Years ago I used a centrifuge made to filter, by centripetal force, metal hydroxides out of a waste treatment slurry. It worked great and it had a scoop which pushed out the toothpaste like sludge while the centrifuge was running. Today they make centrifuges called algae centrifuges to pull solids from liquid. Inside it looks like this;
View attachment 65545
And the flow schematic looks like this
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I can imagine a mercury coated copper sheet inside the collection area as
@goldshark mentioned and possibly the combination of the amalgam formation and the contact provided by a slow flow through a centrifuge, it may just be effective. You could set something like that up on a flow of water containing nano particles of gold and let it just run. I've always been a fan of making money while you sleep!