Rick, a simple filter that will slow down the progression of
investment and metal in your lines is a VW Bug fuel filter. Cost is
around $2.00 last I looked. Simply split the vacuum line and insert
it. It does a great job of keeping the investment out of your pump.
When it starts to plug up, it's an easy replacement. As for a
filter for keeping molten metal out, you could make a filter from
some PVC fittings and pack it with course steel wool. Anything to
slow and cool the metal. Your scrap refiner might not like the
steel wool though.