When using Nitric acid to dissolve gold, problems will arise if this excess Nitric acid is not removed. If you use Sodium Hypochlorite (household Bleach) with Hydrochloric acid to dissolve the gold, denoxing is not necessary as there is no Nitric acid. Excess Nitric acid will prevent the gold from dropping using most methods. In order to remove the excess Nitric acid, a process called denoxing is used. The three primary ways to denox a solution is by boiling down the solution and adding Hydrochloric acid to reconstitute, or adding Sulfamic acid (Sulphamic acid for you British folks) or Urea.
But which is better and when should you use each? Hint: I prefer to use Sulfamic acid and I will explain why. I have ruined a few beakers of gold-containing solution by performing the actions listed in the Difficulty sections. Heed these warnings or risk losing some (or all!) of your gold!
Denoxing using the “Boil-Down” method
Let’s start with denoxing using the boil down method. In this method you essentially boil the Aqua Regia mixture down to a few mL remaining in the beaker. Then, you reconstitute the solution by adding Hydrochloric acid. You can then boil down a second time to ensure all the Nitric acid has been removed.
Benefits:
The benefits of using this method are that it is extremely easy, doesn’t require any excess chemicals that can complicate things later, and if done properly, will remove all the excess Nitric acid.
Difficulties:
If you let it boil down too far, the remaining liquid will “pop” and splash out of the beaker. For this reason, you should watch carefully once the solution gets below about 50mL.
Denoxing using the Urea method
The second method for denoxing is using Urea (CO(NH2)2). This method will only remove the excess nitric acid with no other added benefits. The chemical equation is:
6HNO3 + 5CO(NH2)2 —>8N2 + 5CO2 + 13H20
Benefits:
It does an effective job of removing the excess Nitric acid when the right quantity is used.
Difficulties:
If too much is added, problems will arise (this may be a topic for another blog post) making it very difficult to recover the gold. Additionally, if too much is added at once, you will cause a boilover and lose a lot of your precious metals. This method takes a little bit of practice to get right without boiling over.
Denoxing using the Sulfamic acid method
The third method (and my personal favorite) is denoxing using Sulfamic acid (H3NSO3) . The Sulfamic acid will react with the Nitric acid to create Sulfuric acid, Nitrous Oxide, and water using the following equation:
HNO3 + H3NSO3 → H2SO4 + N2O + H2O
Benefits:
This process provides a dual benefit. First, it gets rid of the excess Nitric acid. Secondly, it creates Sulfuric acid which will react with any Lead in solution. The Lead will react with the Sulfuric acid to produce Lead Sulphate. This dual process is the reason why I prefer to use Sulfamic acid to denox my Aqua Regia solutions.
Difficulties:
Although this method is my favorite, it is just as difficult to perform as the Urea method. It is very easy to cause a boil over if even a few mL too much is added at once.
Conclusion:
My preferred method of denoxing is using the Sulfamic acid method. There are a few difficulties with using Sulfamic acid but when done properly is the best way to denox your Aqua Regia solutions. If you found this blog post helpful, let me know by emailing me at Scott@Ewaste-Nation.com. You can also share on social media! If there are other parts of E-waste Refining that you would like to learn about, let me know!