# Help with smelting and fluxing.



## justinhcase (May 25, 2017)

Hi, chaps!
I have been having fun with high-temperature smelting for want of a better term. 
I have a number of clients that might benefit from a hot recovery instead of a straight wet process.
So I have been working with a 1L crucible (largest my furnace will take.)and using my waste stream as an analog.
Very interesting, good old borax sponges up oxides and other impurities, Feldspathic did not improve viscosity until it was used with fine silicon sand, The waste was treated with sodium hydroxide before drying and ball milling with flux, the use was liberal so there would be NaOH present to reduce any salts that had not yet been converted to forms susceptible to thermal degradation.
The present run has been run for three sessions of three to four hours and I am still finding prill's.
Afte three hour's the metal work on my furnace is glowing red but this still is not enough.
When mixed the flux is like water but it still seems to hold value in suspension.
I even noticed a silvery metallic layer form when the flux was poured out as if oxidation was helping draw out silver.
Sodium nitrate is the one flux I have not tried yet so that will be next to see if the observation has any merit.
Any suggestions for further reading or edifying anecdotes most welcome.
Regards.
J


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## FrugalRefiner (May 25, 2017)

It might help if you tell us what your feed material is, what fluxes you're using, and what collector you're using.

Dave


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## justinhcase (May 25, 2017)

Well, it was my waste barrel.
I took it to be mostly silver chloride with all sorts of other junk, every insoluble compound produced as a byproduct of silver digestion and A.R.
There will be finally divided Au And P.G.M.'s all the way up to Rh.If I managed to do effective processes it should not be a lot.if I have been sloppy there may be a good pay day.
I ran it in a drum mixer with iron and Sulphuric acid for my primary conversion, then a little boile in NaOH to reduce the base metals chlorides before letting it boil dry and powder.
I started the first melt with just borax and Feldspathic but found that quite sandy even at high temperature so have added fine silicon sand which has helped the flow considerably.
The last melt I added cut up copper cable with the original mix, then dropped four square of copper in the top at half hour intervals.


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## 4metals (May 25, 2017)

The waste barrel after cementation with copper or the result of cementation with copper? Or your insoluble sludges that end up on filter papers? Or a mix of both?


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## justinhcase (May 26, 2017)

There was lots of copper in the barrel as a catch-all.
But there were lots of chlorides in the stream so most nitrates should have been converted to chlorides.
There was a good deal of rough silver that went in as metal.
No incinerated filter papers, That is in the next lot once I have worked my way through the waste barrel detritus which I have several k yet to start on. This was just the first test run.
One thing I just thought of ,the mould and catch pan are Iron and stainless steel. I only have small assay cone moulds so used my large loaf mould razed on one end sat in a large stainless steel frying pan. I heated it well then poured the crucible out at the top of the gradient so the flux flowed down hill and over flowed the mould leaving the metal in the lower right angel , I have not yet tried adding Iron to the flux to convert sulphides. may be the silver coloured layer I noted on the slag was from a reaction from the hot slag hitting the iron
There are always more test I would like to run than time in the week to run them.
I have been looking at the rotary furnaces you mentioned as well, interesting rotating an insulated crucible and forcing the burner through the top. I am getting quite a list of kit I will have to build as such kit simply douse not come up second hand in the U.K. 
Regards
J


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## justinhcase (May 27, 2017)

As there were no suggestions I have gone ahead with my next melt.
I opted to try some iron bar instead of saltpeter.
I did not want to use Iron and an oxidizer together as iron oxidizes so easily.
If I have not achieved a complete recovery this time I will try saltpeter to see if that helps.
Also, my store bought furnace had started to leak heat around the bolts that hold the quartz insulation in place.
Even with the hardening agent it had become quite brittle and layered after so meany hours at full heat.
So I have pulled my old homemade furnace out.it is cast refractory cement and should last much longer, take hours to get up to heat but should be more suited to long cooking times.
it has been on for two hours and I will give it until 16-00 until I pour it out.
I have quite a good supplier for gas but at this rate, I may have to buy one of those large LPG tanks that you can fill wholesale.
Regards
J


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## kurtak (May 27, 2017)

justinhcase said:


> Feldspathic did not improve viscosity until it was used with fine silicon sand,



That's because feldspar (feldspathic) if NOT the right flux - you want fluorspar

Feldspar is a flux used in glass making - alumina from feldspar improves product hardness, durability, and resistance to chemical corrosion --- also as a flux in ceramics - the alkalis in feldspar (calcium oxide, potassium oxide, and sodium oxide) act as a flux, lowering the melting temperature of a mixture. 

Fluorspar (calcium fluoride - or fluorite) is used as a flux in smelting to decrease the viscosity of slags.



> Sodium nitrate is the one flux I have not tried



You ONLY need to use a nitrate if you need an oxidize in the mix - normally potassium nitrate is recommended - I can only "assume" sodium nitrate "may" work in place of potassium nitrate - potassium nitrate is VERY hard on crucibles & therefore is not needed/recommended - unless you need the oxidizer



> When mixed the flux is like water but it still seems to hold value in suspension.



That's because you are not using enough collector metal &/or its not fine enough (like cemented silver or copper) &/or mixed well (for even distribution) in what's being smelted &/or your flux/slag was not thin enough



> sodium hydroxide



bad idea - keep it out



> good old borax



You want your basic starting flux to be borax & soda ash (50/50) - not just borax - then to that you add other components (flux ingredients) depending on the chemistry needed (oxidation/reduction) to take place in the crucible during the smelt for the material you are smelting 

 Any suggestions for further reading 

Any good book on fire assay - AND - http://goldrefiningforum.com/phpBB3/viewtopic.php?f=85&t=25592

Kurt


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## justinhcase (May 27, 2017)

kurtak said:


> justinhcase said:
> 
> 
> > Feldspathic did not improve viscosity until it was used with fine silicon sand,
> ...


Thanks Kurt I will digest your instructions and rethink.
Much thanks.
J


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## Topher_osAUrus (May 27, 2017)

Fluorspar is the one you need, not feldspar or feldspathic.

Im thinking that there is one other thinning agent for flux, that being sand. Not sure how that would effect the chemistry of the melt though. If it would at all.


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## Shark (May 27, 2017)

Kurt is very right when he said potassium nitrate is hard on crucible's. Depending on the make up of the crucible, the life can be as short as one use. A while back I tried some silica sand in a flux mix of 50% borax, 40% soda ash and 10% silica sand on copper parts from IC's. Using that, I got a very clean glass from it with no visible residue from the metals. It seems to be the best mix I have tried yet for the copper from IC'S for me. If I am understanding things right, it should be a good starting base for fluxing sulfide ores as well. Not having much experience at smelting, especially in the fluxing area, I am curious as well to get back out and try some more things. Keep us informed on it is going for you.

Edit: And the sand did seem to thin the mix pretty good. Made for a smoother, more fluid type pour, which may be why it seemed thinner.


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## kjavanb123 (May 27, 2017)

Hi

I read in salamander cruicble usage guide, that mention adding fluxes at near the end of melting to lower the attack on cruicble.

Regards
Kj


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## Topher_osAUrus (May 27, 2017)

Had to look it up, but Rose suggests adding sand for fluidity of the slag and help prevent corrosion of the crucible by the oxides. (Pg 389)

The prior page, he states that when graphite crucibles are used. They add bone ash to cover the molten metal, where the layer is thickest around the sides. Holes are made near the center of this cover with an iron rod, and nitre introduced through them, in small amounts at a time. As the fusible oxides are formed, they are absorbed by the bone ash and prevented to some extent from attacking the crucible.

Sounds like a wonderful theory, but I would like to know how well the practice of it works.

Sodium carbonate is just "washing soda", sold in stores under the arm and hammer brand over here. There, I have no clue! 

If lead is present, sal-almmoniac (ammonium chloride) and nitre additions are alternated. Pearlash is used (potassium carbonate) for the problem of tin in the melt.


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## justinhcase (May 27, 2017)

Thank you very much chaps that is all very helpful.
I am getting some good results thanks to your kind indulgence.
It will be interesting to see the difference the soda ash makes.
Regards
Justin


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## justinhcase (May 31, 2017)

kurtak said:


> You want your basic starting flux to be borax & soda ash (50/50) - not just borax - then to that you add other components (flux ingredients) depending on the chemistry needed (oxidation/reduction) to take place in the crucible during the smelt for the material you are smelting
> 
> 
> kurtak said:
> ...


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## kurtak (Jun 1, 2017)

How full did you fill the crucible :?: 

What kind of borax are you using :?: --- off the grocery store shelf laundry borax (like 20 mule team borax) or anhydrous (glass) borax

If you are using off the shelf borax (instead of anhydrous) yes you will have "excessive" foaming - you will have "some" foaming anyway - that's because you have a chemical "reaction" taking place in the crucible (at high temp) --- you WANT this chemical reaction to take place - it is what "dissolves" the unwanted elements in the material you are smelting so they "slag off" (dissolve & go into the slag)

Even with anhydrous borax there is going to be some foaming - that's because borax (& for that matter silica) are acid in nature while soda ash is basic in nature

Smelting is in fact "chemistry" (meaning its "reactive") so a reaction (foaming) is/should be expected in "most" smelting - the exception being when using flux to remove "trace" contaminants when melting actual metals

Have you done any actual reading (as I recommended) about smelting :?: --- (I am talking about "in depth" reading - not just skim reading looking for a simple answer to a simple question)

I ask Justin - because - other then "rare" exception - soda ash is almost always part of the flux - in fact 50/50 borax/soda ash is a "starting" foundation recommendation - MANY flux recipes call for MORE then 50% soda ash &/or the addition of more soda ash (&/or fluorspar) if the flux/slag is not responding as desired (to the starting 50/50 borax/soda ash

Examples of some flux recipes --------

Smelting flux (when smelting for fire assay)

1) 41 grams litharge
27 grams soda ash
18 grams anhydrous borax
2.5 grams charcoal
1.5 grams silica 

2) 41 grams litharge
28 grams soda ash
18 grams anhydrous borax
1.6 grams silica

In assay the litharge is the collector metal --- in smelting (other then for assay) copper or silver would be used in place of the litharge - the point is - soda ash is "more" then 50/50 borax/soda ash

In the smelting thread - for incinerated CBs - 4metals posted as a flux recommendation ----



> To start off I would add a mixture with 10 pounds of Soda Ash, 5 pounds of Borax and 2 pounds of silica



When making up my flux - I "start" with about 40% borax & 60% soda ash & then add other ingredients based on what I am smelting - more often then not - I end up adding additional soda ash (& "some" fluorspar) because I don't like the way the flux/slag is running

I suspect the problem you had with a foam over reaction was ----------

1) you "over" loaded your crucible so did not leave "head room" for the (expected) reaction

2) you used off the shelf borax instead of anhydrous borax which will (greatly) increase foaming due to the water in off the shelf borax

3) you are trying to smelt metal "salts" which are very chemical reactive - they need to under go "reduction roasting" to reduce them to oxides before smelting

Kurt


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## justinhcase (Jun 1, 2017)

I have read a number of articles covering hot recovery but most only give their recommended list of flux mixes.
I did note the foaming but the issue was the complete decomposition of my crucible.
It was only an hour and a half into the run before I saw something was amiss.

The gray powder is actual the lining of my furnace, not the crucible or slag, Very impressed how effectively it took apart castable refractory material, I had thought it quite impermeable
My first flux mix did recover just over 400g of quite dirty silver.
I do not think it is worth the effort to know the exact returns from my waste bin so I will just use them for my next inquartation.

Definitely, something to work at and most interesting observing how the different flux reacts and work with each other.
To learn requires you make mistakes sometimes which is why I work on a small scale experiment first where the safety considerations are easily managed and the amount of value locked up in education can be kept to a minimum.
Sadly I only have a limited amount of time for such interesting work each week so your help and advice save me wasting such a precious commodity.
Still I had enough time for a little extra.


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## justinhcase (Jun 1, 2017)

kurtak said:


> How full did you fill the crucible :?:
> 
> What kind of borax are you using :?:
> 
> Kurt


I only half filled the crucible then topped up once I saw the level go down.
This was a fresh run with a 50/50 flux and my waste barrel as I wanted to compare the results against my original mix.
I was hoping to get the optimal flux for small particle collection where you can observe a clear flux on top by taking a probe sample, bail out the top half of the crucible and recharge with fresh material to cook longer
Reusing the clear flux continually so as to minimize loss.
Probably the single thing that keeps me so engaged is how the practical always has more nuance that any of the literature describes.

I used anhydrous borax from the pottery supplier, sold as glazing.


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## Shark (Jun 1, 2017)

What type crucible were you using? I have been using fused silica crucibles and usually get 2 to 4 runs from them. These are what I started with and have on hand, so I have nothing to compare them with first hand. 

I have also seen the flux eat away refractory cement, it will do it very easily and was quite a surprise. In my case, is was only slight spill and did no real damage. 

Thanks for keeping us up on your progress.


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## justinhcase (Jun 1, 2017)

Shark said:


> What type crucible were you using? I have been using fused silica crucibles and usually get 2 to 4 runs from them. These are what I started with and have on hand, so I have nothing to compare them with first hand.
> 
> I have also seen the flux eat away refractory cement, it will do it very easily and was quite a surprise. In my case, is was only slight spill and did no real damage.
> 
> Thanks for keeping us up on your progress.


Clay Graphite Refractoriness ≥1635 ℃ Carbon content ≥41.46% Porosity ≤32% Bulk density ≥1.71g / cm3 Size：D12.5 x H16cm.
I love the video's of the chaps in third world refinery's, They seem to have a crucible the thicknesses of fine china that can hold up for days.
High-temperature salt degradation is quite a pertinent subject considering all the talk about molten salt reactors.
What kind of alloy could contain such a fluid if relatively inert compounds react so vigorously?
Is silica a better matirial for the job as the flux is quite dependent on fine silica sand for fluidity?
Are there any other alternatives apart from alumina?


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## Shark (Jun 1, 2017)

I don't know if the fused silica is any better or not. I have been working with copper based materials and they have held up decent. I do have one that has a groove in it where the material and flux settles to when in use. It has ate in to it deep enough that I no longer use it. When I started, graphite was suggested as being better, I just haven't tried one yet. I think a very careful examination is needed before each use to be on the safe side no matter what type is used. Heavy use of nitrates and/or soda ash seems to really shorten the life. Maybe in few weeks I can get back out and try some more smelting/melting as I do enjoy it.


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