This is a very simple device that has also stood the test of time; it is very effective for small amounts of material, but using it for more than a couple of hours is very hard work as you have to carry the weight of the hopper with the material in it.
Hopper gun
It works by injecting the air through a jet at the base of the hopper, which literally blows the material out through an orifice plate; it requires at least 10 cfm of air at a fairly constant pressure, so in practice this means either a large classic compressor capable of at least 20 cfm with a decent receiver, or a smaller one than runs continuously, to avoid the pressure variation from the motor stopping and starting.