Here's a process I have learned and modified to get the best results for denoxing AR solutions.
The process is very cheap to operate and once learned and applied correctly it will remove the hassle of denoxing, and dramatically speed up the process of refining gold.
Caveats:
1. I am assuming that all normal safety procedures are both known and followed.
2. I am assuming that the reader has a basic but working knowledge of refining gold using AR.
3. I am assuming that the reader has all the relevant equipment to carry this out.
Basic theory:
Excess Nitric acid in a AR solution that is pregnant with AuCl will prevent the gold being precipitated using your preferred method of choice (SMB, ferrous II Sulphate etc.) The plan is to remove this excess Nitric leaving a solution that is ready to cool, filter, and process in the normal way in order to add your precipitant of choice.
Effectively the use of Sulphamic acid (I am British so I spell it the British way- Americans then to know it as Sulfamic Acid) does this by converting the Nitric acid to Sulphuric Acid, Nitrous Oxide, and water.
The chemical reaction is: HNO3 + H3NSO3 → H2SO4 + N2O + H2O
That's great to know, however let's concentrate on the practical application of the process because it also has some other benefits to the process other than the removal of Nitric, but I will cover those later.
Preparation of Sulphamic Acid solution.
Prepare your Sulphamic acid as follows:
You will need a 1 litre glass beaker, a hot plate, a plastic spoon, an inert stirring rod, water, and a tub of Sulphamic acid crystals.
Add 150ml of water to the beaker, and place on the hot plate. You want this solution to be warm to hot, not boiling.
Add a couple of teaspoons of the crystals to the water, stir and let it dissolve- this will happen relatively quickly to start with.
Keep repeating this process until your solution leaves some crystals in the bottom even after plenty of stirring. Your solution is now close to being saturated. It doesn't HAVE to be this strong but experience will help you learn the required amounts.
In order to learn this properly without any risk of losing gold, I would suggest that you prepare a dummy solution of AR. Remember the purpose of the tutorial is to help you learn to denox a solution so why risk your money if it can be learned without financial risk?
Preparation of "dummy" AR.
Put 100ml of HCl in a 1 litre beaker.
Add 100ml to 150ml of water - tap water is absolutely fine for this.
Add 3ml of Nitric acid (70%) or 6ml of Nitric acid (35%) or the equivalent of "poor man's Nitric."
Put this on a hotplate (WITH A CATCH PAN) and warm up. Again you want a warm to hot solution - as if you were truly dissolving gold but no need to boil.
Once you have the two warm solutions ready then move onto the next stage.
Application of the Sulphamic Acid on the AR.
Gently add a few (and I mean literally a few) ml of the solution you made from the Sulphamic Acid and water to the AR. You will immediately see a vigorous foaming of the AR. Put your Sulphamic solution down and then stir your AR. The initial reaction WILL THEN BECOME A LOT MORE VIGOROUS.
Now, this is why I suggested that you learn using a "dummy" solution. You will possibly add too much on your first attempt and then proceed to watch with little tears of joy, as your AR solution bubbles up and foams over the sides of the beaker.
I can promise you from experience that those little tears of joy are infinitely more preferable than the tears you will cry if your solution had been pregnant. I cried those tears once - and my AR was orange at the time....
When the reaction has abated, repeat it. Then repeat it again, until you get very little reaction after addition of the Sulphamic Acid solution and stirring. At this point leave it alone for 15 minutes and go and do something else. You may have to mix another set of Sulphamic Acid solution and this is a good time to do it.
After 15 minutes, come back, add more Sulphamic Acid solution to the AR, stir and see what happens. You may almost be at the point where the Nitric has been converted by now. You need to repeat the last step until there is no reaction with the AR. At that point you are denoxed and I would leave the solution on the hotplate for 30 minutes, then cool, add ice, and do all the things you normally do.
Learn the process using "dummy" AR or even Nitric and water until you are totally comfortable before applying it to your gold pregnant AR. You will make mistakes, that's fine. You will learn from them in a "risk free" manner."
Learn the process, make a few mistakes, then get comfortable with the application of it and it will serve you well.
Now here are a couple of practical pointers for using "live" AR.
1. Don't filter your AR before denoxing. Decant it from your spent , raw product, wash your spent product down and pour the liquid into your denox glassware with the AR, and denox it "warts and all." You will filter it afterwards.
2. One by product of the process above is that the sulphuric acid produced will precipitate out any lead as lead salts so it can save you adding Sulphuric acid later to do this.
I hope this helps. If I have missed anything, or you'd like further information then please let me know.
Regards
Jon
