I believe you will get the temperature you need to use sulfamic acid to kill off the excess nitric if you add it in the tumbler when the last addition of nitric has gone to completion and the reaction is still hot enough. The question is if the reaction is hot enough to begin with. This is something you will have to physically measure.
Originally I was skeptical about adding sulfamic to the tumbler because it will be difficult to add from the holes you can open up on the end of the tumbler where the exhaust to the scrubber exits. But then I remembered the charging hole. I added a red arrow pointing to the charging hole on a photo Gold Refinery provided.
View attachment 50347This hole is where scrap material is added to the tumbler and it is convenient because it is directly opposite the drain hole where the tumbler empties into the filter. If you stop the tumbler with this port on top, it can be opened and you can measure the temperature. If you have 80ºC or close, you can add sulfamic acid there.
Using the charging hole is good because it will allow you to carefully add the sulfamic and to monitor it closely so it doesn't foam too high.
Re-read the posts in this thread by Geo and Orvi about adding the sulfamic and add it carefully if it is at 80ºC or higher. do not over-add and look for the signs as described to determine when you have added enough.
Then you can add ice through the same hole and let the solution cool before filtering.
This is not as simple as it sounds. It is nearly impossible to know how much of the nitric you added has actually reacted and how much remains in solution. However your working experience can show you the right quantity for subsequent reactions. The nitric (70%) weighs 1.4 grams per ml. You know how many milliliters of nitric you added. Weigh out the quantity of sulfamic acid you will need to neutralize 100% of the acid you used (this will be too much, but it will help you do this properly in the future). Add the sulfamic, paying attention to the way it reacts and stop when it is complete. The activity of the reaction of the sulfamic acid on the solution should provide enough mixing that you need not seal up and spin the tumbler for this part. Simply place the cover over the hole to provide stronger exhaust from the scrubbing duct.
When you are done, weigh the remaining sulfamic that you haven't used. Now you can calculate what percent of the calculated sulfamic acid you actually used. Do this for a few lots and soon you will have a working value for future use. I would be surprised if it is more than 25% of the calculation made for 100% of the acid added.
I realize your system has the capability to add peroxide. I have never used peroxide in this fashion and the 4:1 aqua regia quantities I gave you per ounce of material should provide sufficient oxidizer.
Gold Refiner, please inquire what the wetted parts of the dosing pump provided for peroxide additions are. If compatible, the pump could be used for a slow constant nitric feed which will give you a continuous controlled reaction which could control NOx generation and provide a more constant warming effect for the reaction.