Aqua Regia method with 12-13% Ag _inquart_

[giahylxag]

Here are a couple of posts/threads that may help.

Lou's Washing Procedure

Method for Ultra High Purity Gold

Dave

Wow really appreciate your help. The Library thread is not so big and many knowlegde (beside Hook's book) are scattered around, some threads are acient.

 

[4metals]

It seems getting gold to 99+% even to 99.95+% consistently at a home refining is not hard at all. But it seem really hard to get 99.99% Why? Is this the limitation of AR and one can only get 99.99% by electrolysis?

Getting gold to .9999 is a challenge, but actually getting anything to .9999 pure, whether it's gold or borax or peanut butter is difficult. The reason with gold is some of the metals like to stay together. With gold the major impurities of 9999 metal are copper and silver when run on an ICP. That is if the feedstock is karat gold, as your feed is. Truth be told all of the refining machines out there that claim .9999 gold with single pass aqua regia really aren't telling you the whole truth. But there are some tricks you can add to your standard processing that will get you very close to if not past 9999. But as others have noted, it comes at a cost.

There are some tricks mentioned here like cold acid, which is very effective for removing Silver from the gold. I believe if you know why that is true, it will make more sense to you and you will incorporate it into your process.

First off, silver chloride is soluble in water, very slightly, but still some is soluble. If you have ever filtered a hot or warm solution of aqua regia that dissolved an alloy which had silver in it, and after you filtered it, you let it stay overnight, you will likely see a fine white precipitate sitting on the bottom of the flask. You know you did the filtration properly and used a good grade of filter paper yet that layer of white Silver Chloride is there. It is there because when you filtered it some of that Silver Chloride was dissolved in the acid and when it cooled down it came out of solution. So no matter how well it was filtered, if it was dissolved in the acid it will pass the filter paper.

It turns out that, according to the handbook of chemistry and physics, the solubility of Silver Chloride in hot water is 0.0021 g/100 cc. The solubility of Silver Chloride in cold water is 0.000089 g100 cc. This tells us that the colder the acid is before you filter it, the more of the Silver Chloride is insoluble and will be caught on your filter paper.

For this very reason I tell mid sized refiners I set up to purchase an ice maker. Before they filter the acid after a digestion, it is best to add ice to drop the temperature as cool as you can get it. This step will assure you that any silver carried over from the dissolution in aqua regia will have as much Silver filtered out as possible and it will be reflected in the purity of your gold at the end.

In this particular application the OP has high Silver in the alloy. The debate has been whether to try direct aqua regia and deal with the chlorides and the gold contained, or inquart in nitric first to remove the base metals and silver. If the scrap is inquarted and only the insoluble gold is put up in aqua regia, this will be very similar to doing a second aqua regia refine to raise the purity. You should end up with a beautiful red solution which has a lot of gold and very little else in terms of metal impurities. Usually these are copper and Silver.

The gold from the parting also has the benefit of having been etched out of an alloy and having a lot of surface area and it is a powder like material. This will dissolve quickly in aqua regia and slow nitric additions will result in almost no free nitric when you are done.

The OP said he is able to get SO₂ gas for precipitation. This is probably the best way to drop the gold in as pure a form as possible. Careful attention to the gassing process will assure no copper (as copper 1 chloride) is dropped with the gold at the end of the reaction. It is visible as a white crystal mixed with the gold sponge if there is any dropped and it can be cleaned up by dissolving it away with a strong solution of Hydrochloric Acid which will dissolve out the copper chloride. (this rinse acid can be reused for future aqua regia solutions)

These steps, after inquartation and parting, will get you a long way towards your goal of 9999 gold. And they are easy to implement in your process. There are other steps you can incorporate to get your purity where you need it but these steps should be first.

I assume (and assuming is often dangerous) that your potential buyers will be using an XRF to determine the purity of what they buy from you. Gold processed the way described above usually reads 9999 on these machines. The only true way to determine purity is by ICP which is a long process (compared to the XRF) and it is costly.

In the days before XRF kilo bars were cooled without a torch, when they cooled the surface would cleave and you could see gold crystal structure in the bar. Back then any bars cooled with a torch were not purchased by knowledgeable buyers, only bars with the bars showing the surface cleavage were bought because bars with impurities did not show the crystalline structure. This is still a good way for you to test your own purity for your own knowledge.
Other posts about this cleaving of the bar were posted by both Harold_V and GSP but they called the cleavage a pipe. so if you search for this concept use the word pipe.

 

[4metals]

Are there any PGM's in the normal lots sent out to the refiner? Not enough to be paid on by the refiner but enough to pick up on XRF of your bars shipped out? We should know if PGM's are there if you want to recover them eventually.

 

[giahylxag]

Are there any PGM's in the normal lots sent out to the refiner? Not enough to be paid on by the refiner but enough to pick up on XRF of your bars shipped out? We should know if PGM's are there if you want to recover them eventually.

Most of the stock I have, I can say 90% of the stock don't have any PGM's.
PGM's mostly come from white gold and there are some particular lots (white gold 18k 14k 10k) that has PGM's but they come separated.
Yes I want to recover them eventually but i think it is out of scope right now, I will bring them later.

 

[4metals]

PGM's mostly come from white gold and there are some particular lots (white gold 18k 14k 10k) that has PGM's but they come separated.
Yes I want to recover them eventually but i think it is out of scope right now, I will bring them later.

If you were to process bars with just a few percent of the alloy as Platinum or Palladium, inquarting will take the PGM's with it into the silver and parting that silver will put those metals into the silver nitrate. Generally, as old assaying books say, if you add 10 times the weight of Platinum as Silver (which you will do when you inquart) the Platinum will be parted and dissolve in nitric. Palladium will always dissolve in nitric anyway.

But the good news is when you cement the values (mostly Silver) from the parting liquids with copper. all PGM's in with the Silver will come down as metals and when cast into anodes for your Silver cell, you will get that Platinum in the anode slimes and some of the Palladium will be in the slimes, and some in the electrolyte, recoverable with Dimethylglyoxime.

If you were to do the first digestion in aqua regia, the PGM's would be in the acid after the gold drops and for 6 kilo's that will be about 25 liters plus rinse waters. This will have to be cemented on Copper to collect the PGM's. So in your debate about aqua regia first or inquartation first, choose cementing small amounts of PGM's out of 25 plus liters or concentrating them in anode slimes in your Silver cell.

 

[giahylxag]

Getting gold to .9999 is a challenge, but actually getting anything to .9999 pure, whether it's gold or borax or peanut butter is difficult.

Given the market I can get 0.25 to 0.5% more if i can get from 9995 to 9999. 9999s are sourced for coins and bars, 9995 and below are for jewelry making.
Beside I just want to push further. I have read many infomation about AR process, (on the internet and on this forum, and from many source can't remember) most of them claim 9995 is the standard, but I don't think it is the limit.

 

[4metals]

Given the market I can get 0.25 to 0.5% more if i can get from 9995 to 9999.

So to be clear if you produce 99.95% gold they discount it by .25 to .5% and if you produce 99.99% gold they pay spot? The difference can be as much as $10.50 per ounce!

 

[giahylxag]

So to be clear if you produce 99.95% gold they discount it by .25 to .5% and if you produce 99.99% gold they pay spot? The difference can be as much as $10.50 per ounce!

The difference between 9999 and 9995 is $5~7/ounce and yes it can be high to $10 as demand for coins and bars rise.
Physical market right now here is around $40/ounce higher than spot

 

[4metals]

Well then, that represents some serious money on a weekly basis. I would start out with the precautions defined above and see where your purity ends up.

From there there are options such as chelation which will hold the impurities in solution as the gold drops, resulting in high purity. And then there is cleaning up the gold sponge by tumbling in hot acids depending on the impurities you are eliminating.

So either way you can cost effectively get the gold to 9999 and we can discuss the pro’s and cons of these methods when you get closer to production. Be warned, as your pro chemistry associate may love, these methods delve deeper into chemistry!

 

[nickvc]

Well then, that represents some serious money on a weekly basis. I would start out with the precautions defined above and see where your purity ends up.

From there there are options such as chelation which will hold the impurities in solution as the gold drops, resulting in high purity. And then there is cleaning up the gold sponge by tumbling in hot acids depending on the impurities you are eliminating.

So either way you can cost effectively get the gold to 9999 and we can discuss the pro’s and cons of these methods when you get closer to production. Be warned, as your pro chemistry associate may love, these methods delve deeper into chemistry!

The pros and cons of producing higher quality product have to be considered carefully and can depend on your cash in hand as refining to a higher quality takes longer and uses more chemicals against the increased value placed on it.
If you can afford to have stock bullion on hand you can sell as you buy so you are not risking price movements and if you have enough you can produce the higher quality as the time element will not be so important in your costs.

 

[giahylxag]

The pros and cons of producing higher quality product have to be considered carefully and can depend on your cash in hand as refining to a higher quality takes longer and uses more chemicals against the increased value placed on it.
If you can afford to have stock bullion on hand you can sell as you buy so you are not risking price movements and if you have enough you can produce the higher quality as the time element will not be so important in your costs.

Most of the time I move the gold as soon as possible. I have stock bullions. But for me I "feel" it risky when I sell my stock bullion as I refine the scrap later.
As a middleman, I can move the gold without risk and take a little profit. But as a refiner I can make much more but sure there will be some risk involved.
Really appreciate this forum when there are people can help me when some "wtf" moments happen.

 

[ModernAlchemy]

I had some time today so I prepared this.

The cost of refining a lot of karat gold with silver at 12%. The average size of these lots is 4.5 kg

The first part of the process involves separating the copper and nitric soluble metals, along with the silver from the inquarted alloy. Fortunately, this can be easily done in a stainless-steel vessel which are available larger and at a price much more reasonable than glass. The stainless steel can be heated to heat the acid on either an electric or gas hot plate. And a stainless pot can take a lot more dings than glass.

Melt all of the bars together and add the material you are inquarting with. Eventually this will all be silver. Stir the melt well and pour the metal in a thin stream into a tank of water with a piece of wood sticking out of the water at an angle so you can pour the stream of metal to the point where the wood meets the water. This will help you have smaller granules of alloy which react quicker. These shotted granules are what we will part in the nitric acid.

View attachment 60773

Because of the high silver in the assay this alloy will require inquartation with silver or copper. Given the size of the lot, the extra nitric required for this process will result in an excessive use of nitric acid if it is inquarted with copper. As this process will be to attain the highest purity of gold when processing the gold remaining after inquartation, to assist in this the inquarted alloy needs to be parted twice.

The first parting is done with 1-part nitric acid and 2 parts distilled water. When the reaction is complete the evolution of the red nitric fumes will stop. Based on the quantity of nitric acid added the dissolving out of the should be complete and the metal remaining undissolved should be primarily gold. I always stir the reaction about halfway through with a glass rod to be sure it is well mixed. When the reaction appears to be complete, give it another stir to be sure.

Decant the solution and rinse the gold mud sitting on the bottom with distilled water, swirl the water and gold mud and allow the gold to settle. The gold is heavy, and it behaves well by staying in the pot if you are careful. Save the liquors you have just removed from the first parting process. These will later be filtered, and the silver will be recovered.

Now the second parting solution can be added. This parting liquor is 1 part nitric to 1 part distilled water. Add ¼ of the volume of the acid you added the first time as the volume of the metal will be much less and the purpose of this leach is to clean up any remaining silver of base metals, so your gold mud approaches a purity of .995. Heat this solution and let the gold mud sit in the hot parting acid for about 45 minutes. This acid will have leached out a very small amount of silver and copper and once you learn the process you can filter this acid and re-use it as a first parting acid to save acid costs. The rinse water you add should bring the nitric ratio close enough to the 2:1 of the first acid and save on nitric. Only start doing this once you are comfortable doing the process using new acids each tie until you are comfortable. After the swirling rinse like the first time, decant the liquid for recycling for a second use. The gold mud, which looks just like used coffee grinds at this point should be rinsed through a filter where it is rinsed well with hot distilled water.

I have gone through the calculation to determine the quantities of acid to do the steps above here for you to see.

Starting weight 4.5 kilograms
Gold assay 0.625. gold content 144.625 oz
Silver assay 12%. Silver content 17.361 oz
Remaining metals (Cu, Zn possibly nickel). 100% - (.625 + .12) = .255 which equals 36.892 oz other metals (we assume copper)

Good numbers to remember.

129 ml of 68% nitric acid dissolves 1 ounce of copper
38 ml of 68% nitric acid dissolves 1 ounce of silver
Easy to see why silver is preferred to inquart.

The alloy we start with has 90.422 ounces of gold in it. To make that one quarter of the total inquarted alloy multiply it by 3 and that’s how much we need to dilute the gold with. But there is already copper and silver in the alloy, so we need some subtraction.

Total metal needed to have gold at 25% 271.266 ounces.

But we already have 17.361 ounces of Ag in the alloy and 36.893 ounces of what we assume is copper (and other nitric soluble metals)

So 271.266 ounces minus 17.361 ounces minus 36.893 ounces means we need to add 217.013 ounces of silver

Now to calculate the amount of nitric acid to part this.

We added 217.013 ounces and there were 17.361 ounces of silver in the alloy so we need to add enough nitric to dissolve 234.374 ounces

From the good number to remember above that means we will need
234.374 x 38 = 8906.193 ml of 68% nitric acid.
And…..
36.893 x 129 = 4759.084 ml of nitric to dissolve the copper.

To do the first parting of this lot, Inquarted with silver, requires 13.665 liters of nitric acid. This quantity of nitric acid is diluted with 2 parts of distilled water so your total acid for this parting will be 40.995 liters.

Using the cost numbers you gave for nitric this first parting will cost you (in acid)

EUR 8.86 for the cheaper acid
EUR 40.996 for the better grade acid

The second parting we need the strength of the acid to be 1:1 with distilled water but we only need enough of the acid to cover the gold mud with acid by a little bit. I assume ¼ of the original parting acid quantity. The second parting should cost you in acid;

EUR 2.215 for the cheaper acid
EUR 6.645 for the better grade acid

Two things to consider. The cheaper nitric is cheaper for a reason. Is it clear or discolored. And does it have chlorides in it?

If it has chlorides and can dissolve gold it is a problem, and you cannot use it. If it is just discolored, it is probably from some dissolved metals and it should be OK with rinsing. If it is diluted, you will need more acid.

The method to convert the silver nitrate are covered elsewhere on the forum. As are silver cells if you want to set up a small cell. A silver cell will be more important if you want to go with straight aqua regia because you will hang up gold in the chlorides. And you will want to recover it quickly.

The gold mud is dissolved in a mixture or 4 parts Hydrochloric Acid to 1 part of Nitric acid. Add the gold mud to an appropriately sized glass vessel and add the HCl. For the lot size listed use the rule of thumb that says for every 7.5 ounces of gold to dissolve use 1 liter of aqua regia. Do not add all of the nitric at first, add about 1/3 of the nitric to start. (This example with 90.422 ounces of gold will require 12 liters of aqua regia or 2.41 liters of nitric and 9.6 liters of hydrochloric.). The gold mud will dissolve quickly so go slow with small additions of nitric acid as the reaction slows. This way you will not add too much and you won’t need to neutralize any excess free nitric acid.

The gold solution needs to be filtered and the gold dropped with either sodium metabisulfite or preferably sulfur dioxide gas. Add ice to the solution before you filter it and this will drop out any silver chloride so the filter catches it. The ice will make your gold purer. If your alloys have any lead, add a few CC of sulfuric acid to the digestion to keep it out of your precipitate.



As far as labor is concerned for the lots you are discussing I would leave an hour for melting and inquarting and the silver parting process should be done in 3 hours. The gold digestion, filtration and dropping should be done in under 4 hours. Due largely to the small particle size of the gold mud after parting process.

Since you will not be dealing with gold hanging up in the parting process your losses will be minimal and with the exception of any values in the exhaust (minimal) you will not lose more than 0.0625% in this process. You will find the assay between you and your refiner may vary by as much as .5% unless you run proof assays as most refiners use a standard proof factor of .995 when it should be closer to .998

As with all refining, technique and experience matters, any losses experienced when starting up will get smaller with experience.

You can plug in the labor cost and Hydrochloric Acid cost to get pretty close to your total cost. You also need metabisulfite or Sulfur Dioxide but that isn't very costly.

What a wonderful post Thank you so much. I have 40 grams of scrap 14K gold and would like to attempt this process. Still digesting all the procedures listed here. I was thinking of creating a spreadsheet to calculate the amounts of everything needed for processing. What are your thoughts on this.. Maybe you have something created already? I see your calculations are with 4.5 kilograms. a few questions.

1. Is it ok to use my little furnace for the inquarting? I see many people just using a torch to melt and inquart. I think the furnace would be better, Maybe not as cost-effective, But Im learning so Im not concerned about this.

2. I also saw sreetips use copper for inquarting instead of silver and it came out very nice. I did read in your post that there is already copper in the 14k gold material so that's why you use silver if I am correct. Just curious about why or if it's not a good decision to just use copper. Should silver always be the inquarting material?


3. I am confused about using the stainless steel vessel with the nitric once you have melted and inquarted. I was going to use my 2000ml beaker to do the separation with nitric. and then the rest of the processes with 2000ml beakers as a vessel.

I am sure you have probably covered all of this somewhere. I appreciate the post and look forward to trying this, but not until I am comfortable with all the processes and amounts of material for each process. I think 40 grams of 14K is a good amount for a first try... Any guidance on the amounts of materials for the processes would be greatly appreciated. Thanks in advance.

 

[Yggdrasil]

What a wonderful post Thank you so much. I have 40 grams of scrap 14K gold and would like to attempt this process. Still digesting all the procedures listed here. I was thinking of creating a spreadsheet to calculate the amounts of everything needed for processing. What are your thoughts on this.. Maybe you have something created already? I see your calculations are with 4.5 kilograms. a few questions.

1. Is it ok to use my little furnace for the inquarting? I see many people just using a torch to melt and inquart. I think the furnace would be better, Maybe not as cost-effective, But Im learning so Im not concerned about this.

2. I also saw sreetips use copper for inquarting instead of silver and it came out very nice. I did read in your post that there is already copper in the 14k gold material so that's why you use silver if I am correct. Just curious about why or if it's not a good decision to just use copper. Should silver always be the inquarting material?


3. I am confused about using the stainless steel vessel with the nitric once you have melted and inquarted. I was going to use my 2000ml beaker to do the separation with nitric. and then the rest of the processes with 2000ml beakers as a vessel.

I am sure you have probably covered all of this somewhere. I appreciate the post and look forward to trying this, but not until I am comfortable with all the processes and amounts of material for each process. I think 40 grams of 14K is a good amount for a first try... Any guidance on the amounts of materials for the processes would be greatly appreciated. Thanks in advance.

You need 4 times as much Nitric for the Copper than the Silver, that is why one use Silver to inquart if you can.
If there is little Silver one might go directly to AR.
Most Stainless steels is passified in Nitric and can be used for Nitric digestions.

40g of 14 k is 40 x 14/24 which is 40x 0.583 equals 23.3 grams pure Au.
You want the Gold to be 25% so 23.3 times 4 is 93 grams total.
you need to add 70 grams of Silver or Copper.
And then part it in Nitric.

 

[FrugalRefiner]

You need 4 times as much Nitric for the Copper than the Silver, that is why one use Silver to inquart if you can.

A second reason for using silver to inquart is that if there are any PGMs present in the gold, especially platinum, they will "follow" the silver into solution during the nitric leach, leaving your gold a little cleaner for the AR digest.

40g of 14 k is 40 x 14/24 which is 40x 0.583 equals 23.3 grams pure Au.
You want the Gold to be 25% so 23.3 times 4 is 93 grams total.
you need to add 70 grams of Silver or Copper.
And then part it in Nitric.

You do want a total weight of around 93 grams, but there are already about 17 grams of base metals present in the 14K alloy. Take the 93 gram final weight minus the 40 gram starting weight and you need to add about 53 grams of silver.

Dave

 

[Yggdrasil]

A second reason for using silver to inquart is that if there are any PGMs present in the gold, especially platinum, they will "follow" the silver into solution during the nitric leach, leaving your gold a little cleaner for the AR digest.


You do want a total weight of around 93 grams, but there are already about 17 grams of base metals present in the 14K alloy. Take the 93 gram final weight minus the 40 gram starting weight and you need to add about 53 grams of silver.

Dave

Oops forgot what was there already

 

[4metals]

What a wonderful post Thank you so much. I have 40 grams of scrap 14K gold and would like to attempt this process

Some of the details in this thread are detailed to the OP’s rather unique situation. However inquartation with Silver is a viable method for smaller (even 1.25 ounce lots). Keeps all of your Silver together.
Kadriver or sreetips on you tube, has an excellent video on doing this entire process and if I wasn’t on my phone posting this I would find the link and post it. We rarely suggest you tube video’s but sreetips video’s are excellent.

 

[ModernAlchemy]

Some of the details in this thread are detailed to the OP’s rather unique situation. However inquartation with Silver is a viable method for smaller (even 1.25 ounce lots). Keeps all of your Silver together.
Kadriver or sreetips on you tube, has an excellent video on doing this entire process and if I wasn’t on my phone posting this I would find the link and post it. We rarely suggest you tube video’s but sreetips video’s are excellent.

Thank you 4metals, I am in no rush, so if you can post those videos in the thread when you can, that would be greatly appreciated.

 

[Wiehan]

I would just like to commend everyone who contributed to this thread - so much useful information and learning points were covered. Truly insightful!
Happy 2024 !

 

[ModernAlchemy]

A second reason for using silver to inquart is that if there are any PGMs present in the gold, especially platinum, they will "follow" the silver into solution during the nitric leach, leaving your gold a little cleaner for the AR digest.


You do want a total weight of around 93 grams, but there are already about 17 grams of base metals present in the 14K alloy. Take the 93 gram final weight minus the 40 gram starting weight and you need to add about 53 grams of silver.

Dave

thank you!!

 

[4metals]

Thank you 4metals, I am in no rush, so if you can post those videos in the thread when you can, that would be greatly appreciated.

Sreetips video with Silver inquarting