Removing stones

[semi-lucid]

That includes nitric oxide. The nitric fumes we have to scrub are far from scrubber friendly, the fumes generated by the Miller process are chloride fumes and despite how nasty chlorine is, they are a water soluble fume and easily scrubbed.

Metals

I thought the nitric fumes, NO₂ ?, could be absorbed into water?

john

 

[4metals]

There are multiple nitrogen compounds released in an acid digestion. Since they are formed by specific conditions which can vary in the course of a digestion they are referred to as NO_x. If you bubble the red fume through water at a pH of 12, it will surface as red as it went in. The most effective fume scrubbers break the fumes down to 1 species of nitrogen in one scrubber then pass through 1 or 2 more scrubbers to effectively scrub the fume. Most scrubbers in use today are single pass caustic scrubbers which the EPA will approve but they don't completely scrub the fume. If they clamp down on NO_x emissions, I for one will be searching for alternatives from multi unit scrubbing to peroxide additions to the working acids.

 

[semi-lucid]

Ok, From what I've read, (which is not that much, correct me where I'm wrong) the two species of NO_x formed are NO and NO₂. And I've read that NO reacts with atmospheric oxygen to give NO₂, and that nitric acid is produced by reacting nitrogen dioxide (NO₂) with water.

3 NO₂ + H₂O → 2 HNO₃ + NO

The residual NO would then react with O₂, and could be scrubbed a second time.

My source for all this is Wikipedia.

http://en.wikipedia.org/wiki/Nitric_acid

John

 

[4metals]

In a perfect world where other reactions are not going on all around the reaction you are describing where nitrogen peroxide and water react to form nitric acid and nitric oxide, the reaction would be simpler and more predictable. The nitric is actually a bit more involved in reacting with the hydrochloric acid to bump off a chlorine molecule which allows the acid to digest the gold. There is a bit of nitrosyl chloride action going on which decomposes in water and the soup of the reaction contains hydrogen, chlorine, oxygen all reacting and decomposing with the nitrogen compounds to form nitric oxide (NO) nitrous oxide (N₂O) nitric anhydride (N₂O₅) which decomposes back into nitric acid, nitric trioxide (NO₃), nitrogen dioxide (NO₂) and nitrous anhydride (N₂O₃). All of these gases are swapping oxygen and nitrogen as they decompose in water and reform different compounds depending on pH and different flow characteristics inside the scrubber. The potential different reactions taking place are so difficult to predict that it is easier to consider them transient forms all grouped together and called NO_x. In fact the name has such a neat ring to it our forum administrator adopted the name!

 

[Refiner232121]

Never in my more than 20 years of refining did I use urea. I evaporated routinely, using a button of added pure gold to consume traces of excess nitric. Because of my method, I had to evaporate, so adding the button was added insurance, and permitted evaporating for concentration instead of for expulsion of nitric.

Hi Harold
Can you explain how you did this

button of added pure gold to consume traces of excess nitric.

Thanks

 

[Harold_V]

Real simple. But first a comment. As GSP has alluded, time and again, the wise way to process is to use only the amount of nitric that is required. That can be time consuming, due to the small amount of gold remaining in often large volumes of solutions, with but traces of nitric present. Yes, his method works, but the time loss wasn't in the books for me. I prided myself on a fast turnaround for my customers, many of whom were small operators with little free gold at their disposal. A fast return was important to them. Therefore, I started using the added gold button to save extensive evaporation time.

So then, to use this process, dissolve the values, using the required amount of AR, and heating the solution. That way you are assured of the fastest possible dissolution. Do keep the vessel covered with a watch glass, so vapors condense and drip back into the solution.

When all of the values have been dissolved, temperature is lowered and the watch glass removed. A weighed button of gold is then added to the solution, along with some free HCl, which can react along with any unused nitric to dissolve the added gold button. By the time the solution has been evaporated to a dark red/orange color, any free nitric that may have been present will have been consumed. You can generally see small bubbles coming off the added gold, assuring you that it is working.

If, when you have the solution concentrated, you see no signs of activity at the added gold, it is a good idea to add a small amount of HCl once again. Should you have exhausted the previous addition before the nitric was consumed, this will serve two purposes. The addition of the HCl will help eliminate any nitric that may be present in the way of brown fumes. It will also react with the now concentrated solution to rapidly dissolve more of the gold button.

If, when the second addition of HCl yields no changes, no fumes, or no bubbling at the added gold button, you can safely consider the solution to be free of nitric, so it can then be diluted and filtered in preparation for precipitation.

Remember to dribble a few drops of sulfuric in your solutions when you start the evaporation process. That will serve to precipitate any traces of lead that may have traveled in tandem with your values.

Hope this helps.

Harold

 

[golddie]

Hi Harold
I had to read it a few times so that it would sink in.
I guess when you actually perform the experiment it will be clearer.
This may sound trivial but I am not sure about it
The gold button, what should be the size and also what karat.
Thanks

 

[Harold_V]

The added gold should be pure. That way you know what amount was added to a given lot, for the added gold is weighed after it has been recovered from the solution, subtracting the ending weight from the starting weight. The difference is the amount of gold added to the solution. It's important to do that if you want to know the yield of the lot in question.

As far as size is concerned, the larger the amount you add, the faster it will work. The best scenario is if you add a rolled piece, so it has a large surface area. That can be negative in that as it breaks down, it often does so by making a lot of small pieces, which can be difficult to recover. Instead, I simply added more buttons. I'd use the very small ones, remnants from previous use, first, so they would be totally consumed by the unused nitric, adding a large, new button when they were consumed.

A word of caution. Don't drop buttons in a beaker. They will almost always crack the beaker (don't ask!) :roll: . I used to slide them to the bottom, pinning them to the side of the beaker with a stirring rod.

Don't try to complicate this process. It's simple, and straight forward----you add a known amount of pure gold (that is sacrificial, but recovered when the solution is precipitated) to the solution ----used only to consume the free nitric. By weighing the gold before and after, you know how much you added to the solution. Simple as that.

Harold

edit: corrected typo

 

[golddie]

Hi Harold
I don't know if you have already mentioned this or not.
But I was wondering how you prepared aqua regia.
Also
I was wondering what you did for nitric acid.
Did you buy it ready or make it yourself
Thanks

 

[Harold_V]

When I was refining, buying chemicals wasn't much of an issue. I purchased all of my acids. Nitric, purchased in 55 gallon lots, cost just over $200 per drum. I owned my own stainless 55 gallon drum, so I exchanged for a full one.

The ratio for aqua regia is not commonly agreed upon. Many say 3 parts HCl, 1 part nitric. I used 4 parts HCl, 1 part nitric. I also used water, but that is an option you may or may not use. It should extend the use of the acid, although it will slow its pace a little.

My reasoning behind using a 4:1 mix?

Excess HCl does no harm. By providing a little excess, you insure consuming your nitric, instead of running short and assuming you have exhausted the nitric.

Mix AR only as it is needed. It gasses, with terribly corrosive vapors. If you stopper a vessel tightly, you risk breaking from internal pressure.

Harold

 

[golddie]

Since I am getting so much from this forum.
I might as well give you a trade secret
To remove stones you use carbide disc
It eats the beads and the claws and nothing happens to the diamond
If you have other stones that are softer than diamonds then they will get scathed.

 

[qst42know]

I have never done more than a few stones at a time but I have had good luck with the rod of an ice pick pulled from the handle and sharpened to a tiny jewelers sized chisel point. Sometimes a push is all that is needed or a light tap with a small hammer will cut the end of the stubborn prongs off. A wooden woodworkers clamp will hold all sorts of odd shaped pieces while you work on them. It goes much faster than it sounds.

 

[goldsilverpro]

A third generation jeweler friend once told me that, when removed, 90% of the diamonds under 1 carat, were damaged. I know zero about diamonds so, I'm just repeating his words.

 

[qst42know]

I suspect it is the skilled jeweler who hides cheap flawed diamonds beneath the prong heads. I would think any gold alloy too ductile to damage a diamond. Opposing prongs are merely squeezed tight enough to grip the stone by pliers, peening, or by drawing a burr.

Is a diamond really that brittle or was the stone already cracked or otherwise flawed?

 

[Harold_V]

A third generation jeweler friend once told me that, when removed, 90% of the diamonds under 1 carat, were damaged. I know zero about diamonds so, I'm just repeating his words.

I processed for a pawn shop that removed all substantial stones, but left anything smaller than roughly 20 points. I used to remove them regularly, and sold them for additional profit. Prong set diamonds are easy to remove, but bead set stones are much harder, due to the small exposed beads that hold the diamond. The use of an abrasive parting wheel and a flex shaft, working in a "glove box" to capture the diamonds and wasted values made removal easy. I made more money on the diamonds than I did on the refining. Diamonds so removed are not damaged. Stones of less value are, those that are not as hard as diamond.

I'll share something with you you may find hard to believe.

I screened polishing wastes after incineration, and recovered a fair number of diamonds in the process. What surprised me was when I ran the two 55 gallon drums of polishing wastes that I had accumulated. They were processed with AR for extraction, but I was aware that total extraction was not to be expected. I ran them in my agitation tank with cyanide, but in order to do so, they had to be run through the ball mill, where the pH was changed to basic. I introduced hydrated lime along with the wastes, shifting the pH to roughly 10. The material, upon being discharged from the ball mill, was classified by a fine screen. To my surprise, I recovered something like 100 small diamonds that had been through the ball mill---all without damage. Keep in mind, the ball mill had steel balls up to 2" diameter, and it ran at the proper speed, where the balls were dropped from the apex of the top curve, crashing down on the contents of the mill.

Diamonds are very tough and can take some serious abuse. None of the man made, or naturally occurring abrasives harm diamond in the short term. They are used routinely for dressing grinding wheels made of silicon carbide, which is harder than aluminum oxide by a long shot.

Harold

edit: corrected typo

 

[golddie]

Hi Folks
Thanks for your interesting thoughts
I have been a setter for a long time and I have removed many stones.
When a diamond is of good quality and the cut is of good proportion you can just poke a sharp object and give it a wack from the back of the piece and it will come off.
But if the stone is set very well and the claws are holding the stone well then doing this can break the stone.
After a bit of experience you get the feel of the trade and things move along rapidly

 

[4metals]

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