Non-Chemical Theorethical Process for Dental Lab Dust-Collector Material

[Scrapper-aw-]

I would like to hear some opinions on processing large quantities,(many pounds at a time), of powder, or grinds type material. This material is generated by dental technicians cutting, grinding and polishing metal substructures of dental restorations. The dust from these activities is most often collected by a dust collection system of some sort. This type of scrap can contain all sorts of material, organic and inorganic. It can also contain all types of metal; in particular the silver content averages about 5%. We get alot of this material. Our goal is to digest and extract Au, Ag, Pd, Pt separately.

We have been experimenting with some small batches of this material over the past several months. We never did any of the pre-washes as I have learned about by reading these forums. But that is something I will incorporate into the process from now on. At this point we handled the material as follows.

1. Digest in AR
2. Filter silver chloride-laden inert solid material, (such as silica, gypsum, dirt, and other insoluables), from the precious metal laden acid.
3a. Solids are set aside for future processing.
3b. Acid is then processed for Au, then Pt, then Pd; then it is stored for processing of trace elements and recycling.

The above method is the very abridged version and I can go into plenty of detail if needed, but my main questions are concerning the processing of the solids containing the silver chloride.

This week we were very successful in converting silver chloride into elemental silver using the water/sulfuric acid/iron-zinc-aluminum method. It is a set-it-and-forget-it type of operation that needs minimal monitoring. So I got to thinking about our problem of having silver chloride mixed with other insoluables and how to separate them from each other. I do have some of this material to experiment with, but will not move forward until all avenues of the best ways to handle it are explored.

My theory:

3a.1. Starting with the solids in 3a above, treat them in the same manor as the silver chloride above. This will turn all AgCl to Ag.
3a.2. Filter all solids including Ag from liquid and reserve dilute sulfuric acid for next batch.
3a.3. Place all solids in a nitric bath to disolve all Ag. ( I'm not sure of the nitric concentration necessary here.)
3a.4. Filter all solids, now free of Ag, from Ag laden nitric acid.
3a.5. Dry solids and test for remaining values and if clear neutralize and dispose.
3a.6. Then process acid to recover AgCl and convert as above.

I have thought about using nitric for the first digestion to remove all Ag first, but the Pd will digest as well. And with the processes for Pt and Pd so similar I wanted to keep those two together if possible until it's time for them to be separated.

Q1. When dealing with a powder-type material, do the same digestive properties apply in regards to AR or Inquartation methods? In other words, if a powder's assay shows it to be made up of more than 10% Ag, will the gold be inhibited from digestion as occurs with shot batches?

Q2. In 3a.1. can I in fact process that type of material in the water/sulfuric acid/iron-zinc-aluminum method safely?

Q3. If the answer to Q2 is yes, will processing the AgCl laden solids in the water/sulfuric acid/iron-zinc-aluminum method work with the other materials in the bath along with the AgCl or will they inhibit the conversion to Ag?

Q4. Are there any holes in my theory?

I welcome any and all critiques, suggestions and advice.

Scrap

 

[nickvc]

This material is basically a dental technicians sweep from what i can gather and its processing can be done in the same way as jewellers sweeps. Your first process should be incineration to remove as much of the organic materials as possible then it needs crushing( concrete mixer and steel balls ) and sieving to remove any large metallics and or material that hasnt been reduced.Hoke if i remember advises boiling the powder in water and caustic to release as much of the junk and leaving the values in a more condensed form,this can then be treated by whichever method you decide upon,but as its in very small particles it should be fine to go straight to AR. Harold V treated dental material and his advice would be well worth obtaining as to which way to proceed ,my refining was nearly all jewellery wastes where the PGMs were of insignificant amounts so my thoughts that AR would be fine might not be correct but im sure one of our resident experts will point you in the right direction if im wrong.

 

[Harold_V]

The suggestion to incinerate is dead on, as is the idea of classifying. Treat the material exactly as if you were handling floor sweeps. Large pieces may need inquartation. Unless you have a huge amount of material to handle, you should be able to screen the incinerated waste with classifying sieves, commonly found in the lab. A mortar and pestle will serve perfectly well to break up any material that clumps. The sieves in question are typically 8" in diameter, but there are larger sizes available. They're made of brass, with either brass or stainless screen. Select stainless, for longevity.

Once sieved, a magnet should be employed, with anything removed being placed in the stock pot. That way any values that hitch a ride will be recovered.

Boil the screened material in dilute nitric. That will remove silver and some of the base metals that may be present.

If you find there is a lot of non-metallic material included, a second incineration with a subsequent boil in HCl, along with a good rinse with (tap) water, is a good idea. That insures that the resulting solution will filter well.

Rinse well, then dissolve in AR. Selective precipitation is then used to recover the values. I prefer removing gold first, if for no other reason, it is easier to wash platinum and palladium from precipitated gold than it is to remove gold from the recovered platinum and palladium, each of which readily dissolve in water.

Because a great deal of the metal used in dentistry is not of value, I expect that the recovered values will not be pure. They may all need a second refining to improve quality. You should look upon the first process as a recovery effort, not a refining effort.

Harold

 

[Scrapper-aw-]

Thanks for the replies!

1. Digest in AR
2. Filter silver chloride-laden inert solid material, (such as silica, gypsum, dirt, and other insoluables), from the precious metal laden acid.
3a. Solids are set aside for future processing.
3b. Acid is then processed for Au, then Pt, then Pd; then it is stored for processing of trace elements and recycling.

The above method is the very abridged version and I can go into plenty of detail if needed, but my main questions are concerning the processing of the solids containing the silver chloride.

Maybe I should have worded this differently. I understand the handling of dental sweeps, and the importance of incineration and pretreatment of the material. Step # 1 above is only started after all of that work is finished. Up until last week I didn't know anything about converting AgCl to Ag. I very briefly outlined my procedure above, and I do mean briefly, because I wanted to focus my attention on solving a problem with AgCl being comingled with other undisolved non-metalic solids after an inital digestion in AR.

Harold, Is it your position that if I would incinerate, wash, and digest in dilute nitric that I won't need to convert any Agcl to Ag that is mixed in the leftover non-metalic solids because there won't be any in those solids?

Boil the screened material in dilute nitric. That will remove silver and some of the base metals that may be present.

Will it not also digest the Pd?

And what about the particle size and the Inquartation Vs. AR methods? If I were to take a batch of this sieved and incinerated powder material that is made up of 20% metal, with 20% of that metal being Ag and disolved it in AR would the silver chloride that would form prevent the small particles of Au from being disolved? Because that's what happens with shot all-metal batches.

For that matter what if I were to take some of our "shot", http://dl.dropbox.com/u/3358180/Our Shot 3.jpg , which could be an alloyed batch containing 50% Au, 7% Ag, 15% Pd and 1.5% Pt, and amounts of Cu, Ni, Si, Mo and other metals and digest it first in nitric. Would the small size of my shot or powder, whichever the case may be, allow for a complete digestion of the Ag as if it were a batch of larger sized iquarted shot?

Do the laws of inquartation pertain to powders as they pertain to larger size shot?

On Monday I will try an experiment of a small portion of our tiny shot and see hat happens in nitric.

Scrap :mrgreen:

 

[Palladium]

http://goldrefiningforum.com/~goldrefi/phpBB3/viewtopic.php?f=39&t=1253

 

[Scrapper-aw-]

http://goldrefiningforum.com/~goldrefi/phpBB3/viewtopic.php?f=39&t=1253

Thanks, Pd.....I read that entire thread before I started this one. And I started this one specifically because it did NOT answer the questions I have about converting Agcl that is mixed with other undisolved non-metalic solids after an inital digestion in AR. Each reply to my inital post seems to only address everything else I mentioned in that first post.

:?: Nobody can tell me if undisolved NON-METALIC solids would interfere with or inhibit in some way the conversion of AgCl to Ag using the water/sulfuric acid/iron method of conversion. :?:

Look, even after incineration, HCl wash and a digestion in AR with a powder type of material there will always be non-metalic undisolved solids, (such as gypsum, silica, aluminum oxide, glass beads--all used in the fabrication of dental prosthetics and jewelry), that filter out with whatever amount of Ag was in the lot, now converted to AgCl. I am looking for a way to recover that Ag, or a method of extracting the silver first by iteslf, not with the Pd.

Scrap :mrgreen:

 

[butcher]

ok I'll stick my foot in my mouth,
in my processing these would have been filtered out long ago, and would not be mixed with the silver chloride,
with a good inceneration.
gypsum being calcium sulfate, in an inceneration the SO2 would gas off removing the sulfate, so I would think the calcium would only use a small portion of your sulfuric acid (using the sulfuric and metal for silver conversion), silica I see should not screw things up to bad, and aluminum oxide would only be assisting somewhat in the conversion,
there may still be things mixed in I feel would be seperated in the melt,
If the volume of these were small compared to silver.

why not incenerate, rinse, dissolve silver and Pd in nitric filter, recover these from solution.
then insolubles to aqua regia, denox, some dilution filter, dilute well, get lil bit AgCl HCl if nessasary, filter recover gold decant recover platinum in solution,
the insolubles from filtrates I would suspect values.

I cant talk good with my foot here.

 

[qst42know]

Gypsum is soluble in hot dilute HCL.

Silica, aluminum oxide, and glass beads are fairly inert I don't suppose they would interfere with the conversion if you stirred adequately.

A surplus of the right flux and a cone mold might help to separate the insoluble crap from the silver.

Are you running a silver cell to finish purifying your silver?

 

[Scrapper-aw-]

I cant talk good with my foot here.

Nice Butcher! LOL

why not incenerate, rinse, dissolve silver and Pd in nitric filter, recover these from solution.
then insolubles to aqua regia, denox, some dilution filter, dilute well, get lil bit AgCl HCl if nessasary, filter recover gold decant recover platinum in solution,
the insolubles from filtrates I would suspect values.

Ok Butcher, if I do this, then what about the question about whether powders behave like shot in a digestive reaction?

And what about the particle size and the Inquartation Vs. AR methods? If I were to take a batch of this sieved and incinerated powder material that is made up of 20% metal, with 20% of that metal being Ag and disolved it in AR would the silver chloride that would form prevent the small particles of Au from being disolved? Because that's what happens with shot all-metal batches.

For that matter what if I were to take some of our "shot", http://dl.dropbox.com/u/3358180/Our%20Shot%203.jpg , which could be an alloyed batch containing 50% Au, 7% Ag, 15% Pd and 1.5% Pt, and amounts of Cu, Ni, Si, Mo and other metals and digest it first in nitric. Would the small size of my shot or powder, whichever the case may be, allow for a complete digestion of the Ag as if it were a batch of larger sized iquarted shot?

Do the laws of inquartation pertain to powders as they pertain to larger size shot?

:?: Are we going to have complete digestions of powdered metals and other material regardless of the quantity of silver in the lot? :?:

Scrap

 

[4metals]

If the silver chlorides are in the minority with other material in the sludge it may not reduce completely using the the method you mentioned as it requires the silver chloride be directly in contact with the aluminum or the iron to be reduced. If there is too much non silver chloride in the way, this necessary contact may not happen.

While it is interesting to discuss in house refining of this material, and as a chemist doing it in house has some appeal, I would suspect that from a business standpoint your best bet would be to burn crush and sift the material for sampling and ship it as a prepared sweep to a refiner who will process this material using a copper refining model. This process can usually be performed for a few percentage points per metal and considering the volume, your chemical usage will cost more. The key being your ability to ship a prepared homogeneous product which you have the ability to assay. (and you represent its sampling at the refiner)

Often the challenge we all enjoy of refining is trumped by more cost effective processing by refineries running precious metals as a by product. I think that is the case here.

 

[Harold_V]

Harold, Is it your position that if I would incinerate, wash, and digest in dilute nitric that I won't need to convert any Agcl to Ag that is mixed in the leftover non-metalic solids because there won't be any in those solids?

A great deal depends on the nature of the silver. If it's alloyed with other metals, it might be that the vast majority remains, so it might be a portion of the waste after processing with AR. However, by using the nitric preatreatment, coupled with a succeeding HCl wash, you would eliminate a great deal of the troublesome material, perhaps allowing you to process the resulting waste by furnace. That was the process I used, but it may not appeal to you.

Boil the screened material in dilute nitric. That will remove silver and some of the base metals that may be present.

Will it not also digest the Pd?

Depends. If it is alloyed with gold and is a low percentage of the alloy, some may dissolve, just like some of the silver, but even if you do, by recovering your silver from the operation by using copper to cement, you'd recover the palladium at the same time. Fact is, you'd likely dissolve traces of platinum, too. It would all be recovered in the silver cell, so it can be a good way to concentrate traces.

And what about the particle size and the Inquartation Vs. AR methods? If I were to take a batch of this sieved and incinerated powder material that is made up of 20% metal, with 20% of that metal being Ag and disolved it in AR would the silver chloride that would form prevent the small particles of Au from being disolved? Because that's what happens with shot all-metal batches.

Yes, of course it would. If you're not going to inquart, you must do as 4metals has advised, which is to selectively melt and shot your materials, keeping the silver content low. It's important that you avoid high concentrations of silver if you expect to be successful processing exclusively in AR.

For that matter what if I were to take some of our "shot", http://dl.dropbox.com/u/3358180/Our Shot 3.jpg , which could be an alloyed batch containing 50% Au, 7% Ag, 15% Pd and 1.5% Pt, and amounts of Cu, Ni, Si, Mo and other metals and digest it first in nitric. Would the small size of my shot or powder, whichever the case may be, allow for a complete digestion of the Ag as if it were a batch of larger sized iquarted shot?

Assuming the shot is small enough, you may enjoy success. I routinely processed bench sweeps by boiling in nitric, then directly to AR. The resulting gold was always of poor quality, but it was far easier to process that way than to melt the filings. While a generous amount of silver chloride resulted from the AR digest, indicating that the prolonged nitric boil did not penetrate the filings, I was also working with 14K and greater, so perhaps you have a chance that you'd enjoy better success. I wouldn't bet on it, however, especially considering dental gold alloys can be in excess of 16K.

Do the laws of inquartation pertain to powders as they pertain to larger size shot?

Not sure of your question, but if you intend to inquart fine particles, I would think that you'd enjoy the best results by melting with a good flux, then pouring to a cone mold. Inquart the resulting button, or, perhaps, convert it to the shot you create. Inquarting directly would be somewhat troublesome, I would think, and couldn't be accomplished without fluxing. On the other hand, if you know the ratios, you could rely on the added silver to collect the values. I can see both methods working, but in both cases pouring to a cone mold would be required, to eliminate the flux.

On Monday I will try an experiment of a small portion of our tiny shot and see hat happens in nitric.

Frankly, I can think of no smarter thing to do. What worked for me may not work for you. How you handle your materials may be totally different from the methods I chose. Bottom line here is you should pay close attention to anything offered by 4metals.

Harold

 

[Scrapper-aw-]

Thank you alll for the feedback!

Bottom line here is you should pay close attention to anything offered by 4metals.

Harold

I am paying close attention to all of you! And I do appreciate all of your help.

I have much food for thought right now, and it's time to go to work. I'll reply later today with an update on how my test goes.

Off to work.

Scrap

 

[Oz]

Something I have not seen addressed here considering we are talking about dental sweeps is mercury. I understand that incineration is being practiced in this example but that could be missed by others, and I have not heard the retort word or of other reclamation. Mercury will go into nitric as well as cement on copper along with Pms. I am unsure what will happen with NaOH and Karo or the sulfuric and iron-zinc-aluminum if mercury is present.

A personal curiosity I have is what happens with a lead chloride/silver chloride mix if one uses the NaOH/Karo or sulfuric/iron conversion methods for the silver. What would occur with the lead chloride?

 

[Lou]

If I help you, can I have more details about your atomizer? It's arguably the coolest thing I've seen on this forum and frankly, I think you are my new best friend. I can get small BBs, but not as small as that!! How does it scoop and flow? Is it easily poured from container to container? I know it'll melt a treat in a high frequency induction furnace like most jewelers use...

So, you said that you have Pt, Pd, Ag, Au and probably copper and other base metals. You also mentioned nonmetallics, most likely SiO2 and Al2O3 used in grinding.

This type of material necessitates pretreatment. After pretreatment, you can process it any old way.

Presuming you don't separate the silver electrolytically (take all of this material and add it to some of your silver and run it through a Thum), you should...

1.) Incinerate.

2.) In a stainless stockpot, boil the solution in a concentrated sodium hydroxide solution (50% by weight). This will eliminate nonmetallics like glass/sand/alumina/aluminum/zinc/magnesium etc. Depending on size, this may take two or three hours (usually less). After cooling, dilute by 1/2, and decant the solution. Rinse with more base solution, decant, and repeat. It may be slightly blue in color from dissolving copper oxide. Then pull a small sample out in a Pyrex test tube and add 6M sulfuric acid to it (it will become very hot) until acidic. Hold this up to the light and look for any flocculation--if there is any, that means your last wash has not removed all the gangue.

3.) Boil in 40% nitric acid to remove the silver and palladium. With the cooled solution, precipitate the silver with conc. HCl, adding just enough to precipitate the silver. After that, filter the AgCl off and rinse with 1M HCl until the filtrate runs clear and is no longer yellow (this removes any Pd solution entrained in or stuck on the silver precipitate). The silver chloride should be boiled with hot water and the tested with KI or Na2CrO4 to see if lead is present (lead (II) iodide is canary yellow, lead chromate is orange-yellow; both are sensitive tests). After it is rid of lead or determined to be lead free, you can use any old process on the silver chloride. Remember not to let it dry out!

4.) The palladium solution should be a dark yellow bordering on brown. Boil this down with additional HCl to remove the nitric acid. A rotary evaporator is a great investment here. Once you have H2PdCl6 as a dark brown, heavy syrup, add a saturated solution of ammonium chloride (with a generous table spoon or so of sodium chlorate per 3L, or a cup of sodium hypochlorite commercial bleach, but this is not as good because it is heat sensitive). Do the precipitation hot, then put it all into an ice bath. It'll flocculate and the ammonium hexachloropalladate precipitate will settle as a brick red powder that filter easily. You may rinse once with cold, saturated ammonium chloride solution to remove the supernatant. Test the supernatant with stannous chloride or DMG to ensure that it is Pd-free (I presume you know how this works). The ammonium hexachloropalladate may be dissolved in concentrated ammonia to yielded a yellow solution, which can then be precipitated upon acidification (use HCl) to get additional purity, or it may be reduced right then and there to sponge with hydrazine sulfate (heat to 80*C).

5.) Return to your now Pd and Ag free starting material. What is left is probably gold, platinum, and perhaps a few other noble metals (or things like antimony). I would dissolve the gold in cold cyanide solution, 2% bleach rinse, and then go after the Pt with aqua regia. If BBC (base buffered cyanide) isn't an option or it's Friday and you just don't want to bother, go with aqua regia. Residue is ruthenium (be aware that hypochlorite bleach dissolves it). Follow standard procedure, and drop the gold first (SO2 works, but oxalic is better at avoiding PGM contamination), then go after the Pt and chase the canary.

 

[Harold_V]

Something I have not seen addressed here considering we are talking about dental sweeps is mercury. I understand that incineration is being practiced in this example but that could be missed by others, and I have not heard the retort word or of other reclamation. Mercury will go into nitric as well as cement on copper along with Pms. I am unsure what will happen with NaOH and Karo or the sulfuric and iron-zinc-aluminum if mercury is present.

The source of the material is from a dental lab, not from a dentist's office. I expect there would be no mercury present.

Harold

 

[lazersteve]

Lou,

Very excellent post, as usual!

Scrap,

The silver chloride can be removed with several hot ammonium hydroxide extractions plus stirring, filtered, and the dissolved silver chloirde precipitated with HCl.

Lou covers everything else very eloquently.

Steve

 

[Scrapper-aw-]

If I help you, can I have more details about your atomizer? It's arguably the coolest thing I've seen on this forum and frankly, I think you are my new best friend. I can get small BBs, but not as small as that!! How does it scoop and flow? Is it easily poured from container to container?

Lou, Buddy, how you doin? :mrgreen: How does it scoop???......If it is allowed to dry.?.?..like sugar... :mrgreen: ......How does it flow????.......like sugar.....It's very nice stuff to work with.

Thanks for the help and the walk through. I'll get you some info on my little doughnut.

Here are a couple of experiments that we experienced today at our lab.

First, the comingled insolubles and AgCl that were the catalist for this thread were experimented with today. I took a 5 gram sample of this material and put it in a small graduated beaker with about 80ml of distilled water and sulfuric, 10:1 ratio and added a few corroded nails and a small piece of zinc. We stirred, and waited, and stirred and waited. We started this at about 8:00AM and filtered it at about 3:00 PM. By that time the liquid looked very similar as to when we are converting just AgCl, it was clearing with a dull blue-green-grey color.

Next we decanted and rinsed with distilled water 3 times reserving the liquid to do our next conversion. The supposed Ag mixed with the insolubles were now subjected to 40ml of 70% nitric. It began to fume immedietly, warmed slightly and then calmed after about 3 minutes. We let it rest for another five minutes and then filtered it. Rinsed with distilled water until the rinse ran clear and then some. The liquid from that filtration is shown first .

http://dl.dropbox.com/u/3358180/Nitric from AgCl comingled with insolubles experiment.jpg

Next is the insolubles that were caught on the filter. The X-ray gun was put away for the day or I would have preliminary scan results on this material.

http://dl.dropbox.com/u/3358180/insolubles from comingled experiment.jpg

Then the reaction as we added a little salt to the nitric.

http://dl.dropbox.com/u/3358180/AgCl droping out of solution.jpg

So, after we do some testing on the solids above we'll know if it's cleared of all values or not. And if it is then I am confident to try the same experiment on a much larger scale. We were obviously able to convert some AgCl to Ag, we just need to know if we can recover it all.

The other question I had and experiment we tried today had to do with the behavior of our "shot" in a pretreatment of nitric acid to see how much Ag would digest. I easily scooped up 30 grams of shot .... .... and covered it with 200ml of 70% nitric in a 4l beaker and boiled it for 1 hour. This is the nitric after the hour.

http://dl.dropbox.com/u/3358180/Nitric after 1 hr boil n filter.jpg

Here is the shot during the third rinse with distilled water.

http://dl.dropbox.com/u/3358180/30 grams our shot.jpg

And we all know what this is.

http://dl.dropbox.com/u/3358180/AgCl.jpg

Now for the bad news.....the nitric did not disolve all the silver. When I put AR, 4:1, on top of the gold it digested that 30 grams in about 15 minutes. But it also converted a substantial amount of silver to AgCl. :roll:

 

[Harold_V]

Now for the bad news.....the nitric did not disolve all the silver. When I put AR, 4:1, on top of the gold it digested that 30 grams in about 15 minutes. But it also converted a substantial amount of silver to AgCl. :roll:

That parallels what I experienced with filings. I also noted that the nitric solution appears to be devoid of palladium, a very good sign you are not penetrating the shot, in spite of it being very fine in nature.

You're going to have to get the silver and base metal content high enough to allow full penetration of the nitric. That's what proper inquarting does. Otherwise you're restricted to processing with AR, and controlling the percentage of silver, using selective melting, and dealing with the silver chloride.

Harold

 

[nickvc]

Scrap just a thought if you are going to inquart why not use copper, as your going to treat your own solutions to allow proper disposal you must have some in your solutions already ,so it will be enviromentally friendly, and reduce the amount of AgCl to be dealt with as you can cement the silver out of your nitric,the only problem is that your Pd will be in nitric solutions also,Harold V to the rescue here or one of our cleverer members as ive said before PGMs were never my speciality and i might be wrong in my thinking(not unusual) :lol:

 

[Harold_V]

Scrap just a thought if you are going to inquart why not use copper, as your going to treat your own solutions to allow proper disposal you must have some in your solutions already ,so it will be enviromentally friendly, and reduce the amount of AgCl to be dealt with as you can cement the silver out of your nitric,the only problem is that your Pd will be in nitric solutions also

That wouldn't be a problem, for it could be recovered by cementing with copper, but I'd give considerable thought to the idea of NOT using copper for inquartation, unless I had a ready source of inexpensive nitric. The consumption of nitric is so great with copper that it makes it a poor choice. That is especially true if silver is handled routinely, where its use in inquartation is the first step towards parting in the silver cell. Further, by inquarting with silver, both platinum and palladium would be largely removed from the metal upon dissolution, concentrating it with the silver. It would be recovered in the silver cell. By removing them from the parent metal, the resulting gold would be better in quality.

While major refiners tend to not inquart, it's the magic tool for small refiners. It solves a lot of problems, although, admittedly, by creating a few others.

Harold