Is my HCl dissolving gold?

[VanMarco]

So, I am suspecting that my toilet-grade HCl is actually dissolving gold without anything else present.
I explain.
I dissolved in aqua regia some black dust leftover from dissolving silver.
Test positive for gold. didn't over nitric or anything.
Added smb, precipitated the gold.
Decanted solution, added water, did a hot water boil, twice. (rain water, not tap)
then, i got my HCl and did a 5min HCl boil. somehow, gold powder looked like half what it originally was.
went to weight the gold. 0.06G. It looked too little to me, so i tested the HCl boil solution and.. tested positive for gold.
Put smb in it. gold.. precipitated. weighed the gold 0.05G. So the HCl wash dissolved half of my gold.
I was told that it's possible that HCl may be contaminated by Cl2. I must say that the occasional whiff of the acid while boiling
did smell a bit like chlorine, not just HCl.
Is it possible there's Cl2 in my acid? or what could have caused the gold to be dissolved?
Thanks!

 

[Yggdrasil]

So, I am suspecting that my toilet-grade HCl is actually dissolving gold without anything else present.
I explain.
I dissolved in aqua regia some black dust leftover from dissolving silver.
Test positive for gold. didn't over nitric or anything.
Added smb, precipitated the gold.
Decanted solution, added water, did a hot water boil, twice. (rain water, not tap)
then, i got my HCl and did a 5min HCl boil. somehow, gold powder looked like half what it originally was.
went to weight the gold. 0.06G. It looked too little to me, so i tested the HCl boil solution and.. tested positive for gold.
Put smb in it. gold.. precipitated. weighed the gold 0.05G. So the HCl wash dissolved half of my gold.
I was told that it's possible that HCl may be contaminated by Cl2. I must say that the occasional whiff of the acid while boiling
did smell a bit like chlorine, not just HCl.
Is it possible there's Cl2 in my acid? or what could have caused the gold to be dissolved?
Thanks!

Boiling HCl will pick up Oxygen from the air.
You only need to get it hot and stir.
If your Gold is not clean, it will shrink as the base metals are removed.

 

[nickvc]

I have stated before that just HCl will dissolve a little gold depending on the other elements present and I believe even Hoke makes mention of this.

 

[Yggdrasil]

I have stated before that just HCl will dissolve a little gold depending on the other elements present and I believe even Hoke makes mention of this.

That will usually be in the micro gram scale will it not?
Depending on the amount of powder 0.06g by a 5 minute boil seem excessive.

 

[Yggdrasil]

So, I am suspecting that my toilet-grade HCl is actually dissolving gold without anything else present.
I explain.
I dissolved in aqua regia some black dust leftover from dissolving silver.
Test positive for gold. didn't over nitric or anything.
Added smb, precipitated the gold.
Decanted solution, added water, did a hot water boil, twice. (rain water, not tap)
then, i got my HCl and did a 5min HCl boil. somehow, gold powder looked like half what it originally was.
went to weight the gold. 0.06G. It looked too little to me, so i tested the HCl boil solution and.. tested positive for gold.
Put smb in it. gold.. precipitated. weighed the gold 0.05G. So the HCl wash dissolved half of my gold.
I was told that it's possible that HCl may be contaminated by Cl2. I must say that the occasional whiff of the acid while boiling
did smell a bit like chlorine, not just HCl.
Is it possible there's Cl2 in my acid? or what could have caused the gold to be dissolved?
Thanks!

How was it dissolved and how was it precipitated?

 

[VanMarco]

was dissolved with aqua regia and precipitated with smb. i thought a hot water boil was OK to get rid of these chemicals?

 

[Shark]

It may take multiply rounds of hot water depending on the amount of powders and how fine they are.

 

[Yggdrasil]

was dissolved with aqua regia and precipitated with smb. i thought a hot water boil was OK to get rid of these chemicals?

If you are gong to be sure it need to be roasted.
That goes both ways, from Nitric to HCl or from HCl to Nitric.

 

[nickvc]

That will usually be in the micro gram scale will it not?
Depending on the amount of powder 0.06g by a 5 minute boil seem excessive.

I suspect it may be related to the ratio of acid to gold powder and on the other elements present as I mentioned.

 

[Yggdrasil]

I suspect it may be related to the ratio of acid to gold powder and on the other elements present as I mentioned.

Yes and probably insufficient destruction of Nitric.

 

[andrewhunt38]

Also, the mention of rain water may include carbonic acid depending on the CO2 levels in the area.

 

[orvi]

Also, the mention of rain water may include carbonic acid depending on the CO2 levels in the area.

Using rainwater is clever, when you have good collection system and filters - if you live somewhere far from the busy road or city, it is qualitatively close to distilled water. Also snow.

CO2 does not interfere with any of these processes.

I suspect it may be related to the ratio of acid to gold powder and on the other elements present as I mentioned.

I would say that it also depends on conditions of gold drop. In my experience, well formed deposit of nice brown heavy gold powder never leave positive stannous after HCL wash. But finer, more dark coloured precipitates often do. Not that terrible, but it can be detected.

I personally think that it does have something to do with encapsulation of impurities, or their absorption onto big surface area of the particles. As these "more dark" deposits often liberate more junk after some soaking in HCL as the well formed dense brown precipitates do. I do not believe that SMB or sulfite can reduce metals such as copper or nickel (common in scrap) to their metallic state. But they can stay enclosed/interlocked between grains of gold, and as SMB and sulfites are basic, some temporary precipitate of hydroxy-nitrate can form, which is in situ enclosed and thus trapped. Pouring HCL and simmering eventually release it and then it could redissolve gold.
Just my hypothesis

 

[VanMarco]

Ok, i see, thanks for the kind answers!!

 

[cejohnsonsr1]

You don’t have enough to make any accurate assessment. 0.06 gr is such a small amount that a variation of 0.01 gr from one handling to the next is to be expected. Every time you rinse some of the gold is going to get poured out with the water or acid. That’s the whole reason for temporary waste containers and stock pots.

 

[yellowfoil]

You will not get rid of all HNO3 by boiling it in distilled water. That was most likely the cause of redissolving of your gold in HCl boil. The only way to be sure is roasting. If you don't want to roast, do at least three long boils in distilled water, not rain water. You can make simple solar still to make free distilled water.

 

[Ayham Hafez]

Once upon time, I leached some PCBs in HCL to eliminate tin, after 24 hours I noticed that most of gold pins changed colored to green, even gold fingers did.

I didn't add any thing with the industrial HCl grade, after washing the PCB's I tried to scratch the gold fingers to see the gold layer, I didn't see it, after that I used AR and I got gold but less than expected about 80% of the expected gold.


What was that green layer? The HCL solution color was transparent so I think it didn't dissolve copper , from that time and I only leach PCBs in HCl for only 5 hours.

 

[Yggdrasil]

Once upon time, I leached some PCBs in HCL to eliminate tin, after 24 hours I noticed that most of gold pins changed colored to green, even gold fingers did.

I didn't add any thing with the industrial HCl grade, after washing the PCB's I tried to scratch the gold fingers to see the gold layer, I didn't see it, after that I used AR and I got gold but less than expected about 80% of the expected gold.


What was that green layer? The HCL solution color was transparent so I think it didn't dissolve copper , from that time and I only leach PCBs in HCl for only 5 hours.

Nobody here leach whole PCBs in AR.
Not even in AP.
We remove the fingers or other good parts,
then we put those in AP and after it has done its job we go to AR, HCl/Bleach or HCl/Peroxide

 

[Ayham Hafez]

Nobody here leach whole PCBs in AR.
Not even in AP.
We remove the fingers or other good parts,
then we put those in AP and after it has done its job we go to AR, HCl/Bleach or HCl/Peroxide

AP not work well with magnetic pins,I have a telecom type board that has only gold pins connectors never striped using AP, also lot of recyclers use AR directly with pcb that has gold tracks like cell phone board, always AP decrease the gold recovery rate, lost of gold is much more than the cost of chemicals of using AR.

I did different tests for same amount and type of PCBs with AP and with using AR directly without AP, the recovery rate of AR is more than AP process almost 30% in spite of I process also the black powder from AP as it may contains gold. I only use AP with low grade gold pins and fingers cause in low grade case chemical of AR cost a lot comparing with the recovered gold.

 

[Yggdrasil]

AP not work well with magnetic pins,I have a telecom type board that has only gold pins connectors never striped using AP, also lot of recyclers use AR directly with pcb that has gold tracks like cell phone board, always AP decrease the gold recovery rate, lost of gold is much more than the cost of chemicals of using AR.

I did different tests for same amount and type of PCBs with AP and with using AR directly without AP, the recovery rate of AR is more than AP process almost 30% in spite of I process also the black powder from AP as it may contains gold. I only use AP with low grade gold pins and fingers cause in low grade case chemical of AR cost a lot comparing with the recovered gold.

Well as per the big boys in here has told us, "nobody" runs whole boards in AR directly, but as they are concerned they are also in need to run proper waste management and maybe the ones you know about do not need to do so.
All the big companies run pyro-metallurgical processes, they create an order of magnitude lass waste and give for all practical purposes complete recovery.
Shred, pyrolize with proper afterburner, incinerate, mix with flux and smelt.