How to make an armoured zombie filter?

I tried to make an armour farm using a zombie Mob Spawner (found in a dungeon) and I want to filter zombies which have diamond armour (iron would also do) from unarmoured ones, killing those without armor on the way to a room where I can safely kill those who have some. So far, I tried using small lava channels, but they move too slow in them and die.
Any help for separating armoured zombies from unarmoured ones?

2 Answers

Here is an Armour Filter Farm example:

1. Here is the main redstone mechanism composed of tripwire, wire, redstone repeaters with delay level 4 and a sticky piston attached to a slime block, that pushes the burning zombies out of the fire.
2. This is the spawning platform, which must let zombies pass only through the hole.
3. This is the push-down-into-fire mechanism, that uses tripwire for powering and a villager "helper" to attract zombies into it.
4. This is how the mechanism looks from above.
5. Here is the villager "helper" that attracts zombies.
6. This is the Zombie that burns and is about to be pushed out of fire by a slime block into a hole.
7. This Zombie was unarmoured, so he died while on fire because the water didn't had time to extinguish him.
8. This is the extinguishing system, that uses water to extinguish zombies with an armour level of at least 3 out of 20. The water is blocked by a piston while there is no entity inside the trap. It uses tripwire, linked by wire to a Redstone Torch (NOT Gate) that reverses redstone signal ("powered" becomes "unpowered" and "unpowered" becomes "powered") linked to the piston by wire. Also, the rotten flesh doesn't go to a waste; instead it is collected by hoppers and sent into a chest.
   9. Now another zombie that unlike the other, has a Diamond Helmet, burns in the hole……but unlike the other, he didn't die and the water saved him in time!

So this farm is effective for zombies wearing at least a diamond helmet.
Note that it may not work on baby zombies and, also, multiple zombies "using" it may corrupt the mechanism, so you can add walls at the spawner or a redstone mechanism that allows a single zombie to pass in a certain number of seconds, for example

You should make a standard zombie crusher. The armoured zombies won't die and can after the crushing be transported somewhere else using a water stream. You'll need a bit of redstone knowledge and a zombie crusher design. Almost any design on YouTube will do just fine.