Reusing HCl from Au cleaning for CuCl Leach

[Alentia]

I collect all my spent HCl for Au washes into 4L jags, drop a bit of SMB and collect bits of gold when emptying. I use this HCl to decrease pH in waste treatment barrel from 10 to 7.

However, I end up with excess. While this HCl is almost clean not counting for minor Cu and Fe and other minor contaminants from Au wash, I wonder if this HCl can be re-used for CuCl leach.

From my perspective I do not see any issue with it, but wonder if anyone does it already and if there might be issues I am not aware of.

 

[butcher]

Sodium metabisulfite SMB and water forms a solution of sodium bisulfite and sulfur dioxide gas.
Na2S2O5 + H2O --> 2NaHSO3 + SO2
When we reduce old from solution we change the sodium bisulfite to sodium bisulfate the bisulfite is oxidized to sulfate.
2AuCl3 + 3NaHSO3 +3H2O --> 2Au + 6HCl + 3NaHSO4


I guess what I am trying to show you is the HCl wash will not only Be a solution of HCl but also will contain sulfites or sulfates, this does not mean you cannot reuse the solution to dissolve base metals, but you should consider what is in this solution and how it will react with metals or in the chemical process your are using it. example I could not only be making chloride salts of metals with this solution but also sulfate salts...

 

[Alentia]

Which should not be an issue as long as it does not dissolve any PMs and foils can be collected from CuCl leach solution.

Would there be any issues with sulfates in AP?

 

[butcher]

It would not be a true copper chloride leach, but it will dissolve base metals, the sulfate involved can leave some metals insoluble and as sulfates like silver or lead, but many of the base metals would be soluble as base metal chlorides and sulfates in solution.

 

[Alentia]

When we reduce old from solution we change the sodium bisulfite to sodium bisulfate the bisulfite is oxidized to sulfate.
2AuCl3 + 3NaHSO3 +3H2O --> 2Au + 6HCl + 3NaHSO4

Ok, while we have 3NaHSO4 in reacting with CuCl2 will it produce?:

CuCl2 + NaHSO4 --> CuSO4 + NaCl + HCl

I think "NoIdea' has already raised this in another thread and may join me in this thread being very frugal. I would love expand on it:

http://goldrefiningforum.com/~goldrefi/phpBB3/viewtopic.php?f=48&t=16508

It seems like other users not interested in this as waste or price of replacement acids are no issue.

Will HCl re-engage into producing CuCl2 when CuCl is still present?

I have cleaned my foils in "used HCl wash" and it has cleared all CuCl (grey mud) with air bubbler, but have produced clean copper as sediment. I believe there more reactions are going due to Fe and FeSO4 present in the solution. As we discussed before resulting in clean copper as sediment.

Using clean HCl is troublesome due to HCl gas and cost of fresh acid. Interestingly, used HCl wash with bit of SMB (1-2gr per 3-4L) does not gas as strong as fresh HCl .

 

[g_axelsson]

I only get very small amounts of HCl used in washing gold. If there is a risk of having suspended gold in it I put it in the stock pot. If the gold have dropped in a good way I use the old HCl wash in some AP I have running or another process that I know that any gold from the wash would be collected.
The small amounts of SMB or byproducts from the drop will not affect those processes so much that it matter, most of it went away to the stock pot with the original solution.

Göran

 

[butcher]

I guess I am just having a hard time understanding what the Question or what the goal is.

CuCl2 + NaHSO4 --> CuSO4 + NaCl + HCl

Yes all three products are soluble in solution, so basically there is no reaction, but you can drive off HCl as a gas by distilling it off, and crystallize the blue copper sulfate, and separate it from the salt water.

Will HCl re-engage into producing CuCl2 when CuCl is still present?

Yes we can add HCl acid to copper I chloride to form copper II Chloride.
2CuCl + 2 HCl --> 2CuCl2 + H2
But here we are only talking of these in the reaction. The reactions would be different if there were other things in the mix, like iron, sulfates or peanutbutter.

I wouldn't clean my foils in a used solution.
Although I sometimes will use a used solution to recover metals from scrap.
Gold is expensive, HCl is cheap, washing the gold foils in fresh HCl will not use that much HCl and the wash can be reused somewhere else if you wish.

I am still confused and having a hard time understanding what it is you are trying to do.

Göran is using his used solution to recover metals from a dirty scrap, He will also know how to deal with what he gets from this process.

 

[solar_plasma]

I see an additional problem, if any unexpelled SO₂ should find its way into a dirty AP, which contains tin(II), since SnCl₂ is known to reduce SO₂ into H₂S, which again will precipitate sulfides (actually I believe the excess-SO₂-caused false positive stannous test is a result of brown tin sulfide - does anybody know, if this is right?).

So, I would at least expel any SO₂ by heating, IF there should be any left.

 

[bswartzwelder]

I must be reading something wrong. If you have cleaned your foils in a "used HCl wash" and have produced clean copper as a sediment, isn't that being kind of counterproductive? I thought the idea of using HCl in AP is to dissolve the copper and leave the foils behind. Using HCl in an AR solution we want to dissolve the gold so we can precipitate the gold and leave the other metals behind in the solution. I have to be missing something here.

While I do like the idea of getting the most bang for my buck, I would think reusing chemicals could cause more problems than it is worth. Once you have used your acid, without in depth chemical assays (like a gas chromatograph), you really don't know what you might have introduced as a contaminant. I don't know where you are, but here in the good ole USA, you can get HCl for about $7 a gallon. Not much more than a gallon of gas or milk. According to Hoke, you can have faster reactions by using fresh acids, however, you will end up using more acid in the long run since the acid will not be used up completely. To get the most out of the acid you are using, she basically says to sit back, knock down a favorite beverage and let the acid have the time it takes to complete its job.

 

[solar_plasma]

If the HCl has only been used for the final washes, it is still almost pure compared to an AP pot. Cheap HCl isn't pure anyway. But I would not like to have sulfates and sulfides in my AP.

 

[Alentia]

"Dirty" HCl from Au wash with bit of SO2 has in fact converted any remaining CuCl mud into Cu. It is probably is not a good idea to re-utilize it in such manner.

I pay $30 for 5 gallon drums of HCl and use about 2 of them per month for all my needs. Besides, think about all that NaOH I have to use to neutralize it and time it takes in waste treatment. In addition, anytime I have to use fresh HCl with wide month container (read bucket) HCl gasses are spreading around the shop even with ventilation working at full capacity. I have 100s of pounds of materials to process and reducing waste by a single bit greatly saves time and money.

Put it this way: 1L HCl will normally generate 2L of HCl +H2O. It will take about 2% of my waste treatment drum (minimum half full) occupied for at least a week. It will take about 300gr+ of NaOH to neutralize the acid. 10 such washes/treatments will take 10L of HCL and 20% of my waste treatment drum and 3Kg of NaOH.

 

[solar_plasma]

If the HCl is from final washed, I would just not add smb to it. Then I still use it for dirty dissolving of basemetals, more or less dirty AP or directly for my stockpot, if I like to. Any possibly dissolved gold would be recovered, but I don't think there will be any (a test will be wise).

 

[Alentia]

Good thinking Solar!

Normally there is 0.001-0.01 of a gram per 4L jag. But it may not be worth it to recover it (add SMB) and re-use in AP or foils wash vs time spent for additional acid waste treatment.

And there is a still good chance at least half of it will be recovered when reduced in AP solution.

 

[moose7802]

I personally pour the hcl used in my washes right to my AP jug unless the hcl is really dirty then it goes to stock pot. I figure if I dump any Au off of the washes I will get it when I filter my AP. I also boil my Au in water first to remove any SMB that might be on it then 3×hcl, 3×water then melt.

Tyler