Silver Chloride Conversion Problem - Sulfuric / Iron in Tumbler - Heat / Caking

[snoman701]

I am using approximately a 20% excess stoichiometric quantity of dilute sulfuric acid (10% w/v) to tumble damp silver chloride with iron.

It gets really freaking hot.

Am I using too much iron? I use large ball bearings in various sizes with a little bit of squarish steel cutoffs just so I get tumble action as opposed to everything just rolling along the side of the barrel (5 gallon bucket)

Biggest problem is getting the iron out. The silver chloride / silver powder will cake to the iron, and if you don't get it off, it forms a rust layer that follows the silver, it can also prevent full conversion.

 

[Yggdrasil]

I am using approximately a 20% excess stoichiometric quantity of dilute sulfuric acid (10% w/v) to tumble damp silver chloride with iron.

It gets really freaking hot.

Am I using too much iron? I use large ball bearings in various sizes with a little bit of squarish steel cutoffs just so I get tumble action as opposed to everything just rolling along the side of the barrel (5 gallon bucket)

Biggest problem is getting the iron out. The silver chloride / silver powder will cake to the iron, and if you don't get it off, it forms a rust layer that follows the silver, it can also prevent full conversion.

A magnet will pull it out, won't it?

 

[anachronism]

Have you tried ever Zinc instead?

 

[snoman701]

No.

Getting the iron out isn’t a problem, I just pour through a sieve. The problem is the overheated caked on silver. It didn’t happen when I was doing this on the small scale in a 2 gallon bucket. It’s the scale up.

I don’t have great records of what w/v % sulfuric I ran at that time. It may have been more dilute.

 

[anarxi]

maybe you need a vibrating table.
I do it with hydrochloric acid and aluminum.
it is also an exothermic reaction with a lot of heat release.
but I have microscopic volumes no more than 2 ounces at a time.
but aluminum does not require painstaking cleaning and is cheaper than zinc

 

[4metals]

Why don't you treat the Silver Chloride with caustic to convert it to silver oxide, rinse it well, and direct melt it. The oxide decomposes with heat leaving Silver metal.

 

[snoman701]

Why don't you treat the Silver Chloride with caustic to convert it to silver oxide, rinse it well, and direct melt it. The oxide decomposes with heat leaving Silver metal.

Isn’t there a good chance of ending up with unconverted silver chloride because it’s hard for the caustic to get to? That’s the advantage of the iron tumbling, the grinding action.

are there Limitations on caustic if there is lead chloride present?

 

[snoman701]

I like the sulfuric because the copperas decomposes to water and rust with bubbling and a little extra iron. One less waste stream.

 

[4metals]

are there Limitations on caustic if there is lead chloride present?

Lead hydroxide is insoluble in water so it will come out of solution and remain with the Silver. But Iron will also reduce lead chloride and it will remain with the silver. Any lead dissolved with the weak sulfuric will be insoluble as well in rinse waters.

 

[snoman701]

Lead hydroxide is insoluble in water so it will come out of solution and remain with the Silver. But Iron will also reduce lead chloride and it will remain with the silver. Any lead dissolved with the weak sulfuric will be insoluble as well in rinse waters.

So why do we worry about aldehyde conversion if the next step is pretty much always melting?

This has been a persistent question for me.

Would you recommend just convert with caustic, then flux with borax / soda ash and heat slow to melt?

I have a couple buckets of silver chloride to do, and I’d prefer to keep losses to a minimum.

 

[orvi]

I am using approximately a 20% excess stoichiometric quantity of dilute sulfuric acid (10% w/v) to tumble damp silver chloride with iron.

It gets really freaking hot.

Am I using too much iron? I use large ball bearings in various sizes with a little bit of squarish steel cutoffs just so I get tumble action as opposed to everything just rolling along the side of the barrel (5 gallon bucket)

Biggest problem is getting the iron out. The silver chloride / silver powder will cake to the iron, and if you don't get it off, it forms a rust layer that follows the silver, it can also prevent full conversion.

If the feeds, amounts of chemicals, iron source and shape are the same... Than only the scale can be the issue.
Less exposed surface of iron can lower the rate of reaction = less heat. If you are using same shape of iron pieces with same diameter, logically, my next thought is heat is the problem.

When heated or placed into low-acidity enviroment, iron compounds can decompose into rust-like oxohydroxides. Heat will gradually dehydrate reactive iron hydroxide to various oxohydroxides and even oxides - which aren´t easily solubilized with weak acids. Typical example is iron dissolution in dilute nitric - reaction is usually very rapid, high amount of heat is evolved and in turn rusty iron junk will form - and you can pour in more fresh nitric and it does not do much in terms of re-dissolution.

It is similar to copper hydroxide conversion into copper oxide, or even our favourite silver chloride into silver hydroxide to silver oxide. Here the transition is extremely quick between hydroxide and oxide.

I would try to add less iron for the start and navigate the process through temperature monitoring. In bulk, drums or mixers won´t cool that well naturally - surface area to volume ratio simply dictate the heat dissipation. Maybe some external/internal cooling jacket/snake will help to speed it up.

 

[orvi]

So why do we worry about aldehyde conversion if the next step is pretty much always melting?

This has been a persistent question for me.

Would you recommend just convert with caustic, then flux with borax / soda ash and heat slow to melt?

I have a couple buckets of silver chloride to do, and I’d prefer to keep losses to a minimum.

I do not get "aldehyde" conversion in the context, but I assume you meant conversion all the way to silver metal, if you can just end it at AgCl step.

Pyrometallurgical treatment of AgCl is always a possibility, but chlorides present can make it very unplesant - as sodium chloride is smokey as hell, not to mention silver chloride itself. There always be some losses due to vaporization and incomplete reduction.
Oh how I hate chlorides in melts... wrecking crucibles, smoking, smell, health concerns, precious metal losses...

If you are interested in pyrometallurgical conversion of silver chloride to silver metal, look up process developed by Wasyl Kunda. It involves highly controlled reduction of silver chloride to silver and sodium chloride using sodium carbonate as a base. Key point is temperature control between 500-600 °C. This produce softer caked residue of metallic silver and sodium chloride, which can be crushed and milled/pulverized. This is then leached in water, where everything except silver dissolves. If heated past 630 °C, cake will sinter, shrink and become rock-hard.

I can imagine doing this in some nice electric furnance or induction furnance where you can reliably monitor the temperature. I immediately thought about heat gun for heating - as that is my favourite, reliable and precisely adjustable tool for heating things below 650 °C... unfortunately, this will probably cause unnecessary smoke loss. On the other hand, at these low temperatures, vaporization won´t be that pronounced... worth a shot

 

[snoman701]

Aldehyde conversion is using karo syrup to convert silver oxide to silver.

Concern with kunda method is loss. Losses can be quite high. If you’ve got active scrubbing of furnace exhaust, it’s still getting captured. I don’t have that.

Right now I am cementing silver back out of nitrate solution, but in past I have used chloride precipitation via hcl addition to extend the life of my nitric. It works, and it drastically reduces waste, but you have to deal with AgCl reduction.

In the original post Lou made about sulfuric acid reduction, he said to use punchings, and honestly, I think they are optimum. They tumble instead of roll. When I used to do it in my small tumbler, I used odd size pieces of compact scrap and they worked ok.

 

[4metals]

If you melt the silver oxide from the conversion directly it will be of a high purity, this is why refiners running Silver cells use this method. But if you want to forego the cell and can deal with the quantity of rinsing involved, the complete caustic / corn syrup method will yield 999 Silver.

 

[MGH]

If you melt the silver oxide from the conversion directly it will be of a high purity, this is why refiners running Silver cells use this method. But if you want to forego the cell and can deal with the quantity of rinsing involved, the complete caustic / corn syrup method will yield 999 Silver.

Hello forum (and looking at you specifically, 4metals )

I recently started some trials to convert my accumulated AgCl to metallic silver. I have ~2.5 gallons (volume) of AgCl solids in a five gallon bucket. This first trial was conversion to silver oxide by addition of sodium hydroxide, then melt directly. I obtained some decent looking silver, but I have doubts about a couple steps and the yield was disappointing. I’ll lay out my steps below, and hope someone can point to an obvious error.

Process (my doubtful steps are in bold):

• Transferred AgCl to a 2L beaker, about 950mL volume after settling.
• Rinsed with tap water until rinse water is colorless.
• Added NaOH (solid) and enough water incrementally to keep a fluid slurry
  • Mixed vigorously after each addition with a Cuisinart stick blender
  • Don’t have exact amount of NaOH, probably ~500-700g
  • Solids eventually turned jet black, indicating conversion to silver oxide
• Transferred to a 2.5 gallon plastic bucket for further rinsing
  • Rinsed with tap water until pH is neutral (using pH paper)
  • Solids turned brown during the rinsing with tap water – is this normal?
• Transferred rinsed silver oxide to a shallow pyroceram dish
  • Dried on griddle
  • Solids turned greyish upon drying – is this normal, did the decomposition to elemental silver begin in part? I wouldn’t have thought it was hot enough as it’s below the boiling point of water.
• At this point I have ~700g of [mostly] dry solids
  • I initially thought it was completely dry, but after making a split of the material to melt, I realized it was still slightly sticky and wet.
• I weighed up 200g of the assumed silver oxide into a melt dish
  • The melt dish has been previously used for inquarting. I’m not aiming for super high purity, so wasn’t concerned.
  • Upon melting, with an oxy-acetylene torch:
    • Some white vapor was expelled. I assume this was remaining water.
    • Pools of low-viscosity liquid started to form.
    • I added a little Borax, as I often do when inquarting, but I had never seen such low viscosity flux in the dish.
    • I poured off the low viscosity flux into a 5 gallon pot of water. It settled at the bottom as a dark/black/brown crusty solid.
    • I tested a small amount of this crusty material by heating in dilute nitric, crushing with a glass stir rod, mixing, then adding some HCl. No obvious AgCl was formed, so I don’t think it contained any silver.
    • The molten metal was poured into a graphite mold.
• Final yield of the 200g of assumed silver oxide was 49.39g.
  • Aside from the ugly spot of flux, the metal looks relatively pure.
  • Even though the material wasn’t completely dry, I was still expecting something in the range of 150-190g yield.

I don’t have a good idea of how much AgCl I started with, as I simply transferred a portion from the larger bucket where it had been sitting. If I recovered 49.39g of silver from 200g of the assumed silver oxide, then I’d have ~173g from the entire test batch. I had a separate inquarted karat batch going at the same time, and recorded that ~150g of silver as AgCl has a settled volume of ~350mL, whereas I started this test batch at ~950mL – so my yield seems low all the way around.

Does anything stand out as a red flag in my process above?

Thank you!

Pictures are:
Crusty flux after being poured into water
Melted recovered silver from 200g of assumed silver oxide
Dried assumed silver oxide in pyroceram dish

 

[Yggdrasil]

Hello forum (and looking at you specifically, 4metals )

I recently started some trials to convert my accumulated AgCl to metallic silver. I have ~2.5 gallons (volume) of AgCl solids in a five gallon bucket. This first trial was conversion to silver oxide by addition of sodium hydroxide, then melt directly. I obtained some decent looking silver, but I have doubts about a couple steps and the yield was disappointing. I’ll lay out my steps below, and hope someone can point to an obvious error.

Process (my doubtful steps are in bold):

• Transferred AgCl to a 2L beaker, about 950mL volume after settling.
• Rinsed with tap water until rinse water is colorless.
• Added NaOH (solid) and enough water incrementally to keep a fluid slurry
  • Mixed vigorously after each addition with a Cuisinart stick blender
  • Don’t have exact amount of NaOH, probably ~500-700g
  • Solids eventually turned jet black, indicating conversion to silver oxide
• Transferred to a 2.5 gallon plastic bucket for further rinsing
  • Rinsed with tap water until pH is neutral (using pH paper)
  • Solids turned brown during the rinsing with tap water – is this normal?
• Transferred rinsed silver oxide to a shallow pyroceram dish
  • Dried on griddle
  • Solids turned greyish upon drying – is this normal, did the decomposition to elemental silver begin in part? I wouldn’t have thought it was hot enough as it’s below the boiling point of water.
• At this point I have ~700g of [mostly] dry solids
  • I initially thought it was completely dry, but after making a split of the material to melt, I realized it was still slightly sticky and wet.
• I weighed up 200g of the assumed silver oxide into a melt dish
  • The melt dish has been previously used for inquarting. I’m not aiming for super high purity, so wasn’t concerned.
  • Upon melting, with an oxy-acetylene torch:
    • Some white vapor was expelled. I assume this was remaining water.
    • Pools of low-viscosity liquid started to form.
    • I added a little Borax, as I often do when inquarting, but I had never seen such low viscosity flux in the dish.
    • I poured off the low viscosity flux into a 5 gallon pot of water. It settled at the bottom as a dark/black/brown crusty solid.
    • I tested a small amount of this crusty material by heating in dilute nitric, crushing with a glass stir rod, mixing, then adding some HCl. No obvious AgCl was formed, so I don’t think it contained any silver.
    • The molten metal was poured into a graphite mold.
• Final yield of the 200g of assumed silver oxide was 49.39g.
  • Aside from the ugly spot of flux, the metal looks relatively pure.
  • Even though the material wasn’t completely dry, I was still expecting something in the range of 150-190g yield.

I don’t have a good idea of how much AgCl I started with, as I simply transferred a portion from the larger bucket where it had been sitting. If I recovered 49.39g of silver from 200g of the assumed silver oxide, then I’d have ~173g from the entire test batch. I had a separate inquarted karat batch going at the same time, and recorded that ~150g of silver as AgCl has a settled volume of ~350mL, whereas I started this test batch at ~950mL – so my yield seems low all the way around.

Does anything stand out as a red flag in my process above?

Thank you!

Pictures are:
Crusty flux after being poured into water
Melted recovered silver from 200g of assumed silver oxide
Dried assumed silver oxide in pyroceram dish

I think you created molten Silver Oxide.
It melts at 300C and is very fluid.
You need a reducer in your flux to rob the Oxygen from the Silver.

 

[Shark]

I think you created molten Silver Oxide.
It melts at 300C and is very fluid.
You need a reducer in your flux to rob the Oxygen from the Silver.

Would the addition of carbon take care of this? It seems somewhere along time back I read that wood or another form of carbon would help with this by reducing some or all of the chloride to metal during the melting step.

 

[Yggdrasil]

Would the addition of carbon take care of this? It seems somewhere along time back I read that wood or another form of carbon would help with this by reducing some or all of the chloride to metal during the melting step.

He is melting Silver Oxide as far as I understand. And then he need some kind of reducer I believe.
Carbon of some kind could do the trick, maybe.

 

[MGH]

Yes, I'm melting silver oxide (what I assume to be silver oxide).

Molten silver oxide is one thing I was wondering. But everywhere I've seen on this form is along the lines of simply "melt the silver oxide directly". I could, of course, be assuming too much.

There are a few scientific demonstration videos [on YouTube] where silver oxide is heated to produce silver powder - but these demonstrations stop there. The silver is not actually melted. So I also wonder if the oxide to elemental silver conversion needs to be a more controlled heating step, and I've made things complicated by putting in too much heat too quickly?

Thank you

 

[Yggdrasil]

Yes, I'm melting silver oxide (what I assume to be silver oxide).

Molten silver oxide is one thing I was wondering. But everywhere I've seen on this form is along the lines of simply "melt the silver oxide directly". I could, of course, be assuming too much.

There are a few scientific demonstration videos [on YouTube] where silver oxide is heated to produce silver powder - but these demonstrations stop there. The silver is not actually melted. So I also wonder if the oxide to elemental silver conversion needs to be a more controlled heating step, and I've made things complicated by putting in too much heat too quickly?

Thank you

Not sure.
It is easily melted as it melts at 300C.
Maybe it need stronger heat to decompose or you could try to mix it with Carbon powder.