Aqua Regia method with 12-13% Ag _inquart_

[4metals]

I am buying a very small (3kw) induction furnance, (~150$), I don't have many experiences with induction furnance.

I love induction furnaces but don't get me wrong here, you may not need induction as badly as you will need capacity. I have attached a screenshot from a crucible size for a 3 kw induction furnace on amazon.
The small U shape on the left is the inner crucible which determines the capacity to melt. This thing is tiny, and is probably rated to hold 1 kg of 24 karat gold. Maybe more, I didn't do the math, but it is way too small for your needs if you want to eventually process 3 to 6 kg of alloy a week.

I remember back in the day when I learned about density. The trick question they asked was "What weighs more, a pound of feathers or a pound of lead?" The answer is they both weigh the same but the real point of the question is to show you the difference in density and what a big difference it makes. Well, that feather lead analogy applies to melting metals as well. Silver is roughly half the density of Gold. So that tiny crucible will only hold half a kilo of Silver.

Let me preface this answer by saying I am old and have done this stuff for many years. I refined large quantities for many years before induction became available so I know for a fact that what you want to do will work perfectly well with a gas furnace. Realize gas furnaces take longer to heat up and are louder when running. If you have customers bringing in melts you may appreciate a faster quieter melt. But even then, whole jewelry pieces take up space and that crucible for a 3 kw furnace doesn't have much to start with. You should be thinking about melt capacity and you will see gas melting will get you the capacity you will need for a lot less money. If you decide to inquart with Silver to refine you will need the capacity. I would suggest a gas furnace that takes a #10 crucible. That will handle inquartations and all of your melt needs. Another benefit of gas furnaces is you can put a smaller crucible in the furnace to do smaller melts. That isn't doable with induction.

 

[4metals]

Yes it is for waste treatment. The whole project will collapse if I can't treate waste and fume probably. I am living in the city center.

This sheds an entirely different light on the subject. The inquarting process will increase your nitric usage and require a larger fume scrubber. Don't get me wrong, I have built refining operations in New York City that were permitted and legal. The set up costs were considerable due to both size limitations (NYC refineries are limited in space because of availability and massive rent costs) and the cost of larger scrubbers. So what you want to do is possible. But maybe, considering tumbling and aqua regia can be done with a smaller fume scrubber and we will deal with the gold in chlorides with a silver cell.

There are many threads in the library about building both hoods and fume scrubbers.

 

[4metals]

SO2 is good, you could use SMB and HCl to generate it.
Actually the active component in Sulfite is the SO2.

This is true but you will also 1. generate more waste from the process of releasing the SO₂ and 2. it is usually available in pressurized cylinders which will be more practical for 100+ ounce fine gold precipitations.

 

[giahylxag]

I love induction furnaces but don't get me wrong here, you may not need induction as badly as you will need capacity. I have attached a screenshot from a crucible size for a 3 kw induction furnace on amazon. View attachment 60778
The small U shape on the left is the inner crucible which determines the capacity to melt. This thing is tiny, and is probably rated to hold 1 kg of 24 karat gold. Maybe more, I didn't do the math, but it is way too small for your needs if you want to eventually process 3 to 6 kg of alloy a week.

I remember back in the day when I learned about density. The trick question they asked was "What weighs more, a pound of feathers or a pound of lead?" The answer is they both weigh the same but the real point of the question is to show you the difference in density and what a big difference it makes. Well, that feather lead analogy applies to melting metals as well. Silver is roughly half the density of Gold. So that tiny crucible will only hold half a kilo of Silver.

Let me preface this answer by saying I am old and have done this stuff for many years. I refined large quantities for many years before induction became available so I know for a fact that what you want to do will work perfectly well with a gas furnace. Realize gas furnaces take longer to heat up and are louder when running. If you have customers bringing in melts you may appreciate a faster quieter melt. But even then, whole jewelry pieces take up space and that crucible for a 3 kw furnace doesn't have much to start with. You should be thinking about melt capacity and you will see gas melting will get you the capacity you will need for a lot less money. If you decide to inquart with Silver to refine you will need the capacity. I would suggest a gas furnace that takes a #10 crucible. That will handle inquartations and all of your melt needs. Another benefit of gas furnaces is you can put a smaller crucible in the furnace to do smaller melts. That isn't doable with induction.

Yes, It is very small and probably won't have enough space to process 3kg alloys a week. But it is cheap and i think good enough for testing. Actually I am looking for a 15kw furnance, any above will need me to install a 3-phase electricity source and rewire my warehouse. If I can't melt them at once I think I will melt them in several batches.

 

[giahylxag]

There are many threads in the library about building both hoods and fume scrubbers.

I am reading about it

This sheds an entirely different light on the subject. The inquarting process will increase your nitric usage and require a larger fume scrubber. Don't get me wrong, I have built refining operations in New York City that were permitted and legal. The set up costs were considerable due to both size limitations (NYC refineries are limited in space because of availability and massive rent costs) and the cost of larger scrubbers. So what you want to do is possible. But maybe, considering tumbling and aqua regia can be done with a smaller fume scrubber and we will deal with the gold in chlorides with a silver cell.

I am convinced inquarting -> AR is the proper method for dealing with my scrap. I will have to built a fume scrubber anyway, big or small. I will update the progress later.

 

[4metals]

Everything in refining has it's benefits or it's disadvantages and you will need to weigh them according to your situation. Ferrous sulfate has a benefit of not having the foul noxious odor that metabisulfite has. But ferrous sulfate needs to be mixed with water to prepare it. At the preferred strength it is mixed at 50 grams of the fresh green iron ll sulfate to 100 ml of water. At this strength it will require 100 ml to drop every 7.5 grams of gold. That translates to 414 ml per ounce of gold. In your typical lot of 90 ounces of fine gold that will require 37 liters to drop the gold. That is more liquid and more waste to treat in the long run.

SO₂ as a gas doesn't add any volume to the solution.

 

[giahylxag]

Everything in refining has it's benefits or it's disadvantages and you will need to weigh them according to your situation. Ferrous sulfate has a benefit of not having the foul noxious odor that metabisulfite has. But ferrous sulfate needs to be mixed with water to prepare it. At the preferred strength it is mixed at 50 grams of the fresh green iron ll sulfate to 100 ml of water. At this strength it will require 100 ml to drop every 7.5 grams of gold. That translates to 414 ml per ounce of gold. In your typical lot of 90 ounces of fine gold that will require 37 liters to drop the gold. That is more liquid and more waste to treat in the long run.

SO₂ as a gas doesn't add any volume to the solution.

That seems answer why SO2 is preferred. I can buy a pressurized SO2 tank. But how much gold I can refine given a 40L tanks?

 

[4metals]

Another benefit of SO₂ is you can use it to de-NOx your solutions. If you are careful to not add too much nitric to begin with, you can gas the solution and any free nitric will be consumed by the gold the SO₂ drops and you will see a red cloud while gassing. This stops when all of the free nitric is consumed and the gold continues to drop. This is true even if there is a lot of free nitric but it is wasteful of both nitric and SO₂ gas so careful nitric additions in the dissolve is preferred.

 

[4metals]

But how much gold I can refine given a 40L tanks?

It depends on how efficiently it is dispensed into the acid. But I have seen the cylinders available here in the US, which hold 100 pounds, drop in excess of 1000 ounces.

 

[nickvc]

Another alternative precipitant is ascorbic acid which again avoids the smell from using smb.

 

[4metals]

I have never used ascorbic acid to drop gold. I have heard others speak well of it as a precipitant but I’m too old to start using it now. But not too old to learn new tricks.
How much ascorbic acid is needed to drop a gram of gold? Is it used dry in the acid or dissolved in water? Is is more or less expensive than metabisulfite?

 

[nickvc]

4metals I used it dry but I never measured how much was needed and I believe it is or was more expensive than smb but a lot cheaper than gold.
I used ferrous dry direct to the solution to precipitate the majority of the gold and finished with ascorbic.

 

[4metals]

I only used ferrous diluted with water because I only used ferrous when the solutions were holding PGM’s that I went after once the gold dropped. The PGM’s come out of solution better when using ferrous over smb because smb is a stronger reducing agent and it causes the Platinum and Palladium to change valence states making later selective precipitation less effective.

I do not know how ascorbic acid affects recovery of PGM’s.

 

[giahylxag]

I only used ferrous diluted with water because I only used ferrous when the solutions were holding PGM’s that I went after once the gold dropped. The PGM’s come out of solution better when using ferrous over smb because smb is a stronger reducing agent and it causes the Platinum and Palladium to change valence states making later selective precipitation less effective.

I do not know how ascorbic acid affects recovery of PGM’s.

Trying to warp my head around different types of recipitaion agents is not easy since they have pros and cons. I am testing on ferrous, SMB and SO2

 

[4metals]

I am testing on ferrous, SMB and SO2

Sulfur Dioxide gas will win out for larger lots. For smaller lots it can be a toss up. But first hand impressions will help you make better decisions for your particular situation.

Do you have an XRF? If you do, do your bars indicate any PGM’s?

 

[orvi]

Yes, It is very small and probably won't have enough space to process 3kg alloys a week. But it is cheap and i think good enough for testing. Actually I am looking for a 15kw furnance, any above will need me to install a 3-phase electricity source and rewire my warehouse. If I can't melt them at once I think I will melt them in several batches.

As a person who dealt with induction furnances through my whole refining "career", my advice is go for at least 15 kW one, if you intend to melt on kilogram scale. Induction is reliable, quick, predictable and very very compact. Versatile - you can melt charges up to around 2000 °C (however, at 1700+°C, graphite is burning very quickly), that means you can not only melt gold, but also palladium, platinum and even rhodium. Graphite crucibles tend to be pricier than clay/silica ones. And gold alloys do not "catch" induction on their own that well.

However, the two biggest advantages of induction - attaining very high temperatures + speed of the work - are only really seen if you have that 35+kW one. Yup, 3-phase is necessary for these, but it is well worth the effort, if you plan to upgrade and scale up. Proper induction is the quickest way how to reliably and repeatably melt metals, actual melting table/bench can be easily fitted with pyro-fumehood (which is hard to do with comparable performance gas furnances - you will eventually fry the fan, if it isn´t rated for very hot air) - another bonus, healthier workplace, oxide/metal fumes at least sucked away from you, if not properly scrubbed.

15 kW ones can run on 230VAC 1-phase, but you need to check your breakers and wiring, if it can handle such a power. Chinese manufacturers tend to "overhype" their products, and in my experience this "15kW" machine draw like 8-9kW max. However, that is still plenty of amps

You can also easily run on lower amps breakers, but you should be careful when regulating the power - we handled 35kW furnance on breakers that held like 15-20kW, and it was doable if you know how far you can turn that knob Eventually, we set up AC amp-meter on the input cable to reliably track the consumption - thus we knew how much juice we are drawing and how far we can go till breaker drops

 

[orvi]

Trying to warp my head around different types of recipitaion agents is not easy since they have pros and cons. I am testing on ferrous, SMB and SO2

From my personal experience, SO2 gas is top precipitant in terms of particle size and purity of the dropped gold. If you are generating it from some sulfite + acid, it can be tricky on bigger scale, but if you can legally get the SO2 cylinder, get it. Convenient, just bubble through, visual control, stannous and it is done. If you bubble wisely and slowly, very little SO2 is wasted - as SO2 has very good solubility in water (in comparison with many other common gasses).

You can also use sodium sulfite, not only sodium metabisulfite (SMB). Action is practically the same, however, sodium sulfite is much more basic than SMB. And also you need to use more of it, compared to SMB. This can cause issues during gold drop regarding encapsulation of impurities as pH locally rise. This was thoroughly discussed here on the forum, just search it.

I used to use sodium sulfite for bulk gold precipitations from AR solutions (e-waste), also paired with brute-force de-NOx-ing while solution was still hot. Fact is - sodium sulfite is very cheap chemical in bulk, and that was the reason why we used it regularly. "Dirty" drop from AR + 1 refining via AR+same sulfite drop consistently produced above 999 gold for us from material that was rarely above 1% gold.

However, if you can go nice and professional from the start (SO2 cylinder), go for it (if it is not prohibitively expensive).

Just one last thing - SAFETY is very important regarding SO2 cylinder (and other harmful gasses in pressure cylinders like chlorine, carbon monoxide or ammonia) - be sure and acquire knowledge from the guys who sell these or work with them regularly - to avoid any chance of accidental leaks. SO2 is toxic gas, and need to be handled cautiously. Let them teach you properly how to screw everything tight, how pressure regulator works, how to safely discharge the gas and how to properly stop the gas flow and secure the cylinder.

"Nice" feature of SO2 is the pungent and unbearable smell it has. It is very annoying during regular work, of course, but if any accidental leak is present, you can be sure you will notice

 

[giahylxag]

Hello everyone and happy new year!
Unfortunately I couldn't conduct any test last month.
I am going to make my first test next week.
My friend and I had a hot debate about the optimal process for my gold. He is pro chemistry, but I don't, really I don't know much about chemistry.
Given the gold alloy weight 1kg, with 60%Au and 13%Ag. We are debating 2 options
A: To go directly into the inquarting process. I don't want to say how because everyone here explained throroughly.
B: As my friend proposed. Go with AR, get gold around 90-99%, but lose some gold in the Au/AgCl. Then gather that and make an Au-Ag alloy, then inquart it to retrieve Au.
Both process will recover Ag by electrolysis.
____________________
What is your comment?

 

[Yggdrasil]

Hello everyone and happy new year!
Unfortunately I couldn't conduct any test last month.
I am going to make my first test next week.
My friend and I had a hot debate about the optimal process for my gold. He is pro chemistry, but I don't, really I don't know much about chemistry.
Given the gold alloy weight 1kg, with 60%Au and 13%Ag. We are debating 2 options
A: To go directly into the inquarting process. I don't want to say how because everyone here explained throroughly.
B: As my friend proposed. Go with AR, get gold around 90-99%, but lose some gold in the Au/AgCl. Then gather that and make an Au-Ag alloy, then inquart it to retrieve Au.
Both process will recover Ag by electrolysis.
____________________
What is your comment?

For B you either need to convert the AgCl to Silver or smelt it with flux Bi Carbonate I think.
And the Gold are not in loss just a bit delayed.

 

[4metals]

Silver Chloride is like rust, it forms quickly and coats the surface. Much like rust it slows down further corrosion by inhibiting the ability of the chemistry to reach what you are dissolving. More Silver in the alloy amplifies this process.

It is a really tough choice because 12-13% is high. If you choose the aqua regia method, you will benefit greatly by slowly tumbling the reaction to mechanically remove the Silver Chloride coating. And only trying it will tell you if it is working effectively enough.

The inquartation method is almost fool proof, and you will probably need it to completely dissolve the remnants on the insolubles if you use aqua regia first.

If it were me, I would inquart first because it removes all of the Silver and cleans up the alloy to 99+% Gold quickly. And copper cementation will give you a nice Silver metal for a small Silver cell. And you skip dealing with Silver Chloride processing completely.

I am a big fan of keeping all of the values from a single lot together to make accounting easier, not chasing a few percent of gold all over the shop for days. For security in a refinery that is a good thing, and by inquarting you will know quickly that your recovery meets expectations. That cuts down on theft.