I have a beaker that contains washed sludge from a sulfuric stripping cell set up. I pre treated with dilute nitric acid then washed several times to close to neutral PH. Then I began AR to dissolve all remaining values. I am determined not to apply too much nitric, so have been adding on the weekends with gentle heat, then let cool. When all settles, there is still a grey sludge at the bottom, so more nitric and heat an eye dropper at a time. After this weekend, I have an unexpected result and want a little input before proceeding. There should only be cell sludge, nitric acid , muratic acid, and a stink bug in this jar. Instead of clear gold liquid over grey sludge, I have a pretty gold flake solution over grey sludge. Have I put too much nitric acid and need to add more muratic acid? Why the flakes? Shouldn't the AR dissolve all the sludge even if it contains lead from the cell?
Gold not dissolving in AR
- Thread starter Amber
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Geo
Well-known member
That's a long time. Have you added any more HCl to the mix? Add a few drops of HCl and stir to see if there's any reaction. Nitric acid additions over that period of time could mean you have depleted the free HCl. Try breaking the crust up and adding some more HCl as the addition of HCl can't really hurt or cause a bad reaction.
It is hard to tell from the picture.
Is that a chunk of lead in the beaker? The lead may be dissolving, using up the hydrogen in the HCl acid to make lead chloride solution, the base metals will dissolve before the gold, or can push any dissolved gold back out of solution.
Lead chloride solution will be clear (if other metals like copper are involved), when the acid is depleted the lead chloride can form white insoluble needle like crystals which can break up to a whitish powder, these lead crystals are fairly insoluble in cool water but can be fairly soluble in boiling hot water.
Is that a chunk of lead in the beaker? The lead may be dissolving, using up the hydrogen in the HCl acid to make lead chloride solution, the base metals will dissolve before the gold, or can push any dissolved gold back out of solution.
Lead chloride solution will be clear (if other metals like copper are involved), when the acid is depleted the lead chloride can form white insoluble needle like crystals which can break up to a whitish powder, these lead crystals are fairly insoluble in cool water but can be fairly soluble in boiling hot water.
I think it may have some lead from the cathode, but it is not a chunk. It is a fine powder and moves freely whenbutcher said:
agitated. It should have gold in it still that is not dissolving. I think I have muffed up the AR ratio by adding too much nitric. My instinct is to add hcl as geo suggests OR evaporate all and restart the AR (PIA). Copper can be in there also from the mesh basket. I did attack base metals with dilute nitric before moving to AR, so they should be minimal compared to gold values. I am unclear why GEO mentioned time. isn't there a story about saving a gold medal from the nazis by leaving it in AR? I thought time was your friend? Thanks for enlightenment.
Hydrochloric acid is a gas (hydrogen chloride) dissolved in water, usually above its azeotrope. Given time, the hydrogen chloride will gas off from the solution faster than the evaporation rate of the water it's dissolved in, making the hydrochloric acid weaker.Amber said:
Once gold is dissolved in a chloride solution, you're dealing with a different animal.
Dave
Thanks Dave. I think your answer cleared up my confusion. I have added fresh hcl and the gold flakes have redisolved. I let the undisolved powder settle, decanted the the beautiful gold chloride to another container so I can continue AR slowly on the still undisolved powder. Now I am adding both hcl and nitric in increments to keep the ratio correct. I am making progress. Thanks again for the help.
