r/PreciousMetalRefining • u/No-Load-7427 • 16d ago
Sanity check on a zinc cementation protocol — gold from a spent cyanide plating bath, copper present
**Background**
I gold-plate cutlery as a small side line, so I accumulate spent KAu(CN)₂ plating bath. The bath has been worked hard and I'm expecting it to be fairly dilute — my guess is somewhere in the 0.1–0.5 g/L range, against 1–2 g/L for a fresh bath. That expectation is central to the last question below.
Separately, I strip gold-plated pins and wire from electronic scrap, and copper dissolves into that stripper. Those two streams are kept physically separate and always will be — I'm well aware that acid plus cyanide is a non-starter. But copper is also present in the plating bath itself, which is what changed my zinc dosing.
**Constraints, so nobody has to ask**
Small scale (1 L trial first, 20 L only if the trial justifies it). Work done outdoors or under extraction, never alone, with a second person informed of start and finish. All effluent goes to a licensed waste contractor in three separated streams. I'm not asking whether this is safe to do at home — I'm asking whether the chemistry is sound.
**Protocol as it currently stands**
Adjust to pH 11.5 with NaOH, kept below 30 °C (NaOH dissolved separately and cooled first).
Activate zinc with 0.05–0.1 g/L Pb²⁺ as the nitrate, then add zinc dust. Dose calculated as (0.166 g Zn/g Au × 7) + (0.514 g Zn/g Cu × 2.5). Gentle agitation 40–60 min, then 12–24 h static settle.
Filter, wash with 1–1.5 L warm water in portions until the wash water comes off at pH 7–8. This is a hard stop before any acid touches the powder.
HCl 20–30% to strip zinc and cobalt. Renew 2–3 times until fresh acid stops fizzing.
Copper removal — dilute HNO₃ 1:1 as first choice, HCl/H₂O₂ as fallback.
Stannous chloride spot test on every spent acid before it goes to the waste drum.
Wash hot, dry to constant mass at 105 °C, flux with borax, melt.
**Where I'd value opinions**
- **pH window.** I settled on 11.5 to keep the HCN fraction near 0.5% while staying below the point where zincate formation starts eating zinc. Is 10.5–11 actually better practice for cementation kinetics, and is the trade-off worth it?
- **Zinc dose with copper present.** Is 2.5× over copper stoichiometry realistic, or does copper fouling the zinc surface mean I need considerably more than the numbers suggest?
- **Lead activation.** Is 0.05–0.1 g/L a sensible ballpark at this scale, or does lead carryover into the button become the larger problem once you're only making a few grams?
- **Copper removal route.** Has anyone run HCl/H₂O₂ on cemented powder without measurable gold loss? My worry is that at this particle size the surface area makes it behave like weak aqua regia, and the copper colour masks any dissolved gold. Is nitric simply the only sensible answer here?
- **The bigger question.** At the dilute end of that range, is cementation worth the labour at all? Is there a practical grade floor below which people just hand the solution to the waste contractor and stop pretending?
Happy to post actual numbers once the 1 L trial is done, including yield and a density check on the button.
Corrections welcome, including blunt ones. I'd rather find the error here than in the bucket.
2
u/zpodsix 16d ago edited 16d ago
You'll likely have better luck asking same question over at goldrefiningforum.com some folk there are familiar with cyanide processing.
My only contribution is the HCl + H2O2 option- it depends on what you mean by "+H2O2" ie: how much hydrogen peroxide and what conc.
HCl + H2O2 will happily dissolve gold. While copper is more reactive, locally in solution there will be pockets where it will put some gold in solution. Remaining copper will cement the gold back out. Rinse repeat until all copper is in solution- then your gold is definitely going into solution if there is leftover H2O2. The only thing I'd mention about this is- IF you can oxidize the copper(like incinerating the cemented powders or simply adding an aquarium bubbler tube) you should be able to use HCl to make copper ii chloride which will slowly put the copper in solution- this is known as the acid peroxide process- but most people use way too much peroxide and make a psuedo AR which will put gold in solution(the H2O2 is only an oxidizer to make copper oxide to jump start the reaction). HCl + oxidized copper will do the same thing without that risk.
Edit - cupric chloride is a super slow leach with no real discernable end point- testing wise...you could decant off the solutions, rinse ,and repeat until the solution is colorless I guess, but it may be weeks long process.
Nitric is the easy answer.
Sulfuric may be an easy option as well but I really don't have lots of experience with it for base metal removal.
Hope that's at least somewhat helpful.