Help with copperas precipitation

Hello all,
This is my first time doing this. I am doing a gold recovery from CPUs via Aqua Regia/ FeSO4 method for a chem project.

For more project information see here: https://chemicalreport99.blogspot.com/
Primary sources have been
-Recovery and Refining of Precious Metals by CW Ammen (1997)
-Sheng, Peter P., and Thomas H. Etsell. "Recovery of gold from computer circuit board scrap using aqua regia." Waste management & research 25.4 (2007): 380-383.
-Marsden, J. O., & House, C. I. (2006). The Chemistry of Gold Extraction, Second Edition (Vol. 2nd ed). Littleton: Society for Mining, Metallurgy, and Exploration, Inc.

I added 500mg of Ferrous Sulphate Heptahydrate after the evaporation/denitrification step, and observed a brown cloudy precipitate along with nitrogen dioxide gas bubbling. It turned out I actually had a lot more nitric acid than I thought present, so I added excess Ferrous Sulphate until I had a cloudy brown mixture with unreacted ferrous sulphate crystals at the bottom. I centrifuged the mixture and the cloudy brown mixture did not settle at all. This means that the brown color is in solution and I haven't precipitated anything. I have left the samples to hopefully separate over the weekend, but any interim advice would be appreciated. What happened and how can I get the gold out??

Welcome to the forum.
You could try heating your beaker that seems to settle the gold faster, it may also dissolve the extra ferrous which will be a bonus.

Try a little sulfamic acid with the evaporation.
and/or
Add gold button and heat to concentrate.
and/or
Evaporate down to a very concentrated solution to remove as much water and free acid as possible, rewet with a little HCl, watching for NOx gas, evaporate again the added water and acid, add a little more HCl, watch for NOx gas, continue to evaporate to remove the added 20% HCl water previously added, this should remove most of the free nitric and nitrates from solution.

My guess is you still have too much nitric or nitrate salts in solution.
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Do a little homework on the test for nitrates using iron sulfate and a strong acid like H2SO4, here although HCl was the acid used the reaction can be similar...
Concentrated HCl involves too much water, 20% HCl is about 80% water, and HCl has one ionizable hydrogen ion, where H2SO4 has two ionizable hydrogen ions.

nitrate test
The example here the nitrate salt of zinc in solution is tested for nitrates...
2 Moles of Zn(NO3)2 in solution

Adding FeSO4 and slowly adding concentrated H2SO4 acid to the test tube a brown ring will form in the solution when nitrate salts are present.

2Zn(NO3)2 + 2H2SO4 --> 2ZNSO4 + 2HNO3
the strong acid reforms nitric acid in solution,
then
4FeSO4 + 2H2SO4 --> 2Fe(SO4)3 + 2H2
decomposition of the free nitric from the heat of the reaction of the concentrated H2SO4...
4HNO3 --> 2H2O + 3O2 + 4NO
thus
2Fe2(SO4)3 + 2H2 + 4NO --> 4[Fe(H2O)5NO]SO4 + H2SO4
here the formation of [Fe(H2O)5NO]SO4 gives us the brown ring or a positive test indication nitrates in solution...
(sensitive up to 2,5 microgram and a concentration of 1 in 25,000 parts)


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HCl alone will not remove free nitric acid from solution, it can dilute the solution with its water content and also convert some of the base metal salts like copper nitrate salts in solution to copper chloride and free nitric acid, which can then be decomposed further with the heat of evaporation and concentration of the solution to form NOx gas.

Thank you guys for your help. I will review the literature on the nitrate tests.