FINISHED.
I started writing this before Harold's excellent post. In refining, for each type of material, there are probably at least 6 different methods of doing things. These are good, bad, or stupid.
Please note that both Harold's way and my way have been proven to work. It is best to learn every way of doing something and, then, you can think about what is the best way for you, based on the equipment you have, turnaround time, cost, profits, efficiency, safety, etc. I hate to admit this but, if I ever refine karat gold or platinum group from jewelry or dental, again, I will adopt some of Harold's techniques and incorporate them with mine. I only hope that he has learned some tricks from me, on things like the sulfuric cell.
Did you use real nitric or sodium nitrate?
There is always platinum (from .5% to 8%) and palladium (from .5% to 2%) in dental gold. The amateur looks at the high numbers. The pro looks at the low numbers.
The Main Problem -Silver
I thought all nitric was gone as there were still undissolved appliances at the bottom.
The silver in dental alloys can be quite high (3% to 24% - the average is about 12%). If the silver percentage is over 10%, it becomes very difficult to dissolve all the gold in AR. Silver chloride forms and it coats the gold. This is probably the gray color you now see on some of the metal. The silver chloride stops the action and that's why you thought there was no nitric left. From here, everything snowballed. It takes both hydrochloric and nitric to dissolve gold. If EITHER acid has been used up, the action will stop. So, excess metal on the bottom does not necessarily mean that all the nitric is gone. It could mean that the hydrochloric is all gone and there's still excess nitric.
You should of inquarted, mainly because you must deal with the very possible high silver content.
Thinking there might be some PL, I added some Sodium Chlorate. It boiled over into my heating element, so I unplugged it and dumped the solution back into my big solution.
I added a ton of SMB (I guess the nitric) was not all gone.
I sprinkled ammonium chloride into the solution moving very slowly through the filters and it turned red.
It was a mistake to add any of these chemicals before taking care of the nitric. However, it is less of a problem than you might think.
How I would have done yellow Dental Gold. I am using real nitric acid.
All of the techniques not covered in detail are covered in detail elsewhere on this forum.
INQUART
(1) Weigh the metal
(2) Inquart with silver or copper. First, though, calculate the copper or silver you need to end up with 25% gold. Make sure the molten alloy is well mixed before shotting. Stir with a carbon or quartz stirring rod (jewelry supplies).
DISSOLVE THE COPPER, SILVER, ETC. IN NITRIC
(3) Estimate the amount of nitric nedded to dissolve the Ag. One gram of copper needs about 4.2 mLs of concentrated nitric. A gram of silver will need 1.2 mLs.
(4) Put the metal into the Pyrex dissolving container. You could use plastic and no heat but, it's slower. For small amounts, I prefer glass beakers so, I can heat them.
(5) Cover the metal with distilled water, plus about 1/4".
(6) Heat the solution until you first see whisps of steam coming off. I like the hot plate about medium. Medium low for very small amounts
(7) Measure out the calculated total amount nitric you'll need.
(8 ) Dissolve the Ag, Cu, etc., with nitric. Add about 5 mL of straight nitric. You will immediately get action. It will shortly (few minutes) slow down. Give it a gentle but complete stir. It will flare up a little bit. Let it relax. Add some more nitric. When it's finished, add some more. Repeat until all the the Ag, etc., is dissolved and you only have gold powder. Test this by dragging the powder to the side of the beaker and looking at it. You can, also, mash around with a stir rod, to check for lumps. You may need about 10%, or so, extra nitric. Try not to use too much. This whole thing should take 15 to 30 minutes. The reason to start with only 5 mL, is to prevent it from foaming over. The solution will get quite hot from the dissolving and, you have to be careful of instant foamovers. Towards the end, you can progressively add more nitric at one time. But, never add more until the last addition has died down.
A good reason for working with heat is to eliminate foamovers. When you add a little nitric to a cold solution, it, at first, won't do much. You add a little more and, maybe, a little more. As you've been doing this, the dissolving action is heating the solution up, slowly. When it reaches a critical temp., it suddenly all works at the same time and, boils the solution over. Don't ask me how I know this. Most boilovers are caused by working with cold solutions or, by dumping all the acid in at the same time.
You will probably need more water, as you go, to keep the metal salts from crystallizing. At the end, you want to end up with 50% water. Stir.
(9) Allow to cool and settle over overnight. I like to prop a piece of wood under it, so the solids settle to one corner.
Note: Summary of what we have to this point. It's good to figure out what is where, no matter what you're working on..
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(a) In the solution, we have all of the silver, palladium, and all the base metals. If we used silver to inquart, instead of copper, we will also have some platinum in the solution.
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(b) In the solids, sitting in the solution, there is most or all of the platinum and all of the gold and, some dirt.
(10) Separate the solids from the liquid. You can filter and rinse it or you can pour off the solution with several rinses - pour off - rinse - settle - pour off - repeat. For the best chance at good final purity, filter and rinse well.
DISSOLVE THE GOLD AND PLATINUM IN AQUA REGIA
(11) Put clean solids in beaker.
[/b](12) Cover solids with the muriatic or hydrochloric.[/b] You should calculate how much HCl you need and add about 20%. For one ounce of gold, figure about 120 mL of HCl. It's very important to have extra HCl. But, it's wasteful and more cumbersome to add way too much.
(13) Heat on medium until you see whisps of steam
(14) Calculate and measure out how much nitric you'll need Figure about 20-25 mL, per ounce of gold. Measure on the low side. The goal is to never add any more nitric than is necessary to dissolve the gold and platinum powder.
(15) Add not over 5 mL of nitric Action will start immediately. When it dies down, stir it and add some more nitric.
(16) Repeat adding nitric until the gold/platinum powder has dissolved. Add - wait until reaction slows - stir - repeat. This may or may not take all the nitric. Especially towards the end, allow the action to completely stop before adding more nitric.
Note: The magic of adding a little too much HCl and just enough HNO3 is that, when you finish, there will be none, or very little, extra nitric in the solution. Thus, boiling down or, adding urea, isn't necessary. The first addition of SMB or FeSO4 will start dropping gold!!! If, you used too much nitric, use urea, that you've dissolved in water, until you get a pH of one.
IMPORTANT: Gold in powder dissolves rapidly. If you add too much nitric, the solution can foam over, quicker than you can say Jack Spratt.
The depth of the solution will make a difference in the speed of the dissolving. The deeper it is, the slower things go. The acid is way up here and the solids are way down there. For maximum speed, use a container that will keep the acid under about 3" deep.
To kill or slow down foamovers, buy a small container of Anti-Foam located on the same rack as carpet cleaner rentals in the supermarket. Add 1" of the concentrate to a spray bottle and fill it up with water. Spray a little on the climbing foam. Often, it stops the foam. Sometimes, especially when real hot, it only slows it down. Believe it or not, just water in a misty spray bottle does a pretty good job on foam. Another weird way to to put a little oil on you fingertip and wipe it around the top edge on the dissolving container. I don't like oil in the solution. It tracks over to the next processes. I only use it rarely with large amounts of acid when no values are in the solution.
(17) Heat more strongly at the end but, don't boil. Platinum dissolves a little slow and needs some heat.
(18 ) Check to see if everything's dissolved. Tilt the beaker, stir the solution, let it settle a bit, and, using good light, look at the settled solids, if there are any. If the container is small, pick it up and look at the bottom.
(19) If needed, add more nitric. Heat it up and add only 2 or 3 mLs at a time. Let it settle totally down and stir before adding. Keep checking for solids. Don't add too much.
FILTER AND RINSE
(20) Add 3 times water and let it cool and let the dirt settle.
(21) Filter Carefully pour the solution into the filter. Keep the solids in the beaker. When all the solution has drained through, rinse the solids (dirt) into the same filter. If things slow down, get a fresh filter.
(22) Rinse the filter well.
DROP THE GOLD
(23) Choose a precipitant. There's a long list of things you could use: Ferrous Sulfate; SO2 gas; Sodium Sulfite; Sodium Metabisulfite; Sodium Bisulfite; Oxalic acid; Sodium Nitrite; Hydroquinone; Metol; Etc. The last 4 are only used in special situations. For this job, I should (but, probably wouldn't) first use green Ferrous Sulfate (FeSO4 - Copperas). It will drop the gold, without dropping the platinum group, better than any other precipitant. I could use a sulfite first and then melt, shot, re-dissolve, and final drop with ferrous sulfate.
(24)Drop the gold To drop with one of the 3 sulfites, put a couple of spoonfuls in a one cup Pyrex measuring cup. Fill with hot water and stir until most is dissolved. Pour some of the liquid into the solution, let it react, and stir. If all of the nitric is gone, you should see brown powder forming in the solution. If there is still nitric, the sulfite will react with it before you start dropping gold. Sometimes, there is a false indication of gold dropping. The gold drops locally, where you added the sulfite but, when you stir it, the gold re-dissolves.
At some point, all the gold has dropped out. It shouldn't take much sulfite, if all the nitric was gone. Also, with no nitric, you should get no boilovers. Maybe, use about ounce for ounce, with gold or, a little more. There are some visual indications of this. Without the presence of platinum group, the foam (when stirring) will get white and the yellow color will disappear. With platinum, whose solution is also yellow, this is harder to see. Test the solution, with stannous chloride, for the presence of gold. Add a little more sulfite, stir, and test. Repeat until the gold is out.
(25) Filter, leach and rinse the gold. This has been a controversial subject between Harold and I. I would probably settle first, then, - (a) Filter the tops - (b) Filter the gold in the same filter - (c) Rinse 5 times with very hot water - (d) Remove and boil in HCl - (e) Filter and hot rinse 5 times - (f) Check for the presence of chlorides coming out of the filter, with a drop of silver nitrate made from a little silver dissolved in a little nitric/distilled water - (g) Rinse until no more chlorides - (h) Mix up enough 25% nitric, diluted with hot water, to pretty well fill up the filter. With a vacuum filter, shut it off and bleed the vacuum, first. The slower it drains through, the better. - (i) When it is drained, rinse 5 times with hot distilled water. - (j) Add 50/50 household ammonia, cut with hot water, and put it in the filter. When it has drained through, rinse 5 times with hot water. (k) Rinse 5 times with hot distilled water.
(26) Dry the gold and melt it. Look for instructions on the forum. Note: if the gold powder looks grayish, it has platinum in it.
(27) Rework dirty gold If the melt, the ingot, or the powder looks dirty, it is dirty. Pt makes dirty looking powder and ingots. Sometimes, the Pt.Gp. produces a fern-like pattern on the gold ingot. Always melt and shot the powder before re-dissolving in aqua regia. Use a different precipitant. Learn to use ferrous sulfate.
GRAND SUMMARY
---In the
filter papers, there is likely some precious metals. If you can, keep the gold, silver, and Pt group all separate. Even if the platinum group filters have gold, keep them in the Pt. group pile. You can burn the filters or, you can add some of the best aqua regia filters to the next batch, paper and all. Don't add too many papers, unless you have a lot of solution. The hot acid pulps the paper and aids in filtering. Commercial paper pulp can be purchased for use as a lab filter-aid.
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The spent aqua regia solution. It contains platinum, hydrochloric acid, nitrates, chlorides, sodium, and sulfite. You could drop the platinum out as a black powder with copper buss bars (A trick. If it isn't cementing onto the copper, add a small shot of nitric and stir it in). Or, you could put it into Hoke's stockpot.
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The nitric solution It contains silver, palladium, some of the platinum, copper, zinc, and nitrates. How do you separate the silver from the platinum and palladium? Good question. You could drop the silver, Pt, and Pd, all together, using copper bussbar. You could then melt and run the silver through a silver cell. The platinum doesn't dissolve and stays in the cell's filter cloth. The palladium dissolves and eventually contaminates the silver cell solution. Another way would be to try and drop out the Pt.Gp., selectively, using something like hydrazine sulfate at a certain pH. Maybe, sodium borohydride would drop the silver without dropping the Pt and Pd. Anyone have any other ideas?