Nitric, H202 and Sulfuric initial digestion

I'm no chemist so I'm wondering if anyone could provide an answer?

I was thinking of adding small amounts of concentrated sulfuric acid or maybe diluted sulfuric acid after the nitric digestion is complete or maybe together while nitric is digesting other base metals. For the sole purpose of removing exposed iron.

I remember adding sulfamic acid once to a nitric digestion which I wanted to stop as my recycled nitric contained chlorides. It did dissolve some of the iron attached to magnetic mixer.

The purpose of H202 is simply to help with scrubbing and recycle Nitric.

I processed bench filings about two days ago and even after separation with magnet I had too many iron contaminants end up in AR. I guess the nitric did not digest it due to the presence of H202 which oxidizes the iron for it to passivate.

Why not just filter out the undissolved iron?

Dave

The next step after process will be AR. If I filter it, there is nothing to process in AR.

If I try to magnetize the iron there are lots of dragged gold in it.
Well, if my theory works I wouldn't have to remove iron in the first step thus lessening the drag out gold.

Sorry, my misunderstanding. When you said "I processed bench filings about two days ago and even after separation with magnet I had too many iron contaminants end up in AR. I guess the nitric did not digest it due to the presence of H202 which oxidizes the iron for it to passivate.", I thought you meant you had undissolved, passivated iron after the AR digest.

Dave

Adding sensibly sulfuric acid in concentrated form may help but just by warming

I assume you are using nitric to recover the silver but I can’t see any problem with just dissolving what’s left after straight into AR as it will remove the iron at that point.

nickvc said:

I assume you are using nitric to recover the silver but I can’t see any problem with just dissolving what’s left after straight into AR as it will remove the iron at that point.

Click to expand...

True. AR will be clean if the lot is in atomized form.
The problem with filings and inquarted gold is that the passivated iron and gold actually traps the silver. If one could put the iron out, there will be more silver or other base metals exposed to nitric.

The problem I had was there were too many contaminants in the AR digestion. Iron contaminant was easily removed with HCl wash. Silver chloride wash took about 1L of ammonia for it to wash out. It was just a 130g lot. The real problem is that silver chloride will trap a lot of gold. I had about 3mm thick of silver chloride in a 125mm filter paper.

My bad. Iron readily dissolves in nitric acid as long as it is not concentrated. It is only concentrated nitric acid that will passivate iron.
I think those that didn't dissolve were hardened steel or iron alloyed with gold/silver/copper. I do not really know yet .

I will process another set of filings next week. I'll try to put some sulfuric in it and see how it goes.

You might find that the pieces left could be stainless, also try stirring the powder with a piece of plastic plumbing pipe to expose all the powder to the acid during dissolution.
As an aside keep all the magnetics you remove as there will be some values amongst them