Dissolving iron - how?

[plamenppp]

I found some old Soviet PCB and collected some 5 kg of plastic chips containing bakelitte (a lot of it), tin (which was on their legs), iron legs (8 of them in each chip), extremely thin gold wires, gold plating (the inside surface of the legs), silicon crystal, and soldier containing gold. I burned everything until it became dust mixed with legs and silicon and gold I removed the plated legs with a magnet and stored the dust for later recovery. There is a lot of iron and it is stopping me from using AR directly. I tried to remove the Fe with HNO3 but the result was almost none. I tried with HCL and again the result was almost none. What should I do to remove it?

 

[lazersteve]

Try dilute H2SO4

Steve

 

[plamenppp]

I have 95% H2SO4. How dilute should it be? 20-30-40% ?

 

[qst42know]

10% heated will work.

 

[Renaldas]

Cant we process iron containing pieces in AR? I have done such a way several times, and there were no problems, or, maybe, I miss something?

 

[goldsilverpro]

I have used hot AR to get rid of the iron. The gold won't dissolve until most of the iron is gone, assuming it is iron metal and not an oxide. You can test for gold in solution with stannous chloride. Ideally, you want to dissolve as much iron as possible without dissolving any gold, then pour off this waste solution. Dissolve what remains (gold and remaining iron) using fresh AR and precipitate the gold as normal. You'll probably have to re-do the gold powder since it likely won't be pure.

If you go too far with the first AR (the solution tests positive for gold), you can add a very small amount of additional iron until the stannous chloride shows no gold in the solution. Then proceed as above.

The idea of this double AR is to get most of the iron into the 1st AR, which contains no gold. That way, the 2nd AR will be mainly gold (and far less solution) and it will be much easier to deal with.

As in all AR processes, use excess HCl and try to keep the nitric at a minimum. To do this, I would cover the material with HCl and heat it. Then feed in the nitric a little at a time.

I used this process on large amounts of gold bearing stainless steel in a 50 gallon Pfaudler kettle with great success. It will also work with getting rid of a lot of copper.

 

[arthur kierski]

i do exactly what GSP explained in his thread above-------
it works fine and is better then using magnets, and cheaper then using great quantities of nitric+sulfuric as i used to do to eliminate the iron

 

[lazersteve]

Arthur,

I've found the key to not tossing your gold out along with the iron solution, or even copper for that matter, when using AR this way is to let the solution work until it is very dark colored, then allow it to cool before decanting and adding more AR. The cooling step is very important as it changes the solubility of the iron/copper compounds which forces any dissolved gold back out of the solution as a dark powder.

When you get the feel for it you will know when to stop the reaction and decant. I look for indicators such as:

1. The tiny bubbles turn to large bubbles. Add a small amount of Nitric until saturated.
2. No more NOx gas (red-brown). Add a small amount of Nitric until saturated.
3. Colored crystals forming in the hot solution. Ready to cool and decant.
4. Very dark colored solution, thick almost like ink. Ready to cool and decant.

The above signs will help you recognize when to repeat the process with fresh AR.

Upon cooling you will very likely encounter large amounts of crystals which are easily dissolved with fresh HCl after decanting the bulk of the solution. I find this is a benefit in that the crystals trap the gold powder and foils in the bottom of the beaker so they are not poured off with the waste solution. Be sure you remove all of the crystals with HCl before beginning the next round of Modified Poor Man's AR treatment. Pay close attention to the amount of base metal material that is still undissolved after the crystals are washed away. This is a great indicator of how close you are to completely removing the base metals. You can estimate your progress by tilting the beaker side to side or gently shaking it: if your foils roll around and clink against the glass, they still contain base metals, if they matte together as a single mass and 'tinkle' (sorry for the odd word) they are nearly free of base metals.

I feel this process is more like using nitric made in situ with the sodium nitrate and HCl as opposed to making AR directly. The real trick is balancing the point where the gold is finally dissolved. As GSP stated, the gold will need all of the proper washes afterward and to be redissolved for maximum purity.

I've been doing it this way for several years now.

Steve

 

[arthur kierski]

thanks Steve,i will try to digest your information very carefully----
just a doubt:do you add fresh ar over the old ar(the one that did not forms gold because iron or copper cementation or do you filtrate the powder left and add the new ar to it?

 

[lazersteve]

thanks Steve,i will try to digest your information very carefully----
just a doubt:do you add fresh ar over the old ar(the one that did not forms gold because iron or copper cementation or do you filtrate the powder left and add the new ar to it?

No, after each batch of AR becomes saturated and cooled siphon it off of the solids, wash the solids with HCl until the crystals are all gone, then add more HCl and water followed by small additions of nitrate.

Steve