Problem with melting refined silver and gold in a crucible

[MGH]

I’m also wondering if this was ever resolved. I’m having similar trouble. Here’s the background with my situation:

I can’t claim to have vast experience, but I have melted silver in a melting dish before - starting both with clean sliver wire and cement silver that I had processed myself. The buttons all came out looking decent, needing to have the flux cleaned off with a soak in hot 5% sulfuric acid, but then being nice and white.

I then processed some scrap sterling silver jewelry (from several different sources), about 8 ounces of silver. I tested each piece with Schweter’s solution prior to processing. I dissolved the material in 50/50 nitric acid/DI water, filtered to a clear blue solution, then cemented with clean copper pipe. I rinsed the cemented silver 7-8 times with hot DI water, testing the last two clear rinse waters with ammonia to ensure that no more copper was present. The remaining water was evaporated away on a hot plate, with the silver in a 1000mL beaker.

I obtained what I thought was reasonably pure silver powder, but upon melting the first half ounce, my melting dish turned into a black (and brown, green, and red) mess. I used a MAPP gas torch for the melt, and also used a brand new dish. The problems didn’t become immediately apparent until I started to let the button cool before plucking it out to quench it in some water. At a certain point after taking the heat off, a sort of crust formed across the surface of the button. I figured this was a sign that it was cool enough to grab with the pliers, but the button was still molten and it deformed when I tried to grab it. I waited just a bit more, then grabbed it, but it seemed the flux had hardened quite a bit by that time and it didn’t want to come out. I repeated the melt a few times to see if I could improve my timing, but the same thing kept happening. The only thing I can do is grab the metal at a semi-solid state; otherwise the flux (just using Borax) solidifies and I can’t get the button out. There’s also some metal left stuck in the dish. After quenching the mangled button in water, it has the same discolored crud on it as the melting dish.

I started with a brand new dish, seasoning with Borax before adding any silver powder. A few pictures are attached. I’m using no other acids or reagents other than nitric acid, deionized water, solid copper pipe, and Borax. Any other ideas? Could PGMs in the silver cause these kinds of trouble in the melt?

Thanks,
Matt

 

[lazersteve]

Here's a typical borax color chart:

Borax Colors in Dish

Steve

 

[MGH]

Thanks Steve for the link and the chart. I need to study this for a bit, then I'll post back here when I make some progress.

 

[muconium]

MGH, did you have any resolution to your problem? I'm having the same situation with silver cement from karat-scrap. I used coin silver to inquart. The gold came out great. But the silver melt wasn't and I ended up with a chocolate brown borax residue in my crucible. The flux discolored the silver button as well. I can deal with both with dilute sulphuric, just wondering what the source of the discoloration is. It covers the button when cooling and makes plucking up the button with tongs difficult, as you mentioned.

According to Lazersteve's chart, it's molybdenum? Or nickel?

 

[MGH]

I still haven’t really circled back to this yet. Been thinking and trying to answer some of my own preliminary questions - especially what to consider as the nature of my MAPP torch flame. My searches tell me this is a “cold” reducing flame; not strongly reducing, but more reducing than it is oxidizing because I’m not mixing any oxygen or even air into the fuel.

So under those conditions, my guess about my contaminants is as follows:
Copper, opaque red
Iron, pale green (green might also indicate Chromium or Vanadium)
Nickel, opaque grey
Cobalt (blue)

Muconium, if you’re using a MAPP gas torch like me, then I would guess that your chocolate brown color is actually the mixture of opaque grey from nickel mixed with some red form copper. Molybdenum seems too exotic to expect it in either of our scenarios.

I suppose all these colors simply indicate insufficient washing of my cement silver. My next step will be to re-dissolve an ounce or so of my silver powder and then re-cement. I’m still puzzled though. I stirred at least a half dozen times over three days while this silver cemented, and then washed many times with DI water, even testing the last two washes with ammonia (and obtaining no color change). We will see…

Please feel free to correct flaws in my conclusions or logic. I’ll update again once I get back to the refining bench (Spring is finally here! Time to do some gardening too…). Thanks.

 

[MGH]

I finally got back to some hands-on refining in the last few days. I think my problem with this batch was an incomplete cementation or even just dirty copper. I’ll try to find a good bus bar for the next batch.

Rather than starting over (dissolving in nitric acid again), I washed the cement with hot 5% sulfuric acid. The solution turned blue, and then I rinsed several times with water. I did another wash with more sulfuric, but the second wash did not [apparently] leach out any more contaminant. The melt improved remarkably - resulting in a clean button and clean melting dish. I know this step shouldn’t be necessary; my goal for the next batch will be to do it right the first time. :|