass Buchner funnel with a pore plate. I used the slow filters that I had. Because I have time and it is my first time; I figured I should do it properly. Ok, in hind side that may not have been the best idea though. The filter got clogged relatively quickly and it took 1.5 hour to drain it through the filter. Carefully removed the filter with a small bunch of gold in it (admittedly, a very small bunch...) Rinsed all and cleaned all after having put the filters and the gold in a separate jar. Now filtered it again and again with a slow filter. After having used a slow filter I felt it was a really stupid idea to now use a faster filter. But, this time the filtering went very quickly, as in less then 5 minutes. Done this 3 times over. And the blue liquid left over was very clear by now.

Due to the washing and rinsing the 150ml Nitric Acid had now increased in volume to about 500ml. I guess I need to be more cautious next time I rinse with water. Anyways, I figured I should boil off some of the water. And now I noticed somethings strange. Just before it started boiling, I noticed a white powder precipitating to the bottom of the beaker. I am specifically saying 'just before it started boiling' because I can not see inside once it actually starts boiling. This does not mean that it is not there anymore though. Does any of you have a clue what this may be? Could there be silver in it as well? By the looks of it this should be a very small amount though. In comparison, I guestimate this to be about 10% of the total gold I got in the filter. But me guesstimating on my first run is not a very scientific approach of course. I will keep all the liquids for later and treat further.

Now some reporting on the kit. I know I can be a bit of a snob when it comes to buying tools. However, I had already decided that this was going to be a hobby thing and not (not for now anyways) a money making thing. And when I saw the price difference between proper German kit and some cheap stuff from China I decided to go for the latter anyway. I bought everything online from platforms such as AliExpress. Interesting experience... Got things which arrived broken and encountered some obvious quality issues. I am gonna summarize this in a bulleted list with some questions here and there.

- Since the amount of glassware I am dealing with at this stage a small I figured I should make sure that all joints are the same and I have chosen for 24/29 joints. I noticed that glassware from different manufacturers have a slightly different fit. Not to the point where the joint is compromised, but I can feel it is not a perfect fit. Is this normal? Is this reason for concern?
- On most glassware I can see that it Is a relatively cheap kind of glass. Not like my crystal glasses in the kitchen if you know what I mean. I am a little worried about the glas breaking when heated. Is this concern justified? Are there other concerns I should consider? Safety Concerns I am most worried about, but there may well be other practical issues I have not considered .
- The glass Buchner funnel with pore plate and 1 set of filter paper came together in the kit. I bought some additional filter papers of different grades. Surprise; all filters are slightly too big for the poor plate and consequently creep up on the side wall of the Buchner filter for 2-3mm on one side while the other side is flat down all the way to the edge. I guess this is what happens when you go the cheap way.
- p.s. Filter kit came with 1l erlenmeyer flask, 500ml Buchner funnel, filters and a vacuum hose. All for about 33USD. So that is not too bad I figured.
- Vacuum pump. I was really surprised to be able to buy a small vacuum pump for only 50USD. Obviously I wondered if, and how well, this would work. So when I started the first filtering I noticed the pump was getting seriously hot. So I turned it off and every few minutes or so I turned it back on for 10 seconds. Sure enough after a while the pump did not want to start anymore, but once I let it cool down a bit, it started to work a little better again. Again, quality comes at a price. No bad feelings. It works like that. You get what you pay for I believe. But still annoying. lol.
- During the boiling off of the water I noticed that it only started to boil at about 115-120C. Is this due to the mixture of Nitric with water and a consequently increased boiling point or may this come down to a really bad quality Chinese thermometer? I also notice that once the level was reduced to a little less that the initial amount of Nitric Acid which I had used, it kept fuming. Even when it cooled down. Any suggestions here? I want to keep this for later when I have what I need to analyse it. But it is obvious that I can not keep it in a closed storage container. This will likely blow-up in time when pressure builds up and this is not the kind of stuff I want happening on my watch.
- One more comment on the Blueish liquid. After boiling off the water, it was not the pretty clear blue I described earlier. The colour was kind of the same, but just not clear anymore. It had like a whitish haze over it. Presumably this is the white precipitate I was talking about earlier, but I am not sure. And then, of course, after letting this settle for several hours, the liquid is all clear again and the white precipitate has settled on the bottom. Filtered this 2 times and I clearly caught a lot of precipitate in the filters, but there is still a white haze over the liquid, which than slowly settles to the bottom. But only a very small amount. I don't think I can clear this with filtering, because it is quite obvious that it passes the filter fairly easily. Maybe a separating funnel? I will give that some thought.

p.s. Let me be clear about the colour. It is called aquacyan in Wikipedia. https://en.wikipedia.org/wiki/Aquamarine_(color) when you go to the bottom and on the right of 'Shade of Cyan' click on 'Show' and you will see a colour called 'Aqua/Cyan'. This is the colour of the blueish liquid I was talking about. Describing a colour is difficult, but remembering all these colour names is equally difficult... Who oh who came up with all those names and colours? lmao.

Anyway, this was a lot of fun. I will update next time when I continue, but this will take a little while.

Joey
 
JoeyJoystick said:
Anyways, I decided to remove the base metals with Nitric Acid 68%. I buy a liter for 2.5USD so I do not feel the need to be careful using this for monetary considerations. I added the gold into a beaker and covered it with Nitric Acid.
You should use a mixture of about equal amounts of distilled water and your nitric acid.

I think this was too much acid though, but I do not think that this would cause problems down the line. No major issues anyway. I was impressed at how quickly the base metals dissolved and the colour changed from clear to a very dark greenish liquid. Kind of clear, but still dark-green. The reaction must have stopped within 30 minutes but I left it for 2 days standing. (I have plenty of time...) Surprise! The next day it was clear blue. Almost like aqua-marine-blue with very slight haze of green over it. I believe I have read about this in the past somewhere, but now that I am looking for it, I can obviously not find this. (Any Ideas?)
The green is a combination of the blue copper nitrate and the reddish orange NOx gasses created as the nitric breaks down during the reaction. By the next day, most of the NOx had gassed off leaving the predominantly blue copper nitrate.

Due to the washing and rinsing the 150ml Nitric Acid had now increased in volume to about 500ml. I guess I need to be more cautious next time I rinse with water. Anyways, I figured I should boil off some of the water. And now I noticed somethings strange. Just before it started boiling, I noticed a white powder precipitating to the bottom of the beaker. I am specifically saying 'just before it started boiling' because I can not see inside once it actually starts boiling. This does not mean that it is not there anymore though. Does any of you have a clue what this may be? Could there be silver in it as well? By the looks of it this should be a very small amount though. In comparison, I guestimate this to be about 10% of the total gold I got in the filter. But me guesstimating on my first run is not a very scientific approach of course. I will keep all the liquids for later and treat further.
The solution is primarily waste at this point. It may contain some silver, but otherwise only contains copper and whatever other metals were alloyed with your gold. I would have probably just cemented it with copper to capture any silver, then treated it as waste.

- On most glassware I can see that it Is a relatively cheap kind of glass. Not like my crystal glasses in the kitchen if you know what I mean. I am a little worried about the glas breaking when heated. Is this concern justified? Are there other concerns I should consider? Safety Concerns I am most worried about, but there may well be other practical issues I have not considered .
Regardless of the source of glassware, you should always use a secondary containment vessel in case of breakage. I don't know if you have Corningware available, but it's definitely worth the small investment. I buy all mine at yard sales, usually for about a dollar or two per piece.

- p.s. Filter kit came with 1l erlenmeyer flask, 500ml Buchner funnel, filters and a vacuum hose. All for about 33USD. So that is not too bad I figured.
Is it a regular Erlenmeyer flask or a filter flask? A regular erlenmweyer flask may not withstand the stress of a high vacuum and implode, while a filter flask is thicker and can better withstand the stress.

- Vacuum pump. I was really surprised to be able to buy a small vacuum pump for only 50USD. Obviously I wondered if, and how well, this would work. So when I started the first filtering I noticed the pump was getting seriously hot. So I turned it off and every few minutes or so I turned it back on for 10 seconds. Sure enough after a while the pump did not want to start anymore, but once I let it cool down a bit, it started to work a little better again. Again, quality comes at a price. No bad feelings. It works like that. You get what you pay for I believe. But still annoying. lol.
You want to scrub the fumes before they reach the pump or they will quickly eat up the pump.

- During the boiling off of the water I noticed that it only started to boil at about 115-120C. Is this due to the mixture of Nitric with water and a consequently increased boiling point or may this come down to a really bad quality Chinese thermometer? I also notice that once the level was reduced to a little less that the initial amount of Nitric Acid which I had used, it kept fuming. Even when it cooled down. Any suggestions here? I want to keep this for later when I have what I need to analyse it. But it is obvious that I can not keep it in a closed storage container. This will likely blow-up in time when pressure builds up and this is not the kind of stuff I want happening on my watch.
Probably due to any remaining nitric as well as the copper and other salts now dissolved in the solution. Was the fuming colored as in the BFRC, or was it just steam? You can store it in a container with the lid not tightly closed to allow off gassing. Be sure it is stored somewhere where any acid fumes will not ruin anything nearby.

- One more comment on the Blueish liquid. After boiling off the water, it was not the pretty clear blue I described earlier. The colour was kind of the same, but just not clear anymore. It had like a whitish haze over it. Presumably this is the white precipitate I was talking about earlier, but I am not sure. And then, of course, after letting this settle for several hours, the liquid is all clear again and the white precipitate has settled on the bottom. Filtered this 2 times and I clearly caught a lot of precipitate in the filters, but there is still a white haze over the liquid, which than slowly settles to the bottom. But only a very small amount. I don't think I can clear this with filtering, because it is quite obvious that it passes the filter fairly easily. Maybe a separating funnel? I will give that some thought.
Cement out any remaining values with copper and treat the remaining solution as waste.

Dave
 
FrugalRefiner said:
Is it a regular Erlenmeyer flask or a filter flask? A regular erlenmweyer flask may not withstand the stress of a high vacuum and implode, while a filter flask is thicker and can better withstand the stress.

It came with the filter-set I bought. I'd like to assume that it is made for that purpose. I did put the whole filter-set in a large stainless pot just in case. I wouldn't trust the acid in the stainless pot for long though, but I am sure it would give me ample time to act if the flask had broken. Stil thanks for the tip though. When I need to replace the flask I will keep that one in mind.

Joey
 
FrugalRefiner said:
You want to scrub the fumes before they reach the pump or they will quickly eat up the pump.

Ok, will set-up a scrubber next time. But could this be the reason that the pump started acting up after only 30 minutes or so? Is that not a bit too quick? Or does this come down to quality? (Which is what I suspect to be honest..)

Joey
 
FrugalRefiner said:
JoeyJoystick said:
Anyways, I decided to remove the base metals with Nitric Acid 68%. I buy a liter for 2.5USD so I do not feel the need to be careful using this for monetary considerations. I added the gold into a beaker and covered it with Nitric Acid.
You should use a mixture of about equal amounts of distilled water and your nitric acid.

Can you please motivate the answer so I understand why I should not use 68% but diluted Nitric? Pros & Cons and that kind of stuff.

Dave, Thank you very much for your elaborate response to my post. It helps me big time.

Joey
 
Some metals will passivate (build up an insoluble crust that protects the metal from further dissolution).
Some metals that will normally passivate in concentrated nitric acid, can be attacked better by a dilute nitric solution.

Nitric acid needs to begin to break down before it can dissolve metals, the HNO2 gas in solution attacking metals, basically, the gasses in solution are needed to do the work, diluting the solution helps to form these gases in solution.

Once transformed into ions the water gives a place for the newly formed ions to swim away, while the nitric and its gases in the solution can pull another electron from another metal ion and not saturate the solution where it will hold more soluble ions in solution, and keep the gases in solution to do work.

Dilution besides helping to slow or stop the runaway action, or boilovers, where the concentrated nitric suddenly form NOx gases along with a sudden buildup of heat in the reaction as the nitric acid decomposes to gasses and try to escape all at once escaping the solution without doing any work to dissolve metals. diluting slows the reaction but also will hold more dissolved metal into solution.

the dilute nitric will dissolve and hold more metal ions in solution than a concentrated nitric solution.
 
JoeyJoystick said:
FrugalRefiner said:
Is it a regular Erlenmeyer flask or a filter flask? A regular erlenmweyer flask may not withstand the stress of a high vacuum and implode, while a filter flask is thicker and can better withstand the stress.
It came with the filter-set I bought. I'd like to assume that it is made for that purpose. I did put the whole filter-set in a large stainless pot just in case. I wouldn't trust the acid in the stainless pot for long though, but I am sure it would give me ample time to act if the flask had broken. Stil thanks for the tip though. When I need to replace the flask I will keep that one in mind.

Joey
A filter flask typically has a side arm, i.e., a short barbed outlet on the side of the flask near the top. You can also look at the thickness of the glass. A filter flask will be thicker than a regular Erlenmeyer flask or beaker.

A stainless steel pot will actually withstand nitric quite well. The nitric passivates the chromium in stainless, creating a very resistant coating that protects the metal. Large quantities of nitric are typically shipped and stored in stainless. Having said that, there are lots of different flavors of stainless steel, with varying amounts of nickel, chromium, and other constituents. Their resistance to nitric will vary.


FrugalRefiner said:
You want to scrub the fumes before they reach the pump or they will quickly eat up the pump.
Ok, will set-up a scrubber next time. But could this be the reason that the pump started acting up after only 30 minutes or so? Is that not a bit too quick? Or does this come down to quality? (Which is what I suspect to be honest..)

Joey
It could be either. Nitric fumes, aka NOx, aka the BFRC, are extremely corrosive. 30 minutes is awfully quick, so I'd lean toward the pump having a short duty cycle. But the fumes will help to destroy the pump in short order.

Dave
 
I use a vacuum aspirator bottle to protect my pump.

It is connected between the vacuum flask and the vacuum pump.

Here is an example on ebay. (Not my listing)

https://www.ebay.com/itm/SCIENCEWARE-ASPIRATOR-BOTTLE-NICE-CONDITION-CLEAN-ID-300216-/263917993318

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