You use a transfer node sitting on top of a cobblestone with a lava source block on one side and a water source block on the opposite side. It will generate cobblestone infinitely up until it's internal buffer(1 stack) is filled, and transfer pipes can be used to connect to a pulverizer. Cobble pulverizes to gravel, gravel to sand, sand to dust. I believe dust is the only thing you can sift redstone out of in crash landing, so you'd need three pulverizers sitting side by side feeding each other, and the last feeding autonomous activators.
If you use 3 activators to a sieve, the three pulverizer chain should feed it just enough to keep it going forever with a very small amount of excess. I'd recommend trash cans or other methods of disposal under the pulverizers to get rid of the extra stuff that comes out(the stuff in the lower output slot). That should feed three redstone generators with plenty of redstone to spare, so you're getting 960 rf/t and using 120 rf/t on the pulverizers and whatever the magma crucible uses, however the magma crucible won't run anywhere near constantly on just three redstone generators.