Pretreatment of Jewelers Bench Tray Filings

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notehunter494

notehunter494

Active member
Joined
Dec 26, 2020
Messages
35
Hello All and Happy Birthday America! As per Harold's excellent postings on incineration I have finally done a proper incineration and am ready for pretreatments. Jewelers filings can have almost anything among them and proper incineration and pretreatments are necessary or a mixed bag of junk will follow the gold and foul the drop. I know this from watching kadriver's videos and seeing the trouble he has had with jewelers filings.
Before my nitric acid digestion I am planning on boiling in a 50/50 HCl/water soln., settle, decant, rinse. I will do this once or twice depending on how the solution looks. Then low heat dry and incinerate again to get rid of the chlorine.
Next, as per Butcher, 03/11/2008, in Harold's "Incineration" thread, I want to boil the filings in a strong hydroxide, lye in water. Again I would do this once or twice depending on how the solution looks. Again a low heat dry.

My questions are:
1). Do I have to incinerate after boiling in strong hydroxide? I know incineration after the HCl boil is a must to rid the chlorine and prevent the production of AR during the nitric digestion.
2). Is there a proper order in which to do this? Hydroxide before HCl or the other way around? I am thinking in terms of best way to rid the filings of the "other stuff" in terms of efficiency first, and then if all things being equal in terms of costs. I believe Nitric Acid is the most expensive and hardest to get, then HCl, and hydroxide the cheapest.

Your thoughts and insights are Most Appreciated. Thanks, Tom
 
Hello Tom,

Chloride salts can cause problems during incineration, values like gold or silver, or even platinum group metals can become volatile (burn off as smoke) in heated escaping chlorine gas fumes during an incineration process (improperly controlled).

Slower heating at lower temperatures for a longer period of time can be used to help drive off some of the chloride ions in a fusion process, before bringing the material up to a roasting temperature glowing red hot and stirred in a stream of oxygen or air can be used to drive off the remaining chloride ions and converting the base metal materials to oxides or hydroxides. This option is harder to control without losing some of your values, using the suggested sodium hydroxide wash (conversion) will help remove the chloride ions easier with less possibility of loss during roasting..

Treating the materials containing say some free hydrochloric acid with a slew of other metal chloride salts.

HCl + NaOH --> NaCl + H2O
Hydrochloric acid and sodium hydroxide react to produce sodium chloride (water-soluble table salt) and water, the salt can be washed from the material before incineration, removing much of the chloride ions from the materials such as Jewelers filings,

The base metal chlorides salts (many of which may be themselves insoluble) treated with the caustic soda (NaOH) will form hydroxides and water-soluble sodium chloride salts where most of the chloride ions can be washed from the solids before incinerating the jeweler sweeps.
FeCl2 + 2NaOH --> Fe(OH)2 + 2NaCl
Iron chloride salts and sodium hydroxide are converted to iron hydroxide and sodium chloride (water-soluble material similar to table salt).

2AgCl + 2NaOH --> Ag2O + 2NaCl + H2O
CuCl2 + 2NaOH --> Cu(OH)2 + 2NaCl
PbCl2 + 2NaOH --> Pb(OH)2 + 2NaCl
 
Thanks Butcher. I will start with the hydroxide wash, slow long and easy with a slow dry then incinerate.

Now with that said I am thinking that I do not have to do the HCl wash? Or should I do the HCl wash first and then the hydroxide as the hydroxide will more easily rid the chlorine compounds. The reason I am making the extra work by doing the HCl wash at all is to get rid of any tin or other unknowns that will foul the drop or be carried over with the gold.

For me, with respect to this "adventure", time is not a factor and I want to get it right. Again many thanks for the insights and chemistry. Tom
 
Tom,
I am a bit confused by what you are asking or how to answer the question.

In Hoke's book, she covers the topic well of bench sweeps, the goal is to separate the unwanted from the wanted as much as possible in each step of the recovery process.

Chlorides are just reduced ions or salts of chlorine gas or HCl.
Hydrochloric acid can be oxidized to produce chlorine in a solution.
Chloride salts can also be oxidized to produce HCl.

Many metals can be oxidized (loss of or sharing of its electrons) by changing the elemental metal atom to ions or salts of the metals as chlorides, most metal chlorides are soluble with a few exceptions, the oxidation of the metal atoms by using HCl alone which can oxidize many of the base metals.
With the more noble metals (the less reactive metals) or with the metals we may value more say from copper to the least reactive metals like gold or several of the platinum group metals, HCl alone does not have a high enough oxidation potential to dissolve them or oxidize them to ionic salts, for these metals we can add an oxidizer to the HCl to create a solution of a higher oxidation potential such as one containing chlorine gas such as with using HCl and an oxidizer such as H2O2, HNO3, NaOCl or other oxidizing agents, as we see with the use aqua regia to change metallic gold to an ionic salt of gold dissolved in a solution as chloride of the metal.
 
Thanks Butcher, yes Ms Hokes book I do remember reading about pre treating the filings with hydroxide. I will check it out as I am getting ready to go forward. Many thanks again, Tom
 
I’m a jeweler. incinerate it well. Put it in a tumbler and let it grind to a powder. This will enable a faster reaction due to increased surface area.

use HCL this will digest a lot of the crap

I don’t see the need for Lye. Roasting gets rid of a lot of organics and hcl digest almost all the remaining. after this , filter then
roast it again

then add AR slowly.

works for me.
 
Excellent grainsofgold and thank you for your insights, as a fellow jeweler it puts us on the same page so to speak.

Yes, on page 17 of Hoke she says "Do not use both the caustic and the frying pan. Use one or the other, according to the kind of dirt present" and because I have very clean filings, no grease or shellac, I believe I can skip the hydroxide.

I will proceed with 50/50 HCl boil, settle, decant, rinse, repeat until the solution is clear and then slow dry. Then grind again, incinerate and go with a nitric digestion.

I am starting with a mixture of mostly 18k yellow with a much smaller amount of 14k. Maybe 5% of the whole thing is silver from model making, and unfortunately maybe 2% platinum is in there, in the beginning I did some platinum work at this bench before I set up a platinum only bench. And as a jeweler you know that working 25+ years you can have anything in there as you may have repaired "Aunt Loretta's precious Napier clip back earrings", etc and although it was only and occasional mercy favor over time these add enough junk to the mix to really foul the recovery process.

I have had enough experience with chemistry to know that this starting material, with all the surface area, can lend itself to a real messy run away reaction, even if at the start it seems that nothing is happening. It is a freight train in the making.

Again, Many thanks, Tom
 
To avoid a runaway reaction, master the art of adding just enough nitric to warm HCL in small increments with AR or when adding nitric to distilled water to dissolve silver.

It also keeps the nitrous fumes down and you don't have to get rid of excess nitric for precipitation.

Martijn.
 
Excellent tip Martijn. I have entered it into my notes. Start warm and add slow and incremental, like a titration. Super advice. Many thanks, Tom
 
You haven’t said how much your sweeps weigh -

After you incinerate , mill it then hcl the roast again. i would suggest doing a small run on less than an oz. of feed stock. this way you see how it reacts instead of doing it all at once then saying uh oh.

Sweeps can vary in percentage yield. Keep track of the gross amount and what you bet out. Save that number for a future baseline to work from.

GOG
 
Hello GOG and many thanks. Right now it is 1236 grams after I burned it out in my casting furnace on three 2 inch square 18 gauge 304 stainless shelves I stacked with refractory tiles. It took about 20 two hour burns, holding low red for an hour, to run it all, roughly 60 grams at a time. A labor of love as I never left the furnace alone, it took about two weeks. The result was more of a pyrolysis than incineration, even with the door cracked a little, the material is more black than gray. I believe I lost 30 grams from its out of the tray and prepared state but I will not run it as it is because I saw the problems kadriver had with this material in an early video on bench tray recovery/refining with a burnout oven incineration, it did not go well.

I got ahead of myself but I will get a great incineration as I am following Harold V to the letter and it will go much faster as I believe I can do it in two 600 gram batches. I am excited to see what the weight difference will be in the properly incinerated final product from this pyrolysis as that would represent the amount, on a % basis, of junk kadriver had to put up with.

From that point I hope to put an ounce or less of these filings through the HCl roast reincinerate and nitric digestion. If I produce 3 buttons I will see what kind of XRF readings I am getting. With the silver gone it should be between 14 and 16 karat at least as my jewelry line was 18k yellow and platinum, two tone rings primarily, and what the platinum might read as this is what would be the karat down metal to the gold. At this point I can inquart and refine these samples.

From there I don't know, it depends on how I feel about the XRF results. It would be a commitment to inquart this much metal, and I will only go forward if my scrubber can handle it in 2 ounce batches at a time. If things go smoothly I can see myself running the whole thing as the excitement of participating in the precipitating is infectious, as you all know.

Many thanks, Tom
 
If what you have is just filings from the bench then incineration and a nitric dissolution will remove the silver and most base metals and some PGMs if present , this you can recover by cementing with copper, the remaining powder needs a good rinsing to remove most of the silver containing solution which you add to the other nitric solution and then add HCl to your powders and heat and add nitric in small increments only add more when all reaction ends , if after an addition of nitric you get no reaction then slowly add more HCl , if this causes more reaction then add another amount of HCl once all reaction stops , you can then continue adding nitric as before. Once you have dissolved all that you can allow to cool , filter until your solution is crystal clear and precipitate .
With your precipitate decant any solution off once you have tested it with stannous and add to your stockpot rinse with water until it seems to be clear , then cover with water and boil it , you may need to do this several times until the water is clear , once that’s happened cover the powder with HCl and again boil again you may need to do that several times until it’s colorless then rinse with water until clear and then add water and boil again again several cycles may be needed . This should clean your gold to a very acceptable degree , 999s is possible but you can always simply redissolve if you want higher purity .
 
Hello nickvc and thank you for your advice. You were the first response to my plea when I first joined the GRF and understand my situation.

Yes they are all filings, nice clean stuff with, sadly, a little platinum in there from before I set up a platinum only bench in the shop, maybe 2% of the total.

I would really like to avoid inquarting what would amount to about 7 pounds of silver to do this properly and if I could get three 9's in this manner it is the way for me to go. If the platinum cannot be removed and follows the gold I am hoping the XRF readings will be accurate enough to give me piece of mind.

Well...time for me to stop hoping and start doing. lol

All the Best, Tom
 
Tom I may be wrong but if you use ferrous sulphate to precipitate your gold you may get some platinum but if then you re dissolve and precipitate with ascorbic acid you should leave it in solution , if I’m wrong I’m sure someone will correct me :cool:
 
Many thanks Grains of Gold, yes the pgms will be going into the stock pot, just tiny amounts of iridium and palladium from the platinum alloys.

Can you verify or do you have insights on the procedure nickvc has suggested above using ferrous sulphate to precipitate first and then redissolve and using ascorbic acid for the second precipitate which will leave the platinum in solution. I am thinking I might have 60 grams of platinum in the total so it might be more like 4% of the total ratio to fine gold.

Thank you all for your help. Tom
 
I know ascorbic will precipitate Pd which you may have in the mix if you dealt with much white gold that’s why I suggested the ferrous first , some of our clever chemists may well know if I’m right or wrong :cool:
 
I am not sure how much palladium might be in there. The bulk of my work as a jeweler was my own traditional line of two tone jewelry, 18 karat yellow gold and the white was platinum unless I got a special order to use white gold which was almost never because I kept the price the same. I hated the working properties of white gold vs platinum. That said I probably did hundreds of mercy repairs on whatever over the years but not the thousands of repairs a retail jeweler would experience, so there is not a lot of other pgms present on a percentage basis. For a while, maybe 10 years ago, when the price of platinum was high there was a push to replace it with palladium, which was in the mid $300's per ounce at the time, and I did do some work in the metal but I kept the filings etc. separate.
 
Bench sweeps will always have some Pd if the jeweler was a busy one. Str. hno3 alone will dissolve Pd, and Ag.
 
Rhpos said:
Str. hno3 alone will dissolve Pd, and Ag.
Click to expand...
Actually you will be better served cutting your nitric in half with distilled water.

When I refined commercially the jewelers came to watch the melt and get a sample. So we had to melt them. I always had issues with the flux holding up values because jewelers use carborundum wheels that retained values in the slags. We had to melt with cryolite to break down the carborundum and it always took 2 melts. First to get the bulk of the metal and second to process the slags and pour into a cone mold. Then we melted both the bar and the cone together to sample for the customer.

If the bench sweeps are not pulverized after the incineration, the values retained in the insolubles can be considerable. Only the jeweler who generated the sweeps knows how many wheels are in the bench sweep.

If the jeweler was also a scrap buyer, we always encouraged them to bring us their sterling scrap with the bench sweeps so they would get paid for the silver and we would get a nice inquarted bar which made refining by parting before aqua regia easier. Back when there were a lot more jewelers in the jewelry district in Manhattan, we saw a lot of bench sweeps.
 

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