Recovering precious metals from polishing sweeps

[Harold_V]

How do i go about cementing the silver?

If the solution is relatively free of solids, all you need to do is put in pieces of copper. Don't use wire unless you're sure you will consume all of it. Otherwise as it gets smaller, it breaks in to small lengths, which are virtually impossible to remove because they're hard to find. That sort of defeats the purpose of cementing the silver, which will yield quite good quality, assuming you've done things right.

do I add more water? if so how much more?

If you've added rinse water to the original solution, no, don't add more water. It should already be quite dilute. I used to cement from relatively concentrated solutions. It can be troublesome in that on occasion the silver will plate out as a solid surface, adhering to the copper doggedly. It's best when it cements as fine particles, which yield the general appearance of (Portland) cement, which is the source of the name we use, at least as I understand it.

What is the ideal ph of the solution for cementing?

Don't know that I can answer that. I was never too concerned, and on occasion dissolved some copper because there was free nitric present. It does no harm aside from consuming copper. Eventually it achieves equilibrium where no copper is lost to dissolution from acid, just from cementation.

Can I use zinc dust instead of copper to cement? Thanks.

Yes, you can---but that's a mistake. Zinc will cement copper, too. At first the copper may be consumed, cementing silver, but eventually you'll end up with copper and silver mixed, which defeats the purpose. You'd be far better served to recover the silver as silver chloride than to use zinc----with the sole exception being if you intend to re-use the recovered silver for inquartation, it doesn't make too much difference. The one negative (if you use the recovered material for inquartation) is the reason you use silver instead of copper, however, in that copper requires almost three times as much nitric to digest as an equal amount of silver.

Harold

 

[star122552]

We have refining jewelers polishing dust at home, which is more than gold just sending out big companies but we do try once, waiting for us to try again many times to come to an accurate conclusion.Next time we have a lot of polishing powders we will mix and keep 1 LBS to do at home and the rest we will send to the big company will have accurate results to compare.Thank you guy help me to refine gold.
https://www.youtube.com/watch?v=zpIw3zYJblI
https://www.youtube.com/watch?v=WUMBgzU27Qg

 

[erinda]

Hi Harold,
I have read all your posts about recovering gold from polishing sweeps and I tried to follow all the steps that you have indicated. At the end of the process, the result was not very satisfied and I think something gets wrong with my process but I don't know what!! Can you please help me? I'm describing all the procedure that I did step by step.

- First, I burned all polishing sweeps in an oven (that has an incorporate cyclone for reducing the loss of material)
- After burning I screened the material until it became fine dust
- I weighted 250g dust in a beaker and covered it with HCl and water 50/50 (HCl 33-35%)
- I boiled it for approximate 2 hours and the color of the solution became green, then I cooled it with some water and I filtered it with vacuum pump. Here I start with the first problem.
- The filtration was not very easy and very fine particles of the material past through the filter paper (I can say that I use the same filter paper when I refine old jewelers and I have never had problems)
- I re-filtrated the solution with the same kind of filter paper without vacuum and I collected all the material that has pasted through the filter paper in the first filtration.
-I dissolved all the material in aqua regia. I intended to get 25gr of gold and used a combination of 1g value=1ml of HNO3 and 4ml HCl, so in total, we used 100ml of HCl and 25ml of HNO3.
- After the dissolving process in aqua regia (the solution was boiling without brown fumes and after the addition of some ml of HCl nothing changed, I added 2ml of HNO3 and the brown fumes come again but lighter and didn't last a lot) when all the HNO3 was evaporated I put off the heater and made an addition of cool water.
- The next day I filtrated the solution and I get a considerate quantity of AgCl (white sludge) and the color of the solution was yellow/green.
- After I rinsed several time the AgCl (till the pH of rinse water was approximate 6.8-7). I putted the solution and the rinse water in a beaker and tried to precipitate gold using Sodium metabisulfite.
- I usually add some metabisulfite and stir it well and then check the voltage of the solution. I do this procedure till the voltage is 450mV or min 400mV, this value indicates the reduction of all AuCl in Au.
- I did exactly this procedure of precipitation even this time and result is that I got only 2.5g gold for 100g of burned dust... fare long of what I expect!!
- For the moment I am at this point and of course, I'm going to wash the precipitated gold with HCl and with ammonium hydroxide as you describe.

Can you please tell me what is going wrong in all the procedure that I explained above?
I will appreciate if you can give me any advice!

Thank you,
Erinda

 

[FrugalRefiner]

Erinda, I'm afraid Harold hasn't visited the forum for nearly four years.

Dave

 

[butcher]

The solution you precipitated gold from, I would suspect still held some of your gold, the de-NOxing of the solution is where I would suspect you may be having a problem.

If you use copper in the stockpot you may find some of your gold there.

 

[erinda]

I'm sorry that Harold hasn't visited the forum for so long
I have found very interesting advices and suggestions by him.

Concerning my problem: 'the de-NOxing of the solution'  you mean that I should had added more Sodium metabisulfite  for gold precipitation? 
I have never used copper in the stockpot, can you please tell me how to do this?

Thank you 

 

[Slochteren]

- After I rinsed several time the AgCl (till the pH of rinse water was approximate 6.8-7). I putted the solution and the rinse water in a beaker and tried to precipitate gold using Sodium metabisulfite.

I think your ph is to high, isn't a acidic solution neccesary for SMB to work?

Verstuurd vanaf mijn FIG-LX1 met Tapatalk

 

[erinda]

Yes, you are right the solution need to be acidic.
The water of last rinse had the pH =6.8, and of course i mixed it with the solution, in order to took all the values, and the pH in the moment of gold precipitation was nearly pH=1 

 

[butcher]

Free nitric acid or nitrate salts in solution after dissolving the gold will keep the gold dissolved in solution, and you cannot precipitate the gold, or give the gold back electrons with a reducing agent (like SMB) if nitric keeps taking the electrons away from the gold...

Copper or other base metals involved in the solution can form nitrate salts with free nitric in solution, these base metal salts, the nitrate salts, if acidified later, will reform nitric acid in solution which can then redissolve your gold, or work to keep your gold dissolved in solution...

Sodium Metabisulfite will not effectively de-NOx a solution, it can like urea, if added in excess, can help to remove NO2 gases from solution, but it will not destroy or consume free nitric acid or nitrate salts.

Ferrous sulfate, on the other hand unlike SMB or urea will remove excess nitric and de-NOx a gold chloride solution...

When we dissolve gold we provide a chloride environment in the acidic HCl solution, the gold will not dissolve under normal lab conditions in this environment.

We need a very powerful oxidizer to get the gold electrons to budge from their atoms, to join the chlorides in making a salt of gold chloride in solution.

Most problems people have using aqua regia is they use way too much acid, making it more difficult to get their gold back, the excess acid, which makes concentration or evaporation difficult because of the volumes of extra acids and water involved. They overuse the nitric which is needed to oxidize the gold and then have a hard time eliminating the nitric or its salts from solution in order to test for the gold or to reduce it from solution.

Once the gold is dissolved any excess oxidizer will work hard to keep the gold dissolved, to keep you from precipitating any or all of the gold.
The stannous test cannot reduce the gold, or keep the gold reduced in the test, so basically the stannous chloride test for gold in solution does not work with free nitric in solution...
On the other hand, Copperas, or ferrous sulfate will consume NO2 gas and destroy excess HNO3 in solution, so it will work to test for gold in a solution where SMB cannot (because of the free nitric acid oxidizer in solution).

In order to precipitate the gold, we have to remove the oxidizer nitric acid and its salts (as if the nitrate salts are re-acidified in a later process wash... they will again reform nitric acid and redissolve gold back into solution.

Sulfamic acid will work to remove free nitric acid and de-NOx the solution.

De-NOXing using the method taught by C.M. Hoke:
The heat of the evaporation process will help to distill off free nitric acid, and adding a little HCl (or any acid) will help to reform nitric acid in solution from the nitrate salts involved in the solution.
Continuing this evaporation process this nitric is distilled off upon reconcentrating, another addition of HCl (brown gas evolving indicates nitric still decomposing) evaporation and repeating for a third evaporation HCl cycle is normally enough to eliminate the free nitric and successfully deNOx the solution.

Harold's trick to use gold in solution and heating to consume free nitric acid, this can be used before all of your gold is dissolved (much like adding nitic in drops with heat and letting the gold use up all of the nitric before adding more nitric to consume more gold), or a gold button used for the purpose can be added in order to consume free nitric after all of the original gold is dissolved...

I like to use a combination of the three methods above, using minimum nitric to dissolve the gold, to begin with, using heat and gold to consume free nitric, excess HCl to keep nitrates converted to nitric (or NO2, nitrosyl chloride...
Stop adding drops of nitric before all of the gold is dissolved, using heat and evaporation to use up as much of the nitric before another drop is added, the heat and concentration will normally consume much more gold. (eliminating my need to add a gold button to de-NOx the solution with heating...).

When necessary, I will use sulfamic acid to de-NOx the solution when I overshoot how much nitric I needed.
Copperas is my choice of reducing agents to use to reduce the gold where free nitric acid may be a concern, as it will work to de-NOx the solution where SMB cannot.

Using the stockpot, a busbar of copper will consume free nitric, and displace the gold (or metals less reactive than copper from your waste solutions (your values from solution), this prepares the solution for the waste treatment process, after removing the values from the solution...


The solution then goes to the waste bucket where we use iron metal to cement copper and many of the other toxic metals, after decanting, we adjust the pH of the solution to around 10-11 (adding caustic or alkali) where many of the metals form oxides and hydroxides and will precipitate from solution, again after decanting the sludge, the pH is again adjusted (by adding small additions of acid) to reach a neutral pH of around pH7 leaving a saltwater solution which can be disposed of, or as I like to do, is evaporate the salt waters to dryness and roast them before disposal.

 

[nickton]

Thanks for the great explanation butcher. I was wondering if using something higher than iron to precipitate remaining metals would work. Since sodium is up there on the reactivity chart, could adding salt cement metals? I am not sure if you need to use a solid bar or not. Also lithium from spent batteries might work, or not?

 

[butcher]

Once you get to the metal that reactive they explode in water.

Sodium chloride will precipitate silver from a solution but this is not a displacement chemical reaction.

once we get much above the metals in the series more reactive than aluminum we would not have much of a reason to use them to displace metals lower or less reactive in the series.

The more metals are used more in reactions dealing with electrolysis to move electrons in electrochemical reactions...

 

[nickton]

okay.

That seems to clarify somewhat how my waste treatment experiments are going. I have about six five gallon buckets of pretty clear greenish solution from which I removed cemented black and orange slimes by decanting, after having soaked much of it for a few years with iron and later copper (when I learned it was the better choice). I unravelled a few old lithium batteries and put them in two of the buckets, one of which also had aluminum in it. The other buckets had only aluminum, and in one I added just table salt.

So far it appears the aluminum works best and in those buckets the liquid looks almost clear, with no more green color--which I'm sure was copper. The lithium ones however do not look as clear. I tested with ph strips also to make sure the by now diluted liquids were acidic, which they were, giving pretty bright red colors all around. The salt appears to be not doing much if anything either. Another factor is that the lithium batteries contained copper, on which the black colored lithium was deposited (I presume), as well as some Al, and I guessed that rather than just cementing other metals, this copper was going into solution somehow, and thus keeping the water greenish, but I don't know for sure. I will check it again tomorrow to see if there is any change after I removed the copper strips today. Perhaps the lithium water is more black that green actually. I can't really tell yet.

Anyhow I am certainly leaning toward using aluminum for that final cementation before adding a base to de acidify and precipitate hydroxides (as you mentioned).

I still haven't decided how to process previously dropped solids, some of which I roasted down to a fine reddish powder that attracts to a magnet, and thus must contain ferrous materials. I first want to remove all metals from solution and dispose of cleared liquids.

Hoke said that refining wastes is often the most interesting of all tasks to perform, and thus well worth doing.
I'm beginning to see why now.

 

[erinda]

Thanks butcher for all the information shared above, it is very helpful!!

I putted a copper bar in the waste solution a left it for 20 hours but nothing changed..
Does it means that the waste solution has no value to reduce or do i have to heat the solution for a better result?

Is it possible that the HCl wash (boiling in dilute HCl) maybe has dissolved some of the gold?
I didn't add oxidizer in this process, only HCl 33-35% and deionized water 50/50.
I know that theoretical is not possible to dissolve the gold in HCl but i suppose just in case there is something in HCl as impurity that may cause this.

Thanks
Erinda

 

[butcher]

The copper will use up any oxidizer that may have been present, to make copper chloride, any values will deposit onto the copper (black sludge powders), as the copper gives up electrons to any of the less reactive metals in solution or the metals that are more noble than copper.

If you have not seen any reaction after having copper in the solution overnight, I would suspect the solution had no values to deposit. Testing the solution can also be used to verify that there are no values in the solution.

Edit to correct wording where I said more reactive when I should have said less reactive (I always seem to say or type things backward nowadays they would call me dyslexia, or something like that. Thankfully I have friends here that catch my mistakes so I can correct them.

 

[nickvc]

Erinda in your process you added far too little AR in my opinion, as you boil the solution, one of the few times it’s recommended, as it boils it will expel some of the nitric, trying to do this using just enough acids is false economy,I know I have done many many sweeps in the past and you have to use plenty of acids to dissolve the values, using either the approved sulfamic or urea is needed to denox your solution, if you do this right you should recover 90% of the values on the first pass, if on the second pass you recover more than 10% you didn’t do it right the first time.
With returns you should be looking at around 20 grams of gold per kilo of un burnt sweeps if you use 18 k predominantly and you can adjust that figure to match your actual carat usage.

 

[Deano]

Last year I watched Nick treating jewellers sweeps on an industrial scale as far as sweeps are available in this scale.

It was immediately obvious that he not only had great experience in this treatment, judging by the competence with which he carried out the processing,
but that he had sorted out the best treatment method and could do variations on this method depending on variations in the sweeps themselves.

If I was looking for real world advice on how to treat sweeps I would not go past Nick's advice.

Deano

 

[erinda]

Thank you Nickvc

I appreciate your advice, I'll try it the next time I'll refine gold polishing waste.
Usually when i refine gold karat, I use urea to eliminate the excess nitric acid and seems to have worked...
I'll inform you about the next refine!

Thanks

 

[nickton]

What do you do with hydroxides and oxides after decanting them? Actually I had to use filters rather than a simple decant on my first batch. It was a green/blue color by the way.