Many metals when melted together form alloys, like copper and zinc used to make brass, both of these metals alone have very different melting points of each other or than the brass does, but when melted together the brass alloy will now have a different melting point than any one of its individual metals that formed the alloy as copper and zinc.
You cannot separate the alloy by melting points again, as the new metal alloy formed has a different melting point, they are no longer individual metals but an alloy of the two metals in this case we call brass, the metal in majority is a solvent for the other metal, the solid metal also has a different properties than the individual metals used to make the alloy.
The new alloy may be harder or softer than the original metals used to make this new metal alloy.
The individual metals used to make this alloy will not separate by density in the melt,( I imagine) that the atoms are moving around in the melt so vigorously that all they do is mix together even better, heating of the atoms bumping each other all over the place, as when melting during cooling one metal atom being a solvent of another metal while cooling and solidifying, and when cooled or cooling lock together in atoms of each metal into crystals, binding atoms of one metal into a lock with the other with atoms of both metals marrying then to forming bonds of the crystals.
If this were not the case we would not have many of the metals we have, as any time metals were melted they would separate and would not form many of the very useful alloys we have.
Other wise we could not form alloys by melting two metals, as we would burn one metal up and boil it away before the other melted or got hot enough to become liquid and form the alloy, luckily one metal becomes a solvent for the other metal in the melt, and dissolves it into the melt to form the new alloy, it also has effect on the other metals melting point.
When solder is soldered in electronics to solder gold pins to a copper circuit board, the solder dissolves the gold into the solder, the solder is the solvent, the soldered joint now has a somewhat different property than the pure solder did, now unsoldering the joint will not separate the tin and gold, as they are alloyed together, they cannot be separated by heat of the two metals melting point, or by the metals density.
We can separate them by removing electrons from their atoms with acids, forming salts of the individual metals, (acid + metal = salt) and use the solubility of these salts of metals in solutions to separate them, as a chloride salt, the gold chloride salt is soluble in water, lead chloride salt is not soluble in cold water.
Before you loose valuable gold to experimenting, trying to melt your gold fill, try heating 50/50 solder see if you can make the tin and lead separate by temperature control or melting point of one of the metals, try again to see if you will be able to separate them by density, see if you can melt atoms from one of the group of the metal atoms and not the other, I can tell you it will not work
Now try acids separating tin and lead and separate the two, tin is soluble as a chloride lead chloride is insoluble.
The solder is an alloy and is no longer lead and tin but a new metal alloy composed of lead and tin atoms with different properties than the pure metals atoms used to form the alloy had.
Just as a ring can be an alloy of karat gold, composed of copper and gold, or a mixture of these metal atoms, with a new alloy melting point than either copper or pure gold has.
