An old thread from the moderators forum about formate reduction, gold purity and cleaning up gold

[Harold_V]

I'm convinced it is the agitation process that gives such good base metal removal.

I agree. I know that washing with HCl yields the best results when the solution is vigorously boiled. There has to be a reason for contaminants captured within the precipitated gold to be freed, and I've long believed that they are not freed easily.

Harold

 

[Lou]

Next time I'm using the scanning electron microscope for a job, I'll bring some different purity gold powders. I'm absolutely convinced that it has everything to do with the pore structure in the precipitate:

i.e. more fluffy (blonder) is easier to wash and better to purify.

If we plot a graph of ease of purification with washing methods of crude gold precipitates, what would probably be seen is that the impure gold doesn't wash well.

 

[Lou]

So I decided to try this out on crude gold powder and typical 3N6+ stuff. It really works quite well and I'm very impressed with the results.

I should mention that this also works well (with hot sulfuric and nitric) for crude platinum sponge: 98.211% Pt sponge was upgrade to 99.901 after five hours hot 30% nitric 30% sulfuric and vigorous tumbling. The contaminating elements were nickel and iron and both were removed rather well.

 

[4metals]

I figure if we dicuss our successes in this process it's only fair to discuss the failures.

To do the sulfuric clean up effectively the silver chloride has to be pretty low as sulfuric doesn't dissolve the silver as a chloride so it needs the icing before filtering to get the silver chloride low enough for the desired purity. That usually means a lot of ice to me.

So I thought, what if I precipitated the gold the same way but didn't add ice before filtering, the sponge should have more silver in it because of the silver chloride. Ran a batch, took out about 100 ounces of sponge and melted it, digested a sample, and confirmed it by ICP, high silver.

Then I took the balance that I didn't melt and tumbled it in distilled water for 2 hours while exposing it to a high intensity UV light. My thought was the UV would release the chlorine and leave metallic silver, which will be parted off by the sulfuric agitated leach process. Well it did lower the silver by about 20% but apparently not enough was exposed to sufficient light to reduce it enough for the sulfuric to be effective enough to remove all of the silver.

OH well, thought I could save on the ice step but not this time.

 

[Lou]

This pulping procedure is especially good for iron, nickel, and cobalt.

I have remedied the silver issue using three techniques:


1. ammonia digest to remove AgCl and any copper. The copper forms a tetrammine bright blue complex and the silver forms the colorless diammine, both are highly soluble.

2. hyperchloridic media means the much of the AgCl is present as AgCl₂^-, a complex. Dilute from 10-12 M [Cl^-] to 4 M [Cl^-] causes this complex to break and the silver to drop as AgCl. This is especially pronounced in solutions containing potassium as the counterion. Recall that the graph of silver chloride solubility is parabolic: in dilute chloride solutions it is more soluble, as chloride concentration increases its solubility is suppressed by the common ion effect, but as the [Cl^-] goes up past about 5 M, then it complexes and will ride with the HAuCl4 and contaminate it.

3. temperature modulation. Chilling the solution decreases AgCl solubility.

That said, it's ideal to use ice to cool it while simultaneously diluting it, but it's got a disadvantage as it increases the waste solution volume that needs to be treated. I'm theorizing that we can make a modification to the pulping method: 4metals, why not try pulping the gold powder with HCl with KCl added, preferably hot (to preclude much oxygen dissolving and making Cl₂ which could rob some gold, although it could be done cold with a pinch of sulfite added to preclude this). It is important that the sulfuric is not concentrated and hot, but rather around 20-30% w/v otherwise it may oxidize the HCl to Cl₂ as well. I think this could be used to not just remove the iron and nickel, but also to get the silver as well. Care to give it a try? You should be able to get the silver chloride to complex up and filter it off, and rinse it all down with HCl/KCl.

I would still chill the solution before filtering (you'll get some silver and your SO₂ goes in much better), but perhaps we can avoid the icing out of the silver.

It's amusing that you can actually see the gold quality improving. I started with a dense ugly black gold powder and managed a fluffy light brown in a few hours time and assayed at almost 100X improvement.

 

[4metals]

In 2 weeks I'll be at a refiner who produces around 4 to 500 ounces of fine from stone removal. No cold acid and SMB dropped, it runs around .997. I was tempted to try dropping the digested karat with Versenol 120 to get .9999 but this may do it instead. I have to come up with a cheap tumbler mixer in a size for smaller lots but if it works I will report back.

Better late than never.

I did go to that refiner and I brought an oblique tumbler that held 5 gallon buckets. I found some 6 gallon buckets which were deeper and added a Rheostat to slow down the tumbling. Then a mixer blade was mounted just as I described above.

We used 3 parts hot water to 1 part sulfuric acid and the sulfuric was metered in slowly over about 1/2 an hour. We let it tumble for about an hour until the acid cooled before filtering the gold sponge. It did clean up the gold to +.9999.

The pregnant gold solution, before filtering, was chilled and filtered into a Buchner funnel set up as a super filter using cellite over the filter paper followed by a top sheet of filter paper to help maintain the cellite thickness. This was done to catch as much of the particulate Silver Chloride as possible. As the Sulfuric will not remove any Silver Chloride.

The process is effective in the smaller tumbler holding about 500 ounces of sponge. Considering these days that's close to a million dollars of gold it is a good size for processing. The one caution is be careful setting it up because if it splashes, it is hot sulfuric. For that reason I also slowed the mixer down on a Rheostat.