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.