In a try to clarify some points, I dare to add some comments.
@Harold:The title of the publication cited:
"cornstarch-replaces-cyanide-clean-new-gold-xtraction-method" is misleading. Cyclodextrin (not simply cornstarch) does not extract gold out of ores. It forms a complex with already dissolved gold(III), in the form of the tetrabromoaurate-anion, AuBr
4-, after dissolution of the metal in a HBr/HNO
3-mix, followed by neutralization of most acid with KOH. Together with potassium (K
+)-cations, surrounded by 6 water-molecules, the cyclodextrin-complexed AuBr
4--anions form an only sparingly soluble, crystalline compound.
@Butcher: You're right. Cornstarch has nothing to do with replacing cyanide. The gold, in the precipitation-reaction with cyclodextrin is not reduced, it remaines in the +(III)- oxidation state, as AuBr
4--anion.
SMB has in fact been used, to reduce the coprecipitated gold-cyclodextrin-complex, like written in the original paper:
The co-precipitated α·Br complex—namely, recovered gold—was separated from impurities by filtration and then reduced with a reductant, such as Na2S2O5, to afford the recovered gold metal. The residual gold in the filtrate can be recycled while α-CD can be reused by recrystallisation from the filtrate. This laboratory scale process is highly selective for gold, as well as being economic, fast and feasible.
@Practicability of the process: The investigated process does not apply, to dissolve gold from ores, and thus is not apt to replace cyanide-leaching. The authors of the original paper themselves choose to dissolve small samples of gold-alloy-scrap (250 mg) in a mixture of conc. HBr and conc.HNO
3 (3:1;v/v), similar to treatment with AR, and they didn't investigate really existing ore-samples. Such HBr/HNO
3-mixtures may become very dangerous, being much more unstable, than AR, decomposing spontaneously in an unpredictable way, boiling over explosively
under evolution of huge amounts of bromine-vapours (Br
2) and nitrogen-oxides (NO
x).
In my opinion in gold-recovery/refining this KAuBr
4/cyclodextrin-coprecipitation is not only a cost-intensive, but also an unnecessary additional step, compared to direct reduction of gold, dissolved in aqueous HBr.