Has anyone used a 4 inch column with a filter disk above an electric keg with a pump as a mash tun? Basically a vertical recirculation system for a mash tun. Would be great for wart/beer one keg at a time.
I doubt it, it'd be more like "good for one beer at a time". The amount of pipe you need to house any significant amount of grain would very high and if you made a column big enough the pressure of the grain on the filter would clog it.
If you look at the volume of a 4" column, every 20" holds about 1 gallon. When I do 10 gallon batches of beer, I need about 10-13 gallons of space to mash ~20-24# of grain. That would require 10+ sections, which is not only very expensive, but very tall, hard to load, and having 16+ feet of wet grain would be an awful lot of pressure on the filter disc.
I love the thinking of ways to use the equipment, but there are much less expensive ways of building a proper mash tun.
Thinking the OP had a fermenter somewhere in the middle of that conversation.
At least I hope so.