The best success I ever had with silver up to the current appreciation of my physical silver ETC as in removing silver for waste photographic fixer/developer.
Back in the day I used to own a photographic mini-lab. As you are probably aware, old school film contains silver and the effluent from the developing process contains silver.
As silver is highly cyto-toxic ( old cowboy trick of a silver dollar in the water canteen to keep the water sweet-kill bacteria etc.), it was not allowed to go to the municipal drain. In the early days ( 1990's) we'd collect it in 20L drums, and when we had about 30 of them filled, they would be collected and we'd get about £30-40 depending on the silver price ( about £5/oz at the time iirc). Then the regulations changed and collectors needed certification and had increased costs (so they said!) so it changed to "we'll collect it dispose of it for free", then roll into the early 2000's and they started charging £30 at first , then £50 a collection.
At that point someone developed an ion-exchange filter that went in-line with your effluent pipe and removed the silver to a resin cartridge which allowed the waste to go to drain. The setup cost about £3,500 and it took 6 to 12 months to use up a resin cartridge, at which point you paid £600 for a new one.
Here's the good bit, they removed the silver and paid you the difference between a new cartridge and the recovered silver.
It was an amazing deal, I was being paid about £1,500 each cartridge, I paid of my system in a couple of years and after that it was free money !!