You will want to use clean iron (no rust), and a large surface area ( the nail is small possibly with a coating of rust that could make it hard for the acids and metal ions in solution come into contact with the nails clean fresh metal with an electron to share (the rusted iron metal coating has already given up its electrons and has none to share with the more valuable metal ions in solution).
The nail is small and has a small surface area (increasing how many nails in the solution can increase the surface area of iron in solution to speed thing up... A better solution would be to use a larger piece of soft pure as possible iron metal sheet or bar, with as much of the iron exposed to solution as possible...
A piece of rebar made for concrete used for its skeleton (carbon steel) has more surface area than a nail...
A piece of clean angle iron has more surface area than the nail...
Most iron or structural steel will have some kind of coating phosphate cardinal paint, mill scale from the hot rolling process, or even rust... that will need to be removed for best results.
Some types of nails have zinc galvanized coating, the zinc would replace some of the other base metals that iron alone would not.
Other things can come into play in these reactions, stirring can help to help ions contact the bare metal easier, temperature, pH, free acids, free oxidizers in solution...
The aluminum will displace more metals than iron, it also makes a solution a gooey mess as the aluminum hydrolysis in solution. it also consumes acids as it displaces hydrogen gas or depletes the hydronium ion in solution (because of its place in the reactivity series of metals). in the gooey solution, it can be harder for small particles or clusters of small atoms of valuable metals to settle out of the goo or gelatinous solution...
The aluminum waste is more trouble to deal with than an iron waste, aluminum hydroxide loves moisture and is terribly hard to dry or deal with in the waste stream, hard to filter, difficult to dry, just difficult...
In some manners reminds me of tin and its problems...
Have you spent time studying the reactivity series of metals to understand more about these displacement reactions?
https://www.google.com/search?sxsrf=ALeKk00dumy5GHDrrItGtcPYiaEoo-PwZw%3A1588174142665&source=hp&ei=Pp2pXpS1JJj3-gSBs6yQCQ&q=reactivity+series+of+metals+displacement+readtions&oq=reactivity+series+of+metals+displacement+readtions&gs_lcp=CgZwc3ktYWIQAzIGCAAQFhAeMgYIABAWEB46BAgjECc6BAgAEEM6AggAOgUIABCDAToHCAAQFBCHAjoFCCEQoAE6BwghEAoQoAE6CAgAEAgQDRAeUN4qWILQAWC46gFoAXAAeACAAecDiAHkRpIBCjMuMzUuNy40LjGYAQCgAQGqAQdnd3Mtd2l6&sclient=psy-ab&ved=0ahUKEwiUzPHc-Y3pAhWYu54KHYEZC5IQ4dUDCAk&uact=5
