A video from Picky Plans worthy of discussion

[4metals]

Follow this link to a video by @PickyPlans about smelting ore concentrates. His equipment and methods are worthy of discussion.

 

[PickyPlans]

Follow this link to a video by @PickyPlans about smelting ore concentrates. His equipment and methods are worthy of discussion.

Thank you for mentioning ...! If any forum members have questions, I'd be happy to answer — feel free to ask.

 

[4metals]

I'll start with the cupellation furnace in your shop. Is there a base layer of bone ash in the furnace to absorb the lead and base metals? Is there a way to limit the air so driving the lead slows and absorption increases to lessen the load on the bag house?

 

[PickyPlans]

I'll start with the cupellation furnace in your shop. Is there a base layer of bone ash in the furnace to absorb the lead and base metals? Is there a way to limit the air so driving the lead slows and absorption increases to lessen the load on the bag house?

No, the furnace operates on a slightly different principle compared to the common cupellation method, where lead and base metal oxides are absorbed by a porous substance like bone ash. The furnace base is formed through continuous tapping to fully harden, using a castable chromite-based refractory material known for its high density and resistance to the corrosive effects of molten oxides. Unlike bone ash, this material does not absorb oxides. Instead, the oxides are poured out of the furnace similarly to slags in the first stage of smelting. Both air and fuel flow can be regulated through their respective control taps at the inlets, allowing the adjustment of the flame to either reducing, neutral, or oxidizing conditions depending on the process stage

 

[4metals]

I like the tilt function to allow the skimming into your cone molds, the lead restrictions for airborne lead in the USA would mandate a bag house and since the entire charge of lead is volatilized it would be extreme. I have used set-ups where a removable layer of bone ash made up the bottom and about 80-90% of the lead absorbs into the bone ash. Controlling the air and ability to have an oxidizing or reducing environment can effectively cause the base metal oxides to slag off for skimming. Do the skimmed portions ever hold precious metals from "skimming" too deep. I imagine it is a learned skill to lightly skim the top.

 

[PickyPlans]

I like the tilt function to allow the skimming into your cone molds, the lead restrictions for airborne lead in the USA would mandate a bag house and since the entire charge of lead is volatilized it would be extreme. I have used set-ups where a removable layer of bone ash made up the bottom and about 80-90% of the lead absorbs into the bone ash. Controlling the air and ability to have an oxidizing or reducing environment can effectively cause the base metal oxides to slag off for skimming. Do the skimmed portions ever hold precious metals from "skimming" too deep. I imagine it is a learned skill to lightly skim the top.

The regulations here aren't as strict as in the USA, but to prevent lead pollution and avoid causing problems for nearby industrial workshops, I've used a bag filter system commonly used for collecting wood dust in the wood and MDF industries. It effectively filters out a large portion of lead oxides.

You're absolutely right — two key factors that prevent precious metals from ending up in the slag are the slag's viscosity and the operator's skill in skimming without dipping the rod too deep into the molten metal. However, I grind the slag afterward and separate lead shots from it.

 

[4metals]

Second question, not at all cupellation related. You melt the Silver Chloride with borax and soda ash. The resultant fume is a nasty choking chloride fume. Is there a reason you choose this flux? I have found an addition of carbon to the flux can be effective. (7 parts soda ash, 4 parts borax, 1 part carbon). Mixing the chlorides and flux well and holding the temperature so the Silver Chloride melts and reacts with the flux before going to the silver melting temperature. Still not the kind of melt you want to stand over the furnace and breathe but better.

 

[PickyPlans]

Second question, not at all cupellation related. You melt the Silver Chloride with borax and soda ash. The resultant fume is a nasty choking chloride fume. Is there a reason you choose this flux? I have found an addition of carbon to the flux can be effective. (7 parts soda ash, 4 parts borax, 1 part carbon). Mixing the chlorides and flux well and holding the temperature so the Silver Chloride melts and reacts with the flux before going to the silver melting temperature. Still not the kind of melt you want to stand over the furnace and breathe but better.

I observed that using a graphite crucible yields better results without the need to add carbon. Silver chloride donates chloride ions to sodium from the flux, forming sodium chloride, which melts at around 800°C and produces minimal fumes. Simultaneously, silver oxide forms and spontaneously reduces to metallic silver at high temperatures. Additionally, the graphite crucible itself contributes carbon, which helps prevent fume formation and enhances silver reduction.

 

[4metals]

Graphite or clay graphite? Clay graphite has longer life than pure graphite by far, possibly by not adding to the carbon content. I always used clay graphite crucibles and added the carbon.

 

[PickyPlans]

Graphite or clay graphite? Clay graphite has longer life than pure graphite by far, possibly by not adding to the carbon content. I always used clay graphite crucibles and added the carbon.

Sorry, I meant clay graphite, You are absolutely right,Adding carbon could be a much better idea. I also noticed that stirring the molten charge with an iron rod can improve silver reduction to metal, similar to the effect of adding carbon.

 

[4metals]

With reactions involving solids a big factor is the actual physical contact of the reactants. Mixing carbon into the flux and blending it well with the charge makes it more efficient. Whatever contact with the clay graphite there is, is only on the outside of the charge so more is better. (In this example! Not true with metabisulfite guys!!!!). Stirring with an iron rod is also a unique approach.

 

[snoman701]

my biggest concern is that the yeilds I have seen him produce seem low compared to other reported yields. Have there been any assays done on the slags?

 

[PickyPlans]

my biggest concern is that the yeilds I have seen him produce seem low compared to other reported yields. Have there been any assays done on the slags?

I always keep the slags and perform an XRF assay to determine if re-smelting is needed. On average, I've experienced around 0.2% gold loss during the smelting process, primarily during the cupellation step rather than from the smelting process itself or the resulting slag. The majority of the losses are retained in the cement cupel. The low yield is mainly due to the quality of the starting materials, which, I can source locally. Trying

 

[snoman701]

I have been thinking about it, I think the one I watched was one of your first videos and you lost silver. That's why I was concerned.

 

[PickyPlans]

I have been thinking about it, I think the one I watched was one of your first videos and you lost silver. That's why I was concerned.

In cupellation, silver loss is common because copper interferes during this step, typically resulting in losses of around 5-7%. To mitigate this, I reprocess the cupel after a few runs. Additionally, I always add approximately five times the amount of scrap silver compared to gold in the charge to prevent or minimize gold losses. Silver has a protective action on gold and PGMs

 

[4metals]

In a fire assay of silver bullion the cupellation losses are typically 1.5 - 2% which is why it is always good to run a proof assay. It makes sense you would experience losses in the process to your giant cupel as well.

I would be wary of any XRF result on slags, I would put more faith in a remelt of the slags with a thinning flux and a pour into a cone mold.

The interest in this video for me was more of a proof of concept for the equipment at the scale of your furnace. The spill you had could have explained a lot and typically you wouldn’t experience that.

 

[PickyPlans]

In a fire assay of silver bullion the cupellation losses are typically 1.5 - 2% which is why it is always good to run a proof assay. It makes sense you would experience losses in the process to your giant cupel as well.

I would be wary of any XRF result on slags, I would put more faith in a remelt of the slags with a thinning flux and a pour into a cone mold.

The interest in this video for me was more of a proof of concept for the equipment at the scale of your furnace. The spill you had could have explained a lot and typically you wouldn’t experience that.

Based on the reprocessing of slags, I typically lost an average of **5% silver**, which was later recovered. The primary reason for these silver losses is the **high copper content** in e-waste. As you know, copper is not entirely a noble metal, and during cupellation, the formation of **copper oxide** causes the lead oxide layer to become too thick. This thick oxide layer, due to its **higher melting point** compared to pure lead oxide, mechanically traps around **5% silver** (in such a large furnace) along with approximately **0.2% gold** in the molten oxides.

To recover the silver, the oxides simply need to be **remelted with silica**, forming **lead silicate**, while adding **iron pieces** to the melt. In this environment, the reduction of metals (mainly copper and lead) produces **thinner molten oxides** in the cupellation step, allowing for **almost 99% silver recovery**.

The key advantage of using such a furnace is that the resulting **lead and copper oxides are obtained in pure form**, without contamination from bone ash or cement. This makes the resmelting process much easier and requires only **minimal acidic fluxes**. A small amount of **silica sand** is enough to form lead silicate, which not only protects the furnace bottom from corrosion but also produces an alloy with a **higher lead-to-copper ratio**. Make recovery more efficient in the downstream processes.
P.S. You can see pure lead/copper oxides discharged from the furnace...and not absorbed into a cupel.

 

[orvi]

I observed that using a graphite crucible yields better results without the need to add carbon. Silver chloride donates chloride ions to sodium from the flux, forming sodium chloride, which melts at around 800°C and produces minimal fumes. Simultaneously, silver oxide forms and spontaneously reduces to metallic silver at high temperatures. Additionally, the graphite crucible itself contributes carbon, which helps prevent fume formation and enhances silver reduction.

Carbon from the actual crucible is far more expensive reducing agent than adding ground up charcoal or coal.

Chloride smoke is very bad stuff and with this kind of smelting approach, you will always lose some silver. In the worst form - that can actually harm you. Wet route using sodium hydroxide and than some sugar/glycerine/glucose-fructose syrup is way better in terms of losses and also harm to you/enviroment. But yeah, it is more steps. Factoring in the cost of crucible erosion, it may be worth considering wet route alternatives

 

[PickyPlans]

In a fire assay of silver bullion the cupellation losses are typically 1.5 - 2% which is why it is always good to run a proof assay. It makes sense you would experience losses in the process to your giant cupel as well.

I would be wary of any XRF result on slags, I would put more faith in a remelt of the slags with a thinning flux and a pour into a cone mold.

The interest in this video for me was more of a proof of concept for the equipment at the scale of your furnace. The spill you had could have explained a lot and typically you wouldn’t experience that.

Based on this chart from Edward Bugbee's *Textbook of Fire Assaying*, showing that more silver is lost at the end of the cupellation process, I replaced the charge mold with another mold at the final stage to collect approximately the last one-third of the molten oxides, allowing silver to be recovered later on.

 

[PickyPlans]

Carbon from the actual crucible is far more expensive reducing agent than adding ground up charcoal or coal.

Chloride smoke is very bad stuff and with this kind of smelting approach, you will always lose some silver. In the worst form - that can actually harm you. Wet route using sodium hydroxide and than some sugar/glycerine/glucose-fructose syrup is way better in terms of losses and also harm to you/enviroment. But yeah, it is more steps. Factoring in the cost of crucible erosion, it may be worth considering wet route alternatives

Thank you for sharing your knowledge @orvi ! Yes, the wet route is indeed a better option, and recently I've found that reduction using ascorbic acid shows great promise, though it requires heating the solution. However, I've never heard of using glycerin — thanks for the suggestion! It seems like an easily accessible and abundant alternative for me.