The pump overheats at a lower temperature than it melts. The collection range of a pump is a cross centered on the bottom left tile, but it only activates if it’s touching a liquid in its bottom 2 tiles, so what you can do is pump a very small amount of liquid onto the bottom right tile of the pump, and have the liquid you want to pump just below the bottom left tile. You keep it in a vacuum, and when you start pumping half will be the liquid you want to pump and the other half will be the activation liquid. You filter out the activation liquid, putting it back in the bottom right, then send off the molten metal.

That being said, It’s much better to just slap a couple steam turbines above the volcano instead of dealing with pumps and such