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Non-Chemical Theorethical Process for Dental Lab Dust-Collector Material
- Thread starter Scrapper-aw-
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Scrapper-aw-
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- Dec 12, 2009
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- 34
Harold_V said:
I am coming to the conclusion that we will probably NOT use the inquatration method for our normal process. After the experiments I conducted yesterday I believe it would be less labor intensive and use less materials to go right into AR with all our material. The preliminary assay on the grey solids after trying to convert AgCl to Ag that has been comingled in the solids shows that it was under 1% Ag from a starting sample of 5 grams. We did not re-convert the AgCl from that filtration as we should have. We will conduct another experiment to determine the conversion rate of Agcl to Ag when in a comingled slurry. If this is a complete conversion then it could be a viable method for us to use. The liquid in this method is reusable.
Shot, even as small as ours is, in hot nitric does not digest all silver. I have to do an experiment with a powder lot to see if there is any difference. My gut is telling me the results will be the same as with the shot.
Lou said:
Ok Lou, in your second step you talk about the caustic boil to rid all the non metalics. But after experimenting with our AgCl polluted solids that are fiiltered after AR digestion, I am inclined to recover the Ag in that manor and refine further as necessary. And here's my reasoning: By following steps 2 and 3 in your outline we will be producing about 3 times as much waste liquid. And 1/3 of it will be a caustic waste, (unless there's something else we can do with this waste?). I'm not saying that your methods won't work, I'm sure they will, but I am looking at the most cost effective operation possible over all. It may be cheaper and less labor intensive to use the method above to recover the Ag.
Still thinking.....can't you smell the smoke?
Scrap :mrgreen:
Sir,
You only think you'll be producing more liquid. Only step 2 necessitates an actual increase in volume of solution, and it's really quite trivial.
The caustic boil is the most effective way to remove nonmetallics, particularly sand. You can even skip the incineration since concentrated lye will dissolve damn near any organic grease or contaminant, including paper, wood, and other cellulosic materials. I should also say, you do not throw away this solution---store it in a 5 gallon polypropylene carboy (don't store it in glass, it'll etch the glass) to protect it from atmospheric CO2 for it can be reused several times. When you notice a drop in its performance (indicative of more washes needed to pass the hydrolysis test), you can neutralize some acid waste with it, whereupon all the silicate and aluminate present will drop out as alumina and silicon dioxide--these are non hazmat wastes, and aren't readily soluble in acids, save HF, straight to the landfill. Base can always go down the drain as can most base-soluble metals (i.e. aluminum and zinc, but they won't be present because they already precipitated when you used the base solution to neutralize some acid waste).
Step 3 is the proper way to do it and get the highest purity of silver out of the gate, and it also ensures complete recovery of palladium. Something I forgot to mention is that you should do a platinum check (either stannous or ICP) after you've precipitated out the palladium. Occasionally Pt will codissolve with silver and palladium in nitric and it's good practice.
If you're worried about solution volumes, I can tell you there are processes that can do what you want and do not involve any aqueous chemistry, thus making trivially small amounts of concentrated waste, or just solid waste.
While I'm giving hints, if you're annoyed with silver chloride in your aqua regia dissolution, saturate your aqua regia with KCl (it's the cheapest chloride salt to work very well, although ammonium chloride works too but only after the nitric is gone, otherwise your NOCl species oxidizes the ammonium cation). This is an old and common trick for the analytical chemist.The KCl complexes the silver chloride to AgCl2- and basically means that your already small material will dissolve even quicker. This silver is present in solution and you can precipitate and filter out your gold, and then dilute with cold water and the silver chloride will precipitate. This works well on silver alloys that have too little silver to do a nitric parting, but too much silver to effectively/quickly dissolve in aqua regia. One caveat, this is NOT a good method if you have platinum present--as you're reducing the gold from Au(III)-->Au(I), some platinum is going from (IV)-->(II). The problem with that? Well, that pretty red K2PtCl4 is made from KCl, and H2PtCl6 in the presence of a reducing agent (like sulfites, or borohydride, or formate, etc.) it is a weakly soluble salt so it precipitates...right into your gold. Hot HCl can rinse it out, but why end up in that position? K2PtCl4 is a good way to gravimetrically determine platinum though!!
Adding KCl to hot HCl is also a good way to rinse out silver chloride, lead chloride, copper, and cobalt chloride (most transition metals). To protect your gold from loss due to volatilization from residual salt (in case you didn't know, NEVER strongly heat metallic gold, or any PGM with a halide salt), add ammonia till basic, then you can heat because all of the chloride will leave as ammonium chloride early on in the crucible.
Lou
You only think you'll be producing more liquid. Only step 2 necessitates an actual increase in volume of solution, and it's really quite trivial.
The caustic boil is the most effective way to remove nonmetallics, particularly sand. You can even skip the incineration since concentrated lye will dissolve damn near any organic grease or contaminant, including paper, wood, and other cellulosic materials. I should also say, you do not throw away this solution---store it in a 5 gallon polypropylene carboy (don't store it in glass, it'll etch the glass) to protect it from atmospheric CO2 for it can be reused several times. When you notice a drop in its performance (indicative of more washes needed to pass the hydrolysis test), you can neutralize some acid waste with it, whereupon all the silicate and aluminate present will drop out as alumina and silicon dioxide--these are non hazmat wastes, and aren't readily soluble in acids, save HF, straight to the landfill. Base can always go down the drain as can most base-soluble metals (i.e. aluminum and zinc, but they won't be present because they already precipitated when you used the base solution to neutralize some acid waste).
Step 3 is the proper way to do it and get the highest purity of silver out of the gate, and it also ensures complete recovery of palladium. Something I forgot to mention is that you should do a platinum check (either stannous or ICP) after you've precipitated out the palladium. Occasionally Pt will codissolve with silver and palladium in nitric and it's good practice.
If you're worried about solution volumes, I can tell you there are processes that can do what you want and do not involve any aqueous chemistry, thus making trivially small amounts of concentrated waste, or just solid waste.
While I'm giving hints, if you're annoyed with silver chloride in your aqua regia dissolution, saturate your aqua regia with KCl (it's the cheapest chloride salt to work very well, although ammonium chloride works too but only after the nitric is gone, otherwise your NOCl species oxidizes the ammonium cation). This is an old and common trick for the analytical chemist.The KCl complexes the silver chloride to AgCl2- and basically means that your already small material will dissolve even quicker. This silver is present in solution and you can precipitate and filter out your gold, and then dilute with cold water and the silver chloride will precipitate. This works well on silver alloys that have too little silver to do a nitric parting, but too much silver to effectively/quickly dissolve in aqua regia. One caveat, this is NOT a good method if you have platinum present--as you're reducing the gold from Au(III)-->Au(I), some platinum is going from (IV)-->(II). The problem with that? Well, that pretty red K2PtCl4 is made from KCl, and H2PtCl6 in the presence of a reducing agent (like sulfites, or borohydride, or formate, etc.) it is a weakly soluble salt so it precipitates...right into your gold. Hot HCl can rinse it out, but why end up in that position? K2PtCl4 is a good way to gravimetrically determine platinum though!!
Adding KCl to hot HCl is also a good way to rinse out silver chloride, lead chloride, copper, and cobalt chloride (most transition metals). To protect your gold from loss due to volatilization from residual salt (in case you didn't know, NEVER strongly heat metallic gold, or any PGM with a halide salt), add ammonia till basic, then you can heat because all of the chloride will leave as ammonium chloride early on in the crucible.
Lou
The saturating the aqua regia with KCl is much more effective with heat. If you still have insoluble silver chloride when using the KCl saturated AR, crank it up to a near boil. Much more silver chloride dissolves when hot than cold, or even warm for that matter.
qst42know
Well-known member
I take it this technique makes AR more effective on karat gold alloys containing silver.
What are the limits of it's use?
Will this permit direct digestion of 18k in AR known to be high in silver?
I've never successfully had it work on 18K gold or green gold. Although the patent claims it will, the procedure is for assaying in field labs where cupellation and inquartation are not practical. I believe it was used to assay gold silver alloys from Inca archeological sites in the field. It does have a noticeable effect on karat gold with normal 8% silver but aqua regia alone is very effective alone and any gold trapped in the chlorides is easily assayed after the chloride is reduced and melted into a bar.
If it would work on 18K it would be sweet, I've never seen any benefit.
If it would work on 18K it would be sweet, I've never seen any benefit.
