# Incineration after HCL pretreatment



## kurt (Apr 2, 2012)

OK it’s been awhile since I last posted anything but I have just been to darn busy – have hardly had time to log on every day & read but I at least do that.

Anyway – there is always the mention of the need to incinerate after doing a HCL pretreatment in order to get rid of residue HCL (as in the case of using HCL to get ride of tin from solder)

Incineration suggest fire (as in the use of a torch) &/or at least heat to the point of getting metals to red hot --- so my question is – is it really necessary to “incinerate” to rid the residue HCL – or is it just a matter of applying a “high” heat – like using a hot air gun on high - or some sort of a “roasting” set up.

How much heat is really needed to rid the HCL residue?

It seems like (& I could be wrong) back when I first joined the forum I read something posted by Harold where he talked about “force drying” to rid the residue HCL

Kurt


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## Palladium (Apr 2, 2012)

There is a difference between force drying and incineration. When he referred to force drying he was probably talking about hcl as in the washing procedures. Hcl decomposes at a lower temperature into H and cl than a metal salt does. That’s where incineration comes in at. By heating to a red glow you beak down these salts and free the nitrate or chloride or what have you. This requires a much higher temperature to accomplish than force drying to rid something of residual chlorine or hcl. I think that’s right.


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## Dr. Poe (Apr 2, 2012)

To reply to this I would have to guess why you were using the HCl and of what metals would chlorinate as insoluble chlorides.
Please elaborate so we may help you. Dr. Poe :roll:


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## kurt (Apr 3, 2012)

Thanks Palladium – that makes perfect sense to me & is what I thought. The reason I asked was because I keep reading post’s that say to incinerate after using HCL because you can not wash out all the HCL & if your next step is nitric you end up with a weak AR due to residual HCL.

So the reference has been to “incinerate” to rid residual HCL – not to rid (or brake down) residual salts. 

Dr Poe – read my post again – second sentence where I said --- (as in the case of using HCL to get ride of tin from solder)

Here is the actual case – I am processing 5 lbs of gold plated pins – about 10 – 15% of them had heavy solder on them like the pile on the left. So, after incinerating the first time to rid any oils &/or bits of plastic I did a pre-treatment with HCL to get rid of the solder – then washed with water till a litmus test showed no more acid. The pile on the right is what they look like after the HCL treatment & I am not seeing a need to re-incinerate at this point (I did “force” dry) even though I have read many post saying you should to rid the residue HCL

Kurt


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## kurt (Apr 3, 2012)

Here is a picture of the whole pile of pins - after HCL pretreatment (to rid solder) & "force" drying

I am not seeing a need to incinerate these before going to nitric treatment --- or am I wrong???

(they were incinerated once - before going into HCL treatment)


Kurt


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## butcher (Apr 3, 2012)

The problem I see with incinerating chloride salts, gold chlorides are volatile (yellow fumes), and silver chloride can also evolve in fumes as white smoke.

Because of this I always, wash the chloride salts in a solution of sodium hydroxide, neutralizing acidic powders, and rinse the NaCl salts (salt water) that forms, with plenty of hot water washes, my thinking here is that I will help to make oxides of silver and base metals and remove most of the chlorides in these powders, that I feel would otherwise cause me a loss of values in fumes in the incineration process.

Rinsing of the salt NaCl is also important because gold can be volatile if incinerated with NaCl (table salt).


(If lead as chlorides are involved in the powders they are to be washed out in boiling hot water (before I would have neutralize the chloride powders prior to roasting them).

Lead chloride is insoluble in cold water but it is pretty soluble in boiling hot water, silver chloride is insoluble in both cold or hot water (although the silver chloride takes time to settle in solution, and also if solution is being stirred by the boiling action the silver would be stirred up and not settle, so keeping the water hot to keep lead chloride soluble and also letting the silver settle and giving the silver time to settle is needed to separate these two metals, (or lead from the other chloride salts).


The incineration will drive off the acid portion that formed the metal salt; (incineration drives off this acid portion from the salt as a gas of that acid),

Example copper metal dissolved in HCl/H2O2 gives a salt of copper, in solution Copper II Chloride (Cu2Cl), or as insoluble powders as white Copper I Chloride (CuCl), If we incinerated the copper chloride red hot we would drive off the chlorides as a gas, and also convert the copper to copper oxide, as we expose these copper powders to air or oxygen, it is important to crush the powders and clumps, and stir the powders to get good exposure to oxygen, I am not exactly sure at what temperature all of the chlorides would convert to oxides But I do know it would be very close to red hot.
Sulfate would need red hot and held at that temperature for a period of time almost an hour, as you need greater than 700 degrees before the sulfate salts even begin to break down to sulfur dioxide gases.

Also incineration helps to oxidize most of the metals, this makes it easier to dissolve the metals into solution later, and metals like tin will be much more easily removed in steps later, this also greatly reduces your chances of loosing gold in the tin laden solutions, makes filtering solution less of a major pain in the backside (tin after incineration is remove with boiling hot HCl and hot water rinses), (if you made a chloride metal salt from using HCL, incinerating before using nitric will keep you from dissolving gold when you do not want to, the chloride salts and HNO3 would form aqua regia if the chloride salts were not incinerated to metal oxides (removing chlorides), before using nitric acid.

Do you see the similarity to the SSN leach? (Where saturated salt NaCl and nitric acid form aqua regia, and will dissolve gold)

Understanding why and when to incinerate is very important to success in recovery and refining, Harold has done a great Job of helping me to understand this, ore and electronic scrap the incineration method is extremely important tool, and it can be a major difference in getting values or loosing your values.


Edited to add this thought:
I would Incinerate the pins again you could have traces of lead and tin salts (chlorides), that you did not, (or cannot) rinse out.

Note: a metal and an acid will make a neutral salt, rinsing off acids and testing pH to see that these salts are neutral does not remove these salts, and the neutral tin chloride salts can again be a problem when you add nitric to the pins (if you do not incinerate them again), the tin chloride can make aqua regia, the tin make a gel in nitric (hard to filter), and carry tin through the solution to the later stages of processing, basically making you loose values.

Tin in a chloride solution of dissolved gold forms stannous chloride in this solution, this reduces the gold (to colloids of gold they will not settle), and since the colloids of gold in solution are already reduced to metal (that will not settle), a stannous test will not show you a positive test of this gold metal held in solution, making throwing out your gold into the waste stream a costly mistake, easily avoidable with a quick use of the torch now.


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## dtectr (Apr 3, 2012)

IMHO
When harold speaks of Incineration between steps, I believe that it often refers to recovered and partially refined gold powders, or as in the case we're about to witness, using hot nitric on a tin-containing bronze alloy, which will result in the creation of metastannic acid. And contrary to Hoke, concentrated sulfuric acid merely "takes up", not "dissolves" it, Incineration converts it to tin oxide which is readily soluble in copious amounts of boiling HCl-


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## Harold_V (Apr 4, 2012)

Here's a little exception to the "rule" of incineration after a digest with HCl. 

By rinsing carefully, when the product is something like pins, you will have effectively eliminated the vast majority of the chlorides. In theory, you can then proceed to nitric acid directly. Here's what will happen, and it's totally acceptable. What little of the chlorides that may remain will, indeed, create traces of AR. That will dissolve a trace of the gold, but because you have an overwhelming amount of base metal present (remember, we're talking about pins here, not polishing wastes from the jeweler's bench), the traces of dissolved gold will be cemented and recovered. It is actually an acceptable procedure, assuming you have fully eliminated all of the tin. Simply ensure that all traces of solids are recovered from the nitric digest.

I would not recommend the incineration process be avoided with polishing wastes. 

Harold


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## kurt (Apr 5, 2012)

Thank you Harold - like Palladiums answer that also made sense to me.

The other factor here is that when you do the nitric digestion to recover the foils from the brass you get quite a bit of the nasty old tin paste anyway – which of coarse does require incineration & HCL treatment before going to AR

In fact that brings up another question – I recently bought the “Fire Assay Home Study Course” from Action Mining Services in which they talk about the “need to roast” ores as a pretreatment to doing your assay. First a reduction roast at 750 F for one hour then a oxidize roast at 1350 F for one hour

So – rather then actual incineration (using torch to heat foils to red hot to convert tin past &/or other salts to oxide for HCL treatment) can you instead follow this same “roasting” procedure to accomplish the same task?

Put foils in crucible – cover crucible – put in furnace – roast one hour at 750 F (reduction roast) – remove crucible cover – roast one more hour at 1350 F (oxidize roast)
& then proceed with HCL treatment – then on to AR treatment.

Kurt


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## kurt (Apr 5, 2012)

dtectr said:


> IMHO
> When harold speaks of Incineration between steps, I believe that it often refers to recovered and partially refined gold powders, or as in the case we're about to witness, using hot nitric on a tin-containing bronze alloy, which will result in the creation of metastannic acid. And contrary to Hoke, concentrated sulfuric acid merely "takes up", not "dissolves" it, Incineration converts it to tin oxide which is readily soluble in copious amounts of boiling HCl-



Yep - I have already tried the sulfuric method to - not the way to go for getting rid of the tin paste - IMHO

Kurt


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## Harold_V (Apr 6, 2012)

kurt said:


> So – rather then actual incineration (using torch to heat foils to red hot to convert tin past &/or other salts to oxide for HCL treatment) can you instead follow this same “roasting” procedure to accomplish the same task?


Certainly. The heat source isn't too important, so long as you achieve the temperature and there's abundant air present to oxidize. In fact, I incinerated in a pan, using a gas burner. I used a torch only at the end, to raise the temperature to a dull red heat. For me, that was far more convenient than using a furnace, plus it was all accomplished in my fume hood, so I didn't have to deal with toxic vapors and gasses. 

By the way, going in the reverse direction (from nitric acid to HCl) I don't generally suggest you do that without the incineration process. Considering an ounce of nitric will dissolve an ounce of gold, plus the fact that the nitric digest will have, most likely, eliminated any of the base metals that would cement any dissolved gold, I suggest you always incinerate. It's amazing that, in spite of careful washing, traces of nitric remain. 

Harold


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