electrostatic seperator build questions
At least in principle, aluminum is not a good electrode material. The reason is that when exposed to air, aluminum quickly (that is, almost instantaneously) forms an aluminum oxide layer, and alumina is an excellent insulator. As a result, your particles, unless they pierce the oxide, will not pick up a charge.

More importantly, I'm dubious about using electrostatic separation with plant material. As a rough estimate, I'd expect the differential sensitivity to be explained by moisture content, in which case a simple density separation will be easier and less dangerous. I bring up danger, because if your plant material produces organic dust during the process you run the risk of dust explosions and fires.