Not getting all collector metal back after smelting ore

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boarteats

Well-known member
Joined
Mar 31, 2018
Messages
48
I'm smelting iron oxide heavy, gold bearing material using Chapman flux with some fluorspar added as flux thinner. I'm using silver as a collector metal to pull out gold during smelting process. After smelting, I end up with a silver/gold amalgam. Using specific gravity as a measure, i estimate that the resulting metal is about 20% gold on average.

Here's the problem. At times, I end up with less metal by weight than I started with. I must be leaving some silver and probably gold in the slag and I don't know why. I could use some help to develop a better understanding why such a problem might arise. What are the smelting process variables that I should be examining/tweaking?

Some more details describing my current approach.
1. Time: After material is molten, I hold at for 30 mins.
2. Temp: Max temp is 2700 to 2800 degrees Fahrenheit.
3. Flux to concentrate ratio: 1.5 to 1.
4. Fuel: forced air, propane.
5. Crucible: clay bonded graphite.
 
Collect in a cone.

Is your slag fluid?

Break up some of the slags to look for bb's.

What form is the silver in before melting??


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snoman701 said:
Collect in a cone.

Is your slag fluid?

Break up some of the slags to look for bb's.

What form is the silver in before melting??

Thanks @snowman701: The slag is very fluid due to the addition of fluorspar (aka Chapman flux thinner). I crush up all slag and retrieve any BBs already. I don't have a conical mold but will make one and see if that helps.

I do roast the ore before smelting.

I use either .999 silver or Sterling silver depending on what I have sitting around. When using Sterling, i account for the fact that is 92.5 percent silver when recording weight.
 
Something just occurred to me while thinking about this problem.

Quite a lot of silvery material floats to the top of the molten material after I pour it. I had assumed that this was graphite from the crucible. However, this didn't happen when I attempted to smelt the concentrate without using a collector metal (total fail).

Maybe a stretch, but could something be binding with the silver preventing it from precipitating out of solution as slag cools?
 
I would guess you have chlorides in the ore. Increase soda ash, cover the crucible and heat slower to red hot.


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snoman701 said:
I would guess you have chlorides in the ore. Increase soda ash, cover the crucible and heat slower to red hot.


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Great! Will give the above a try. Will also roast material longer in case sulfides are a contributing factor. Appreciate the info, @snowman701.
 
Why are you smelting at such a high temp? I would start with not opening the door to look at the molten state, it doesn't hurt anything to leave it in for a set time and leave it alone. If you still see matte then you know that next time you need to leave it in longer. Document your times and temp so you can adjust your next smelt.
 
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