Cementing with Zinc didn't go as planned.

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4metals said:
The process detailed by @tmj883 is the textbook description. My formula is about a pound of ferrous sulfate to a quart of warm water. (which, if you do the head math is very close) The only reason I ever use ferrous is if the lot has a significant quantity of platinum group metals in it which I want to recover after the gold. It is always in a dirty solution of a dissolve awaiting a first drop.

One thing I do not like about ferrous sulfate is the fact that it does go bad, fortunately visibly so. Whenever I purchased a sack of ferrous sulfate, I immediately weighed out 1 pound portions and vacuum sealed them in a food saver pouch, this prevented the ferrous from degrading, keeping it nice and green.

The fact tmj883 pointed out about 100 ml for every 7.5 grams of gold is probably a stoichiometric reality, but in practical reality who really knows how much gold they expect from a lot of e-scrap? (assayed karat is a different story) This leaves the stannous test as the only practical real time indicator of your endpoint as Jon pointed out.
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It's incredibly effective with ewaste. You can have a completely black solution that's almost like warm treacle because it's so full of multiple metals and yet it drops the gold like a metaphorical stone.

In this circumstance it makes absolutely no difference whatsoever if the Ferrous Sulphate isn't 100% perfectly green. Picking out the bits that arent is fine. Incidentally I find sealing it in a 20 litre plastic white container keeps it perfect for ages.

I'll probably cause a few sharp intakes of breath here but for dirty drops in a 3 or 5 litre glass I crumble it up by hand and put it in as crystals. Stir vigorously, test and add more if required. That way there's no additional volume of waste. If you have a solution with a lot of gold in it (30g plus) then you're going to be adding an awful lot of volume to it in order to add the required amount of liquid Ferrous Sulphate to drop out all the gold.

If there are excess crystals in your dropped gold- then wash it with hot water and they remove themselves from the equation.
 
anachronism said:
I crumble it up by hand and put it in as crystals. Stir vigorously, test and add more if required.
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The other thing I don't like about ferrous is the volume increase in overall waste liquors. For refining karat gold scrap with enough PGM by assay Ferrous was used but as a dry granule. Considering the quantity of gold in karat refining the overall liquid volume added will be over a gallon per ounce so with production quantities granular was the only practical solution. Using the ferrous made subsequent PGM recovery easier and more complete making it worth the hassle.

And the lack of undesirable nasal stimulation with ferrous is appreciated. I never worried about the odor because I always had good exhaust and scrubbing, but for backyard downwind refiners it matters!
 
Please direct any criticism to Ms. Hoke as this methodology is derived directly from her. It is stoichiometricaly twice the amount of copperas needed to drop the gold given. This is done to promote the formation of large particles that are easier to recover. It does not drag down excessive contaminates. And yes, it is a starting point.
 
sulfuric acid (electrolyte for car batteries) perfectly restores ferrous sulfate solution.
turning it from brown to green...

remember
pour acid into water
 

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