What dissolves first?

[bpcp7208]

So I have 82% Au and 18% Ni. I have started dissolving it and leached it once already and am continuing with new solutions. I did a hot nitric bath for quite some time, tested then drained it into my stock pot then did it again until there was almost no green. The first refining process had a slight green hue, no it only has that beautiful golden color. My question is, the product at the bottom of my beaker all Au? Do I need to dissolve it or could I filter then melt the remaining powder? If I do that, of course I'll only melt a small amount then test its purity. Just curious if I should safe my acids and try that or should I continue refining it?

Separate question, is there a max as to how much you can dissolve in a solution? I know it should be about "120mL per gram" but the last time I processed I got about 16 grams out of 700mL.

 

[Lino1406]

1. Check the remaining powder, the same way you got 82:18
2. The limit of silubility is written in books, changes with temperature

 

[nickvc]

With a mixed metal powder there is no guarantee as to what’s in it, if your aim is really pure gold I would recommend dissolving all the remaining powder using the method of your choice, as it’s all fine material it will dissolve in most combinations easily.
If you are happy with what you have then filter rinse well and melt.

 

[nickton]

I would certainly filter out the precipitate at the bottom of your beaker, then figure out what it is, and probably discard it since it shouldn't be gold. It's probably the Ni, and possibly some lead or trace PGMs. The separated auric chloride could then be further processed.

I assume you tested it with stannous chloride, and from your description it is unclear how you got the gold into solution. You must have also boiled the material in Nitric acid before putting it into solution.

That is a fairly expensive way to dissolve base metals. If it was from e waste usually refiners on this site use the AP leach method first, then filter out gold foils to be dissolved in aqua regia.

 

[g_axelsson]

Separate question, is there a max as to how much you can dissolve in a solution? I know it should be about "120mL per gram" but the last time I processed I got about 16 grams out of 700mL.

120 ml/gram is a very weak solution. Rule of thumb (+/- 50% depending on conditions) is that it takes 3 ml HCl and 1 ml nitric acid to dissolve a gram of gold. More than that per gram of base metal. Any deviation far off those numbers means you are probably wasting chemicals.

Gold chloride, or more correctly chloroauric acid ( https://en.wikipedia.org/wiki/Chloroauric_acid ), is really soluble in water. A liter of water can hold 3.5 kg of chloroauric acid, which is about 2 kilo of gold. The volume will be more than a liter though, probably closer to two liters when it is mixed together. So a concentrated solution is close to a gram gold per ml of liquid.

The most concentrated solution I have ever worked with was 53 g gold in 200 ml of liquid. 150 ml HCl, 25+12 ml nitric and some water added when filtering.


In the end, you have to ask for your self, what is the goal. Do you value the feeling of accomplishment when you have that 99.95% pure golden button or are you only in for the money? A bonus with refining before selling is that you know the purity so there is less of a risk of getting cheated on the sale price. Normally you will get the same payment whether your gold is 95% or 99.995% pure. The payout from a gold buyer is based on gold content, not purity.

If you're just in for the money then I would just boil the powder it in nitric acid, wash and melt. If it looks okay I would settle with that. Your powder looks so fine that the acid will get most of the nickel without dissolving the gold. A test melt will tell a lot.

Göran